To assess the normal values of the antero-posterior (AP) diameter of the posterior interosseous nerve (PIN) of the elbow as it passes beneath the arcade of Frohse and to search for PIN-diameter differences between the upstream, entry point and downstream of the arcade.

Thirty asymptomatic patients prospectively underwent bilateral B-mode ultrasound of the PIN of the elbow. There were 15 men and 15 women with a mean age of 30.2±5.31 (SD) years (range: 26–43 years). Of these, 23 patients were right-handers (23/30; 77%) and 7 were left handers (7/30; 23%). AP diameter of the PIN was measured in long axis at three different locations including the entry point of the arcade, 5-mm upstream and 5-mm downstream the arcade. A comparison between the three measurements was performed using paired t-test.

The mean AP diameters of the PIN were 0.83±0.21 (SD) mm (range: 0.43–1.31mm), 0.6±0.17 (SD) mm (range: 0.29–1.16mm) and 0.49±0.13 (SD) mm (range: 0.26–0.86mm) at 5-mm upstream, entry point of the arcade and 5-mm downstream the arcade of Frohse, respectively. Significant drops in PIN diameter were found between upstream and the arcade (−0.23mm; 27%; P<0.001), between the arcade and downstream (−0.11mm; 17%; P<0.001), and between upstream and downstream the arcade (−0.34mm; 40%; P<0.001).

Disparity in AP diameter of the PIN of the elbow in the arcade of Frohse is a normal finding and should not be erroneously interpreted as entrapment when present alone.