## 2 Materials and Methods  
For the canine femoral models, 120 transverse computed tomography scans of the femur were obtained from the cadaver of a male dog weighing 22 kg. The dog was healthy and was not associated with any abnormality of the femur. This investigation was approved by the Institutional Animal Care and Use Committee, Faculty of Veterinary Medicine, Chiang Mai University, Chiang Mai, Thailand (Ref. No. R7/2558). The entire left femur was scanned using the 3D scanner EinScan-SE from SHINING 3D (EinScan SE, Shining 3D, Hangzhou, China) and the EinScan-S series v2 software. This canine femur model consists only of cortical bone. A canine femur with a transverse fracture was developed with the use of SolidWorks 2018 (SolidWorks Corporation, Concord, MA, USA), and the fracture gap between the proximal and distal bone fragments was assumed to be 1-3 mm [23,25]. In the current research, three types of femoral shaft fractures were considered, including a proximal shaft fracture, a middle shaft fracture, and a distal shaft fracture, as shown in Figure 1.  
Each type of canine femoral shaft fracture was fixed using a composite interlocking nail system, including traditional fixation techniques: proximal and distal shaft fracture fixed with two proximal locking screws (2P), two distal locking screws (2D), one proximal locking screw with one distal locking screw (P-D), and two proximal locking screws with two distal locking screws (2P-2D); and new fixation techniques: proximal and distal shaft fracture fixed with two middle locking screws (2M), one proximal locking screw with one middle locking screw (P-M), one middle locking screw with one distal locking screw (M-D), two proximal locking screws with two middle locking screws (2P-2M), one proximal locking screw with two middle locking screw and one distal locking screws (P-2M-D), and two proximal locking screws with two middle locking screws and two distal locking screws (2P-2M-2D). The composite interlocking nail was implanted from the proximal fragment to the distal fragment. In the current study, three types of canine femoral shaft fractures fixed by 11 types of fixation techniques, as seen in Figure 2 (proximal and distal), and 4 types of fixation techniques, as seen in Figure 3 (middle), were analyzed.  
The material properties were as follows: The 3D models of the canine femoral fracture fixed with the composite interlocking nail system were transferred to the commercial software ANSYS R14.5 Workbench (ANSYS, Inc., Canonsburg, PA, USA). The canine femoral model length was about 180 mm, and the canine femur made from the isotropic material for the cortical bone with a Young's modulus of 15 GPa, a Poisson's ratio of 0.3, and a density of 2 g/cm\({}^{3}\)[26,27]. The 3D-printed composite interlocking nail and locking screws used in this study were made from PLA and PCL in a 70:30 mixing ratio with hydroxyapatite 15% _w/w_ (PLA/PCL/15 HA). The test specimens were 3D-printed from the extruded composite filaments using an FFF printer. The mechanical properties (Table 1) were obtained from  
Figure 1: The three types of canine femoral shaft fractures.  
the mechanical tests of [PERSON] et al. (2017), with a Poisson's ratio of 0.34 and a density of 2 g/cm\({}^{3}\)([PERSON] et al., 2017). The composite interlocking nail had a diameter of 6 mm and a length of 120-160 mm, with a 2.7 mm diameter ([PERSON] et al., 2017), and the new hole position was located on two proximal holes, two middle holes, and two distal holes (Figure 4). The locking screws were modeled without threads, with a diameter of 2.7 mm.  
Figure 2: The finite element analysis (FEA) models with proximal and distal canine femoral shaft fractures and fixation techniques.