Biomechanics of the Distal Radioulnar Joint During In Vivo Forearm Pronosupination

Abstract

Background Ulnar variance (UV) and center of rotation (COR) location at the level of the distal radioulnar joint (DRUJ) change with forearm rotation. Nevertheless, these parameters have not been assessed dynamically during active in vivo pronosupination. This assessment could help us to improve our diagnosis and treatment strategies. Questions/purposes We sought to (1) mathematically model the UV change, and (2) determine the dynamic COR's location during active pronosupination. Methods We used biplanar videoradiography to study DRUJ during in vivo pronation and supination in nine healthy subjects. UV was defined as the proximal-distal distance of ulnar fovea with respect to the radial sigmoid notch, and COR was calculated using helical axis of motion parameters. The continuous change of UV was evaluated using a generalized linear regression model. Results A second-degree polynomial with R ² of 0.85 was able to model the UV changes. Maximum negative UV occurred at 38.0 degrees supination and maximum positive UV occurred at maximum pronation. At maximum pronation, the COR was located 0.5 ± 1.8 mm ulnarly and 0.6 ± 0.8 mm volarly from the center of the ulnar fovea, while at maximum supination, the COR was located 0.2 ± 0.6 mm radially and 2.0 ± 0.5 mm volarly. Conclusion Changes in UV and volar translation of the COR are nonlinear at the DRUJ during pronosupination. Clinical Relevance Understanding the dynamic nature of UV as a function of pronosupination can help guide accurate evaluation and treatment of wrist pathology where the UV is an important consideration. The dynamic behavior of COR might be useful in designing DRUJ replacement implants to match the anatomical motion.

Keywords: DRUJ; center of rotation; kinematics; radioulnar joint; ulnar variance.

Fig. 1

The biplane videoradiography experimental setup. The orientation between sources was 110 degrees, and the source-to-hand distance was approximately 90 cm. A doorknob device was affixed to the trestle and subjects' wrist joint was at the center of the field-of-view of the X-ray sources.

Fig. 2

The coordinate systems of the radius and ulna in volar view of the ulna. x-axis (directed proximally), y-axis (directed radially), and z-axis (directed volarly) are shown for both bones. The ulnar variance was defined as the proximal-distal distance between the ulna fovea (origin of the ulnar coordinate system) to the sigmoid notch of the radius at the neutral pose. In this figure, the ulnar variance is negative.

Fig. 3

Examples of tracked radius and ulna for one subject overlaid on one of the radiographs in the functional neutral position, mid- and full pronation (25 and 50 degrees), and mid- and full supination (30 and 60 degrees). The highlighted sections are the digitally reconstructed radiographs of the bones after tracking and optimization process. The features of the bones in the radiographs are enhanced using the Sobel and intensity image filters.

Fig. 4

The change in ulnar variance (UV) from its value at the neutral position was nonlinear relative to forearm pronosupination. The most negative UV was noted at an average of 38.0 degrees of supination. The solid line is demonstrating the fitted model, and shaded area are demonstrating its 95% confidence interval.

Fig. 5

Center of rotation (COR) moved volarly in supination, but it was stationary throughout pronation. The solid lines demonstrate the average COR location at every 2 degrees of pronation or supination, and the dashed lines demonstrate the standard deviation.

Fig. 6

Center of rotation moved volarly for eight (out of nine) participants from maximum pronation to maximum supination. Each arrow is directed from maximum pronation to maximum supination.