INTRODUCTION
Atlantoaxial dislocation (AAD) can cause severe neurologic deficit which make patients disabled or neck pain results from kyphosis at upper cervical spine [
1-
6]. Pathologies of AAD were various such as trauma, inflammation, congenital anormaly, and iatrogenic causes [
1-
10]. The treatment of AAD has always been a major concern for spine surgeon. Since transarticular screw fixation and interlamina fusion was introduced by Mergel and Brook, posterior C1–2 fusion has been known as an effective treatment for AAD [
11,
12]. As Ham’s technique emerged, cervical polyaxial screw and rod fixation system is widely used because of its simplicity of the technique and low risk of vertebral artery injury [
13].
As these surgical techniques have been popularized, some surgeons gradually interested in postoperative changes in cervical sagittal balance [
14,
15]. C1–2 fusion led to reciprocal changes in subaxial spine according to its angle, and it is considered to be an important factor for sagittal alignment in subaxial spine [
16]. Moreover, some studies stated that these surgical techniques are associated with complex cervical deformities such as postoperative regional kyphosis, postoperative subaxial kyphosis (PSK), or hyperlordosis [
17,
18].
Previous studies focused on evaluating the relationship between postoperative C1–2 angle and subaxial sagittal alignment, but there were a few studies stated that the association between postoperative radiologic parameters related with sagittal balance and clinical outcomes [
19,
20]. In our study, we analyzed the correlation between radiologic parameters in cervical spine before and after posterior C1–2 fusion. Furthermore, we investigated how these changes in radiologic parameters affect clinical outcomes in patients treated posterior C1–2 fusion.
DISCUSSION
Atlantoaxial fusion is frequently associated with sagittal realignment in subaxial spine, and many authors stated the negative correlation between ΔC1–2 CA and ΔC2–7 CA after surgery [
14,
15,
17,
18]. However, we observed different results of correlation between ΔC1–2 CA and ΔC2–7 CA. Postoperative sagittal realignment in subaxial spine was occurred in our study. However, the reciprocally negative correlation between ΔC1–2 and ΔC2–7 CA wasn't found although preoperative C1–2 and C2–7 CA represented the negative correlation each other. We found that the reciprocally negative correlation between ΔO–C2 and ΔC2–7 CA, instead of ΔC1–2 CA. We studied why this phenomenon happened in our study unlike other studies. First, ΔC1–2 CA was compensated by ΔC2–7 CA as well as ΔO–C1 CA to maintain the horizontal gaze. The preoperative range of motion (ROM) of C1–2 angle in normal people has about 6° when flexion and extension [
23], and compensate for the change of subaxial spine in available ROM. However, postoperative C1–2 angle is fixed after surgery, and it plays the constant no room to change. It seems that the O–C1 angle plays as a buffer angle to maintain the horizontal gaze. ΔC2–7 CA was compensated by ΔO–C1 CA instead of the constant C1–2 angle. It seems that postoperative C1–2 CA as the constant does not work on cervical sagittal realignment. ΔO–C2 CA was actually the parameter obtained by adding a constant to ΔO–C1 CA. Therefore, ΔO–C2 CA was correlated with ΔC2–7 CA negatively, not ΔC1–2 CA excluding ΔO–C1 CA. Second, we observed a radiologic feature of patients in this study. The ratio of C1–2 CA and C2–7 CA was different to normal ranged patients. Some authors stated that normal values of C1–2 angle ranged from 25.6° to 28.9° and it accounted for 75%–80% of cervical standing lordosis [
24,
25]. However, our patients showed the proportion of cervical lordosis of C1–2 CA was about 52.5%, suggesting that C1–2 CA constitutes a relatively small proportion of cervical lordosis, and the proportion of C2-7 CA is predominant in cervical lordosis compared with others. Therefore, ΔC2–7 CA is increased relatively compared to other studies. There is the possibility that this feature might contribute that the reciprocal correlation between ΔC1–2 and ΔC2–7 CA was not significant.
