Improving predictability of post-operative knee joint line obliquity in high tibial osteotomy: A new radiographic parameter correcting for foot position.

Purpose: Joint line obliquity is crucial for the long-term success of a high tibial osteotomy (HTO), particularly when large corrections are made. The relationship between the size of correction and changes in joint line obliquity (KJLO) is complex, often leading to the preference for a double-level osteotomy to manage significant post-operative obliquity. We aim to improve the prediction of knee joint line orientation changes and correction size by examining the influence of foot position. The goal is to develop a linear model to predict post-operative KJLO, which could assist in determining whether a single- or double-level osteotomy is necessary to prevent excessive post-operative joint line obliquity.

Methods: A retrospective radiographic analysis was conducted on patients who underwent HTO surgery between April 2016 and April 2017. Ninety-one patients were randomly selected for radiographic measurements on long-leg radiographs pre- and 3-month post-operative. A novel radiographic parameter was introduced to realign the foot of the operated side to the midline. The foot position relative to the midline was assessed by determining the angle formed when rotating the natural foot position onto the midline using the centre of the hip as a rotation point. The angle obtained by subtracting this from the KJLO results in a corrected KJLO for midline foot position (aKJLO).

Results: Predictive model of differences between pre- and post-operative values of MPTA (ΔMPTA [medial proximal tibial angle]) and KJLO (ΔKJLO) was less predictive (0.185 [p < 0.001]) than ΔMPTA and ΔKJLO corrected for foot position, namely ΔaKJLO (0.688 [p < 0.001]). Adding more parameters did not significantly improve the linear model's predictions.

Conclusion: Predictability of aKJLO could be significantly enhanced. With this new parameter, inter- and intra-variability of foot position is bypassed. A safe MPTA position of 91° can be presumed; however, inter-individual variability in limb adaptation following correction remains uncertain.

Level of evidence: Level IV.