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° 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° of abduction, and the posterior capsule between 40° and 90° of abduction. The anterior part of the rotator cuff and the upper part of the anterior capsule were essential in the first 40° of abduction. Cutting the capsular structures only, we found that the entire anterior capsule resisted anterior displacement for 70-90° of abduction, and the entire posterior capsule from 50-90° of abduction. For posterior displacement, the entire posterior capsule was important from 60 to 90° 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.