Keywords
foal - orthopaedics - equine - fracture - carpus
Introduction
The accessory-carpal bone in the equid is united with the parent articulation via
three short and individual accessory-carpal ligaments. Proximally, a short ligament
extends from the lateral aspect of the accessory-carpal bone and extends to the distal
end of the radius behind the groove for the lateral digital extensor (accessory-radial ligament). A middle ligament connects the accessory-carpal bone to the ulnar carpal bone (accessory-ulnar ligament). The distal ligament is composed of two distinct bands which pass from the distal
margin of the accessory to the fourth carpal bone (accessory-fourth carpal ligament) and the proximal end of the fourth metacarpal bone (accessory-fourth metacarpal ligament).[1] Injury associated with the accessory-carpal ligaments has not been previously detailed
in the veterinary literature.
Case Description
A 45-day-old Paint colt (130 kg) presented for a weight bearing lameness on the left
forelimb. The foal was brought in from pasture 10 days prior to presentation with
a noticeable lameness and swelling surrounding the carpus. The foal was managed on-farm
with cold hosing, support bandaging, stall confinement and flunixin meglumine (2.2
mg/kg, orally) for the first 3 days of the lameness. The lameness and carpal swelling
did not improve, leading to orthopaedic examination.
On admission, the foal was bright, alert and responsive. Vital parameters were within
normal limits. The colt foal was able to ambulate on all four limbs; however, a marked
lameness was present when weight was placed on the left forelimb (AAEP Grade 4/5).[2] On musculoskeletal palpation, moderate effusion of the radiocarpal joint and slight
effusion of the middle carpal joint were present. Additionally, subcutaneous oedema
could be palpated diffusely surrounding the dorsal and lateral aspects of the carpal
region. Hoof tester application to the foot was negative. Passive carpal flexion was
limited by a marked painful response by the foal. The remainder of the limbs, as well
as the umbilicus, palpated within normal limits. Haematology was submitted with no
abnormalities identified.
Arthrocentesis and Joint Cytology
The left front carpus was prepared with alternating chlorhexidine and alcohol antiseptics.
Arthrocentesis of the radiocarpal joint was performed and approximately 1.5 mL of
joint fluid was aspirated. Grossly, the joint fluid was orange to red in colour and
turbid. Cytology was performed and characterized by a total nucleated cell count of
1,330 cells/μL, total protein (TP) of 3.8 g/dL and lactate of 3.1 mmol/L (normal total
nucleated cell count range: <1000 cells/μL, normal TP range: < 2.0 g/dL, normal lactate: < 3.9
mmol/L).[3] Morphology identified a mixed collection of nucleated cells comprised of 87% large
mononuclear cells (resembling quiescent macrophages), 8% lymphocytes and 5% non-degenerate
neutrophils. No micro-organisms were identified. Based on cytology, joint sepsis as
a differential was deemed unlikely. The middle carpal joint fluid was aspirated, and
grossly the fluid appeared straw coloured and clear. Cytology of the middle carpal
joint was not performed.
Ultrasound Examination
Under sedation, ultrasound identified increased joint fluid with proliferative synovium
within the radiocarpal joint and irregular bone margins associated with the distal
aspect of the cranial radius. Generalized hypoechogenicity and increased thickness
of the subcutaneous tissue overlying the carpus were also noted. The middle carpal
joint on ultrasound imaged within normal expectations.
Radiographs
A complete radiographic series of the left front carpus was obtained. The cuboidal
bones were observed to be normal in shape and ossification. On the dorsopalmar projection
and superimposed with the accessory-carpal bone, an abnormal radiolucent line associated
with the proximal articular margin of the ulnar carpal bone was observed extending
distal and laterally ([Fig. 1A]). Additionally, a separate mineralized curvilinear structure was noted palmaromedial
to the distal radial epiphysis ([Fig. 1B]). The dorsolateral margin of the distal radial epiphysis and proximal aspect of
the radial carpal bone were also discontinuous with well-defined separate mineral
fragments present dorsally.
Fig. 1 (A) Dorsopalmar radiograph illustrating a radiolucent line associated with the proximal
articular margin of the ulnar carpal bone extending distal and laterally (circled);
(B) Palmarolateral–dorsomedial radiograph illustrating faintly mineralized curvilinear
focus palmaromedial to the distal radial epiphysis (palmarolateral–dorsomedial projection,
abnormality circled) and osteochondral fragmentation off the dorsolateral aspect of
the distal radial epiphysis and proximal aspect of the radial carpal bone. DLPMO,
dorsolateral palmaromedial oblique.
