Background: Carpal tunnel syndrome is a very common pathology. The global incidence is 3.9%, and in Mexico, it affects 3.4% of women and 0.6% of men. Although the standard approach for carpal tunnel release is described without traversing muscle fibers, these are common, and their exact origin is unknown.

Methods: This retrospective study analysed clinical records of patients undergoing open carpal tunnel release. Conservative treatments and other surgical procedures were excluded. The presence of anomalous muscle fibers over, under, or through the transverse carpal ligament was assessed.

Results: Out of 23 hands from 19 patients, 69.6% presented anomalous muscle fibers. They were classified into large and small muscles, with 56.3% corresponding to large muscles. Although no anatomical variations of the motor branch of the median nerve were found, they were not intentionally sought.

Conclusion: The presence of anomalous muscle fibers in the carpal tunnel is common in the studied population, exceeding international rates. Although no correlation with anatomical variations of the motor branch of the median nerve was found in this study, the relationship between these fibers and variants has been documented previously. Caution is emphasized during tunnel release to prevent injuries to the motor branch of the median nerve in the presence of such fibers. Specific clinical implications are still unknown. The need for more extensive studies is suggested to better understand the role of these fibers in median nerve compression