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CASE REPORT
Amelioration of caudal thoracic
syringohydromyelia following surgical
management of an adjacent arachnoid cyst
W. Oxley and J. Pink
Willows Referral Service, Highlands Road, Shirley, Solihull, West Midlands B90 4NH
A nine-year-old male, neutered, pug was presented for investigation of progressive ambulatory paraparesis and pelvic limb ataxia of three months’ duration. Magnetic resonance imaging was suggestive of caudal thoracic syringohydromyelia with an adjacent intradural arachnoid cyst. The cyst was
marsupialised following dorsal laminectomy. Neurological status had improved 10 weeks following
surgery when repeat magnetic resonance imaging revealed reduced spinal cord compression both as a
result of resolution of the cyst and reduction in size of the syringohydromyelia. At 17 months following
surgery, the dog showed further improvements in neurological status, exhibiting mild pelvic limb ataxia
and proprioceptive deficits. Improved cerebrospinal fluid flow following surgery may have played a role
in the improvement in both conditions. The presence of syringohydromyelia in this context does not
preclude a favourable clinical outcome following surgical management.
Journal of Small Animal Practice (2011)
DOI: 10.1111/j.1748-5827.2011.01146.x
Accepted: 12 September 2011
INTRODUCTION
Syringohydromyelia has been widely reported in dogs in association with Chiari-like malformation, but has also been described
less commonly in more caudal locations in association with
other pathologies including arachnoid cysts (Galloway and others 1999, Skeen and other 2003, Jurina and Grevel 2004, Foss
and Berry 2009). Although clinical improvements in dogs with
Chiari-like malformation often follow medical and surgical interventions (Rusbridge and others 2006), associated reductions in
syrinx dimensions have not been reported (Skerrit and Hughes
1998, Vermeersch and others 2004, Rusbridge 2007). The term
syringomyelia refers to a syrinx located within the parenchyma of
the spinal cord, whilst hydromyelia describes dilation of the central canal (Milhorat and others 1995, Rusbridge and others 2006).
As such definitive syrinx classification is challenging premortem,
the terms syringomyelia or syringohydromyelia have been widely
used irrespective of precise syrinx location (Rusbridge and others
2000, Cappello and Rusbridge 2007).
Spinal arachnoid cysts have been reported as a cause of spinal
cord compression with increasing frequency in the veterinary literature, with almost 100 cases described since the first report in
1968 (Gage and others 1968, Parker and Smith 1974, Parker and
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others 1983, Bentley and others 1991, Dyce and others 1991,
McKee and Renwick 1994, Hardie and others 1996, Bagley and
others 1997, Cambridge and others 1997, Shamir and others
1997, Ness 1998, Frykman 1999, Galloway and others 1999,
Vignoli and others 1999, Webb 1999, Hashizume 2000, Rylander
and others 2002, Gnirs and others 2003, Skeen and others
2003, Jurina and Grevel 2004, Chen and others 2005, Sessums
and Ducote 2006, Goncalves and others 2008, Foss and Berry
2009). They are also reported as a relatively uncommon cause of
myelopathy in humans (Lee and Cho 2001). Despite this there
remains considerable confusion in both the human and veterinary literature as to the precise aetiology and structure of spinal
arachnoid cysts, with the term being used indiscriminately to
describe a number of different fluid-filled structures within the
vertebral canal. Indeed, the term “cyst” is itself confusing because
these structures generally do not comprise closed, epithelial lined
cavities (Nabors and others 1988, Hamburger and others 1998).
Although “pseudocyst” has been proposed as an alternative (Jurina
and Grevel 2004), in this report the term “cyst” will be used to
maintain consistency with the existing human and veterinary
literature.
This report describes a reduction in syringohydromyelia
dimensions following surgical management of an adjacent spinal
intradural arachnoid cyst (SIAC) in a pug.
1
W. Oxley & J. Pink
CASE REPORT
A nine-year-old male, neutered, pug weighing 7·2 kg was presented for investigation of progressive pelvic limb incoordination
and weakness of three months’ duration. On clinical examination, the dog exhibited moderate ambulatory paraparesis and
hindlimb ataxia. Neurological examination of the cranial nerves
and thoracic limbs was unremarkable. The cutaneous trunci
reflex was absent caudal to the 13th rib bilaterally but there was
no evidence of spinal hyperaesthesia. Pelvic limb segmental spinal
reflexes were normal although severe bilateral symmetrical proprioceptive deficits were present. The perineal reflex was intact.
These findings were suggestive of a T3-L3 myelopathy; differential diagnoses at this stage included intervertebral disc disease,
neoplasia, discospondylitis, inflammatory central nervous system
(CNS) disease and arachnoid cyst.
The results of haematology and serum biochemistry were within reference ranges. Following premedication with 0·02 mg/kg
acepromazine (IM; ACP Injection, Novartis Animal Health) and
0·2 mg/kg methadone hydrochloride (IM; Physeptone, Martindale Pharmaceuticals), general anaesthesia was induced with 2
mg/kg propofol (Rapinovet, Schering-Plough) and maintained
with isofluorane in oxygen. Magnetic resonance imaging (MRI)
of the thoracic and lumbar spine was performed (GE Signa HDe
1·5 T MRI scanner). Sagittal and transverse T1- and T2-weighted
sequences were acquired as well as a dorsal 3D FIESTA sequence.
Slice thickness was 2 mm (sagittal and 3D FIESTA) and 1·5 mm
(transverse).
MRI images are presented in Fig 1. There was a focal, extramedullary, intradural accumulation of cerebrospinal fluid (CSF)
located dorsal to the spinal cord over the caudal aspect of the
T11 vertebral body which was causing moderate to marked cord
compression. Craniocaudal, dorsoventral and laterolateral measurements were 4·1, 1·8 and 3·1 mm, respectively; differential
diagnoses included arachnoid cyst, synovial cyst, epidermoid
cyst and cystic neoplasia. An area of T2-weighted hyperintensity,
(B)
(A)
(C)
(D)
FIG 1. Preoperative T2W magnetic resonance imaging (MRI) images showing a focal, extramedullary, intradural accumulation of cerebrospinal fluid
(CSF) located dorsal to the spinal cord over the caudal aspect of the T11 vertebral body resulting in moderate to marked cord compression. An area
of T2-weighted hyperintensity extends from the level of the T8-T9 intervertebral disc space to just beyond the site of cord compression. There are
small-volume intervertebral disc protrusions at T11-T12 and T12-T13 with loss of the ventral subarachnoid space at T11-T12 and minimal right-ventral
extradural cord compression at that site. (A) Sagittal image. (B) Transverse image at the level of the T10-11 intervertebral disc space. (C) Transverse
image at the level of the caudal aspect of the T11 vertebral body. (D) Transverse image at the level of the T11-12 intervertebral disc space
2
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Amelioration of thoracic syringohydromyelia
consistent with syringohydromyelia, extended from the level of
the T8-T9 intervertebral disc space to just beyond the site of
cord compression, with the greatest dilation being at the level
of the T10-T11 intervertebral disc space. Craniocaudal length
was 25 mm and maximum diameter was 2·5 mm. Small-volume
intervertebral disc protrusions were evident at T11-T12 and
T12-T13. At T11-12 this was resulting in loss of the ventral subarachnoid space although this was preserved dorsally. There was
minimal right ventral extradural cord compression at T11-T12
associated with the protruded disc material.
A dorsal laminectomy of T11 was performed using a highspeed bur [Surgairtome II; Hall (Linvatec)] following a standard
approach (Toombs and Waters 2003, Piermattei and Johnson
2004). Caudally the laminectomy was similar to a Funkquist
Type B laminectomy with preservation of the cranial articular processes of T12 and the majority of the caudal articular
processes of T11 (Toombs and Waters 2003); cranially the
articular processes were preserved. The surface of the dura was
grossly normal and a midline durotomy was performed with the
aid of loupes (Keeler Standard Loupes 2·5x; Keeler Ltd.). Incision through a single, thickened layer of tissue released a large
volume of CSF, apparently under some pressure. The spinal cord
appeared depressed especially in the midline. A biopsy of the
meningeal layer was placed in 10% buffered formalin and submitted for histopathology. The margins of the durotomy were
sutured loosely to the laminal periosteum and facet joint capsule
bilaterally with 0·7 m polydioxanone (PDS II; Ethicon) before
routine closure. Recovery from surgery was unremarkable with
unchanged neurological status the following day.
Histopathology findings indicated the meningeal layer to
be composed of dense fibrous connective tissue. There was no
evidence of inflammatory infiltration. The surgical and histopathological findings were consistent with a dilation of the subarachnoid space bounded ventrally by pia mater and dorsally by
fibrosed arachnoid and dural layers.
Neurological examination four weeks following surgery
revealed mild ambulatory paraparesis and ataxia. The cutaneous
trunci reflex was abnormal as previously described; severe pelvic
limb proprioceptive deficits also remained. Neurological examination after 10 weeks revealed persistent mild ambulatory paraparesis and ataxia although this had subjectively improved. A reduced
cutaneous trunci reflex of normal extent had returned bilaterally,
and mild pelvic limb proprioceptive deficits were present. Repeat
MRI was recommended to quantify the degree of persistent spinal cord compression associated with the arachnoid cyst and to
exclude progression of the disc protrusions. MRI was performed
under general anaesthesia using the previously described protocols.
The dorsally located CSF accumulation over the caudal aspect
of the T11 vertebral body was no longer visible (Fig 2). There
was diffuse T2-weighted hyperintensity affecting the majority of
the cord at that level; differential diagnoses included glial scarring, parenchymal oedema and myelitis. The area of T2-weighted
hyperintensity consistent with syringohydromyelia had decreased
significantly in size to 6 mm × 1·2 mm × 1·2 mm and was located
at the level of the body of T11. There was no gross progression
of disc protrusion, and no further treatment was recommended.
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(A)
(B)
(C)
FIG 2. T2W magnetic resonance imaging (MRI) images obtained 10
weeks following surgery showing no evidence of cerebrospinal fluid (CSF)
accumulation dorsal to the spinal cord and reduction in size of syringohydromyelia. There is diffuse T2-weighted hyperintensity affecting the cord
at the level of the caudal aspect of the T11 vertebral body. (A) Sagittal
image. (B) Transverse image at the level of the cranial aspect of the T11
vertebral body. (C) Transverse image at the level of the caudal aspect of
the T11 vertebral body
At the time of writing (17 months following surgery) the dog’s
neurological status has remained stable with ongoing mild bilateral pelvic limb ataxia.
DISCUSSION
The incidence of concurrent syringohydromyelia and arachnoid
cysts in dogs is unknown. MRI findings have been reported in
only 21 cases of arachnoid cysts in dogs (Galloway and others
1999, Rylander and others 2002, Gnirs and others 2003, Skeen
3
W. Oxley & J. Pink
and others 2003, Jurina and Grevel 2004, Chen and others
2005, Sessums and Ducote 2006, Goncalves and others 2008,
Foss and Berry 2009), four of which had syringohydromyelia
(Galloway and others 1999, Skeen and others 2003, Jurina and
Grevel 2004, Foss and Berry 2009). In humans partial occlusion
of the subarachnoid space causing obstruction of CSF flow is
considered a key factor in the aetiopathogenesis of syringohydromyelia (Batzdorf 2005, Mauer and others 2008). This may
occur at the craniocervical junction (most commonly in association with Chiari malformations) (Levine 2004) but also more
caudally, secondary to arachnoid cysts, adhesive arachnoiditis,
neoplasia, spinal malformations and intervertebral disc protrusions (Klekamp 2002, Batzdorf 2005, Holly and Batzdorf 2006).
