Biomechanical analysis of the lumbar spine on facet joint force and intradiscal pressure--a finite element study

Abstract

Background: Finite element analysis results will show significant differences if the model used is performed under various material properties, geometries, loading modes or other conditions. This study adopted an FE model, taking into account the possible asymmetry inherently existing in the spine with respect to the sagittal plane, with a more geometrically realistic outline to analyze and compare the biomechanical behaviour of the lumbar spine with regard to the facet force and intradiscal pressure, which are associated with low back pain symptoms and other spinal disorders. Dealing carefully with the contact surfaces of the facet joints at various levels of the lumbar spine can potentially help us further ascertain physiological behaviour concerning the frictional effects of facet joints under separate loadings or the responses to the compressive loads in the discs.

Methods: A lumbar spine model was constructed from processes including smoothing the bony outline of each scan image, stacking the boundary lines into a smooth surface model, and subsequent further processing in order to conform with the purpose of effective finite element analysis performance. For simplicity, most spinal components were modelled as isotropic and linear materials with the exception of spinal ligaments (bilinear). The contact behaviour of the facet joints and changes of the intradiscal pressure with different postures were analyzed.

Results: The results revealed that asymmetric responses of the facet joint forces exist in various postures and that such effect is amplified with larger loadings. In axial rotation, the facet joint forces were relatively larger in the contralateral facet joints than in the ipsilateral ones at the same level. Although the effect of the preloads on facet joint forces was not apparent, intradiscal pressure did increase with preload, and its magnitude increased more markedly in flexion than in extension and axial rotation.

Conclusions: Disc pressures showed a significant increase with preload and changed more noticeably in flexion than in extension or in axial rotation. Compared with the applied preloads, the postures played a more important role, especially in axial rotation; the facet joint forces were increased in the contralateral facet joints as compared to the ipsilateral ones at the same level of the lumbar spine.

Figure 1

The (a) jagged and (b) smooth contour lines of a DICOM image.

Figure 2

Comparison of the calculated results with previous studies.

Figure 3

Sensitivity analysis for the intradiscal pressure versus the inner fiber strength, under preload 460N, at level L2/L3.

Figure 4

The changes in intradiscal pressure in the (a) L1/L2 and (b) L4/L5 disc of present study and Rohlmann et al. [9]for the intact lumbar spine.

Figure 5

(a) Front view/(b) Back view of von Mises stress distribution and (c) front view/(d) back view of von Mises strain distribution in the lumbar spine (without ligaments) under an evenly distributed load of 460 N over the superior surface of the L1 vertebral body in a standing posture.

Figure 6

Facet joint forces at various levels under different combinations of preloads and loadings in extension.

Figure 7

Facet joint forces at various levels under different combinations of preloads and loadings in left rotation.

Figure 8

Facet joint forces at various levels under different combinations of preloads and loadings in right rotation.

Figure 9

Intradiscal pressures at various levels of the lumbar spine under preloads of (a) 300 N, (b) 460 N, and (c) 600 N and different loadings, including forward/backward bending moments and left/right rotation moments of 5 Nm, 10 Nm, 15 Nm, and 20 Nm.

Figure 10

Intradiscal pressures at (a) levels L1/L2 and L4/L5, and (b) levels L2/L3 and L3/L4 under a preload of 460 N.

Figure 11

Facet joint forces at various levels under a preload of 460 N and left/right rotation moments of 5 Nm and 10 Nm, (L) and (R) denote left facet joint and right facet joint, respectively.