The dorsal interossei and the palmar interossei insert into the dorsal extensor hood at the level of the proximal phalanx via their interosseous tendons, contributing to both MCP joint abduction (dorsal interossei) and adduction (palmar interossei) and to PIP and DIP joint extension via the lateral bands. Their dual insertion into the base of the proximal phalanx and the extensor expansion allows them to act simultaneously as MCP flexors and IP extensors.
The interosseous extensor contribution is the basis of ulnar nerve motor deficit pattern: loss of interosseous and hypothenar intrinsic muscles from ulnar nerve palsy removes the primary IP joint extensors, producing clawing of the ring and little fingers (clawing is less severe at the index and middle as median nerve lumbricals compensate). The ulnar intrinsic minus posture (MCP hyperextension, IP flexion) results from unopposed extrinsic extension at the MCP combined with unopposed FDP at the IP level. Intrinsic reconstruction (lasso procedure) creates a tenodesis restraining MCP hyperextension.
Ulnar nerve injury eliminates the interosseous intrinsic contribution to ring and little finger MCP flexion and IP extension, producing clawing at these digits from unopposed extrinsic flexor at the IP and extensor at the MCP; intrinsic reconstruction using a lateral band or tendon graft lasso prevents MCP hyperextension and restores IP extension.
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