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Distraction osteogenesis is an effective method for generating large amounts of bone in situ for treating pathologies such as large bone defects or skeletal malformations, for instance leg-length discrepancies. While an optimized distraction procedure might have the potential to reduce the rate of complications significantly, our knowledge of the underlying mechanobiological processes is still insufficient for systematic optimization of treatment parameters such as distraction rate or fixation stiffness. We present a novel numerical model of lateral distraction osteogenesis, based on a mechanically well-controlled in vivo experiment. This model extends an existing numerical model of callus healing with viscoplastic material properties for describing stress relaxation and stimuli history-dependent tissue differentiation, incorporating delay and memory effects. A reformulation of appositional growth based non-local biological stimuli in terms of spatial convolution as well as remeshing and solution-mapping procedures allow the model to cope with severe mesh distortions associated with large plastic deformations. With these enhancements, our model is capable of replicating the in vivo observations for lateral distraction osteogenesis in sheep using the same differentiation rules and the same set of parameters that successfully describes callus healing in sheep, indicating that tissue differentiation hypotheses originally developed for fracture healing scenarios might indeed be applicable to distraction as well. The response of the model to modified distraction parameters corresponds to existing studies, although the currently available data is insufficient for rigorous validation. As such, this study provides a first step towards developing models that can serve as tools for identifying both interesting research questions and, eventually, even optimizing clinical procedures once better data for calibration and validation becomes available. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

ABSTRACT The outcome of secondary fracture healing processes is strongly influenced by interfragmentary motion. Shear movement is assumed to be more disadvantageous than axial movement, however, experimental results are contradictory. Numerical fracture healing models allow simulation of the fracture healing process with variation of single input parameters and under comparable, normalized mechanical conditions. Thus, a comparison of the influence of different loading directions on the healing process is possible. In this study we simulated fracture healing under several axial compressive, and translational and torsional shear movement scenarios, and compared their respective healing times. Therefore, we used a calibrated numerical model for fracture healing in sheep. Numerous variations of movement amplitudes and musculoskeletal loads were simulated for the three loading directions. Our results show that isolated axial compression was more beneficial for the fracture healing success than both isolated shearing conditions for load and displacement magnitudes which were identical as well as physiological different, and even for strain-based normalized comparable conditions. Additionally, torsional shear movements had less impeding effects than translational shear movements. Therefore, our findings suggest that osteosynthesis implants can be optimized, in particular, to limit translational interfragmentary shear under musculoskeletal loading. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:865-872, 2014. [ABSTRACT FROM AUTHOR]

Abstract: Assessment of fracture healing is a common problem in orthopaedic practice and research. To determine the effectiveness of certain treatments, drugs, mechanical loads, or rehabilitation regimes, the strength of the fracture callus must be determined. Both clinically and experimentally, there is a need to noninvasively and quantitatively evaluate fracture callus quality during healing. The objective of this study was to develop a method to assess fracture stiffness using micro-computed tomography (micro-CT) and finite element analysis. The method was developed and validated with plastic phantoms of various cross sections and known material properties, tested experimentally in four-point bending and torsion. The method was then applied to fractured rat femurs after 3 and 4 weeks of healing tested experimentally in torsion (50 femurs total). Micro-CT scans were made of the fracture calluses to determine three-dimensional geometry and material properties for the finite element models. Experimentally measured torsional rigidities were compared to finite element solutions. Finite element model predictions of callus rigidity correlated significantly better with experimental torsional rigidity than other common measures of healing progress such as callus area, bone mineral density, or area moment of inertia. Using FEA to predict mechanical properties of the callus could prove to be a useful tool in fracture-healing studies. [Copyright &y& Elsevier]

Abstract: For numerical simulations of biological processes the assignment of reliable material properties is essential. Since literature data show huge variations for each parameter, this study presents a method for determining tissue properties straight from the investigated specimens by combining electronic speckle pattern interferometry (ESPI) with finite element (FE) analysis in a two-step parameter analysis procedure. ESPI displacement data from two mid-sagittal ovine fracture callus slices under 5N compressive load were directly compared to data from FE simulations of the respective experimental setup. In the first step a parameter sensitivity analysis quantified the influence of single tissues on the mechanical behavior of the callus specimens. In the second step, material properties (i.e. Young''s moduli and Poisson''s ratios) for the most dominant material of each callus specimen were determined through a parameter sampling procedure minimizing the mean local deviations between the simulated (FE) and measured (ESPI) equivalent element strains. The resulting material properties showed reasonable ranges downsizing the variability of previous published values, especially for Young''s modulus which was 1881MPa for woven bone and 16MPa for cartilage in average. In conclusion, a numerical method was developed to determine material properties straight from independent fracture callus specimens based on experimentally derived local mechanical conditions. [Copyright &y& Elsevier]

Abstract: This study evaluated the feasibility of assessing continuous strain distributions on fracture callus cross-sections with an electronic speckle pattern interferometry (ESPI) system. Mid-sagittal callus cross-sections were harvested from ovine tibiae. One low stiffness (LS) specimen and one high stiffness (HS) specimen were selected to evaluate the feasibility for strain acquisition over a range of callus properties. The HS specimen was 147 times stiffer in compression than the LS specimen. ESPI captured continuous strain distributions on both specimens. Peak strain was located adjacent to cortical boundaries in the osteotomy gap. In response to 5N compression, peak compressive strain of 5.8% in the LS specimen was over two orders of magnitude higher than peak compressive strain of 0.013% in the HS specimen. In conclusion, ESPI-based strain acquisition enables reproducible quantification of strain distributions on callus cross-sections. Such measurements may support validation of computational models and evaluation of experimental results in fracture healing research. [Copyright &y& Elsevier]