## 4 Discussion
For proximal shaft fractures of 1 mm, 2 mm, and 3 mm, the difference in the maximum principal stress of the new fixation techniques was lower than that of the traditional fixation technique group, and the maximum principal stresses in the interlocking nail were lower than the tensile stresses of the 3D-printed composite specimen (PLA/PCL/15 HA) in a proximal shaft fracture of 1-2 mm (Figures 15 and 16 top row). However, in the middle hole (Figure 13 top row, Figure 14), these stresses reached values that were very close to the tensile stresses of the 3D-printed composite specimen (52.05 MPa). For distal shaft fractures of 1 mm, 2 mm, and 3 mm, the maximum principal stresses in the interlocking nail were greater than the tensile stresses of the 3D-printed composite specimen in all models (Figures 15 and 16, middle row). Such findings confirm the in vivo study in which all the interlocking nails failed at one of the locking holes nearer to the fractured line at the interface. Previous research also reported that interlocking nails frequently failed at the locking hole close to the fracture line [35; 45]. The first locking hole at the distal site is the hole that is thought to be most at risk, especially if it is near the fracture site [35]. Figure 13 (middle row) shows that the maximum principal stresses occur in the first locking hole in all models. For middle shaft fractures of 1 mm, 2 mm, and 3 mm, the maximum principal stresses in the interlocking nail were lower than the tensile stresses of the 3D-printed composite specimen in all models (Figures 15 and 16 bottom row), as well as the stress that occurs in the area between the middle locking holes near the fracture site (Figure 13, bottom row). For the six locking screws in the proximal shaft fracture, according to the multilevel factorial design, gap type, fracture gap, nail length, and fixation techniques are the main factors that affect the maximum principal stress of the composite interlocking nail. Therefore, this paragraph discusses the relationship between the main factors. The maximum principal stresses in the interlocking nail were lower than the tensile stresses of the 3D-printed composite specimen (52.05 MPa) in conditions 1, 3, 4, 5, 6, 8, and 9. In contrast to the proximal shaft fracture, the maximum principal stresses in the interlocking nail were greater than the tensile stresses of the 3D-printed composite specimen (52.05 MPa) for the distal shaft fracture (Figure 17). In distal shaft fractures, the distal locking hole has a greater bending load than the middle locking hole because it is nearer to the fracture site and causes more interlocking nail failure [46; 47]. A key factor in reducing the stress of these interlocking nails was the nail length necessary to be pushed down to the subchondral zone to increase the distance between the fracture locking hole [35]. At the distal shaft fracture, the interlocking nail with a length of 160 mm had less stress than the interlocking nails with lengths of 120 and 140 mm. Thus, smaller diameter nails with insufficient length should be avoided (where nail length does not reach the subchondral zone). Modifications to the designs that increase the distal thickness of the interlocking nail could decrease nail failure. The new fixation techniques have decreased the maximum principal stresses of the composite interlocking nail in canine shaft fractures. Generally, each fixation technique has advantages and disadvantages. Due to modeling assumptions, there are some limitations. One of the limitations is the assumption that bone is a homogeneous isotropic material. This assumption is justified in this case as we modeled compact bone only. It cannot be assumed that the apparent density of the compact bone varies too much from point to point or that Young's modulus has a single value. Moreover, the compact bone in long-bone diaphysis is transversely isotropic, but the assumption of bone isotropy was a reasonable approach to analyzing implant behavior.