INTRODUCTION: Even though there have been many clinical studies regarding the facet orientation/facet tropism and disc degeneration, no biomechanical study has been reported about this aspect. Therefore, the purpose of this study was to investigate the association between the facet orientation/facet tropism and the corresponding/adjacent disc pressure, using the finite element (FE) model.
METHODS: Four validated lumbar FE models (L2‐5) were simulated, in which each model had a different facet joint angle at L2/3 segment such as 50°, 55°, 60°, and tropism (50°/60°), based on the data of L2/3 facet angle from previous study. Under flexion, extension moment, and anterior shear force, the intradiscal pressure and range of motion at all segments were analyzed, and compared among four models.
RESULTS: Under flexion moment, there was no difference in the intradiscal pressure at all levels among 50°, 55° and 60° models. Only tropism model yielded the 10% increase of intradiscal stress at L2/3 level, compared with other models. Under extension moment, most sagittally oriented, 60° model led to 2.0%, 1.7%, and 7.4% increase of intradiscal pressure at L2/3, L3/4, and L4/5, compared with the 50° model, and the tropism model have 5.7%, 1.7%, and 10.4% increase of intradiscal pressure at the L2/3, L3/4, and L4/5, respectively, compared with the 50° model. Under anterior shear force, the change of intradiscal pressure amounted to 11.1%, 22.2%, 27.7% increase at L2/3 segment in the 55°, 60°, and tropism model, respectively, compared with 50° model. The change of range of motion had similar trends to the change of intradiscal pressure.
DISCUSSION: Facet tropism leads to an increase of corresponding disc pressure, while facet sagittal angle have a little effect on the increase of corresponding disc pressure under pure moments. However, both sagittally oriented facet and tropism can make corresponding disc be vulnerable to anterior shear force.