Finite element investigation of the effect of Spina bifida on loading of the vertebral isthmus

Introduction Lumbar spondylolysis is a fatigue fracture of the pars interarticularis and correlates with Spina Bifida Oculta (SBO) in 67%. Hpothesis Load is normally transferred across the arch in axial rotation. Bifid arch results in increased strain across the isthmus of the loaded inferior articular process. Aim of investigation Finite element (FE) analysis of altered load transfer in combined axial rotation and anteroposterior shear in SBO potentially predisposing to fatigue fracture of the pars interarticularis. Methods FE models of natural and SBO (L5-S1) including ligaments were axially load to 1kN and an axial rotation of 3° applied. Bilateral stresses and strains on intact and SBO lateral inferior lines of the L5 isthmus were assessed and compared. Results Under 1000N axial load: Maximum von Mises stress observed on left and right lateral inferior lines of L5 isthmus were 0.13 and 0.24 MPa, with maximum equivalent strain values of 1.56 and 2.91 (strain, for natural spine and SBO, respectively. Combined with 3° axial rotation (rotation of spinal processes toward right lateral side): Left lateral L5 isthmus stresses increased to 0.49 and 0.77 MPa for natural spine and SBO, respectively. Right lateral L5 isthmus values increased to 0.67 and 0.95 MPa for natural spine and SBO, respectively. The percentage increase in SBO strains compared to the natural spine on the L5 isthmus were +57.9 and +40.2%. Conclusion Significant load transfer occurs through the vertebral arch in axial rotation. In SBO this load transfer is lost and mechanical demand on the isthmus is significantly increased. Strain increases across the L5 isthmus in axial rotation by +40.2% to +57.9% compared to normal and may predispose to fatigue fracture.