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Posterior interosseous nerve entrapment

Case contributed by Brendan Cullinane
Diagnosis almost certain


Referred for ultrasound of the right wrist after a fall. As he also described occasional forearm pain, the exam was extended to assess areas of potential pathology more proximally, including the elbow.

Patient Data

Age: 15 years
Gender: Male

Image A demonstrates the pathological side. Here the posterior interosseous nerve (PIN) appears swollen and deviates sharply as it enters the arcade of Frohse (thin arrow). There is a wedge-shaped scar superficial to it, which appears to alter the course of the nerve.

Image B: normal appearance of PIN, gradual change in its course and absence of the echogenic scar seen on the contralateral side.

On the pathological side, there was point tenderness at the arcade with pain occasionally radiating down the forearm. These symptoms could not be reproduced on the contralateral side.

The overall impression was of PIN entrapment at the arcade of Frohse. 

Case Discussion

Ultrasound demonstrated evidence of posterior interosseous nerve (PIN) entrapment at the arcade of Frohse (supinator muscle) caused by a band of scar tissue. 

In the setting of trauma to the wrist and hand, it was not possible to assess for finger drop and radial deviation of the wrist on extension.

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