Introduction
Postoperative alignment problems, including kyphotic deformity in the cervical spine, are factors that worsen the postoperative prognosis and reduce the patient’s satisfaction in relation to the surgical procedure [
1,
2].
Many reports have investigated bony and soft tissue factors as the causes of this postoperative alignment disorder [
3,
4]. In particular, the resection of the lamina alone in bony elements does not impair cervical spine stability, and resection of the facet joint causes cervical spine instability [
3,
5]. Regarding the soft tissue component, studies have focused on muscle and joint capsules [
4,
6], with many reports suggesting that damage to the muscles attached to C2 causes alignment disorders [
7,
8]. Additionally, other studies suggested that damage to the muscles attached to C7 causes the appearance of disorders [
9]. Furthermore, it has been reported that damage to the articular capsule alone can cause cervical instability [
6]. Muscle-preserving selective laminectomy is a technique that allows decompression without damaging the facet joint, the joint capsule, or muscles attached to C2 [
10]. Therefore, we have reported that cervical spine alignment is well preserved after this technique [
11–
13].
However, in this technique, the spinous process is dissected at its base; this damages the insertion of the cervical semispinalis and multifidus muscles that attach to the spinous process [
10]. We call the extension mechanism of the cervical spine—which consists of the spinous process and the muscles attached to it—as the extension unit. In a muscle-preserving selective laminectomy, one extension unit is damaged in single laminectomies, and two extension units are damaged in consecutive double laminectomies [
12]. To-this-date, no other study has clarified the extent to which damage to this extension unit affects the maintenance of local alignment of the cervical spine. Accordingly, alignment analyses after muscle-preserving selective laminectomy without damage to the facet joint or articular capsule is suitable for clarifying the effects of damage to this extension unit alone.
Additionally, while prior studies stated that the attached muscles to the C2 and C7 spinous processes are important for maintaining postoperative cervical alignment [
7,
9], no other studies have clarified the importance of the attached muscles between the C3 and C6 spinous processes for maintaining cervical alignment. Furthermore, no other publication has described the differences at the level of this group of muscles. In a muscle-preserving selective laminectomy, procedures are usually performed within the range of C3 to C6 to select the decompression level. The technique is suitable for the investigation of the influences of muscles on postoperative cervical alignment within this range [
12].
This study compared the postoperative alignment of muscle-preserving, selective double laminectomy with the maximum possible preservation of posterior stabilizing tissues as a function of the decompression level. We investigated the influence of the extension unit of the spinous process and its attached muscles on postoperative alignment.
Discussion
In this study, the lower local angle decreased after consecutive double laminectomies with the exception of the C3–C4 laminectomy cases. The C3–C4 laminectomy group exhibited lower local kyphosis before surgery, and the kyphosis did not progress postoperatively. This was probably owing to the preoperative kyphosis. The results of this study revealed that the local kyphosis at the disk caudal to the decompression level progressed after consecutive double laminectomies. This result supports the fact that the deep extensor muscles of the cervical spine maintained their alignments as a dynamic stabilizer muscle group [
4,
17]. To our knowledge, this study is also the first report to show that the deep extensor muscles of the cervical spine influenced the maintenance of local alignment of the cervical spine. Although the mechanism for this phenomenon is unclear, we hypothesized the mechanism. When the spinous process was split longitudinally and the local extension unit was damaged, muscle tone around the facet joint at the muscle origin was weakened and the stabilization mechanism around the joint was impaired. This resulted in abnormal local alignment. We reported that after muscle-preserving selective laminectomy, caudal lordosis of the cervical spine decreased, and the cephalad lordosis increased to compensate; these changes maintained the alignment of the entire cervical spine [
13]. In this study, we could show that the mechanism of the caudal reduction in cervical lordosis was caused by local kyphosis at the decompressed caudal intervertebral space. However, in a relatively less invasive double laminectomy, this local kyphosis was compensated by an effect on the cephalad side, and the overall cervical alignment became more lordotic postoperatively. Additionally, the recovery rate after this procedure was reasonable, and the results of the surgery were satisfactory.