In dogs Chiari-like malformation is the most widely described
craniocervical junction lesion associated with syringohydromyelia, although brainstem tumours have also been reported (da
Costa and others 2004, Jung and others 2006). More caudally,
syringohydromyelia has been described in association with spinal
dysraphism (Furneaux and others 1973), vertebral malformation (Chauvet and others 1996) and intervertebral disc disease
(McGrath 1965), as well as arachnoid cysts.
Numerous theories have sought to describe a common aetiopathogenesis for syringohydromyelia in these diverse circumstances, and detailed reviews covering the human (Klekamp 2002) and
veterinary (Rusbridge and others 2006) literature are available.
Although the precise mechanisms remain unclear, there is recent
evidence to suggest that altered CSF flow may affect the drainage of
parenchymal extracellular fluid (ECF) with subsequent oedema and
syrinx formation (Klekamp 2002), a process promoted by parenchymal injury due to transmedullary transmission of CSF pulse
pressure waves (Rusbridge and others 2006). A prior theory, the
hydrodynamic hypothesis, proposed that subarachnoid CSF flow
obstruction elevated intra-ventricular pressure and forced CSF into
the central canal, resulting in hydromyelia (Gardner and Goodall
1950). In humans, however, greater pressures within syrinxes than
the adjacent subarachnoid space (Heiss and others 1999) are inconsistent with syrinx pressurisation from the fourth ventricle, and rostral central canal stenosis is present in most humans over 30 years
of age (Brodbelt and Stoodley 2007). The situation in dogs and cats
may not be entirely analogous because the central canal may remain
patent (Fitzgerald 1961, Rusbridge and others 2006); hydromyelia
occurred in up to 69% of dogs (Hall and others 1980, Yamada
and others 1996, Chuma and others 1997) and all cats (Becker
and others 1972, Rascher and others 1987) following experimental subarachnoid space occlusion, and occurs in combination with
hydrocephalus with severe fourth ventricle outflow obstruction in
dogs (Itoh and others 1996, Kirberger and others 1997) and cats
(Okada and others 2009). In addition, high syrinx pressures have
not been measured in clinical cases in dogs or cats.
In humans, spinal arachnoid cysts have been described in extradural, dural and intradural locations. Spinal extradural arachnoid
cysts are reported relatively frequently in humans and represent
accumulations of CSF within a thin-walled sac within the epidural
space (Nabors and others 1988). This sac is a diverticulum of the
arachnoid mater and thus communicates with the subarachnoid
space via a pedicle which penetrates a congenital or acquired dural
4
defect (Nabors and others 1988, Liu and others 2007). Spinal
dural cysts are rare lesions resulting from developmental duplication or splitting of an area of dura mater with accumulation of
CSF (Hamburger and others 1998). Neither spinal dural cysts, nor
extradural arachnoid cysts have been reported in animals.
Spinal intradural arachnoid cysts (SIACs) in humans result
from abnormalities of the arachnoid mater and subarachnoid
space including the arachnoid trabeculae and may be acquired
or congenital (Pradilla and Jallo 2007). Congenital SIACs in
humans are rare (Wang and others 2003) and may represent CSF
accumulation within the septum posticum (Perret and others
1962) or pathologically distributed arachnoid trabeculae (Agnoli
and others 1982). These typically discrete structures may be true
cysts with a cuboidal cell lining and amenable to complete resection (Mohindra and others 2010). Acquired SIACs in humans
occur in association with adhesive arachnoiditis (Petridis and
others 2010) which may be secondary to spinal surgery, myelography, infection, subarachnoid haemorrhage and spinal trauma
(Lee and Cho 2001, Wang and others 2003). The resultant
adhesions affecting the subarachnoid space may be discrete or
extensive (Batzdorf 2005), take the form of a dense focal web
(Mallucci and others 1997, Paramore 2000), or incorporate discrete pockets of CSF (Kazan and others 1999, Tumialan and others 2005). MRI has been used to measure CSF flow in patients
with arachnoid adhesions (Mauer and others 2008, Gottschalk
and others 2010); sagittal cardiac-gated phase-contrast CSF flow
studies have revealed reduced CSF flow at the level of surgically
confirmed arachnoid adhesions. Postoperative imaging revealed
normalisation of CSF flow after adhesion resection, as well as
reduced dimensions of associated syringomyelia in the majority
of patients (Mauer and others 2008). Increased CSF hydrostatic pressure rostral to an area of arachnoid adhesions may result
in subarachnoid space dilation and cord compression (Paramore 2000), a concept supported by fluid dynamics modelling
(Bilston and others 2006). Such CSF accumulations are erroneously described as cysts, contributing to the confusion surrounding the nomenclature of these lesions.
Parallels exist between descriptions of SIACs in the veterinary
and human literature. A congenital SIAC has never been specifically described in a dog, although cases in puppies (Ness 1998) and
related individuals (Ness 1998, Frykman 1999) have been reported. There are four reports of the removal of entire cysts (Bentley
and others 1991, Frykman 1999, Skeen and others 2003, Foss
and Berry 2009), a finding more consistent with congenital rather
than acquired SIACs. Although detailed descriptions of lesions are
limited in the veterinary literature, the majority of those available
are consistent with acquired SIACs. The most detailed anatomic
description of a canine SIAC (Dyce and others 1991) describes
a dilated subarachnoid space beneath a thickened dura, open
cranially but sealed caudally by pia-arachnoid adhesions, findings almost identical to those described by Paramore in humans
(Paramore 2000). Caudal or circumferential adhesions have been
described in four further reports (Gage and others 1968, Frykman
1999, Galloway and others 1999, Gnirs and others 2003), and
there are 19 specific descriptions of subarachnoid space dilation
(Dyce and others 1991, McKee and Renwick 1994, Ness 1998,
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Amelioration of thoracic syringohydromyelia
Gnirs and others 2003). It has been suggested that the predisposition of high cervical and thoracolumbar sites to SIAC formation
is linked to their relatively high mobility leading to chronic meningeal microtrauma and adhesive arachnoiditis (Gnirs and others
2003, Skeen and others 2003). SIACs have also been reported in
association with caudal cervical spondylomyelopathy (Skeen and
others 2003), vertebral deformity (Bagley and others 1997) and
disc protrusion (Chen and others 2005), conditions potentially
associated with increased mobility. SIACs in dogs following spinal
surgery (Galloway and others 1999) and fracture (Skeen and others 2003) may also have been the result of arachnoid adhesions.
The link between acquired adhesive arachnoiditis, reduced
CSF flow and syringohydromyelia formation is less well established in dogs. This association was considered in a recent case
report of syringohydromyelia where mild disc protrusion (affecting the subarachnoid space only, as in this case) was proposed as
a possible cause of arachnoid adhesions and reduced CSF flow
(Cagle 2010). An attempt to quantify CSF flow using MRI at
the site of an SIAC has been made in one dog (Gnirs and others
2003). CSF flow at the site of the SIAC (analysed by the 2D Fourier transform steady-state free precession technique) was found
to be normal. Prior myelography had however revealed an abrupt
end to the contrast medium column, a finding inconsistent with
normal subarachnoid fluid flow. Further MRI studies, possibly
utilising a sagittal cardiac-gated phase-contrast technique, may
shed light on this apparent discrepancy.
The recognition of partial subarachnoid space obstruction
as a key factor in the aetiopathogenesis of syringohydromyelia
has led to a recent trend in human surgery towards restoration
of CSF flow rather than traditional shunting procedures (Mallucci and others 1997, Batzdorf 2005). In the context of SIACs,
microsurgical removal of obstructions to CSF flow is considered
the treatment of choice in humans (Mauer and others 2011).
SIAC resolution and reduction in syrinx dimensions have been
reported with this approach (Holly and Batzdorf 2006, Mauer
and others 2008, 2011). The surgical management of SIACs in
dogs has focussed on decompression of the spinal cord rather
than restoration of CSF flow, with the additional aim of prevention of recurrence by dural fenestration or marsupialisation. The
prognosis for short-term improvement or resolution of neurological signs is favourable, probably as a result of acute cord decompression. Deterioration at greater than one year after surgery has
been reported in four dogs (Frykman 1999, Skeen and others
2003), possibly as a result of fibrosis of the dural defect (McKee
and Renwick 1994). Follow-up MRI has not been previously
reported following surgical management of concurrent syringohydromyelia and SIAC, but has been described following foramen magnum decompression for Chiari-like malformation
where syrinx size was unchanged at follow-up up to 3·8 years
(Vermeersch and others 2004, Rusbridge 2007). Two reports
have described reduction in size of a cervical syrinx following
treatment of a compressive extramedullary lesion at the foramen
magnum (da Costa and others 2004, Takagi and others 2005).
In both cases MRI revealed concurrent ventriculomegaly, and
the syrinx probably represented communicating hydromyelia, a
situation documented by postmortem examination in dogs with
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brainstem meningioma (Jung and others 2006) and epidermoid
cyst (MacKillop and others 2006).
In the current case, these concepts may support a role for
reduced CSF flow in the aetiopathogenesis of both the subarachnoid space dilation/SIAC and the syringohydromyelia. It is proposed that arachnoid adhesions may have occurred secondary
to the T11-12 disc protrusion, although an unrelated cause is
possible. Resultant obstruction of CSF flow could have caused
rostral subarachnoid space dilation and syringohydromyelia, the
latter either through altered ECF dynamics or as a result of communicating hydromyelia. The authors suggest that improvement
of CSF flow following surgery (albeit through marsupialisation
rather than restoration of subarachnoid space patency) contributed to the resolution of the subarachnoid space dilation/SIAC
and reduction in syrinx size.
To the authors’ knowledge, reduction in syrinx dimensions and
concurrent clinical improvement following surgical management
of an SIAC in a dog has not previously been reported. This case also
demonstrates that the presence of syringohydromyelia and mild
disc protrusion in this context do not preclude a favourable clinical outcome following surgical management. Although technically
demanding procedures such as microsurgical dissection of arachnoid adhesions and duraplasty are probably unnecessary in dogs,
consideration of subarachnoid space patency and CSF flow may
prove a useful component of a rational surgical approach to these
conditions and lead to improved long-term surgical outcomes.
Conflict of interest
None of the authors of this article has a financial or personal
relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
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Journal of Small Animal Practice
•
© 2011 British Small Animal Veterinary Association
A comparison of complication rates and clinical outcome between tibial plateau levelling
osteotomy and a modified cranial closing wedge osteotomy for treatment of cranial cruciate
ligament disease in the dog
Bill Oxley, MA VetMB CertSAS, Toby J. Gemmill, BVSc MVM DSAS(Orth) Diplomate ECVS, Alasdair
Renwick, BVMS DSAS(Orth), Dylan Clements, BSc BVSc PhD DSAS(Orth) Diplomate ECVS, W. Malcolm
McKee, BVMS MVS MACVSc DSAO
Abstract
OBJECTIVE: To report complication rates and clinical outcomes following tibial plateau levelling osteotomy
(TPLO) and a modified cranial closing wedge osteotomy (mCCWO) for treatment of cranial cruciate ligament
disease in dogs.
