Introduction/Aim: Restoring the natural alignment of the knee during total knee arthroplasty, has gained lots of interest among knee surgeons. By using conventional instruments, a slight undercorrection was realized in high degree of varus knees. Since the notion of constitutional varus knees, surgeons accepted these undercorrections. In robotic surgery, the alignment will not become slight varus when we aim to neutral. So, the surgeon has to implement the exact alignment target. In this study, we investigate how changes in Hip Knee Ankle (HKA)-angle can be declared. If we found those correlations, the originally natural alignment of our patients can be predicted.

Materials And Methods

Hundred patients who had undergone sequential full-length radiographs between 2014 and 2020 in the ZOL Genk hospital were selected. 13 alignment parameters were measured as well as the Kellgren-Lawrence grade was determined. A correction for rotation was done by using the formula of Maderbacher.

Results

The HKA-angles increases by a higher Kellgren-Lawrence grade. A higher Kellgren-Lawrence grade is also associated with a decrease in tibial Joint line angle (TJLA) and an increase in joint line convergance angle (JLCA). HKA-angle, medial proximal tibial ankle (MPTA) and JLCA have the biggest changes over time.
A decrease in HKA-angle was associated with an increase in MPTA (p=0,0008). An increase in HKA-angle was seen by a increase of JLCA (p=0,0008). A multiple regression model was found whereby the change of HKA = change of mLDFA x 0,40645 – change of MPTA x 0,71659 – change of TJLA x 0,13549 + change of JLCA x 0,53781 + 0,61195.

Discussion

By using sequential full-length X-rays, within patient differences could be found. So real influences of changes in alignment parameters can be investigated. A difference in HKA-angle is mostly correlated with bony changes at proximal tibial level (MPTA) and an increase in ligamentous imbalance, represented by a higher JLCA. So, restoring the natural alignment in varus knees implicates mainly a correction on the tibial cut and a decrease in JLCA.

Conclusion

Varus OA knees progress into more varus because of progressive medial tibial erosion (MPTA) and progressive lateral ligament laxity (JLCA).