Anatomy of the lateral intermuscular septum of the arm and its relationships to the radial nerve and its proximal branches: Laboratory investigation

Object. Other than very simple descriptions of the existence of the lateral intermuscular septum (LIS), the literature offers almost nothing about its detailed anatomy, relationships to the radial nerve, and proximal branches. To further elucidate its morphological characteristics, the present cadaveric study was performed. Methods. The lateral arm was bilaterally dissected from 25 adult fixed cadavers (50 sides). Specifically, a detailed evaluation of the LIS was made, and this structure's attachments and relationships to the radial nerve were analyzed and measured. Results. In addition to the previously described muscles arising from the LIS, the authors identified the extensor carpi radialis brevis muscles as partially arising from this structure. The deep and posterior portion of the deltoid tendon was confluent with the superior aspect of the LIS. The mean thickness of the LIS was 1.0 mm. Distally, the LIS attached strongly to the lateral epicondyle of the humerus and became confluent with the annular ligament encircling the head of the radius. The distal attachment of the LIS was confluent with the capsule of the elbow joint. All radial nerves traveled through a defect (mean diameter 1 cm) in the LIS. With traction on the nerve from proximal and distal to this defect, there was free excursion. In 85% of the specimens, however, the posterior antebrachial cutaneous nerve traveled through a tunnel within the LIS and pierced the septum at a mean of 5 cm proximal to the lateral epicondyle. The lower lateral brachial cutaneous nerve proximally pierced the LIS near its origin, occurring a mean of 3.2 cm distal to the LIS's origin from the humerus. Conclusions. To the authors' knowledge, the details regarding the LIS and its relationships to the radial nerve have not been reported. Such information may be of use to surgeons who operate in this region, for example, during neural repair or entrapment procedures.