IMPACT OF DISTAL ULNAR RESECTION AND ELBOW JOINT POSITIONS IN RELATION TO LOAD TRANSMISSION ACROSS THE RADIOCAPITELLAR JOINT

Introduction: The ulna is often resected to treat injuries resulting from trauma, congenital anomalies, as well as a result of procedures for tumors and arthritis in the wrist. However, there is no literature discussing the impact of ulnar shortening on force transmission. This study looks at how force is transmitted across the radiocapitellar joint (RCJ) after varying lengths of ulnar resection. Methods: 4 fresh-frozen cadaveric forearms (mean age 44.5 years) each with an intact elbow and distal humerus were secured onto a mounting plate, allowing for range of elbow motion to be tested: 45 degrees and 90 degrees in flexion, as well as supination, pronation, and neutral positioning of the forearm. Tests were done with varying ulnar conditions: normal intact, resected by 2 cm, resection of a quarter of the length, and resection of half of the length. Step loading was applied to a maximum of 150 N and resultant force across the RCJ was measured using a joint pressure sensor. Data from the four ulnar conditions were compared using repeated measure ANOVA with statistical significance set at p=0.05. Results: With the radioulnocarpal joint intact, the average force transmitted to the RCJ was 94.3±72.3 N with the elbow in 45-degree flexion and 123.6±69.3 N with 90-degree flexion and neutral pronation/supination. 45-degree supination caused a decrease in radiocapitellar force to 87.2±49.2 N and 81.7±56.2 N respectively, while 45-deree pronation increased the force to 123.2±67.5 N and 131.7±66.8 N respectively. In comparison to the control, an increase of statistical significance was found in supinated 90-degree flexion and supinated 45-degree flexion for both quarter- and half-length ulnar resections. In 45-degree flexion with neutral supination/pronation, the RCJ force was significantly higher than the intact after 2-cm ulna resection (p<0.03). Conclusion: This study demonstrates that with an intact ulna, 63% of applied axial force was transmitted to the RCJ when the elbow was flexed at 45, and 85% at 90 degrees. It also showed that the RCJ force transmission decreases with supination and increases with pronation. As the ulna is progressively shortened, increased force is transmitted across the RCJ, especially in elbow flexion and supination. A pronated position was observed to be protective of the RCJ.