The role of the coronoid process in elbow stability

Background: The current treatment of coronoid process fractures of the ulna is based on the classification system of Regan and Morrey. We found no bio mechanical studies that specifically addressed the role of the coronoid pro cess in elbow stability. In the present investigation, the elbows of cadave ra 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 so ft tissue surrounding the elbow including the skin, was left intact. An axi al load compressing the elbow joint was applied along the shaft of the fore arm in the sagittal plane. A displacement of fifteen millimeters per minute was applied until a load of 100 newtons was attained. Each elbow was teste d in 15, 30, 45, 60, 75, 90, 105, and 120 degrees of flexion. Next, less th an 25 percent, 25 to 50 percent, or more than 50 percent of the coronoid pr ocess was fractured with an osteotome under radiographic guidance, and the testing was repeated. Each elbow served as its own control, and one elbow w as used for two tests; therefore, a total of seven situations were investig ated. The difference in displacements between the intact and osteotomized e lbows was measured. Results: There was no significant difference, at any flexion position, in p osterior axial displacement between the intact elbows and the elbows in whi ch 50 percent or less of the coronoid process was fractured (type I and typ e II) (p = 0.43). There were significant differences, across all flexion po sitions, 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 si gnificant increase in displacement compared with specimens with a type-I or type-II fracture (p = 0.012). Specifically, from 60 to 105 degrees of flex ion, a significant increase in posterior translation of up to 2.4 millimete rs was found (p < 0.05). Conclusions: In response to axial load, elbows with a fracture involving mo re than 50 percent of the coronoid process displace more readily than elbow s with a fracture involving 50 percent or less of the coronoid process, esp ecially when the elbow is flexed 60 degrees and beyond. Clinical Relevance: These data provide biomechanical support for the curren t treatment of coronoid process fractures and may help to explain why type- III coronoid process fractures often have a poor prognosis.