# Biomechanical Effect of 3D-Printed Foot Orthoses in Patients with Knee Osteoarthritis
## 4 Discussion  
The primary aim of this study was to investigate the biomechanical alterations in knee and ankle joint kinetics as a result of personalized 3D-printed FOs. The key findings of this study are that under the Shoe + FO condition, the first and second peak KAMs and the peak AIM decreased; but no significant differences were observed in the KFM and AEM. Besides this, the mean value and standard deviation can be used to calculate the effect size. The Shoe + FO condition exhibited a more lateral shifting compared to the Shoe condition, with a medium-to-large effect size. Figure 5: Average center-of-pressure path of two different experimental conditions during level walking. The dashed line represents the long axis of the foot. Figure 6: External joint moments for (**a**) knee frontal, (**b**) knee sagittal, and (**c**) ankle frontal plane. The black, solid line represents the Shoe condition; the gray dashed line represents the Shoe + FO condition. Changes in the joint moment are due to the magnitude of the GRF, the lever arm distance between the GRF vector and the joint center, or both [23]. Moreover, the magnitude of the GRF is associated with walking speed [24]. In the present study, no significant differences were observed in the walking speed between the two experimental conditions. Therefore, we can assume that the alternations of the joint moments by the 3D-printed FOs were due to changes in the COP path. The reduction of the KAM was caused by the lateral shifting of the COP, decreasing the moment arm around the knee joint, when using lateral wedges [10]. Under the Shoe + FO condition, a similar pattern of COP trajectory was observed; therefore, the first and second peak KAMs decreased. This result is in agreement with those of previous studies [12; 25]; there were significant changes in the reduction of the peak KAM in response to lateral wedges with arch support. However, the 3D-printed FOs in this study were used without any additional lateral posting wedges. The 3D-printed FOs may provide lateral support, due to the geometric shapes. A similar design of FO was presented in our previous study [20]. The FOs reduced the moments at ankle joints in individuals with flexible flatfoot, but no significant difference was observed in the knee and hip joint moments. The reason for this difference might be that the individuals with flexible flatfoot do not have severe problems of malalignment of the lower limbs. Therefore, we believe that the 3D-printed FO maintained the foot posture in the neutral position as much as possible during walking, which changed the COP path and contributed to correcting the malalignment of lower limbs. The use of lateral wedges increased the AEM and ankle version angle. However, the lateral wedge with an arch support tended to reduce the ankle version angle, while keeping the AEM equal to the level of lateral wedge without an arch support [26]. Although the ankle version angle is not reported in this study, the peak AEM was not affected by the 3D-printed FOs. To correct the malalignment of lower limbs, the 3D-printed FOs were semi-rigid and manufactured in photosensitive polymer resins by a 3D printer. In addition, the FOs were made based on the 3D scanning method, which captured the patients' foot shape in the subtalar neutral position. The joint moments in the frontal plane are primarily responsible for the dynamic stability of the lower extremities [27]. Using 3D-printed FOs as interventions affected ankle moments that were highly associated with dynamic stability by decreasing the AIM. The 3D-printed FOs did not influence the AEM but decreased the AIM and KAM; therefore, it may be reasonable to assume that the 3D-printed FO allowed the patients with knee OA to walk in a more natural manner. To investigate the dynamic loading of the knee, the KFM should be considered [6]. However, 3D-printed FOs did not significantly affect the KFM in the present study. [PERSON] et al. [28] used instrumented prostheses to record in vivo tiboifemoral contact forces during several activities.