The goal of this study was to investigate the correlation between various anatomical parameters and the extent of bone resection in TKA when MA and conventional instruments are used. The safety and reliability of the MA technique has been proven for decades [1, 11], so it is considered the “gold standard” for TKA [12, 13]. Regrettably, MA cannot fully take into account the patient's natural lower extremity alignment and joint line orientation. In fact, significant variation in the overall coronal limb alignment and femoral and tibial joint lines has been proven to exist in osteoarthritic knees . Therefore, we speculate that the interaction of the above two factors leads to inaccurate primary bone resections when TKA is performed using conventional instruments.
Based on the classic MA philosophy, the distal femoral and proximal tibial resection should be perpendicular to the mechanical axis of the femur and tibia, respectively. The resection thickness should be equal to the corresponding component thickness to guarantee proper soft tissue tension in the full range of motion without extra soft tissue release [16, 17]. In most patients, however, neither the femoral condyles nor the tibial plateaus are perpendicular to the mechanical axis. As a consequence, the resection thicknesses of the femoral condyles is not always the same, as is the case with the tibial plateau .
In femoral bone preparation, when a conventional instrument is used, a valgus alignment jig, together with an intramedullary rod, is usually used to determine the orientation of the distal femoral cut. Theoretically, the distal femoral resection thickness is determined using an adjustable jig, but the actual resection thickness of each condyle is determined based on which condyle contacts the jig first. In most cases, the medial part of the jig contacts the articular surface first; consequently, the resection thickness of the medial femoral condyle is usually equal to the desired thickness, while the resection thickness of the lateral femoral condyle is usually less than that of the medial condyle. On the other hand, the resection thickness of the medial tibial plateau is usually less than that of the lateral plateau, especially in most knee OA cases with neutral or varus alignment. Therefore, the amount of bone removed is generally less than that to be replaced with components.
Although the discrepancy in medial and lateral EGs could be corrected with osteophyte removal or ligament release, a relatively narrower EG necessitates additional distal femur cut to accommodate the implants, and a wider EG indicates undesirable bone loss, a thicker polyethylene insert or distal femoral augments might be required to achieve extension stability and appropriate joint line levels. The reason for this may be a combination of MA operative principles, high variation in joint line orientation, constitutional varus or valgus alignment, and possible surgical technical error.
Researchers have begun to consider the correlation between anatomical alignment parameters of the lower extremities and different TKA alignment techniques. In one study , Hirschmann et al. combined individual’s HKA, femoral mechanical angle (supplementary angle of the mLDFA) and tibial mechanical angle (same as MPTA) to divide the knee joint into a group of “functional knee phenotypes.” These phenotypes were matched with the target of different TKA alignment techniques, and it was found that only 5.6% of males and 3.6% of females met the MA alignment target. In another study , Lin et al. investigated the relative parameters of lower limb alignment in relation to one another in a young non-osteoarthritic population, and all the subjects were categorized based on 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 alignments in Hirschmann’s and Lin’s studies were defined as “-1.5°<HKA<1.5°” and “-3°<HKA<3°”, respectively, rather than HKA=0°.
To enhance the clinical relevance of the current correlational analysis, all subjects included in our study were patients diagnosed with knee OA, and all patients were divided into the valgus or varus knee groups based on whether their HKA was greater than 0°. Through the measurement of alignment parameters, all varus knees were found to have MPTAs less than 90°, and all valgus knees had mLDFAs less than 90°. These findings are consistent with those of previously published literature, and the major contributors to valgus knee and varus knee are valgus of the distal femur and varus of the proximal tibia, respectively.
