The central slip is the continuation of the EDC tendon that inserts on the dorsal base of the middle phalanx to extend the PIP joint. It is flanked by the lateral bands from the intrinsic muscles. Disruption from direct laceration, crush, or PIP dislocation produces boutonniere deformity: the PIP flexes (lateral bands slip palmar and become PIP flexors) while the DIP hyperextends (lateral bands tighten distally). Elson test (resisted PIP extension with the finger draped over a table edge) confirms central slip disruption.
PIP joint extension — the exclusive active extensor of the PIP joint through the central slip; its failure allows the lateral bands to migrate palmar to the PIP axis producing boutonniere deformity
The Elson test: with the PIP joint at 90 degrees over a table edge, resisted extension produces DIP hyperextension (rigid DIP) if the central slip is intact (force transmitted to DIP via lateral bands), and a floppy DIP if the central slip is ruptured (lateral bands are the only extensors). This distinguishes complete from incomplete central slip tears. Acute closed central slip injuries are managed with PIP extension splinting for 6 weeks.
Boutonniere deformity from central slip disruption managed with PIP extension splinting acutely and central slip reconstruction for chronic deformity.
