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, second peak KAMs and the peak AIM decreased, but no significant differences were observed in the KFM and AEM.
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 [19]. Moreover, the magnitude of the GRF is associated with walking speed [20]. 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, 21]; 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. One explanation for the change in the COP path is that the foot posture was maintained in the neutral position as much as possible by the FO.
The use of lateral wedges increased the AEM and ankle eversion angle. However, the lateral wedge with an arch support tended to reduce the ankle eversion angle, while keeping the AEM equal to the level of lateral wedge without an arch support [22]. Although the ankle eversion angle did not report in this study, the peak AEM was not affected by 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 [23]. Using 3D-printed FOs as interventions affected ankle moments 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. Trepczynski et al. [24] used instrumented prostheses to record in vivo tibiofemoral contact forces during several activities. They suggested that the KFM considerably contributed to medial knee contact force only during the activities with high knee flexion, such as sit-to-stand-to-sit, squat, and stair negotiation. In this study, the patients with knee OA were only asked to walk on a level floor; therefore, most of the alteration of medial knee loading can be explained by the KAM. Moreover, both the first and second peak KAMs were reduced significantly by the experimental interventions in this study. The magnitude of the peak KAM is associated with increased disease severity [25], pain [26], rate of progression [27], and cartilage thickness [28]. Thus, 3D-printed FOs can still produce positive immediate biomechanical effects in patients with knee OA.
The design and manufacture of custom 3D-printed FOs consist of three main steps: foot geometry capture, FO design, and FO manufacture. The time taken for both the 3D scan of feet and the FO geometric design was approximately 6 and 10 minutes, respectively. The print time of each FO was approximately 6 hours. Nevertheless, the traditional fabrication process for FO normally takes from 7 to 14 days, depending on the manufacturer. However, the fabrication costs for a pair of custom-made FOs is from 194 to 485 USD in Taiwan. The photosensitive polymer resins used were approximately 160 mL during printing, and they cost approximately 12 USD. The 3D scanner and SLA 3D printer cost 4810 USD and 1295 USD, respectively. Although the capital costs of 3D scanners are higher, the consumable costs for the traditional fabrication of FOs are higher. However, the use of a low-cost 3D scanner, such as a Microsoft Kinect system (Microsoft, Redmond, WA, USA), and a low-cost 3D printer could fabricate custom FOs similar to traditionally made FOs [29]. The method provides a solution to digital design and manufacture that potentially overcomes the limitation of conventional subtraction manufacturing. In addition, an increasing amount of free open-source software is available, such as the Meshmixer software that we used; clinical staff can use such software, as it does not require a high level of engineering skill.
Some limitations of the study should be considered. First, the present study focused on the immediate effects of 3D-printed FOs in patients with knee OA and did not evaluate long-term responses. Second, kung fu shoes were used as the standard shoes in this study. Kung fu shoes have a low-sided cloth upper part and a flat, hard plastic sole. It remains unclear whether 3D-printed FOs would alter the biomechanical effects in different types of shoes.