Changes in Muscle Mass and Bone Density and Their Relationship After Total Knee Arthroplasty

<b>Purpose:</b> To investigate the effects of total knee arthroplasty (TKA) on muscle mass and bone density in end-stage knee osteoarthritis (OA). <b>Methods:</b> This prospective study was conducted on 111 patients with Kellgren-Lawrence grade 4 knee OA who underwent TKA after failing to respond to conservative treatment for more than 3 months at a single institution from June 2022 to May 2023. Appendicular lean mass index (ALMI) and bone mineral density (BMD) were measured using dual-energy X-ray absorptiometry before and every 6 months after surgery. The average follow-up period was 15.5 ± 2.31 months (range, 11.6-24 months). <b>Results:</b> During the follow-up period after TKA, the ALMI increased relatively continuously and consistently. The BMD of the L-spine and proximal femur did not change significantly until 12 months after TKA surgery but began to increase steeply after 12 months and slowed down after 18 months. The increase in muscle mass showed a significant positive correlation with the increase in BMD. <b>Conclusions:</b> Muscle mass gradually increased after TKA for end-stage knee OA, whereas bone density also increased but not until 12 months after surgery. The significant positive correlation between the increase in muscle mass and bone density suggests that the increase in muscle mass after TKA may be one of the causes of the increase in bone density.