Mechanics and Differential Healing Outcomes of Small and Large Partial-Thickness Injuries of the Rat Rotator Cuff

Abstract

The size of rotator cuff tears affects clinical outcomes following rotator cuff repair and is correlated with risk of re-injury. This study aimed to understand how defect size influences the structural and mechanical outcomes of the injured rotator cuff attachment in vivo. We used our previously established model of full-thickness injury of the rotator cuff tendon-bone attachment in Long Evans rats to compare differences in healing outcomes between small and large defects. Biomechanical properties, gross morphology, bone remodeling, and cell and tissue morphology were assessed at 3- or 8-weeks of healing. At the time of injury (no healing), large defects had decreased mechanical properties compared to small defects, and both defect sizes had decreased mechanical properties compared to intact attachments. The mechanical properties of the defect groups were comparable after 8-weeks of healing and significantly improved compared to no healing but failed to return to intact levels. Local bone volume at the defect site was higher in large compared to small defects on average and increased from 3- to 8-weeks. Contrastingly, bone quality, measured as bone volume percentage and trabecular morphometry, of the total epiphysis and greater tubercle decreased from 3- to 8-weeks of healing and these changes were not dependent on defect size. Qualitatively, we observed that large defects had increased disorganized collagen and neovascularization compared to small defects. In this study, we demonstrated that not only small but also large defects do not regenerate the mechanical and structural integrity of the intact rat rotator cuff attachment following healing in vivo. Statement of Clinical SignificanceOur rat model of full-thickness rotator cuff tears may be beneficial to understand and prevent tear enlargement in vivo.