The cause of chronic LBP in cases without clear and serious anatomic pathology is not known. The serious structural lesions such as tumors, infection, fractures, and severe deformities are frequently painful and fortunately can be diagnosed with modern imaging studies. However, these patients with serious structural problems are uncommon. Much more commonly people have back pain episodes of varying degrees and either do not seek care or are treated symptomatically without a pathoanatomic diagnosis. Why some people with common backache become patients with serious disability is of enormous clinical and public health importance. Previous studies suggested that structural factors, exposure to mechanical stressors, psychological factors and social circumstances could be correlate with the development of chronic LBP.1,2,4−6,9,29 However, there are many debates regarding the risk factors of chronic LBP and it is very difficult to define them because many complicated parameters are involved.29−32 Therefore, an understanding of the relationship between pathoanatomic abnormality and advanced degeneration is of importance from a clinical and public health perspective.
Many researchers believed that LBP, as a result of lumbar degeneration, begins in the intervertebral disc, followed by spine malalignment and facet joint degeneration.33−35 It is quite likely that the intervertebral disc and facet joints contribute to the initial degenerative process. In this study, we analyzed the associations between facet joint parameters (FO and FT), and chronic LBP in a Chinese population sample. To our knowledge, this work represents the first study to date to evaluate the associations between facet joint parameters and chronic LBP.
The facet joints are the only synovial joints in the spine, with hyaline cartilage overlying subchondral bone, a synovial membrane and a joint capsule. The intervertebral disc and the facet joints form a three-joint complex. As an important part of the three-joint complex in the posterior area of the spinal column, the lumbar facet joint has a far-reaching influence on the spine. Many biomechanical studies have considered that the intervertebral disc and the two facet joints carry loads together in the normal lumbar spine.36−38 Any abnormality of one joint could affect the others, which might cause asymmetric stress transmission to both facet joint and corresponding disc, and this leads to stress concentration at particular regions of disc and facet joint.
Biomechanically, the facet joints primarily share the load in compression, extension, and torsion of the lumbar spine and protect the disc against torsion. Previous studies have found that FO and FT significantly influence the biomechanics of the corresponding segment.24,37,39−41 Some scholars proposed that a more sagittal orientation of the facet joint promoted anterior gliding by reducing resistance to anterior shear forces.37,41 Kim et al.37 studied three models at different FOs (50o, 55 o, and 60o relative to the coronal plane) and one model with FT (50 o on the right, 60 o on the left). The three models with different FOs did not differ in the intradiscal pressure gradient but the FT model had the greatest increase in intradiscal pressure and facet contract force, suggesting that tropism is what makes it more vulnerable to anterior sheer force than orientation. In addition, when tropism was present, the motion segment was found to have a tendency to rotate towards the more oblique joint when axial loads were applied. This asymmetric axial rotation caused by tropism can place additional torsional loads on the intervertebral discs which can lead to intervertebral disc injury and degeneration.
Several studies have examined the relationship between FO, FT and facet joint osteoarthritis (OA).12,22,42−44 Grogan et al.42 found an association between FT and facet sclerosis, although no association was found between FT and a composite score of cartilage degeneration and sclerosis. Conclusions from this study are limited due to the very small sample size (n = 22) and the somewhat arbitrary criteria used to grade degeneration. Liu et al.44 studied asymmetric facet joint OA and its relationship to FT and ligamentum flavum thickening using CT scans of L3-4, L4-5, and L5-S1 levels of the patients. The investigators did conclude that there was a positive correlation between FT, asymmetric facet joint OA, and ligamentum flavum thickness. Fujiwara et al.43 found a significant association between FO and facet joint OA, but a negligible association between FT and OA in 111 Japanese patients. In a population-based CT study, a significant association between sagittal orientation and OA of the lumbar facet joints at the L4–L5 spinal level was found in 188 individuals. However, no association was found between FT and facet joint OA at any spinal level.22
Data on studies investigated the effects of FO and FT on chronic LBP appear only rarely in the literature. In the current study, we compared the FO and TO between the non-LBP and LBP groups in an Chinese population sample, and found that there was a significant correlation between FT and chronic LBP. We considered that both sides of the facet joints and intervertebral disc together constitute the spinal three-joint complex. When the lumbar spine is flexed and twisted, if both joints are asymmetric, the stress of the three-joint complex is imbalanced. Resistance on the sides of the vertebral body is different, and the vertebral body will deviate from the original trajectory, thus pulling the rear of the intervertebral disc. Such a loading imbalance may accelerate the degeneration of the facet joints and intervertebral discs. Therefore, FT may play a more important role in the pathogenesis of chronic LBP.
Our study has several limitations. This was a cross-sectional observational study on facet angle. our study was limited by geometrical considerations. Even if facet joints often were not planar, our measurements did not take into account the complex three-dimensional geometry of the facet joints and their relationship with the disc and facet joint degeneration. Due to these limitations, future studies should focus on more sophisticated biomechanical factors of the lumbar spine, and further explore the correlation between biomechanical factors and chronic LBP. Accordingly, we plan to perform next the biomechanical analysis of facet configuration such as FT and FO in finite element models of lumbar spine.