On the Lumbosacral Spine Geometry Variation and Spinal Load-Sharing: Personalized Finite Element Modelling

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Abstract

Computational modeling of the lumbar spine provides insights on kinematics and internal load development and distribution along the spine. Geometry (size and shape) of the spinal structures and more particularly sagittal curvature of the spine governs its response to mechanical loading. Thus, understanding how inter-individual sagittal curvature variation affects the spinal loadsharing between spinal components (discs, ligaments and facet joints) becomes of high importance. The load-sharing is an indicator of how spinal components interact together in a harmonic synergy to maintain its normal function.

This study aimed to investigate how the inter-individual sagittal curvature variation affects spinal load-sharing in flexed and extension postures using geometrically personalized Finite Element (FE) modeling.

This research used three lumbosacral spines with different curvatures: one hypo-lordotic (HypoL), one normal-lordotic (Norm-L) and one hyper-lordotic (Hyper-L) spines with low, normal and high lumbar lordosis (LL), respectively. A 3D nonlinear detailed FE model for the Norm-L spine with realistic geometry was developed and validated against a wide range of numerical and experimental (in-vivo and in-vitro) data.

The model was subjected to compressive Follower Load (FL) combined with moment to simulate flexed and extended postures. Load-sharing was expressed as percentage of total internal force/moment developed along the spine that each spinal component carried. These internal forces and moments were determined at the discs centers using static equilibrium approach and included the applied load and the resisting forces in the ligaments and facet joints. Sensitivity of the model predictions to a wide range of FL (500-1100N) and moment (0-20Nm) magnitudes was performed. Optimal magnitudes that minimized the deviation of the model predictions from in-vivo data were determined by optimization.

Additional FE models were developed for the Hypo-L and Hyper-L spines. Their kinematics and load-sharing in flexed and extended postures were compared.

The kinematics, intradiscal pressure (IDP) and articular facet joint force (FJF) predicted by the FE model were in a good agreement with previous FE results and in-vivo and in-vitro data. The sensitivity analysis revealed that the intervertebral rotations (IVRs), disc moment, and the increase in disc force and moment from neutral to flexed posture were more sensitive to moment magnitude than FL magnitude in case of flexion. The disc force and IDP were more sensitive to the FL magnitude than moment magnitude. The optimal ranges of FL and flexion moment magnitudes were 900N-1100N and 9.9Nm-11.2Nm, respectively. To obtain reasonable compromise between the IDP and disc force, our findings recommend that FL of low magnitude must be combined with flexion moment of high intensity and vice versa.

The Hypo-L spine demonstrated stiffer behavior in flexion but more flexible response in extension compared to the Norm-L and Hyper-L spines. The excessive LL stiffened response of the Hyper-L spine to extension but did not affect its resistance to flexion compared to the Norm-L spine.

Result showed that contribution of the facet joints and ligaments in supporting bending moments produced additional forces and moments in the discs. Results demonstrated that internal forces produced by FL and flexion were mainly carried by the discs (75%) and posterior ligaments (25%) while contribution of ligaments in supporting internal moment was higher (~70%) compared to the discs (~20%). Role of the facet joints was negligible except at level L5-S1. This force-sharing was almost similar in all the three spines. In the case of FL and extension, the discs, ligaments and facet joints shared spinal force with proportion of 55%, 20%, 25% respectively in the Hypo-L spine while facet joints contribution did not exceed 10% at levels L1- 4 and reached up to 30% at levels L5-S1 in the Norm-L and Hyper-L spines. The facet joints carried up to 63% of the internal moment in the Hyper-L spine.

This study demonstrated that spinal load-sharing depends on applied load and varies along the spine. It also depends on spinal curvature. The three spines studied demonstrated that interindividual curvature variation affects spinal load-sharing only in extended posture while no noticeable difference between the spines was found in flexed posture. Analyzing response of additional spines in each category under different loading conditions such as gravity load in future studies may reveal more significant effects of inter-individual curvature variations.

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