The third lumbrical is bipennate, arising from adjacent FDP tendons. Its ulnar nerve innervation means it is lost in ulnar nerve palsy, contributing to ring finger clawing. The ring finger's lumbrical function is shared between ulnar (primary) and occasionally a median contribution, making ring finger claw deformity variable in severity across patients with ulnar palsy.
| Origin | Adjacent sides of the second and third FDP tendons — bipennate |
|---|---|
| Insertion | Radial side of the extensor expansion of the ring finger |
| Nerve Supply | Ulnar nerve deep branch (C8, T1) — variable; occasionally has median nerve contribution |
| Blood Supply | Third palmar metacarpal artery |
| Actions | Flexes the MCP joint of the ring finger; Extends the PIP and DIP joints of the ring finger |
|---|
The third lumbrical's innervation variability (predominantly ulnar, occasional median dual supply) explains why ring finger claw severity varies in ulnar nerve palsy. Patients with dual supply show less clawing. The Martin-Gruber anastomosis (median-to-ulnar crossover in the forearm) may supply the third lumbrical via an anomalous median pathway, which must be accounted for in electrodiagnostic studies.
Not palpable. Tested by ring finger MCP flexion with IP extension.
Loss of third lumbrical extension of the ring finger IP joints from ulnar nerve injury producing ring and little finger clawing, with severity modified by any anomalous median nerve supply to the third lumbrical.
