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

## TL;DR

Chronic ACL injuries lead to fibrotic changes and matrix degradation, marked by increased MMP-13 and TGF-β1, which could influence surgical timing and treatment strategies.

## Contribution

The study identifies a shift in ACL remnant biology from acute to chronic injury, linking MMP-13 and TGF-β1 to fibrotic and matrix-degradative states.

## Key findings

- Chronic ACL remnants show disorganized ECM and higher MMP-13 and TGF-β1 expression.
- Higher marker expression correlates with worse clinical outcomes and residual pivot-shift positivity.
- Bioinformatic analysis links markers to ECM-receptor interaction and fibrosis pathways.

## Abstract

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 (<3 months) and chronic (>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.