Ulnar Subluxation of the Extensor Pollicis Longus Tendon as a Cause of Congenital Clasped Thumb: A Case Report.

A 15-year-old male patient presented with difficulty extending his right thumb due to subluxation of the extensor pollicis longus (EPL) tendon. Physical examination showed that active motion of the thumb's metacarpophalangeal (MP) joint was 76° in flexion and -58° in extension. During active extension, the EPL tendon shifted onto the ulnar aspect of the MP joint, and the extensor pollicis brevis (EPB) tendon shifted onto the central dorsal course of the MP joint. After repositioning by adducting the thumb into the palm to extend the MP joint (dynamic tenodesis effect), active extension of the MP joint increased to -12°. Corrective surgery was performed under local anesthesia and radial sensory nerve block. Following plication of the attenuated dorsal capsule and sagittal band between the EPB and EPL, we confirmed that the patient could flex and extend the thumb smoothly. One year postoperatively, the extension deficit of the MP joint had not recurred.