Patients
This study was approved by the ethics committee (IRB00006761-2018192) and was performed according to the principles of the Declaration of Helsinki. We recruited consecutive patients who underwent posterior lumbar interbody fusion (PLIF) for lumbar degenerative disease at our hospital between December 2010 and April 2012. Written informed consent was obtained from all patients. All patients met the following inclusion criteria: patients who received PLIF with the lower instrumented vertebra located in the lumbar or sacral regions, and fusion levels of ≤4. The exclusion criteria were as follows: patients (1) who underwent other spinal surgeries, (2) with coronal deformity and adjacent segment instability, (3) with severe lower back pain affecting sitting and standing position or an Oswestry Disability Index of >40, (4) with hip or knee joint contracture, (5) with vertebral fracture, (6) with neuromuscular disorders, (5) with severe osteoporosis, and (7) with internal fixation breakage or pseudarthrosis formation.
Assessment
All enrolled patients underwent comprehensive history taking and a physical examination. Sex, age, body height, and body mass index (BMI) were recorded. Computed tomography (CT) was then performed to evaluate the fusion, and X-ray imaging of lumbar extension and flexion was performed to assess adjacent segment stability.
For each patient, lateral full standing and sitting radiographs of the spine were obtained with a constant distance between the subject and the radiographic source (Figure 1). For standing radiographs, the patients were instructed to stand as straight as possible, with the fingers touching the homolateral collar bones10. In the erect sitting position, they were asked to flex their hips and knees to 90°, and sit as straight as possible, with the fingers touching the homolateral collar bones11. In the natural sitting position, the patients were instructed to sit as naturally as they would on a chair while relaxing the trunk. A height-adjustable stool without a back-rest was provided such that the height could be adjusted to achieve a standardized posture, with the feet placed flat on the ground. If the patients’ feet could not touch the ground after adjusting the stool height, a wooden step was provided.
The radiographs were digitized, and all measurements were performed by means of imaging software (Centricity RIS/PACS, GE Healthcare), based on standard measurement techniques by two senior spine surgeons, and the average of their results was recorded. The following parameters were measured preoperatively and at the final follow-up (Figure 2): (1) Pelvic parameters: pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS); (2) Local curvature: lumbar lordosis (LL), fusion segment lordosis (FSL) i.e. the angle between the upper and lower endplates of the fusion level (endplate of S1 in lumbosacral fusion), upper residual lordosis (URL) i.e. the angle between the upper endplate of L1 and the upper endplate of the fusion level, lower residual lordosis (LRL) i.e. the angle between the lower endplate of the fusion level and the upper endplate of S1, and thoracic kyphosis (TK); (3) Global parameters: T1 pelvic angle (T1PA) i.e. the angle between the line from the femoral head axis to the centroid of T1 and the line from the femoral head axis to the middle of the S1 endplate. The changes in LL, unfused adjacent segment lordosis, and PT between standing and natural sitting were calculated.
Statistical analysis
All data were analyzed using SPSS software (version 17.0; SPSS, Chicago, IL). Inter-observer reliability was assessed using the intraclass correlation coefficient (ICC). An ICC of ≥0.80 was considered to indicate excellent reliability. Continuous variables are expressed as means ± standard deviation. An adaptation of the Kolmogorov-Smirnov test was applied to assess the normality of the distribution. The changes in parameters were analyzed using a one-way analysis of variance (or the Kruskal-Wallis test) with post hoc comparisons performed among different positions. We set the physical and surgical factors (sex, age, body height, BMI, lumbosacral fusion, fusion levels, and PI) as explanatory variables. Correlations between the changes in lordosis and these factors were analyzed using Spearman correlation analysis. Finally, multiple regression analyses with a forward stepwise procedure were conducted. A p value of <0.05 was considered to indicate a statistically significant difference.