# Functional shape of the proximal radioulnar joint: anatomical characterization through Alpha and Beta Angles

## TL;DR

This study introduces new anatomical measurements for the proximal radioulnar joint and finds a significant inverse relationship between two angles that affect forearm stability.

## Contribution

The study introduces the Alpha and Beta Angles as novel anatomical parameters for the proximal radioulnar joint.

## Key findings

- The mean Alpha Angle was 139.82° with high variability among individuals.
- A statistically significant negative correlation exists between Alpha and Beta Angles.
- Higher Beta Angles suggest increased radial head coverage and greater joint stability.

## Abstract

The proximal radioulnar joint (PRUJ) plays a critical role in forearm rotation through the articulation between the radial head (RH) and the ulnar notch. While previous studies have emphasized the importance of congruency for joint stability, limited data exist on anatomical variations in healthy individuals and their potential biomechanical implications. This study aimed to describe novel anatomical parameters of the PRUJ.

A retrospective analysis of 104 elbow computed tomography scans from patients without elbow instability or deformities was performed. Measurements were taken on standardized axial computed tomography slices using two novel angular metrics: the Alpha and Beta Angle. Linear regression was used to assess the association between these angles.

The mean Alpha Angle was 139.82° (standard deviation 10.08°), while the mean Beta Angle was 81.25° (standard deviation 11.04°), both showing high interindividual variability. A statistically significant negative correlation was found between Alpha and Beta Angles (β = −0.53, 95% confidence interval: −0.72 to −0.34, P < .0001).

A larger Alpha Angle implies a less concave notch and reduced bony constraint of the RH, increasing the reliance on ligamentous structures, particularly the annular ligament, for joint stability. Conversely, a higher Beta Angle, indicating increased RH coverage, may confer greater intrinsic stability.

The study identifies a significant inverse relationship between the Alpha and Beta Angles of the PRUJ, supporting the concept that ulnar notch morphology affects RH coverage and forearm stability. These reference values in healthy elbows may serve as a baseline for future comparisons in pathological conditions and inform the diagnosis and management of joint instability and degeneration.