Surgical anatomy of deep branch of the radial nerve at the Arcade of Frohse: geometric relationships relevant to posterior interosseous nerve entrapment.

PURPOSE: Posterior Interosseous Nerve (PIN) syndrome results from compressive neuropraxia of the deep branch of the radial nerve (DBRN) at or around the Arcade of Frohse (AOF), manifesting purely as motor deficits. Accurate localization of the nerve remains challenging due to limited surface landmark data. This study aimed to establish reliable geometric surface landmarks to guide clinicians in diagnosing and managing PIN syndrome. METHODS: Sixteen upper limb specimens from eight cadavers (7 male, 1 female) underwent standardized dissection. Key measurements were recorded, including lengths of DBRN at various anatomical intervals, nerve width at three points, and distances from bony landmarks (medial and lateral epicondyles) to the AOF. Texture of the proximal and distal arcades of the supinator was also documented. RESULTS: The mean length of DBRN from its origin to AOF was 39.35 ± 7.9 mm. The mean distance from the radio-capitular joint (RCJ) to the AOF was 23.06 ± 4.1 mm, and the mean length within the radial tunnel was 41.59 ± 8.5 mm. DBRN width increased from 1.89 ± 0.69 mm at origin to 3.41 ± 1.05 mm at the AOF, consistent with compressive remodelling. A geometric triangle ("PIN Triangle") was defined using the medial epicondyle, lateral epicondyle, and the AOF as vertices, with the lateral epicondyle-to-AOF distance averaging 41.36 mm. The AOF was musculotendinous in 44%, tendinous in 25%, and membranous in 31% of specimens. CONCLUSION: The PIN Triangle is a scalene triangle defined by the medial epicondyle, lateral epicondyle, and the AOF, with its apex approximately 41 mm distal to the lateral epicondyle, provides a reproducible surface landmark for diagnostic palpation and arthroscopic planning for PIN syndrome.