Long-term observation of marrow adipose tissue and trabecular bone in the rat proximal tibial epiphysis after anterior cruciate ligament reconstruction: effects of immobilization and non-weightbearing.

Anterior cruciate ligament (ACL) injury and subsequent reconstruction induce marrow adipose tissue (MAT) accumulation accompanied by bone loss. Short-term immobilization or non-weightbearing after ACL reconstruction further promotes MAT accumulation. However, it is unclear if combining immobilization and non-weightbearing synergistically promotes MAT accumulation. Additionally, it is unknown whether MAT increase induced by immobilization or non-weightbearing can be reversed through remobilization or reloading. We aimed to address these questions. ACL-reconstructed rats were divided into four groups: no intervention, immobilization, non-weightbearing, or immobilization plus non-weightbearing. Immobilization and non-weightbearing were applied for 2 weeks, after which all rats were allowed to move unrestricted. Intact rats were used as controls. The marrow adiposity and trabecular bone in the proximal tibia were histologically assessed at 2-, 4-, and 12-weeks post-surgery. ACL reconstruction induced MAT accumulation and trabecular bone loss accompanied by increased osteoclastogenesis. Two weeks of immobilization and non-weightbearing after ACL reconstruction individually promoted MAT accumulation, but the combined use of these interventions had a similar impact on MAT accumulation as either of each intervention. Importantly, the increased MAT induced by immobilization or non-weightbearing did not reverse even after remobilization or reloading. Neither immobilization, non-weightbearing, nor both conditions combined after ACL reconstruction further decreased trabecular bone compared to no intervention. These findings suggest no synergistic effect of immobilization and non-weightbearing on MAT accumulation, and MAT accumulation induced by 2 weeks of both immobilization or non-weightbearing did not decrease even after at least 10 weeks of remobilization or reloading. MAT accumulation due to both immobilization and non-weightbearing did not have negative effects on trabecular bone.