Doxycycline-mediated inhibition of matrix metalloproteinases improves healing after rotator cuff repair

Background: Recent studies demonstrate a potentially critical role of matrix metalloproteinases (MMPs) and their inhibitors in the pathophysiology of rotator cuff tears. Hypothesis: Doxycycline-mediated MMP inhibition after rotator cuff repair will improve tendon-to-bone healing. Study Design: Controlled laboratory study. Methods: Rats (n = 183) underwent acute detachment and repair of the supraspinatus tendon and the animals were divided into 4 groups: In controls (n = 66), the supraspinatus was repaired to its anatomical footprint. In experimental groups, an identical surgery was performed with doxycycline (130 mg/kg/d) administered orally at (1) preoperative day 1 (n = 66), (2) postoperative day (POD) 5 (n = 28), or (3) POD 14 (n = 23). Animals were sacrificed at 5 days, 8 days, 2 weeks, and 4 weeks. Tendon-bone interface was evaluated with histomorphometry. Enzyme-linked immunosorbent assay for local MMP-13 activity was performed at 8 days and 4 weeks. Biomechanical testing of the healing enthesis was performed at 8 days, 2 weeks, and 4 weeks. Serum doxycycline levels were measured at sacrifice. Statistical analysis was performed using unpaired t tests and 2-way analysis of variance (P <.05). Results: Serum doxycycline levels were significantly higher in all treated groups compared with controls (1830 ± 835 vs 3 ± 3 ng/mL, respectively; P <.001). Doxycycline-treated animals demonstrated greater metachromasia and improved collagen organization at the healing enthesis at POD 5 (P <.06), POD 8 (P <.03), and 2 weeks (P <.04). The MMP-13 activity was significantly reduced in doxycycline-treated compared with control animals at POD 8 (6740 ± 2770 vs 10400 ± 2930 relative fluorescent units [RFU], respectively; P <.02) but not at 4 weeks (3600 ± 3280 vs 4530 ± 2720 RFU, respectively). The healing enthesis of animals started on doxycycline preoperatively or at POD 5 had an increased load to failure compared to controls at 2 weeks (13.6 ± 1.8 and 13.2 ± 1.94 N vs 9.1 ± 2.5 N, respectively; P <.01). Conclusion/Clinical Relevance: Modulation of MMP-13 activity after rotator cuff repair may offer a novel biological pathway to augment tendon-to-bone healing.