Influence of glenohumeral prosthesis geometry and placement on shoulder muscle forces

The authors studied the influence of a changed geometry of the glenohumeral joint on the function of the muscles with the use of a shoulder prosthesis with an anatomic design. The changed geometry is characterized by 4 parameters: orientation of the glenoid, radius of the humeral head, position of the glenohumeral joint's geometric center in relation to the scapula, and position of the glenohumeral joint's geometric center in relation to the humeral shaft. The effect of changes in these 4 parameters was investigated with an inverse dynamic 3-dimensional musculoskeletal model of the shoulder. This was done at 60 degrees and 90 degrees of abduction and flexion. Gravity was the only external force on the arm. The magnitudes of the introduced changes are assumed to be a realistic representation of a changed geometry due to the implantation of a prosthesis. In most situations, the effect of a change in the 4 parameters on the exerted muscle force was small compared with the maximum force of a muscle. However, in relation to the initial reference force in a muscle, changes with an average of 50% occurred. Changes in the geometric center's position relative to the humerus are especially important, because they are closely related to the retroversion angle and can cause changes in force of up to 300%.