Introduction: Saving the L5-S1 motion segment may have significant functional benefits for patients undergoing thoracolumbar fusion for adult spinal deformity (ASD). We aimed to (1) understand how the L5-S1 disc initially adapts to surgical realignment and (2) determine whether MRI characteristics are correlated with reactive changes in the L5-S1 disc.
Methods: A retrospective review of ASD patients who underwent long fusion (≥5 levels) to L5 from 2013-2023 was performed. Modic type changes and Pfirrmann grade were determined on preoperative MRIs. Regional and global spinopelvic parameters, segmental lordosis, L5-S1 coronal Cobb angle, ventral and dorsal disc height, anterior osteophyte formation was assessed on preoperative and immediate standing postoperative XRs. Mean L5-S1 disc angle loss was used to stratify into “reactive” and “non-reactive” cohorts for comparisons.
Results: 17 patients (mean 54.2±14 years, BMI 27.5±5 kg/m2; 71% female, avg 7.0 [6.4-7.6] levels fused). No L5-S1 disc showed Modic changes, 5 (29%) were Pfirrmann grade 2, 12 (71%) grade 3. Two (12%) had ventral osteophytes. Despite an average postoperative increase in L1-L5 lordosis (-10.2±14.3°), L5-S1 lordosis decreased (6.8±7°), leading to no overall change in LL and an increase in L1PA (p=0.007) and PT (p=0.024). Mean loss of L5-S1 lordosis was 6.8±7°, splitting patients into “reactive” (53%) and "non-reactive” (47%) groups. Patients with reactive discs had higher PI (64.4±9.1 vs. 49.5±9.1, p=.035) and underwent larger L1-L5 correction (-15.2±15° vs. -4.6±13°, p=.131) but lost more L5-S1 lordosis (11.7±5° vs. 1.2±4°, p<.001). Pfirrmann grade, L5-S1 Cobb angle, and ventral osteophytes were not associated with disc reactivity.
Conclusion : After long fusion to L5, changes in the L5-S1 disc cannot be predicted by MRI characteristics. Surgeons should use caution when considering fusion to L5 in patients with high PI, as the necessary lordosis may not be maintained. Future research should focus on the long-term clinical implications of radiographic L5-S1 changes.