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Background: The purpose of this study was to evaluate the role of the radial head and the coronoid process as posterolateral rotatory stabilizers of the elbow and to determine the stabilizing effect of radial head replacement and coronoid reconstruction.Methods: The posterolateral rotatory displacement of the ulna was measured after application of a valgus and supinating torque (1) in seven intact elbows, (2) after radial head excision, (3) after sequential resection of the coronoid process, (4) after subsequent insertion of each of two different types of metal radial head prostheses (a rigid implant and a bipolar implant with a floating cup), and (5) after subsequent reconstruction of the coronoid with each of two different techniques in the same cadaveric elbow.Results: The posterolateral rotatory laxity averaged 5.4° in the intact elbows. The surgical approach used in this study insignificantly increased the mean laxity to 9°. Excision of the radial head in an elbow with intact collateral ligaments caused a mean posterolateral rotatory laxity of 18.6° (p < 0.0001). Additional removal of 30% of the height of the coronoid fully destabilized the elbows, always resulting in ulnohumeral dislocation despite intact ligaments. Implantation of a rigid radial head prosthesis stabilized the elbows. However, a mean laxity of 16.9° persisted after insertion of a floating prosthesis (p < 0.0001). The elbows with a defect of 50% or 70% of the coronoid, loss of the radial head, and intact ligaments could not be stabilized by radial head replacement alone, but additional coronoid reconstruction restored stability.Conclusions: The results of this study suggest that the coronoid and the radial head contribute significantly to posterolateral rotatory stability.Clinical Relevance: Replacement of the radial head with a rigid implant seems to restore stability better than does replacement with a floating prosthesis. Elbows with a defect of ≥50% of the coronoid combined with a radial head defect can be stabilized with coronoid reconstruction and radial head replacement. Additional, clinical studies are necessary to validate these experimental findings.