Spino-pelvic alignment influences disc hydration properties after AIS surgery: a prospective MRI-based study

To analyze the disc hydration and volumetric changes of the intervertebral disc after scoliosis surgery depending on the sagittal spino-pelvic organization.

We conducted a prospective MRI study in 45 patients with surgically treated adolescent idiopathic scoliosis (AIS) with a minimum 2-year follow-up. Fusion ended at L1 (n = 13), L2 (n = 14), L3 (n = 11) or L4 (n = 7). Total disc (Vd) and nucleus volumes (Vn) were extrapolated from 3D reconstruction using a custom-made image processing software (Biomechlab^®, Toulouse, France). Nucleus and external disc contours were semi-automatically detected on turbo spin echo T2-weighted sequence joined 3-mm sagittal cuts. Disc hydration was extrapolated from the nucleus–disc volume ratio (Vn/Vd). Radiographic sagittal parameters were measured pre- and post-operatively on full spine standing views (pelvic incidence, sacral slope, L1S1 lumbar lordosis). Lumbo-pelvic congruity was calculated by the ratio LL/SS according to Stagnara.

Mean PI of the cohort was 55° (34°–85°). After surgical correction, lumbar lordosis was slightly increased by 3° (p = 0.02) decreasing lumbo-pelvic congruity from 1.37 to 1.27 (p < 0.01). When pelvic incidence was less than 55° (mean PI 46°), nucleus volumes have increased on average by 30 % compared to the preoperative status in the unfused lumbar discs, while the total disc volumes has remained stable. Five-year follow-up (n = 13) confirmed the constant improvement of the disc hydration ratio. When PI was high (mean PI 64°), volumetric changes were very mild and significant changes in nucleus volumes and disc hydration ratio concerned only the intermediate lumbar levels (L2L3, L3L4 and L4L5).

This prospective MRI study showed a significant and sustainable improvement in T2 hypersignal of the disc, indirectly indicating improvement of disc hydration content after AIS surgery. Analysis of disc volumetric changes according to the pelvic incidence suggests that these changes are under the influence of the sagittal spino-pelvic alignment. PI seems to play a key role in the homeostasis of the discs under fusion and should be taken into account for preoperative planning. The restoration of the lumbo-pelvic congruence may help to limit early degenerative changes in the free-motion segment discs after AIS surgery. Hydration content was less sensitive to surgery when PI was high, suggesting higher shear stress in the lower discs. Longer follow-up is required to confirm this hypothesis.