Investigation of variations in the innervation of the thenar muscles by the median nerve

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Abstract
Introduction This study aims to investigate the anatomical variations in the median nerve and its branches, specifically focusing on their distribution to the muscles and the points at which they enter the muscles. In addition, the branching level of the ramus muscularis of the median nerve supplying the thenar muscles was examined to explore potential correlations with carpal tunnel syndrome. Understanding these variations is crucial for minimizing iatrogenic nerve damage during surgical procedures in the carpal tunnel region. Methods Dissections were conducted on 15 forearm and wrist specimens, including 8 right-sided cadavers and 7 left-sided cadavers. The muscle branch entry levels into the thenar region were measured, and the nerve was classified according to Lanz’s classification system during the opening of the flexor retinaculum. Parameters such as the height of the first branch of the median nerve relative to the z-line (interstyloid line), the relationship of the nerve’s muscular branch to the flexor retinaculum, the level of entry of the muscle branch into the muscle (relative to the z-line), muscle length, and the ratio of the entry level of the first branch to muscle length were examined. Results Four types of median nerve branching were observed according to Lanz’s classification. The extraligamentous type was the most common, found in 53% of the specimens. In preligamentous types, the branches emerged on average 13.37 ± 5.69 mm inferiorly, except for those relative to the z-line. The entry level of the muscle branch into the muscle was most frequently located in the upper third of the thenar region. The average length of the thenar region was measured to be 11.40 ± 1.02 mm. The high frequency (53%) of the extraligamentous type in our study aligns with the findings in the general literature. However, the discovery of an additional preligamentous accessory motor branch in 46% of cases is a noteworthy observation, highlighting a higher rate than previously documented. Conclusion These branching patterns suggest the need for careful consideration of accessory branches during surgical procedures in the carpal tunnel region. In particular, during carpal tunnel surgeries, it is recommended to assess the relationship between the motor branch of the median nerve and the flexor retinaculum before releasing it, as variations in branching could have implications for surgical outcomes.

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