Hypothesis: Pediatric trigger thumb is characterized by flexion deformity of the interphalangeal joint of the thumb. It usually occurs spontaneously in the childhood and can be resolved spontaneously without any treatments. The purpose of this study is to verify the causes of the flexion deformity of the thumb. Methods: We enrolled 42 patients who were diagnosed as pediatric trigger thumb with unilateral involvement and underwent ultrasonographic measurements of the flexor pollicis longus tendon at the level of A1 pulley and 1 cm proximal to the A1 pulley. The measurements were repeated in the contralateral normal side. The average age at the time of measurements was 33 months. Results: In the side of pediatric trigger thumb, the average anteroposterior and radioulnar diameters of the flexor pollicis longus tendon at the level of A1 pulley were 2.9 and 2.3 mm, respectively, and those 1 cm proximal to the A1 pulley were 4.5 and 2.7 mm, respectively. In the contralateral normal side, the average anteroposterior and radioulnar diameters of the flexor pollicis longus tendon at the level of A1 pulley were 3.0 and 2.0 mm, respectively, and those 1 cm proximal to the A1 pulley were 3.3 and 1.8 mm, respectively. The average ratio of the anteroposterior diameter of the flexor pollicis longus tendon 1 cm proximal to the A1 pulley compared with the A1 pulley was 1.58 in the side of pediatric trigger thumb, and 1.08 in the contralateral normal side. The average ratio of the radioulnar diameter of the flexor pollicis longus tendon 1 cm proximal to the A1 pulley compared with the A1 pulley was 1.17 in the side of pediatric trigger thumb, and 0.95 in the contralateral normal side. Conclusions: In the comparison of the cross-sectional configuration of the flexor pollicis long tendon and inner area of the A1 pulley, the anteroposterior and radioulnar diameters of the flexor pollicis longus tendon were more increased than those of inner area of the A1 pulley. The anteroposterior diameter of the flexor pollicis longus tendon was 58% longer than that of inner area of the A1 pulley, and the radioulnar diameter of the flexor pollicis longus tendon was 17% longer than that of inner area of the A1 pulley. The flexor deformity of the interphalangeal joint can be caused by increased anteroposterior diameter of the flexor pollicis longus tendon.
