Abstract
Introduction
Decreasing endplate porosity has been proposed as a risk factor for intervertebral disc degeneration, because it interferes with disc metabolite transport. However, endplate porosity has recently been shown to increase with age and disc degeneration. We hypothesise that this increase reflects adaptive remodelling in response to altered loading from adjacent discs.
Methods
Nineteen cadaver motion segments (61–98 yrs) were compressed to 1kN while a pressure-transducer was pulled across the mid-sagittal diameter of the disc. Stress profiles indicated nucleus (intradiscal) pressure (IDP) and maximum stress in the anterior and posterior annulus. Subsequently, micro-CT was used to evaluate endplate porosity along the antero-posterior diameter of the adjacent endplates. Data were analysed using ANOVA and linear regression.
Results
Endplate porosity averaged 67% centrally (where IDP averaged 0.85MPa) and decreased steadily to 48% and 53% in the anterior and posterior periphery, where maximum stresses were 1.37MPa and 1.33MPa, respectively. In each region, porosity was inversely related to IDP (or maximum stress) with R2 = 0.49, 0.32 and 0.31 respectively (P<0.001 in each case). Porosity was 3% higher in the inferior (weaker) endplate of the disc compared to the superior endplate (P=0.07).
Conclusion
In old spines, strong correlations between endplate porosity and stress in each anatomical region indicate mechanically-adaptive remodelling. Regional differences in endplate porosity (across the antero-posterior diameter) probably reflect the varying nutritional demands of nucleus and annulus, as well as adaptations to loading from an adjacent decompressed disc. In younger age-groups, high loading could possibly reduce endplate porosity, promoting disc degeneration.
