The terminal extensor tendon is a thin, flat structure measuring approximately 1 mm thick and 4-5 mm wide. This tendon occupies the sparse space between the bone and dorsal skin and inserts onto the dorsal lip of the distal phalanx, well proximal to the germinal nail matrix. At the DIP joint, the tendon’s excursion is only several millimeters from full joint extension to 80° of flexion.
As its name implies, the terminal extensor tendon is the terminal extension of the dorsal mechanism, which is a complex crossing of fibers at the proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints. Powered by the lumbrical and interosseous muscles, the dorsal mechanism flexes the MCP joint and extends the PIP and DIP joints.