Low back pain is a clinical and public health problem affecting more than half of the population. Disc degeneration was noted as a major source of low back pain. Reviews of epidemiological studies revealed that long-term exposure to sedentary work posture and whole body vibration was associated with disc degeneration and low back pain. The prolonged effects from these confined conditions may initiate mechanical failure or lead to insufficient nutrition of the intervertebral discs resulting from disc fluid loss.

In order to understand the effects of mechanical loading on the fluid flow within the lumbar motion segment, a three-dimensional poroelastic finite element model was developed to study the responses of the motion segment to static and vibrational loading with an attempt to find implications on the associated pathological disc changes. The finite element mesh of a L4-L5 lumbar motion segment was reconstructed using the commercial finite element package ABAQUS v6.1. The geometry of the model was obtained from reconstruction of CT scans, which consisted of two vertebrae, two endplates, an intervertebral disc, two facet joints and seven associated ligaments. The biphasic material property was applied to the cancellous bone, endplates and intervertebral disc. The model was employed to predict the biomechanical parameters such as intradiscal loading, deformation and facet load under compression, sagittal and lateral bending and axial torsion. The diurnal variations in load sharing among different components of the motion segment resulting from fluid volume change were predicted under simulated sitting posture and whole body vibration. The finite element analysis showed that the intervertebral disc was the main structure in supporting compressive load. The facets shared a minor portion of loads under compression while major in resisting extension, lateral and torsional moments. Relatively high compressive stresses were predicted in the endplates as compared to the intervertebral disc. Under vertical compression, maximum fibers stresses were located at the postero-lateral region of the annulus that complies with clinical observation in which annulus rupture was prevalent within this region. The annulus fibers experienced high tensile stress under torsion especially combine with compression. Ligaments were important in resisting flexion moment and the interspinous and supraspinous ligaments sustained the largest tensile strains. Capsular ligaments were most active in supporting torsional moment.

Under a static situation, the loads carried by the annulus and the facets were increased with time. Meanwhile, the pore pressure of the fluid phase decreased as the structure deformed and fluid expelled from the disc. The load was gradually transmitted to the solid skeleton of the tissue. Under vibrational loading, the rate of creep increased as a result of large fluid volume loss especially from the nucleus. The fluid phase played a more important role in supporting or assimilating the applied cyclic compression because of a higher increase in pore pressure than that of the effective stress. The results showed that the fluid flow of the intervertebral disc was dependent on both the frequency and duration of loading. The finite element analysis indicated that there might be an optimal loading period, which is important in facilitating fluid flow of the intervertebral disc so as to enhance the disc nutrition and metabolism.