The Radial Tunnel:
Five Compression Areas of the Radial Nerve
The radial nerve divides into two branches: the posterior interosseus (PIN) and the superficial radial nerve. This split is highly variable in the elbow and compression neuropathies may include a combination of motor and sensory findings.
Radial Tunnel Syndrome is primarily a pain syndrome distinct from lateral epicondylitis and associated with compression of the PIN (sometimes also called the deep branch of the radial nerve) as well as recalcitrant symptoms (Resistant Tennis Elbow). The radial tunnel originates at the radiocapitellar joint where the nerve overlies the joint capsule. The medial border of the tunnel includes the brachialis muscle and biceps tendon. The lateral border and roof of the tunnel is formed by the extensor carpi radialis longus and brevis (ECRL and ECRB) muscles. The tunnel continues into the supinator muscle and ends at the distal border of the supinator muscle. The superficial radial nerve may be entrapped in the radial tunnel so that radial nerve sensory findings may also present.
There are five potential nerve compression areas in the radial tunnel
- Under the fibrous proximal arch of the supinator muscle (the Arcade of Frohse). This is the most frequent site of compression in the radial tunnel.
- Fibrous bands around and anterior to the radial head and radiocapitellar joint capsule.
- Under a recurrent fan of blood vessels (the Leash of Henry).
- At the fibrous (medial proximal) edge of the ECRB
- The distal aspect of the supinator where the PIN or deep branch exits the muscle.
Causes of compression in the radial tunnel are many and may include a direct blow, fracture (including surgery to the proximal radius), repetitive or forceful movements at the forearm, wrist and hand, vascular anomalies, synovitis as with rheumatoid arthritis, lipomas and ganglia.
Clinical diagnosis of Radial Tunnel Syndrome is difficult as there are rarely significant findings on imaging or electrodiagnostic studies. The location of pain is one clue though as the maximal point of tenderness is usually 2-5 cm below the lateral epicondyle (differentiating it from lateral epicondylitis). Note that the two conditions sometimes appear concurrently.
A C7 radiculopathy will present similarly but there will also be weakness of the triceps and wrist flexors.
A radial nerve entrapment in the spiral groove will leave the triceps intact but affect the brachioradialis, and extensors below (why humerus fractures must be evaluated for radial nerve palsy)
References:
- Hammer W (1999). Functional Soft Tissue Examination and Treatment by Manual Methods, 2nd Ed. Aspen Publishers, Gaithersburg MD.
- Hazni R. et al. Anatomic Landmarks for the Radial Tunnel. Open Access Journal of Plastic Surgery. 2008;8:e37.