The aim of this anatomical study was to describe the position of the posterior interosseous nerve (PIN) with respect to the elbow joint capsule and the midline of the radial head, with particular attention on the changes occurring with forearm movements.

The PIN innervates the supinator and the extensor muscles of the wrist and the digits and its lesion may have dramatic consequences. Measurements of PIN course have been provided from open surgical approaches, but arthroscopic descriptions are lacking.

Eleven fresh-frozen cadaver specimens were dissected under arthroscopy. Attention was paid to the possibility of identifying the PIN immediately after anterior capsulectomy. The distance between the nerve and the most anterior part of the radial head was then measured with a graduated hook inserted via the midlateral portal with the forearm in neutral position, full pronation and full supination.

The PIN was identified as immediately extracapsular only in one specimen. In the other specimens, a thick layer of adipose tissue surrounded it. The relative position of the PIN to the midline of the radial head changed with forearm pronosupination, moving from lateral in supination to medial in pronation in all cases. A statistically significant increase in the distance of the PIN from the radial head in full supination was recorded, as compared to neutral position and full pronation.

At the level of the radiocapitellar joint, the PIN does not lay just extracapsular but a thick layer of adipose tissue may hide it. Furthermore, movements of the forearm change significantly its position with respect to the radial head. The use of retractors can be helpful in protecting this nerve from accidental injury when performing procedures on the radial head.