## 1 Introduction  
The metallic interlocking nail system has been widely used to treat canine femoral shaft fractures [1; 2]. The biomaterials used in interlocking nails are titanium and stainless steel [3; 4; 5]. Nevertheless, the removal of a metallic interlocking nail is still necessary due to long-term implantation complications [6]. Biological support to living cells is not activated, and bones are not regenerated in the interlocking nail cavity according to the bioinert properties of these materials [7]. Thus, the metallic interlocking nail will leave a blank cavity inside the healed bone. To eliminate the limitations of the metallic interlocking nail, alternative biodegradable materials such as polylactic acid (PLA) and polycaprolactone (PCL) were used to fabricate interlocking nail components for the model of a canine, porcine, and ovine femoral fracture [8; 9; 10]. During the early state and the complete healing state of bone fractures (4 and 12 weeks after implantation), there were no differences concerning the fracture healing, gait pattern, or tissue reaction between the PLA and stainless-steel interlocking nails in an animal experiment [9; 11]. In a rabbit humerus osteotomy model, PCL intramedullary pins presented low inflammation compared to stainless-steel pins. All PCL-immobilized osteotomies have been healed with an extensive production of periosteal cells [12]. However, the PCL fixation was initially weaker than the stainless-steel fixation by mechanical testing. To enhance the properties of the fabricated materials, copolymers or mixed composites were presented [13]. PLA and PCL are additionally combined as a copolymer providing the necessary abilities for applications in the field of tissue engineering, for example, biocompatibility, biodegradability, non-toxicity, and environmentally friendly tissue [12; 13]. Currently, PLA and PCL are commercially available for fused filament fabrication (FFF) 3D printing applications [14], which is the promised fabrication technique for medical applications [15; 16]. The previous study presented the possibility of an interlocking nail fabrication from FFF 3D-printed PLA/PCL/hydroxyapatite (HA) composites [17]. Hydroxyapatite (HA), the main mineral in mammalian bone, was used to improve osteoconductivity and induce bone growth [18; 19]. A common treatment for a canine femoral shaft fracture is an interlocking nail system with a traditional fixation technique [20]. Due to its improved fixation and reduced loss of soft tissue compared with traditional plating, this technique has biomechanical advantages [21]. However, some traditional fixation techniques have rigidly locked proximal as well as distal fragments by the locking screws and could not transfer the canine weight from the proximal fragment to the distal fragment. Furthermore, the distance between the screws, which fixes only the proximal or distal fragments without fixation in the middle, makes it easy for the stress to reach the middle of the interlocking nail [22; 23]. Thus, the new fixation techniques have added middle locking screws and made it more stable to fasten the middle fragments, which allow for better support of the stresses in the middle, which normally occur in the interlocking nail without the middle holes [24]. Based on a literature review, there have been no research investigations using a finite element analysis (FEA) of the biomechanical characteristics of traditional locking nails in canine femoral shaft fractures. We hypothesized that the new technique of fixation would reduce the implant failure risk and stress of the interlocking nail. Moreover, new fixation techniques could produce different results. In this study, traditional and new fixation techniques were used for three types of canine femoral shaft fractures. Additionally, to estimate the biomechanical performances, the interlocking nail stress and fixation stability were calculated.