Our study design and reporting followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement . This prospective study was approved by the Research Ethics Committee of Guangdong Provincial People’s Hospital. Patients who provided written informed consent and underwent cervical open-door laminoplasty for DCM at our hospital between April 2017 and April 2020 were enrolled. The Inclusion criteria were as follows: (1) Diagnosis of multilevel DCM with imaging findings and clinical symptoms; (2) Posterior cervical open-door laminoplasty without fusion. The exclusion criteria were as follows: (1) Patients with a history of stroke, Parkinson's disease, poliomyelitis, or other diseases that affect the neurological outcome; (2) Previous cervical surgery or traumatic spinal cord injury. After excluding 86 patients with basic information deficiencies, 177 patients met all the inclusion criteria and, among them,158 patients were successfully followed up [insert Additional File 1].
The smoking status of patients was obtained by preoperative questionnaire, which contained data collected from the intake forms given to all patients at the initial visit. The research draws on the World Health Organization (WHO) global standard for smoking, where those who answered "Yes" to "Do you currently smoke tobacco? " were classified into the active smoking group (AS group). Patients who had never smoked before were asked, “How many days per week are you exposed to second-hand smoke usually?” Those who answered “none” were classified into the non-smoking group (NS group), and all others were classified into the passive smoking group (PS group) . Besides, we also classified former smokers who did not smoke for a week before surgery into the NS group. After the surgery, active smokers were asked not to smoke, and those who were exposed to second-hand smoke were asked to maintain their distance from the smoke.
All the operations were performed by four of our institution's spinal surgeons. The procedure of open-door laminoplasty was described in previous studies [6, 29–31]. The patient is placed in a prone neutral position, and then the C3 to C7 laminae are exposed by dissection of the bilateral paracervical muscles through a posterior midline incision. A high-speed burr is used to create grooves at the junction of the Lamina and facet joints, and the ventral cortex of one lamina was cut. Instead of being cut, the ventral cortex of the other lamina is grooved as a hinge, and the cut lamina of the opposite lamina is raised and secured in the expanded position with titanium miniplates and screws. The open-door laminoplasty procedure is plated alone and no bone graft or fusion equipment was placed on the hinge or the open side.
Clinical outcome parameters
The primary outcome was the recovery rate of JOA at the last follow-up. Secondary outcomes included the JOACMEQ score and the NRS for neck and arm pain. The JOA score consists of six domain scores (upper and lower extremity motor function [4 points], upper and lower extremity sensory function [2 points], trunk sensory function [2 points], and bladder function [3 points]), and is used to evaluate the severity of preoperative and postoperative cervical spondylotic myelopathy [32, 33]. The primary outcome was evaluated using the recovery rate described by Hirabayashi et al, as follows: recovery rate (%) = [post-operative JOA score – preoperative JOA score]/[17-pre-operative JOA score] × 100 [5, 34–37]. We used the MCID, which is defined as the smallest change that can be recognized as “clinically meaningful” by a patient, to evaluate the success of interventions. Previous studies have indicated that the MCID for JOA recovery rate is 52.8% [34, 38, 39]. Depending on whether the JOA recovery rate reaches the MCID, the patients were classified into either the recovery rate reached the MCID (≥ 52.8% recovery rate) or the recovery rate did not reach the MCID (< 52.8% recovery rate). Patients with JOA recovery rate ≥52.8% were counted. The JOACMEQ score consists of six subscores: upper extremity function, lower extremity function, bladder function, visual analogue scale (VAS ) score of the upper extremity, VAS score of the body trunk, and VAS score of the lower extremity, which primarily evaluates patient-reported outcomes such as quality of life and function of the cervical spine, upper extremities, lower extremities, and bladder [29, 33]. We also prospectively collected demographic information (age, BMI, gender), medical history (symptom duration, history of diabetes and hypertension, OPLL), radiological data (C2-7 Cobb angle and K-line status on a neutral lateral view of the cervical spine, MRI T2 high intensity), and the numerical rating scale (NRS) for neck pain and arm pain before surgery and at the final follow-up (at least 6 months after surgery).
The alignment of the cervical spine was assessed by measuring the C2-7 Cobb angle, defined as the angle between the inferior endplate of C2 and the inferior endplate of C7. The K-line is defined as a line connecting the C2 and C7 centers of the cervical spinal canal and is widely used to determine the surgical approach for patients with Ossification of the Posterior Longitudinal Ligament. According to the criteria proposed by Fujiyoshi et al., we divided patients with cervical OPLL into two groups: K-line (+) group that OPLL did not exceed the K-line and K-line (−) group that did exceed it. We recorded patients who showed high signal intensity at the compressed segment of the T2 weighted magnetic resonance images.
Continuous variables and frequencies are expressed as means ± standard deviation, and categorical variables are expressed as percentages. One-way analysis of variance (ANOVA) was used for normal distribution data, and the Kruskal-Wallis H test was used for non-normal distribution data. For each group, the pre-and postoperative JOA scores were assessed by paired t-test. The postoperative JOA scores among the three groups were compared using repeated-measures ANOVA. For patients with the JOA recovery rate unreached the MCID, the odds ratio with 95% confidence interval (CI) of each risk factor was calculated using univariate logistic regression analysis. The categorical recovery rate of JOA (whether the recovery rate reached the MCID) was used as a dependent variable. Age, duration of disease, and pre-operative JOA score were assigned as continuous independent covariates. Smoking status and OPLL were assigned as categorical independent covariates. Multivariate logistic regression with a forward stepwise procedure (P < 0.05 for entry) was performed to identify correlations between risk factors for those with the JOA recovery rate unreached the MCID. All statistical analyses were performed using SPSS version 26.0. P-values < 0.05（two tailed）were considered significant.