Human In Vivo Disc Mechanics in Flexion, Extension, and Diurnal Loading With Aging and Degeneration

Background: The intervertebral disc serves a critical role in spine function, but with aging and degeneration the disc undergoes changes that alter disc mechanics, and degeneration is often implicated in low back pain. The objective of this study was to quantify and evaluate the impact of aging and degeneration on in vivo lumbar disc mechanics across the lifespan. Methods: loading for 84 subjects (evenly distributed by sex and age group from 18 to 83 years old). Intervertebral disc deformations were evaluated between the reference and loading states by change in wedge angle, anterior-posterior (A-P) Shear displacement, and axial strain. Results: Flexion induced the greatest changes in the lower discs of young subjects, but this shifted toward the central discs in older subjects. Flexion induced a gradient of anterior shear displacement in the upper levels and posterior shear displacement in the lower levels that was maintained across all ages. Extension had opposite trends in A-P shear displacement with respect to level, with young subjects having greater posterior shear displacement at L1-L2 and greater anterior shear displacement at L5-S1 than older subjects. Extension also had large changes in wedge angle between L1 and L3. Diurnal loading consistently induced compression across all disc regions. Diurnal change in disc height was age-dependent, with greater disc height loss in younger subjects. Conclusions: Using MRI repeated in different loading states, we established a reference for expected disc behavior with controlled loading with aging, degeneration, and by disc level. Deviations in lumbar disc mechanical behavior from the reference baseline provided by this work may provide a useful marker of disc pathology.