4.1 The structure and function characteristics of IVD and facet joints
The IVD consists of three structurally various tissues: the annulus fibrosus (AF), the nucleus pulposus (NP), and two cartilaginous endplates that connect the disc with the adjacent vertebrae 11,12. Throughout the lifetime, IVD undergoes a morphological and functional degenerate. LDH refers to a series of clinical symptoms caused by lumbar IVD degeneration and the herniation of nucleus pulposus, causing stimulation or compression of neighboring tissues. Among all cases, the highest incidence is at L4-5 and L5-S1 level with a prevalence of 95%.13 LDH seriously threatens the health of the elderly and middle-aged. However, with the increasing pressure of daily life, the amount of activity and load on the spine have slowly increased, resulting in a younger trend of LDH, and there is an increasing number of patients with LDH in the youth population. Many youth patients having LDH do not have a long labour-time or previously trauma as a cause. If the relevant risk factors are intervened before the start of the disease, it is believed that the number of young patients with LDH will be significantly reduced.
The facet joints are the only synovial joints in the lumbar spine. Both sides of the facet joints and corresponding IVD form a three-joint complex. Several previous studies have considered that the two facet joints carry loads together with the IVD in the lumbar spine 3,14,15. These studies showed that any deformity of one facet joint could affect the other and might cause asymmetric stress transmission to both facet joints and IVD, eventually leading to abnormal stress applications in regions of the IVD and facet joints. Facet joints are considered to share loads mainly during perpendicular motion of the lumbar spine and also protect the corresponding IVD from the excessive rotational motion 16. The role of facet joint abnormality in the progression of LDH has been consistently studied during recent years 6–8,17−20. Nevertheless, it remains unclear whether the changes of the facet joint geometrical parameter (FO and FT) act as the original morphological variations or it should be recognized as a result of reconstruction.
4.2 Relationship between FO, FT, and LDH in corresponding segment
In 1967, Farfan et al. 21 first proposed that asymmetry of the facet joints was a possible cause of LDH. Since then, various studies have yielded conflicting results concerning the association of facet joint parameters and the pathogenesis of LDH 22–26. Van Schaik et al. 27 were the first to use CT scans to address this issue. They measured facet asymmetry in 100 patients with backache or sciatica or both, and found that there was an equal distribution of herniation to the side of both the more coronally oriented and more sagittally oriented facet joint. With greater degrees of asymmetry, there was a greater incidence of unilateral disc protrusion towards the side of the more coronally oriented joint. Park et al. 28 compared far lateral and posterolateral LDH and found that the differences in the degree of FT and disc degeneration might be considered a key factor in distinguishing the development of far lateral LDH from that of posterolateral LDH. Noren et al. 10 also concluded that facet joint asymmetry is a risk factor for the development of disc degeneration and herniation at all lumbar levels. Karacan et al. 29 observed that the patients with LDH had more asymmetry and sagittal orientation of the facet joints, and these alterations were more evident in the taller patients. More recently, Wang et al. 30 concluded that the measurement on different portions of facet joint may result in discrepancy on FT identification, and asymmetry between ipsilateral cephalad and caudad facet portions is also associated with L4-5 LDH in older patients.
In contrast, other scholars believe that the facet joint parameters and LDH do not correlate, and that lumbar facet joint asymmetry is a congenital structural manifestation, which is not due to age or degeneration 31–34. Lee et al. 34 assessed 149 levels in 140 adolescents aged between 13 and 18 years and 119 levels in 111 adults aged between 40 and 49 years with LDH. They reported no significant difference in FT between the herniated and the normal discs in both the adolescent and adult groups, except at the L4-L5 level in the adults. They demonstrated that FT did not influence the development of herniation of the lumbar disc in either adolescents or adults. Cassidy et al.33 found that there was no difference in the distribution of the more coronally or sagittally facing facet joints with respect to the side of lateral herniation. These results do not support the hypothesis that facet asymmetry is associated with LDH. Another study by Vanharanta et al. 31 also showed that there was no association between FT and lumbar disc diseases including herniation and degeneration. In the present study, we compared the FO and TO between the LDH and N-LDH groups, and found that there was a significant correlation between these two parameters and LDH at the L4-5 and L5-S1levels.
