MRI Assessment of the Early Disc Degeneration Two Levels above Fused Lumbar Spine Segment: A Comparison after Unilateral and Bilateral Transforaminal Lumbar Interbody Fusion (TLIF) Procedure

Background: Adjacent segment degeneration (ASD) has become a great concern as a late complication in patients following fusion spine surgery with a potential need for revision surgery. Segments above the fused spine have higher mobility and they are especially prone to accelerated disc degeneration. The purpose of our study is to investigate early disc degenerative changes two levels above a surgically fused lumbar spine segment and to compare MRI analyses after unilateral and bilateral TLIF procedures. Methods: A total of 117 patients were included in this cross-sectional retrospective single center study (after bilateral TLIF surgery: n = 91, and after unilateral TLIF: n = 26). In both groups, the average patient age was similar: 62.84 years (SD = 12.53) in the unilateral TLIF group and 60.67 years (SD = 11.89) in the bilateral TLIF group. On average, MRI was performed 2.5 years after surgery (SD = 2.09). The modified eight-level Pfirrmann grading system was used for the assessment of disc degeneration severity. Descriptive statistics and the Mann-Whitney test were used to show differences in the Pfirrmann grades regarding the after-surgery period and the patient age. The Wilcoxon signed-rank test results were used to display differences in the Pfirrmann grades before and after surgery. Results: The comparison of mean values, regardless of the type of surgery, shows that this mean value is on average higher in the first segment adjacent to the fused spine segment. The assessment of the intervertebral disc structure in BIL TLIF is higher in both the first and the second segment. Early disc degeneration progression is subtle yet detectable (UNI TLIF 9.28% vs. BIL TLIF 16.74%). The assessment of the intervertebral disc structure is on average lower in patients aged less than 50 years at time of surgery compared with patients aged more than 50 years in UNI TLIF, and higher in the BIL TLIF group, for both the first and the second segment. Conclusion: Patients who had undergone unilateral TLIF fusion surgery have a lower rate of early disc degenerative changes. Considering a significantly higher rate of progressive disc degenerative changes in the elderly with bilateral fusion surgery, extra caution is required in the selection of appropriate surgical technique.