Stiffness of the First Annular Pulley in Normal and Trigger Fingers

Abstract

The cause of snapping in trigger finger is not clearly understood. The purpose of this study was to investigate the influence of stiffness of the first annular pulley on snapping in trigger finger patients with sonoelastography, a new technique for quantitative assessment of the stiffness of soft tissues. We first studied 20 healthy volunteers (all women, aged 24-78 y) to define the change of stiffness with aging and with specific fingers. We then studied 10 digits of 9 patients (all women, aged 60-78 y) with trigger finger that involved middle or ring fingers. Thickness and stiffness were evaluated by sonoelastography. Stiffness was estimated by strain ratio of subcutaneous fat to the pulley (F/P strain ratio). The patients were treated with corticosteroid injection, and the measurement was repeated 3 weeks later. In the healthy volunteers, both thickness and F/P strain ratio showed a positive correlation with age. There was no difference between the middle and ring fingers. In trigger finger patients, the pulley thicknesses of the affected finger and the adjacent normal finger before corticosteroid injection were 0.99 ± 0.19 mm and 0.60 ± 0.14 mm (mean ± SD), respectively. The F/P strain ratios of the affected finger and the adjacent normal finger before the injection were 4.2 ± 1.3 and 2.4 ± 0.63, respectively. Three weeks after the injection, snapping disappeared in all patients, the pulley thickness decreased to 0.61 ± 0.15 mm, and the F/P strain ratio decreased to 2.5 ± 0.68. The cross-sectional area for flexor tendons within the pulley did not change after the injection. Increased stiffness and thickening of the A1 pulley are considered to be causes for snapping in trigger finger, and corticosteroid injection can alleviate snapping by changing these 2 features.