Regional alterations in bone microstructure, muscle, and fat of the knee following anterior cruciate ligament rupture: a laboratory study in rabbit model

To investigate the temporal alterations in bone, muscle, and fat tissues in an anterior cruciate ligament transection (ACLT) rabbit model. A total of 24 male New Zealand White rabbits underwent ACLT in the right knee. The animals were euthanized at 3, 6, 9, and 12 weeks post-ACLT (n = 6 per time point), and bilateral knee joints were harvested. An additional 6 healthy rabbits served as a normal control group. Magnetic Resonance Imaging (MRI) was performed to quantify the signal-to-noise ratio for assessing thigh fat and muscle content. Micro-computed tomography (Micro-CT) was used to evaluate bone mineral density (BMD) and trabecular microstructure of the knee joints. Histological evaluation with hematoxylin and eosin (HE) staining was conducted to examine morphological changes in the medial/lateral femoral condyles (MFC/LFC) and medial/lateral tibial plateaus (MTP/LTP). At 3 weeks post-surgery, the knee with ACLT showed an increase in thigh fat content of 1.6 ± 1.2% and a decrease in muscle content of 2.0 ± 1.2% compared to the contralateral side ( p < 0.05). At 6, 9, and 12 weeks post-ACLT, the BMD of the knee with ACLT was significantly lower than that of the contralateral ( p < 0.05), with the most pronounced reduction observed in the LFC. The BMD of the LFC on the affected side was lower than that on the contralateral side by 97.53 ± 50.83, 200.53 ± 52.4, and 108.0 ± 67.42 mg/cc at 6, 9, and 12 weeks post-ACLT, respectively ( p = 0.017, < 0.001, and 0.048). Furthermore, compared to the contralateral side, the ACLT side exhibited increased trabecular spacing and decreased trabecular thickness at 3 weeks post-ACLT, whereas at 9 and 12 weeks, it showed decreased trabecular spacing and increased trabecular thickness. HE staining further revealed that the number of trabeculae was significantly lower at 9 and 12 weeks than at 3 and 6 weeks post-ACLT. After ACLT in rabbits, the knee showed disuse atrophy at 3 weeks post-ACLT, with trabecular thinning, increased spacing, muscle loss, and fat accumulation. At 6 weeks post-ACLT, BMD decreased significantly, most notably in the LFC, and this reduction persisted through 12 weeks. At 9 and 12 weeks post-ACLT, a remodeling response was observed, characterized by trabecular thickening and reduced spacing.