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MRI Web Clinic - August 2005

Posterior Interosseous Nerve Syndrome

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Clinical history: Persistent forearm pain in a 72 year-old male after a fall. Axial fat suppressed T2-weighted (1a and 1b) and sagittal fat suppressed proton density-weighted (1c) images are shown below. What are the findings and what is your diagnosis?




Figure 1:

Axial fat suppressed T2-weighted (1a and 1b) and sagittal fat suppressed proton density-weighted (1c) images





Figure 2:

(2a) An axial fat suppressed T2 weighted image just distal to the radiocapitellar joint demonstrates localized fluid (arrow) anterior to the radius. Edema is noted in the supinator muscle (arrowheads). (2b) A more distal axial fat suppressed T2-weighted image demonstrates diffuse volume loss of the supinator muscle (arrowheads) with diffusely increased signal intensity compatible with edema. (2c) A sagittal fat suppressed proton-density image confirms the presence of a ganglion cyst (arrow) anterior to the radial head.


Posterior interosseous nerve entrapment by a ganglion at the level of the radial head.


Radial nerve compression or injury can occur at any point along the course of the nerve within the upper extremity. Entrapment of the radial nerve or its branches is most common within the proximal forearm and at the elbow. Variations in anatomic structures at this level, particularly the supinator muscle, are an important cause of radial nerve entrapment syndromes.

The radial nerve bifurcates just above the level of the elbow, dividing into motor and sensory branches (3a,4a,5a). Compression of the radial nerve and its branches at the elbow can therefore result in motor, sensory, or mixed deficits. The motor branch (posterior interosseous nerve) is particularly vulnerable to compressive injury, and compression of this branch may result in a variety of clinical presentations.


Figure 3:

(3a) A lateral rendering of the elbow demonstrates the bifurcation of the radial nerve (R) into the posterior interosseous nerve (PIN) and superficial radial nerve (SR). The posterior interosseous nerve (PIN) passes between the superficial (Ss) and deep (Sd) heads of the supinator muscle before exiting into the posterior compartment.


The radial nerve is formed from the posterior cord of the brachial plexus, with contributions from C6, C7, C8, and T1. The nerve passes between the medial and lateral heads of the triceps muscle, continuing distally along the lateral side of the arm. Approximately 10 cm above the elbow, the radial nerve pierces the lateral intermuscular septum and continues distally between the brachialis and brachioradialis muscles(4a). Just proximal to the radiocapitellar joint, the radial nerve bifurcates into the superficial radial nerve and deep radial nerve (posterior interosseous nerve)(3a,5a). The superficial radial nerve is a sensory branch, and innervates the skin of the thumb, index, and middle fingers. The posterior interosseous nerve is a motor branch, and supplies the wrist and finger extensors. This branch passes through the supinator muscle between its superficial and deep heads (6a), exiting into the posterior compartment of the forearm.


Figure 4:

(4a) Above the elbow the radial nerve (arrow) lies between the brachioradialis (Brd) and brachialis muscles (Br) and is typically outlined by a small layer of fat on axial T1-weighted images. The biceps (B) and extensor carpi radialis longus (ECRL)muscles are also indicated.


Figure 5:

(5a) An axial T1-weighted image just above the elbow joint demonstrates that the radial nerve has bifurcated into the superificial radial nerve (arrowhead) and posterior interosseous nerve (arrow). The brachioradialis (Brd), brachialis (Br), the extensor carpi radialis longus (ECRL) muscles and the biceps tendon (B) are indicated.


Figure 6:

(6a) An axial T1-weighted image distal to the radiocapitellar joint demonstrates the posterior interosseous nerve (arrow) between the superficial (Ss) and deep (Sd) heads of the supinator muscle. The superficial radial nerve (arrowhead) courses between the supinator and brachioradialis muscles into the distal forearm.

In the proximal arm, the radial nerve innervates the medial and lateral heads of the triceps and the anconeus. In the distal arm the radial nerve innervates the brachialis, brachioradialis, and extensor carpi radialis longus and brevis muscles. The posterior interosseous nerve provides motor innervation to the supinator muscle and the extensor muscles of the wrist and hand, including the extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, extensor pollicis longus, extensor pollicis brevis, abductor pollicis longus, and extensor indicis proprius muscles.

Entrapment of the radial nerve proximal to its bifurcation produces both motor and sensory deficits resulting in pain in the forearm, weakness of finger and wrist extension, and eventual muscle atrophy. Entrapment distal to the radial nerve bifurcation results in distinct clinical presentations depending on the branch affected1.

