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IN DEPTH: PROXIMAL SUSPENSORY DESMITIS
Proximal Suspensory Desmitis in the Forelimb
and the Hindlimb
Sue Dyson, FRCVS
Author’s address: Centre for Equine Studies, Animal Health Trust, Lanwades Park, Kentford,
Newmarket, Suffolk CB8 7UU. © 2000 AAEP.
1.
Introduction
The suspensory ligament (SL) can be divided into
three separate regions which are subject to injury:
the proximal part, the body, and the branches.
For clinical purposes in the forelimb the proximal
part extends from approximately 4 to 12 cm distal to
the accessory carpal bone, and in the hindlimb from
approximately 2 to 10 cm distal to the tarsometatarsal joint.
In the forelimb, the SL originates from two heads
which rapidly fuse. In the hindlimb this division is
less obvious. The ligament contains a variable
amount of muscular tissue (2–11%), which tends to
be bilaterally symmetrical.1 In the forelimb the SL
originates from the palmar carpal ligament and the
proximal aspect of the third metacarpal bone,
whereas in the hindlimb it originates principally
from the proximoplantar aspect of the third metatarsal bone. In the forelimb the SL is innervated by
the palmar metacarpal nerves, derived from the lateral palmar nerve, which receives fibres from both
the ulnar and median nerves.2 The hindlimb SL is
innervated by the plantar metatarsal nerves, derived from the tibial nerve. The proximal SL is
closely related to the palmar outpouching of the
middle carpal joint in the forelimb,3 and the plan-
tar outpouching of the tarsometatarsal joint in the
hindlimb.4
Proximal suspensory desmitis (PSD) is a common
injury in both forelimbs5–9 and hindlimbs10,11 of
athletic horses, and may occur unilaterally or
bilaterally.
2.
Proximal Suspensory Desmitis in the Forelimb
PSD results in a sudden onset lameness, which can
be remarkably transient, resolving within 24 h unless the horse is worked hard. Lameness varies
from mild to moderate and is rarely severe unless
the lesion is extensive. Bilateral PSD may result in
loss of action rather than overt lameness. This occurs more commonly in flat racehorses, probably due
to failure of recognition of earlier, subtle unilateral
lameness. Lameness is usually worse on soft
ground, especially with the affected limb on the outside of a circle, and, when subtle, may be more easily
felt by a rider than seen by an observer. Lameness
may not be apparent at working trot, but may be
detectable at medium or extended trot. Recognition of these features in the history may be important since lameness often resolves rapidly and it
may be undesirable to work the horse hard to reproduce lameness, with the inherent risk of worsening
NOTES
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IN DEPTH: PROXIMAL SUSPENSORY DESMITIS
Fig. 1. Transverse ultrasonographic images of the proximal
metatarsal region of a nine year old Grand Prix showjumper with
severe right hindlimb lameness, which was alleviated by plantar
metatarsal (subtarsal) analgesia. Intra-articular analgesia of
the tarsometatarsal joint was negative. The proximal suspensory ligament of the right hindlimb is enlarged compared to the
left and is diffusely hypoechoic, consistent with proximal suspensory desmitis.
Fig. 2. The same horse as Fig. 1. Lateral nuclear scintigraphic
images of the left and right hock and proximal metatarsal regions.
Uptake of the radiopharmaceutial is symmetrical in the proximal
metatarsal regions and of normal distribution.
the injury. Lameness is often transiently accentuated by distal limb flexion.
In the acute phase there may be slight edema in
the proximal metacarpal region, localized heat, and
distension of the medial palmar vein, but these features may be transient or absent. Pressure applied
to the SL against the palmar aspect of the third
metacarpal bone may elicit pain. Forced extension
and protraction of the limb may elicit pain.
The feet should be evaluated carefully since frequently foot imbalance is a predisposing factor.
Local Analgesic Techniques
If PSD is suspected, perineural analgesia of either
the lateral palmar nerve3 (2 ml mepivacaine) or the
medial and lateral palmar metacarpal nerves (2 ml
per site) is indicated. This should result in sub138
Fig. 3. Lateral radiographic view of the left hock and proximal
metatarsal region superimposed over a nuclear scintigraphic image of a ten year old Grand Prix showjumper with moderate left
hindlimb lameness, which was substantially improved by subtarsal analgesia of the plantar metatarsal nerves. There were no
detectable radiographic or ultrasonographic abnormalities suggestive of proximal suspensory desmitis.
stantial improvement in, or alleviation of, lameness
within 10 min, assuming PSD is the only cause of
lameness. However, neither technique is necessarily specific. Blockade of the lateral palmar nerve
also has the potential to alleviate pain associated
with a lateral source of pain in the more distal limb.
