Changes in the flexor pollicis longus tendon during spontaneous resolution of pediatric trigger thumb.

The present study aimed to evaluate conformational changes in the flexor pollicis longus (FPL) tendon and inner space of the A1 pulley during spontaneous resolution of pediatric trigger thumb (PTT) and to compare them versus the normal contralateral side. We enrolled 36 patients with unilateral PTT who underwent ultrasonography twice between January 2016 and July 2020 and showed >10° improvement in thumb interphalangeal (IP) joint extension lag. Ultrasonography was used to measure the anteroposterior (AP) and radioulnar (RU) diameters of the FPL tendon proximal to the A1 pulley and of the inner space of the A1 pulley. On the side of the PTT, the average extension lag in the thumb IP joint significantly improved, from 35.6° to 14.2°. The AP and RU diameters of the FPL tendon increased by 0.3% and 1.9%, respectively, and those of the inner space of the A1 pulley increased by 15.3% and 5.0%, respectively. In the contralateral normal thumb, the AP and RU diameters of the FPL tendon increased by 12% and 9.3%, respectively, and those in the inner space of the A1 pulley increased by 9.9% and 4.6%. During improvement in IP joint extension lag, the mismatch between the enlarged FPL tendon and the inner space of the A1 pulley was reduced by their asymmetric growth.