Long-term Effect of a Single Subcritical Knee Injury: Increasing the Risk of Anterior Cruciate Ligament Rupture and Osteoarthritis

Background: Rupture of the anterior cruciate ligament (ACL) is a well-known risk factor for the development of posttraumatic osteoarthritis (PTOA), but patients with the "same injury" can have vastly different trajectories for the onset and progression of disease. Minor subcritical injuries preceding the critical injury event may drive this disparity through preexisting tissue pathologies and sensory changes. Purpose: To investigate the role of subcritical injury on ACL rupture risk and PTOA through the evaluation of pain behaviors, joint mechanics, and tissue structural change in a mouse model of knee injury. Study Design: Controlled laboratory study. Methods: Ten-week-old male C57BL/6J mice were allocated to naive control and subcritical knee injury groups. Injury was induced by a single mechanical compression to the right hindlimb, and mice were evaluated using joint histopathology, anteroposterior joint biomechanics, pain behaviors (mechanical allodynia and hindlimb weightbearing), and isolated ACL tensile testing to failure at 1, 2, 4, or 8 weeks after injury. Results: Subcritical knee injury produced focal osteochondral lesions in the patellofemoral and lateral tibiofemoral compartments with no resolution for the duration of the study (8 weeks). These lesions were characterized by focal loss of proteoglycan staining, cartilage structural change, chondrocyte pathology, microcracks, and osteocyte cell loss. Injury also resulted in the rapid onset of allodynia (at 1 week), which persisted over time and reduced ACL failure load (P = .006; mean +/- SD, 7.91 +/- 2.01 N vs 9.37 +/- 1.01 N in naive controls at 8 weeks after injury), accompanied by evidence of ACL remodeling at the femoral enthesis. Conclusion: The present study in mice establishes a direct effect of a single subcritical knee injury on the development of specific joint tissue pathologies (osteochondral lesions and progressive weakening of the ACL) and allodynic sensitization. These findings demonstrate a predisposition for secondary critical injuries (eg, ACL rupture) and an increased risk of PTOA onset and progression (structurally and symptomatically).