Partial-width injuries of the rat rotator cuff heal with fibrosis.

PURPOSE: Identify the healing outcomes following a partial-width, full-thickness injury to the rotator cuff tendon-bone attachment and establish if the adult attachment can regenerate the morphology of the healthy attachment.

HYPOTHESIS: We hypothesized that a partial-width injury to the attachment would heal via fibrosis and bone remodeling, resulting in increased cellularity and extra-cellular matrix deposition, reduced bone volume (BV), osteoclast presence, and decreased collagen organization compared to shams.

MATERIALS AND METHODS: A partial-width injury was made using a biopsy punch at the center one-third of the rat infraspinatus attachment. Contralateral limbs underwent a sham operation. Rats were sacrificed at 3 and 8 weeks after injury for analyses. Analyses performed at each time point included cellularity (Hematoxylin & Eosin), ECM deposition (Masson's Trichrome), BV (micro-computed tomography; microCT), osteoclast activity (Tartrate Resistant Acid Phosphatase; TRAP), and collagen fibril organization (Picrosirius Red). Injured and sham shoulders were compared at both 3 and 8 weeks using paired, two-way ANOVAs with repeated measures (Sidak's correction for multiple comparisons).

RESULTS: Cellularity and ECM deposition increased at both 3 and 8 weeks compared to sham contralateral attachments. BV decreased and osteoclast presence increased at both 3 and 8 weeks compared to sham contralateral limbs. Collagen fibril organization was reduced at 3 weeks after injury compared to 3-week sham attachments.

CONCLUSIONS: These findings suggest that a partial-width injury to the rotator cuff attachment does not fully regenerate the native structure of the healthy attachment. The injury model healed via scar-like fibrosis and did not propagate into a full-width tear after 8 weeks of healing.