For now, the study targeting at Europeans showed that the weight gained on the knee ranged from 266.7 g to 380 g of patients after TKA, and for some patients, the weight of implants was six times higher than that of removed tissues during the procedure[10, 11]. The large increase of knee joint weight after the procedure brought great challenges to the postoperative rehabilitation of patients. The data indicate that the strength of quadriceps femoris muscles and hamstring muscles on the affected side of patients was significantly lower than that on the healthy side in the short-term after TKA. In addition ,deficits in muscle torque and power and in the extensor muscle cross-sectional area were present 10 months after knee replacement[12]. Poor muscle strength may not support the unexplained weight gained, which may be one of the reasons for active activity limitation of the lower extremities of patients after TKA. Secondly, for most patients with KOA, the proprioceptive function of the knee joint had decreased in different levels[13, 14], and it would inevitably cause injury to the joint capsule, muscle tendon and ligament which contain knee joint proprioceptors after the procedure[15]. Therefore, some patients may present pathological gait patterns and have difficulty in performing basic functional tasks and in maintaining balance and postural control, even one year after surgery. While the weight gained on the knee joint after TKA would possibly deepen the impact caused by the proprioceptive change: that the tibia’s forward and backward mobility in the joint was increased due to the removal of cruciate ligaments and the decrease of knee muscles strength, and the weight gained would increase this mobility inertia, which would not only accelerate the fretting wear of joint prosthesis, but also increase the risk of postoperative joint dislocation[16]. Besides, for some patients undergoing patellar resurfacing, the weight change on the femural condylar and patella could cause the change of stress load of patellofemoral joint: the increased femoral weight would directly increase patellofemoral joint shear force, which may be one of the causes of patellar trajectory changes and persistent anterior knee pain[17].
It has crucial meaning to properly reduce the weight of prosthesis components. At present, there are mainly three schemes to reduce the weight of prosthesis components. Firstly, lighter materials with the same size and volume of the knee joint prosthesis should be used. Polyethylene tibial components prosthesis has been applied to clinical practice, which can significantly reduce the weight of the prosthesis on the tibial plateau, but some studies have shown that all-polyethylene prosthesis is poor in hardness and may generate higher stress and micromotions than metal-backed tibial components[18]. Secondly, the volume of the non-load-bearing part of the prosthesis should be reduced. Some scholars assume that the weight of the femoral prosthesis can be greatly reduced by drilling holes on the non-load-bearing surface of the femoral prosthesis without affecting the load-bearing performance of the prosthesis. Sudesh et al.[11] used finite element modelling (FEM) technique to carry out the simulation study on the radius and length of the drill hole in the prosthesis. It has shown that using drills on implants can reduce the implant weight of approximately 25 g. The new type of prosthesis still have better performance under 2000N load. Thirdly, part of the uncemented prosthesis has been applied to clinical practice that can reduce the weight of cement, but the weight of the uncemented prosthesis has not been studied yet. There was no significant difference in the short-term usage life, joint function recovery and complications between the uncemented prosthesis and cemented prosthesis after the procedure, but the long-term follow-up is still needed to evaluate the long-term outcome of the uncemented prosthesis[19].
Our study first involves the new parameter of BMD, related to the weight of the knee joint prosthesis and the weight of removed host tissues when studying the outcomes of TKA. Given that weight is inextricably linked to density, we hypothesized that the lower BMD, the lighter weight of removed host tissues. We test this hypothesis with three different brands of prostheses, and our results show that the weight of removed host tissues is lower for the patients with lower BMD, while the weight change of prosthesis components and the knee joint are not statistically different. Nevertheless, we suggest that patients with lower BMD should choose the lighter prosthesis to replace. The severity of knee osteoarthritis is always negatively correlated with the BMD[20, 21], that is, compared with patients with the normal BMD, patients with lower BMD may experience more pain and have poor strength of muscles and tendons around the affected knee. According to the research, the proximal tibial BMD will reduce after TKA, and joint load capacity will also decrease in the short-term after the procedure, so the unreasonable weight gained on the knee will have a negative impact on the postoperative recovery of patients[22].
This study has some limitations. Firstly, the BMD in our study was synthetically calculated by the hip joint Ward's triangle BMD and lumbar spine L1-L4 BMD, rather than directly calculating the local knee joint BMD on the affected side, and the knee joint BMD on the affected side is always lower than that on the healthy side, so our BMD results may be higher than the real one. Secondly, the prosthesis type included in this study was posterior-stabilized knee prosthesis, which has additional gasket central column and requires osteotomy in the intercondylar fossa compared to posterior cruciate knee prosthesis, so the weight of the knee joint with this type of prosthesis may change more after the procedure, and it needs to be further studied when more prosthetic types are included. In addition, our preliminary studies have shown that the weight of host removals of patients with rheumatoid arthritis is significantly lower than normal patients with KOA. The aim of the study was to provide a standardized TKA data for reference in the future studies. Therefore patients with Rheumatoid arthritis (RA), with severe bone defects, or with special types of prostheses (e.g., prostheses including tibial extension stem) had been excluded in our study.