DESCRIPTION (provided by applicant): Healing following rotator cuff repair remains a significant clinical challenge, and repair failure rates of 20-70% continue to be reported. We postulate that the weak relationships between current measures of structural integrity and such clinical outcomes are due to an imprecise understanding of tendon healing, which limits advances in treatment and rehabilitation strategies aimed to improve healing and outcomes. Structural outcomes currently evaluated with MRI or ultrasound indicate only if the repair is intact, attenuated, or failed, but are inadequate to discern the extent to which the tendon has "failed with continuity", i.e., undergone significant tendon retraction in the absence of a discernible defect (gap) in repair continuity. This proposal's objective is to challenge and expand our current definition of rotator cuff healing by investigating tendon retraction - broadly defined as medial translation of the repaired tendon away from the bone with or without a defect - as a common and clinically predictive structural outcome following rotator cuff repair. The approach is to characterize tendon retraction using an array of implanted markers, and investigate its relationships to pre- operative tissue quality (MRI), post-operative repair integrity (MRI) and clinical outcomes in a prospectively enrolled cohort of 125 patients. Pre- and post-operatively, patients will complete validated outcome questionnaires at 6 weeks and repeat these, as well as undergo MRI (repair continuity), CT imaging (tendon retraction) and strength testing, at 3, 6, 12 and 24 months. Specific Aim 1 will characterize the magnitude, patterns, and timing of tendon retraction for 2 years after rotator cuff repair, and assess its relationship to pre- operative tisue quality and post-operative repair integrity (both by traditional MRI). Specific Aim 2 will assess the extent to which tendon retraction after rotator cuff repair is associated with and can improve prediction of 1- and 2-year strength and patient reported outcomes (PROs). We expect to show that tendon retraction is common, occurs early post-operatively, with or without repair continuity as assessed by traditional imaging, and significantly correlates with clinical outcomes. The immediate and highly significant consequence of this finding would be a paradigm shift in our understanding of tendon repair healing, now incorporating the magnitude, timing and location of tendon retraction as well as the continuity of the repaired tissue. This information would yield a more precise understanding of rotator cuff tendon healing, allowing for advances in treatment strategies that improve surgical healing and clinical outcomes and result in more durable rotator cuff repairs over time.