1. The pathological anatomy of total rupture of the ulnar collateral ligament of the metacarpo-phalangeal joint of the thumb has been studied at operation in thirty-nine patients.
2. It has been found that the ligament, when ruptured distally (the most usual type) is often displaced so that the ulnar expansion of the dorsal aponeurosis—here referred to as the adductor aponeurosis—becomes interposed between the ruptured end of the ligament and the site of its attachment on the phalanx; the ligament gets folded over to a greater or lesser extent and the ruptured end sticks out beyond the proximal edge of the adductor aponeurosis. Such displacement of the ligament with interposition of the adductor aponeurosis was found in twenty-five of the thirty-nine cases. The ligament may also be displaced without the adductor aponeurosis being interposed (with a gap between the ruptured end and the site of itsattachment on the phalanx).
3. Variations in the pathological anatomy are illustrated by more detailed descriptions of nine cases. In one of these the adductor aponeurosis had got caught between the two surfaces of the joint, and in another the adductor aponeurosis was found ruptured as well as the ulnar collateral ligament.
4. On the basis of dissection studies on forty-two fresh post-mortem specimens, a rational explanation is given of the pathological anatomy.
5. The results of the present investigation on clinical and anatomical material provide a strong argument in favour of operative treatment for total rupture of the ulnar collateral ligament of the metacarpo-phalangeal joint of the thumb.
6. Aspects of diagnosis and operative technique are discussed.