STUDY DESIGN: Prospective cohort study.
SAMPLE POPULATION: Dogs weighing between 20kg and 60kg with unilateral cranial cruciate ligament disease
treated by either TPLO (n=97) or mCCWO (n=74).
METHODS: Clinical and radiographic assessments including lameness score, morphometric measurements and
tibial plateau angle (TPA) were made prior to surgery and at eight weeks following either TPLO or mCCWO. Long
term outcome assessment by owner questionnaire or interview was undertaken at a minimum of six months postoperatively.
RESULTS: Significant differences in lameness scores between groups were not identified at short or long term
follow-up. Major complication and re-operation rates did not differ significantly between groups (TPLO 7.2% and
6.1%; mCCWO 9.5% and 5.4%). Median post-operative TPA did not differ significantly between groups (TPLO
group 5.5°; mCCWO group 6.5°). At >6 months owner assessed lameness, disability, quality of life and
satisfaction were not different between groups and were good in 90-97% of cases.
CONCLUSIONS: TPLO and mCCWO are associated with similar complication rates and clinical outcomes when
performed by surgeons experienced with the surgical techniques.
CLINICAL RELEVANCE: Both techniques can be considered equally appropriate for the treatment of cranial
cruciate ligament disease in dogs weighing between 20kg and 60kg.
Introduction
Cranial cruciate ligament (CCL) disease is one of the more common causes of pelvic limb lameness in dogs1.
Although the precise aetiopathogenesis of this condition remains unclear2, the resultant stifle instability has
become accepted as an important pathophysiological mechanism leading to the development of progressive
osteoarthritis and, in some cases, meniscal injury3,4. A key component of this instability was defined by Slocum
and Devine as cranial tibial thrust (CTT), a cranially directed tibiofemoral shear force generated as axial
compression acts on the caudally inclined tibial plateau during weight bearing5. Whilst definitive descriptions of the
forces acting across the normal and cranial cruciate deficient canine stifle remain elusive3, most surgical
approaches aim to counteract CTT either through static or dynamic stabilisation during the stance phase of gait.
Cranial closing wedge osteotomy (CCWO) was the first dynamic stabilisation technique described for the
treatment of CCL disease6. Post-operative tibial plateau angles were not reported in the original paper6, although a
subsequent in vitro biomechanical study demonstrated elimination of CTT following CCWO at a tibial plateau
angle (TPA) of 4-6°7. Slocum subsequently described a radial tibial plateau levelling osteotomy (TPLO)8 which
also results in neutralisation of CTT at a similar TPA9,10. Since both approaches appear capable of providing
dynamic stifle stabilisation other factors must be considered when choosing between these techniques, and
indeed other biomechanically appropriate surgeries. Such factors are diverse and include functional outcome,
complication rates, technical difficulty, incidence and severity of unwanted effects of surgery (e.g. limb shortening),
and surgeon preference.
Assessment of long term function in clinical cases is challenging11, and in the context of TPLO and CCWO has
predominantly been limited to owner assessed outcomes which do not indicate a superior outcome following either
technique. Corr and Brown reported 92% ‘good’ or ‘excellent’ outcomes with both techniques12. Similar outcomes
have been reported at rates of 86%13 and 94%6 following CCWO, and 94%8 and 94.7%14 following TPLO.
Complication rates reported following TPLO in four large retrospective cases series published between 2003 and
200614-17 were analysed by Boudrieau to give an overall complication rate of 26.3%18. Two large more recent
retrospective studies reported lower complication rates of 9.7% and 14.8%4,19. Re-operation rates have been
reported at between 5% and 9%4,15,17. Much less data is available for CCWO although relatively high complication
rates of 31-36%12,20 and re-operation rates of 12-18%6,12,20,21 have been reported with this technique following
treatment of CCL rupture in medium and large dogs.
We hypothesised that the outcomes and complication rates of TPLO and a modified CCWO (mCCWO) for the
treatment of CCL disease would be similar when performed by surgeons experienced in the relevant technique,
when assessed in a prospective cohort study.
Materials and Methods
Inclusion criteria, and data collection and surgical protocols were determined prior to instigation of the study in
June 2009. Dogs eligible for inclusion weighed between 20kg and 60kg, were presented with naturally occurring
unilateral CCL disease, and had no evidence of any other concurrent stifle pathology on physical examination and
radiographic evaluation. Dogs presented for revision CCL disease surgery were excluded, as were dogs
presented with any evidence of bilateral CCL disease. Consecutive cases meeting these criteria were enrolled
until the termination of the study in March 2011. Cases re-presented for treatment of contralateral CCL disease
within the study period, or prior to completion of long term follow up, were retrospectively excluded. During the
study period two surgeons performed TPLOs, and two performed mCCWOs; within each group one surgeon was
a boarded specialist and one an experienced resident. All surgeons had completed a minimum of 200 tibial
plateau levelling procedures prior to the start of the study. Case allocation to a given surgeon occurred as
reception staff (who were unaware of the existence of the study) arranged routine appointments with the next
available surgeon.
Pre-operative evaluations
Following clinical examination, investigations including synoviocentesis, goniometry of the affected stifle22,
measurement of mid-thigh muscle circumference, and digital radiography were performed either under heavy
sedation or under general anaesthesia prior to surgery. Caudocranial and mediolateral radiographs were centred
on the stifle but included the tarsus23. Projections were repeated if there was not complete superimposition of the
femoral condyles on the mediolateral projection. Conventional tibial plateau angle24 was recorded and an
osteophyte score attributed based on maximum osteophyte size (none, mild <1mm, moderate 1-3mm, severe
>3mm). Additional data collected included duration of lameness and surgeon assessed lameness score (from 0 to
10, where 0 corresponded to no observable lameness and 10 to non-weight bearing lameness).
Surgical planning
In both the TPLO and mCCWO groups target TPA was 5°. The planned osteotomy position was marked on a
digital radiograph and measurements made in relation to bony landmarks to maximise intra-operative accuracy. To
minimise translation of the tibial plateau, TPLO osteotomies were centred as near as possible to the base of the
intercondylar eminence25 (Figure 1). Three millimetre blade size increments were available (between 18mm and
30mm). Rotation distance was planned according to a standard published table (New Generation Devices, Glen
Rock, NJ).
MCCWOs were planned using a novel method (Figure 2). An isosceles triangle shaped wedge was positioned as
proximally as possible whilst preserving sufficient bone stock for plate fixation, and remaining an adequate
distance distal to the tibial tuberosity. This distance was a minimum of five millimetres for dogs under 25kg, and a
minimum of 10 millimetres for heavier dogs. The planned wedge angle varied with TPA to compensate for greater
tibial long axis shift associated with larger wedges26 (Table 1).
Surgical technique
Standard premedication was with acepromazine (0.01-0.03mg/kg; ACP Injection, Novartis Animal Health) and
methadone hydrochloride (0.3mg/kg; Physeptone, Martindale Pharmaceuticals) by intramuscular injection.
Methadone was re-administered as necessary during and following surgery. Meloxicam (0.2 mg/kg; Metacam,
Boehringer Ingelheim) or carprofen (4 mg/kg; Rimadyl, Pfizer Animal Health) were administered subcutaneously at
premedication. Clavulanate potentiated amoxicillin (20 mg/kg; Augmentin, Glaxo-SmithKline) was administered
intravenously at induction and repeated every 90 minutes during anaesthesia.
For both TPLO and mCCWO, dogs were positioned in lateral recumbency with the affected side adjacent to the
table. Following hanging limb surgical preparation, standard four quarter draping was performed prior to
application of an adhesive iodine-impregnated drape which was subsequently sutured to the wound edges. A
standard medial approach to the stifle and proximal tibia was made and a craniomedial sub-patellar arthrotomy
performed. The integrity of the cranial cruciate ligament was assessed; rupture was recorded as partial if taut
fibres were palpable, and complete when the remaining ligament was flaccid or fully ruptured. Stifle distractors
were placed and the menisci visually examined and probed with a meniscal hook (Veterinary Instrumentation,
Sheffield, UK). Meniscal injuries were treated by resection of the damaged portion; meniscal release was not
performed. Caudal and lateral subperiosteal elevation and placement of moistened gauze swabs was performed
prior to osteotomy in all cases. A jig (Slocum Enterprises, Eugene, OR) was used for all TPLOs but not mCCWOs.
TPLO osteotomies were made using an in-line oscillating saw (De Soutter, Aston Clinton, UK) with New
Generation Devices TPLO blades (Glen Rock, NJ). MCCWO osteotomies were performed using a MiniDriver
K220 sagittal saw; an attempt was made to preserve a small ‘hinge’ of intact caudomedial cortex. For TPLO,
reduction was maintained using a cranially positioned K-wire placed adjacent to the patellar tendon; the K-wire
was removed following plate application. For mCCWO, 0.8mm or 1mm orthopaedic wire (Veterinary
Instrumentation, Sheffield, UK) was placed through 1.1mm or 1.5mm drill holes positioned cranially immediately
proximal and distal to the osteotomy surfaces; the osteotomy would reduce about its caudal hinge as this was
tightened. This wire was placed as a loop tightened with a single twist medially, and was not removed. Care was
taken to ensure the cranioproximal drill hole was not adjacent to the insertion of the patella tendon. In all cases an
appropriately sized locking TPLO plate was applied (Synthes, West Chester, PA). All proximal screws were
locking screws; distal screws were either all standard cortical screws or included one or two locking screws
according to surgeon preference and plate size. Routine layered closure was performed following lavage, and a
sterile adhesive dressing applied for 24 hours. Surgical time and any intra-operative complications were recorded.
Post-operative radiographs were assessed with respect to TPA and completeness of osteotomy reduction; the
widths of any osteotomy gaps were recorded. Post-operative radiographs from the same cases illustrated preoperatively are presented in Figure 3. Dogs were routinely discharged on the day following surgery; the presence
or absence of consistent weight bearing on the operated limb was recorded.
All dogs received a five day course of either clavulanate potentiated amoxicillin (20mg/kg; Synulox, Pfizer Animal
Health) or cephalexin (20mg/kg; Rilexine, Virbac Animal Health) and a four week supply of either meloxicam
(0.1mg/kg PO SID; Metacam) or carprofen (2mg/kg PO BID; Rimadyl). Lead-only exercise for 10-15 minutes three
times daily was advised until follow-up examination after eight weeks. Recommended physiotherapy comprised
passive range of motion exercises performed by the client three times daily for the first two weeks following
surgery.
Complications
Complications were recorded either following re-presentation of a patient or following notification from an owner or
referring veterinary surgeon; in addition owners were questioned regarding complications at scheduled eight week
re-examination and at long term follow-up.
All known major complications were investigated by the surgeon responsible for the case. Joint sepsis was
diagnosed on the basis of synovial fluid neutrophilic pleocytosis and / or positive bacterial culture. Implant related
infection was diagnosed when synoviocentesis was unremarkable but aspiration adjacent to the plate revealed
neutrophilic pleocytosis and / or positive bacterial culture. All cases of late meniscal injury were diagnosed at
arthrotomy. Implant failure was diagnosed when radiography revealed gross post-operative alteration in the
orientation of the osteotomy fragments considered to be responsible for lameness.
