Fracture Risk in Ulnohumeral Arthroplasty—A Biomechanical Study

Ulnohumeral arthroplasty, also known as the Outerbridge-Kashiwagi procedure, was popularized after reports of successful results in 1978, and has long been a means of management for ulnohumeral arthritis. However, there are concerns over the loss of integrity of the distal humerus as a result of fenestration. The purpose of this study was to examine the relationship between the size of fenestration and fracture risk. Using a validated fourth-generation sawbones model, load to failure and site of fracture were investigated following incrementally increasing distal humeral fenestration sizes. Each sample was subjected to a uniform extension stress on a materials testing system, with 5 samples run for each group. The experimental groups began with a fenestration size of 10 mm and increased by 3 mm increments up to 31 mm. Load at failure and site of fracture were recorded for each sample. Forty-five fourth-generation sawbones samples were tested. Average load at sample failure was equivalent for each fenestration group up to 25 mm. At 28 mm, average load to failure began to decrease, and was statistically significant beginning between 28 mm and 31 mm. At 28 mm, 4 of 5 samples fractured through the fenestration, and at 31 mm, all 5 samples fractured through the fenestration. This change in fracture site became statistically significant between 25 mm and 28 mm. Distal humeral fenestration does compromise its structural integrity; however, for resection in the range of 10–25 mm, there is no increased risk of fracture. On the basis of this biomechanical model, the authors do not recommend any activity limitations after initial surgical recovery, but do recommend against distal humeral fenestrations larger than 25 mm when performing this procedure.