Experimental and computational study of the behaviour of trabecular bone-cement interfaces.

Vertebroplasty is a treatment for vertebral compression fractures in which cement is injected into the vertebral body to relieve pain and stabilise the fracture site. Conflicting reports in the literature as to its efficacy indicate that further biomechanical evaluation of vertebroplasty is necessary to optimise the treatment variables (cement injection location, volume and composition) and to better understand which patients vertebroplasty will benefit. Finite element (FE) methods provide a means by which this can be undertaken under controlled conditions which are not possible experimentally, but existing vertebral FE models poorly reproduce the behaviour of cement-augmented vertebrae. The aim of this study was to develop an improved, clinically practical FE method of representing the behaviour of the interface between the bone and cement augmentation. Appropriate homogenous finite element (hFE) micro-computed tomography (μCT) greyscale-modulus and yield strain relationships were derived for un-augmented ovine lumbar vertebral trabecular bone. Similar ovine vertebral bone specimens were then fractured and augmented with poly(methyl methacrylate) cement, and novel methods and equipment were developed to enable the imaging of these specimens using μCT as they were deformed to failure in axial compression. Proprietary software was then used to determine the specimen strain distribution from the images. hFE models that incorporated an explicitly modelled interfacial region were generated from the images and parametric studies undertaken to derive the most appropriate interfacial properties. Good agreement with the corresponding load-displacement and strain distribution data was achieved. Finally, a preliminary study was conducted in which the new method of representing the interface was incorporated into existing hFE models of whole cement-augmented vertebrae. The predicted strain-distribution seen within the modified whole vertebral models more closely matched the behaviour of the earlier interfacial specimens, though this has yet to be validated experimentally against cement-augmented whole vertebrae.