Anterior and posterior shoulder instability. A cadaver study.

In a cadaver study of 10 glenohumeral joint specimens, the anterior and posterior displacement of the humeral head was recorded after cutting parts of the rotator cuff and capsular structures applying a constant force to the humerus. The posterior structures were important for anterior stability in the first 40 degrees of abduction. Anterior subluxation was changed to luxation in the first half of abduction, but only after lesions to the anterior part of the rotator cuff and upper half of the anterior capsule. For posterior displacement, the posterior part of the rotator cuff was found significant from 0-90 degrees of abduction, and the posterior capsule between 40 degrees and 90 degrees of abduction. The anterior part of the rotator cuff and the upper part of the anterior capsule were essential in the first 40 degrees of abduction. Cutting the capsular structures only, we found that the entire anterior capsule resisted anterior displacement for 70-90 degrees of abduction, and the entire posterior capsule from 50-90 degrees of abduction. For posterior displacement, the entire posterior capsule was important from 60 to 90 degrees of abduction. Clinically, a large lesion to the posterior structures seems to be essential for any major anterior displacement, and posterior displacement leading to subluxation only seems possible in connection with a major anterior injury.