Systematic characterization of flexor tendon and tendon sheath malposition in proximal phalangeal thumb polydactyly

INTRODUCTION: . METHODS: Patients with proximal phalangeal thumb duplication who underwent primary surgery and had high-quality intraoperative records with images of tendon and sheath anatomy were included. Duplicated thumbs were classified as articulated/bony-connected or non-articulated types according to the articulation and developmental status of the radial thumb component and were further stratified based on the duplication range. Fibrous structures bridging the radial and ulnar thumbs overlying the flexor pollicis longus tendons were defined as aberrant pulleys. Displacement of the flexor pollicis tendon insertion was assessed relative to the palmar longitudinal midline. RESULTS: Consistent anatomical patterns were observed within each duplication range. Skin levels 1 (mid-distal phalanx) to 2 (interphalangeal joint) were associated with marked displacement of the flexor pollicis longus tendon insertion, whereas skin level 2.5 (distal third of the proximal phalanx) showed moderate displacement. In contrast, the insertion was near normal at skin level 3 (mid-proximal phalanx). Across duplication ranges and developmental types, tendon sheaths demonstrated eccentrically displaced attachments on the proximal phalanx. CONCLUSION: The duplication range offers a practical framework for systematically understanding malposition of the flexor pollicis longus tendon and tendon sheath in proximal phalangeal thumb polydactyly and may facilitate preoperative risk stratification and surgical planning for tendon centralization and pulley reconstruction. LEVEL OF EVIDENCE: IV.