Teaching Case

A 60-year-old woman presented with progressive pain in her right elbow following immobilisation after total shoulder arthroplasty. There was no other significant medical history. The pain radiated to her upper arm and forearm, without any associated sensory or motor deficits. Palpation of the radial proximal forearm exacerbated the pain. These clinical findings warranted further investigation with ultrasound imaging.

Ultrasound imaging of the right forearm revealed a spindle-shaped, hypoechoic thickening of the posterior interosseous nerve (PIN) at the supinator arch, also known as the “arcade of Frohse”. The thickening measured 6 x 1.5 mm, with the nerve returning to its normal appearance further distally in the radial tunnel (Figures 1a and 1b). No other abnormalities were observed on ultrasound imaging. An electromyography study was also performed and did not detect any abnormalities. These findings suggest entrapment of the PIN, a known cause of radial tunnel syndrome [1].

Radial tunnel syndrome (RTS) is caused by entrapment of the posterior interosseous nerve (PIN), a deep motor branch of the radial nerve, within the radial tunnel. This condition results in pain over the proximal radial forearm without neurological deficits, despite it involving a motor nerve [1]. This distinguishes RTS from a similar condition known as posterior interosseous nerve syndrome (PINS), which is characterised by motor function loss [2]. Unlike PINS, electrodiagnostic examinations in RTS typically do not demonstrate abnormalities [2]. The distinction between RTS and PINS is somewhat controversial, as both conditions appear to stem from the same underlying issue, and are sometimes used interchangeably in the literature [1,3]. Clinically, RTS may be difficult to distinguish from a tennis elbow, as pain at the lateral epicondyle is common in RTS [1].

The PIN is susceptible to entrapment at several sites, with the supinator arch being the most common location [1]. This arch, also known as the arcade of Frohse, is a tendinous structure at proximal margin of the superficial layer of the supinator muscle [1]. Other potential entrapment sites include fibrous bands at the humeroradial joint, a vascular arcade at the radial head known as the “leash of Henry”, the edge of the extensor carpi radialis brevis, and the distal edge of the supinator muscle [1,3]. Masses and osseous deformities should also be considered as they may also cause entrapment of this nerve.

Imaging is primarily used to rule out other pathologies that can cause lateral elbow pain [1]. As shown in this case, ultrasound can demonstrate the thickening of the PIN, which is a typical finding in compressive neuropathies [4]. However, it may be difficult to reliably differentiate this thickening from a neurogenic tumour based on ultrasound imaging alone. MR imaging can show muscle denervation along the distribution of the PIN, although this finding is not pathognomonic [2]. Moreover, it lacks the spatial resolution to reliably assess the nerve itself [2].

In this case, the diagnosis of PIN entrapment was made based on imaging findings and clinical examination. Surgical decompression was recommended and accepted by the patient. The surgery confirmed entrapment of the PIN, and a release procedure was performed. After surgery, the patient’s symptoms completely resolved.