Effect of rotator cuff integrity on deltoid efficiency following reverse shoulder arthroplasty: a biomechanical model

In reverse shoulder arthroplasty (RSA), controversy exists over whether to repair the subscapularis and infraspinatus muscles. The objectives of this study were to investigate the effect of rotator cuff deficiencies and repairs on deltoid force in RSA and assess for any interaction with humeral implant neck-shaft angle. This was achieved using a biomechanical model of an RSA shoulder to test abduction in the scapular plane. Combinations of rotator cuff deficiencies and repairs were simulated by modifying the forces applied to the subscapularis and infraspinatus using actuator driven cables. Testing was performed with both 135 and 155-degree humeral neck-shaft angle implants. It was found that the amount of deltoid force required to abduct the arm in the scapular plane increases with intact subscapularis and infraspinatus muscles, and that decreasing the humeral neck-shaft angle may play a role in minimizing this force.