• 1 year old thoroughbred colt
• Originally evaluated by its primary care veterinarian (record not available).
• Referred to the Large Animal Surgery Service at University of Florida for a CT of the carpi.
• Limited pre-anesthetic evaluation: Normal physical examination and bloodwork.
• No lameness seen at a walk when observed ambulating around the stall.

RIGHT CARPUS:

• Along the distal palmaromedial aspect of the ulnar carpal bone, two well-defined and sharply marginated small osseous fragments are identified. These fragments are located at the proximal attachment site of the lateral palmar intercarpal ligament, which can be seen on the soft tissue images. At the same level, within the ulnar carpal bone, there are well-defined concave defects with irregular margins and moderate sclerosis.
• Within the antebrachiocarpal and middle carpal joints, the joint capsule is mildly distended with fluid, seen in greater volume at the level of the described osseous fragments, in the distomedial recess of the middle carpal joint. In the same region, within the middle carpal joint, soft tissue septations are identified.
• The physes are open, consistent with the young age of the patient.

LEFT CARPUS:

• The physes are open, consistent with the young age of the patient.
• The osseous and soft tissue structures are normal.

• Right ulnar carpal bone fragmentation with sclerosis and mild antebrachiocarpal and middle carpal joint effusion and synovial proliferation. These findings are most consistent with acute lateral intercarpal ligament avulsion fractures with associated synovitis. The possibility this finding represents an anatomical variant is also considered.

Lateral palmar intercarpal ligament (LPIL):

• Proximal attachment predominantly on the distal part of the palmaromedial surface of the ulnar carpal bone.1,2
• Extends in a distomedial and slightly palmar direction to insert on the the proximal palmarolateral surface of the third carpal bone , with a few thin branches inserting on the palmaromedial surface of the fourth carpal bone.1,2
• Beinlich et al. (2004) previously described the radiographic and histologic characteristics of lateral palmar intercarpal ligament avulsion fractures in horses.2 The reported histologic appearance of the fragments confirmed trauma as an underlying etiology. 2
• Injuries to the LPIL are less common than medial intercarpal ligament injuries.2,3
• Injuries isolated to the LPIL are not fully understood.4
• Increased tension on the palmar half to two third of the LPIL was observed on extension of the carpus in a cadaveric study.1
• A more recent study described small osseous fragments at a similar location without evidence of fracture in 17% of 30 mature horses. These fragments were thought to represent separate centers of ossification or dystrophic mineralization in the lateral palmar intercarpal ligament.5
• In the described case, there was adequate conspicuity of the carpal ligaments on images reformatted using a soft tissue algorithm.

• The patient transferred back to the primary care veterinarian for treatment.

1.  Whitton, R. Chris, Pat H. McCarthy, and Reuben J. Rose. “The intercarpal ligaments of the equine midcarpal joint, Part 1: The anatomy of the palmar and dorsomedial intercarpal ligaments of the midcarpal joint.” Veterinary Surgery 26.5 (1997): 359-366.
2. Beinlich, Christopher P., and Alan J. Nixon. “Prevalence and response to surgical treatment of lateral palmar intercarpal ligament avulsion in horses: 37 cases (1990–2001).” Journal of the American Veterinary Medical Association 226.5 (2005): 760-766.
3. Beinlich, Christopher P., and Alan J. Nixon. “Radiographic and pathologic characterization of lateral palmar intercarpal ligament avulsion fractures in the horse.” Veterinary radiology & ultrasound 45.6 (2004): 532-537.
4. Whitton, R. Chris, Nicholas J. Kannegieter, and Reuben J. Rose. “The intercarpal ligaments of the equine midcarpal joint, part 3: clinical observations in 32 racing horses with midcarpal joint disease.” Veterinary Surgery 26.5 (1997): 374-381.
5. Nagy, Annamaria, and Sue Dyson. “Magnetic resonance anatomy of the carpus of the horse described from images acquired from low‐field and high‐field magnets.” Veterinary radiology & ultrasound 52.3 (2011): 273-283.