Externally applied force helps reduce bowstring effect of flexors in patients with carpal tunnel release surgery

Background: Patients with severe carpal tunnel syndrome (CTS) undergo carpal tunnel release (CTR) surgery to alleviate pressure in the carpal tunnel. However, the subsequent lack of the transverse carpal ligament (TCL) causes the bowstring phenomenon of the flexor tendons and increases the potential incidence of trigger finger. Objective: This study aimed to investigate the effects of various compressive forces on the flexor tendon and identify the appropriate force needed to mitigate the bowstring effect of those flexors. Design: Cross-sectional repeated measures comparison. Method: Thirteen CTS patients who underwent CTR surgery were asked to flex the middle finger while applying different external compressive forces, just contact, 4N, and 8N force, over the carpal tunnel. Images of the flexor tendon within the carpal tunnel and at the metacarpal phalangeal (MCP) joint were recorded via ultrasound. Result: Results show that the compression force limited the volar migration of the flexor tendon under maximal voluntary contraction (MVC) conditions. Entrance angles between the flexor tendon and metacarpal bone also decreased as the external compressive force increased. Conclusions: Findings of this study may indicate that applying compression force on the carpal tunnel is useful for CTS patients and can inhibit the volar shift of the flexor digitorum superficialis (FDS) tendon after surgery, which may further prevent trigger finger.