# Biomechanical Effect of 3D-Printed Foot Orthoses in Patients with Knee Osteoarthritis
## 4 Discussion
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, squatting, 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 [29], pain [30], rate of progression [31], and cartilage thickness [32]. Thus, 3D-printed FOs can still produce positive immediate biomechanical effects in patients with knee OA. The calculations of knee joint moment need a good accuracy for capturing lower limb kinematics and calculating joint centers. A previous study investigated the reliability of five different knee-joint-center estimation techniques (femoral epicondyle, femoral condyle, tibial ridges, plug-in-gait model, and functional technique) [33]. The knee joint center for the femoral epicondyle, femoral condyle, and tibial ridges configurations were determined by the midpoint between the medial and lateral femoral epicondyles, femoral condyle, and tibial ridge markers. However, they found that no significant differences in the peak knee adduction between femoral epicondyle and functional configurations. In addition,knee joint moments were associated with the highest reliability in all three planes for the femoral epicondyle configuration. Therefore, the femoral epicondyles technique can be used to estimate the knee joint center in this study. 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 min, respectively. The print time of each FO was approximately 6 h. Nevertheless, the traditional fabrication process for FO normally takes from 7 to 14 days, depending on the manufacturer. The conventional method of custom-making an FO is to take a plaster cast of the foot, but the use of plaster casts may have some reliability problems [34]. By contrast, the 3D printer can produce an orthosis with high dimensional accuracy [35]. In the customized FOs market, the cost 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. Based on the cost of the podiatrist's time and business overheads, the cost for the plaster-cast method is from 20.90 to 37.11 USD [36]. The cost for the 3D scan is from 7.48 to 11.22 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 the Microsoft Kinect system (Microsoft, Redmond, WA, USA), and a low-cost 3D printer could fabricate custom FOs similar to traditionally made FOs [37]. 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, patient satisfaction was not assessed in this study. Moreover, the present study focused on the immediate effects of 3D-printed FOs in patients with knee OA and did not evaluate long-term responses. The effects on pain and sensations of comfort should be evaluated over a longer period in future works. 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.