Quantitative Measurements of the Cross-sectional Configuration of the Flexor Pollicis Longus Tendon Using Ultrasonography in Patients With Pediatric Trigger Thumb

Purpose Pediatric trigger thumb is regarded as an acquired condition characterized by flexion deformity of the interphalangeal joint of the thumb. However, the exact etiology and pathoanatomy of this condition remain unknown. The purpose of this study was to evaluate crosssectional configurations of the flexor pollicis longus (FPL) tendon and the area under the A1 pulley quantitatively using ultrasonography. Methods In this study we enrolled 43 patients, 23 boys and 20 girls, with unilateral pediatric trigger thumb. We measured the anteroposterior (AP) diameter, radioulnar diameter, and cross- sectional area of the FPL tendon at the level of the greatest AP diameter of the FPL tendon proximal to the A1 pulley and those of the inner dimensions of the A1 pulley using ultrasonography. The measurements were repeated on the contralateral side. Average age at the time of the measurements was 32 months. Results Average AP and radioulnar measurements of the FPL tendon were 13% and 55% larger than those of the inner dimensions of the A1 pulley in the trigger thumb. The average AP measurement in the area under the A1 pulley was notably larger in the trigger thumb than on the normal side. Conclusions Using ultrasonographic measurements, we were able to identify enlargement of the FPL tendon proximal to the A1 pulley in the symptomatic thumb, compared with the area under the A1 pulley in the symptomatic thumb or FPL tendon on the contralateral side. Developmental mismatch between the FPL tendon and the area under the A1 pulley is a possible cause of pediatric trigger thumb.