# A Review of the Impacts of Implant Stiffness on Fracture Healing
###### Abstract  
The bone healing process is influenced by various physiological factors. Fracture fixation traditionally relied on rigid metallic implants. However, excessively rigid constructs can lead to complications, necessitating revision surgery. This review focuses on approaches to improve bone healing by introducing adequate interfragmentary movement (IFM) at the fracture site. IFM promotes secondary fracture healing and callus formation. Studies suggest that rigid fixation may impair fracture healing by inhibiting callus formation and causing stress shielding. Titanium alloy locking plates have been shown to be biomechanically superior to stainless steel. Flexible fixation and techniques to regulate implant stiffness are crucial for managing fractures with bridge plating. Materials with a lower Young's modulus balance biomechanical properties. A novel TiNSbSn alloy with a low Young's modulus has been developed to address stress shielding issues. It is effective in promoting osteosynthesis, bone healing, and superior mechanical properties compared with materials with higher Young's moduli. The enhanced formation of bone and callus associated with TiNSbSn alloy suggests its promise for use in fracture treatment plates. Understanding the biomechanics of fracture healing, optimizing fixation stiffness, and exploring innovative materials like TiNSbSn alloys, are crucial for advancing approaches to accelerate and enhance bone healing. fracture; interfragmentary movement; low Young's modulus; osteosynthesis; TiNSbSn alloy 2  
of interfragmentary movement (IFM). Furthermore, these mechanical forces directly impact the localized stress and strain at the fracture site, consequently affecting a complex and sequential series of biological pathways involved in the healing process. Optimizing the speed and strength of fracture healing is challenging. However, adjusting the mechanical parameters of the implants can enhance bone healing and alleviate stress shielding concerns. This has spurred the development of implants with reduced stiffness, dynamic features, and functionally graded characteristics [10; 11; 12]. Internal fracture fixation has long depended on the use of metallic implants, whose mechanical properties stay fairly unchanged over their duration of use. Throughout much of the 20 th century, fracture reconstruction commonly utilized plates equipped with non-locking screws. These devices necessitate a substantial degree of bone integrity, relying on the friction generated between the plate, screws, and bone to promote primary bone healing. However, the introduction of locking plate technology in the 1990s marked a significant shift in the approach to fracture management. This innovation provided stability that does not depend on the condition of the bone, making it especially useful for patients with osteoporosis or for treating highly fragmented fractures [3; 10]. The substantial rigidity associated with locking plate reconstruction has given rise to distinct failure mechanisms, notably screw cutout, which frequently necessitates revision surgery [13]. Figure 1 shows a case of fusion failure caused by excessively rigid fixation using locking plates. Cutoff occurrences are typically observed in older patients, indicating that excessively rigid constructs may pose challenges in the context of compromised bone quality. Adequate autologous bone grafting is often necessary for the treatment of committed fractures, but the invasiveness of autologous bone grafting may be problematic in fractures in elderly patients. Insufficient autologous bone grafting and gaps between bone fragments limit bone healing and, further, excessively rigid implants lead to inadequate IFM, ultimately causing stress shielding and unfavorable remodeling [14]. Effective secondary fracture healing requires controlled IFM at the fracture site. Independent studies have shown that rigid locking plates do not create an environment conducive to this process [15; 16].