What is the reason for the inconsistency in the results of many studies on the association between facet joint parameters and LDH? A lot of variables could affect the conclusion of the study, such as the method used to measure the facet joint angle, the definition of FT, the type of control group, and so on. The age was also considered an important factor which influences the results of correlation between facet joint parameters and LDH. Because the changes of the facet joint parameters may be a result of LDH, especially in older patients with a long history of LDH, but this hypothesis needs to be verified. The IVD degeneration is more prevalent with increasing age. These degenerative changes such as decreased disc height index and increased sagittal range of motion further alter the biomechanics of the motion segment. As a result, it has been propagated that the facet joints are overloaded and become more susceptible to anterior shearing forces leading to facet joint remodeling and the development of LDH. At present, it is difficult to know whether the changes of the facet joint parameters act as the primary morphological variations or it should be recognized as a secondary reconstruction. In the present study, to eliminate the influence of age and prove the initial influence of facet joint parameters on IVD of the corresponding level, we limited patients' age between 18–35 years with the clinical history within two years which is distinguished from previous studies. Our study demonstrated that FO and FT are significantly associated with LDH at both L4-5 and L5-S1 levels. LDH group had a significantly greater degree of FT and lowered FO than N-LDH group. These differences were perceived to be statistically significant may be attributed to secondary changes because of changes in the facet parameters. It is to say LDH may be a result of facet joint parameters abnormality, especially in youth patients without a long labour-time or previous trauma. Lower FO and higher FT may be regarded to be risk factors for the development of LDH in the youth population.
Many biomechanical studies have found that facet joint parameters significantly influence the biomechanics of the corresponding segment 17,20,35−38. Some scholars proposed that a more sagittal orientation of the facet joint promoted anterior gliding by reducing resistance to anterior shear forces 17. 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 IVDs which may plausibly contribute IVD or facet injury and degeneration. At the same time, the more sagittal orientation facet joint leaves angular motion as well as rotation to compensate for the other. Gradually, this rotational movement indirectly applies tensile stress on the annulus of the IVD, leading to protrude on the sagittal side 39. In the present study, we found there is a significant correlation between the location of LDH and different FO of bilateral facet joints in L4-5 level. The IVD is more likely to protrude on the ipsilateral side where the FO is lower in L4-5 level. Our study raises the probability that the IVD has the inclination of herniated toward the sagittally oriented facet joint whenever there happened to be a combination of sagittal and coronal orientation in L4-5 level. However, a few previous studies have refuted this hypothesis 40. This judgment still needs further confirmation with clinical and biomechanical research. Besides, our study demon strated that FT is more significantly related to LDH at the L4-5 level than L5-S1 level. Meanwhile, there is a significant association between the different locations of LDH and different FO of bilateral facet joints in L4-5 level while not in L5-S1 level. We speculated the reason is that the IVD in L5-S1 level below the iliac crest, the ilium could restrict segment movement, thus reduce the pressure and shear forces on the IVD.
4.3 limitations
There were several limitations concerning the current study. Firstly, this was a retrospective nonrandomized case-control study conveyed only on specific groups. Secondly, we had not selected sagittal balance, paraspinal muscle volume, pelvic and angular parameters, and other demographic features that may individually influence the development of LDH. Thirdly, our study was limited by geometrical considerations. The articular surface of the facet joint was viewed as a flat plane, which was not the most suitable representation of the real geometry. Our study did not take into account the complex three-dimensional geometry of the facet joints and their relationship with the lumbar spine degeneration. Future prospective, longitudinal, multi-modal imaging, and biomechanics studies are needed to further assess the feasibility and accuracy of this study.