# A Review of the Impacts of Implant Stiffness on Fracture Healing
## 4 Clinical Applications of Low Young's Modulus Metallic Materials to Accelerate Fracture Healing  
Titanium alloy fracture treatment plates have been reported to have a higher fracture healing rate than stainless steel fracture treatments [39]. We will focus on the elastic modulus of Ti6 Al4V alloy, a typical titanium alloy clinically applied in orthopedics owing to its excellent biocompatibility and corrosion resistance. [PERSON]'s moduli of the Ti6 Al4V alloy and commercially pure titanium (CP-Ti) are 117 and 107 GPa, respectively, which are lower than those of stainless steel (205 GPa) [45; 46]. Indeed, [PERSON]'s moduli of Ti6 Al4V alloy and CP-Ti, although lower than that of stainless steel, are still considerably higher than that of the human cortical bone, which typically ranges from 11 GPa to 20 GPa [47]. The discrepancy in [PERSON]'s moduli between the prosthesis and cortical bone results in stress shielding, which impedes the efficient transfer of load from the implant material to the bone, thereby affecting the load-sharing dynamics [48]. Fracture treatment devices that exhibit excessive stiffness have been reported to lead to suboptimal outcomes, including delays and failures in the process of bone healing [17; 34]. In contrast, previous studies have demonstrated that when evaluating fracture healing through imaging, intramedullary nails composed of titanium alloy with a low Young's modulus facilitate the process of fracture healing and the restoration of bone strength [49; 50]. A novel TiNbSn alloy characterized by [PERSON]'s modulus as less than 50 GPa was developed to address the issue of stress shielding [51]. The tensile strength of the TiNbSn alloy is equivalent to that of the Ti6 Al4V alloy. Moreover, upon implantation into the rabbit femoral bone marrow cavity, the TiNbSn alloy exhibited biocompatibility that was comparable to that of the Ti6 Al4V alloy [52]. Cell culture tests showed reduced cytotoxicity [52]. Previous studies have documented the excellent corrosion resistance of TiNbSn alloys [53; 54]. Results from studies on rabbit and murine tibia fractures treated with intramedullary nails indicate that nails fabricated from the TiNbSn alloy are more efficacious in facilitating osteosynthesis compared to materials possessing higher Young's moduli [55; 56; 57]. The effect of the TiNbSn alloy compared with CP-Ti in promoting osteosynthesis in a rabbit tibial osteotomy model is presented in Figure 2[58]. The results demonstrated that in the TiNbSn alloy group, the bone healing process was linked to heightened callus formation, improved structure of newly formed bone, greater bone strength in the early phase, and enhanced bone strength and stiffness during the late phase of bone healing. Adequate IFM and controlled axial motion are provided by elastic intramedullary nails composed of a TiNbSn alloy with a low Young's modulus, and these motions may contribute to better bone healing and superior mechanical properties. Additionally, the impact of the fracture plate's low Young's modulus on osteosynthesis was examined. Radiological, histological, and mechanical evaluations have suggested the outstanding effectiveness of the TiNbSn alloy on bone repair compared to both CP-Ti and Ti6 Al4V alloy [11; 58]. The TiNbSn alloy is a titanium alloy with a low Young's modulus and could be equipped with a locking mechanism [58]. Utilizing an implant fabricated from a material with a lower Young's modulus, which facilitates more effective load sharing with the bone compared to an implant made of Ti6 Al4V, could confer advantages for bone healing and potentially diminish the risk of stress shielding. Specifically, a TiNbSn alloy plate, characterized by its low Young's modulus, demonstrated superior new bone formation and produced a stiffer callus at the osteotomy site, particularly during the initial phases of the healing process, in comparison to plates made from Ti6 Al4V alloy and CP-Ti [11; 58].