Abstract
Bone fractures commonly repair by forming a bridging structure called callus, which begins as soft tissue and gradually ossifies to restore rigidity to the bone. Virtual mechanical testing is a promising technique for image-based assessment of structural bone healing in both preclinical and clinical settings, but its accuracy depends on the validity of the material model used to assign tissue mechanical properties. The goal of this study was to develop a constitutive model for callus that captures the heterogeneity and biomechanical duality of the callus, which contains both soft tissue and woven bone. To achieve this, a large-scale optimization analysis was performed on 2363 variations of 3D finite element models derived from computed tomography (CT) scans of 33 osteotomized sheep under normal and delayed healing conditions. A piecewise material model was identified that produced high absolute agreement between virtual and physical tests by differentiating between soft and hard callus based on radiodensity. The results showed that the structural integrity of a healing long bone is conferred by an internal architecture of mineralized hard callus that is supported by interstitial soft tissue. These findings suggest that with appropriate material modeling, virtual mechanical testing is a reliable surrogate for physical biomechanical testing.
Highlights
Bone fractures commonly repair by forming a bridging structure called callus, which begins as soft tissue and gradually ossifies to restore rigidity to the bone
In a recent ovine study, we demonstrated that a virtual torsion test outperforms subjective methods like radiographic scoring for predicting the progress of healing and that it is a reliable surrogate for postmortem physical torsion testing in intact tibiae[12]
The optimization process was successful in identifying a value for ρcut at each level of Esc that minimized the difference in torsional rigidity between the virtual and physical biomechanical tests (Fig. 2B–D)
Summary
Bone fractures commonly repair by forming a bridging structure called callus, which begins as soft tissue and gradually ossifies to restore rigidity to the bone. In subject-specific finite element models, elementwise material properties are mapped from the underlying scan data, such that regions of bone with lower density are assigned lower modulus values This technique is highly effective for mimicking physical testing of intact bones but has a clear limitation when extended to be used in specimens with callus. Our previous ovine study found that extending a density-modulus scaling law developed for cortical bone to include regions of callus produced strong correlations between virtual and physical biomechanical tests, but that the virtual tests over-predicted the measured torsional rigidity in osteotomized specimens by an average of 58%12 This over-prediction of rigidity when callus is present suggests that a constitutive material model derived for cortical bone does not completely capture the mechanical behavior of callus and that more work is needed to virtually replicate the in vivo biomechanics of healing fractures
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