The risks of influencing middle carpal joint pain are
less than using the subcarpal approach, but local
anaesthetic solution may diffuse and result in improvement in lameness associated with the middle
carpal joint.a Perineural analgesia of the palmar
metacarpal nerves may alleviate pain associated
with either the middle carpal or carpometacarpal
joint, due to local diffusion or inadvertent deposition
of local anaesthetic solution into the palmar outpouching of the middle carpal joint capsule. A false
negative result may be achieved either due to inadvertent injection into the carpal sheath, or failure of
the local anaesthetic solution to diffuse proximally
to the most proximal extent of a lesion. Although
the SL receives innervation from fibres from both
the median and ulnar nerves, perineural analgesia
of the ulnar nerve usually resolves lameness associated with PSD.
Intra-articular analgesia of the middle carpal
joint may result in either partial improvement or
complete alleviation of pain associated with the
proximal suspensory ligament in some cases (15/25
horses, 60%).a Using a dorsal approach to the middle carpal joint rather than a palmarolateral approach should theoretically reduce the risks of
diffusion of local anaesthetic solution to the proximal SL and palmar metacarpal nerves; however, in
practice there appears to be little difference. Comparison of the relative responses to middle carpal
joint analgesia (6 ml mepivacaine; assessed 10 min
after injection) and perineural analgesia of the lateral palmar nerve or the palmar metacarpal nerves
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IN DEPTH: PROXIMAL SUSPENSORY DESMITIS
is potentially useful, but it can be highly misleading.
Generally a horse with lameness due to PSD shows
a better response to perineural analgesia than intraarticular analgesia, but this is not universal. Similarly, primary middle carpal joint pain is usually
best improved by intra-articular analgesia, but this
is not always the case. The clinician should evaluate the response to these local analgesic techniques
in the light of:
●
●
●
The use of the horse and thus the likelihood of
the site of injury.
Other clinical signs (e.g., distension of the middle carpal joint capsule, pain on passive manipulation of the carpus)
The degree and character of the lameness
Perineural analgesia of the palmar nerves at the
level of the base of the proximal sesamoid bones
often results in lameness due to PSD appearing
worse. Perineural analgesia of the palmar nerves
(at mid metacarpal level) and palmar metacarpal
nerves (distal to the button of the second and fourth
metacarpal bones) (4 point block) often results in
some improvement in pain associated with PSD,
presumably due to proximal diffusion of local anaesthetic solution via lymphatic vessels or along fascial
planes.
It should be borne in mind that there may be more
than one source of pain contributing to lameness.
PSD and concurrent foot pain occur quite commonly.
There may also be concurrent hindlimb lameness,
especially in the contralateral hindlimb, so it is important both to assess and to re-evaluate the horse
as a whole.
Differential Diagnosis
PSD should be differentiated from middle carpal
joint pain, being aware that especially in young
Thoroughbred racehorses lesions may occur in both
locations simultaneously. Degenerative joint disease of the carpometacarpal joint occasionally occurs. Pain associated with palmar cortical fatigue
fractures of the third metacarpal bone8,12–14 responds similarly to local analgesic techniques, however lameness tends to be more severe, worse on
firm ground and often deteriorates the further the
horse trots. Avulsion fractures of the origin of the
SL occur less frequently and tend to be associated
with more persistent and severe lameness.1
Diagnostic Ultrasonography
Diagnostic ultrasonography is essential for accurate
diagnosis of PSD. The limb should be evaluated in
both transverse and longitudinal planes and careful
comparison should be made with the contralateral
limb. High quality images are required since lesions can be subtle and easily missed if the gain
controls are too high and the transducer is not focused on the SL. Cross-sectional area measurements may be extremely valuable since, especially
in acute cases, enlargement of the ligament may be
the only detectable ultrasonographic abnormality.
Bear in mind that muscular tissue appears less
echogenic than ligamentous tissue and that proximally the SL originates in two halves. Therefore,
the proximal aspect of a normal SL is not necessarily
of uniform echogenicity in transverse images.1
In longitudinal images in the most proximal part of
the metacarpal region it may be difficult to orientate
the transducer perpendicular to the line of the fibers, thus the SL may appear less echogenic than
further distally. Previous injuries to the SL may
not resolve fully to restore normal, uniform echogenicity. Be aware that poor local analgesic technique may result in aspiration of air, which will
create artefacts.
Abnormalities associated with PSD include:
●
●
●
●
●
●
Enlargement of the cross-sectional area. This
may result in reduction of space between the
SL and the palmar cortex of the third metacarpal bone, or reduced space between the SL and
the accessory ligament of the deep digital
flexor tendon.
