Abstract
Aim
Biomechanical models of the shoulder have been used to measure forces and glenohumeral pressures. Their results have been found to vary. The aim of this study was to produce a biomechanical model to replicate the biomechanical principles of the glenohumeral joint and to measure the centre of pressure on the glenoid through a mid-range of arm movement with an intact and a compromised rotator cuff.
Method
The model consisted of anatomic saw-bones of a scapula and proximal humerus with calibrated extension springs to mimic rotator cuff muscles. Glenoid pressures were measured using pressure sensitive film. The joint was examined through a mid-range of movement with an intact rotator cuff and a supraspinatus deficiency.
Results
In the normal cuff model, in neutral, the centre of pressure was in the centre of the glenoid and migrated inferiorly on abduction, rotation and 45° of flexion. The only exception to this was 90° flexion and 35° extension. Concavity compression force rose in internal/external rotation, was steady on flexion/extension but dropped on abduction. In the supraspinatus-deficient model, the centre of pressure dropped to the inferior lip in neutral and rose on any movement with extremes of flexion and abduction, resulting in subacromial impingement. Concavity compression force rose slightly on flexion and extension. On abduction, the force rose as much as three times that of the normal cuff.
Discussion
The results suggest that the humeral joint reaction force rests in the centre of the glenoid and is driven inferiorly on arm movement. Loss of supraspinatus reverses this pattern and leads to impingement. These results would be in keeping with osteoarthritic patterns in vivo and may have a bearing on glenoid prosthesis design.
Conclusion
The glenohumeral joint demonstrated inferior migration of the humeral reaction force on elevation of the arm. Cuff pathology leads to breakdown of this mechanism.