Almost all minor complications were assessed and treated by referring vets. When available a specific diagnosis
was recorded (e.g. seroma or wound self-trauma). All other reported episodes comprised non-specific wound
problems such as minor peri-incisional swelling or erythema which resolved rapidly with symptomatic treatment.
These cases were recorded as minor incisional complications.
Short-term follow-up
Re-examination was scheduled at eight weeks following surgery and was undertaken by the surgeon responsible
for the case. Clinical examination included lameness scoring and measurement of stifle standing angle. Mid-thigh
muscle circumference measurement, stifle goniometry and radiographs were obtained under heavy sedation.
Radiographic assessment included TPA, implant integrity and osteotomy healing. Osteotomy healing was graded
as either complete (remodelled callus at all cortices and / or osteotomy indistinct), good (active bridging callus,
osteotomy mostly blurred or filled with callus), poor (some evidence of bone healing but unbridged cortices and/or
osteotomy gaps), or none.
Long term follow-up
Long term follow-up was undertaken at a minimum of six months following surgery. All owners were sent a postal
questionnaire (Appendix 1). Owners were firstly asked to list any complications and to assess the pain associated
with surgery as none, mild, moderate or severe. The same choices were given for the severity of current lameness
and current disability associated with the operated limb. Finally overall quality of life and owner satisfaction were
rated as good, fair or poor. Non-responding owners were telephoned and the same questions asked.
Statistical analysis
Continuous data were analysed for normality by the Kolgomorov-Smirnov test. Data with normal distribution were
analysed by the Students t-test (independent data) or paired Students t-test (continuous data). Data with a nonparametric distribution were analysed using the Mann-Whitney U test. Ordinal data was analysed with the Mann
Whitney U test (independent variables) or the Wilcoxian signed rank test (paired data). Categorical data were
analysed with the Chi squared test, or Fishers exact test if the expected frequency of data in any cells was <5.
Statistical significance was set at p<0.05, and the False Discovery Rate correction was applied to all comparisons
to correct for the multiple permutation error rate27. All analyses were performed using a commercially available
statistical software package (Minitab for Windows, Coventry, UK).
Results
Signalment
During the study period 100 dogs met the inclusion criteria for inclusion in the TPLO group; three of these dogs
were re-presented for contralateral CCL disease and were excluded leaving a total of 97 stifles (55 left, 42 right).
Median age was 58 months (95% CI = 51.5, 64.5; range, 16-144 months) and median weight was 36kg (95% CI =
34, 38; range, 21-60kg). There were 50 male (37 neutered) and 47 female (39 spayed) dogs. Median duration of
lameness prior to surgery was 72 days (95% CI = 55.5, 97; range, 2-365 days) and median osteophyte score was
1.5. For the mCCWO group 79 dogs met the inclusion criteria; five were excluded due to subsequent contralateral
CCL disease leaving a total of 74 stifles (34 left, 40 right). Median age was 60.5 months (95% CI = 53.5, 68;
range, 12-120 months) and median weight was 36.5kg (95% CI = 34.5, 38.5; range, 20-56kg). There were 38
male (32 neutered) and 36 female (35 spayed) dogs. Median duration of lameness was 67 days (95% CI = 52, 95;
range, 2-365 days) and median osteophyte score was 1.5. Breeds represented on more than five occasions
included (n; TPLO / CCWO); Labrador retriever (39;21/18), crossbreed (30; 13/17), rottweiler (25;13/12), golden
retriever (21; 16/5), boxer (17; 10/7), dobermann (9; 6/3), German shepherd dog (9; 3/6), husky (6; 2/4) and
Staffordshire bull terrier (6; 5/1). Seventeen other breeds were represented five or fewer times. There was no
significant difference between TPLO and mCCWO groups with respect to side affected (P = 0.41), age (P = 0.91),
weight (P =1.00), gender (P =1.00), neuter status (P = 0.13), duration of lameness (P = 1.00), osteophyte score (P
= 0.30) and breed (comparing the common breeds listed above; P = 1.00).
Follow-up interval and availability
Short-term follow-up data was available for 70/97 TPLO cases (72%) and 57/74 mCCWO cases (77%). Median
intervals to short term follow-up were 58 days (95% CI = 56.5, 60; range, 41-105 days) for the TPLO group and
56.5 days (95% CI = 54, 59.5; range, 40-96 days) for the mCCWO group. Long-term follow-up data was available
for 90 TPLO cases (93%) and 69 mCCWO cases (93%). Median intervals to long term follow-up were 458.5 days
(95% CI = 417, 494; range, 202-782 days) for the TPLO group and 448 days (95% CI = 407.5, 492; range, 180731 days) for the mCCWO group.
Surgical data
The rate of medial meniscal injury was not significantly different between groups (P = 0.44; TPLO group 29%,
mCCWO group 38%). The rate of complete versus partial cruciate ligament rupture was not significantly different
between groups (P = 0.07; TPLO group 71% complete, mCCWO group 86%).
Median pre-operative TPA was not significantly different between groups (P = 1.00; TPLO group 24.5° (95% CI =
23.5, 25; range, 15-38°), mCCWO group 24° (95% CI = 23.5, 25; range, 15-35°)). Median post-operative TPA
was not significantly different between groups (P = 0.06; TPLO group 5.5° (95% CI = 5, 6; range, 2-13°), mCCWO
group 6.5° (95% CI = 6, 7; range, 3-10°)). In both groups there was a significant increase in median TPA between
surgery and short-term follow-up (TPLO group median follow-up TPA 7° (95% CI = 6, 7; range, 2-20°), P < 0.001;
mCCWO group 8° (95% CI = 7.5, 8.5; range, 2-13°), P < 0.001).
Median surgical time for the TPLO group 67.5 minutes (95% CI = 67.5, 70; range, 55-100 minutes) and for the
mCCWO group 73.5 minutes (95% CI = 71.5, 75; range, 54-100 minutes); these were significantly different (P =
0.001). Plate selection is summarised in Table 2; significantly fewer broad plates were applied in the TPLO group
(21%) than the mCCWO group (42%) (P = 0.01). Radiographically apparent osteotomy gaps were significantly
less frequent in the TPLO group (31%) than the mCCWO group (59%) (P = 0.002); this data is summarised in
Table 3. In the mCCWO group lateral gaps were most frequent and had a median width of 1.1mm; in the TPLO
group gaps were usually cranial had a median width of 1mm.
The rate of intra-operative complications was not significantly different between groups (P = 0.88). In the TPLO
group three such complications occurred (3%) and included fibular fracture during rotation, a hole drilled in the
fibula (which subsequently fractured), and misdirection and failure to lock of a proximal screw. In the mCCWO
group two intra-operative complications occurred (3%) and included misdirection and failure to lock of a proximal
screw and osteotomy malreduction (with 7° tibial varus deformity). With the exception of the latter case (where
long term owner assessed lameness was mild), none of these cases experienced known post-operative
complications or were assessed to be lame by their owners at long-term follow-up.
The severity of owner assessed pain at the time of discharge was not significantly different between groups (P =
0.64); these results are summarised in Table 4.
Lameness
Median pre-operative lameness score was 5.5/10 for both groups (P = 0.99). Median lameness score at short term
follow-up was 1/10 for both groups (P = 1.00) and differed significantly from before surgery for both groups (P <
0.001).
At long term follow-up owner assessed scores for lameness and disability due to the affected limb did not differ
significantly between groups (P = 1.00 and 0.88 respectively). For the TPLO group these were considered absent
or mild in 95.5% (lameness) and 97.7% (disability) of cases. For the mCCWO group lameness and disability were
both considered absent or mild in 92.7% of cases. Client assessed outcome scores are summarised in Table 5.
There was no significant difference between groups with respect to the frequency of consistent weight bearing on
the operated limb on the day following surgery (P = 1.00). This was noted in 73% of TPLO cases and 75% of
mCCWO cases.
Goniometry and thigh muscle circumference
At short term follow-up there had been significant increases in stifle extension in both the TPLO and mCCWO
groups (P < 0.001); this was associated with significantly increased stifle ROMs in both groups (Table 6). Median
standing angle at short term follow-up was not significantly different between groups (P = 0.18; TPLO group 135°
(95% CI = 135, 137.5), mCCWO group 134° (95% CI = 132.5, 135)). There was no significant difference in thigh
muscle circumference between pre-operative values and those at short term follow-up in the TPLO group (P =
0.19) or the mCCWO group (P = 0.43).
Complications
There was no significant difference between groups in the rate of major complications (P = 1.00) or the reoperation rate (P = 0.72). In the TPLO group major complications were seen in seven cases (7.2%). These
comprised four late meniscal injuries (LMI), one of which also had joint sepsis and another of which also had
grade 2/4 medial patellar luxation (MPL), one further case of joint sepsis, one confirmed implant associated
infection requiring implant removal, and one implant failure. The implant failure comprised lateral osteotomy
collapse with fibular fracture and resulted in 11° tibial valgus and a TPA of 20°; revision surgery was declined, and
there was moderate persistent lameness. All other cases required surgical treatment, giving a re-operation rate of
6.1%. Long term owner assessed lameness was available in the following cases; LMI alone - mild (1), absent (1);
LMI and MPL - mild; implant sepsis - absent. In the mCCWO group major complications were seen in seven cases
(9.5%) and comprised three septic joints (all of which resolved with extended antibiotic treatment), and four late
meniscal injuries (treated by debridement at arthrotomy) giving a re-operation rate of 5.4%. Long term owner
assessed follow-up was available for two of the sepsis cases where lameness was absent (1) or mild (1), and all
LMI cases where lameness was absent (2), mild (1) and severe (1).
There was no significant difference in the rate of reported minor complications between groups (P = 0.41). In the
TPLO group these were reported in 16 cases (17.2%) and comprised seroma (7), minor incisional complications
(5), wound self-trauma requiring re-suturing (3), and one case with moderate serosanguinous wound discharge
persisting for 72 hours. In the mCCWO group minor complications were reported in eight cases (10.8%) and
comprised minor incisional complications (4), wound self-trauma requiring re-suturing (3), and one case of implant
associated infection which resolved with antibiotic treatment.
There was a significant difference between groups in osteotomy healing grade at short term follow-up, with the
mCCWO group more advanced (P = 0.01) (Table 7); specific treatment of delayed union was not necessary in any
case.
Owner assessed quality of life and satisfaction
At long term follow-up overall owner satisfaction and owner assessed overall quality of life (QOL) scores did not
differ significantly between groups (P = 1.00).
For the TPLO group owner assessed QOL scores were available for 89/97 cases (92%); grades were as follows good (n=81; 91%), fair (n=7; 8%), poor (n=1; 1%). In three cases QOL was influenced by other factors
(neurological disease (n=2), other orthopaedic disease (n=1)). If these cases were excluded from analysis
outcomes were as follows - good (n=81; 94%), fair (n=5; 6%), poor (n=0). In this group overall owner satisfaction
was reported in 90/97 cases (93%); grades were as follows - good (n=81; 90%), fair (n=7; 8%), poor (n=2; 2%).
