Frohse's arcade is not the exclusive compression site of the radial nerve in its tunnel.

INTRODUCTION: The radial tunnel is a musculo-aponeurotic furrow which extends from the lateral epicondyle of humerus to the distal edge of the supinator muscle. The superficial head of the supinator muscle forms a fibrous arch, the arcade of Frohse (AF), which is the most common site of compression of the radial nerve motor branch. The latter is less commonly compressed by the adjacent muscular structures. This tunnel syndrome might be worsened with repeated pronation and supination of the forearm. The double object of that work was: (1) to define the radial nerve anatomic landmarks, (2) to determine the anatomical relationship of the radial nerve main trunk and branches to the peripheral osseous and muscular structures in the anterior aspect of the elbow joint in order to identify which of these conflicting elements are likely to cause a compressive neuropathy.

MATERIAL AND METHODS: The study design involved the dissection of 30 embalmed cadaveric upper limbs. Anatomic and morphometric investigations of the radial nerve, its terminal and motor branches were carried out. The presence of adhesions between radial nerve and joint capsule, tendons and aponeurotic expansions of epicondylar muscles and supinator arch was investigated. All measurements were taken in both pronation and supination of the forearm.

RESULTS: Neither macroscopic radial compressive neuropathy at the level of the supinator arch nor adhesions between the radial nerve and the joint capsule were found. In four cases (13%), dense fibrous tissue surrounded the radial nerve supply to extensor carpi radialis brevis (ECRB). The fibrous arch of the supinator muscle arose in a semi-circular manner and was noted to be tendinous in 87% of the extremities and of membranous consistency in the remaining 13%. The length of the AF averaged 25.9 mm. The angle formed by the radial shaft and the supinator arch was 23 degrees. Neither fibrous structures nor adhesions of the deep branch of the radial nerve (DBRN) along its course through the supinator muscle were observed.

DISCUSSION: Anatomic studies have revealed a variable rate of occurrence of a tendinous AF, which range from 30 to 80% (87% in our study) according to authors. It is reported to be a predisposing factor to the development of chronic entrapment neuropathy of the DBRN, especially if it is thick and provides a narrow opening for passage of the DBRN. The tendinous consistency of the supinator arch is believed to develop in adults, in response to repeated rotary movements of the forearm. Repetitive pronation and supination of the forearm induces compression of the radial nerve and its branches between two inextensible structures. The fibrous AF and the proximal end of the radius (radial head and radial tubercle). This condition is aggravated by the supinator muscle repeated activity. Repetitive compression might then promote histological changes in radial tunnel content and progressive development of a local fibrous zone. We also observed that the radial nerve supply to ECRB could be entrapped between the superolateral aspect of the ECRB and the superior edge of the supinator muscle.