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PATELLAR FRACTURES IN CATS
Author : MARK LONGLEY, SORREL LANGLEY-HOBBS
Categories : Vets
Date : November 3, 2014
MARK LONGLEY BVM&S, GPCertSAS, MRCVS
SORREL LANGLEY-HOBBS MA, BVetMed, DSAS(O), DipECVS, FHEA, MRCVS
discuss this uncommon condition in young cats, looking at studies into the diagnosis and treatment
options available, and request help from practitioners
Summary
Feline patellar fractures are a relatively rare condition affecting young cats. Fractures occur usually
in the absence of significant trauma and have a transverse configuration in the mid to proximal third
of the patella. Bilateral fractures are common, although not always concurrently. Sclerosis has
been noted in affected bones prior to fracture. Persistent deciduous teeth are also a feature of the
condition, but may have been removed prior to first fracture. Affected cats frequently go on to
develop spontaneous fractures in other bones in later life. A high proportion of affected siblings
may be suggestive of a genetic disorder. Repair is difficult with a high failure rate associated with
use of pin and tension band. Radiographic non-union is a common feature, even following surgical
stabilisation. Speculation of an underlying systemic bone pathology has led to research into
potential causes of brittle bones. Research into the optimum fixation method is also underway.
THE patella is the largest sesamoid bone in the body and is located in the tendon of
insertion of the quadriceps femoris muscle.
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It functions to confer mechanical advantage to the extensor mechanism of the hindlimb by
redirecting the extensor forces, thus increasing the lever arm of the quadriceps muscles. The
patella, along with the parapatellar fibrocartilages, have a protective function on the quadriceps
tendon by increasing the contact area of the tendon in the femoral trochlea (Evans, 1993),
distributing the contact forces.
Patellar fractures are an uncommon presentation in veterinary practice, representing approximately
0.1 per cent of all fractures (Harari, Person and Berardi, 1990). In humans, patellar fractures are
relatively common, estimated as one per cent of all fractures (Schnabel et al, 2009). Typically, most
fractures of the patella have been considered to be traumatic in origin and this is true in humans
and dogs. Traumatic fractures are also documented in cats (Langley-Hobbs, Brown and Matis,
2008), but, in recent years, a more common, although still rare, syndrome of spontaneous patellar
fractures has been recognised in cats (Langley-Hobbs, 2009), the aetiology of which is poorly
understood (Figure 1).
Stress fractures have also been reported in dogs (White, 1977) and in humans (Mason et al, 1996).
Reports of bipartite/multipartite patellae exist in the literature (Denny and Butterworth, 2000) and
are considered a congenital, clinically insignificant condition resulting from separate centres of
ossification failing to fuse. However, it is unclear, particularly in cats, whether these entities may
represent chronic, clinically silent, spontaneous patellar fractures.
Traumatic fractures have more variable morphologies with longitudinal, polar and comminuted
configurations all being reported (Langley-Hobbs, Brown and Matis, 2008). Mechanisms for
fracture occurrence may be an avulsion type injury during jumping or landing or due to direct
trauma.
Spontaneous patellar fractures as seen in cats are almost invariably transverse in the mid to
proximal third (pole) of the bone and are frequently bilateral, with a mean interval to fracture of the
contralateral patella being six months (Langley-Hobbs, 2009). Typically, the presentation is of a
young cat with either an acute onset of hindlimb lameness or a more insidious onset. Varying
degrees of displacement may be evident on radiographs. A survey of cases (Langley-Hobbs, 2009)
found many affected cats had persistent deciduous teeth (15 per cent) – an uncommon finding in
the general cat population. Furthermore, a significant proportion of cases go on to develop
spontaneous fractures of other bones, the most common being proximal tibia, lateral humeral
condyle, and ischium/acetabulum (Figure 2). Clinicians identifying a patellar fracture in a cat should
consider taking radiographs of the contralateral stifle and pelvis also. Sclerosis of the patella (and
tibia) has been observed prior to fracture occurring, leading to the conclusion these are stress
fractures. Stress fractures may be as a result of increased workload on a normal bone that has not
had sufficient time to accommodate or as a result of normal workload on a bone weakened by
disease. In general, bone density of affected cats appear normal radiographically, although
occasional cases have been seen by the author with marked osteosclerosis either multifocally or
more generally.