For the mCCWO group owner assessed QOL scores were available for 69/74 cases (93%); grades were as
follows - good (n=62; 90%), fair (n=4; 6%), poor (n=3; 4%). In three cases QOL was influenced by other factors
(other orthopaedic disease (2) and epilepsy (1)). If these cases were excluded from analysis outcomes were as
follows - good (n=62; 94%), fair (n=1; 1.5%), poor (n=3; 4.5%). In this group overall owner satisfaction was
reported in 69/74 cases (93%); grades were as follows - good (n=64; 93%), fair (n=2; 3%), poor (n=3; 4%).
Discussion
In the absence of compelling evidence favouring a specific surgical option for treatment of CCL disease the topic
has remained the subject of debate and ongoing research. In recent years this has focused on TPLO and tibial
tuberosity advancement, with other techniques receiving less attention. Our results indicate that outcomes and
complication rates of mCCWO are similar to those following TPLO, both within the context of a prospective cohort
study, and in comparison with recent large retrospective studies4,19.
CCWO technique has changed very little since its original description by Slocum in 19846. Performed as an
adjunct to hamstring muscle advancement, a cranially based bone wedge was removed from the proximal third of
the tibia. This wedge was asymmetric since its base was perpendicular to the long axis of the tibia, resulting in a
cranial cortical ‘overhang’ following reduction with caudal cortical alignment. Where descriptions are available,
subsequent in-vitro7,26 and clinical12 reports have employed a similar technique, with wedges excised around the
level of the distal tibial crest7,12,13,20,26. Tibial long axis shift (TLAS) associated with distal osteotomy position and
caudal cortical alignment26 results in under-correction of TPA unless the wedge angle is increased to at least
TPA+5°7. In our study wedge geometry was modified to reduce TLAS. Firstly, the isosceles design permitted a
more proximal osteotomy position (due to reduced wedge size for a given angular correction28 and the transverse
orientation of the wedge). Precise osteotomy planning relative to the tibial tuberosity further optimised proximal
positioning. Secondly, optimal cranial and caudal cortical alignment is achieved, further reducing TLAS. Variable
post-operative TPA has been reported following traditional CCWO12,20 and has been cited as a disadvantage of
the technique3. In this study appropriate and predictable TPAs (median 6.5°; range, 3-10°) were achieved with
wedge angles between TPA-5° and TPA-2° despite a wide pre-operative TPA range (15-35°). This is attributed to
minimisation of the unpredictable effect of TLAS and precise pre-operative wedge planning. Additional cited
disadvantages of traditional CCWO include unaesthetic craniocaudal tibial angulation21 and patellar baja3 as a
result of limb shortening. Such changes are mitigated by reduced wedge size29 and are likely to be small; a recent
investigation of a cranial neutral wedge osteotomy demonstrated tibial shorting of only 2.5mm with the traditional
CCWO technique29.
Inclusion criteria were designed to include medium and large breed dogs with naturally occurring cruciate disease.
Dogs weighing over 60kg were excluded to maintain implant consistency between groups since the authors
currently employ double plating of mCCWOs in dogs >60kg. Dogs weighing under 20kg were excluded since
previous studies have shown good outcomes following CCWO30,31; we would routinely perform mCCWO rather
than TPLO in most of these cases. Similarly bilaterally affected cases were excluded to avoid the potential
confounding effects of bilateral disease.
Subjective osteophyte scoring systems have been used to monitor the progression of osteoarthritis in the canine
stifle32,33. Although this approach is associated with moderate inter-observer reliability33, we chose to further
standardise osteophyte scoring on the basis of maximal osteophyte size (using a modification of a system used to
assess the radiographic severity of canine elbow osteoarthritis34). Osteophyte scores derived in this way did not
vary between groups; pre-operative TPA, the number of meniscal injuries, and the frequency of partial cruciate
ruptures were also equivalent between groups and did not differ widely from reported values4,19. The rate of intraoperative complications was not significantly different between groups. Misdirection and failure to lock of a
proximal screw occurred in both groups and was not noted during screw placement in either case. Although crossthreading may confer a degree of angular stability, it is probable that non-locking screw function would result. Invitro torsional testing of a hybrid locking plate construct with one conventional and two locking screws per
fragment found no difference to an all locked construct35, although reduced torsional stability with only one locking
screw per fragment has also been reported36. In our two cases TPA was unchanged at short-term follow-up and
post-operative complications were not encountered. We did not see haemorrhage from the cranial tibial artery in
any case. Whilst there is evidence that caudolateral placement of swabs to protect the artery may be unnecessary
during TPLO37,38, a higher risk of vascular trauma may exist during mCCWO due to the oblique saw blade
orientation necessary for caudolateral corticotomy, and swab placement is therefore prudent.
Median post-operative TPA was not significantly different between groups but was slightly further from the target
of 5° in the mCCWO group (6.5°) than TPLO group (5.5°). On this basis a slight increase in wedge angle from
those presented in Table 1 could be considered, although it seems improbable that this difference would be of
clinical significance. Possibly of greater importance is the ability of the mCCWO technique to achieve predictable
post-operative TPA values (range 3-10°). Imprecision during osteotomy execution and the unpredictable effect of
TLAS may explain the observed deviation from target TPA. Post-operative TPA range following TPLO was in fact
slightly wider (2-13°), although still within the range associated with good clinical outcomes in a previous study39.
Failure to achieve planned proximal fragment rotation was recorded in three out of four of our TPLO cases where
post-operative TPA was greater than 10°. This occurred despite a pre-operative TPA of 18° in one case, and may
have been associated with ankylosis of the proximal tibiofibular joint40 or cranial centring of the osteotomy
(resulting in fibular tethering)25. MCCWO is apparently less susceptible to this difficulty, although osteotomy
reduction can become more challenging at wedge angles >25°.
Small osteotomy gaps were commonly noted following mCCWO and most frequently affected the lateral cortex.
The potential adverse biomechanical effects of a small transcortical gap were not manifest either in terms of
implant failure in any case or poor osteotomy healing. This may have been due to the splinting effect of the fibula
or the robust mechanical properties of the construct following locking plate application. MCCWO osteotomy gaps
were usually associated with an incomplete osteotomy; on occasions when the caudomedial cortical ‘hinge’ failed
complete reduction usually resulted, and could be maintained easily during plate application by the cranial wire.
Small TPLO osteotomy gaps were almost always cranial and occurred in 31% of cases. These probably
developed during proximodistal compression of a slightly asymmetric osteotomy (resulting from use of a nonbiradial blade). Complications relating to these small cranial gaps were not encountered. Medial and lateral gaps
were unusual, in contrast to a recent report where these occurred in 26% of cases40.
In both groups median TPA increased by 1.5° between post-operative and short-term follow-up radiographic
assessments. Previous studies have reported median TPA increases of 1.5°41 and 1.8°40 following TPLO; after
CCWO a median increase of 3° has been reported20. Causative factors remain uncertain, although in a recent
paper greater TPA increases after TPLO were noted when fibular fracture had occurred, possibly due to construct
instability40. Relatively high rates of implant failure have been reported following CCWO stabilised by standard
dynamic compression plates and may also be suggestive of suboptimal construct stability12,20. On the basis of our
results the use of a locking TPLO plate for CCWO stabilisation is recommended. Preservation of an intact
caudomedial cortical hinge may further improve postoperative CCWO stability. Compared to TPLO the larger
proximal mCCWO fragment permitted significantly greater use of the broad 3.5mm locking plate, again potentially
enhancing stability. This may however have been one factor responsible for the slightly greater median surgical
time in the mCCWO group, as well as the necessity for a second osteotomy. There was no evidence that this
adversely affected surgical outcome or complication rate, although such differences may have become apparent
in a larger series.
In humans thigh circumference has been shown to correlate with isokinetic strength42 and quadriceps muscle
mass43, and has been used in both humans44 and dogs45,46 as a measure of limb use and function following
cruciate surgery. In one study mean thigh circumference following TPLO was reduced after three weeks but had
surpassed pre-operative levels by seven weeks46. Conversely Monk reported unchanged thigh circumference at
six weeks following TPLO in dogs not receiving physiotherapy47. With only two datum points per dog our results
could fit either scenario, although they do demonstrate equivalence between TPLO and mCCWO groups with
respect to this measure of post-operative limb function.
Following TPLO significant long term reductions in mean stifle range of motion (ROM) have been
demonstrated46,48, which in some cases are associated with lameness particularly when extension is
compromised49. In the shorter term however dogs receiving intensive physiotherapy following TPLO exhibit
significant increases in total stifle ROM46 and stifle extension47, possibly as a result of resolution of acute stifle
swelling associated with cruciate injury46. Our findings demonstrate similar increases in stifle ROM following both
mCCWO and TPLO (predominantly as a result of increased extension), despite the modest nature of the
physiotherapy regimen. We additionally showed that the post-operative standing angle of the stifle did not differ
between groups and was approximately 135°, a similar value to that measured in normal dogs50,51. Both these
findings are consistent with a kinematic study which showed increased stifle extension angles during the swing
phase of gait after both TPLO and CCWO, but unchanged extension angles during the stance phase52. Thus
concerns that CCWO may result in stifle hyperextension (to compensate for patella baja) with resultant gait
abnormalities appear unfounded7. In fact reduction of patellar tendon angle via decreased TPA whilst stifle angle
remains constant will reduce CTT53 and may be key to the ability of CCWO to dynamically stabilise the stifle; a
similar mechanism has been demonstrated following TPLO54. The effect of patella baja is unknown, although did
not appear to adversely affect long term outcomes in this cohort.
Long term outcome assessment following veterinary orthopaedic surgery is challenging55. Kinetic and kinematic
studies provide objective measures of limb function and have been used to assess outcome after cruciate
surgery46,56. Gait analysis cannot however assess broader, and clinically relevant, questions including vet and
owner assessments of the overall benefit of a particular intervention11,12. Such subjective outcome measures
should ideally be validated, for example by the development of a disease-specific health measurement
instrument57. In common with this study, long term owner scored outcomes following TPLO have typically been
assessed using combinations of unvalidated Likert scales and derivations of the Bristol Osteoarthritis in Dogs
questionnaire12,14,19,58 (which has shown reliability for owner assessments of generic and disease-specific
outcomes following cruciate surgery59). Gait analysis equipment was unfortunately not available for this study, and
recall for long term veterinary assessment was impractical given our widely distributed referral population. Thus
whilst acknowledging the limitations of our outcome assessments we feel they are comparable to previous studies
and permit comparison between our mCCWO technique and TPLO both within this study and with previous
reports. Higher rates of long term follow-up were achieved in this study (92-93%) compared to previous reports
(69-78%)12,14,19. Our results indicated that at long term follow-up owner assessed patient quality of life and overall
satisfaction following mCCWO and TPLO were both good in≥90% cases and poor in ≤4% . Indeed, poor
outcomes were frequently due to health problems unrelated to the cranial cruciate ligament disease. Equivalent
90-95% good overall owner reported outcome and / or satisfaction rates have been reported following TPLO14,19
and CCWO12. In our study owner assessed lameness at long term follow-up was mild or absent in 92.7%
(mCCWO) and 95.5% (TPLO); similar rates have been previously reported following TPLO14 and CCWO13.
