In the lumbar spine, each type of MC has a different effect on the segmental mobility, and a specific type of MC may be the cause of low back pain and an indication of fusion [3, 5-7, 12]. As more and more scholars begin to pay attention to cervical spine MC [8, 9], it is also of important clinical significance to explore the influence of different types of cervical spine MCs on segmental mobility. Considering that there is a close relationship between MC and DD, and the segmental mobility is determined by both endplate and intervertebral disc degeneration [3, 12], this study aims to explore the kinematic characteristics of cervical spine MC based on the grading of DD, and to analyze the relationship between MC type and intervertebral disc degeneration grade.
1. Prevalence and distribution of the cervical MCs
The prevalence of cervical MCs is lower than those of the lumbar spine, resulting in fewer related studies. According to our results, 20% patients have cervical MCs and most of them are MC II, which is similar to previous reports (16.9%~19.2%) [8, 9, 13].
Most of the MCs are found on C4/5, C5/6 and C6/7, especially on C5/6, which is also consistent with the literatures [8, 9, 13-17]. The reason to explain why C5/6 is the most common level to exhibit MCs is not difficult because C5/6 is believed to have larger segmental motion range than any other levels [18, 19]. Similarly, it can also explain why MC is often observed on C4/5, because C4/5 also has great segmental mobility [20].
The reason for why more MCs are found on C6/7 than C4/5 in our study is interesting. Several authors have reported the effects of MCs on cervical sagittal balance and many of them suggested that patients with MCs have larger cervical curvature, larger T1 slope or C7S [21-23]. Higher T1 slope or C7S needs more CL to keep horizontal balance, which may cause more stress and hypermobility at the bottom of the cervical spine (C5/6 and C6/7), resulting in unusual mechanical stress and more endplate degeneration [21].
2. MC I indicate small sROM and advanced DD grade.
MC I are considered as the early phase of endplate degeneration, although only a few authors clearly reported the effects of MC I on segmental motion, they are still believed to cause segmental instability in the lumbar spine [5, 24-27]. However, in this study, we find that the segments with MC I present lower sROM than those with MC 0 in the group of DD D and DD E, which is actually consistent with the conclusions of Hayashi et al [9]. Moreover, the VAS scores of neck pain in the patients with MC I are similar to that of patients with MC II and III (Fig. 4c), indicating that MC I may not be the cause of unstable neck pain.
According to previous reports, the reason why cervical MC I can cause segmental motion reduction is because MC I are always accompanied by advanced DD grades that can significantly decrease segmental motion [9, 19]. Our results also show that the sDD scores of MC I are significantly higher than those of MC II and similar to those of MC III (Fig.4a), more importantly, we even find that between the segments with the same DD grade, the segments with MC I present much smaller sROM than those with MC 0 and MC II, indicating that MC I itself has the effect of reducing sROM (Fig.3a).
As the CL, C7S and tDD of the patients with MC I are relatively large (Fig. 4b and 5), the reason why MC I is always accompanied by high-grade DD can be explained by cervical spine aging, because the increase in CL and C7S is considered to be a compensatory change in the aging process of the cervical spine [28].
3. MC II indicate unchanged sROM and it is the transition phase of endplate degradation between MC I and III
Though MC II are accompanied by lower sDD, tDD scores than those of MC I and MC III (Fig. 4a and 4b), we still consider MC II as the middle transition phase between MC I and III for the following reasons:
(1) During the degeneration process from DD A to DD D, though sROM keeps decreasing, the difference is not actual significant, but when disc degenerate from DD D to DD E, sROM reduction becomes statistically significant [19]. This theory is also supported by our result which is that, when disc degenerate from grade D to E, sROM of MC 0 and I decrease significantly (Fig. 3a). As a result, in the segments with advanced DD (especially grade E), sROM are primarily determined by the status of endplate degeneration instead of the disc, which has been proven by us in the lumbar spine [3]. In this study, when MC I turn into MC II in DD E, sROM increase significantly (Fig. 3a), which is the same as that of MC II in the lumbar spine, that is, MC II is a transitional phase between MC I and III that can increase segmental mobility [3];
(2) Both MC I and II are unstable and they can convert to each other [2], as a result, though MC I have higher DD scores than those of MC II in our study, several of them may be actually converted from MC II, indicating that they may have longer degeneration process than expected. In addition, many segments with MC I-II mixed type are found in our study, which can also support this theory.
4. MC III indicate the final phase of endplate degeneration process, which can significantly reduce sROM
MC III is the final and stable phase of endplate degeneration process and is wildly agreed to significantly decrease segmental angular and translational motion in the lumbar spine [2, 3]. According to our results, similar results are found in the cervical spine, that is, MC III indicate small sROM and high-level DD grade. However, the patients with MC III have significantly smaller CL and C7S than those with MC I, indicating that high-grade DD may be due to the straight cervical spine rather than cervical spine aging [28].
5. Effect of MCs on tROM
The tROM of patients with MCs are affected by many factors. In our study, neck pain degree and disc degeneration level of the entire cervical spine are considered the most important.
Our results show that no statistical difference in tROM is found between the patients with MC I, II and III, and they are all smaller than the normal range [29]. Though pain killers were routinely used before patients underwent imaging studies, the effects of pain on decreasing cervical motion cannot be completely ruled out which may finally cause the similarity of tROM between the patients with MC I, II and III [Fig. 4c]. In addition, we find that the sDD scores of the segments with MCs are higher than that of the segments without MCs, and MC I and III have similar high sDD and tDD scores [Fig. 4a and b], which indicate that the overall high degree of degeneration of the cervical spine limits the tROM of the patients with MCs.