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Siblings are quite frequently affected and this has led to the suspicion of a genetic aetiology. The
high incidence of retained deciduous teeth and development of other spontaneous fractures has
led to the hypothesis this may be a form of osteogenesis imperfecta (OI). OI is the most common
cause of spontaneous fractures in humans and has also been reported in dogs. Most cases of OI
are associated with defects in genes for collagen synthesis (COL1A, COL2A), although defects in
genes for the molecular chaperones involved in collagen folding can result in the disease
(Drogemuller, 2009). In humans, a high proportion of affected individuals suffer from dental
abnormalities (dentinogenesis imperfecta; Rauch and Glorieux, 2004). OI has been reported in
cats, but this was not confirmed definitively; animals in the reported case series did not present
with patellar fractures or dental abnormalities (Omar, 1961).
Diagnosis
Reported clinical signs associated, with patellar fracture are lameness with stifle pain, crepitus,
swelling, decreased range of motion, increased stifle flexion, stiffness and difficulty rising.
In many cases, diagnosis is based on history, pain on palpation of the patella and standard lateral
and craniocaudal radiographic views. However, care must be taken to ensure fractures identified
on radiographs are not an incidental finding (Figure 3) and other common concurrent causes of
lameness should be considered (for example, cranial cruciate ligament disease or hip disease). A
skyline radiographic view may be helpful in some cases where small fragments or longitudinal
fracture are suspected; views taken with the stifle flexed can be useful in detecting minimally
displaced small fracture fragments. Patients suspected of trauma should be assessed thoroughly
for the presence of concurrent injuries.
Treatment
Treatment is focused on achieving a functional extensor mechanism and minimising degenerative
joint disease. Cases with minimal displacement may be treated conservatively with strict rest and
appropriate analgesia, but displacement may occur later. Due to the articular nature of these
fractures, significant displacement is probably best managed surgically.
The location of the patella in the tendon of insertion of the quadriceps muscle group results in a
challenging biomechanical environment for fracture repair since it is subject to substantial
distractive forces during normal movement. A variety of fixation methods have been described and
several biomechanical studies have been performed in human surgery to assess the various
methods (Burvant et al, 1994; Carpenter et al, 1997; Schnabel et al, 2009). No studies have been
performed in companion animals to assess the various repair methods, however, and the method
employed is therefore based on the fracture configuration and surgeon preference.
A commonly utilised technique in humans involves the pin and tension band principle, although
various other methods have been assessed. Patient size in cats will limit implant choice, so direct
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extrapolation from human literature should be made with caution (Figure 4). Pin and tension band
repair was found to be associated with a very high failure rate (80 per cent) in cats and, therefore,
is not recommended. It is postulated this may be because the drilling of an already weakened bone
may lead to fragmentation (Langley-Hobbs, 2009; Figure 5). A circumferential wire may be a better
option in these patients if surgery is to be attempted. However, pin and tension band was used in
the majority of surgically managed patients in this study and, therefore, there is limited evidence to
support the use of any one alternative technique. This subject is one of ongoing research by the
author.
Longitudinal fractures may be repaired with a mediolaterally placed pin and tension band.
Comminuted fractures may be reconstructed by fixation with small k-wires. Small, non-reducible
fragments may be removed (partial patellectomy) with good outcome (Carb, 1975). Outcome has
been reported as “good” following total patellectomy in some reports (Harari, 1990). Despite this, it
should only be performed as a salvage option as other authors have shown permanent dysfunction
results, as is the case in humans. Tears in the retinaculae should be sutured as this may contribute
to the strength of the repair (Weber et al, 1980).
