DETERMINATION OF LOAD TRANSMISSION AND CONTACT FORCE AT FACET JOINTS OF L2–L3 MOTION SEGMENT USING FE METHOD

In the human spine, it is well known that facet joints play a significant role in load transmission and providing stability. It has also been hypothesized to be one of most probable sources of low back pain. Experimental determination of the load-bearing role of lumbar intervertebral joints, such as the facets joints, under axial compression has not been a straightforward task. In this study, the role of the facets in load transmission through a L2–L3 motion segment under axial compression is investigated using a L2–L3 finite element (FE) model, incorporated with an accurate three-dimensional geometry of facet joints with the inclusion of surface-to-surface continuum contact representation. The effects of osteoarthritis on facet force and biomechanical behaviors are also investigated by assuming friction at the facet joints. The study shows that the facet joints resisted 8% more in load for joints with osteoarthritics as compared with the normal joints. High percentage increase in contact facet force was also predicted for joint with osteoarthritics deformity. The use of the analytical FE model provided yet another efficient alternative for predicting the load transmission and contact force for degenerative joints, so as to provide a better understanding of the biomechanics of the spine as well as the pathophysiology of the various spinal disorders and degenerative conditions.

Effect of facetectomy on lumbar spinal stability under sagittal plane loadings

STUDY DESIGN

A study using an anatomically accurate finite-element model of a L2-L3 motion segment to investigate the biomechanical effects of graded bilateral and unilateral facetectomies of L3 under flexion and extension loadings.

OBJECTIVE

To predict the amount of facetectomy on lumbar motion segment that would cause segmental instability, therefore enhancing the understanding concerning the role of the facet under sagittal loadings.

SUMMARY OF BACKGROUND DATA

This study provides a quantitative study on the role of facets in preserving segmental lumbar stability. Previous analytical models lack of three-dimensional structural characterization and insufficient element representation for facet joints.

METHODS

A validated finite-element L2-L3 model was subjected to sagittal loadings at 7.5 Nm. Effects of ligaments and facets were examined to establish their relative importance on segment response. The effect of iatrogenic changes (graded unilateral and bilateral facetectomy) was then investigated under these loadings to predict the alterations in terms of gross external (angular and coupled) responses, flexibilities, and facet load.

RESULTS

This study shows the importance of preserving ligaments to prevent rotational instabilities for motion segment under flexion. The effect of the facetectomy on the motion segment is insignificant under flexion. In extension, unilateral facetectomy and resection on contralateral facet markedly alters the rotational motion and flexibilities as well as coupled motions. Also, unilateral complete facetectomy with resection of less than 100% on contralateral facet generates high facet load.

CONCLUSIONS

Clinically, this study suggests that it may be appropriate to incorporate additional stabilization procedure in restoring the spinal strength and stability for surgical intervention of unilateral complete facetectomy and resection on contralateral facet. The exploitation of the finite-element method to simulate clinically related situations permits an improved understanding of lumbar spinal stability to assist in defining clinical expectation for various forms of surgical intervention of the operative procedures.

Finite-Element Analysis for Lumbar Interbody Fusion Under Axial Loading

A parametric study was conducted to evaluate axial stiffness of the interbody fusion, compressive stress, and bulging in the endplate due to changes in the spacer position with/without fusion bone using an anatomically accurate and validated L2-L3 finite-element model exercised under physiological axial compression. The results show that the spacer plays an important role in initial stability for fusion, and high compressive force is predicted at the ventral endplate for the models with the spacer and fusion bone together. By varying the positioning of the spacer anteriorly along anteroposterior axis, no significant change in terms of axial stiffness, compressive stress, and bulging of the endplate are predicted for the implant model. The findings suggest that varying the spacer position in surgical situations does not affect the mechanical behavior of the lumbar spine after interbody fusion.

The Biomechanics of Lumbar Graded Facetectomy Under Anterior-Shear Load

In this paper, an anatomically accurate three-dimensional finite-element (FE) model of the human lumbar spine (L2-L3) was used to study the biomechanical effects of graded bilateral and unilateral facetectomies of L3 under anterior shear. The intact L2-L3 FE model was validated under compression, tension, and shear loading and the predicted responses matched well with experimental data. The gross external (translational and coupled) responses, flexibilities, and facet load were delineated for these iatrogenic changes. Results indicted that unilateral facetectomy of greater than 75% and bilateral facetectomy of 75% or more resection markedly alter the translational displacement and flexibilities of the motion segment. This study suggests that fixation or fusion to restore strength and stability of the lumbar spine may be required for surgical intervention of greater than 75% facetectomy.