Computed Tomography and Contrast Arthrography
Based on suspicion of a traumatic insult, and the difficulty characterizing the injury
from radiographs alone, computed tomography (CT) of the carpus was performed. In preparation
for the procedure, the colt foal was sedated with xylazine (50 mg) intravenously and
induced with ketamine (300 mg) intravenously. The foal was placed in a sternal position
on the CT table and held in place using a combination of rope ties and white adhesive
tape. The CT field of view of both forelimbs extended from the distal aspect of the
radial diaphysis to the proximal metaphysis of the third metacarpal bone. Before and
after positive contrast arthrography was performed using iohexol (diluted in a 1:1
ratio with saline, 6 mL volume injected into the left radiocarpal joint) for both
bone and soft tissue algorithms. Transverse images were reconstructed in dorsal and
sagittal planes for analysis.
Best appreciated on the transverse images, a sagittally oriented hypoattenuating linear
defect was noted in the midbody of the ulnar carpal bone ([Fig. 2]). The defect was wider at the palmar margin and tapered dorsally. Several separate
mineral foci were noted on the palmar aspect of the ulnar carpal bone in the region
of the accessory-ulnar ligament. Additionally, the articular margins of the palmar
aspect of the ulnar carpal bone and dorsal aspect of the accessory-carpal bone were
moderately discontinuous with irregular margins. Multiple small separate fragments
were also visualized associated with the dorsal radial epiphysis (in the region of
the accessory-radial ligament), intermediate carpal bone and medial and lateral styloid
processes.
Fig. 2 Transverse slice of proximal row of carpal bones demonstrating (A) normal right proximal row; and (B) sagittally oriented hypoattenuating linear defect in the midbody of the left ulnar
carpal bone (A = accessory carpal, U = ulnar carpal, I = intermediate carpal, R = radial
carpal).
Diagnosis
Based on clinical examination and collective diagnostic imaging, the final diagnosis
was a biarticular, non-displaced sagittal plane fracture of the ulnar carpal bone
with micro-comminution and associated avulsion fragments originating from the accessory-ulnar
and accessory-radial ligaments and dorsal recess of the radiocarpal joint in the left
front. Secondary non-septic synovitis of the radiocarpal joint and subcutaneous oedema
was also present.
Treatment
Surgical fixation of the ulnar carpal bone utilizing internal lag screw fixation in
conjunction with arthroscopic surgery of the dorsal recess of the joint was offered.
The ability to perform palmar carpal arthroscopy in a foal to remove palmar-based
fragments was anticipated to be difficult. Conservative management was also offered
and forecasted to have a fair prognosis for athleticism. The potential for radiocarpal
osteoarthritis developing and requiring future joint support was discussed with the
owner during consideration of how to manage the injury. The owner elected conservative
management.
Under sedation, the radiocarpal joint was flushed with 3 L of sterile saline to remove
inflammatory mediators and micro-debris present within the articulation. Two 14-gauge
needles were placed along the lateral and medial aspects of the extensor carpi radialis
tendon, and a third 18-gauge needle was placed into the palmar recess of the carpus
from a lateral approach. Following conclusion of joint lavage, 44mg of hyaluronic
acid was administered intra-articularly in conjunction with 80mg of gentamicin. A
firm cotton bandage was placed overlying the carpus to provide stiffness and maintain
the carpus in extension.
The foal was placed on a 14-day course of oral firocoxib (0.1 mg/kg per os Semel in
Die). The owner was instructed to keep the foal on strict stall rest for a total of
3 months from time of presumed injury. Starting at 2 weeks post-injury, range of motion
exercises were performed daily by the owner by passively flexing the carpus to the
point of physical resistance and/or aversion response from pain. This was continued
as a daily exercise for ∼90 days.
Outcome
The owner was contacted by phone at 4 weeks post-examination. The foal was showing
improvement in comfort, although lameness was not fully resolved. Range of motion
during flexion of the carpus, as estimated by the owner, was ∼30 degrees (0 degrees
indicating carpus in normal extension while weight bearing). At 60 days, the range
of motion during flexion of the carpus, as estimated by the owner, was ∼90 degrees.
The foal was noted to be sound at the walk at 60 days. At 140 days, the range of motion
during flexion of carpus was considered normal during veterinary examination. Comfort
had improved such that the owner had initiated training on the lunge-line. Recheck
radiographs at 140 days demonstrated moderate-to-severe subchondral sclerosis in the
region of the accessory-ulnar joint and enthesis of the accessory-ulnar ligament as
best noted on the dorsolateral palmaromedial oblique view ([Fig. 3]). The previously defined mineralized feature palmaromedial to the distal radial
epiphysis was superimposed with the palmarodistal aspect of the radius and exhibited
smooth margins. The previously noted mineral features along the dorsolateral distal
radius and proximal radial carpal bone were no longer identified.
