Smaller radioulnar window is associated with a distal biceps tendon rupture in patients with limited forearm rotation: a 3-dimensional computed tomography comparison study of proximal impingement caused by radial tuberosity hypertrophy—a single-center case series

Abstract

It has been suggested that hypertrophy of the radial tuberosity may result in impingements leading to either lesion of the distal biceps tendon or rotational impairment. The two studies on hypertrophy of the radial tuberosity had contradictory results and did not study the distance between the radius and ulna: the radioulnar window. Therefore, this comparative cohort study aims to investigate the radioulnar window in healthy subjects and compare them with subjects suffering from either non-traumatic onset rotational impairment of the forearm or non-traumatic onset distal biceps tendon ruptures with a rotational impairment of the forearm using dynamic 3D CT measurements in order to attain a comprehensive understanding of the underlying etiology of distal biceps tendon ruptures. We hypothesize that a smaller radioulnar window increases the risk of having a non-traumatic onset distal biceps tendon rupture and/or a rotational impairment compared to healthy individuals. This study measured the distance between the radius and ulna at the level of the radial tuberosity using entire forearm computed tomography (CT) scans of fifteen patients in the Amphia Hospital between 2019 and 2022. Measurements of healthy subjects were compared to subjects with non-traumatic onset rotational impairment of the forearm and subjects with a non-traumatic onset distal biceps tendon rupture with a rotational impairment of the forearm. The Wilcoxon signed-rank test was used for individual comparisons, and the Mann-Whitney U test was used for group comparisons. A significant difference was found between the radioulnar window in the forearms of the subjects with a distal biceps tendon rupture (mean: 1.4, SD 0.4 mm) compared to the radioulnar window in forearms of the healthy subjects (mean: 4.8, SD 1.4 mm, p = 0.018). A trend towards smaller radioulnar windows in the rotational impairment groups was also observed, although it was not significant (p > 0.05). The radioulnar window in the forearms of the subjects with a distal biceps tendon rupture with a rotational impairment was significantly smaller than in the forearms of the healthy subjects. Therefore, patients with a smaller radioulnar window have a higher risk of rupturing their distal biceps tendon. Non-traumatic onset rotational impairments of the forearm may also be caused by a similar mechanism. Future studies are needed to back this finding further.