Anatomy, Kinesiology, and Pathophysiology
Structural map
Know the six dorsal extensor compartments - 1: APL and EPB (de Quervain); 2: ECRL/ECRB (intersection syndrome); 3: EPL; 4: EDC and EIP; 5: EDM; 6: ECU. Know the flexor zones (Verdan I–V) and the pulley system, where A2 and A4 are the critical pulleys, A1 is the site of trigger finger, and A2 is lost in FDP avulsion. The carpal tunnel contains the median nerve plus the flexor tendons and FPL; Guyon's canal carries the ulnar nerve and artery.
Muscle mechanics
The lumbricals flex the MCP joints and extend the IP joints. Dorsal interossei abduct (DAB, three muscles); palmar interossei adduct (PAD, four muscles). Test intrinsic tightness by stabilizing the MCP in extension and passively flexing the PIP: limited flexion indicates intrinsic tightness. The tenodesis effect - wrist extension producing passive digit flexion - is a quick check of tendon continuity and balance.
Nerve geography
Median: thenar muscles (APB, opponens, superficial FPB) and palmar sensation to the radial three and a half digits. Ulnar: most intrinsics, FCU, and FDP to the ring and small fingers. Radial: wrist and digit extension with dorsal radial hand sensation.
Pathophysiology
Use Seddon's classification: neuropraxia (myelin, no wallerian degeneration, recovery in weeks), axonotmesis (axon lost, endoneurial tube intact, regenerates about 1 mm/day), neurotmesis (complete, needs surgery). Tendon healing has intrinsic and extrinsic contributions; excess extrinsic healing drives adhesion.
Traps
Mixing median and ulnar innervation of the thenar versus intrinsic muscles; naming A1 instead of A2/A4 as critical; confusing neuropraxia with axonotmesis.
Rule of thumb
"DAB and PAD, lumbricals flex the knuckles and extend the tips," and compartment numbers 1 through 6 run radial to ulnar on the dorsum.