Trigger Thumb

Excerpt

The thumb is anatomically distinct from the other fingers, consisting of a metacarpal bone and proximal and distal phalanges. Unlike the other fingers with proximal, middle, and distal phalanges, the thumb may also have sesamoid bones. The thumb is unique from other fingers due to its distinctive ability to perform 2 distinct movements—opposition and apposition. Furthermore, the thumb can flex, extend, abduct, and adduct at the metacarpophalangeal (MCP) joint, making it a distinctive and versatile part of the hand.

Trigger thumb or stenosing flexor tenosynovitis of the thumb is characterized by a narrowing of the flexor tendon sheath. This constriction results in the flexor tendon catching as it attempts to glide smoothly through the relatively stenotic sheath. Trigger thumb or trigger finger is caused by a size mismatch of the diameter between the swollen flexor tendon and its sheath at the level of the metacarpal head. The outcome is a noticeable clicking or popping sensation during attempts at thumb extension.

The extrinsic flexor pollicis longus (FPL) and intrinsic flexor pollicis brevis (FPB) drive thumb flexion and contribute to the flexion process. During finger flexion and extension, pulleys along the tendon sheath secure the flexor tendons to the finger bone. This pulley system optimizes the force production and efficiency of the flexor tendon. The FPL tendon travels within its sheath, passing through the A1, oblique, and A2 pulleys, with the A1 pulley positioned most proximally and the A2 pulley situated most distally. Notably, the A1 pulley is located on the metacarpal bone, overlapping the MCP joint and the base of the proximal phalanx.

Trigger thumb typically results from the thickening of the A1 pulley, impairing the free movement of the FPL tendon. This results in pain, reduced functionality, and a clicking or popping sensation on the attempted extension of the thumb. Both adults and children can experience trigger thumb. Trigger thumb are primarily acquired in adults and are considered a rare developmental anomaly in children. The treatment for this condition starts with splinting and physical therapy. Intralesional steroid injections are suitable for patients not responding to conservative management. In cases where conservative measures prove insufficient, surgical release of the A1 pulley may be necessary to manage the condition effectively.