We retrospected a total of 687 patients, of all ages, who came to the inpatient department of spinal surgery at Renji hospital and were scheduled for lumbar surgeries between January and December 2017. We divided these patients into five groups according to age: below 35 years, between 36 and 45 years, between 46 and 55 years, between 56 and 65 years, and above 66 years. Then, we randomly selected 20–30 samples(a ratio of 6:1) from each group and mixed them up to create a final sample group. A total of 122 patients remained in the analysis (Fig. 1). We registered the age and sex of all 122 patients, who all underwent magnetic resonance imaging(MRI) and computerized tomography(CT) scans of the lumbar spine prior to surgery. The inclusion criteria were: (1) no history of spinal surgery; (2) no recent history of severe lumbar trauma; (3) no abnormal radiological findings, such as vertebral fractures, space occupying lesions of the lumbar spine, or apparent spinal deformities (e.g., scoliosis); and (4) no history of systemic diseases (rheumatic diseases of the spine or carcinoma).
The study protocol was approved by the ethics committee of Renji Hospital, School of Medicine, Shanghai Jiao Tong University. Informed consent was were obtained from each patient prior to the imaging examination.
Magnetic Resonance Imaging Protocol
As reported previously by Yu10, all T2-weighted images were acquired using the same 3.0 T imaging system (Magnetom; Siemens, Erlanger, Germany) with a repetition time of 3220 ms and an echo time of 120 ms. Slice thickness was 4 mm. The acquisition matrix was 512 × 512 and the field of view was 310 mm. We then obtained and evaluated Original Digital Imaging and Communications in Medicine files from transverse oblique MRI images parallel to the superior end plate of L4 and L5.
Computed Tomography Protocol
Preoperative patients who were eligible for CT underwent imaging with an 8-slice multidetector CT scanner (Lightspeed Ultra; GE, Milwaukee, Wisconsin). Each patient underwent unenhanced lumbar CT performed with a sequential scan protocol with slice collimation of 8 × 2.5 mm (120 kVp, 320/400 mA for 0.220 lb body weight) during a single end-inspiratory breath hold (typical duration, 18 s). For the lumbar scan, 256 contiguous 2.5 mm slices of the lumbar region were acquired, covering a 150 mm area above the level of S1. The evaluation of CT scans was performed with blinding to clinical and personal data.
Disc Degeneration Assessment
Disc degeneration assessment used the disc degeneration grade described by Pfirrmann in 20019. Observers analyzed the L4-L5 lumbar intervertebral disc from each patient on T2-weight sagittal MRI images using Picture Archiving and Communication Systems (PACS), version 11.4 (Carestream health, Shanghai, China)(Fig. 2) with Pfirrmann’s original article to confirm the grade at the time of evaluation. Independently, more than half of the selected grade was recorded. If there was a dispute, then images were reevaluated until more than half of the observers agreed.
Laminar Slope Angle Measurements
Laminar slope angle was measured with PACS. We reconstructed pre-operation CT in three dimensions and proofread the central axis in parallel with the direction of the spinous process on the axis CT image and with the postural tilt of the spine on the coronal image. We then chose the sagittal images at the level of the tip of the unilateral facet joints (Fig. 3). Evaluators analyzed the selected sagittal CT images using our redefined method similar to Bai-ling’s on lateral radiographs18 We drew two separate lines connecting the tip of the superior facet with the base of the inferior facet and connecting the midpoints of the anterior and posterior vertebral cortices; we then measured the intersecting angle between the two lines. We selected L4 and L5 axis position on sagittal CT images, measured the corresponding LSA, and calculated the absolute value of the difference between the two adjacent segments of LSA (Fig. 2)
Muscle Measurements And Analysis
Muscle measurements and analysis were carried out with PACS and Image J software, version 1.42q (National Institutes of Health, Bethesda, Maryland) following the method described in our previous article10. We selected the axial slice at the level of the L5 vertebral body upper endplate to calculate L4-5 muscle cross section area(CSA) (Fig. 2). Intramuscular fatty infiltration was obtained with a widely accepted muscle-fat index, which represents the ratio of mean signal intensity in a region of lean muscle tissue relative to the signal intensity in a homogeneous region of fat (Fig. 2).
Thickness Of The Ligamentum Flavum
The thickness of the ligamentum flavum was measured on axis T2-weight MRI images with PACS. We located the spinal level of the L4-5 intervertebral spaces on sagittal T2-weight MRI images and selected the axial slice at the level of the L5 vertebral body upper endplate.13,19−21 We then drew two parallel lines along the direction of the ligamentum flavum and chose the maximum distance between the dural side and the dorsal side. The maximum thickness of the LF was measured on both the right and left sides (Fig. 2)
To avoid bias, two radiologists and two surgeons were blinded to the study design; consequently, when measuring the parameters, they all ignored the laminar slope angle. To ensure the objectivity of the results, all measurements were repeated by the radiologists and surgeons 2 weeks after the initial evaluation. The mean of the data was then used in the primary analysis.
Statistical analyses were performed with SPSS software, version 24.0 (IBM, Armonk, New York). The association between age and measurement indexes were determined by the independent-samples T test. The association between laminar slope angle and other parameters were determined by Pearson correlation. Partial correlation was used to analyze the correlation of other remaining variables while controlling age variables. The reliability of the measured parameters was evaluated using intraclass correlation coefficients. Significance was set at P < 0.05 and values represent mean ± standard error.