Comparison of complication rates between studies is hampered by inconsistent categorisation, inadequately
detailed reporting, and, in many cases, low rates of long term follow-up. A recent editorial proposed categorisation
of major complications as those requiring re-operation or medical treatment to resolve60. Significantly this definition
includes joint sepsis which we feel is appropriate given the typical morbidity associated with this problem and the
potential for suboptimal long term outcome61. We did however categorise as minor those complications considered
to be associated with minimal patient morbidity or potential for long term adverse sequelae, including self-trauma
requiring resuturing, minor incisional complications and implant associated infection rapidly responsive to
antibiotics. On this basis major complications occurred in 9.5% of cases following mCCWO and 7.2% of cases
following TPLO. In two recent large series major complication rates following TPLO were 6.6%4 and 4.2%19. The
slightly higher major complication rates in our study may reflect the inclusion of medically managed joint sepsis
cases in this category. Re-operation rates in our study were relatively low (5.4% mCCWO, 6.1% TPLO) and
comparable to those reports (6.6%4, 4.2%19). With specific regard to CCWO, Corr reported re-operation in four
cases (18%; 3 implant failure, 1 LMI)12, whilst Kuan reported a re-operation rate of 12.3% including four cases of
implant failure causing osteotomy instability and three tibial fractures20. Tibial fractures and implant failure have
also been reported following CCWO at rates of 9%21 and 4%13. Major complication rate and re-operation rate
following our mCCWO appear to be lower than previously reported and similar to TPLO; implant failure and tibial
fracture, relatively common in previous CCWO reports, were not encountered.
Minor complication rates in our study (mCCWO 10.8%, TPLO 17.2%) are higher than those recently reported
following TPLO (8.2%4 and 5.5%19). The reasons for this are unclear. With only two exceptions these cases were
not re-examined by the authors creating potential for misdiagnosis and misinterpretation of the normal appearance
of the healing incision. It is also probable that the prospective design of the study, and the high rates of short and
long-term follow-up, resulted in a high detection rate of such episodes. Technical errors could also have been
responsible, although this seems less probable given that almost all such complications were apparently
associated with healing of a standard medial surgical incision.
True randomisation of cases was not possible although it is improbable that bias was introduced in this manner. A
further potential source of bias was the follow-up assessment of their own cases by surgeons who were by
definition aware of which procedure had been performed in that dog. It could also be argued that all four surgeons
should have performed both techniques, although the approach adopted aided surgical consistency within groups.
A control group was not included, although the aim of this study was to compare a novel technique with an
established one, rather than to prove efficacy of either technique per se. Contralateral stifles were assessed
clinically only, and it is possible that cases with very early contralateral cruciate disease which would otherwise
have been excluded were missed; however it is improbable that such cases would have significantly affected
overall results.
In conclusion, we found that in the treatment of dogs weighing between 20 and 60kg with naturally occurring
cranial cruciate ligament disease a modified CCWO yielded predictable post-operative TPAs and similar
complication rates and long term functional outcomes to TPLO. Both techniques can therefore be considered as
equally appropriate for the treatment of these cases.
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57. Brown DC, Boston RC, Coyne JC, et al: Ability of the canine brief pain inventory to detect response to
treatment in dogs with osteoarthritis. J Am Vet Med Assoc 233:1278-1283, 2008.
58. Boyd DJ, Miller CW, Etue SM, et al: Radiographic and functional evaluation of dogs at least 1 year after tibial
plateau leveling osteotomy. Can Vet J 48:392-396, 2007.
59. Innes JF, Barr AR: Can owners assess outcome following treatment of canine cruciate ligament deficiency? J
Small Anim Pract 39:373-378, 1998.
60. Cook JL, Evans R, Conzemius MG, et al: Proposed Definitions and Criteria for Reporting Time Frame,
Outcome, and Complications For Clinical Orthopedic Studies in Veterinary Medicine. Vet Surg 39:905908, 2010.
61. Clements DN, Owen MR, Mosley JR, et al: Retrospective study of bacterial infective arthritis in 31 dogs. J
Small Anim Pract 46:171-176, 2005.
Figures
Figure 1 - Pre-operative radiograph to illustrate TPLO planning. The osteotomy is centred as close as possible to
the intercondylar eminence whilst maintaining sufficient bone stock for plate application. The osteotomy diverges
from the cranial tibial cortex to preserve a broadening wedge of bone supporting the tibial tuberosity, and exits the
caudal tibial cortex at approximately 90°. Post-operative radiographs for this dog are presented in Figure 3.
Figure 2 - Pre-operative radiograph to illustrate mCCWO osteotomy planning. The wedge was an isosceles
triangle drawn perpendicular to the cranial cortex. Distance x (between the tibial tuberosity and proximal
osteotomy) was modified to preserve sufficient bone stock for plate application and an adequate distance distal to
the tibial tuberosity (a minimum of five millimetres for dogs under 25kg, and 10 millimetres for heavier dogs). Postoperative radiographs for this dog are presented in Figure 3.
a
c
b
d
Figure 3 - Post-operative radiographs following mCCWO and TPLO. Mediolateral projection following mCCWO
(a); note intact caudomedial cortical ‘hinge’ and alignment of cranial cortex. Caudocranial projection following
mCCWO (b); note wire loop secured with single medial twist. Mediolateral (c) and caudocranial (d) projections
following TPLO.
Tables
TPA
Wedge angle
≤20º
TPA - 5º
21-25º
TPA - 4º
26-30º
TPA - 3º
31-35º
TPA - 2º
Table 1 - Planned wedge angles based on pre-operative tibial plateau angle (TPA)
Synthes locking TPLO plate size
TPLO
mCCWO
2.7mm
2 (2.1%)
2 (2.7%)
3.5mm small
2 (2.1%)
3 (4.1%)
3.5mm standard
73 (75.3%)
38 (51.4%)
3.5mm broad
20 (20.6%)
31 (41.9%)
Table 2 - Summary of Synthes locking TPLO plate selection in TPLO and mCCWO groups
TPLO
mCCWO
Cortex affected
Number
Median gap (mm)
95% CI (mm)
None
67 / 97 (69%)
-
-
Cranial
26
1
0.8, 1.25
Medial
1
1.1
-
Lateral
0
-
-
Caudal
3
0.45
-
None
30 / 74 (41%)
-
-
Cranial
5
0.8
0.5, 1.8
Medial
18
1.2
1, 1.45
Lateral
38
1.1
0.95, 1.3
Caudal
6
1.45
1, 1.8
Table 3 - Location and width of radiographically apparent osteotomy gaps in TPLO and mCCWO groups. In the
mCCWO group gaps could affect one cortex only (n=25) or occur concurrently at two (n=15) or three (n=4)
cortices, whilst in the TPLO group multiple cortices were never affected.
Pain grade
TPLO
mCCWO
No pain
4 (4.5%)
6 (8.7%)
Mild
35 (39.3%)
19 (27.5%)
Moderate
34 (38.2%)
27 (39.1%)
Severe
16 (18%)
17 (24.6%)
Table 4 - Owner assessed post-operative pain in TPLO and mCCWO groups
Lameness
Disability
TPLO
mCCWO
TPLO
mCCWO
None
62 (70.4%)
49 (71%)
61 (69.3%)
46 (66.6%)
Mild
22 (25%)
15 (21.7%)
25 (28.4%)
18 (26.1%)
Moderate
4 (4.5%)
4 (5.8%)
2 (2.3%)
3 (4.3%)
Severe
0
1 (1.4%)
0
2 (2.9%)
Table 5 - Long term follow-up owner assessed scores for lameness and disability
Pre-operative (degrees)
Short term follow-up (degrees)
(95% CI)
(95% CI)
Flexion
37.5 (35, 38.75)
35 (32.5, 37.5)
0.16
Extension
147.5 (145, 150)
157.5 (155, 160)
<0.001
ROM
110 (107.5, 112.5)
120 (117.5, 123.5)
<0.001
Flexion
33.5 (32, 35)
35.5 (33.5, 37.5)
0.04
Extension
140 (138.5, 142.5)
149 (146.5, 151.5)
<0.001
ROM
106 (104, 109)
113 (110, 117)
0.001
Group
TPLO
mCCWO
p-value
Table 6 - Comparative pre-operative and short-term follow-up median goniometry values (with 95% confidence
intervals)
Grade
TPLO
mCCWO
Complete
17 (24.3%)
29 (50.9%)
Good
42 (60%)
23 (40.4%)
Poor
11 (15.7%)
5 (8.8%)
None
0
0
Table 7 - Osteotomy healing grades at short term follow-up; healing was graded as either complete (remodelled
callus all cortices and / or osteotomy indistinct), good (active bridging callus, osteotomy mostly blurred or filled with
callus), poor (some evidence of bone healing but unbridged cortices and/or osteotomy gaps), or none.
Appendix 1 - Postal questionnaire sent to all owners
Owner assessed outcomes following cruciate ligament surgery
For multi-choice questions please circle the best answer. Please feel free to write any additional comments.
Dog’s Name
...........................................................................
Today’s Date ...........................................................................
At the time of surgery
•
Can you rate the severity of pain associated with the operation?
None
•
Mild
Moderate
Severe
Did your dog suffer any set-backs or complications following surgery (e.g. wound problems,
suddenly worsening lameness, infections)? Please give details.
...........................................................................................................................................................................................................................
At the present time
•
Can you rate the severity of current lameness affecting the operated limb? ***
None
•
Moderate
Severe
Can you rate the severity of current overall disability as a result of the operated limb? ***
None
•
Mild
Mild
Moderate
Severe
Can you rate your dogs current overall quality of life? ***
Normal
Good
Fair
Poor
*** If these answers are not none / normal please indicate if the current problems are as a result of the
cruciate ligament rupture / surgery or a different problem.
...........................................................................................................................................................................................................................
Overall Impressions
•
Could you indicate your overall satisfaction with the operation and your dogs recovery?
Good
Fair
Poor
Precision of a novel computed tomographic method for the quantification of femoral varus in the dog
and an assessment of the effect of femoral malpositioning
Abstract
OBJECTIVE: To assess the precision of a novel protocol for the determination of femoral varus angle (FVA) using computed
tomography (CT) in dogs, and to quantify the effect of femoral rotational and sagittal plane malpositioning on measured
FVA.
STUDY DESIGN: Cross-sectional study.
SAMPLE POPULATION: Femora (n=66) from dogs which had undergone pelvic limb CT examination for patellar instability
(26) or other reasons (10).
METHODS: Three observers measured the FVA of each of 66 femora on three separate occasions. Standardised
orientation of a volume rendered image was achieved by superimposition of the caudal and distal aspects of the femoral
condyles on a lateral projection, definition of a sagittal plane axis, and finally rotation through 90 degrees to yield a cranial
projection. Intra- and inter-observer variability were estimated using the intra-class correlation coefficient. The effect of
variation in rotational and sagittal plane orientation on measured FVA was subsequently quantified using six femora with
FVAs between -0.4° and 19°.
RESULTS: Intra-class correlation coefficients for the three observers, indicating intra-observer variation, were 0.982, 0.937
and 0.974. The intra-class correlation coefficient of the means of the results from each observer, indicating inter-observer
variation, was 0.976. Consistent linear variations in measured FVA occurred as a result of rotational malpositioning in all six
tested femora, and as a result of sagittal plane malpositioning in femora with FVAs ≥7.9°.