Discussion
The carpus is a complex articulation composed of multiple cuboidal bones supported
by various carpal ligaments. The tendons of the ulnaris lateralis and flexor carpi
ulnaris insert along the proximal aspect of the accessory-carpal bone. Three short
ligaments support the accessory-carpal bone with the dorsal radial–cuboidal articulation.
Proximally, a short ligament extends from the lateral aspect of the accessory-carpal
bone and extends to the distal end of the radius behind the groove for the lateral
digital extensor (accessory-radial ligament). A middle ligament connects the accessory-carpal bone to the ulnar carpal bone (accessory-ulnar ligament). The distal ligament is composed of two distinct bands which pass from the distal
margin of the accessory to the fourth carpal bone (accessory-fourth carpal ligament) and the proximal end of the fourth metacarpal bone (accessory-fourth metacarpal ligament) ([Fig. 4]).[1]
Fig. 3 Palmarolateral–dorsomedial radiograph taken at day 140 post-injury illustrating mature
sclerosis associated with the previously demarcated radiolucent line within the proximal
articular margin of the ulnar carpal bone (circled).
Fig. 4 Anatomical illustration of the accessory-ulnar (proximal ligament, yellow), accessory-carpal
(middle ligament, green) and accessory-fourth metacarpal ligaments (distal ligaments,
blue and red) that support the accessory-carpal bone to the remainder of the carpus
(Illustration by Dr. R.B. Modesto).
The initial injury was not observed; however, hyperextension injury is presumed when
evaluating diagnostic images and understanding limb biomechanics. Specifically, carpal
hyperextension induces compression dorsally and places the palmar soft tissue structures
under tensile forces. In this case, compression dorsally manifested as osteochondral
crushing within the radiocarpal joint is evident along the cranial-distal radius and
dorsal-proximal radial carpal bone. A similar manifestation occurs in racehorses that
sustain osteochondral fragmentation from carpal hyperextension during high-speed racing.[4] When the palmar carpus is placed under tension, distracting forces are applied to
the accessory-carpal bone. In this instance, avulsion fragmentation was evident at
the interface of the accessory-ulnar and accessory-radial ligament. Furthermore, propagation
of forces secondary to ligament avulsion presumably created a complete, non-displaced
sagittal plane fracture of the ulnar carpal bone. To our knowledge, accessory-carpal
ligament avulsions have not been previously reported in the horse. Hyperextension
of the carpus can create frontal, transverse or comminuted fractures of the accessory-carpal
bone itself in adult horses. These primary accessory-carpal bone fractures also occur
secondary to the high tensile forces exerted through the flexor carpi ulnaris and ulnaris lateralis insertions during carpal hyperextension.[5]
Surgical stabilization, using lag screw fixation, is considered the best treatment
option for the management of articular, non-displaced carpal bone fractures in adult
horses.[6] Neonates have a tremendous ability to create bone callus, and have capacity for
further growth of the entire osseous articulation. Conservative management was forecasted
to result in a fair prognosis for healing and future athleticism, as it was hypothesized
that the non-displaced fracture of the ulnar carpal bone would heal via secondary
bone healing. Stabilization of the avulsed accessory-carpal ligaments was predicted
possibly via fibrosis. As such, passive range of motion exercises were considered
paramount for rehabilitation to ensure that restriction of joint capsule movement
did not develop as a sequela.
Other diagnostic imaging modalities, such as magnetic resonance imaging, could have
better characterized soft tissue injuries. However, the duration of anaesthesia required
for magnetic resonance imaging is prolonged compared with CT, wherein the entire procedure
was achieved via one dose of intravenous anaesthesia. Ultrasound was not repeated
following CT. Potentially, ultrasound examination of the palmar and lateral aspect
of the joint would have demonstrated avulsion injury associated with the accessory-carpal
bone ligaments.
In conclusion, this case report highlights an unusual traumatic carpal bone injury
secondary to presumed carpal hyperextension in a neonatal foal. Computed tomography
was essential in fully understanding the location and extent of osseous injury. This
report supports conservative management in foals as a therapeutic option for non-displaced
cuboidal bone fractures, as well as avulsion injuries associated with the ligamentous
attachments of the accessory-carpal bone itself.
