Abstract
The stiffness of fracture fixation devices together with musculoskeletal loading defines the mechanical environment within a long bone fracture, and can be quantified by the interfragmentary movement. In vivo results suggested that this can have acceleratory or inhibitory influences, depending on direction and magnitude of motion, indicating that some complications in fracture treatment could be avoided by optimizing the fixation stiffness. However, general statements are difficult to make due to the limited number of experimental findings. The aim of this study was therefore to numerically investigate healing outcomes under various combinations of shear and axial fixation stiffness, and to detect the optimal configuration. A calibrated and established numerical model was used to predict fracture healing for numerous combinations of axial and shear fixation stiffness under physiological, superimposed, axial compressive and translational shear loading in sheep. Characteristic maps of healing outcome versus fixation stiffness (axial and shear) were created. The results suggest that delayed healing of 3 mm transversal fracture gaps will occur for highly flexible or very rigid axial fixation, which was corroborated by in vivo findings. The optimal fixation stiffness for ovine long bone fractures was predicted to be 1000–2500 N/mm in the axial and >300 N/mm in the shear direction. In summary, an optimized, moderate axial stiffness together with certain shear stiffness enhances fracture healing processes. The negative influence of one improper stiffness can be compensated by adjustment of the stiffness in the other direction.
Highlights
Fractures typically heal successfully, five to ten per cent of all fractures show complications such as healing delays or nonunions [1,2,3]
Characteristic maps of bending stiffness correlated to fixation stability To create characteristic maps of healing outcome resulting from fixation stability, healing processes were simulated for a total of 96 different combinations of axial and translational shear fixation stiffness for two gap sizes: most in vivo studies in sheep apply gaps of 2–3 mm, a gap size of 3 mm was chosen, a small gap size of 1 mm was investigated
The findings presented go beyond the in vivo experimental conclusions of Epari et al [21], who found that enhanced healing outcomes can be achieved especially through optimization of the axial stiffness to moderate values and limitation of the translational shear flexibility
Summary
Five to ten per cent of all fractures show complications such as healing delays or nonunions [1,2,3]. A measure for the mechanical conditions within the fracture site is the interfragmentary movement (IFM), which under physiological loading [11] in fractured human tibiae is highly complex and consists of axial motion, bending, and torsional and translational shear [12,13]. To stabilize long bone fractures against these loading influences, surgeons use either external fixation, plate fixation, or intramedullary nailing [14]. Each of these different fixation methods show different predominant IFM directions. Intramedullary nails can create remarkable shear movements in the fracture gap caused by the play of the nail within the medullary canal [16]
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