Basic science on shoulder instability

Abstract

Shoulder instability is a complex issue involving a multitude of static and dynamic components. This chapter gives an overview of the basic science studies on shoulder instability. The important parameters in biomechanical testing models are peak contact pressures over the glenohumeral joint and the required force to dislocate the shoulder. It is discussed how joint biomechanics are altered by labral detachment and bone loss. These changes in joint biomechanics are a possible cause of both recurrent instability and the development of degenerative changes of the shoulder. Next, it is addressed if the altered joint biomechanics can be restored to normal with a Bankart procedure and when a bone graft procedure is justified. Therefore, it is important to understand the concept of bipolar bone loss and how this influences the “critical size defect.” These are the most important conclusions that can be drawn: A Bankart lesion results in a decreased contact area, increased contact pressures accross the joint, and a decreased force required to dislocate the shoulder. These can be restored with Bankart repair if there is no glenoid bone loss. Anterior glenoid bone loss progressively increases the mean contact pressures in the glenohumeral joint, with an antero-inferior shift of peak contact pressures toward the antero-inferior quadrant. A 30% glenoid defect results in a fourfold (or 390%) increase in contact pressure in the antero-inferior quadrant. The “critical size” of bony glenoid defects which justifies a bone graft procedure is difficult to define, but can be as small as 15% of the glenoid diameter, or even less (10%, or 2-4 mm) in the case of humeral head defects, i.e., bipolar bone loss. Joint biomechanics can be restored to normal with a Latarjet procedure, provided that the bone graft is placed flush with the glenoid articular surface.