Causes / typical injury mechanism: Ulnar collateral ligament (UCL) injury of the thumb metacarpophalangeal (MP) joint is one of the most common ligamentous injuries to the thumb, with an estimated annual incidence of 50 in 100 000 individuals. The UCL is located along the ulnar border of the thumb MP joint and, along with the accessory UCL, is one of the major stabilizers of the thumb during pinching and grasping. Instability can result from acute trauma, failure to recognize or appropriately treat an acute injury, or chronic repetitive stress to the ligament, and thus can present as an acute or chronic instability (colloquially known as "skier's thumb" and "gamekeeper's thumb," respectively). As such, a thorough history and examination is critical to diagnosis.

Acute UCL injury of the thumb, or skier's thumb, is caused by an acute valgus (radially directed) force on an abducted thumb, resulting in partial or complete rupture of the UCL. This injury is commonly the result of skiing, sporting, bicycle, or motorcycle accidents.

A notable variant of acute thumb MP joint UCL injury known as a Stener lesion occurs when the adductor pollicis aponeurosis becomes interposed between the avulsed UCL and its insertion at the base of the proximal phalanx, resulting in inadequate healing and persistent laxity or instability. This variant has been reported to occur in 64%-88% of complete ruptures; thus, distinguishing between complete and partial rupture on examination is critical.

Chronic instability of the thumb MP joint UCL can result from untreated acute ruptures, failure to recognize a Stener lesion, or chronic repetitive stress to the ligament. This injury is often referred to as gamekeeper's thumb, a diagnosis originally used to describe Scottish gamekeepers who fractured the necks of rabbits between their thumbs and index fingers.

Classic history and presentation: Typical presentation includes tenderness, ecchymosis, and swelling along the ulnar border of the thumb MP joint. Pinch grip and valgus stress across the thumb MP may cause pain and instability. Avulsion fracture of the ulnar base of the proximal phalanx, and less commonly of the metacarpal head, can be an associated finding on x-ray.

Patients with chronic instability typically present with thumb pain, swelling, and weakness that is exacerbated by forceful pinch activities. Laxity or gross instability can similarly be seen on valgus stress.

Risk factors: Risk factors for acute injury include activities that predispose to acute UCL injury, such as cycling, skiing, and ball-handling or stick-handling sports. Risk factors for chronic injury include undiagnosed Stener lesions or activities that predispose to chronic, repetitive UCL stress.