Poor demarcation of the margins of the SL,
especially the dorsal margin.
Focal or diffuse areas of reduced echogenicity.
These may extend less than 1 cm proximodistally and occupy from less than 10% to up to
the entire cross-sectional area of the ligament.
Focal anechoic core lesions.
Reduced strength of fiber pattern.
Focal mineralization.
In a three-year-old Thoroughbred that had had sustained PSD at two years of age, there may be recurrent mild lameness and it may not be possible to
discern any structural abnormality other than enlargement of the SL.
The degree of ultrasonographic abnormality
(cross-sectional area involved and proximodistal extent of the lesion) usually reflects the severity of the
lameness. In acute cases the ultrasonographic abnormalities may be very subtle. They may deteriorate over the next 10 to 14 days and re-evaluation
may be useful to confirm the diagnosis.
In horses with an avulsion fracture of the origin of
the suspensory ligament, the avulsed fragment is
usually readily identifiable and is generally associated with only a very focal lesion in the SL itself,
usually restricted to the dorsal aspect.
Radiography
There are usually no detectable radiographic abnormalities of the third metacarpal bone in acute cases
of PSD. In chronic cases increased opacity of the
proximal aspect of the third metacarpal bone may be
seen in dorsopalmar views. This sclerosis should
be differentiated from that associated with a palmar
cortical fatigue fracture.15 In a lateromedial projection there may be sub-cortical sclerosis in the
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IN DEPTH: PROXIMAL SUSPENSORY DESMITIS
proximal palmar aspect of the third metacarpal
bone. These secondary bony changes in a forelimb
are associated with a more guarded prognosis.
Nuclear Scintigraphy
Nuclear scintigraphy is generally unnecessary for
diagnosis provided that good quality ultrasonographic images are obtained, but may give additional information about associated bone turnover at
the insertion of the SL. Pool phase images using
technetium 99m MDP are rather insensitive, and
may be negative. Abnormal uptake may actually
reflect very early bone uptake. Bone phase images
may or may not show increased uptake of the radiopharmaceutical in the palmar aspect of the third
metacarpal bone. Negative scintigraphic images
do not preclude the presence of PSD. Abnormal
uptake in the bone phase seen in the absence of
ultrasonographic abnormality is more likely to reflect primary bony pathology.
Treatment
Most cases of acute forelimb PSD respond well to
box rest and controlled walking exercise for 3
months. Attention to correct foot balance is important. A premature resumption of work usually results in recurrent injury. Approximately 90% of
horses resume full athletic function without recurrent injury.1 More chronic cases may require more
prolonged rehabilitation, and in a small proportion
lameness is persistent. Extracorporeal shock wave
treatment (3 treatments at 2 week intervals) has
been successful in some chronic cases, which had
failed to respond to conservative management.b
In some horses the lesions disappear completely
ultrasonographically. In others there may be some
increase in echogenicity, but uniform echogenicity is
never restored. Rest should be continued until the
ultrasonographic appearance remains stable.
3.
Proximal Suspensory Desmitis in the Hindlimb
PSD in the hindlimb results in either an insidious
onset or sudden onset lameness which may be mild
or severe. In contrast to the forelimb, lameness
may persist and remain severe despite restriction to
box rest. This is probably due to a compartmentlike syndrome and pressure on the adjacent plantar
metatarsal nerves. In view of the chronicity of
some lesions when first identified, and the finding of
secondary radiological changes in sound horses it is
likely that some lesions exist sub-clinically, or are
associated with a low grade lameness that goes unrecognized. The incidence of bilateral lesions is
higher than in forelimbs. Horses with either
straight hock conformation and/or hyperextension of
the metatarsophalangeal joints appear predisposed
to injury. Such conformational abnormalities were
identified in 9 of 42 horses (21%) with hindlimb
PSD, but in only 4 of 50 consecutive horses (8%) with
hindlimb lameness unrelated to the suspensory ap140
paratus.11 Hyperextension of the metatarsophalangeal joint may also develop as a sequel to PSD.11
In acute cases there may be localized heat and
swelling and pain on pressure applied to the SL, but
frequently there are no localizing clinical features.
Lameness is often characterized by a reduced
height of arc of foot flight, with or without intermittent catching of the toe. The cranial phase of the
stride may be shortened. Lameness may be accentuated by either proximal or distal limb flexion.
Bilateral lesions may result in poor hindlimb action
rather than obvious hindlimb lameness. Like
many hindlimb lamenesses, lameness is often more
obvious when the horse is ridden, especially when
the rider sits on the diagonal of the lame limb.
