Strict Early Knee Stabilization Favors Spontaneous Anterior Cruciate Ligament Healing: Implications for Non-Operative Management from a Murine Rupture Model

Purpose: Anterior cruciate ligament (ACL) injury is a common sports-related injury traditionally considered to have limited spontaneous healing potential; however, accumulating evidence suggests that the ACL can heal under appropriate mechanical conditions. This study aimed to determine how different degrees of anterior tibial translation (ATT) restraint affect ACL healing and tissue maturation. Methods: Male and female C57BL/6 mice (n = 142) underwent non-invasive ACL rupture and were randomly assigned to controlled ATT (CATT) models with strict (Tight-CATT) or mild (Loose-CATT) restraint, or to an uncontrolled ATT group (ACL-R). Knees were assessed immediately and at 4 and 8 weeks after injury. Outcomes included ACL healing rate, collagen organization, COL1A1 localization, knee stability, and failure load. Results: Tight-CATT initially restrained ATT to near-intact levels, but this restraint loosened by 4 weeks. After adjustment for sex and time point, the odds of ACL healing were significantly higher in the Tight-CATT group than in the Loose-CATT group (adjusted OR, 5.06; 95% CI, 1.29-19.78; p = 0.020). No ACL-R knees healed. Collagen fiber coherency and failure load of healed ACLs did not differ between the Tight- and Loose-CATT groups and remained below intact levels. Conclusion: Milder ATT restraint during the acute phase was associated with a lower ACL healing rate and did not enhance tissue maturation or mechanical strength of the healed ACL. These findings suggest that adequate ATT restraint during the acute phase is important for successful ACL healing, whereas progressive mechanical loading after healing may be required to optimize ligament function.