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The Role of the Coronoid Process in Elbow Stability: A Biomechanical Analysis of Axial Loading*

Background: The current treatment of coronoid process fractures of the ulna is based on the classification system of Regan and Morrey. We found no biomechanical studies that specifically addressed the role of the coronoid process in elbow stability. In the present investigation, the elbows of cadavera were tested before and after fracture of the coronoid process to assess the stabilizing contribution of the coronoid process under axial loading. Methods: Six fresh-frozen cadaveric elbows were tested mechanically. All soft tissue surrounding the elbow, including the skin, was left intact. An axial load compressing the elbow joint was applied along the shaft of the forearm in the sagittal plane. A displacement of fifteen millimeters per minute was applied until a load of 100 newtons was attained. Each elbow was tested in 15, 30, 45, 60, 75, 90, 105, and 120 degrees of flexion. Next, less than 25 percent, 25 to 50 percent, or more than 50 percent of the coronoid process was fractured with an osteotome under radiographic guidance, and the testing was repeated. Each elbow served as its own control, and one elbow was used for two tests; therefore, a total of seven situations were investigated. The difference in displacements between the intact and osteotomized elbows was measured. Results: There was no significant difference, at any flexion position, in posterior axial displacement between the intact elbows and the elbows in which 50 percent or less of the coronoid process was fractured (type I and type II) (p = 0.43). There were significant differences, across all flexion positions, in posterior axial displacement between the intact elbows and the elbows in which more than 50 percent of the coronoid process was fractured (type III) (p = 0.006). Specimens with a type-III fracture also showed a significant increase in displacement compared with specimens with a type-I or type-II fracture (p = 0.012). Specifically, from 60 to 105 degrees of flexion, a significant increase in posterior translation of up to 2.4 millimeters was found (p < 0.05). Conclusions: In response to axial load, elbows with a fracture involving more than 50 percent of the coronoid process displace more readily than elbows with a fracture involving 50 percent or less of the coronoid process, especially when the elbow is flexed 60 degrees and beyond. Clinical Relevance: These data provide biomechanical support for the current treatment of coronoid process fractures and may help to explain why type-III coronoid process fractures often have a poor prognosis.