It occurs when the aponeurosis of the adductor pollicis muscle becomes interposed between the ruptured ulnar collateral ligament (UCL) of the thumb and its site of insertion at the base of the proximal phalanx.

[1] In 1962, Bertil Stener described a lesion which he observed to occur in a subset of patients suffering from gamekeeper's thumb.

In these patients, the distal attachment of the UCL was traumatically avulsed from its site of insertion at the base of the proximal phalanx of the thumb.

The severed end of the ligament would become trapped under the aponeurosis of the adductor pollicis muscle and therefore be unable to return to its proper anatomic position.

Specifically, the animals were placed on the ground, and their necks were broken as the gamekeeper exerted downward pressure with the thumb and index fingers.