MRI‐based measurement of in vivo disc mechanics in a young population due to flexion, extension, and diurnal loading

Background Intervertebral disc degeneration is often implicated in low back pain; however, discs with structural degeneration often do not cause pain. It may be that disc mechanics can provide better diagnosis and identification of the pain source. In cadaveric testing, the degenerated disc has altered mechanics, but in vivo, disc mechanics remain unknown. To measure in vivo disc mechanics, noninvasive methods must be developed to apply and measure physiological deformations. Aim Thus, this study aimed to develop methods to measure disc mechanical function via noninvasive MRI during flexion and extension and after diurnal loading in a young population. This data will serve as baseline disc mechanics to later compare across ages and in patients. Materials & Methods To accomplish this, subjects were imaged in the morning in a reference supine position, in flexion, in extension, and at the end of the day in a supine position. Disc deformations and vertebral motions were used to quantify disc axial strain, changes in wedge angle, and anterior–posterior (A‐P) shear displacement. T2 weighted MRI was also used to evaluate disc degeneration via Pfirrmann grading and T2 time. All measures were then tested for effect of sex and disc level. Results We found that flexion and extension caused level‐dependent strains in the anterior and posterior of the disc, changes in wedge angle, and A‐P shear displacements. Flexion had higher magnitude changes overall. Diurnal loading did not cause level‐dependent strains but did cause small level‐dependent changes in wedge angle and A‐P shear displacements. Discussion Correlations between disc degeneration and mechanics were largest in flexion, likely due to the smaller contribution of the facet joints in this condition. Conclusion In summary, this study established methods to measure in vivo disc mechanical function via noninvasive MRI and established a baseline in a young population that may be compared to older subjects and clinical disorders in the future. We developed and applied noninvasive MRI methods to measure disc mechanical function during flexion and extension and after diurnal loading in a young cohort. The disc strain, change in wedge angle, and anterior–posterior shear, were affected by disc level. Flexion produced the most significant correlations between disc degeneration measures and disc functional measures.