INTRODUCTION :
Facet tropism(FT) is defined as the angular asymmetry between the left and right facet joint orientation. FT is likely to cause biomechanical changes in lumbar movement, which accelerates degenerative changes in adjacent intervertebral discs or joints. However, the relationship between facet tropism and lumbar degenerative changes is still controversial. Many clinical studies attempted to correlate FT with lumbar disc degeneration, but few biomechanical tests of this concept have been made. The purpose of the present study is to investigate both clinical and biomechanical relationships between FT and lumbar disc herniation. We used provocative discography data and tried to delineate the correlation between FT and discogenic pain clinically. For biomechanical analysis, we used a finite element (FE) model of the single lumbar spine unit. Since FT is most commonly found at L4-L5 vertebral segment and the most common levels for a herniated disc are L4-L5 and L5-S1, we decided L4-L5 segment for the three-dimensional FE model.
Methods:
A total of 97 patients who received provocative discography from 2012 to 2018 in our center were evaluated. Each patient received provocative discography in one or more lumbar levels. Bilateral facet angles were measured using the patient’s magnetic resonance imaging. The correlation was measured using generalized estimating equation and the correlation between FT and each vertebral segment was calculated using chi-square test.
We developed a three-dimensional FE model of the L4 and L5 vertebrae using computed tomography scanning and the other components including intervertebral disc, posterior elements, articular facet joints, ligaments and the surrounding muscles were created. Material and element properties were selected from various sources in the literature.
We obtained the mean facet angle at L4-L5 from the previous studies, and based on this, we developed F40 and F45 models with symmetrical facet joint angle 40° and 45°, respectively. In addition, a FT model was developed with 40° at the right side and 50° at the left side.
Each model (F40, F45 and FT) was loaded with forces and moments, applied to the centre of mass of the L4 vertebra. The axial compression, flexion-extension and the lateral bending moments were considered and the intradiscal pressure was calculated.
Results:
In account for each lumbar segment, FT showed significant relation only at the L4/5 level (p-value 0.020) but not at L3/4 and L5/S1 levels. Biomechanical results from the analysis of each models will be soon presented. We hypothesize that the intervertebral disc pressure may be significantly higher in the FT model compared to F40 and F45 models.
Conclusion: FT plays a role in discogenic pain and the impact was higher in the L4-5 lumbar segment. FT may result in increased intradiscal pressure, leading to lumbar disc herniation.
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