# A Review of the Impacts of Implant Stiffness on Fracture Healing
## 2 Importance of Interfragmentary Movement in Fracture Healing  
The stiffness of the fracture fixation and weight bearing dictates the mechanical environment. The healing process may be compromised if fixation is excessively flexible or overly rigid. Specifically, when high-strain forces are applied at a fracture site (indicating inadequate stability), fibrous tissue persists, and the formation of a stabilizing bony callus is impeded. Persisting under such conditions for a prolonged period may result in the formation of fibrous non-unions [19; 20]. Earlier studies suggested that rigid fixation yields the most favorable clinical outcomes, subsequently contributing to the widespread adoption of nailing and plating to achieve stable internal fixation [8]. Nonetheless, several studies have indicated that overly rigid fixation can paradoxically hinder the process of fracture healing [21]. The rigidity required for fixation is also affected by the size of the gap between bone fragments. A study using live sheep determined that for larger fracture gaps (2 mm or 6 mm), a more rigid fixation method was preferred, while for a smaller gap (1 mm), better healing occurred under higher interfragmentary strain [20]. In contrast, a more recent computer simulation suggested that higher interfragmentary strain could enhance callus formation, which is crucial for indirect fracture healing in larger gaps (3 mm), but the same level of strain in smaller gaps (1 mm) might result in excessive fibrous tissue formation, possibly leading to non-union [22]. The primary concerns raised revolve around potential hindrances to external callus formation, the exacerbation of maintaining a fracture gap due to bone end resorption, and overprotection of the healing bone from regular stress, which can lead to adverse remodeling (stress shielding). Consequently, these studies have identified numerous mechanical factors that influence fracture healing, such as the magnitude and direction of the IFM, type of fracture, and fracture geometry [23; 24]. IFM denotes the displacement occurring between fracture fragments as a consequence of both physiological and external loads applied to the fractured limb. Upon loading a fractured bone, the fracture fragments undergo displacement relative to one another, producing a multiaxial strain that exhibits spatial variation across the fracture gap [3]. IFM plays a pivotal role in the process of fracture healing, being significantly influenced by the stability of the fixation and the load applied by the musculoskeletal system. Moderate levels of IFM facilitate successful bone healing, marked by the formation of a fracture callus [25]. In contrast, excessive IFM can hinder the healing process or may even lead to the development of non-unions [26; 27]. Various combinations of axial forces, bending, and torsional moments can occur, depending on the load sharing and muscular strain. The two primary factors influencing IFM are the rigidity of the fracture stabilization implant and the superficial area on the fracture fragments. These factors determine tissue strain and cellular responses within the fracture site. The magnitude and direction of the IFM within the fracture site are contingent on the load due to weight bearing, muscle forces, and stiffness characteristics of the selected device [9; 28; 29]. The strain induced by IFM is dispersed across the fracture surface and varies depending on the fracture geometry. Communuted fractures, for example, can accommodate relatively higher motion levels since the strain is distributed over a larger surface area of the fractured fragments. The extent of callus formation during the healing process is contingent upon the magnitude of the IFM [1; 20]. When the IFM surpasses a critical level, the blood vessels formed at the fracture site face repeated disruption, impeding their establishment and preventing the development of stable tissues [20].