Chronic ACL Injury Drives a Fibrotic and Matrix-Degradative Shift: A Multi-Level Analysis of MMP-13 and TGF-β1

Background and Objectives: The biological state of anterior cruciate ligament (ACL) remnant tissue may influence postoperative healing, yet the molecular changes associated with injury chronicity remain poorly defined. This study evaluated MMP-13 and TGF-β1 expression in human ACL remnants to characterize their regenerative or fibrotic potential. Materials and Methods: ACL remnants from acute (6 months) injuries were analyzed using histology, immunohistochemistry, and QuPath-based digital quantification. Clinical outcomes were correlated with marker expression. Protein–protein interaction and KEGG enrichment analyses were performed to identify extracellular matrix (ECM)-related pathways associated with MMP-13 and TGF-β1. Results: Chronic ACL remnants exhibited disorganized ECM structure with significantly higher MMP-13 and TGF-β1 expression across all digital metrics, including DAB-positive area, cell density, optical density, and H-score (p < 0.01). Higher expression of both markers correlated with lower IKDC and Lysholm scores and greater residual pivot-shift positivity. Bioinformatic analysis identified 39 shared proteins enriched in ECM-receptor interaction, TGF-β signaling, and fibrosis-related pathways, aligning with the degenerative phenotype observed in chronic tissue. Conclusions: ACL remnant biology evolves from a reparative profile in acute injuries to a fibrotic, matrix-degradative state in chronic injuries. MMP-13 and TGF-β1 serve as indicators of remnant quality and may help guide timing of surgery and future biologic strategies aimed at improving ACL reconstruction outcomes.