## 4 Discussion  
In the current research, the maximum principal stresses on the canine femoral shaft fracture stabilized with the 3D-printed composite interlocking nail were estimated effects (gap type, fracture gap, nail length, and fixation techniques) based on a computer-based finite element analysis. Failure in implants is one surgical complication of interlocking nails in femoral shaft fracture treatment, and, according to previous research, interlocking nail failure occurs frequently at the middle shaft [32, 33, 34]. In comparison with the traditional fixation technique, the use of a composite interlocking nail system with new fixation techniques revealed lower maximal principal stresses. The new fixation techniques added middle locking screws, fixing middle fragments to be more stable, allowing better support for the stresses in the middle that usually occur in the interlocking nail without the middle holes, and permitting contact between the proximal and distal fragments. Increasing the locking hole distance can prevent the failure of the interlocking nail [35, 36]. Therefore, the fractured bone can share the canine weight of the composite interlocking nailing system. Nevertheless, some traditional fixation techniques (P-D and 2P-2D) were used to stabilize the proximal fragments and the distal fragments by locking screws rigidly, thus the proximal fragment cannot transfer the canine's weight to the distal fragment [25]. Moreover, the distance between the screw positions that fix only the proximal or distal fragments (2P and 2D) without fixation in the middle of the canine bone makes the stress to the middle of the interlocking nail easy. That is why the new fixation techniques decreased the maximum principal stresses, which were not reduced by the traditional fixation technique. The stabilization of a fractured canine femur is related to several factors, consisting of the length and diameter of the composite interlocking nail system [37], fracture type [38], fracture gap, and the condition of the soft tissue of the canine femur [39]. This research established a relationship between the different fixation techniques and fixation stability. The composite interlocking nail system with the new fixation technique had higher stability, as the proximal, middle, and distal locking screws (P-2M-D and 2P-2M-2D) remained in place. However, the proximal and distal fragments were not interconnected. In patients with bone formation problems, the lack of contact could lead to a fracture nonunion [40]. The new fixation techniques (2M, P-M,  
Figure 14: Maximum principal stress distribution on the composite interlocking nails with a length of 120 mm at a 1 mm proximal shaft fracture with the fixation techniques using the traditional P–D fixation technique and the new P–M fixation technique (left), and at a 1 mm distal shaft fracture with the fixation techniques using the traditional P–D fixation technique and the new M–D fixation technique (right). 2P-2M, 2M-2D, and M-D) have decreased the gap between the fractures and allowed proximal-distal contact between the fragments. The promotion of bone callus formation was reported [41]. The new fixation techniques remove the distal or proximal locking screws, but the wrong side would produce different clinical results. This rule states that the proximal locking screws can be removed (M-D and 2M-2D) for a distal shaft fracture and that the distal locking screws can be removed (P-M and 2P-2M) for a proximal shaft fracture [42; 43; 44]. For the two and four locking screws, according to the multilevel factorial design, gap type, fracture gap, and fixation techniques are the main factors that affect the maximum principal stress of the composite interlocking nail. Thus, this section discusses the relationship between these main factors because the nail length does not significantly impact the maximum principal stress. The fixation techniques were divided into two groups: traditional fixation techniques and new fixation techniques.