This retrospective cohort study included cervical OPLL patients treated with anterior cervical surgery from November 2007 to July 2021. Patient data were obtained from the electronic medical record database of our hospital and patients or their guardians, including complications. The study was approved by the Ethics Committee of Changzheng Hospital (Shanghai, China). This study followed the reporting guideline of Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) for cohort studies.
Of the enrolled 206 patients according to the inclusion criteria: age between 35 and 80 years; patients with cervical OPLL detected by imaging scan; and clinical manifestations failed to improve through conservative treatment. The exclusion criteria included: patients with incomplete clinical data; and failed to follow up postoperatively.
Seventeen patients withdrew from this study due to missed follow-up data. Finally, 189 patients were included for analysis using a sealed envelope (Figure 1). The demographic data including age, gender, body mass index (BMI), days of hospital stay, duration of symptoms, history of alcohol and smoking use, diabetes, hypertension, and history of surgery are summarized in Table 1. The radiological data including the OPLL classification and COR of patients are summarized in Table 1. Mean follow-up time of the study was 67.67±35.89 months (51.79 months in ACOE group vs 87.51 months in ACOP, p<0.001) (Table 1).
Operative procedures
The exposure was the different type of operation received by OPLL patients, which comprised ACOE and ACOP. The ACOE was performed as described previously [15]. During surgery, most of the vertebral body was bitten off, with the posterior wall of the vertebral body and the ossified mass (OM) preserved, or the intervertebral disc to the OM removed. The OM was floated by using a high-speed burr to drill along the edges of the OM to the depth of the dura. The posterior wall of the vertebral body and the OM were removed with Kocher or Allis clamp. The ossification was separated from the dural sac with a sharp nerve dissector. The remaining vertebral posterior wall-ossification complex (VPWOC) was removed en bloc. The width of the dissection was the area of least compression of the edge of the OM, which could ensure protection of the SC and preserve the dura during elevation of the ossified ligament. The extent of resection was a little lager than the segment responsible for the patient's chief complaint or the segment with obvious cord or nerve root compression. For dura defects but intact arachnoid without CSF leak, no repair was performed. In case CSF leak occurred, the dura would be repaired with a dura guard patch to protect the SC. The dural ossification (DO) would be preserved as much as possible if no SC compression by the DO was observed; otherwise, the DO would be resected en bloc.
In the ACOP surgery, the OPLL was removed in piecemeals using a Kerrison rongeur or a neural stripper with a hook and a sharp scalpel after exposing the ossification.
The operation was performed by the same team of surgeons. All ossifications were resected by senior spine surgeons. All procedures were performed in accordance with the standard procedures and pre-operative planning.
Outcome measures
The perioperative data including operative time, volume of intraoperative blood loss, and complications, such as CSF leak, pain, C5 palsy [defined as muscle power of the deltoid by at least one grade using manual muscle testing (MMT) with potential biceps involvement and without deterioration of lower extremity function[16].], hoarseness, internal fixation displacement (IF failure), hematoma, Horner Syndrome.
All participants were followed up on the outpatient basis or by telephone interviews in July 2023. The primary outcome was the improvement of postoperative neurological function assessed by the Japanese Orthopaedic Association (JOA) score [15]. The recovery rate was calculated according to the following formula: JOA recovery rate = (final JOA score - preoperative JOA score) / (17 - preoperative JOA score) × 100% [17].The JOA recovery rate ≥75 % was defined as excellent, 50-74 % as good, 25-49 % as fair, and <25 % as poor [10]. The pain intensity was measured by the visual analogue scale (VAS) score using a numerical rating scale.
All patients had preoperative cervical CT scan. And OPLL was classified basing on sagittal CT scans as segmental, continuous, localized, and mixed type. COR was defined as the maximum ratio of OM thickness to the sagittal diameter of the spinal canal on CT scans. IF failure was defined as the internal fixation sinking >2 mm or departure from the original fixation position on radiographs between immediate and follow-up postoperative radiograph [18].
Statistics
Data were analyzed by R software (version 4.2.1). Descriptive statistics were calculated to determine the mean post-operative JOA, VAS score and JOA recovery rate. Because the comparison between ACOE and ACOP groups was stratified by severity of cord compression, the balance of baseline characteristics was also assessed in patients with severe compression (COR ≥50%). Differences in continuous data as JOA, VAS scores, mean operative time, volume of blood loss, and baseline including age, BMI, days of hospital stay, and timing of surgery between ACOE and ACOP groups were calculated using two sample t test. And categorical data as gender, medical history, and complications were computed by Chi-square or Fisher’s exact test. All tests were 2-tailed, with a significance level of P < 0.05.
The propensity scores for the surgical procedures were calculated based on the age, gender, body mass index, days of hospital stay, duration of symptoms, history of alcohol and smoking use, diabetes, hypertension, history of surgery, preoperative JOA score, COR, and time of follow-up. Patients who underwent ACOE or ACOP were matched on the basis of propensity scores and provided that the caliper value ≤ 0.02.