Distally extended muscle belly of the flexor digitorum within the carpal tunnel: is it a risk factor for carpal tunnel syndrome?

Background The distally extended muscle belly of the flexor digitorum (MB-FD) within the carpal tunnel has been suggested as a plausible cause for carpal tunnel syndrome (CTS) based on cadaveric studies. Purpose To evaluate whether MB-FD within the carpal tunnel is a risk factor for CTS by comparing asymptomatic volunteers and patients with CTS. Material and Methods Presence, length, and cross-sectional area of MB-FD within the carpal tunnel were evaluated in 63 wrists of 32 asymptomatic volunteers and 52 wrists of 33 patients with CTS using ultrasonography. Length of MB-FD within the carpal tunnel was measured as the distance between the distal end of MB-FD and the palmar margin of the distal radius longitudinally. On a transverse scan, the cross-sectional area of MB-FD at the proximal limit of the carpal tunnel was measured. Results MB-FD in the carpal tunnel was found in 39 (62%) asymptomatic volunteers and 35 (67%) patients with CTS ( P = 0.549). There was a female preponderancy of MB-FD within the carpal tunnel in asymptomatic volunteers ( P = 0.044), but not in patients. Length and cross-sectional area of MB-FD within the carpal tunnel were significantly greater in male-CTS patients than in male volunteers ( P = 0.022 and 0.012, respectively). These qualities were risk factors for CTS in men, though not in women. Conclusion The presence of distally extended MB-FD within the carpal tunnel was not a risk factor for CTS. In men, a longer and thicker MB-FD within the carpal tunnel increased the risk for CTS.