In our case series, patients in the varus knee group were divided into 2 subgroups based on whether their mLDFA was less than 90°, while those in the valgus knee group were similarly divided with reference to the MPTA. Usually, the coronal deformities in subgroups 2 and 4 are more serious than those in the other two subgroups because both the femoral and tibial bony variations contribute to the coronal deformity. Lateral bowing of the femoral shaft may be an important factor leading to the relatively larger mLDFA in subgroup 2[22, 23]. According to the requirements of the MA technique, bone resection must be perpendicular to the femoral and tibial mechanical axes. Therefore, when determining the level of the bone resection, the lateral condyle and lateral tibial plateau should be used as references in varus knees in subgroup 2. Consequently, an appropriate gap can be easily achieved in the lateral compartment, but extensive ligament release is inevitable in the medial compartment because of the reduced extent of bone removal and intrinsically tight medial collateral ligament . A similar situation occurs in valgus knees in subgroup 4, for which the bone resection levels are determined by the medial condyle and medial tibial plateau. Extensive soft tissue release or even constrained prostheses can be used to achieve acceptable mediolateral balance.
To the best of our knowledge, no prior study has focused on the relationship between joint line orientation and the extent of bone resection performed. One study found a significant correlation between the leg axis and the optimal tibial resection thickness, and the optimal resection thickness in valgus knees was significantly less than that in neutral or varus knees; however, no joint line orientation parameters were considered. Inconsistent with the results of the above study, the amount of bone resection was not related to the HKA in this study. The reason for this difference may be because joint line orientation parameters and simulated resection data for only subgroups 1 and 3 were included in this correlational analysis.
Patients in subgroup 1 had varus knees with valgus femurs and accounted for the majority of all included patients (61.3%). Correlational analysis showed that both MPTA and mLDFA were positively correlated with the extent of bone resection. The results of the subsequent regression analysis showed that mLDFA, rather than MPTA, can have a significant effect on the amount of bone resection, with regression coefficients of 0.556 and 0.098, respectively. This was truly an unexpected finding. The results of the regression analysis revealed that for every 2° increase in mLDFA, the amount of bone resection increases by approximately 1 mm. Assuming an average varus joint line inclination of 3°, when the mLDFA approaches 90°, the EG increases by 1.5 mm, and when the mLDFA is 85°, the EG decreases by 1.5 mm. Considering MPTA, although it is correlated with bone resection amount to a certain degree, its impact is so weak that it can be ignored.
Patients in subgroup 3 had valgus knees with varus tibias and accounted for the majority of the valgus knees. Only MPTA has been shown to be related to the extent of bone resection. In contrast with mLDFA in subgroup 1, MPTA had a weaker influence on the bone resection amount, and both the regression coefficient and R2 were smaller than those of mLDFA in subgroup 1. The small number of patients in subgroup 3 might contribute to this observation.
The accuracy of the simulated bone cut was verified by intraoperative caliper bone resection in this study. Although only 10 patients were enrolled for intraoperative verification in this study, each case had 4 numerical variables: medial femoral condyle, lateral femoral condyle, medial tibial plateau and lateral tibial plateau. Therefore, a total of 40 paired measurements were tested for intraoperative verification, meaning that the power for distinguishing a difference of 0.5 in ICC exceeds 90% . The retrospective medical record review did show that the actual bone resection adjustment during surgery was correlated with the corresponding joint line parameters. Although data visualization can provide useful information, we found that not all bone resection adjustments occurred in subgroups 1 and 3. This result revealed that the degree of operative error 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. There may be some selection bias, as we excluded some patients due to rotation of the lower extremities in full-length weight-bearing radiographs, and abnormal rotation of the lower extremities results in inaccurate measurement of the MPTA . In addition, because of the limited sample size, it may be that not all patients diagnosed with knee OA can be classified into 4 subgroups. One study reported a valgus knee with an mLDFA as large as 92° . Second, some special types of extra-articular deformities, such as tibial or femoral shaft bowing with progression of knee OA, were not used as alignment parameters. Although tibial or femoral bowing could dramatically change the joint line orientation, 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 were used for radiographic measurement rather than 3D CT, and some studies have proven that the latter has higher accuracy . However, the cost effectiveness of radiographs is much greater. As a routine before TKA, some meaningful findings in preoperative radiographs would be more helpful for guiding clinical practice.