Posterior cervical extensor muscles and sagittal parameters
Currently, the relationship between cervical sagittal alignment and PCEMs has been paid attention to by many scholars(27-29). Past studies show that in some patients non-Hirayama disease (HD)(30), cervical sagittal alignment is abnormal and is accompanied by PCEMs atrophy. There is a paucity of research on HD. In our study, we found similar results in HD patients. We have utilized imaging parameters to describe the cervical sagittal alignment and used muscle cross-sectional area represent muscle strength. These methods have been used in previous studies (15, 17).
Strong PCEMs maybe lead to better cervical sagittal alignment
Cervical muscle strength is lower in HD patients relative to normal people, and there is a mismatch between flexor muscles and extensor muscles(23). Change in extensor muscle strength may affect cervical sagittal alignment. T1s is an important cervical sagittal parameter and is related to the thoracic entrance angle(31, 32). The degree of motion of the thoracic vertebra is small and prone to change. Our data show that T1s was closely related to the area of the PCEMs. Since T1s was a relatively fixed value(33), it was closely related to the entrance angle of the thoracic cage and was not easily changed by changes in the cervical spine. Our data suggest that T1s may not be affected by change in the strength of the PCEMs, but the larger the T1s, the more cervical vertebra is required to participate in lordosis in order to maintain horizontal gaze and larger cervical lordosis (CL) requires stronger support of the PCEMs. At present, the relationship between T1s and the strength of PCEMs is not clear in HD patients. In future, it may serve as an indicator of posterior cervical muscle strength. There was no obvious correlation between SVA and PCEMs strength. Lee et al. showed that T1s, CL and SVA are closely related, and that SVA has a compensatory role between T1s and CL(33). Thus, there was no clear correlation between SVA and PCEMs strength.
The C2-C7 Cobb angle was smaller than reported by past studies but it did not significantly correlate with PCEMs strength(34-36). These figures are influenced by factors like age and posture. According to the previous study(37), we predicted that Cobb angle might be closely related to the strength of PCEMs but corresponding results were not obtained after measurement. After analysis, since the number of vertebrae in C2-C7 did not match the measurement site of muscle, we only measured the muscle’s CSA at the C5-C6 level, which does not represent muscle strength of the entire cervical spine. There was no obvious correlation between the muscle area and the C2-C7 Cobb angle. Therefore, we defined "Local kyphosis" and measured the local kyphosis angle. This analysis showed that that in HD patients, LKD tended to occur in the C3-6 segment and was closely related to the CSA of PCEMs (the greater the extensor strength, the smaller the LKA, Figure2d). Meanwhile, we found that the C2-C7 Cobb angle of LKD patients was smaller than that of non-LKD patients (Table 3). There was significant correlation between PCEMs strength and cervical spine kyphosis. Most of the vertebrae involved in kyphosis are C3-C6, and PCEMs atrophy can cause abnormal cervical curvature, especially of the extensor. In HD patients, cervical muscle CSA is smaller than in normal people, and there is flexor and extensor muscle mismatch(23). We speculate that the atrophy of cervical posterior extensor muscle in HD is a reason for abnormal cervical sagittal alignment.
Treatment considerations for HD
We for the first time show association between posterior cervical extensor muscle and cervical kyphosis in HD patients. Neck muscles are crucial in maintaining stability of the cervical spine. The relationship between neck muscles and cervical spine curvature has been shown in some patients with non-Hirayama disease. PCEMs weakness is a cause of adolescent idiopathic kyphosis(38) .Extensor muscle strength is weaker than flexor muscle strength in people with poor cervical curvature, as has been reported for HD(23). The cervical spine is associated with PCEMs much like a "bow" and "string". Numerous studies have highlighted the importance of strengthening muscles for spinal sagittal balance(30, 39, 40). Based on the above studies, we believe that strengthening the PCEMs should be an important part of HD treatment.
There are some limitations in this study. First, the lack of patients make our data not meet the normality, so we could only use rank correlation, which reduces the level of evidence. Second, the study is limited by its retrospective nature as it lacks healthy controls. Thus, further studies are needed to support the theory we put forward. Finally, only the C5-C6 level was selected for the muscle area and a simplified measurement method was used, not the true cross-sectional area of the entire back of the neck muscles. Because some parameters of the cervical spine sagittal position are used to evaluate the overall condition of the cervical spine, and CSA does not represent the overall cervical extensor, even partial adjustment of the measurement indicators may affect our results.