The C2–C7 angle decreased after the C3–C4 laminectomy, and the C2–C7 SVA was also increased after the C3–C4 laminectomy. Based on the findings from this study, it is suggested that the cervical alignment deteriorated after the C3–C4 laminectomy. In addition, multivariate logistic regression analysis showed that the decompression level was significantly associated with C2–C7 angle decreases >10°. This result indicated that the deep extensor muscles on the cephalad side contributed more profoundly to the maintenance of the overall cervical alignment. The lever arm of cervical extension became larger in the cervical extension unit on the cephalad side. Thus, it can be inferred that the cervical deep extensor muscles attached to the spinous process on the cephalad side are more able to maintain cervical lordosis. In fact, many previous studies have described how damage to the most cephalad extension unit C2 worsens cervical alignment [
7,
8]. Given that C2 is the most cephalad region to which the cervical extension mechanism attaches, it is easy to infer that it is the most active level in the maintenance of cervical lordosis. Using cadavers, Takeuchi showed that when C3 laminoplasty was performed, the cervical semispinalis muscle attached to C2 interfered with the posterior spinous processes and spacers [
18]. The same authors showed that the cervical semispinalis muscle attached to C2 can be preserved by performing a C3 laminectomy and demonstrated its usefulness [
8]. The C3 laminectomy technique does not require separation of the muscle attached to C2. Accordingly, our technique also does not require separation of the muscle attached to C2 during C3–C4 laminectomy. Thus, it is unlikely that the alignment loss in this study after C3–C4 laminectomy appeared owing to damage to the C2 attached muscles caused by the C3–C4 laminectomy.
Multivariate logistic regression analysis showed that the preserved disk height at C2/C3 was significantly associated with C2–C7 angle decreases >10°. To the best of our knowledge, there is no report on the relationship between disk height at C2/C3 and postoperative alignment changes in the cervical spine. Given that C2/3 is the most cephalad disk, it is the last disc that can undergo compensatory alignment changes for alignment changes in the caudal part of the spine. The remaining disc height at C2/C3 may contribute to the tendency for alignment changes, but this mechanism remains unclear and requires further investigation.
To-this-date, no other study has clarified the contributions of C3 to C6 extension units to maintain cervical lordosis, and the results of this study indicate that as the extension unit becomes more cephalad, the ability to maintain cervical lordosis is enhanced. In this study, cervical alignment was maintained in patients who underwent C6–C7 laminectomies, thus suggesting that the deep extensor muscles attached to C6 and C7 were not involved in maintaining cervical lordosis; this was probably because the lever arm was considered the shortest.
In this study, patients who underwent C3–C4 laminectomies, i.e., patients with cephalad stenoses, were mostly elderly. Previous studies have also confirmed that elderly patients were more likely to suffer from cephalic stenoses [
19]. It can be assumed that age affects postoperative changes [
20]. In fact, C2–C7 SVA was high preoperatively in C3–C4 laminectomy cases, which may be related to postoperative changes in SVA. However, multivariate logistic regression analysis of the factors responsible for the decrease in the C2–C7 angle by more than 10° and the increase in C2–C7 SVA by more than 15 mm showed that age and preoperative C2–C7 SVA were not significant aggravating factors, thus suggesting that the difference in the decompression level was the primary factor for postoperative alignment changes. The study of age-matched cervical spine alignment also showed a significant decrease in the C2–C7 angle and an increase in the C2–C7 SVA postoperatively in C3–C4 laminectomy cases. Considering all these results, it is suggested that as the number of cephalad extension units increased, its importance in maintaining cervical lordosis increased.
The multivariate logistic regression analysis of the factors contributing to the increase in the C2–C7 SVA >15 mm showed that the preoperative ROM was a significant factor. Many previous studies have reported that preoperative alignment abnormalities affect the postoperative period [
1,
2]. However, there are no reports that show that preoperative ROM affects sagittal plane imbalance. We have reported that cervical alignment was maintained by compensatory lordosis on the cephalad side even when there was loss of alignment on the lower cervical spine after selective laminectomy [
13]. Regarding SVA, when alignment loss appeared in the lower part of the cervical spine, it should be compensated on the cranial side, but if the ROM was small, the compensatory action may be hard to be worked. This study suggests that cervical spine mobility is necessary to maintain the sagittal plane balance and that the remaining cervical ROM is advantageous in compensating for abnormal alignment.
In this study, we used a less invasive technique with a narrow decompression level of two adjacent laminae, and showed that the extension unit alone affected the caudal local alignment. Therefore, as wider decompression with wider exposure is commonly used in conventional laminoplasty [
21,
22], it can be expected that the effect of capsule and joint damage other than the extension unit may work strongly in laminoplasty. Moreover, there are limitations in directly applying the results of this study to conventional laminoplasty. In addition, the ROM restriction that occurs after conventional laminoplasty may affect the maintenance of postoperative alignment [
21,
22], and this may be an issue for additional investigations.
The influence of selection bias cannot be ruled out based on the findings of this retrospective study. Furthermore, the lack of entire spine imaging evaluations which dis not consider spinopelvic parameters is a limitation of the study. The small number of cases limited the reliability of the results of multivariate logistic regression analysis, and the influences of the older age of C3–C4 laminectomy cases could not be eliminated. The absence of a control group for comparison precludes an accurate evaluation. It is important that our technique is compared in the future with fusion or other techniques of decompression to clarify the difference in the occurrence of alignment changes owing to differences in the surgical invasion. A prospective comparative study with a large number of patients is desirable to elucidate the mechanism of alignment changes following surgery.