As a part of the posterior structure of the lumbar spine, the facet joints share the pressure and restrict the rotation. The relationship between the morphology of facet joints and lumbar disc herniation has been controversial. Lumbar facet joint asymmetry (FT) refers to the asymmetry of bilateral facet joint angles. Some studies believe that the asymmetry of lumbar facet joints is the influencing factor for the occurrence of LDH [1-4]. Other studies have suggested no association between facet joint asymmetry and LDH [5].
Some studies suggested that lumbar facet joint asymmetry was associated with the occurrence of LDH, Wang et al. [3] measured 65 patients with LDH by MRI and compared them with 30 normal people. They found that 20 patients with LDH had FT, while only 3 patients in the control group had FT. Chadha et al. [6] studied 60 patients with LDH, and through MRI examination, found that 25 cases of L4/5 intervertebral disc protrusions, 35 cases of L5/S1, 6 cases of FT in the observation group, 3 cases of FT in the control group (P = 0.145), 13 cases of FT in the L5/S1 observation group, and 1 case in the control group (P = 0.0094).
Our study showed that LDH was associated with FT. On axial MRI, the incidence rates of FT in LDH patients were 60%, 66.3%, 56.3%, and 61.7% in L3-L4, L4-L5, L5-S1, and all segments, respectively, while those in the control group were 17.6%, 19.6%, 9.8%, and 15.7%, all of which were statistically significant. Previous studies mostly focused on the axial position of CT or MRI and did not observe the sagittal position, which limited the observation results to a plane rather than a three-dimensional measurement. For the first time, our study measured FT at the sagittal position and found that the incidence of FT in LDH patients was 60%, 22.5%, 57.8%, and 39.6% in L3-L4, L4-L5, and L5-S1 segments, and 17.6%, 14.7%, 11.8%, and 15.7% in the control group, respectively, except for L4-L5 (p=0.176). All the differences were statistically significant. The sagittal plane can be used to evaluate the morphology of the facet joints.
The cause of LDH due to FT is still unclear. Many studies have shown that the direction of the symmetric articular surface can affect the stress of facet joints [7-8]. Kim et al. [9] through the finite element analysis, the simulation analysis was carried out on the lumbar FT, found under the stretching and torque, the FT model relative to the symmetry model, the pressure in the lumbar increased significantly, the moment an intervertebral fusion, the intervertebral disc pressure on buckling, stretching, torsional and lateral bending are significantly increased, increased by 98.5%, 91.2%, 75.5%, and 76.5% respectively. They believe that the increased stress will lead to disc degeneration, leading to the occurrence of LDH. Masharawi et al. [10] show that the presence of FT led to the rotation of articular motion and that the facet joints facing the coronal plane would limit forward and back displacement, and then rotation toward the sagittal position would occur, which was more obvious during flexion and extension, and the occurrence of rotation increased the risk of damage to the fibrous ring, leading to intervertebral disc protrusion. Other studies have suggested that FT leads to asymmetric stress transfer between facet joints and corresponding intervertebral discs, resulting in stress concentration, which may accelerate intervertebral disc degeneration and lead to LDH [11]. At the same time, the adjacent segment FT of the fusion segment will increase the phenomenon of stress concentration, which may promote the occurrence of adjacent vertebral disease after lumbar fusion [12]. The presence of FT may also lead to the asymmetry of the paraspinal muscles, which may also be the cause of LDH [13]. In our study, FT was found not only in the horizontal but also in the sagittal position in patients with LDH, which may be due to the rotation of the facet joints. Previous studies have shown that rotation of the facet joints causes stress changes in the disc, which in turn causes disc degeneration. However, due to the retrospective study, we were unable to determine the causal relationship between LDH and FT.
Although FT is defined as the asymmetry of bilateral facet joints, the specific criteria are still different. In their study, Cyron et al. defined the difference value as 1 ° [3,14], while Noren defined it as 5 ° [1]. In other studies, asymmetry was defined as the difference value greater than 7 ° [15]. Many recent studies show the difference as 10° [16-17]. The FT angle is too small may be measurement error cause [6], in this case, will no doubt be introducing more FT, in turn, affect the results, so we study the FT is defined as the difference on both sides of the small joint angle acuity 10 °, and the difference of greater angle (e.g., 15 ° or higher) are relatively rare in the cases, which leads to fewer cases and is not conducive to statistical analysis.
Some studies believe that the influence of FT and LDH is related to age. Wang et al. studied the adolescent LDH population and believed that the incidence of FT in patients with LDH was higher than that in the normal population [2]. Kalichman et al. [18] studied 188 middle-aged and elderly people and found that 76.7% of males and 66.3% of females had facet joint asymmetry, but there was no statistical difference between them.
Changes in FT may change with time. Researchers followed up the facet joints of 54 patients with LDH. Within two years, 7 cases of FT disappeared, but 7 new FT appeared, which may require further follow-up of our study to observe the causal relationship between FT and LDH.
There are still some deficiencies in our study. First, the sample size of LDH in the L3-4 segment is small, so more observation samples should be included. Second, we applied a retrospective study. Prospective studies should be included in future studies to further determine the relationship between FT and LDH.