ACDF was achieved in cases with radiographic signs of instability or no motion at the target levels, with or without facet degeneration. To date, there have been few reports on the biomechanical effect of ACDF involving three or more levels and its clinical indications and contraindications are unclear yet. The main indication of multilevel ACDF dued to symptomatic multilevel cervical degenerative disc disease with radiculopathy or myelopathy with strict and accurate reflection of neuro-dysfunction on body-dominating area [3,12]. Whether through ACP or SLC system, 3-level ACDF have been proved reliable and effective on decompression, stability and rebuilding of alighment, especially with the more advanced operating-skill and consequently extended sugical-indications on CSM . The newer launched SLC was approved by some researchers for its zero-profile design, simple inplanation, probable fewer comlications such as dysphagia contrasted to former-born ACP system. While it holds debatable since the qulified cost-effectiveness brought by ACP keeps irreplaceable . Therefore, a long-term observation on multilevel SLC versus ACP in this study may support strength on the viewpoints.
In this tudy, the sagittal alignment parameters are comparable between groups after surgery and most are of no significances except T1SCL. Although specific approaches are largely determined by surgeon based on nonuniform surgical-indications, there is a agreement that ACP are likely to be used on cases with straighter spine and imbalanced inclination since kyphotic deformities can be corrected by plate streching , which is one of the somewhat discrepancies on indications between ACP and SLC. The improvement of CL and T1S at final with no significants suggests a well-reconstruction in both groups. As a retrospective study, the change of alignment is induced to extremely avoid selection and reporting bias. The indifference of variable changes between ACP and SLC further consolidates the equivalent efficacy in alignment reestablishment. A main concerning after ACDF was the potential ASD with increased rigidity. Studies have shown that anierior plate is more likely to accelerate degenerative changes in adjacent segments . However, a meta-analysis performed by Zhang et al.  showed no difference in ASD incidence between the SSC and ACP groups with a multilevel ACDF. Our data showed a long-term incidence of 70% on ASD with no intergroup significance, in consistence with Zhang et al. In addition, no case here underwent reoperation for adjacent segment pathology indicated a comparable and limited impact on ASD progression.
The rebuilting of sagittal alignment was emphasized as a main goal in multilevel ACDF and cervical curvation acquires improved in this series after integrating data of the two group with little heterogeneity. Cervical alignment could be corrected by release of anterior tissue, removal of osteophyte, prepocess of disc and endplate bed, the shape of cages and sustaining of implants [5,18]. In addition, three levels occupies majority of the overall cervical spine and correction on operated segments is more suitable for stress distribution physiologically . Alberto et al.  found a straight cervical spine was related to increased SVA and larger T1SCL,suggesting an interact among parameters. The loss of CL forced an increase of T1S-CL tending to imbalance, imitating the larger PI-LL in loss of LL with pelvis retroversion, ensuing the gravity of head shifts forward to keep horizontal sight with excessive C2-C7 SVA. Cervical balance was evaluated by various factors, where cervical imbalance was defined as T1S > 40° or C2-C7 SVA > 40 mm without consensus [6,21]. Grasso et al.  proposed T1S was an important parameter while T1S-CL, like PI-LL, was a better representative than T1S alone, the latter involving two indexes reflecting a further interlocking status. Hyun et al.  firstly determined analogous relationship existed in posterior cervical fusion using T1S-CL, where the threshold of C2-C7 SVA was 50 mm corresponded to a T1S-CL value of 26.1°.
There is correlation between NDI and parameters at final visit but not at preoperation. It is considered that QOL of patients with CSM was affected by many factors before surgery, especially the severity of compression on spinal cord and no single factor can absolutely dominate neuro-function. Then the compression is elimated by the same surgeon, NDI was associated with appropriate sagittal alignment, in this condition, cervical balance status approximates physiological anatomy with most comfortable biomechanics and minimal power consumption, as well as lower tension of paraspinal muscle . The identified relationship of NDI instead of JOA dues to JOA was the patient-reported assessment tool to address each of these domains. Unlike JOA, the NDI is a mix of functional- and pain-status inquiries and more fits the significance of QOL [24-25].
The reasonable range of C2-C7 SVA of 29.2 mm corresponded to T1S-CL of 20.7° is narrower than posterior cervical fusion reported by Hyun et al., which emphasizes the different threshold in various approaches. Staub et al. implied normative CL can be predicted via T1S-CL= 16.5° ± 2° based on 103 patients with cervical spine deformity. It is considered that there were differences between anterior and posterior approach on surgical-indications, internal-fixtions, posterior muscle striping and preservation of ligament complex, where the solid fixtion by lateral mass screw and bowstring effect from posterior muscle requires wider range of T1SCL [5,27]. In addition, spinal canal volume changes with flextion-extension posture while posterior such as LP enable a steady enlargement of spinal canal compared to ACDF , allowing larger deviation from neutral alignment without affecting QOL. Other factors such as no debongding on ossification of posterior ligament, disc and osteophyte in posterior approach probably tolerates extensive T1S-CL and C2-C7 SVA. Furthermore, the cutoff value of NDI is 25 in publications while most cases (95.9%) are lower than 25 after ACDF, which will bring reporting bias on definiting threshold of T1S-CL and consequently NDI of 20 is adopted.
This study firstly identifies the significance of cervical sagittal parameters in three-level ACDF. Then the cutoff value was quantified in this population through the relationship with QOL, just like PI-LL for evaluation of lumbar-pelvic matching. When T1S-CL ˃20.7° after ACDF, the cervical spine will be involved in imbalance with a straight alignment and a predictive unsatisfactory QOL, which supports strategy for specific program for ACDF [9,29]. The conclusions proposes partial theory for better exploration on the overall sagittal spine-pelvic biomechanics. There are some limitations: Firstly, the sample size of both groups are small and a larger population may support a strength verification. Then, the observation focus on regional parameters on cervical spine instead of the whole spine, which probably bring distinctive points through effect of biomechanics chain. The comparisons are mainly performed on alignments, without other radiological parameters such as range of motion and complications,which were mentioned in publications and not replenished in. The threshold of T1S-CL in this study are just suitable for 3-level ACDF on CSM, the rationality for other types of cervical spine disease need further exploration.