Abstract
Numerous in vitro studies have utilised bone models for the assessment of orthopaedic medical devices and interventions. The drivers for this usage are the low cost, reduced health concerns and lower inter-specimen variability when compared to animal or human cadaveric tissues. Given this widespread exploitation of these models the push for their use in the assessment of spinal augmentation applications would appear strong. The aim of the research was to investigate the use of surrogate-bone vertebral models in the mechanical assessment of vertebroplasty.
Nine surrogate-bone whole vertebral models with an open-cell trabeculae configuration were acquired. Initial μCT scans were performed and a bone marrow substitute with appropriate rheological properties was injected into the trabeculae. Quasistatic loading was performed to determine the initial fracture strength in a manner previously used with human cadaveric vertebrae. Following fracture, vertebroplasty was undertaken in which there was a nominal 20% volume fill. Following augmentation the VBs were imaged using uCT and then subjected to an axial load using the same protocol.
The surrogate models had a substantially thicker cortex than that of human osteoporotic vertebrae. During compression, the surrogate-bone models did not exhibit the characteristic ‘toe-region’ observed in the load-deformation profile of cadaveric vertebrae. The mean initial and post-augmentation failure strength of the surrogate vertebrae were 1.35kN ± 0.15kN and 1.90kN ± 0.68kN, respectively. This equates to a statistically significant post-vertebroplasty increase by a factor of 1.38. In comparison with human osteoporotic bone, no significant difference was noted in the relative increase in fracture strength between the artificial and human VB following augmentation.
Despite the apparent equivalence of the strength and stiffness of the artificial vertebrae compared to that of the cadaveric specimens, there are significant differences in both pre- and post augmentation behaviour. In particular, the load-deformation curve shows significant differences in shape particularly at the toe end and in post failure behaviour. There are also issues surrounding where the marrow and cement flows during the injection process thus affecting the final distribution of the cement.
Correspondence should be addressed to Dr Roger Bayston, Division of Orthopaedic and Accident Surgery, Queen’s Medical Centre, Nottingham, NG7 2UH, England.
