Anatomical Characterization of the Motor Branch to the Fourth Lumbrical: A Cadaver Study.

Purpose: The branching pattern of the deep motor branch of the ulnar nerve (DBUN) in the hand is complex. The anatomy of the motor branch innervating the fourth lumbrical (4L), where paralysis results in a claw hand deformity after ulnar nerve injury, is not well defined. This cadaver study focused on mapping and defining anatomical landmarks in relation to the motor branch to the 4L.

Methods: Ten fresh-frozen cadaver hands were dissected. The DBUN was exposed and followed distally, identifying all motor branches, until the branch to the 4L was seen. The small finger flexor tendons were reflected distally to enable visualization of the entire 4L motor branch and its distal entry point. The origin of the 4L motor branch was mapped in relation to fixed anatomical landmarks in the hand.

Results: A consistent motor branch to the 4L and third palmar interosseous (3PI) muscles was seen originating from the DBUN in all specimens. The mean number of motor branches innervating the hypothenar muscles proximal to the 4L/3PI motor branch was 2.1 (range 1 to 5). The mean distance from the origin of the DBUN to the 4L/3PI motor branch was 2.9 cm (range 2.5 to 4.2 cm). The mean length of the 4L/3PI branch was 3.1 cm (range 2.3 to 4.5 cm). In seven specimens, the 4L/3PI branch demonstrated an intramuscular course through the 3PI before terminating in the 4L. In three specimens the 4L/ 3PI branch ran on the volar surface of the 3PI before terminating in the 4L.

Conclusions: A consistent 4L/3PI motor branch was mapped and characterized in all specimens.

Clinical relevance: These findings provide a guide for intraoperative localization of the 4L/3PI motor branch and also may provide further evidence to explain findings seen after distal nerve transfers to treat ulnar nerve injury.