In this study, we demonstrated that the femoral condyle was shifted laterally by an average of 5.99 mm relative to its position where the implant with the medial pivot design was tentatively placed in normal alignment. Interestingly, the lateral shift was significantly correlated with the MPTA (r = − 0.553). According to Bellemans et al., approximately 20% of young adult volunteers have constitutional varus knees with a natural mechanical alignment of 3° varus or more [6]. These observations suggest that the lateral shift of the femoral condyle will be greater when patients with constitutional varus alignment receive TKA with the medial pivot design. The lateral shift of the femoral condyle alters the coronal positional relation between the femur and tibia, which may affect balance of ligaments including MCL, PCL, LCL and ITT. Imbalance of those ligaments may cause inappropriate tension of ligaments during knee motion and lead to poor clinical outcomes.
As most patients receiving TKA, except for a bi-cruciate ligament-retaining type, lack ACL function, the tibial insert has a medial concave design to increase stability and allows medial pivot motion. However, this study shows that the medial concave of the insert restricts the position of the femoral component and shifts laterally after TKA. Those are concerns in TKA with the medial pivot design. Clinical studies comparing flat vs. concave inserts have been made and reported [20, 21]. Uvehammer et al. demonstrated that the Hospital for Special Surgery knee score and the patients’ opinion based on their preoperative expectations were not very different [20]. On the other hand, a two-year matched pair cohort study demonstrated that the clinical outcomes of the medial pivot design used in CR-TKA were more favorable than those of the flat surface design [21]. It is difficult to draw conclusions about which is better, but the absence of significant differences in clinical scores shown by Uvehammer et al. suggests that even the flat surface allows medial pivot motion when appropriate ligament balances are achieved.
The current study clearly showed that the lateral shift of the femoral condyle was associated with the MPTA in simulations of TKA from the young Japanese knee radiographs. To maintain the normal positional relation between the femur and tibia after TKA, it is important to consider pre-arthritic alignment of each patient [22,23,24,25]. To date, studies have reported anatomical variations between the femur and tibia in young adult knees [10,11,12, 14,15,16,17,18, 26]. In a report including 23 Japanese and 47 Caucasian healthy young subjects, Japanese subjects had a significantly greater varus alignment than Caucasians, while women exhibited a more valgus alignment than men [16]. Racial and gender differences in knee-joint obliquity also were reported by Tang et al. Compared with Caucasian subjects, Chinese subjects had significantly larger medial inclination of the knee joint (knee-joint obliquity) and female Chinese subjects had significantly more varus alignment than female Caucasians [18]. Furthermore, Hirschmann et al., using 3D-reconstructed CT scans, demonstrated that there was variability of knee phenotypes in young non-osteoarthritic knees. They showed that in males, the most frequent combination (knee phenotype) was a neutral phenotype in the femur and a neutral phenotype in the tibia, while in females, it was a neutral femoral phenotype and a valgus tibial phenotype [26]. Taken together, these observations suggest that there are racial and gender differences in normal knee alignment and a more individualized approach will be necessary to achieve better clinical outcomes after TKA.
In terms of coronal lower limb alignment in Japanese normal knees, Nakano et al. showed that the femoral condylar orientation and tibial plateau inclination in young and middle-aged (15–54 years-old) participants were greater in males than females [27]. For male participants in their study, the tibial plateau inclination was 85.1 ± 2.4° in the young and 85.6 ± 2.1° in the middle-aged populations. These angles seem to be smaller than the measurements for similar aged populations by Hirschmann et al. who showed that the tibial mechanical angle was 86.7 ± 2.3° for males and 88.0 ± 2.4° for females [26]. In the current study, the estimated lateral shift of the femoral condyle was greater in males than in females. Although there was no significant difference in the MPTA between males and females (Table 1), small component sizes (group 1) were used in females alone and large sizes (group 3) were used in males alone. This may affect differences in the lateral shift of the femoral condyle between males and females. Taking these findings into account, when planning TKA for Japanese patients, individual tibial plateau inclination should be considered to reconstruct pre-arthritic alignment especially for male patients.
In this study, we showed that the lateral shift of the femoral condyle in the coronal plane would occur after TKA using the medial pivot design. Although the data were not shown, the similar shift would occur in the axial plane as an asymmetric shape and femorotibial joint line with an oblique 3o is also incorporated in the axial plane of the implant design (Fig. 1). This may also cause inappropriate tension of ligaments including MCL, PCL, LCL and ITT in flexion.
Mediolateral femoral component position also affects patellar shift and femoral roll-back. Studies using fresh cadaver specimens demonstrated that following mediolateral translation of the femoral component, the patella was significantly shifted and tilted in the same directions, while retropatellar pressure was not significantly altered [28]. More than eight years of follow-up of TKA by van de Groes et al. showed that a medialization of ≥ 5 mm resulted in a significantly lower anterior knee pain score [29]. These results imply that a lateral shift of the femoral condyle increases the risk of patellofemoral problems such as anterior knee pain in the long-term.
This study has some limitations. First, the sample size was relatively small, and the subjects included patients with meniscus (24 cases) or ligament (64 cases) injuries whose knee alignment may not have been completely normal. Second, the lateral shift of the femoral component was measured on radiographs using 2D-templates of only one knee prosthesis (the FINE total knee) and not compared with other prostheses. As the estimated lateral shift is affected by implant size variations, it will change when other prostheses are used in simulations. Similar studies using 3D-templates are desirable to better understand the positional relation between the femur and tibia. The present data clearly showed that a lateral shift of the femoral condyle would occur after mechanically aligned TKA with the medial pivot design, which may affect ligament balance; however, biomechanical analyses were not undertaken in this study. Future studies using computational simulation or cadaver specimens are required to evaluate ligament balance or knee kinematics.