Biomechanical duality of fracture healing captured using virtual modeling and validated in ovine bones

Abstract

Bone fractures commonly repair by forming a bridging structure called callus, which begins as soft tissue and gradually ossifies to restore the rigidity of the bone. In large animals and humans, this process typically requires only a few months to complete. Occasionally bones can fail to heal, resulting in nonunions that are difficult to diagnose and treat, partly because early detection of poor healing remains challenging. We present a translational technique for measuring what really matters in fracture healing – the structural integrity of the bone – using image-based virtual mechanical testing. This work shows that physical mechanical tests of osteotomized ovine tibiae can be reliably recapitulated in a simulation environment only when the soft-hard mechanical duality of the healing zone is accurately captured by the constitutive material model. A large-scale optimization analysis was performed on thousands of 3D finite element models derived from computed tomography (CT) scans of 33 osteotomized sheep. This identified a piecewise material model that successfully replicated the postmortem torsion testing data by differentiating between soft and hard callus based on material density alone. The results suggest that most of the structural integrity of a healing bone is conferred by an internal architecture of mineralized hard callus that is supported by interstitial soft tissue. We conclude that with appropriate material modeling, virtual mechanical testing is a reliable surrogate for physical testing and that this technique has high translational potential for diagnostic testing of nonunion.