Development of a Porcine FE Model for the Investigation of Vertebral Laminae Strains Resulting From Facet Tropism

As many as 85% of adults experience back pain that interferes with their work and leisure activities, and 25% of people between the ages of 30–50 years report lower back symptoms. Much of the pain and discomfort in later life results from an untreated condition during adolescence. Stress fractures of the vertebral lumbar laminae are given the clinical name spondylolysis. This vertebral defect is an acquired fracture with 7% prevalence in the paediatric population. This fracture has a mechanical aetiology, and fair evidence exists to support the role of facet tropism (geometric asymmetry) as a predisposing factor. Tests were carried out on porcine lumbar vertebrae, on which a series of angular asymmetries were simulated. Strain was recorded using 3-element stacked rosette strain gauges placed on the vertebral laminae. These tests showed that as each subsequent step of asymmetry is applied there is an increase in both Von Mises stress and strain on the ipsilateral side, this increase has a complex non-linear progression and pathological values for strain (>3000µε) are recorded indicating potential damage, which is supported by an average 17% reduction in facet/laminae stiffness (N/mm). An FEA model of the vertebra was created using µCT scans and published formulae linking bone mineralisation to material properties. This model successfully replicated both the facet/laminae stiffness (N/mm) and strains that were measured during test. The degree to which facet asymmetry is a predisposing factor and the knowledge of potentially pathological strain levels in the vertebra are important parameters when evaluating new implant devices and surgical techniques.