The posterior interosseous nerve (PIN) is a motor branch of the radial nerve. It is particularly vulnerable to compression as it passes through the supinator muscle, especially at the arcade of Frohse. Most anatomical studies describe its course using fixed distances from deep landmarks, such as the radial head, which limits clinical applicability due to interindividual variation in limb dimensions. Therefore, a surface-landmark–based, ratio-driven approach may provide a practical and reproducible method for localising the PIN in surgical settings.

In this study, upper limbs from 15 adult cadavers were dissected. With the forearm in pronation, the lateral epicondyle to radial styloid line (LR line; distance a ) and the lateral epicondyle to a point 1 cm radial to ulnar styloid (LU line; distance b ) were marked as surface reference lines. The PIN was identified between the brachioradialis and extensor carpi radialis longus muscles, and distances from the lateral epicondyle to its entry ( c ) and exit ( d ) points from the supinator were measured, along with the intramuscular travel length ( Tr ). Ratios a/c and b/d were calculated.

Mean distances were: a 24.54 cm, b 22.82 cm, c 4.26 cm, d 8.54 cm, and Tr 5.83 cm. The mean a/c and b/d ratios were 5.76 and 2.67, respectively. PIN entry and exit points are consistently projected near the LR and LU lines.

Ratio-based surface mapping is a practical technique for estimating PIN location in forearm peripheral nerve surgeries. However, additional validation in actual patients is needed.

V (Cadaveric anatomical study).