In Wassel type 5 and 6 the opponens pollicis muscle must be transferred to the ulnar metacarpal.

It can occur with an injury of the median nerve either at the elbow or the wrist, impairing the thenar muscles and opponens pollicis muscle.

The palmar (medial) surface exhibit a blunt ridge which separates a larger lateral part - the origin of the opponens pollicis muscle - and a smaller medial part - the origin of the lateral head of the first dorsal interosseous muscle.