Introduction

• Cause: racing injury, falls traumatic impact, eg kicks, polo mallets; <10% of all carpal fractures; more common in steeplechasers and jumpers.
• Signs: acute forelimb lameness Musculoskeletal: gait evaluation, carpal flexion pain, palmar carpal swelling, carpal sheath effusion .
• Diagnosis: palpation, carpal flexion test Musculoskeletal: manipulative tests, ulnar nerve block Forelimb: perineural analgesia, radiology Forelimb: radiography, ultrasonography of carpal sheath Ultrasonography: bone/joints.
• Treatment: prolonged rest (6-12 months) with weight bearing results in fibrous union, surgical repair rarely used or successful, carpal sheath tenoscopy in chronic cases with impingement of secondary complications.
• Prognosis: good.
• See also:
  • Fractures overview Musculoskeletal: fracture.
  • First aid for fractures Musculoskeletal: fracture - first aid.
  • Carpal chip fractures Carpus: fracture - chip.
  • Carpal comminuted fractures Carpus: fracture - comminuted.

Pathogenesis

Etiology

• Trauma:
  • Falls particularly after jumps where the horse lands on a flexed forelimb.
  • Hyperextension on landing after jumps.
  • Direct injury from kicks or blows, eg polo mallets.

Predisposing factors

Specific

• Although fractures can occur in any plane of the bone, the most common configuration is in the vertical frontal plane through the lateral groove formed by the long tendon of the ulnaris lateralis muscle.
• Once the fracture has occurred the constant pull of the flexor muscles on the fracture fragments causes distraction and instability with subsequent fibrocartilaginous non-union.

Pathophysiology

• The exact mode by which trauma leads to accessory bone fractures is presently unclear although there are a number of theories.
• External direct trauma is possible but skin/hair damage in the area is rare in fracture cases.
• 'Bowstring' effect of ulnaris lateralis, flexor carpi ulnaris and flexor tendons with the leg partially flexed and fully loaded has been suggested.
• The ‘nutcracker effect’ can occur where the accessory carpal bone is caught between the third metacarpal bone and the radius during a fall on the flexed limb. Contact lesions have been noted on the caudal aspect of the radius in some cases

Timecourse

• Most cases present as acute onset lameness but some may be presented with chronic lower-grade lameness when there are complications such as carpal sheath constriction or tenosynovitis Carpus: sheath tenosynovitis.

Diagnosis

Treatment

Outcomes