In contrast with patients who receive primary total hip arthroplasty, approximately 20% of those who receive TKA are not satisfied with their outcomes (11, 12). Although its underlying reason is not clear, alignment technique of TKA is beginning to be considered to be related to the patient's postoperative outcomes and dissatisfaction (13, 14). With the application of more accurate alignment assistant instruments, the conventional MA technique has begun to be challenged in recent years (7, 15, 16), because MA cannot fully respect the patient's natural alignment of lower extremities and joint line orientation, and a significant variation in overall coronal limb alignment and in femoral and tibial joint lines had been proved to exist in osteoarthritic knees(17).
The safety and reliability of MA technique has been proved for decades (1, 18), so it is considered to be the “gold standard” for TKA (19, 20). This opinion is particularly important for developing countries. Taking China for example,because more accurate alignment-assistant instruments, such as navigation, patient-specific instrumentation (PSI) or robot assistant technology, can only be available in few hospitals(21, 22). In order to reduce the disastrous consequences caused by poor alignment after TKA, a more feasible and more reliable alignment method should be addressed when using conventional instruments for TKA. Therefore, the goal of this study was to investigate the correlation between the orientation of the joint line and the quantities of bone resection in TKA when MA and conventional instrument were used.
With respect to the classic MA philosophy, the distal femoral and proximal tibial resection should be perpendicular to the mechanical axis of femur and tibia, respectively. The resection thickness should be equal to corresponding components thickness to guarantee proper soft tissue tension in full range of motion without extra soft tissue release(23, 24). In most of patients, however, neither femoral condyles nor tibial plateau are perpendicular to the mechanical axis. As a consequence, the resection thicknesses of femoral condyles were not always the same, so was the tibial plateau(25).
In the femoral bone preparation, when a conventional instrument was used, a valgus alignment jig, together with an intramedullary rod, was usually applied to determine the orientation of the distal femoral cut. Theoretically, the distal femoral resection thickness was determined by adjustable jig, but actual resection thickness of each condyle was determined by which condyle contact with the jig first. In most of cases, the medial part of jig contacted the articular surface first, as a consequence, the resection depth of medial femoral condyle was usually equal to the desired thickness, while the resection thickness of lateral femoral condyle was usually less than that of medial condyle. On the other side, the resection thickness of medial tibial plateau was usually less than that of lateral plateau, especially in most of knee joint arthrosis with neutral or varus alignment. So, the amount of bone removed generally is less than that to be replaced by the components.
Although the discrepancy of medial and lateral EG could be corrected by osteophyte removal or ligament release, a gross narrower or wider EG would be caused by too little or too much bone removal. The reason for above clinical scenario is the combination of MA operative principles, surgeon’s individual error and high variability of joint line orientation.
In one study (26), Hirschmann et al. have combined the individual HKA, femoral mechanical angle (supplementary angle of mLDFA) and tibial mechanical angle (same as MPTA) to divide the knee joint into a group of “functional knee phenotypes”. The phenotypes were matched with the target of different TKA alignment techniques, and it was found that only 5.6% of the males and 3.6% of the females met the MA alignment target. In another study (27), Lin et al. investigated relative parameters of the lower limb alignment in relation to each other in a young non-osteoarthritic population, and all subjects were categorized on the basis of HKA, mLDFA and MPTA. In Lin’s study, all subjects could be classified into one of five types of alignment: 2 types of neutral alignment, 2 types of varus alignment and 1 type of valgus alignment. The coronal neutral alignment in Hirschmann’s study and Lin’s study were defined as “-1.5°<HKA < 11.5°” and “-3°<HKA < 3°”, respectively, rather than HKA = 0°.
For the convenience of the evaluation of the correlation between the joint line orientation and the amount of bone resection in extension when MA technique was used, and in order to make the correlation more practicable in clinical situation, all subjects included in our study were patients diagnosed with knee OA, and all patients were divided into valgus knee group and varus knee group based on whether HKA is greater than 0° or not. During the measurement of the alignment parameters, all varus knees were found to have MPTA less than 90°, and all valgus knees had mLDFA less than 90°. This finding is consistent with previous published literatures, the major contributors to valgus knee and varus knee are valgus of the distal femur(28) and varus of the proximal tibia(7), respectively.
