Mobility of the Flexor Pollicis Longus Tendon After Volar Locking Plate Fixation of Distal Radius Fractures: A Prospective Ultrasound Study

Abstract Introduction: Distal radius fractures are among the most common upper extremity fractures encountered in clinical practice. Flexor pollicis longus (FPL) tendon rupture is a well-documented complication of volar locking plate (VLP) fixation. This prospective study used transverse ultrasound (US) to evaluate FPL tendon mobility on the distal radius during finger motion in patients treated with VLP fixation, comparing the operated side with the contralateral non-operated side. Methods Twenty-two patients (18 men, 4 women; mean age [42 ± 12] years) who underwent VLP fixation for distal radius fractures were evaluated at 6 months postoperatively. Transverse US was used to measure the position of the FPL tendon as X and Y coordinates relative to a fixed bony reference point on the distal radius at 60-degree wrist dorsiflexion, with the thumb interphalangeal (IP) joint in flexion, neutral, and extension positions. Operated and non-operated sides were compared. All surgical procedures were performed by or under the supervision of the senior author, and all US examinations were conducted by a single experienced musculoskeletal sonologist. Results The FPL tendon showed a trend toward greater volar (Y-axis) and ulnar (X-axis) displacement on the operated side across all three thumb positions; however, none of these differences reached statistical significance (p > 0.05 for all comparisons). The mean X-coordinate on the operated side was 1.26 cm in flexion versus 1.41 cm on the non-operated side (p = 0.198). The mean Y-coordinate was 0.25 cm versus 0.16 cm (p = 0.087). The mean IP joint range of motion (ROM) of the thumb was significantly lower on the operated side compared to the non-operated side (p = 0.001). No FPL tendon ruptures or clinically significant complications were observed in any patient during the study period. Conclusion Following VLP fixation, the FPL tendon demonstrated a non-significant volar and ulnar shift relative to the contralateral side, contrary to the dorsal shift reported in previous studies. This finding may be attributed to meticulous pronator quadratus repair, early postoperative mobilization, and plate positioning proximal to the watershed line. This study is the first to report a volar FPL shift pattern using a contralateral comparison design, highlighting the influence of surgical technique on tendon kinematics. US remains a valuable, non-invasive tool for postoperative monitoring of FPL tendon position. Larger studies incorporating fracture reduction parameters are needed to confirm these findings.