The correlation of each radiologic parameter was summarized in
Table 3. The important findings of this correlation test is described to 3 things. One thing is that the change of upper cervical spine (ΔO-C2 CA) is correlated with the change of subaxial cervical spine (ΔC2-C7 CA) and this relationship does not be affected before and after surgery. Second thing is that ΔC1–2 CA is associated with cervical kyphosis and flattening of cervical curvature. Third thing is that ΔC2–7 CA is a subjective radiologic parameter affected by various radiologic parameters.
ΔC1–2 CA was associated with ΔO–C2, ΔC1–7 CA, and ΔC2–7 SVA. Moreover, ΔC1–7 CA was negatively correlated with ΔVAS (
Fig. 3A). Therefore, it can be expected if ΔC1–7 CA decreased after surgery, neck pain would be unchanged or worse. In addition, ΔC2–7 SVA had a positive correlation with ΔJOA score (
Fig. 3C). Therefore, cervical sagittal alignment is significantly related to neck pain as well as cervical myelopathy. Some studies supported our results. Shimizu et al. [
26] found that a significant correlation between the degree of cervical kyphosis and the amount of cord flattening leading to decreased vascular supply. Cervical sagittal malalignment is strongly related with neck pain. Tang et al. [
27] also reported that C2–7 SVA directly correlated with NDI and cervical myelopathy. As a result, intraoperative C1–2 angle determined by surgeon was an important factor to affect not only cervical sagittal realignment but also VAS and JOA score.
ΔC1–7 CA was associated with ΔC1–2 CA, ΔC2–7 CA, ΔT1 slope. This correlation is taken for granted that C1–7 CA was the parameter including C1–2 and C2–7 CA. ΔT1 slope was changed according to ΔC1–7 CA.
ΔC2–7 CA also correlated with ΔO–C2 CA, ΔC1–7 CA, ΔT1 slope, ΔC1–7 SVA, and ΔC2–7 SVA. It was the most subjective radiologic parameter that was correlated with various others and also associated with VAS and JOA score such like ΔC1–2 CA. Nevertheless, surgeons can adjust ΔC1–2 CA as determining intraoperative C1–2 CA under C-arm fluoroscopy, but ΔC2–7 CA cannot be controlled intraoperatively and be predicted during follow-up. Therefore, surgeons should carefully observe the change of C2–7 CA in the patient after posterior C1–2 fusion.
ΔT1 slope was relative with ΔC1–7 CA, ΔC2–7 CA. This change in T1 slope explains that ΔT1 slope was complementary to the change of cervical spine.
ΔC2–7 SVA was correlated with ΔC1–2 CA, ΔC2–7 CA, ΔC1–7 SVA, JOA score. ΔC2–7 SVA was affected simply not only ΔC2–7 CA, but also ΔC1–2 CA. ΔC1–7 SVA was correlated with ΔC2–7 SVA each other. However, ΔC1–7 SVA showed the difference to ΔC2–7 SVA in that there was not correlated with ΔC1–2 CA. This different point interestingly affected that ΔC1–7 SVA was not significant with JOA score.
ΔPADI correlates with ΔO–C2 CA, and it was a factor associated with ΔNDI. ΔPADI increased significantly after surgery, it pointed out that most patients obtained enough reduction of AAD intraoperatively. This point explained why the reduction of AAD is important in the improvement of quality of life. Several Authors emphasized the importance of enough reduction of AAD, but this was still controversial. Jun et al. [
28] suggested that complete reduction of AAD could obviate the need for direct decompression. Goel and Shah [
29] introduced that facet manipulation and fixation in irreducible AAD facilitated reduction of AAD. Otherwise, Wang et al. [
30] stated that sufficient decompression by laminectomy and solid fusion for AAD is more important than complete reduction for treatment of AAD. Lang et al. [
31] also reported that incompletely reduced AAD had comparable clinical outcomes with those with complete reduction. Nevertheless, complete reduction of AAD without laminectomy can provide patients with sufficient fusion-bed for bone graft. Because of this advantage, we removed the capsule of C1–2 facet joint and distracted the facet joint by osteotome to release sufficiently in case of irreducible AAD. Finally, we obtained sufficient reduction of AAD in 94.7% of patients except 2 cases performed C1 laminectomy.
