Glenohumeral contact forces in reversed anatomy shoulder replacement

A major requirement to design an implant is to develop our understanding of the applied internal forces during everyday activities. In the absence of any basic apparatus for measuring forces directly, it is essential to rely on modelling. The major aim of this study was therefore to understand the biomechanical function of subjects with the reversed anatomy Bayley-Walker prosthesis, using an inverse dynamic shoulder model. In this context, the muscle and joint forces of 12 Bayley-Walker subjects were compared to those of 12 normal subjects during 12 activities of daily living. Maximum glenohumeral contact forces for normal and Bayley-Walker subjects were found to be 77% (+/-15) and 137% (+/-21) body weight for lifting a 2kg shopping bag, and the least forces 29% (+/-4) and 67% (+/-8) body weight for reaching to opposite axilla, respectively. For normal subjects, middle deltoid, supraspinatus and infraspinatus were found to be the most active muscles across the subjects and tasks. On the other hand, for implanted subjects with a lack of rotator cuff muscles, the middle deltoid and coracobrachialis muscles were found to be the most active. The biomechanical model can therefore be used in order to gain knowledge about the pathology as well as possible post surgical rehab for subjects with reversed shoulder replacement.