The product that the investigators are currently using to try to improve the outcome of surgery for massive rotator cuff tears is called a patch. The patch provides a scaffold to support the muscles of the rotator cuff. Studies of rotator cuff surgery using similar patches have found that recovery is improved and there is a lower rate of postsurgical problems. The aim of this study is to look at whether using the patch improves pain and the clinical function of the shoulder after surgery. In addition, the investigators would like to understand how the patch works. To do this the investigators will scan (take images of) the shoulder using magnetic resonance imaging to look at the muscle damage in the shoulder before and after surgery.