To determine if there is an association between flexor digitorum and lumbrical muscle intrusion into the carpal tunnel and carpal tunnel syndrome (CTS).
513 manual laborers (1,026 wrists) were evaluated with ultrasound to determine if those with CTS had more muscle intrusion into the carpal tunnel than those without CTS. 190 of the participants without CTS at baseline (363 wrists) were followed over 1 year to determine if muscle intrusion at baseline predicted the development of CTS.
Participants with CTS had more muscle within the carpal tunnel with the wrist in the neutral (P = 0.026) and flexed positions (P = 0.018) than those without CTS. Baseline muscle intrusion did not predict the development of CTS at 1 year.
Muscle intrusion into the carpal tunnel is associated with CTS, but muscle intrusion alone does not predict the development of CTS over the course of a year.