CONCLUSIONS: The reported protocol for the measurement of FVA in dogs is repeatable and reproducible. Small
variations in femoral orientation, as might be expected with conventional radiography, lead to clinically significant alterations
in measured FVA.
CLINICAL RELEVANCE: The use of CT should be considered for measurement of FVA in dogs with patellar instability.
Introduction
Medial patellar luxation (MPL) is a frequent cause of lameness in small breed dogs1, and is recognised with increasing
frequency in a number of larger breeds2,3. It has been recognised for some time that MPL may occur in association with a
number of anatomic abnormalities affecting the entire pelvic limb4,5. Despite the potential complexity of the
aetiopathogenesis, in most cases quadriceps mechanism malalignment can be considered to be the result of malpositioning
of the tibial tuberosity (tibial torsion, rotation or malformation), a femoral deformity (torsion or frontal plane angulation), or a
combination of these factors6. Re-alignment of the quadriceps mechanism by tibial tuberosity transposition has been the
mainstay of traditional surgical management7, and has generally been associated with good outcomes8,9. Significant
reluxation8 and major complication3,9 rates may however result from failure to recognise femoral varus as a significant factor
contributing to quadriceps mechanism malalignment3,10,11.
Treatment of MPL associated with femoral varus by distal femoral osteotomy (DFO) was first described in 193512 and has
recently been the subject of renewed attention13,14. Reliable quantification of femoral varus is a prerequisite for the
identification of potential DFO candidates, and several recent studies have characterised the reliability of radiographic and
computed tomgraphic femoral varus measurements in terms of precision and accuracy11,15,16. The precision of a test is a
description of the variation in results yielded on repeated testing of the same sample; this is assessed by evaluation of
repeatability (intra-observer variability) and reproducibility (inter-observer variability). The accuracy of a test is a description
of how close the measured value is to the true value; this requires that a ‘true’ value be both identifiable and measurable,
thus providing an unequivocal gold standard against which new tests may be assessed. Previous studies have
demonstrated reasonable precision but unacceptable accuracy when compared to a reference femoral varus angle (FVA)
value determined from digital photographs of dissected femora11,15,16.
Traditional radiographic measurement of FVA is sensitive to rotational malpositioning of the femur due to the natural
procurvatum of the bone; external rotation (supination) will increase apparent varus, and vice-versa5,16. This difficulty has led
to the development of a number of criteria for determination of appropriate femoral orientation; these include parallel
orientation of the femur to the long axis of the pelvis, centring of the patella on the femur, cortical bisection of the fabellae,
and visibility of the tip of the lesser trochanter11,15-17. Obtaining optimal projections can be frustrating, especially in countries
where ionising radiation regulations preclude the use of manual positioning. In addition, positioning will be influenced by
normal and / or pathological variations in the position and size of the patella, fabellae and lesser trochanter. Such effects are
likely to reduce both the precision and accuracy of FVA measurement, and may partially explain the failure of earlier studies
to validate these techniques. The magnitude of inaccuracy introduced by femoral malposition has not been quantified and is
therefore of unknown significance with respect to surgical decision making as well as planned angular correction if DFO is
performed.
We hypothesised that as a result of improved standardisation of femoral orientation, a novel CT protocol would permit
precise determination of femoral varus angle in dogs. We further hypothesised that relatively small variations in femoral
orientation (both with respect to rotation and inclination in the sagittal plane) would result in clinically significant alterations in
measured FVA.
Materials and Methods
Clinical records were searched to identify dogs which had undergone CT examination including one or both femora at our
referral hospital between October 2009 and November 2011. Exclusion criteria included incomplete imaging of the bone and
the presence of pathological changes likely to invalidate measurement of femoral varus (such as neoplasia affecting
relevant bony landmarks and most fractures). All other femora were included in the study.
In all cases contiguous 0.625mm slices were acquired using a 16 slice GE Brightspeed scanner; images were viewed on a
workstation with screen resolution 1280 x 1024 using GE volume viewer version 3.0 software. Femoral images were
prepared using a volume rendered and surface shaded preset. Using magnification, the tibia was cropped taking care to
avoid inadvertent modification of the contour of the femoral condyles. The pelvis was also cropped (including the portion of
the femoral head within the acetabulum) along with any other bony structures (typically the contralateral limb and tail). Each
isolated femur was anonymised via assignation of a unique number and was saved in a random orientation for subsequent
measurement.
In the first part of the study three boarded orthopaedic surgeons measured the FVA of each femur on three separate
occasions. The order in which the femora were measured on each occasion was determined using a web-based random
sequence generator a. A standardised protocol for FVA measurement was used (Figure 1). Using magnification, the femur
was orientated to achieve superimposition of the most caudal and most distal points of the medial and lateral femoral
condyles. A line was drawn connecting the most caudal points of the femoral condyles and the proximal femur (typically the
lesser trochanter, but occasionally the caudal aspect of the greater trochanter). The image was rotated in the sagittal plane
until this line was vertically orientated whilst maintaining condylar superimposition; the image was subsequently rotated to
result in a 15° caudal inclination with respect to vertical. The image was then rotated through 90° in the transverse plane to
yield a cranial view of the femur. The distance between the most distal apparent extent of the intertrochanteric fossa and the
most proximal apparent extent of the intercondylar notch was measured. The proximal anatomic femoral diaphyseal axis
(PAA) was defined as a line connecting the centre of the diaphysis at 50% and 70% of that distance from distally. In rare
cases the more proximal point coincided with the diaphyseal flare of the lesser trochanter; when this occurred the most
a
www.random.org, School of Computer Science and Statistics, Trinity College, Dublin, Ireland.
proximal possible point distal to the lesser trochanter was estimated and used. The distal transcondylar axis (TCA) was
defined as a line connecting the most distal points of the femoral condyles. FVA was defined as the angle between the PAA
and a line perpendicular to the TCA. Magnification was used to facilitate measurement of diaphyseal width, identification of
the most distal points of the femoral condyles, and placement of digital callipers for angular measurement. FVA was
recorded as a positive value in cases of femoral varus and a negative value in cases of femoral valgus.
Following completion of the first part of the study a fourth boarded orthopaedic surgeon assessed the effect of femoral
rotation on measured FVA. Six femora were selected with mean measured FVAs of approximately 0°, 4°, 8°, 12°, 16° and
20°. The described protocol was followed until a cranial view of each femur was obtained. Additional 2° incremental
rotations were then performed between -20° (external femoral rotation; supination) and +20° (internal femoral rotation;
pronation) prior to FVA measurement as described above. An experienced surgical resident subsequently assessed the
effect of femoral inclination in the sagittal plane on measured FVA using the same six femora. For each femur the described
protocol was used to measure FVA as femoral inclination was varied in 2° increments from zero to 34 degrees of caudal
inclination.
Intra-observer and inter-observer agreement were assessed with the intraclass correlation coefficient. The correlation
between continuous variables was assessed using Pearson’s correlation coefficient. Statistical significance was set at P <
0.05.
Results
66 femora from 36 dogs met the inclusion criteria and were included in the first part of the study. These included 30 paired
left and right femora and six unpaired femora (three left and three right). In 23 cases the presenting problem was uni- or
bilateral medial patellar luxation and in three cases lateral patellar luxation. In other cases the diagnosis was tibial deformity
(3), pelvic limb soft tissue neoplasia (2), and one case each of diaphyseal femoral neoplasia, suspected Osgood-Schlatter
disease, capital physeal fracture, inflammatory arthropathy and pelvic limb gait abnormality.
Mean FVAs (from nine observations) for all but one femur ranged between -0.4° and 20.3°; the remaining femur had a
mean FVA of -10.5°. Intra-class correlation coefficients for each of the three observers were 0.982, 0.937 and 0.974
respectively. The intra-class correlation coefficient of the means of the results from each observer was 0.976. The difference
between the smallest and greatest of the three FVA values measured by each observer for each femur was calculated; the
mean values of these ranges for each observer (with standard deviations) were 1.30° +/- 0.80°, 2.37° +/- 1.40°, and 1.48°
+/- 0.84°. The mean of all 198 values was 1.7° +/- 1.14°. For each observer the absolute magnitude of these ranges varied
between 0° and 3.5°, 0.4° and 7.9°, and 0.3° and 3.7°. The relative frequency of ranges of increasing magnitude is shown
in Figure 2; these were ≤2.9° in 174 of 198 measurements (88%), and ≤3.9° in 189 measurements (95%).
Femora selected for the second part of the study had mean FVAs (from nine observations) of -0.4°, 3.8°, 7.9°, 11.0°, 15.8°,
and 19.0°; these were defined as ‘true’ FVA values. For each of these six femora measured FVA varied with rotation in an
almost linear fashion between 20° of internal and 20° of external rotation (Figure 3); in each case a very high degree of
correlation between these variables was noted (Table 1). Regression equations demonstrated variation of measured FVA
with rotation by a factor of 0.44 to 0.51; thus regardless of true FVA, approximately one degree of error in measured FVA
results from every two degrees of femoral rotational malpositioning.
Figure 4 shows the effect of increasing femoral inclination in the sagittal plane on measured FVA for each femur. For femora
with true FVA values of 7.9°, 11.0°, 15.8°, and 19.0°, measured FVA varied in an almost linear fashion between zero and
34° of caudal inclination. For these femora regression equations demonstrated an increase in measured FVA of
approximately one degree for every six degrees of caudal inclination, a relationship apparently unaffected by the degree of
true femoral varus. For the femur with 3.8° of true varus, measured FVA was apparently unaffected by the degree of caudal
inclination. For the femur with -0.4° of true varus, there was a weak inverse correlation between inclination and measured
FVA. Correlation coefficients and P-values in each case are presented in Table 2.
Discussion
In the assessment of dogs with MPL, quantification of femoral varus is an important element in the selection of potential
DFO candidates, a treatment which has been shown to be successful where FVA is high13. We were able to demonstrate
good reliability and repeatability of a novel CT method for the determination of FVA in dogs. In addition we showed that
relatively small errors in femoral positioning, as might occur during conventional radiography, led to potentially clinically
significant errors in measured FVA.
