Structural finite element analysis of the lumbar spine applied to the conceptual design of a compliant interbody cage implant

Abstract

A detailed solid model of an entire lumbar segment is reconstructed through a reverse engineering approach based on a real patient Computed Tomography DICOM scan. The main structurally and functionally relevant elements have been considered in the model, including the L1-S1 vertebrae, intervertebral discs, ligaments, and the simplified structural and bracing effects of the principal lumbar muscles. Using the model, a structural Finite Element (FE) analysis is setup to provide an insight of stresses and range of motion of the “spinal mechanical system” under typical everyday as well as accidental loads. Organic bone, discs, ligaments physical and structural material data have been retrieved, and both static and fatigue load cases have been taken into account, collecting real world data from a review of literature works and of information available from accessible databases. A validation of the FE model is provided based on the so called Yamamoto protocol first, and then using experimental measurements performed on real patients, again taken from literature works. The numerical model proved to be accurate in the description of the stress states of the different parts of the spine and in reproducing the overall range of motion of the lumbar segment. A tentative design modification of rigid interbody cages and vertebral fixation systems used in present spinal fusion neurosurgery is then proposed, aimed at providing a certain level of compliance to the implant. This should restore a limited range of motion of the two adjacent vertebrae hosting the implant itself, reducing the possibility of subsequent adjacent segments diseases. The new cage and fixation system have been integrated in the FE model to replace a prospective “damaged” L4-L5 disc. The results of the numerical simulation demonstrated that the implant can withstand static and fatigue everyday life loads, as well as accidental overloads without failing. Additionally, the analysis showed that a limited mobility is maintained between the fused vertebrae.