Facet joint motion analysis of L4-5 by using finite element modelling

Abstract

BACKGROUND: Transfacet joint screw fixation has been used to facilitate spinal fusion. However, facet screws offer inferior stability compared to pedicle screw fixation. Establishing restraints on range of motion is always essential to maintain segment stability during fusion. This study aims to develop a finite element model of lumbar segments so as to investigate the effect of facet joint fixation on overall segment motion. METHODS: A finite element model of L4-L5 spinal segment was created by laser scanning digitizer. Testing conditions included compressive preloads from 0 to 1200N, pure moments up to 15Nm for flexion, extension and axial rotation, and horizontal forces of ±600N simulating shear loading. To study the effect on overall motion, additional clamping forces of 0-2000N were applied normal to the facet joint surfaces. Quantitative measures on the angle of rotation were supported with statistical analysis at P<0.05. RESULTS: The angles of rotation were reduced in the spinal segment with clamping forces under all loading conditions except for pure flexion. However, the differences between trials were not significant (P=0.832). Linear regression revealed that association existed between angle of rotation under extension and additional clamping force (R=0.781, P=0.022). CONCLUSIONS: The concept of applying clamping forces at facet joints as a means of fixation method is manifested and the angle of rotation under extension is found to be associated with additionally applied forces. Further study will focus on facet kinematics so as to help develop an effective facet joint fixation device.