## 4 Discussion
The effects of the screw threads were ignored for all the locking screws, and cylindrical rods were created for the locking screws. The main stresses of the composite locking nail method may also have been underestimated. Figure 15: The maximum principal stress of the two locking screws in the composite interlocking nails with different fixation techniques and a different fracture gap in a proximal shaft fracture (top row), distal shaft fracture (middle row), and middle shaft fracture (bottom row). Figure 16: The maximum principal stress of the four locking screws in the composite interlocking nails with different fixation techniques and a different fracture gap in a proximal shaft fracture (top row), distal shaft fracture (middle row), and middle shaft fracture (bottom row). Figure 17: The maximum principal stress of the six locking screws in the biocomposite interlocking nails with different fracture types, different fracture gap, and different interlocking nail length. The mechanical properties and degradation ability of polymers are related to the material composition, molecular weight, density, glass transition point, and melting temperature [48; 49; 50]. These properties are the boundary for the material's applications, and it could be improved to be used beyond its estimations by several techniques such as chemical treatments, thermal treatments, and blending with other materials. In the previous study, PCL blended with PLA could decrease the molecular weight reduction during the degradation period (up to 20 weeks), which prolonged the mechanical conditions and prevented degradation compared with a neat PLA [51]. According to the literature reviews, canine bone fracture healing times are approximately 16 weeks [52; 53]. Therefore, the interlocking nails must provide proper mechanical properties during this healing period. In our previous study, the HA was added to accelerate bone formation by increased osteoblast proliferation [17]. Therefore, the healing period should be decreased as well. Moreover, the biodegradation test conditions were set to mimic the living body conditions, which performed under the material's boundary conditions [17]. On day 30, the degradation was approximately 0.25% and did not present any significant difference compared with day 7 or day 15. The prediction equation of PLA/PCL/15 HA's biodegradation was Y = 0.0115X +0.1898 (where Y is the degradation percentage, and X is the degradation days) [17]. According to this equation, the degradation percentage of 16 weeks of implantation (bone completely healed) is approximately 1.48%. The slight degradation of the composite materials and the accelerated bone formation by HA might eliminate the mechanical problem of the PLA/PCL/15 HA interlocking nail during the healing period. However, further investigations are still required. Due to the biodegradability, biocompatibility, low cost, and reduced stress protection or stress protection atrophy that can occur using rigid metal interlocking nails [12; 13], a 3D-printed composite material was tested in the present study [17; 54; 55]. The advantages of 3D-printed composite materials for bone fracture treatments were reported and include avoiding a later implant removal operation [56; 57].