Posterior Interosseous Nerve Entrapment-an uncommon entity

A 13 years-old female was referred to the plastic surgery for a left-hand deformity characterized by persistent flexion of the index through small digits, and persistent flexion of the wrist with ulnar deviation. There was no history of trauma, infection, or neuromuscular disorders. The patient's mother underwent an uncomplicated pregnancy, labor, and delivery.

Dedicated T1 weighted MRI imaging of the left forearm revealed marked atrophy within the extensor forearm muscles predominantly involving the extensor pollicis longus, extensor digiti minimi, extensor carpi ulnaris, and extensor digitorum. No associated fatty infiltration and lack of muscle oedema to suggest a longstanding process (Fig.1a,b,c).

In addition, there was a focal lesion/fusiform thickening measuring 4 x 3 mm of the posterior intraosseous nerve (PIN), deep to the brachioradialis muscle belly and proximal to the supinator muscle. The thickening appeared isointense to the muscle on T1 weighted imaging, hyperintense on T2 weighted and gradient recovery sequences, without evidence of blooming (Fig.2 and 3). The radial nerve proximal to the elbow and its superficial branches appeared normal. 

Based on the above MR findings, the reporting radiologist raised the possibility of a neurogenic tumour versus focal nerve thickening of posterior interosseus nerve secondary to a fibrous band with concurrent atrophy of extensor forearm musculature. Patient subsequently underwent decompression of the left PIN and nerve graft/transfer of the supinator branch to PIN. Intraoperatively the PIN was bulbous in shape with glassy washout appearance and was compressed at the level of the two heads of supinator/arcade of Frohse. No neurogenic tumor was identified. 

Entrapment of the PIN represents a rare neuropathy with an incidence of 0.03%, with a paucity of literature reported in the pediatric population (1). To our knowledge this case represents the first reported case of PIN entrapment secondary to the ligament of Frohse in a pediatric patient. 

The two forearm supinator muscle layers, superficial and deep, form the supinator canal along the lateral antebrachial region, just distal to the cubital fossa. The posterior interosseous nerve, which branches from the radial nerve, passes through the supinator canal. The superficial layer of the supinator muscle forms the entrance of the supinator canal. A fibrous semicircular arch can be formed at the entrance of the supinator canal, which is known as the "arcade of Frohse" (2,3).

The most common cause of entrapment of the PIN is at the level of  arcade of Frohse (4). Clinically, compression of the PIN can manifest with inability to extend the metacarpophalangeal joints of the finger and thumb, and weakness of thumb extension at the interphalangeal joint. Wrist extension is usually preserved due to radial nerve innervation of the extensor carpi radialis longus proximal to the terminal branch. Sensory manifestations include pain and decreased sensation overlying the radial and dorsal aspect of hand (4,5). 

The arcade of Frohse can be identified on MRI as a hypointense band at the proximal edge of the supinator. Diagnosis of entrapment on MRI can be challenging given the small size of the PIN and arcade of Frohse. Often, muscle atrophy in the distribution of the PIN (abductor pollicis longus, extensor carpi ulnaris, extensor digiti minimi, extensor digitorum, extensor indicis, extensor pollicis brevis, and extensor pollicis longus) are the most apparent findings (2,7). As with other nerve entrapments, on MRI, the PIN can demonstrate focal fusiform/ nodular thickening, bright T2 weighted signal and occasionally, contact with the fibrous band of the ligament of Frohse if identified (2,3,7). 

In our case, the ligament of Frohse is visible, however, it appears thin compared to the ligament of Frohse in adults(7). This may suggest that in pediatric patients with features of PIN entrapment, the ligament of Frohse may not always appear as an obvious band, and a thin hypointense band contacting the PIN should raise the possibility of fibrous entrapment by the ligament of Frohse. At the patient's first postoperative clinical visit, their weakness had not resolved, expectedly given the chronicity of the patient's symptoms. Physiotherapy and a follow-up clinic visit was planned for a later date.