Purpose: This in-vitro study examined the effect of simulated Colles fractures on load transmitted to the distal ulna, using an in-line load cell. Our hypothesis was distal radial fracture malposition will increase distal radial ulnar joint (DRUJ) load relative to the native position of the radius.

Method: Eight fresh frozen upper-extremities were mounted in a motion simulator which enabled active forearm rotation. An osteotomy was performed just proximal to the distal radioulnar joint, and a 3-degree of freedom modular appliance was implanted which simulated Colles type distal radial fracture deformities. This device allowed for accurate adjustment of dorsal angulation and translation (0, 10, 20 and 30 degrees dorsal angulation and 0, 5 and 10mm dorsal translation both isolated and in combination). A 6-DOF load cell was inserted in the distal ulna 1.5 cm proximal to the ulnar head to quantify DRUJ joint forces. Distal ulnar loading was measured following simulated distal radial deformities with both an intact and sectioned triangular fibrocartilage complex (TFCC).

Results: The maximum resultant transverse distal ulnar load occurred during active forearm pronation and supination. Increasing magnitudes of dorsal angulation and translation of the distal radius increased loading in the distal ulna. For pronation with the ligaments intact, the transverse resultant load for the non-fracture, native positioning was significantly lower (p< 0.05) than the majority of malpositioned cases except for the translations only (not combined with angulation). However, all fracture orientations for supination had an increased effect on the resultant loading (p< 0.05) when ligaments were intact. Greater forces were measured in the distal ulna when the TFCC intact relative to TFCC sectioning. Sectioning the TFCC eliminated the effect of fracture malposition for both pronation and supination. The range of maximum transverse force for intact pronation and supination was between 118& #61617;34N and 130& #61617;39N, respectively. Similarly, for sectioned pronation and supination, the maximum transverse forces were and 93& #61617;40N and 89& #61617;24N, respectively.

Conclusion: Malpositioning of distal radial fractures in dorsal translation and angulation was found to increase forces in the distal ulna, which may be an important source of residual pain following malunion of Colles fractures. Healing of the distal radius in an anatomic position resulted in the least forces. Sectioning the TFCC released the tethering effect of the radius on the ulna, decreasing DRUJ force. This is the first study of its kind to attempt to quantify the forces at the DRUJ as a result of Colles fractures, and these early findings provide important baseline information related to the biomechanics of the DRUJ.