Main findings
As far as we know, this imaging study was performed for the first time to compare the differences in lumbar sagittal parameters of LDD with/without MII. In this study, a variety of commonly used and individualized measurement parameters were used for a more comprehensive evaluation of lumbar changes on X-ray. The results of lumbar sagittal parameters revealed that LDD combined with MII had a more severe degree of lumbar degeneration than LDD alone and MII may be the severe radiographic representation in the process of lumbar degeneration. Moreover, the overconcentration of load caused by the smaller LL, SS, and IVA may be a reasonable explanation to answer why MCs are more common at the L5/S1.
Prevalence of MCs
Due to the differences of the study population and sample, the incidence of MCs varied greatly. The result of this study indicated that the incidence of MII among patients with LDD with grades 3–5 of the Pfirrmann classification was 29.81%, which was higher than other relevant studies [
17,
18]. The main reason is that most of the patients with the moderate to severe disc degeneration were recruited in our study. Of course, we could not rule out the effects of the other factors that we did not have the statistical analysis in this study, such as age, BMI, career, and lifestyle.
The distribution of MII mainly occurred at the lower two lumbar levels among the different lumbar segments [
17]. Also, MCs were more likely to be observed at L5/S1 than L4/5, as reported by a recent systematic review [
19]. The lower lumbar levels would be prone to bear the higher mechanical loading than the upper lumbar spine [
17].
Studies have shown that the prevalence of male with MCs was higher than that of female [
20]. This may be associated with a higher rate of males engaging in moderate or heavy physical work that led the lumbar spine to bear more repetitive stress loads than females. Oppositely, Xiao et al. [
18] reported that although there was no statistical significance in gender, females had the higher likelihood to be with MCs than males. This study came to the same conclusion as they did. The possible reason why the rate of females with MII was higher than males could be obtained that it may be associated with osteoporosis which was caused by changes in hormone levels of the female patients at the age of high morbidity of MCs.
Sagittal parameters
The LL, SS, and IVA formed by the lumbar curvature that is one of the key physiological arch in maintaining the posture of the human spine are the important indicators for the imaging measurement of the lumbar spine at present. There were only few studies which reported the relationships between lumbar sagittal parameters and MCs. Farshad-Amacker et al. [
21] performed a long-term follow-up study to look for the predictors for the development of lumbar degeneration in terms of LDD and MCs, but they did not find the relationships between LL, SS, and the development of MCs. However, another study [
15] with the contradictory results reported that the endplates with MCs in degenerative thoracolumbar/lumbar kyphosis were negatively correlated with LL (
r = − 0.562,
P = 0.012) and SS (
r = − 0.46,
P = 0.048). Obviously, the different conclusions of the above studies were based on different research subjects. Our study found that patients with MII on the basis of LDD had smaller LL and SS angles, which means that patients with MII tend to straighten the whole lumbar spine. The straightening of the lumbar vertebral curvature is the protective response so that the human body can well adapt to the degenerative changes of the lumbar spine. Almost all of the MCs were associated with the degeneration of lumbar structures, and the straightening of physiological curvature would inevitably increase the stress load on the vertebral endplate, leading to the occurrence of MCs.
In general, IVA and LL maintain a positive correlation. In our study, patients with MII had significantly smaller IVA and LL compared with patients in the DD group. However, a significant difference only occurred at the segment of L5/S1 between both groups compared with the IVA at other lumbar levels. The smaller IVA in the MC group allows the patient to concentrate more stress on the endplate while standing. Therefore, combined with the above comparison of LL and SS, this may explain the reason why the L5/S1 segment is more prone to MCs on the basis of lumbar degeneration.
The loss of intervertebral height on the lumbar radiograph is a common clinical sign of LDD, which has been reported as a positive correlation with MCs, whether by semi-quantitative or quantitative measurement [
22,
23]. Their reports were consistent with our results of IHI. However, previous studies [
3,
4,
24] have revealed that MCs always occurred at sites of disc degenerative disease. Therefore, the lack of a control group with disc degeneration may lead to the statistical error in those studies. In addition, because the intervertebral height is influenced by the different population, age, sex, BMI, posture, and so on, the results of the direct measurement are not superior to that of IHI, which is considered to be individualized in measuring the height of intervertebral space [
25]. The lower IHI in the MC group suggests that MII plays a positive role in the process of lumbar degeneration.
The endplate is curved in the normal lumbar spine. With the aggravation in lumbar disc degeneration, the endplate tends to flatten itself to adapt the biomechanical changes [
26]. This self-protective mechanism can shift the stress load from the central region of the endplate to the surrounding endplate, so as to decrease the damage of the vertebral body and endplate [
18]. It also explains the reason for the increase of the average ECA in the DD group in this study. However, in the MC group, the cranial and caudal ECA of the patients were significantly smaller than the DD group, but this did not indicate that the MII had the effect of remodeling the normal morphology of the vertebral endplates. Conversely, this may be a sign of partial endplate and/or vertebral collapse caused by the inflammation and repetitive pressure loading. Above, the results could allow us to believe that the reduction in ECA may be a more severe sign of lumbar degeneration for the patients with MII.
Limitations
As with any clinical study, our study has certain limitations. Firstly, a relatively small sample, especially in the MC group, may have decreased the statistical power. Secondly,it is regrettable that we did not set up more detailed subgroups due to the limitation of small sample. In addition, because of the limitations of patient’s economy and local medical insurance policies, the lack of whole spine radiographs has forced us to abandon the measurement and analysis of pelvic parameters. Therefore, a large sample, multicenter imaging study would be more helpful to analyze comprehensively the difference in the spinopelvic parameters between LDD alone and LDD combined with MCs and the different types of MCs.