Local Analgesic Techniques
Perineural analgesia of the plantar nerves (mid
metatarsal level) and plantar metatarsal nerves
may result in slight improvement in lameness due to
proximal diffusion of the local anaesthetic solution.
Lameness is usually substantially improved by perineural analgesia of the medial and lateral plantar
metatarsal nerves distal to the tarsus, but may not
be alleviated fully. It is difficult to deposit the local
anaesthetic solution as proximal as ideal due to the
shape of especially the fourth metatarsal bone.
False negative results may be obtained due to inadvertent injection into either the tarsal sheath or the
tarsometatarsal joint capsule. Sub-tarsal analgesia can influence tarsometatarsal joint pain, and
occasionally (2/24 horses, 8%11) intra-articular analgesia of the tarsometatarsal joint alleviates pain
associated with PSD. Perineural analgesia of the
tibial nerve alone alleviates pain associated with
PSD, without influencing tarsal pain.
Diagnostic Ultrasonography
High quality ultrasonographic images are essential
for accurate diagnosis. Large vessels plantarolateral to the SL may result in broad linear anechoic
artefacts within the SL, which complicate interpretation. In large Warmblood horses in particular
the SL is situated deeply and the ultrasound transducer must be focused accordingly. The SL should
be imaged in both transverse and longitudinal
planes. To examine the most proximal part of the
SL in transverse images it may be helpful to rock the
transducer slightly medially and laterally in order to
obtain the best quality images. In a normal horse
the proximal part of the hindlimb SL is more uniformly echogenic than in the forelimb.
In PSD in the hindlimb focal anechoic areas are
relatively unusual. More commonly there is enlargement of the SL, with poor demarcation of its
borders and a diffuse reduction in echogenicity of
part or all of the cross-sectional area of the ligament
(Fig. 1). Ectopic mineralization occurs more often
in hindlimbs compared to forelimbs. An irregular
contour of the plantar contour of the third metatarsal bone may reflect entheseophyte formation. In
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IN DEPTH: PROXIMAL SUSPENSORY DESMITIS
some horses, especially those with abnormal conformation, the lesions may progress despite box rest.
Radiography
Diagnosis should never be based on radiography
alone since some sound horses have some sclerosis of
the proximal aspect of the third metatarsal bone.
In horses with chronic active PSD this tends to be
more extensive. In the dorsoplantar view there is
increased opacity of the proximal aspect of the third
metatarsal bone, often more obvious laterally. In a
lateromedial projection there is sub-cortical sclerosis and alteration of the trabecular pattern of the
proximoplantar aspect of the third metatarsal bone
due to endosteal new bone, extending up to 4 cm
proximodistally. The plantar cortex may itself be
thickened and in addition there may be entheseophyte formation on its plantar aspect. However
in some acute cases no radiological abnormality is
detectable.
Nuclear Scintigraphy
Not all horses with PSD have detectable abnormalities if examined using nuclear scintigraphy (Fig. 2).
Pool phase images appear to be rather insensitive.
In bone phase images there may be increased uptake of the radiopharmaceutical in the proximoplantar aspect of the third metatarsal bone in some, but
not all cases of PSD. This should be differentiated
from those horses with primary bony pathology with
no detectable ultrasonographic abnormality of the
SL (Fig. 3).
Differential Diagnosis
PSD should be differentiated from pain associated
with the tarsometatarsal joint, and primary stress
reactions in the third metatarsal bone.
Treatment
The prognosis for PSD in the hindlimb has generally
been poor. Only 6/42 horses (14%) seen in a referral practice were able to resume full work without
detectable lameness for at least 1 year, all of which
had been lame for less than 5 weeks.11 All these
horses showed marked clinical improvement within
3 months of the onset of lameness. Two additional
horses resumed full work, but suffered lameness in
another limb. Seven horses improved substantially and were able to work, despite persistent mild
lameness. Twenty-four horses (57%) had persistent or recurrent lameness. Results from a first
opinion practice were also disappointing with only
10 of 17 horses (58%) resuming work.1
Acute cases (less than 4 – 6 weeks duration) of
PSD respond reasonably to local infiltration with
corticosteroids, aimed to reduce inflammation and
therefore swelling and thus minimize the risk of the
development of a compartment syndrome (see below). Chronic cases have a very guarded prognosis
regardless of the treatment. Lameness tends to
persist unchanged even following prolonged box
rest, which is unusual for a primary soft tissue lesion. In some cases lesions are progressive. Local
infiltration with corticosteroids, glycosaminoglycan
polysulphate, sodium hyaluronan or actovegin and
Traumil has given disappointing results. Some
horses have worked satisfactorily while being
treated with phenylbutazone, without apparent deterioration of clinical signs. Tibial neurectomy performed in three horses enabled all to return to full
athletic function (show jumping and horse trials).
