Biomechanical considerations of the normal and rotator cuff deficient shoulders and the reverse shoulder prosthesis

SummaryThe treatment of combined glenohumeral arthritis and massive rotator cuff deficiency remains a unique challenge. Prosthetic reconstruction is aimed at restoring the normal kinematics of the glenohumeral joint. The reverse shoulder prosthesis medialises the centre of rotation of the glenohumeral joint, thereby lengthening the deltoid lever arm. The centre of rotation is also fixed at the glenoid bone prosthesis interface and this has the effect of reducing the torque on the glenoid component and lessens the risk of glenoid loosening. To further enhance the function of deltoid, the humerus is lowered relative to the glenoid, thereby increasing the deltoid tension. Although the preliminary results have been encouraging, there are still some unresolved issues such as prosthetic joint instability, scapular notching, wear of the polyethylene cup, fixation of the humeral stem and the lack of external rotation following surgery.