The Intrinsic Hand: Intrinsic-Minus (Claw) and Intrinsic-Plus Deformity

The interossei (dorsal = abduct/DAB, palmar = adduct/PAD) and the lumbricals insert into the lateral bands/extensor expansion, giving them a unique action: they FLEX the MCP joints (they pass volar to the MCP axis) and EXTEND the IP joints (they pass dorsal to the IP axes). The lumbricals are unusual in arising from the FDP tendons and inserting on the radial lateral band, linking the flexor and extensor systems; they generate little force but are rich in muscle spindles and contribute to proprioception and precision pinch. Ulnar nerve supplies most intrinsics (all interossei, the ulnar two lumbricals, hypothenar and adductor pollicis); the median nerve supplies the radial two lumbricals and most thenar muscles. When the intrinsics fail, the MCP/IP balance is lost - and the direction of the resulting deformity depends on whether they are too WEAK (minus) or too TIGHT (plus).

When the intrinsics are paralysed, the long extensors hyperextend the MCP joints and the long flexors flex the IP joints - the claw. A low ULNAR nerve palsy claws the ring and little fingers (the ulnar two lumbricals/interossei), while a combined low ulnar + median palsy claws all four fingers. The ULNAR PARADOX is high-yield: a HIGH ulnar lesion claws LESS than a LOW one, because a high lesion also paralyses the FDP to ring/little, reducing the IP flexion that produces the claw. Associated ulnar signs include Wartenberg's (abducted little finger), Froment's (FPL substitutes for a weak adductor pollicis on pinch) and Jeanne's signs. The commonest worldwide cause of intrinsic-minus claw is LEPROSY; other causes are Charcot-Marie-Tooth disease and T1/lower-trunk brachial plexus lesions.

The BOUVIER manoeuvre plans surgery: passively block MCP hyperextension (hold the MCP flexed) and ask the patient to extend the fingers. If the IP joints now extend (Bouvier-positive), the extensor mechanism is intact and a STATIC MCP-blocking procedure works - an MCP volar capsulodesis or the Zancolli LASSO (an FDS slip looped around the A1/A2 pulley to flex the MCP). If the IP joints still will not extend (Bouvier-negative), a DYNAMIC transfer that inserts into the lateral bands/extensor mechanism is required - e.g. the FDS four-tail (Stiles-Bunnell) or Brand transfers (ECRB/ECRL + tendon graft) to restore MCP flexion and IP extension. Always correct a fixed PIP flexion contracture first (splint/ release), and prevent contracture early with an anti-claw (lumbrical-bar) splint that holds the MCP flexed.