Flexor muscle incursion into the carpal tunnel: a mechanism for increased carpal tunnel pressure?

PURPOSE: To explore the hypothesis that the extrinsic finger flexor muscles have the potential to move into the proximal end of the carpal tunnel with wrist extension. METHODS: The most distal muscle fibres from the deep and superficial finger flexors were measured relative to the pisiform bone in 18 cadaveric specimens. Muscle excursions during wrist extension were calculated using regression equations previously reported in the literature. RESULTS: The mean distances from the pisiform were 9.3 and 4.9 mm for the deep and superficial flexors, respectively. Ten flexor muscle bellies were at the level of or distal to the pisiform bone in the anatomical position, while 17 of 36 were within 5 mm. DISCUSSION: The excursions expected with wrist extension indicate that many muscles have the potential to enter the carpal tunnel, especially those within 5 mm of the pisiform bone. Comparing the expected excursions to recent pressure data, corroborating support for the pressure increase is found. CONCLUSION: Although not directly measured, the results of this study indicate incursion of the flexor muscles into the carpal tunnel space, particularly with wrist extension, is a plausible mechanism for increased carpal tunnel pressure. RELEVANCE: Proposing a mechanism by which carpal tunnel pressure is elevated during wrist and finger extension is a stepping stone to determining the etiology of the disease itself. Finding that the flexor muscle bellies appear to enter the carpal tunnel with wrist extension indicates that use of the flexor muscles should be avoided when the wrist and fingers are extended.