The PSK that occurred 23.7% of patients was not correlated with clinical outcomes. There are some studies that PSK was one of causes for postoperative neck pain [
32]. We also agree that neck pain was associated with PSK. In the present study, patients with PSK showed the tendency to complain of severe neck pain. However, deterioration of neck pain was also observed in patients without PSK in our study. This point resulted in the failure to prove the statistical significance that PSK is related with neck pain. Yoshimoto et al. [
18] also observed similar results of ours. The author stated that 12 patients among 44 patients without any progression of PSK complained neck pain aggravated after surgery. For this reason, there was no significance found between PSK and clinical outcomes.
Several studies reported the PSK between the changes in subaxial alignment and intraoperative C1–2 angle [
17,
18,
33]. Toyama [
34] investigated 75 cases of interlaminar bone grafting with wiring and reported that straight, kyphotic, and swan neck deformities occurred after surgery and recommended that the optimum postoperative C1–2 CA is 20°. Kato et al. [
35] also recommended that the optimum postoperative C1–2 CA should be 20° in patients with the preoperative C1–2 angle of 0°–20° or < 0°, but perform an in situ angle in patients with a C1–2 angle of ≥ 20°. Some authors stated that surgical overreduction of C1–2 CA would be associated with PSK instead of optimal C1–2 angle [
34-
36]. These statements will be useful to recover the lordosis of subaxial spine and decrease the kyphosis of subaxial spine postoperatively. However, there is no consensus for optimal C1–2 fusion angle because physiological cervical sagittal alignment is different individually. The understanding of postoperative sagittal alignment is still insufficient. PSK is the result from multiple factors associative with cervical sagittal realignment such as age, postoperative C1–2 angle, the extent of surgical dissection, compensation of adjacent segmental angle. Therefore, it is careful to define the optimal postoperative C1–2 CA. Nevertheless, there are several things that spine surgeon should pay attention to obtain better clinical outcomes during surgery. At first, most patients with AAD have kyphotic C1–2 angle, and it is important that kyphotic C1–2 angle change to physiological lordotic angle. It is because a decrease in the C1–2 and C0–2 angle may likely induce a reduction in the pharyngeal space and can be a predictor of postoperative dysphagia, which is not compensated by the middle or lower cervical spine. At second, it is difficult to adjust C1–2 angle to target angle intraoperatively. We tried to fix postoperative C1–2 CA to 20° under C-arm fluoroscopy, but in some patients C1–2 CA were fixed more or less than 20°. Therefore, the greatest care must be taken to determine C1–2 fixation angle during surgery. Finally, C1 slope should be posteriorly slanted. It is not only because posteriorly slanted C1 slope is important to maintain the C1–2 segment lordosis, but also because the posteriorly slanted C1 slope and kyphotic angulation of the C0–1 segment allows some degree of freedom for neck extension as the space between the occiput and C1 posterior arch and allows some rooms for upper cervical extension to prevent the collision of occiput and implant.
The weaknesses of this study are its retrospective design and small sample size. In addition, patients had various pathologies, which included RA, congenital anomalies, and osteoarthritis. This study included 2 irreducible AAD patients. We performed the releasing of C1–2 facet in these patients, but we could not obtain sufficient reduction of AAD, which may have made a different sagittal realignment comparing to other patients. Moreover, C1–2 constructs for posterior fusion was not monotonous, it composed of hybrid structures such like C2 pedicle, lamina, and pars screws. Although we compressed or distracted the rod under C-arm fluoroscopy to control appropriate C1–2 CA, some patients obtained postoperative C1–2 CA showed a large deviation around 20°. Patient numbers 6 and 36 obtained kyphotic C1–2 angle of 4.8° and 9.4°. Patient numbers 12 and 38 obtained lordotic C1–2 angle of 34.3° and 36°. It is difficult for us to adjust intraoperative C1–2 CA closely as looking images in C-arm fluoroscopy. Finally, the long-term radiographic and clinical outcomes more than one year were not evaluated in the present study.