Effects of radial head excision and distal radial shortening on load-sharing in cadaver forearms

Background: The present study was performed to measure;changes in radioulna r load-sharing in the cadaveric forearm following two orthopaedic surgical procedures that often have varying results: radial head excision and distal radial shortening. A better understanding of the biomechanical consequence s of those procedures could aid surgeons in obtaining a more satisfactory c linical outcome. Methods: Miniature load-cells were inserted into the proximal part of the r adius and the distal part of the ulna in twenty fresh-frozen cadaveric fore arms. Load-cell forces, radial head displacement relative to the capitellum , and local tension within the central band of the interosseous membrane we re measured simultaneously as the wrist was loaded to 133.5 N in neutral pr onation-supination and neutral radioulnar deviation. Testing was repeated a fter incremental distal radial shortening and after removal of the radial h ead. Results: With the elbow flexed to 90 degrees and in valgus alignment (the r adial head in contact with the capitellum), the mean force in the distal pa rt of the ulna was 7.1% of the applied wrist force and the mean force in th e in-terosseous membrane was 4.0%. With the elbow in varus alignment (a mea n initial gap of 1.97 mm between the radial head and the capitellum), the r espective mean values were 27.9% and 51.2%. After excision of the radial he ad, the mean force in the distal part of the ulna increased to 42.4% of the applied wrist force and the mean force in the interosseous membrane increa sed to 58.8%, in both varus and valgus elbow alignment. The mean distal uln ar force increased with progressive distal radial shortening in both varus and valgus elbow alignment after 6 mm of radial shortening, the distal ulna r force averaged 92.4% (in varus alignment) and 60.9% (in valgus alignment) . Equal distal load-sharing between the radius and ulna occurred after appr oximately 5 mm of radial shortening with the elbow in valgus alignment and after approximately 2 mm of radial shortening with the elbow in varus align ment. In valgus alignment, the force in the interosseous membrane was negli gible after all degrees of radial shortening; in varus alignment, the mean force in the interosseous membrane decreased from 51.2% (0 mm of distal rad ial shortening) to 0% (6 mm of distal radial shortening) because of progres sive slackening of the interosseous membrane. Conclusions: Radial head excision shifted the applied wrist force that norm ally would be transmitted to the elbow, through radial head-capitellar cont act, to the interosseous membrane. The resulting proximal radial displaceme nt created an ulnar-positive wrist and increased distal ulnar loading. Radi al shortening and ulnar lengthening procedures have been designed to shift the applied wrist force from the distal part of the radius to the distal pa rt of the ulna; it is commonly assumed that these procedures have equivalen t biomechanical effects. We found that radial shortening resulted in slacke ning of the interosseous membrane, thereby negating its ability to transmit load through the forearm. Slackening of the interosseous membrane would no t be expected with distal ulnar lengthening procedures. Clinical Relevance: When the radial head has been fractured or excised, the mechanical status of the interosseous membrane is critical to the load-sha ring process. If the interosseous membrane remains intact, distal ulnar loa ds will be limited to less than half of the applied wrist force; if the int erosseous membrane has been damaged, nearly the entire applied wrist force will be shifted to the ulna. The amount of radial shortening or ulnar lengt hening performed at the time of surgery during joint-leveling procedures ha s been largely empirical. We found that distal ulnar load increased by appr oximately 10% for each millimeter of radial shortening.