Ultrasound acting as a guiding light in the diagnosis of posterior interosseous nerve entrapment at ligament of Frohse – A case report

Posterior interosseous nerve (PIN) entrapment syndrome is a relatively rare compression neuropathy which leads to weakness of the extensor muscles of hand. This becomes a cause of morbidity to the patient with resultant difficulty in conducting day-to-day activities. Thickening of proximal edge of the superficial head of the supinator muscle which forms the ligament of Frohse can lead to compression of the PIN as it enters the supinator canal. Although nerve conduction studies can indicate the diagnosis, they cannot identify the exact site and cause of compression which is crucial for deciding the further management. Magnetic resonance imaging can identify the cause of entrapment in some cases; however, this is extremely difficult given the small size of the PIN and arcade of Frohse. Ultrasound overcomes these limitations and provides a cost and time effective, efficient, and sensitive diagnostic tool for detecting nerve entrapment. Here, we would like to emphasize the emerging importance of ultrasound by describing a case of ligament of Frohse thickening causing PIN entrapment which was effectively diagnosed by ultrasound and confirmed on surgical exploration, treated with surgical ligament release with a full recovery.