A biomechanical evaluation of the restraints to posterior shoulder dislocation

The biomechanics of posterior dislocation of the shoulder was evaluated using nine cadaver shoulders. Each was arthroscoped, roentgenograms were obtained, and then each was dislocated in a testing device. Force displacement data was obtained during testing, and posttesting roentgenograms, arthroscopic evaluation, and subsequent open dissections were performed to evaluate the pathology created. Although instability was created in all cases with displacement of the humerus to the diameter of the humeral head, force displacement curves did not show an inflection point, implying a continuum between subluxation and dislocation. All shoulders had posterior Bankart lesions or posterior capsular lesions, or both. Anterior pathology was not seen. Posterior instability is most likely a continuum between subluxation dislocation with progressive injury to the posterior capsule and attachments such as the labrum as the principal restraint to posterior displacement.