ACDF allows direct decompression of neural structures, reconstruction of cervical lordosis and stabilization of the operated segments[13]. Since the SSC system as a new-designed implant has been used for CSM, multiple studies mainly concentrated on the comparison between SSC and ACP in terms of surgery-related complications and clinical outcomes by a short- to middle- term cohort, having illustrated favorable outcomes[14–15]. Yun et al [16] conduct a 2-year follow-up on 2-level contiguous ACDF and showed compatible clinical outcomes and capacity of lordosis-maintenance between SSC and ACP. Shi et al [17] described a favorable outcomes on SSC on complications between the two procedures for 3-level CSM with 3-year follow-up. With different design concept between SSC and ACP system, the mechanism on cervical alignment reconstruction and on operated- or adjacent-segment effect still posed challenge. Therefore, this study focused on more elaborated measurements on global cervical alignment, operated- and adjacent- segment lordosis, as well as ASD, which firstly, with a long-term visit, demonstrated that ACP was slightly superior to SSC on CL and OPCL improvement, but with little and comparable impact on adjacent segment in both procedures.
Restoration of CL was achieved by posterior osteophytectomy, opening of the posterior longitudinal ligament and suitable selection of implants’ sizes and shapes [6,16]. Our series suggested an effective outcomes on CL improvement by SSC and ACP. When comparing CL improvement between the two approaches, Chen et al [4] indicated SSC was inferior at restoring CL and may not provide better sagittal CL reconstruction in 3-level fixation with 24-36m follow-up. The titanium alloy plate was positioned in anterior vertebral line and the anterior intervertebral disk height might decrease less than posterior; in addition, the CL was restored by pulling the involved vertebrae towards the prebent lordotic ventral plate, which could make the segmental angle more improved [6]. While the zero-profile anchored spacer, consisting of a cage and single or two anchoring clips, showed less ability to restore CL than ACP [18]. On the other hand, a larger improvement on OPCL added more weights on CL reconstruction in three-level ACDF.
There was somewhat decrease on UCL and LCL in ACP group at IPO and FFU, which was considered a compensation by the adjacent segment for a much larger OPCL with ACP system to keep cervical balance. Secondly, biomechanical studies have revealed that SSC provides less stiffness of cervical spine as locking plate does in 2 or 3-level instrumentation [19]. As a consequence, SSC more closely matched the physiological elastic modulus of the vertebrae and resulted in greater load transfer to the interbody cages. The excessive stiffness of the metallic plate may incur stress shielding and uncomfortable kinematics on adjacent segments[20]. Then, the application of ROI-C in skip levels could maintain activity at a normal level between skip levels, thus not sacrificing any normal motion segments[21]. In addition, hence accurate anterior osteophytectomy was essential to smoothen anterior surface of the adjacent vertebrae for ACP, which has also been advocated as a possible risk factor on loss of UCL and LCL[22]. In total, however, there was no much influence on adjacent segment lordosis in both approaches in this study.
One concern with multilevel ACDF was the potential for ASD with increased rigidity. Studies have shown that the presence of a plate was more likely to accelerate degenerative changes in adjacent segments[16,23]. However, a meta-analysis performed by Zhang et al [24] showed that there was no statistically significant difference in ASD incidence between the SSC and ACP groups with a multilevel ACDF with a short-term visit. Therefore, the precise cause of ASD was still unknown. During a long-term follow-up, our series provided comparable incidence of ASD between SSC and ACP although with a rate of more than 60%. While it was reported multi-level procedures may not be at a greater risk of developing ASD compared to single-level procedure[14]. Besides, the fact that no case with adjacent segment pathology for secondary surgery in either group and no significant change of UAL and LAL at FFU indicated the two approaches put little and comparable impact on the progression of ASD.
The comparisons on short-term clinical outcomes between SSC and ACP remained controversial. Njoku et al [8] and Yan et al [2]favored SSC had better effect on human pain relief and neurological function improvement. While Tong et al [25]showed SSC and ACP exerted similar efficacy in improving the functional and radiologic outcomes through a 2-year follow-up in a meta-analysis. Our data showed a significant improvement in clinical outcomes at the final visit contrasted with POP, even with that at immediate post-operation. The explanation might be the appropriate physiotherapy after surgery, the subjective adaption of patients and the edema elimination of nerve root promoted a further step on qualified adjusted life year.
It was considered that improvement and preservation of CL was a main goal in multilevel ACDF. Sagittal malalignment after ACDF may cause postoperative axial pain and worsening of neurologic defificits[21]. But a meta-analysis performed by Luo et al.[11] showed there was no correlation between clinical outcomes and cervical sagittal alignment. Our study favored that whether the CL improved or not, it acquired long-term and satisfactory clinical function recovery. Indeed, was it really important to get an improvement of CL? Posterior approaches such as laminoplasty were usually performed to address multi-level compression, but their use was limited to postoperative complications and loss of lordosis because of excessive posterior muscle stripping, impairment of muscle-ligament complex and reduction of muscle adhesion, which was associated with axial pain[26]. While ACDF kept the superiority in minimal incision and invasion, interstitial-space approach and preservation of posterior complex, leading to little kyphosis-derived symptom. Despite restoration of lordosis in a kyphotic cervical spine being a goal of surgery, there was no consensus about the optimal value of CL to achieve[27]. Moreover, it was reported that increased rigidity of adjacent segment and abnormal sagittal balance after ACDF promotes ASD[28]. While in our study, neither in ACP group nor in NIM subgroup, the incidence of ASD was comparable contrasted to another, which administrated ASD might not be directly derived from increased rigidity of adjacent segment or sagittal imbalance and was consistent with the opinion of Zhang et al[24].
In this study, no identified correlation was drawn between clinical outcomes and the change of CL after both procedures. Despite different types of cage and fixation, both procedures decompressed spinal cord directly by removing the anterior pathogenic compression[24]. Although the volume of spinal canal could be effected by CL[7], there was adequate compensated space for spinal cord recruitment after decompression. Then, there was severe dysfunction and long history before ACDF and the patient could acquire instantly complaint relief after surgery instead of what the change of CL brought. Moreover, the change of CL in most cases, although with CL increase or reduction, was in a acceptable range, which might be below the threshold of accelerating alignment-related dysfunction[8]. Therefore, the improvement of CL after SSC or ACP seemed not so essential as reported.
There were some limitations in our study. Firstly, the sample of both groups were little and a larger population could support a higher grade of evidence. Then, global cervical alignment was expressed by CL, which could not ideally describe total shapes of cervical spine such as sigmoid-S-type since it might effect the spinal canal volume [29] and result in a shape-derived compression. Finally, the conclusion was suitable for 3-level ACDF on CSM but might not for other types such as cervical spondylotic radiculopathy, where the nerve root could be effected by the height of intervertebral foramen result from the change of cervical alignment.