Posterolateral rotatory instability of the elbow has been attributed to disruption of the ulnar part of the lateral collateral ligament. Forty fresh cadavera were studied to define the ligamentous anatomy of the lateral aspect of the elbow specifically as it relates to rotatory instability. The dissections revealed a broad conjoined insertion of the lateral collateral and annular ligaments onto the proximal aspect of the ulna. This insertion was bilobed (type I) in twenty-two specimens and broad (type II) in eighteen specimens. Serial sectioning studies revealed primary and secondary stabilizers of the lateral aspect of the elbow. In addition to the lateral collateral ligament and the annular ligament, the extensor muscle origins provide stability through fascial bands and intermuscular septa.CLINICAL RELEVANCE: The findings of this study suggest that post-traumatic posterolateral rotatory instability of the elbow is the result of attenuation or disruption of both the ligamentous and the muscular origins from the lateral epicondyle of the humerus. Injury to these structures should be avoided during operative exposures for procedures such as débridement of the elbow for recalcitrant lateral epicondylitis, resection of the radial head, or capsular release for a stiff elbow. If immobilization is necessary postoperatively, the forearm should be held in pronation.