Elbow position affects distal radioulnar joint kinematics

Abstract

Purpose: Previous in vivo and in vitro studies of forearm supination-pronation suggest that distal radioulnar joint kinematics may be affected by elbow flexion. The primary hypotheses tested by this study were that, in vivo, ulnar variance changes with elbow flexion and forearm rotation, and the arc of forearm rotation changes in relationship to elbow flexion.

Methods: Changes in radioulnar kinematics during forearm supination-pronation and elbow flexion (0 degrees to 90 degrees ) were studied in 5 uninjured subjects using computed tomography, dual-orthogonal fluoroscopy, and 3-dimensional modeling. Analysis of variance and post-hoc testing was performed.

Results: Proximal translation of the radius was greatest with the elbow flexed to 90 degrees with the arm in midpronation. With the arm in midpronation, the translation of the radius was significantly greater at 0 degrees versus 45 degrees of elbow flexion (0.82 +/- 0.59 mm vs 0.65 +/- 0.80 mm, F: 4.49, post hoc: 0.055; p = .05) and significantly smaller at 45 degrees versus 90 degrees of elbow flexion (0.65 +/- 0.80 mm vs 0.97 +/- 0.35 mm, F: 4.49, post hoc: 0.048; p = .05). Proximal translation of the radius in midpronation was significantly greater than when the forearm was in a supinated position when the elbow was at 0 degrees or 90 degrees flexion (F: 14.90, post hoc: <0.01; p < .01, F: 19.11, post hoc: <0.01, p < .01). The arc of forearm rotation was significantly decreased at 0 degrees compared with 90 degrees of elbow flexion (129.3 degrees +/- 22.2 degrees vs 152.8 degrees +/- 14.4 degrees , F: 3.29, post hoc: 0.79; p = .09). The center of rotation shifted volarly and ulnarly with increasing elbow extension.

Conclusions: Elbow position affects the kinematics of the distal radioulnar joint. The kinematics of the distal radioulnar joint are primarily affected by forearm rotation and secondarily by elbow flexion. These findings have clinical relevance to our understanding of ulnar impaction, and how elbow position affects the proximal-distal translation of the radius. These findings have implications for the treatment of ulna impaction, radiographic evaluation of the distal ulna, and future biomechanical studies.

Figure 1

Dual fluoroscopic imaging system. Subject with elbow at 45° flexion and forearm in neutral rotation. Images were captured at elbow flexion angles of 0°, 45°, and 90° with the forearm at maximum supination, mid-supination, neutral, mid-pronation, and maximum pronation.

Figure 2

The coordinate systems used to describe radioulnar kinematics. The ulnar y-axis, a line the center of the ulnar head to the center of the ulna styloid, was used to delineate supination from pronation.

Figure 3

Change in ulna variance was measured from the neutral position for each angle of elbow flexion. Ulna variance was measured as the distance along the ulnar z-axis between the center of the radial coordinate system and the center of the ulnar coordinate system. Proximal translation of the radial coordinate system relative to the ulna coordinate was defined as negative, whereas distal translation was positive.

Figure 4

Center of rotation was determined by finding the best fit circle of the center points of the radial coordinate system. Deviations from this circular path were random and minor.

Figure 5

Proximal-distal translation of the radius relative to the ulna. Proximal translation of the radius relative to the ulna was defined as negative, whereas distal translation was positive.

Figure 6

Center of rotation (COR) of the radius about the ulna shifted with elbow flexion. There was a significant ulnar shift in the COR from 45° to 90° of flexion.