Lateral bands

The lateral bands are the two paired components into which the extensor mechanism divides over the proximal phalanx. They run dorsolaterally along the proximal interphalangeal joint and unite distally to form the terminal extensor tendon, which inserts on the dorsal base of the distal phalanx. The position of the lateral bands relative to the PIP joint axis determines the balance of extension and flexion: when the central slip is intact the lateral bands sit dorsal to the axis; when it ruptures they migrate volarly, generating the boutonnière deformity.