A standardised and universally applicable system for the assessment of long bone conformation in humans has been
described18. This system has been adopted for the quantification of femoral varus in dogs; the angular relationship of the
PAA and the TCA in the frontal plane has been expressed as either the anatomic lateral distal femoral joint angle
(aLDFA)11,17 or the femoral varus angle (FVA)16. It has been recognised for some time that due to the normal procurvatum of
the canine femur, rotation about an axis perpendicular to the transverse plane will not affect measured TCA but will modify
the relative orientation of the PAA, thus artefactually altering measured FVA16. Standardisation of sagittal plane orientation
has previously been considered unnecessary (on the basis of the relatively similar diameters of the femoral condyles11 and
results of a previous study16); our findings however indicate that, at least in femora with FVAs≥ 7.9°, this variable will also
artefactually alter measured FVA. Thus standardisation of sagittal plane, as well as rotational, femoral orientation should be
considered a prerequisite for the consistent quantification of FVA. Quantification of sagittal plane orientation is challenging
using conventional radiography, and definition of rotational orientation is also difficult due to the paucity of definitive
radiographic landmarks on a conventional craniocaudal projection. This has led to the use of other criteria including patellar
and fabellar position relative to the condylar cortices, and visibility of the tip of the lesser trochanter11,15-17. The consistency
of lesser trochanter size and position is to our knowledge unknown; in addition, since patellar and fabellar position may vary
with respect to the femur (especially when femoral conformation is abnormal or when MPL is present)19, this approach is
likely to introduce variability in measured FVA. It should be noted that such variability is not apparent in previous studies
where observers measured FVA from the same set of radiographs11,15,16; it is likely that intra- and inter-observer variation
would have been higher had each femur been both radiographed and measured by each observer. When isotropic spatial
resolution is achieved, CT permits reorientation of the femoral image without loss of resolution rendering patient positioning
irrelevant, and facilitates definition of landmarks for rotational and sagittal plane orientation which would be impossible using
conventional radiography. When viewed from laterally, femoral rotation will cause relative displacement of the caudal
aspects of the medial and lateral femoral condyles. When these are superimposed, rotational orientation is defined and a
90° rotation gives a standardised frontal plane image. The low intra- and inter-observer variability reported here is attributed
to the standardisation of condylar orientation inherent in the measurement protocol, as well as consistent orientation in the
sagittal plane and the independence of measured FVA from patient positioning.
Given the wide range of naturally occurring femoral conformations included in this study it is reasonable to assume that
similar precision could be expected in the assessment of clinical cases with MPL. It should however be noted that the
described protocol, and the design of the study, is subject to a number of limitations. Firstly, the relationship between the
FVA measured using this CT protocol and the previous radiographic technique (and hence accepted ‘normal’ values) is
unknown. However, due to breed variation, the multifactorial nature of MPL, and our incomplete knowledge of the
contribution of FVA to the development of MPL, at present it is probably best to consider ‘normal’ values as a guide only.
Secondly, we made no attempt to quantify femoral torsion which may play a role in the aetiopathogenesis of some cases of
MPL6. Measurement of femoral torsion has been described following CT15 and may be warranted in addition to FVA
assessment in clinical cases of MPL. Thirdly, the 15° angle of caudal inclination used here gives only an approximation of
femoral inclination (and hence femorotibial contact) at a normal standing angle. Measurement of FVA at this angle should
yield the most clinically relevant value and has been previously recommended11, and also allowed clear visibility of the distal
aspects of the femoral condyles. Fourthly, we did not attempt to validate our technique by means of assessment of accuracy
by comparison with a gold standard test, as we do not believe that such a test exists at this time. A gold standard test would
be capable of quantifying the extent of ‘true’ FVA and this requires that a definition of ‘true’ FVA must first exist. Regardless
of the sophistication and intrinsic accuracy of the imaging technique employed (estimated at +/- 1° in this study b), derivation
of an absolute value for FVA will always necessitate the use of landmarks which are, ultimately, arbitrary. We would
therefore argue that a gold standard test for FVA probably cannot exist, and, in turn, that the concept of absolute
b
GE Advantage Windows Workstation User Guide
quantification of FVA is illusory. Although, prima facie, this concept appears to confound the clinical application of FVA
measurement, it must be recognised that, from diagnostic and therapeutic perspectives, the critical value required by the
clinician is the angular difference between the FVA of the femur in question and a normal femur. The clinical reliability of this
angle is a function of the precision of the FVA measurement technique used, rather than the absolute numeric FVA values it
yields (a function of the accuracy of the technique). In an analogous situation, conventional radiographic measurement of
tibial plateau angle (TPA) has been shown to be a precise20, but inaccurate, representation of the anatomic tibial plateau
angle21; tibial plateau levelling procedures are therefore routinely planned on the basis of the difference between precise,
but inaccurate, TPA values. This comparison may also be pertinent with respect to the absolute precision of FVA
measurement using our CT protocol; our findings suggest that 95% of measured FVA values will fall within +/- 3° of the
mean, compared with +/- 6.5 to 8° for TPA values20. It is also probable that this level of precision exceeds that which can be
achieved surgically both in terms of DFO13 and other closing wedge ostectomies22.
In the second part of our study we were able to quantify the error in measured FVA associated with rotational and sagittal
plane femoral malpositioning. The relationship between measured FVA and femoral rotation was linear throughout the
tested range (between 20 degrees of internal and external rotation) and was minimally affected by the degree of true
femoral varus. It is likely therefore that, at least in the femora measured, the degree of femoral procurvatum is independent
of femoral varus and thus introduces a consistent error in measured FVA (of almost one degree for every two degrees of
rotation). For femora with FVAs≥ 7.9° the relationship between measured FVA and femoral sagittal plane orientation was
also linear throughout the tested range (between zero and 34 degrees of caudal inclination); measured FVA increased by
approximately one degree for every six degrees of inclination, a relationship apparently unaffected by the degree of true
femoral varus. This relationship did not hold true for femora with true FVAs of -0.4° and 3.8°, where measured FVA was
respectively either slightly deceased or was unaffected by increasing femoral inclination. This discrepancy may be explained
by consideration of the effect of altered femoral inclination on both the PAA and TCA. When viewed from cranially,
increased caudal femoral inclination results in foreshortening. In femora exhibiting varus the proximal diaphysis is medially
offset from the condyles in the frontal plane; as foreshortening occurs the angular relationship of these portions of the bone
therefore alters, modifying the PAA, and increasing measured FVA. A recent study quantified variations in the diameters of
the medial and lateral femoral condyles23 which would be expected to result in decreased measured FVA as femoral caudal
inclination increases due to modification of the TCA. This effect is subtle, but may explain the weak inverse correlation
between inclination and measured FVA for the femur with true FVA of -0.4°, as well as the absence of measured FVA
increase for the femur with true FVA of 3.8°. In a previous study femoral inclination was reported to have no effect on
measured FVA, most probably since the 5.8° average FVA fell below the range where this effect appears to become
significant16. At present it is difficult to estimate the clinical significance of these findings. Firstly, the degree of variation in
rotational and sagittal plane positioning following conventional radiography that would be unapparent or considered
acceptable by a suitably qualified observer is unknown. On the basis of our observations during the acquisition of data for
this study it is our opinion that small variations in positioning are difficult to detect from a craniocaudal radiographic
projection. Secondly, insufficient information is currently available regarding indications for, accuracy of, and outcomes
following DFO to estimate the importance of a given variation in initial FVA measurement. Small errors in measured FVA
could however lead to inappropriate selection of surgical technique (especially if specific threshold FVA values13 are applied
inflexibly in the selection of DFO candidates), as well as compromising the ability of DFO to realign the quadriceps
mechanism. Regardless of these considerations the use of a technique not susceptible to errors arising from femoral
malpositioning would seem optimal. The main limitation of the second part of this study was the use of single observations
of a limited number of femora. This was considered acceptable given the very low intra- and inter-observer variability
associated with the protocol employed, although it is probable that multiple assessments of a greater number of femora
would have permitted more detailed assessment. However, in the context of this study, our aim was to demonstrate the
importance of standardisation of rotational and sagittal plane orientation rather than to precisely quantify errors resulting
from femoral malpositioning.
In conclusion, we demonstrated very low intra- and inter-observer variability in the determination of FVA in dogs using a
novel CT method. Using this approach we quantified the errors introduced into FVA measurement by femoral rotational and
sagittal plane malpositioning as might occur during conventional radiography. The authors suggest that this CT protocol
should be considered for the measurement of FVA and subsequent clinical decision making in dogs clinically affected by
patellar luxation.
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Figures
1a
1b
1c
1d
1e
1f
1g
1h
Figure 1 - Protocol for standardised femoral orientation. Perfect superimposition of the most caudal and most distal points of
the femoral condyles is achieved (a). An axis is drawn between the most caudal points of the femoral condyles and the
proximal femur(b). The image is rotated in the sagittal plane until this axis is vertically orientated (c), and then rotated further
such that the axis is caudally inclined by 15°(d). The image is then rotated through 90° in the transverse plane to yield a
cranial view of the femur(e). The centre of the diaphysis is identified at 50% and 70% of the distance between the most
distal apparent extent of the intertrochanteric fossa and the most proximal apparent extent of the intercondylar
notch(f).These points are connected to define the proximal anatomic femoral diaphyseal axis (PAA); the transcondylar axis
(TCA)is defined as a line connecting the most distal points of the femoral condyles (g). Femoral varus angle(angle y) is the
angle between the PAA and a line perpendicular to the TCA (h).
70
Observer 2
Number
Observer 1
Observer 3
0
0-0.9
1-1.9
2-2.9
3-3.9
4-4.9
5-5.9
6-6.9
7-7.9
Range (degrees)
Figure 2 - The relative frequency of the ranges of each series of three FVA measurements made by each observer
Measured
FVA (degrees)
FVA -0.4° (y = -0.4911x; R² = 0.993)
FVA 3.8° (y = -0.5119x; R² = 0.995)
FVA 7.9° (y = -0.4595x; R² = 0.987)
FVA 11.0° (y = -0.4380x; R² = 0.988)
FVA 15.8° (y = -0.4930x; R² = 0.990)
FVA 19.0° (y = -0.4399x; R² = 0.986)
30
25
20
15
10
5
0
-20
-15
-10
-5
0
5
10
15
20
Rotation
(degrees)
-5
-10
Figure 3 - Variation of measured FVA with rotation for each of six femora with differing average true FVAs. The gradient and
R² value for each linear regression line are shown in the key
Measured
FVA (degrees)
25
20
15
10
5
0
0
-5
5
10
15
FVA -0.4° (y = -0.067x; R² = 0.840)
FVA 7.9° (y = 0.180x; R² = 0.974)
FVA 15.8° (y = 0.174x; R² = 0.949)
20
25
30
35
Inclination
(degrees)
FVA 3.8° (y = -0.002x; R² = 0.002)
FVA 11° (y = 0.162x; R² = 0.977)
FVA 19° (y = 0.172x; R² = 0.953)
Figure 4 - Variation of measured FVA with caudal inclination for each of six femora with differing average true FVAs; the
gradient and R² value for each linear regression line are shown in the key
Tables
Average true FVA
-0.4°
3.8°
7.9°
11.0°
15.8°
19.0°
Correlation coefficient
(95% CI)
-0.996
-0.997
-0.993
-0.994
-0.995
-0.993
P-value
(-0.999 to (-0.999 to (-0.997 to (-0.998 to (-0.998 to (-0.997 to
-0.991)
-0.993)
-0.983)
-0.984)
-0.987)
-0.982)
< 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001 < 0.0001
Table 1 - Correlation coefficients and P-values describing the relationship between measured FVA and rotation for each of
six femora with differing average true FVAs
Average true FVA
-0.4°
3.8°
7.9°
11.0°
15.8°
19.0°
Correlation coefficient
(95% CI)
-0.917
-0.044
0.987
0.988
0.974
0.976
(0.965 to
0.995)
(0.968 to
0.996)
(0.931 to
0.991)
(0.936 to
0.991)
P-value
(-0.969 to (-0.500 to
-0.786)
0.432)
< 0.0001 0.864 < 0.0001 < 0.0001 < 0.0001 < 0.0001
Table 2 - Correlation coefficients and P-values describing the relationship between measured FVA and caudal inclination for
each of six femora with differing average true FVAs