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Healing outcomes of meniscal repair are better in younger than in older. However, exact mechanisms underlying superior healing potential in younger remain unclear from a histological perspective. This study included 24 immature rabbits and 24 mature rabbits. Tears were created in the anterior horn of medial meniscus of right knee in each rabbit. Animals were sacrificed at 1, 3, 6, and 12 weeks postoperatively. We performed macroscopic and histological evaluations of post-meniscal repair specimens. Cells were counted within a region of interest to confirm cellularization at tear site in immature menisci. The width of cell death zone was measured to determine the region of cell death in mature menisci. Apoptosis was evaluated by TUNEL assay. Vascularization was assessed by CD31 immunofluorescence. The glycosaminoglycans and the types 1 and 2 collagen content was evaluated by calculating average optical density of corresponding histological specimens. Cartilage degeneration was also evaluated. Healing outcomes following untreated meniscal tears were superior in immature group. Recellularization with meniscus-like cell morphology was observed at tear edge in immature menisci. Superior recellularization was observed at meniscal sites close to joint capsule than at sites distant from the capsule. Recellularization did not occur at tear site in mature group; however, we observed gradual enlargement of cell death zone. Apoptosis was presented at 1, 3, 6, 12 weeks in immature and mature menisci after untreated meniscal tears. Vascularization was investigated along the tear edges in immature menisci. Glycosaminoglycans and type 2 collagen deposition were negatively affected in immature menisci. We observed glycosaminoglycan degradation in mature menisci and cartilage degeneration, specifically in immature cartilage of the femoral condyle. In conclusion, compared with mature rabbits, immature rabbits showed more robust healing response after untreated meniscal tears. Vascularization contributed to the recellularization after meniscal tears in immature menisci. Meniscal injury fundamentally alters extracellular matrix deposition.

Purpose To compare outcomes after arthroscopy in FAIS patients with preoperative asymptomatic gluteal tendinosis (GT) to a control group with no gluteal tendinosis. Methods A retrospective analysis was performed using data from FAIS patients who had arthroscopy between 2016 and 2018. Asymptomatic GT was diagnosed using hip MRI without clinical symptoms. Patients with asymptomatic GT were 1:1 propensity-score matched to patients without GT. Patient-reported outcomes (HOS-ADL, iHOT-12, mHHS), VAS scores, radiographic measures, performed procedures, complications, and revision surgery were compared and analyzed for both groups.Results A total of 105 asymptomatic GT hips and 105 hips without GT were found using propensity-score matching. When compared to preoperative levels, both groups demonstrated significant improvements in PROs and VAS scores at the final follow-up. Besides, there were no significant differences in preoperative scores, final outcome scores, or score improvements between the groups. Patients in the GT group were significantly less likely to achieve the MCID for the VAS score (72.4% vs 83.8%, p = 0.045). However, there were no other differences in the rate of meeting the PASS and MCID between the study and the control groups. Conclusion It was demonstrated in this study that FAIS patients with asymptomatic gluteal tendinosis can expect to experience similar good short-term patients-reported outcomes as compared with patients without gluteal tendinosis. Level of evidence III.