This study describes outcomes of the control group only, from a multicenter, prospective randomized trial of subjects that underwent microscopic limited lumbar discectomy (only the herniated part of the disc was removed) and were followed for 2 years . This study retrospectively reviewed the prospective data to analyse the natural course of MC in serial imaging. In addition, a possible clinical relevance of MC was assessed. The clinical trial (registered at ClinicalTrials.gov—NCT 01,283,438) was approved by local ethics committees, and informed consent was obtained from all individual participants included in the study.
Two hundred seventy-eight patients (enrolled between December 2010 and October 2014) between 21 and 75 years of age with the presence of lumbar radiculopathy due to a primary, single-level disc herniation between L1 and S1, and failure of at least 6 weeks of nonsurgical treatment was eligible for the study. Exclusion criteria included prior index-level surgery, spondylolisthesis with > 25% slip, scoliosis of greater than 10 degrees (both angular and rotational), insulin-dependent diabetes, and contraindication for MRI. Further details were described in the study protocol published previously .
Baseline characteristics are presented in Table 1. Complete data sets including MRI images and evaluation of MC at the index level were available at 1 and 2 years for 89.9% (250/278) and 85.4% (239/278) (Fig. 1). The mean volume of nucleus removal was 1.3 ml.
Clinical follow-up was done at 6 weeks, 3 months, 6 months, 1 year, and 2 years. MRI of the lumbar spine was performed preoperatively and at 1 and 2 years.
According to the imaging guidelines for the study, T1 and T2 images were required. Recommended axial slice thickness was 3–4 mm (sagittal ≤ 5 mm), a gap equal or less than 1 mm (sagittal ≤ 1.5 mm), and a maximum slice interval of 5 mm (sagittal 6.5 mm).
According to the standard protocol, only true X-ray mode was used (no fluoro mode). For the lateral lumbar spine, 75–90 kVp and 15–150 mAs were preferred. Neutral–lateral, anteroposterior, and flexion/extension views were taken.
Magnetic resonance and conventional images were read by two board-certified radiologists at an independent imaging core laboratory (Intrinsic Imaging, LLC, Bolton, MA, USA) who were blinded to patient outcomes. Disagreement between raters was resolved with adjudication by a third radiologist.
Radiological and MRI assessment
Within three months prior to surgery, MRI with T1- and T2-weighted axial and sagittal images were performed. Neutral AP, lateral, and flexion–extension radiographs were performed within sixty days prior to surgery. Postoperative imaging was obtained at each annual follow-up (12 and 24 months). An independent radiological core laboratory analysed all radiological imaging. In this analysis, we concentrated on angular motion (index and adjacent), translational motion (index and adjacent), spondylolisthesis, and MC.
The Modic classification describes three types of different vertebral endplate changes on T1- and T2-weighted MRI [2, 15]. To define the alterations of signal intensity changes at the index level during follow-up distinctly, there was the need for further subdividing each type into “new/increased”, “not changed”, or “decreased” signal intensity relative to baseline (Table 2).
Subjects’ function and pain was assessed using the Oswestry Disability Index (ODI)  and visual analogue scales for back, right, and left leg pain using a 100-mm scale. The postoperative scores of the assessments were compared to baseline scores.
Baseline patient characteristics, including age, BMI, sex, smoking history, together with VAS (for back and leg pain), and ODI are presented as medians and interquartile ranges for continuous variables and percentages for categorical variables. In order to gain an impression of Modic changes over time (i.e. presurgery, after 12, and after 24 months), itemized data of Modic subtypes “new/increased”, “not changes”, and “decreased” were pooled to Modic Types 1, 2 or 3, respectively, for reasons of clarity and comprehensibility. Results are represented as percentage values. Furthermore, an analysis of potential confounders, which could be a contributory cause of a specific Modic Type or Modic change over time, was carried out by the use of group comparisons between Modic Types and Modic changes over time regarding all collected patient characteristics. Initially, for each Modic Type the distribution of a potential confounder was visualized by boxplots for a continuous variable (e.g. age, BMI, size of annular defect, radiological findings) and by barplots for a categorical variable (e.g. gender, smoking,) with reference to the time point, i.e. presurgery, after 12, and after 24 months. Afterwards, possible significant differences between Modic Types regarding potential confounders were verified by using Student’s t test (in case of a two-group comparison) or ANOVA and, subsequently, Tukey’s post hoc test (in case of more than two groups) for continuous data. In case of categorical data, Chi-square test was employed. Additionally, to analyse confounding variables regarding the conversion of Modic Types from one, e.g. after 12 months, to another, e.g. after 24 months, more precisely, group comparisons were employed. To subtype within one preoperative Modic Type, e.g. within group “preoperative Type 0”, group comparisons between group “Type 0 after 12 months” and “Type 1 after 12 months” and so on, were made in the same way as described before. In case of non-normal distributed data and unbalanced groups, Mann–Whitney U test was employed. Statistical significance was set at p < 0.05. Statistical analyses were performed using version 3.3.2 of the R statistics package.