Biomechanics of spinal metastases

The lack of suitable models for prediction of the vertebral body (VB) failure load for a variety of pathologies hampers the development of indications for surgical and pharmaceutical interventions and the assessment of novel treatments. Similar models would also be of benefit in a laboratory environment in which predictions of failure load could aid experimental design when using cadaveric tissue. Finite element modelling shows great potential but the expertise required to effectively deploy this technology in a clinical environment precludes its routine use at the present time. Its deployment within the laboratory environment is also time consuming. An alternative approach may be the use of composite beam theory structural analysis that takes into account both vertebral geometry and the bone mineral density (BMD) distribution and they are utilised to predict the loads at which vertebrae will fail.
As a part of this work, vertebrae suffering from three distinct pathologies (osteoporosis, multiple myeloma (MM) and metastases) were tested in a wedge compression loading protocol (WCF) as a determinant for vertebroplasty treatment. MM bone was first tested for changes at the bone tissue level by means of depth-sensing micro-indentation testing. In the second part more than one hundred VBs were subjected to a destructive in-vitro WCF experiment, while CT images were used for in-silico structural and morphological assessment. In the last part, two vertebroplasty cements, calcium phosphate and PMMA, were tested.
At the tissue level MM bone shows rather moderate changes which are of such small magnitude that alone would not be sufficient to change the overall vertebral strength. Relatively good predictions of VB strength were obtained when using image-based fracture prediction suggesting that bone distribution and pathological alterations to its structure make a significant contribution to overall VB strength. The results of VB reinforcement using either of the cements show increased strength while stiffness was restored only when PMMA cement was injected in lower porosity samples.