Background: The etiology of pediatric trigger thumb is unknown, although ultrasound in adults has shown thickening of the A1 pulley leading to constriction of the flexor pollicis longus (FPL) tendon. The purpose of this study is to characterize the underlying cause of the pediatric trigger thumb and factors responsible for resolution utilizing sonography.
Methods: A prospective analysis of children with trigger thumbs was conducted from May 2008 through June 2010. All children were initially treated with splinting. Surgical release of the A1 pulley was performed at the family's request. Bilateral dynamic ultrasonography was performed at presentation and follow-up until resolution of triggering. Ultrasound images were evaluated for tendon gliding, echotexture, cross-sectional area, and anatomic variations.
Results: There were 35 trigger thumbs in 28 patients. Ten thumbs resolved spontaneously. Eight patients (9 thumbs) underwent surgical release of the A1 pulley. One child who underwent bilateral release achieved only unilateral resolution. Ultrasound imaging of all 56 thumbs demonstrated normal echotexture of the FPL without evidence of inflammation or trauma. Triggering always occurred at the A1 pulley, and there was focal enlargement of the FPL but no definite ultrasound abnormality of the A1 pulley. Surgical release allowed the thickened tendon to pass smoothly, which coincided with resolution of triggering. Two of 3 patients with unilateral triggering presenting with a trigger ratio (cross-sectional area of involved maxFPL to uninvolved FPL) <1.5 converted to bilateral trigger thumbs. An FPL size for age graph was created for nontriggering thumbs in unilateral patients.
Conclusions: The pediatric trigger thumb is a developmental condition with normal echotexture noted in all FPL tendons without inflammation or trauma. Triggering occurs when the cross-sectional area of the FPL exceeds the cross-sectional area at the A1 pulley, and it resolves when this size disparity is eliminated. Patients with unilateral triggering and a trigger ratio <1.5 on the uninvolved thumb are at risk for developing triggering bilaterally.
Level of evidence: Level 2 diagnostic study.