The effect of High Tibial Osteotomy on medial and lateral knee contact forces: a musculoskeletal modeling analysis during daily activities on patients with varus malalignment and unicompartmental osteoarthritis.

High tibial osteotomy has been used as a joint preserving technique to reduce the loads on the medial compartment of the knee and slow down the osteoarthritis progression in patients with varus deformity. However, there is scarce evidence in the effectiveness of high tibial osteotomy in reducing medial knee contact forces and redistributing loads among knee compartments, especially during motor activities different than walking. Therefore, the aim of this thesis was to analyze the effect of high tibial osteotomy on the distribution of medial and lateral knee contact forces during walking, stair ascent and stair descent in patients with medial compartment knee osteoarthritis and varus deformity. Thirty-eight patients with knee varus deformity and medial knee osteoarthritis were included in this study, as part of a larger project at the Rizzoli Orthopedic Institute in Bologna. The patients were divided into two groups: the Operated Group, who underwent high tibial osteotomy, and the Control Group, who received conservative treatments. Pre-treatment and 1-year follow-up data were collected for both groups. Specifically, full-leg weight bearing and Rosenberg 45° knee flexion radiographs, along with motion capture data including 3D marker trajectories, ground reaction forces and EMG activities during walking, stair ascent and descent were acquired. A personalized full-body musculoskeletal model, which includes image-based tibiofemoral alignment and contact point locations, was used to calculate knee contact forces via an optimization-based inverse-dynamics workflow in OpenSim. To analyze the effect of high tibial osteotomy, we analyzed statistically significant differences in knee contact forces and medial-to-total force ratio throughout the stance phase (statistical parametric mapping with non-parametric paired t-tests). Additionally, we investigated statistically significant differences among the force peaks (Mann-Whitney U-tests). Our results showed that high tibial osteotomy has an overall significant effect on the distribution of medial and lateral joint loads, while maintaining the total knee contact forces unchanged. We found a significant decrease in medial contact forces and a significant increase in lateral knee contact forces for most of the stance phase of the three motor tasks of the Operated Group. The force peaks between pre- and post-surgery could reach a median decrease of 0.46 body-weight in the medial force and a median increase of 0.52 body-weight in the lateral force during walking. Furthermore, the average medial-to-total force ratio showed that the medial compartment carried the 58.4\%, 50.1\%, and 50.2\% of the total force during walking, stair ascending, and stair descending, respectively, compared to 73.8\%, 60.5\%, and 61.4\% pre-surgery. On the other hand, control patients did not exhibit any load redistribution. Finally, the indirect model validation process showed a significant correlation between predicted muscle activations and recorded EMG activities, thereby demonstrating that our customized model effectively predicts the knee contact forces. The present study demonstrates the significant impact of high tibial osteotomy on the distribution of knee joint loads, resulting in a general unloading of the medial compartment and the increase of the lateral forces. Moreover, high tibial osteotomy produces an almost complete balance of forces between the two compartments in all three analyzed tasks. These mechanical changes are associated with clinical improvements in osteoarthitis, as they decrease the load concentration in the medial compartment, typical of knee varus patients. Further investigation could explore the impact of additional subject-specific parameters, such as musculotendon insertion points, on knee contact force estimation. Moreover, results on the full cohorts and the inclusion of a healthy group for comparisons will provide additional insights into the efficacy of high tibial osteotomy.