Prize Winner : What Influence Does Sustained Mechanical Load Have on Diffusion in the Human Intervertebral Disc ? An In Vivo Study Using Serial Postcontrast Magnetic Resonance Imaging

Study Design. An in vivo study of the effects of mechanical loading on transport of small solutes into normal human lumbar intervertebral discs (IVD) using serial postcontrast magnetic resonance imaging (MRI). Objective. To investigate the influence of a sustained mechanical load on diffusion of small solutes in and out of the normal IVD. Summary of Background Data. Diffusion is an important source of disc nutrition and the in vivo effects of load on diffusion in human IVD remains unknown. Methods. Forty normal lumbar discs (on MRI) in 8 healthy volunteers were subjected to serial post contrast (Gadoteridol) 3 Tesla MRI in 2 phases. In phase 1 (control), volunteers were scanned at different time points – precontrast and 1.5, 3, 4.5, 6, and 7.5 hours postcontrast injection. In phase 2, 1 month later, the same volunteers were subjected to sustained supine loading for 4.5 hours. MRI scans were performed precontrast (preload) and postcontrast (postloading) at 1.5, 3, and 4.5 hours. Their spines were then unloaded and recovery scans performed at 6 and 7.5 hours postcontrast. In house software was used to analyze images. Results. Repeated-measures ANOVA and pairwise comparisons at different time points in the central region of the loaded disc (LD) compared to the unloaded discs (UD) revealed significantly lower signal intensity ratios (P1.5h:P3h:P4.5h<0.001:<0.001:<0.002) indicating reduction in transport rates for the LDs. Signal intensity ratios continued to rise in LD for 3 hours into recovery phase,whereas UD at the same time point showed a decrease (mean ± SD = 0.08 ± 0.08 vs. −0.21 ± 0.03). Conclusion. Sustained supine creep loading (50% body weight) for 4.5 hours retards transport of small solutes into the center of human IVD and it required 3 hours of accelerated diffusion in recovery state for LD to catch-up with diffusion in UD. The study supports the theory that sustained mechanical loading impairs diffusion of nutrients entering the disc and quite possibly accelerates disc degeneration.