A 32-year-old man presents to the urgent care clinic with right thumb pain after a skiing accident earlier today. He explains that he fell onto his outstretched hand while holding his ski pole, causing his thumb to be forcibly pulled away from his other fingers. On physical examination, there is swelling and ecchymosis over the ulnar aspect of the first metacarpophalangeal (MCP) joint. There is significant tenderness to palpation over the ulnar collateral ligament (UCL). When the clinician stabilizes the metacarpal and applies a valgus stress to the proximal phalanx, there is 35° of joint opening with no clear end-point, compared to 10° on the uninjured left side. The patient has weakened pinch strength.
Which of the following is the most likely diagnosis?
The correct answer is:
D) Gamekeeper’s thumb
The patient is presenting with an acute injury to the ulnar collateral ligament (UCL) of the thumb, commonly referred to as gamekeeper’s thumb or Skier’s thumb. The mechanism of injury is typically a forced abduction and hyperextension of the thumb MCP joint, such as when a ski pole grips the webbing of the thumb during a fall. The UCL is critical for providing stability to the thumb during pinch and grasp activities. A complete tear results in significant weakness when the patient tries to pinch their thumb and index finger together. On physical examination, the hallmark is laxity of the MCP joint during valgus stress testing. A deviation greater than 30°, or a 15° difference compared to the contralateral side, suggests a complete rupture. A significant complication to be aware of is a Stener lesion, where the adductor pollicis aponeurosis becomes interposed between the ruptured ligament and its insertion point, preventing spontaneous healing and necessitating surgical repair.
Answer choice A: Bennett fracture, is incorrect. This is an intra-articular fracture at the base of the first metacarpal. While it involves the thumb, it typically presents with bony tenderness at the carpo-metacarpal (CMC) joint rather than the MCP joint, and is diagnosed via radiography.
Answer choice B: Boxer's fracture, is incorrect. This is a fracture of the fifth metacarpal neck, usually sustained by punching a hard object. It does not involve the thumb.
Answer choice C: De Quervain tenosynovitis, is incorrect. This is an overuse injury involving the abductor pollicis longus and extensor pollicis brevis tendons. It presents with pain at the radial styloid and a positive Finkelstein test, rather than joint instability following trauma.
Answer choice E: Scaphoid fracture, is incorrect. This injury typically follows a fall on an outstretched hand and presents with tenderness in the anatomical snuffbox. While the pain is near the thumb, it involves the carpal bones and would not cause laxity at the MCP joint.
Key Learning Point
Gamekeeper’s thumb is a rupture of the ulnar collateral ligament of the thumb MCP joint caused by forced abduction. Diagnosis is made by identifying laxity or the absence of a firm end-point during valgus stress testing. Weakness in pinch strength is a common functional deficit. Management for partial tears is immobilization with a thumb spica splint, but complete tears or Stener lesions require surgical consultation.
