# Biomechanical Effect of 3D-Printed Foot Orthoses in Patients with Knee Osteoarthritis
## 2 Materials and Methods  
This single-group, pretest-posttest trial was conducted to evaluate the efficacy of using 3D-printed FOs as the intervention in patients with medial knee OA. The present study focused on the biomechanical effects of 3D-printed FOs and analyzed data for the participants who wore and did not wear the 3D-printed FOs. All participants completed all orthotic tests on the same day. The study was approved by the regional ethics committee and conducted according to the Declaration of Helsinki (protocol code: YM106118E, 31 March 2018). Sample size was based upon our previous work [20]. The result indicated that a sample size of fourteen would be enough to detect a 5% difference in peak KAM, with a statistical power of 80% and a 5% level of significance. Fifteen patients (2 males and 13 females; age, 60.13 \(\pm\) 6.10 years; height, 1.59 \(\pm\) 0.05 m; and weight, 70.65 \(\pm\) 14.16 kg) with radiographically diagnosed medial knee OA (Kellgren-Lawrence grade I or II) were recruited for this study. All participants underwent physical examination and met the American College of Rheumatology criteria [21]. Predominant lateral or patellofemoral OA, or a history of planned hip or knee replacement, hip or ankle arthritis, rheumatoid arthritis, and inability to walk without a cane or walker were the exclusion criteria for this study. Figure 1 presents the procedure of 3D-printed FO manufacturing. To design and fabricate 3D-printed FOs, non-weight-bearing 3D surface scans of both feet were taken with the foot and ankle in the subtalar neutral position by using the EinScan-Pro handheld 3D scanner (Shinging 3D Tech Co., Ltd., Hangzhou, China). The generated 3D foot model was exported as an STL file. The 3D foot model was smoothed and edited using Meshmixer software (Autodesk Inc., San Rafael, CA, USA). In the software program, we used the  
plantar surface as the shell of FO (Figure 2), extracted the shell of FO, and extruded it to produce a 2.5-mm-thick FO. All FOs were manufactured in photosensitive polymer resins with material properties conducive to producing semi-rigid FOs (tensile strength, 45 MPa; Young's modulus, 1500 MPa; elongation at break, 30%; shore hardness, 84 D) by using a stereolithography (SLA) 3D printer (Moai; Peopoly, CA, USA). The SLA 3D printer uses a laser to cure photosensitive polymer resins into 3D objects. Figure 3 demonstrates the rear and medial view of the 3D-printed FO. The bottom of the 3D-printed FO is not a flat surface. Because of the  
Figure 1: Procedure of 3D printing for foot orthoses. Figure 2: (**a**) The shell ends near the metatarsal heads. The distance between the shell end and the first metatarsal head is approximately 10% of foot length. (**b**) Medial shell ends near navicular tuberosity. (**c**) Lateral and (**d**) rear edges of shell. geometric shapes of the 3D-printed FO, it may provide not only medial support but also lateral support. Reflective marker placement was described in the previous work [20]. Moreover, two clusters of three noncollinear markers on rigid plates were mounted on the skin of the thigh and shank (Figure 4). Prior to data collection, the participants were allowed to become familiar with the two experimental conditions: walking in standard shoes (Shoe) and walking in standard shoes embedded with 3D-printed FOs (Shoe + FO). In this study, we used kung fu shoes as the standard shoes. The kung fu shoe was made with minimal lining, a soft toe box, and a flat hard plastic sole. The participants completed five trials of level walking at the self-selected speed under each condition. The order of the two experimental conditions was randomized for each participant. An eight-camera motion analysis system operating at 100 Hz (Vicon MX T20; Vicon Motion Systems Ltd., Oxford, UK) and three force plates operating at 1000 Hz (AMTI, Advanced Mechanical Technology Inc., Watertown, MA, USA) were used to measure the kinematic data and GRF. All data were processed using a custom-written software program (MATLAB 2018a; MathWorks Inc., Natick, MA, USA).