Patients with compression of the superficial branch of the radial nerve may complain of pain in the distal forearm and hand parasthesias, the clinical entity known as Wartenberg syndrome. In contrast, posterior interosseous nerve syndrome manifests with the gradual onset of weakness of muscles supplied by the posterior interosseous nerve. Pain may be present but is often not a primary feature, and there is no sensory deficit. On physical exam, the patient has weakness of extension of the digits and wrist. Five potential sites of compression of the posterior interosseous nerve have been identified (7a,8a). Of these, the proximal tendinous edge of the supinator muscle (arcade of Frohse) is the most frequent site of posterior interosseous nerve entrapment (7a,8a)2.


Figure 7:

(7a) This anterior rendering of the elbow demonstrates the potential sites of posterior interosseous nerve entrapment. They are: the arcade of Frohse (1), the radiocapitellar capsule (2), small recurrent vessels that cross the posterior interosseous nerve (leash of Henry) (3), the fibrous edge of the extensor carpi radialis brevis (4), and the distal margin of the supinator muscle (5).


Figure 8:

(8a) The posterior interosseous nerve (arrowhead) is identified between the deep head of the supinator (Sd) and the tendinous proximal edge of the superficial head of the supinator muscle (arcade of Frohse) (arrow).

Like posterior interosseous nerve syndrome, radial tunnel syndrome is felt to be the result of entrapment of the posterior interosseous nerve, and many authors feel that radial tunnel syndrome in fact represents early posterior interosseous nerve syndrome. Radial tunnel syndrome is thus a somewhat controversial diagnosis3. However, the distinction is useful, because in radial tunnel syndrome, unlike posterior interosseous nerve syndrome, no motor deficits are observed4.The potential sites of compression in radial tunnel syndrome are the same as for posterior interosseous nerve syndrome. Patients with radial tunnel syndrome typically present with pain over the lateral forearm with repetitive elbow extension and forearm rotation. Because the pain distribution and the mechanism are similar, radial tunnel syndrome is frequently misdiagnosed as lateral epicondylitis.

In patients with posterior interosseous nerve syndrome or radial tunnel syndrome, the anatomic variants that cause entrapment are often difficult to visualize with MRI. However, MRI is sensitive to the muscle edema, atrophy, and fatty infiltration which accompany nerve entrapment. MRI easily depicts the distribution of muscle involvement, thus assisting in localizing the level of entrapment (9a). In addition, MRI can detect other causes of entrapment including tumors, ganglia, radiocapitellar synovitis, bicipital bursitis, fractures, and dislocations of the radial head (10a).


Figure 9:

(9a) An axial fat-suppressed T2-weighted image in the proximal forearm demonstrates edema of the supinator and extensor carpi ulnaris in this patient with proximal posterior interosseous nerve entrapment.


Figure 10:

(10a) An axial T1-weighted image through the proximal forearm demonstrates a large lipoma (black arrow) in the supinator muscle(S) causing compressive neuropathy of the posterior interosseous nerve, which is not identified. The superficial branch of the radial nerve (white arrow) is identified.

Initial treatment of radial nerve entrapment is conservative, consisting of activity modification, anti-inflammatory medication and functional splinting. Decompressive surgery is reserved for cases that progress or do not improve within 6-12 weeks, or for individuals with an underlying compressive lesion. Radial tunnel syndrome is usually treated conservatively for up to 12 weeks.


Compression of the posterior interosseous nerve can result in a variety of clinical symptoms. The onset of pain or weakness is often insidious, resulting in a confusing clinical presentation. MRI is useful in identifying muscle signal changes indicative of denervation, contributory anatomic factors, and masses or other lesions that may result in nerve entrapment.


1 Lubahn JD, Cermak MB: Uncommon nerve compression syndromes of the upper extremity. J Am Acad Orhtop Surg 1998 Nov-Dec;6(6):378-86.

2 Spinner M. The arcade of Frohse and its relationship to posterior interosseous nerve paralysis. J Boint Joint Surg [Br] 1968;50(4):809-12.

3 Rosenbaum R. Disputed radial tunnel syndrome. Muscle Nerve 1999;22(7):960-7.

4 Barnum M, Mastey RD, Weiss AP, Akelman E. Radial tunnel syndrome. Hand Clin 1996;12(4):679-89.

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