A theoretical analysis and finite element simulation of fixator–bone system stiffness on healing progression

Abstract

Introduction: The unilateral external fixator has become a quick and easy application for fracture stabilization of the extremities; the main value for evaluation of mechanical stability of the external fixator is stiffness. The stiffness property of the external fixator affects the local biomechanical environment of fractured bone. Methods: In this study, a theoretical model with changing Young's modulus of the callus is established by using the Castigliano's theory, investigating compression stiffness, torsional stiffness and bending stiffness of the fixator-bone system during the healing process. The effects of pin deviation angle on three stiffness methods are also investigated. In addition, finite element simulation is discussed regarding the stress distribution between the fixator and bone. Results: The results reveal the three stiffness evaluation methods are similar for the fixator-bone system. Finite element simulation shows that with increased healing time, the transmission of the load between the fixator and bone are different. In addition, the finite element analyses verify the conclusions obtained from the theoretical model. Conclusions: This work helps orthopedic doctors to monitor the progression of fracture healing and determine the appropriate time for removal of a fixation device and provide important theoretical methodology.

Development of Knowledge-Based Engineering System for Structural Size Optimization of External Fixation Device

“…During exploration, external fixation device is loaded with complex stress, which can be considered as a combination of pressure, bending and torsion. In most of the studies of external fixation devices using FEM numerical methods or experimental methods, separated studies are done for: axial pressure testing, sagittal or anterior-posterior (AP) bending, and torsion [20][21][22][23]. The basic external load of external fixation device is axial pressure [24].…”

“…The main goal of usage of external fixation device is fast healing. From a literature review [20,27,28], it can be concluded that optimal biomechanical environment, for optimal healing of a bone, among other things, is based on the following two points of view: limited periodical axial displacements of bone segment at the fracture gap stimulate its healing after inflammation. This process is called dynamization.…”

“…The place which is closer to the bone segment model is marked with MP−, and the place on another side is marked with MP+ (Figure 6, detail A). The full numerical structural analysis of external fixation device, except the analysis of displacements at fracture gap, includes also the analysis of principal stress at characteristic places of design [14,20]. During FEM and experimental analysis, values and directions of principal stress are calculated at two control places at the middle part of the fixator tree.…”

Optimization of a Patient-Specific External Fixation Device for Lower Limb Injuries

“…These different external fixators, designed with different stiffness and stability characteristics, can be classified according to their frame configurations as unilateral, bilateral, quadrilateral, triangular, semi-circular, and circular [ 13 , 14 , 15 ]. The stiffness of the external fixators is critical to heal the fracture quickly and properly [ 9 , 16 ], but the optimal stiffness has not been identified yet. A more rigid frame can lead to a non-union or a delayed union, while a more flexible frame can lead to mal-union, non-union or pin–bone interface issues [ 17 ].…”

“…The mechanical performance of external fixation devices has been investigated by finite element analysis (FEA), considering either individual components of the frame or the entire assembled frame [ 16 , 18 , 19 , 20 ]. Sternick et al [ 21 ] used FEA to study the relationship between the stiffness of the external fixator and the number of pins and concluded that the stiffness of the fixator with four pins is 19% and 42% higher than the stiffness of external fixators with three and two pins.…”