CDDD is a chronic, acquired deterioration of the cervical spine that can cause neck pain, radiculopathy, and/or myelopathy20. HS can be tailored to rebuild cervical stability at the target level, depending on the degree of degeneration of the different segments. Theoretically, an adequate range of motion is achieved at the level of joint replacement, and fixation is achieved at the level of joint fusion 21. Cervical instability due to disruption of the posterior tension band of the cervical spine has been identified as the cause22. The maintenance and reconstruction of cervical balance after HS and ACDF is due in large part to the protection of the posterior cervical ligamentous complex and fewer incisions. In a prospective review of studies, Chen et al23 found that both hybrid surgeries resulted in satisfactory neurological recovery. Compared with cervical laminoplasty, anterior hybrid surgery preserves cervical lordosis and has a lower rate of late complications. The safety and efficacy of HS in one- and two-level cervical spondylosis has been demonstrated 24. Three-level HS is now less studied, and few articles have been devoted to the results of sagittal parameter changes after three-level HS. However, sagittal parameters are critical to postoperative recovery outcomes in patients undergoing cervical spine surgery.
The cervical spine is a relatively complex structure with many factors affecting its alignment and balance. For patients undergoing spine surgery, elucidating compensatory mechanisms can be a key point in properly reconstructing cervical spine alignment. Cervical sagittal alignment is considered a diverse and significant component of the overall sagittal alignment. Cervical sagittal balance is associated with the development of cervical spine-related disorders and a decrease in health-related quality of life25. Ibrahim et al26 showed in a 2-year randomized controlled trial that maintaining cervical sagittal balance significantly reduced chronic cervicogenic headache in patients. A recent large retrospective study showed an increasing trend in cervical sagittal imbalance27. Therefore, it has become increasingly important to assess and correct cervical sagittal alignment during surgical treatment. Therefore, in this study, the effects of different surgical approaches on postoperative cervical sagittal parameters in patients with three-level CDDD were compared. The results showed that HS surgery was superior to ACDF in maintaining CL and SA, while there was no difference between them in other parameters. This result may suggest that three-level hybrid surgery is superior to ACDF in cervical sagittal alignment, while it may delay the development of ASD in the long term.
HS is the combination of ACDR and ACDF and is now one of the most common surgical procedures for treating patients with cervical spondylosis. Compared to ACDF, HS has the same outcomes and functional recovery for patients with cervical disc disease, with significantly better preservation of cervical ROM, suggesting that HS is an effective alternative treatment for multilevel cervical spondylosis28. Cervical sagittal alignment has been a heavily debated and controversial issue. Xu et al29 compared three-level HS (Prodisc-C and MC+) and ACDF through more than 5 years of follow-up, and most patients achieved cervical balance with HS and ACDF, but there was no difference in cervical sagittal alignment parameters between the two. However, in our previous research, we more accurately classified three-segment HS (Prestige-LP and Zero-P) as type I versus type II and showed that type II was superior in terms of cervical lordosis and range of motion12. The different prostheses and cages used may be the main factor in this difference. For three-level CDDD, it has been shown in many studies that HS can restore the anterior lordosis of the entire cervical spine and the surgical segment, preserving the range of motion of the cervical replacement segment and restoring the biomechanical function of the cervical spine30,31. In this study, a comparison of postoperative cervical sagittal parameters between HS and ACDF showed that HS is superior to ACDF in maintaining cervical curvature, while similar cervical sagittal balance can be obtained in all other aspects. Although the follow-up period for three-level HS is relatively short, the use of HS may be a more accepted procedure by more surgeons in the future. More studies are underway for improving the effectiveness of HS.
Multiple disorders of the cervical spine can lead to an imbalance in the sagittal alignment of the cervical spine32,33, and the linkage and interplay between cervical sagittal alignment parameters was demonstrated in this study. In both ACDF and HS, reconstruction of the CL is performed, making both cervical lordosis and T1S more pronounced. Both of these affect SVA simultaneously, with a greater CL causing a posterior shift of the head's center of gravity resulting in a lower SVA and a greater T1S causing an anterior shift of the head and cervical center of gravity resulting in a higher SVA. This is a compensatory mechanism to maintain horizontal gaze in response to changes in the overall sagittal alignment34. By showing that SVA is elevated after surgery in this retrospective study, the effect of T1S on SVA will be greater than the effect of CL on SVA, and the patient's head and cervical spine are anteriorly displaced after surgery.
C2S is a recently considered single, simplified measure of cervical deformity, similar to the T1S-CL measure. Shen et al35 showed that a greater C2S was associated with the presence of preoperative adjacent segmental pathology. C2S can adequately describe cervical deformity because of the association between O-C2A and CL, and both are closely related36. Mathematically, it can also be explained as follows: T1S - CL = T1S - (C7s - C2S), whereas in many patients, the T1 and C7 slopes are approximately equal, which means that they cancel each other in the equation, leaving only the C2S as the only variable to measure the aberration. Similarly, a similar trend as well as a significant correlation between C2S and T1S-CL was observed in both type I and type II groups in this study. The results suggest that the different number of replacement segments in HS does not affect the correlation between T1S-CL and C2S, and both are well-balanced. Similarly, in both type I and type II groups, this study indicates that CL is negatively correlated with T1S-CL, C2S and O-C2A, while T1S is positively correlated with T1S-CL.
This study has several limitations. First, it was a retrospective, single-center study, which may have been biased. In addition, the mean age of the patients in each group varied, which may have biased the results. Second, the sample size was relatively small, especially for patients in the type II group undergoing HS, and the follow-up period was relatively short. Third, only the Zero-P and Prestige-LP systems were included in the study. In the future, prospective, multicenter, large-scale studies with different prostheses should be conducted to confirm these results. Fourth, the focus of this study was the radiological results of HS; therefore, its clinical outcomes were not considered.