The impact of different alignment strategies on bone cuts for neutral knee phenotypes in total knee arthroplasty

The main findings of this study were that depending on the alignment chosen, the four either common or exemplary neutral (NEU) knee phenotypes caused either none or partly significant changes in joint line obliquity and offset. In phenotypes 1 and 2, which both represent together over a third of all neutral phenotypes according to the cohort of OA patients assessed, the changes in distal offset were below 4 mm medially and laterally regardless the alignment strategy chosen, which can be considered as an approximate threshold for offset changes with negligible clinical relevance. However, the evidence for this threshold is mostly based on experience and scarce evidence. The only available evidence comes from a systematic review by van Lieshout et al. [22]. Here, the authors showed a correlation between an elevated joint line and a worse clinical outcome. The authors, thus, recommended avoiding an increase in the joint line of more than 4 mm. However, the study considered symmetrical changes in joint line height. In the present study, the changes in joint line height were asymmetrical. Therefore, it is still under debate how much change in the medial and/or lateral joint line heights a knee can tolerate or compensate. Truly personalised alignment concepts such as unrestricted kinematic alignment do not lead to changes in joint line obliquity, but the long-term implant survival of particularly an obliquely aligned femoral component is still not sufficiently investigated. Hence, it needs a safe transition from mechanical to personalised alignment. It is therefore currently a compromise of what the fixation of the TKA can tolerate and what the optimal alignment for the best function is.

In the present study for phenotype 1 NEUHKA0° VARFMA 90° VALTMA90° (Fig. 4) and phenotype 2 NEUHKA0° NEUFMA 93° NEUTMA87° (Fig. 5), whichever alignment strategy is chosen is unlikely to have a relevant effect on clinical outcome because the changes in joint line height are less than 3 mm in each of the compartments. Thus, for phenotypes 3 NEUHKA0° VALFMA 96° NEUTMA87° and especially 4 NEUHKA0° VALFMA 99° VARTMA84°, the choice of alignment strategy could potentially have an impact on clinical outcome by altering the joint line obliquity, respectively, lead to an asymmetric change of the joint height. The alteration of the joint line alignment may therefore also lead to patellofemoral problems as the elevated joint lines change the direction of the load as well as contact forces at the patella [11, 20]. In general, there seems to be a consensus in the orthopaedic surgeon community that not every anatomy is considered healthy or constitutional, respectively, that certain configurations are biologically inferior and, therefore, should not be reproduced. However, as shown in the simulations, the more extreme joint configurations require greater bone resections for the systematic alignment strategies such as MA or AA and therefore involve a greater risk of a detrimental alteration of the flexion axis, the joint line orientation and a possible adverse change in the knee kinematics. The Classification of Coronal Alignment of the Knee (CPAK) classifies knee phenotypes into 9 different types based on their arithmetic HKA and joint line obliquity and recommends which alignment should be used depending on the CPAK type [16]. The CPAK types II, V and VIII correspond to neutral limb alignment. CPAK type V corresponds to phenotype 1 NEUHKA0° VARFMA 90° VALTMA90° and CPAK type II corresponds to phenotype 2 NEUHKA0° NEUFMA 93° NEUTMA87°. The authors’ conclusion to apply MA to CPAK type V and AA to CPAK type II is consistent with the results of the study presented here, as selected strategies do not require a change in the obliquity of the joint line and are therefore more likely to achieve a balanced soft tissue envelope without the need for ligament release.

How oblique a TKA can be placed without increasing wear is still unclear. Radiostereometric analysis demonstrated that varus alignment of the tibial component, but interestingly not the overall alignment of the limb, causes greater migration of the tibial implant [21]. However, other long-term studies contradict this result and see no connection between coronal alignment and implant survival [1]. Perhaps custom-made implants allow more extreme implant positions without increased risk of loosening and without the problem of patellofemoral joint malalignment that can occur with standard knee implants. However, clear evidence of a benefit is still lacking here as well [12, 17].

The fact that the anticipated improvements in clinical outcome of KA-TKA compared to MA-TKA have not yet been observed or have remained unclear supports the findings from the present simulation study [14]. It appears that the data analysis should be done for neutral, varus and valgus phenotypes separately. Otherwise, possible differences in varus or valgus phenotypes are not found as these are obscured by the fact that NEU phenotypes show no clinically relevant and significant differences. “One stands in the forest, and due to the fog one does not see the trees.” Hence, it is clearly recommended to report patients` outcomes as well as comparisons between different surgical techniques as well as alignment philosophies by NEU, VAR or VAL phenotypes separately. This is in agreement with a study by Luan et al. [13] who found that KA may actually have better outcomes in more severe varus patients.

The present study has some limitations. Only the effects of alignment on four exemplary neutral phenotypes in the coronal plane were investigated. In further steps, however, such simulations could be carried out for many more phenotypes and made more easily accessible with a corresponding software solution. Personalised alignment strategies are under constant development and a mix of different strategies is currently used [15]. This could not be considered in this study. The effects in the sagittal and axial planes were not investigated. The study deals with the alignment of the extended leg. However, the coronal alignment also has a significant influence on the flexion gap geometry. However, this would complicate things further, yet should be investigated in more depth in future studies.

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