The Association Between Annular Pulley Morphology and Maximal Fingertip Force.

Abstract

The digital pulley system enables efficient finger flexion. The A2 and A4 pulleys are biomechanically critical, with injury causing tendon bowstringing and impaired grip. Tendon-bone distance (TBD) is widely used as a surrogate for pulley injury, with a threshold of 2 mm often cited as diagnostic. However, this assumes a limited capacity for physiologic adaptation. Emerging evidence suggests that increased TBD and pulley thickness may reflect adaptive remodeling, similar to other ligamentous structures. The purpose of this study was to quantify pulley morphology in a broad sample of individuals and explore the potential for structural adaptation. We hypothesized that pulley morphology would correlate with climbing experience and force production. A cohort of 56 uninjured recreational climbers and 18 controls was assembled. Each underwent ultrasound of eight digits to assess TBD at the A2 and A4 pulleys, pulley thickness, flexor tendon thickness, and volar plate thickness at the proximal interphalangeal and distal interphalangeal joints. Maximal voluntary contraction (MVC) was measured. Linear mixed-effects models assessed correlations among morphology, MVC, and climbing status while controlling for other factors. Tendon-bone distance exceeded 2.0 mm in 39/542 fingers (7.2%; 22/74 participants) at A2 and 83/541 fingers (15.3%; 34/74 participants) at A4. Increased MVC was associated with increased A2 TBD (0.10 mm per 10 kg MVC) and A2 (0.02 mm per 10 kg MVC) and A4 pulley thickness (0.03 mm per 10 kg MVC), but not flexor tendon thickness. Climber status was associated with increased distal interphalangeal volar plate thickness (+0.20 mm). This study demonstrates that fingertip force production is associated with annular pulley morphology. The structural changes observed are consistent with the mechanical loading demands, supporting the hypothesis of use-dependent adaptation. The use of patient-specific cutoff values for injury diagnosis warrants further investigation to minimize the risk of overdiagnosis.