Alteration of Subchondral Bone Density Distribution Across the Knee Joint Before and After Anterior Cruciate Ligament Reconstruction.

BACKGROUND: Anterior cruciate ligament (ACL) injury alters knee kinematics and contact mechanics, increasing posteromedial (PM) loading and the risk of knee osteoarthritis. Because subchondral bone density reflects long-term joint stress, its distribution may indicate chronic loading changes. Previous studies have shown higher proportions of high-density area (HDA%) in the PM tibial subchondral bone of ACL-injured knees than in healthy ones. However, it remains unclear whether ACL reconstruction restores normal tibial subchondral bone density distribution. PURPOSE/HYPOTHESIS: The purpose of this study was to evaluate changes in tibial subchondral bone density after ACL reconstruction. It was hypothesized that reconstruction would reduce the elevated PM HDA% observed after injury. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: Seventeen patients (7 men, 10 women) who underwent unilateral double-bundle ACL reconstruction were included. Computed tomography-osteoabsorptiometry (CT-OAM) was used to quantify the proportion of the proximal tibial surface occupied by HDA (top 20% of Hounsfield units) within 6 sagittal subregions: anteromedial (AM), centromedial (CM), PM, anterolateral, centrolateral, and posterolateral. The medial ratio represented the percentage of total tibial HDA in the medial compartment. Clinical assessments included side-to-side laxity, pivot-shift test, International Knee Documentation Committee score, Lysholm score, and Knee injury and Osteoarthritis Outcome Score (KOOS). RESULTS: Anterior laxity improved from a mean of 3.3 mm (95% CI, 2.5 to 4.0 mm) to 0.4 mm (95% CI, -0.8 to 1.6 mm) postoperatively, and all pivot-shift tests were negative (P < .05). Clinical scores improved significantly. CT-OAM revealed decreased PM HDA% and increased CM HDA% (Holm-adjusted P = .003 and P = .011, respectively). The AM increase was nonsignificant after correction (P = .055). No significant lateral changes were noted. The medial ratio rose from 59.2% to 63.7% (P = .032) and correlated negatively with KOOS Sports (r = -0.561; P = .037). CONCLUSION: ACL reconstruction reduces PM overloading by decreasing PM HDA%, but the increased medial ratio suggests a shift of loading toward the medial compartment, which may relate to poorer sport-specific function.