Structure and functional characteristics of the intervertebral disk and facet joints
An intervertebral disk consists of three structurally different tissues: the annulus fibrosus, the nucleus pulposus, and two cartilaginous endplates that connect the disk to adjacent vertebrae [23, 24]. Throughout a person’s lifetime, their intervertebral disks undergo morphological and functional degeneration. Degeneration of the intervertebral lumbar disks can result in herniation of the nucleus pulposus, which causes the inflammation or compression of neighboring tissues and a series of clinical symptoms. The highest incidences of disk herniation are for the L4–L5 and L5–S1 disks, with a combined incidence of 95%. LDH is a serious threat to the health of the older and middle-aged adults. However, with the increasing pressure of daily life, the amount of activity and load on the spine have slowly increased. This has resulted in adults being more likely to experience LDH at a younger age—there is an increasing number of patients with LDH in the youth population. Because LDH experienced by youth patients usually do not have a long-term physical labor occupation or previous trauma as a cause, if relevant risk factors are identified early on, it may be possible to intervene prior to the start of the disease to prevent LDH.
The facet joints are the only synovial joints in the lumbar spine. Both sides of the facet joints and the corresponding intervertebral disk form a three-joint complex. Previous studies have shown that the two facet joints, with the intervertebral disk, carry loads in the lumbar spine [11, 26, 27] and that any deformity in one facet joint could cause asymmetric stress distribution to all three structures. Facet joints share the vertical compression loads of the lumbar spine and also protect the intervertebral disks from being subjected to excessive rotation . The role of facet joint abnormality as a cause of LDH has been well studied during recent years [8–10, 15, 29–31]. Nevertheless, it remains unclear whether changes in facet joint geometry (FO and FT) are natural morphological variations or the result of reconstruction.
Relationships between facet orientation, facet tropism, and lumbar disk herniation in the corresponding segment
In 1967, Farfan et al.  first proposed that facet joint asymmetry was a possible cause of LDH. Since then, studies have yielded conflicting findings about facet joint parameters and the pathogenesis of LDH [17, 33–36].
Van Schaik et al.  measured facet asymmetry in 100 patients with backache, 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 asymmetries, there was a greater incidence of unilateral disk protrusion toward the more coronally oriented joint. Park et al.  compared far lateral and posterolateral LDH and found that differences in FT and disk degeneration might able to distinguish far lateral herniation from posterolateral LDH. Noren et al.  also concluded that facet joint asymmetry is a risk factor for lumbar disk degeneration and herniation. Karacan et al.  observed that patients with LDH had greater asymmetries and more sagittally oriented facet joints and that these alterations were more evident in taller patients. More recently, Wang et al.  concluded that measurements taken at different parts of facet joints may result in discrepancies in FT identification; asymmetry between ipsilateral cephalad and caudad portions of the facet is associated with L4–5 LDH in older adult patients.
In contrast, others have found evidence to support that lumbar facet joint asymmetry is congenital and not due to age or degeneration [14, 41–43]. Lee et al.  assessed 149 intervertebral disks in 140 adolescents (13 and 18 years) and 119 intervertebral disks in 111 adults (aged 40 and 49 years) with LDH and found that there was no significant difference in FT between herniated and normal disks in both groups, except for at the L4–L5 level in the adults. Thus, FT had not influenced the development of herniation of the lumbar disk in either adolescents or adults. Cassidy et al. found that there was no difference in the distribution of more coronally or sagittally oriented facet joints with respect herniation laterality. Vanharanta et al.  showed that there was no association between FT and lumbar disk diseases, such as herniation and degeneration.
In the present study, we compared FO and FT between patients with LDH and patients in a control group; we found that there was a significant relationship between these parameters and LDH at the L4–L5 and L5–S1 levels.
What is the reason for the inconsistencies between the results of previous studies? Many factors, such as the method used to measure facet joint angle, the definition of FT, or the type of control group could affect study findings. The age of the study population was also considered an important factor which influences the results of correlations between facet joint parameters and LDH. Changes in facet joint parameters may be the result of prior LDH, especially in older patients with a long history of LDH, but this hypothesis needs to be verified. Intervertebral disk degeneration is more prevalent with increasing age, and changes such as decreased disk height index and increased sagittal range of motion alter the biomechanics of vertebral segments. As a result, facet joints can be overloaded and become more susceptible to anterior shearing forces, which leads to facet joint remodeling and LDH[21, 29].
At present, it is difficult to know whether changes in 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 intervertebral disk of the corresponding level, we only included patients between 18 and 35 years of age with a clinical history less than 2 years, which is distinguished from previous studies[9, 21]. We demonstrated that FO and FT were significantly associated with LDH at both L4–L5 and L5–S1 levels. The LDH group had significantly greater FT and lower FO than those in the control group. These differences may be attributed to secondary changes that lead to LDH in young adult patients as a result of facet joint parameter abnormalities, especially in young adult patients who have not engaged in long-term physical labor or experienced previous trauma. Higher FT may be risk factors for LDH in young adult patients.
Biomechanical studies have found that facet joint parameters significantly influence the biomechanics of a corresponding segment [29, 31, 44–47]. Sagittal orientation of the facet joints promote anterior gliding by reducing resistance to anterior shear forces , and when tropism is present, segments were found to have a tendency to rotate toward the more oblique facet joint under axial loading. This can place additional torsional loads on the intervertebral disk and plausibly contribute to intervertebral disk or facet injury and degeneration. Nonetheless, the sagittal orientation of facet joints allow a trade-off between angular motion and rotation. Gradually, rotational movement indirectly causes tensile stress in the annulus of the intervertebral disk, leading to protrusion on the sagittal side . In the present study, we found that there was a significant relationship between herniation laterality and FO at the L4–L5 level; intervertebral disk herniation tended to protrude ipsilateral to the side with a lower FO, which could suggest that herniation tended toward the sagittally oriented facet joint whenever there was a combination of sagittal and coronal orientation. However, one previous study provided contradictory evidence; therefore, further clinical and biomechanical research is required.
We also found that FT is significantly related to LDH at the L4–L5 segment but not at the L5–S1 level segment, and that, likewise, there was a significant association between the herniation laterality and FO at the L4–L5 segment but not at the L5–S1 level segment. We speculated the reason for this is that the L5–S1 intervertebral disk is below the iliac crest, which may restrict the motion of that segment; and thus, there may be reduced stress and shear forces in the L5–S1 intervertebral disk.
Limitations and Meaning
This study had several limitations. First, this was a retrospective nonrandomized case-control study with specific groups. Second, we did not use sagittal balance, paraspinal muscle volume, pelvic and angular parameters, and other demographic features that may individually affect the occurrence of LDH. Third, our study was limited by geometric considerations. The articular surface of the facet joint was viewed as a flat plane, which is not the most suitable representation of the complex three-dimensional geometry of the facet joints and their relationship with the lumbar disk degeneration.
Our recent biomechanical research identified the biomechanical influence of facet joint parameters on corresponding segments in the lumbar spine; therefore, in this study, we aimed to determine the relationship between facet joint parameters and LDH in young adult individuals to better understand the mechanisms of degeneration and the progression of degenerative disk disease. To the best of our knowledge, this is the first study to provide evidence to support that there are relationships between the facet joint parameters of a given segment and corresponding disk herniation laterality.