Various methods have been attempted to try to reduce strain on the repair. A patellotibial wire can
be used, although this will eventually break, making removal necessary if irritation results.
Polypropylene leader line has a better resistance to cyclic loading and may be preferable. Some
surgeons use a transarticular external skeletal fixator to immobilise the joint; however, this does not
necessarily reduce the strain on the repair during weight bearing (Das et al, 2014). Spica splints
will produce a similar effect.
A study found many cats had a good prognosis with conservative management (Salas and
Popovitch, 2011), although as yet unpublished data, obtained by the author, suggests this is not
always the case – particularly if substantial fragment displacement exists. Removal of persistent
deciduous teeth is recommended to prevent severe dental complications that may occur as a
consequence of failed eruption. The authors are aware of several cases that have developed
severe maxillary osteomyelitis, possibly as a consequence of persistent teeth. Persistent teeth are
defined as deciduous teeth that persist in the mouth beyond the time of normal exfoliation.
Retained teeth are permanent teeth that have failed to erupt in the normal time frame. Both
persistent deciduous teeth and failed eruption have been seen in cats with patellar fractures.
Details of the dental abnormalities present in these patients have been poorly defined and require
further investigation.
Radiographic non-union is a common finding in cats with patellar fractures, but healing has been
documented in a small number of cases (Hermer et al, 2012). Fibrous union results in most cases.
Postoperative care should consist of restricted and controlled exercise for four to six weeks. Some
authors recommend the use of bandaging to restrict movement, but these are generally difficult to
keep in position and may cause problems if slippage occurs. This may result in more load being
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applied to the fracture site. Spica splints and Schroeder-Thomas splints are an option to reduce
this problem, but these external coaptation devices may cause other bandage or immobilisation
problems (Das et al, 2014). Controlled mobilisation of the joint at an early stage may be beneficial,
but overuse of the affected limb may increase the risk of repair failure. Gentle, passive range of
motion exercises are probably best in the early stages following repair. Cage rest is probably best
due to the high risk of repair failure and limiting activity in cats is difficult.
In general, prognosis following fracture repair is good, but this will depend on the level of disruption
of the extensor mechanism and severity of damage to the articular surface and ensuing
degenerative joint disease. Good outcomes appear typical in cats following patellar fractures;
however, it should be noted only limited follow-up data has been obtained. Long-term outcome and
development of osteoarthritis in cats with displaced fractures is unknown.
The authors are performing a study into spontaneous patellar fractures in cats to try to establish an
underlying cause and to develop optimal treatment recommendations. A biomechanical study is in
progress looking at different methods of patellar fracture repair. We are eager for veterinary
surgeons in general practice to contact us if they see a case. We are collecting samples to allow
further investigation into this condition: surplus EDTA blood, when blood has already been
collected as part of the clinical investigation of the case, or buccal swab for genetic analysis.
If surgery is being performed, then a thin sample of excess fascia latae for tissue culture and
collagen analysis is requested. We are also keen to receive any persistent or retained teeth that
are removed (irrespective of the presence of fractures) to perform histopathological analysis on. In
the event of an affected animal being euthanised or dying, then we would seek to obtain samples
of bone (patella and femur – frozen, not formalinised). A website has been set up with information
for vets and owners (www.langfordvets.co.uk/felinepatellafracturestudy ). We request anyone who
has seen a case to visit the website and complete an online questionnaire. Anyone planning
surgery on an affected cat is requested to contact the author to arrange culture
media/consumables to be sent. We would also appreciate copies of radiographs if possible.
A bank of DNA samples from affected cats is being collected in the US by Margret Casal,
University of Pennsylvania, with the aim of performing a genome-wide association study once
enough samples have been collected. The aim of the tissue research is to assess whether affected
cats have a collagen defect, such as that in osteogenesis imperfecta.
