High radial nerve entrapment neuropathy: an anatomical cadaver study and case report.

Background: The pathogenetic mechanism of high radial nerve entrapment neuropathy involves the fibrous arch of the lateral head of the triceps brachii and lateral intermuscular septum. However, the details of these anatomical structures remain unknown. We described the anatomical course of the radial nerve in the upper arm and its relationship with the lateral head of the triceps brachii and lateral intermuscular septum.

Methods: Eleven freshly frozen cadavers (22 limbs, 7 females, and mean age: 87.3 years) were used. The elbow joint was placed at 90° flexion and a lateral incision was made. Specifically, the tendons of the lateral head of the triceps brachii, deep fascia, and lateral intermuscular septum are continuous at the attachment and form a complex. The radial nerve runs obliquely through the fibrous tunnel formed by this complex, and the entry and exit portions form fibrous arches. Additionally, the distance from the lateral humeral epicondyle to the entrance (X1) and exit (X2) of the fibrous tunnel and fibrous tunnel length were measured. The tunnel was incised and the radial nerve was exposed. Specifically, the radial nerve was observed from the lateral aspect of the humerus. R1, R2, and R3 were defined as the points where the radial nerve crosses the posterior aspect, lateral center, and anterior aspect of the humeral shaft, respectively. Their distances from the lateral humeral epicondyle were also measured.

Results: The distance from the lateral humeral epicondyle to X1 and X2 was 145 ± 15 (121-185) and 91 ± 14 (72-122) mm, respectively, while the fibrous tunnel length was 55 ± 12 (28-83) mm. Additionally, the distances from the lateral epicondyle to R1, R2, and R3 were 143 ± 18 (103-177), 107 ± 13 (75-142), and 79 ± 14 (59-105) mm, respectively. No significant correlation was found between the humeral and tunnel lengths.

Conclusion: The lateral head of the triceps brachii and lateral intermuscular septum form a complex that creates a fibrous tunnel. Additionally, the radial nerve traverses obliquely within this fibrous tunnel on the humerus, forming fibrous arches at both its entrance and exit. The lateral head of the triceps brachii-lateral intermuscular septum complex can be a source of compression in cases of high radial nerve entrapment neuropathy. Such neuropathy may result from a plane of compression. Therefore, we advocate for surgery in high radial nerve entrapment neuropathy cases that are resistant to conservative treatment. Dissecting the entire length of the fibrous tunnel is also important during surgery.