In our case series, patients in varus knee group were divided into 2 subgroups based on if individual mLDFA is less than 90° or not, while valgus knees were similarly divided with reference to MPTA. Usually, the coronal deformities in subgroup 2 and subgroup 4 are more serious than ones in other two subgroups, because both the femoral and the tibial bony variations contribute to the coronal deformity. Lateral bowing of femoral shaft may be an important reason leading to the relatively greater mLDFA in subgroup 2(29, 30). According to the MA technique requirement, bone resection should be perpendicular to the femoral and tibial mechanical axis. Therefore, when determining the level of bone resection, lateral condyle and lateral tibial plateau should be used as a reference in varus knees in subgroup 2. As a consequence, appropriate gap could be easily achieved in the lateral compartment, but extensive ligament release was inevitable in the medial compartment, because of the less bone removal and intrinsically tight medial collateral ligament (31). A similar situation would occur in valgus knees in subgroup 4, in which the bone resection levels are determined by medial condyle and medial tibial plateau. Extensive soft tissue release or even constrained prostheses would be used for achieving acceptable mediolateral balance. Hence, only the data of simulated resection in subgroup 1 and subgroup 3 were used for further analysis.
As far as we know, no study focusing on the relationship between the joint line orientation and the amount of bone resection has been performed before. One study found a significant correlation between leg axis and the optimal tibial resection thickness, and the optimal resection thickness in valgus knees were significantly less than that in neutral or varus knees, but no joint line orientation parameters was investigated (8). Inconsistent with the results of the above study, the amount of bone resection was not related to HKA in current study. The reason for this inconsistent result may be due to the fact that only joint line orientation parameters and simulated resection data of subgroup 1 and subgroup 3 were included for correlation analysis.
Patients in subgroup 1 have varus knees with valgus femurs, and account for the majority of all included patients (61.3%). Correlation analysis showed that both MPTA and mLDFA are positively related to the amount of bone resection. The subsequent regression analysis results showed that mLDFA rather than MPTA can have a significant effect on the amount of bone resection, with regression coefficient of 0.556 and 0.098, respectively. This was truly an unexpected finding. The interpretation of the results of regression analysis showed that for every 2° increase in mLDFA, the bone cut amount will increase by about 1 mm. Given joint line inclination of 3° of varus as average value, when mLDFA is approaching 90°, the EG will increase by 1.5 mm, and when mLDFA is 85°, the EG will decrease by 1.5 mm. For MPTA, although there is a certain degree of correlation with bone resection quantity, its impact is so weak that can even be ignored.
Patients in subgroup 3 have valgus knees with varus tibia, and account for the majority of the valgus knees. Only MPTA had been proven to be related with the bone resection amount. In contrast with mLDFA in subgroup 1, MPTA has a weaker influence on the bone resection, both the regression coefficient and R2 are smaller than mLDFA in subgroup 1. The small number of patients in subgroup 3 might be part of the reason.
The effectiveness of simulated bone cut has been verified by intraoperative calipered bone resection in this study. The retrospective medical records queries did show that the actual bone resection adjustment during surgery had a certain correlation with the corresponding joint line parameters. Although data visualization can show us some useful information, we also found that not all bone resection adjustments occurred in subgroup 1 and subgroup3, this phenomenon presented that the operation error still has a significant impact on the accuracy of bone resection.
Our study has several limitations. First, a small number of patients were included in our study. And there may be some selection bias as we excluded some patients due to rotation of lower extremities in full-length weight bearing radiogram (32). In addition, because of the limited sample size, it may be that not all patients diagnosed with knee OA could be classified into 4 subgroups. One study reported that a valgus knee with mLDFA as large as 92° (33). Secondly, some special type of extra-articular deformities, such as tibial or femoral shaft bowing with the progression of knee OA, were not used as alignment parameters. Although tibial or femoral bowing could change the joint line orientation dramatically, such deformities rarely need correction during primary TKA, so our study did not focus on these special extra-articular deformities. Finally, full-length weight bearing radiographs rather than 3D CT were used for radiographic measurement, and some study have proven that the latter has higher accuracy (34). Actually, the cost effectiveness of radiograph is much better. As a routine before TKA, some meaningful finding in preoperative radiographs would be more helpful for guiding clinical practice.