Synovial Fluid as a Window into Early Cartilage Remodeling After Autologous Matrix-Induced Chondrogenesis

Background and Objectives: Autologous matrix-induced chondrogenesis (AMIC) is an established technique for the treatment of focal cartilage defects of the knee, with well-documented clinical outcomes. However, the biological processes underlying early postoperative cartilage remodeling remain poorly characterized, and the role of synovial fluid biomarkers in this setting is not well defined. This study aimed to assess short-term changes in selected synovial fluid and serum biomarkers of cartilage turnover after AMIC and to examine their associations with clinical outcomes. Materials and Methods: Fifteen patients undergoing AMIC for focal knee chondral or osteochondral defects were prospectively enrolled. Synovial fluid and serum samples were collected intraoperatively and at 6 and 12 weeks postoperatively. Concentrations of matrix metalloproteinase-3 (MMP-3), tissue inhibitor of metalloproteinases-2 (TIMP-2), cartilage oligomeric matrix protein (COMP), and procollagen type II C-terminal propeptide (PIICP) were measured using multiplex flow luminescence immunoassay. Clinical outcomes were evaluated using the International Knee Documentation Committee (IKDC) and Lysholm scores preoperatively and at 6 and 12 months. Results: Both IKDC and Lysholm scores improved significantly during follow-up. Absolute biomarker concentrations in synovial fluid were low and did not change significantly over time. Nevertheless, higher MMP-3 levels, higher COMP concentrations, and a higher MMP-3/TIMP-2 ratio were associated with poorer clinical improvement. Correlations between synovial fluid and serum biomarker levels were generally weak. Total synovial fluid protein increased postoperatively but did not account for the low biomarker concentrations. Conclusions: Early biomarker profiles after AMIC were characterized by low absolute concentrations without significant temporal changes. However, associations of COMP, MMP-3, and the MMP-3/TIMP-2 ratio with clinical outcomes suggest that relative biomarker patterns may reflect early intra-articular remodeling. Synovial fluid analysis may provide more informative insight into local joint biology than serum measurements in this setting. These findings should be interpreted cautiously and require confirmation in larger cohorts.