Extracorporeal shock wave therapy (lithotripsy) appears to be helpful in some cases.ab Fasciotomy has
also been successful in some horses.c
Gross Pathology and Histopathology
Post mortem examinations have been performed on
both hindlimbs of eight horses, 6 with unilateral
lameness and 2 with bilateral lameness.11,16d
Abnormalities of the SLs were confined to the lame
limbs. There was gross enlargement of the SLs,
with thickening of surrounding fascia and periligamentous tissues, especially on the plantar aspect.
Histological changes in the SL included hypercellularity and acellular areas, haemosiderin deposition,
fibrosis, hyalinization of collagen, an increased
number of fibrous septae, some with blood vessels,
neovascularization, and chondroid metaplasia.
Although chondroid metaplasia was seen at the ligament bone interface in both lame and sound limbs,
intra-ligamentous chondroid metaplasia was only
seen in the lame limbs.
There was evidence of compression of adjacent
peripheral nerves in the lame limb of 5 horses.
Abnormalities of the plantar metatarsal nerves included thickening of the perineurium, perineural
fibrosis, reduction or absence of nerve fibers, and
Renaut bodies.
These changes support the theory of PSD in the
hindlimb resulting in a compartment syndrome.
References and Notes
1. Dyson S. The suspensory apparatus. In: Rantanen N,
McKinnon A, eds. Equine Diagnostic Ultrasonography. 1st
ed. Baltimore: Williams & Wilkins, 1998:447– 474.
2. Muylle S, Desmet P, Simoens P, et al. Histological study of
the innervation of the suspensory ligament of the forelimb of
the horse. Vet Rec 1998;142:606 – 610.
3. Ford T, Ross M, Orsini P. A comparison of methods for
proximal metacarpal anaesthesia in horses. Vet Surg 1988;
18:146 –150.
4. Dyson S, Romero J. An investigation of injection techniques
for local analgesia of the equine distal tarsus and proximal
metatarsus. Equine Vet J 1993;25:30 –35.
5. Marks D, Mackay-Smith M, Leslie A, et al. Lameness resulting from high suspensory disease (HSD) in the horse.
Proc Amer Assoc Equine Pract 1981;24:493– 497.
6. Genovese R, Rantanen N, Hauser M, et al. Diagnostic ultrasonography of equine limbs. Vet Clin North Am [Equine
Pract] 1986;2:145–226.
7. Huskamp B, Nowak M. Insertion desmopathies in the
horse. Pferdheilkunde 1988;4:3–12.
8. Dyson S. Some observations on lameness associated with
pain in the proximal metacarpal region. Equine Vet J 1988;
6(Suppl):43–52.
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IN DEPTH: PROXIMAL SUSPENSORY DESMITIS
9. Dyson S. Proximal suspensory desmitis: clinical, ultrasonographic and radiographic features. Equine Vet J 1991;23:25–31.
10. Dyson S. Proximal suspensory desmitis in the hindlimb:
42 cases. Br Vet J 1994;150:279 –291.
11. Dyson S. Proximal suspensory desmitis in the hindlimb.
Equine Vet Educ 1995;7:275–278.
12. Ross M, Ford T, Orsini P. Incomplete longitudinal fracture
of the proximal palmar cortex of the third metacarpal bone in
horses. Vet Surg 1988;17:82– 86.
13. Lloyd K, Kobluk P, Ragle C, et al. Incomplete palmar fracture of the proximal extremity of the third metacarpal bone in
horses: ten cases (1981–1986). J Am Vet Med Assoc 1988;
192:798 – 803.
14. Pleasant R, Baker G, Muhlbauer M, et al. Stress reactions
and stress fractures of the proximal palmar aspect of the
142
third metacarpal bone in horses: 58 cases (1980 –
1990). J Am Vet Med Assoc 1992;201:1918 –1923.
15. Butler J, Colles C, Dyson S, et al. Clinical Radiology of the
Horse. 2nd ed. Oxford, Blackwell Science, 2000;152;158 –
159, 162–163.
16. Dyson S. Problems encountered in equine lameness diagnosis with special reference to local analgesic techniques, radiology and ultrasonography. Newmarket, R. & W. Publications Ltd, 1995;31–54.
a
Dyson, S. Unpublished data 2000.
Boening, J. Personal communication 1999.
Ross, M. Personal communication 2000.
d
Dyson, S. Unpublished data 2000.
b
c
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