Recently, there has been a rising debate on MA or KA techniques. The data in the literature was inconsistent, which makes surgeons confused about making decisions. This may be explained that we should choose alignment techniques individually. The most important finding of the present study was that patients with preoperative varus deformity more than 10 degrees were more suitable for KA-TKA, while patients with BMI more than 30 kg/m^2 would benefit more from MA-TKA.
There have been numerous studies on the comparison between the MA-TKA and the KA-TKA. Several studies have suggested a substantial portion of the normal population didn't have a neutral mechanical alignment. Bellemans et al. reported 32% of men and 17% of women had varus knees with a natural mechanical axis of 3° varus or more [11]. Nam et al. suggested that only 31 % of knees had both a neutral mechanical alignment and the absence of joint line obliquity [12]. Thus, several scholars hold the view that KA-TKA may restore normal knee kinematics. Faschingbauer et al. indicated KA-TKA could achieve similar kinematics of the patellofemoral joint relative to the normal state [13]. Blakeney et al. suggested KA-TKA reproduced more closely normal gait of healthy controls compared to MA-TKA due to the restoration of the individual's knee kinematics and ligament tension in KA-TKA [14]. Ishikawa et al. revealed more significant femoral rollback and more external rotation of the femoral component in KA-TKA than Ma-KTA by using a musculoskeletal computer simulation [15].
Although the studies above provided evidence in favor of KA-TKA, the present study found the clinical outcomes were comparable between the two groups, which were similar to data in the recent literature. Luo et al. conducted a meta-analysis including nine randomized controlled trials with 1170 KA-TKAs and 1171 MA-TKAs. This meta-analysis suggested the KSS, knee injury, and osteoarthritis outcome score (KOOS), EuroQoL 5-dimension questionnaire (ED-5D), ROM, and complications were similar for KA-TKA and MA-TKA. In a recent study by McEwen et al. [16], they prospectively enrolled 41 patients who were scheduled to undergo simultaneous TKAs. They randomized one side using MA and the other side using KA. With a minimum of 2-year followup, although more patients preferred their KA knees, they suggested no difference in ROM or functional scores between groups. Additionally, there is a lack of data on comparisons the long-term clinical outcomes. Ishikawa et al. [15] suggested KA-TKA increased patellofemoral and tibiofemoral contact stresses by using finite element analysis, which may impair long-term outcomes. Berend et al. reviewed 3152 TKAs for osteoarthritis with a mean 5-year followup and indicated that varus tibial component alignment more than 3.0 degrees had a 17-fold risk of subsequent tibial component aseptic loosening [17]. Another study with the mean followup of 7.6 years found failure was least likely to occur in patients with a neutral alignment of both the tibial and the femoral component [18].
Several studies have reported the survival rate of TKA was lower in obese patients than nonobese patients [19–21]. However, the interaction between obesity and alignment on the survival of TKA remains unknown. The present study revealed that obese patients might be more suitable for MA-TKA than KA-TKA. Interestingly, our results were similar to a previous study by Berend et al. [17]. They found BMI alone was not associated with failure, but BMI more than 33.7 kg/m^2 combined with varus tibial component more than 3 degrees had a 168-fold risk of subsequent failure. The possible reason may be that overloading of the knee occurs in patients with high BMI and varus axis, resulting in more significant impact loading across the tibial component, therefore, caused patients' discomfort and might increase component loosening and lower implant survival rate. Additionally, we found patients with preoperative HKA angle more than 10 degrees varus may benefit more from KA-TKA than MA-TKA. The reason remains unclear. It may be explained that patients with severe deformity frequently possessed critical contracture of knees. Thus, these patients need more soft tissue releases that may impair patients' satisfaction.
Several limitations should be noted. First, this is a single-institution study, and thus its findings need external validation. Additionally, long-term outcomes were unknown. Second, only a single implant manufacturer was used in the present study, which may limit the generalizability of the findings. Third, the sample size may be inadequate for conducting some statistical analyses, and the possibility of a type-II error exists. Fourth, several parameters were not considered in the present study. Lastly, there is potential variability in patient selections among surgeons. Given the lack of evidence and literature on who to undergo a MA-TKA or KA-TKA, there is no standardized protocol, and thus surgeon's preference may be a factor, which may introduce bias.