We greatly appreciate any time that can be afforded by colleagues to help with this project. We are
always happy to receive enquiries and provide advice if required.
References
Burvant J G et al (1994). Evaluation of methods of internal fixation of transverse patella
fractures: A biomechanical study, Journal of Orthopaedic Trauma 8(2): 147-153.
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Carb A (1975). A partial patellectomy procedure for transverse patellar fractures of the dog
and cat, Journal of the American Animal Hospital Association 11(5): 649-657.
Carpenter et al (1997). Biomechanical evaluation of current patella fracture fixation
techniques, Journal of Orthopaedic Trauma 11(5): 351-356.
Das S et al (2014). Patella ligament rupture in the cat: repair methods and patient outcomes
in seven cases, Journal of Feline Medicine and Surgery Jul 29. pii: 1098612X14544345
doi:10.1177/1098612X14544345
Denny H R and Butterworth S J (2000). The Stifle. In A Guide to Canine and Feline
Orthopaedic Surgery (4th edn), Blackwell Science, Oxford: 512.
Drogemuller C et al (2009). A missense mutation in the SERPINH1 gene in Dachshunds
with osteogenesis imperfect, PLoS Genet 5(7): e1000579 doi:10.1371/
journal.pgen.1000579
Evans H E (1993). Miller’s Anatomy of the Dog (3rd edn), W B Saunders Co, Philadelphia:
206-207.
Harari J S et al (1990). Fractures of the patella in dogs and cats, Compendium on
Continuing Education 12(11): 1,557-1,562.
Hermer J V et al (2012). Healing of patella fractures in two kittens, Veterinary Comparative
Orthopaedics and Traumatology 25(2): 155-159.
Langley-Hobbs S J (2009). Survey of 52 fractures of the patella in 34 cats, Veterinary
Record 164(3): 80-86.
Langley-Hobbs S J, Brown G and Matis U (2008). Traumatic fracture of the patella in 11
cats, Veterinary Comparative Orthopaedics and Traumatology 5(21): 427-433.
Mason R W et al (1996). Patellar fatigue fractures, Skeletal Radiology 25(4): 329-332.
Omar A M (1961). Osteogenesis imperfecta in cats, Journal of Pathology and Bacteriology
82(2): 303-314.
Rauch F and Glorieux F H (2004). Osteogenesis Imperfecta, Lancet 363(9418):
1,377-1,385.
Salas N and Popovitch C (2011). Surgical versus conservative management of patella
fractures in cats: a retrospective study, Canadian Veterinary Journal 52(12): 1,319-1,322.
Schnabel B et al (2009). Biomechanical comparison of a new staple technique with tension
band wiring for transverse patella fractures, Clinical Biomechanics 24(10): 855-859.
Weber M J et al (1980). Efficacy of various forms of fixation of transverse fractures of the
patella, Journal of Bone and Joint Surgery 62(2): 215-220.
White R A S (1977). Bilateral patellar fracture in a dog, Journal of Small Animal Practice
18(4): 261-265.
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Figure 1a and 1b. Transverse patellar fracture in a cat.
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Figure 2. Radiographs of spontaneous tibial (2a) and acetabular (2b) fractures of cats with a
history of patellar fractures.
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Figure 3. Chronic patellar fracture. Chronic fractures may occasionally be found as an incidental
finding.
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Figure 4. Suggested methods of patellar fracture repair in cats adapted from human literature. A:
circumferential wire. B: tension band. C: pin and tension band. D: circumferential and tension band
combined. (NB. The use of pin and tension band is not recommended in cats due to high failure
rates with this technique.)
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Figure 5. Pin and tension band repair of spontaneous patellar fractures was associated with a high
failure rate and is not recommended.
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Figure 6. Postoperative craniocaudal view of feline patella repaired with circumferential wire and
patellotibial wire.
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