If the articular facets of the vertebra grow in an asymmetric manner, the developed geometry causes an asymmetry of loading. When the loading environment is altered by way of increased activity, the likelihood of acquiring a stress fracture may be increased. The combination of geometric asymmetry and increased activity is hypothesised to be the precursor to the stress fracture under investigation in this study, spondylolysis. This vertebral defect is an acquired fracture with 7% prevalence in the paediatric population. This value increases to 21% among athletes who participate in hyperextension sports. Tests were carried out on porcine lumbar vertebrae, on which the effect of facet angle asymmetry was simulated by offsetting the load laterally by 7 mm from the mid-point. The aim of the study is to investigate whether an increase in the coronal orientation of one facet leads to an increase in strain in the corresponding vertebral lamina. Strain in the laminae was recorded using six 3-element stacked rosette strain gauges placed bilaterally. Results show that a significant linear predictive relationship exists between load offset and average strain level in the vertebral laminae with p values of 0.006 and 0.045 for principal strains ε and ε on the right-hand side, and p-values of 0.009 and 0.001 for principal strains ε and ε on the left-hand side ( R all >0.9). This study concludes that facet angle asymmetry does lead to a difference in strain in the vertebral laminae. Change in principal strain as a result of facet asymmetry has a linear relationship and an asymmetry threshold exists beyond which compressive strain on the more coronally oriented facet can be increased by up to 15%.

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.

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