INFLUENCE OF GLENOHUMERAL PROSTHESIS GEOMETRY AND PLACEMENT ON SHOULDER MUSCLE FORCES

The authors studied the influence of a changed geometry of the glenohu meral joint on the function of the muscles with the use of a shoulder prosthesis with an anatomic design. The changed geometry is characteri zed by 4 parameters: orientation of the glenoid, radius of the humeral head, position of the glenohumeral joint's geometric center in relati on to the scapula, and position of the glenohumeral joint's geometric center in relation to the humeral shaft, The effect of changes in thes e 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 fo rce on the arm. The magnitudes of the introduced changes are assumed t o be a realistic representation of a changed geometry due to the impla ntation 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 ref erence force in a muscle, changes with an average of 50% occurred. Cha nges in the geometric center's position relative to the humerus are es pecially important, because they are closely related to the retroversi on angle and can cause changes in force of up to 300%.