Quantification of Intervertebral Disk Cartilaginous Endplate Morphology using MRI

Abstract

IntroductionThe IVD cartilaginous endplate (CEP) is a 450 mm to 800 mm thick layer of hyaline cartilage,2 which functions as a mechanical barrier between the pressurized nucleus pulposus and the vertebral endplate, and as a gateway for nutrient diffusion to the avascular disk.1–3 With age and degeneration, the CEP becomes thinner, calcifies, and may have reduced porosity,4 and it has been implicated as a contributor to degeneration.1–3 As the CEP is not well imaged on MRI, key geometric parameters of the CEP have not been obtained noninvasively. Thus, the objective of this study was to measure geometric properties of the CEP with MRI.Materials and MethodsLumbar motion segments (n = 24), from 11 human cadavers (age 64.9 ± 10.0 years) were imaged on a Siemens 7T MRI scanner using a custom RF coil. Disk degeneration was graded via mid-sagittal T2 mapping. Full volume imaging of each disk at 200 mm isotropic resolution was achieved via 3D FLASH (TR/TE = 9/3.7 ms) in 6 minutes (Fig. 1). Geometric parameters (area, circumference, anterior-posterior width, lateral width) were measured in the axial and mid-sagittal planes for both the CEP and the entire disk using OsiriX software. Disk height was measured using custom-written Matlab code.5Evaluation of CEP thickness was done on a mid-sagittal image of each disc. Semiautomated image analysis was conducted using custom-written Matlab code in which signal intensity profiles across the CEP thickness were captured at the center of the CEP and at ± ¼ and ± ¾ of its A-P length.Comparisons between superior and inferior CEP geometry were established using paired, two-tailed t-tests (p < 0.05). Data, except thickness, was normalized by dividing CEP geometry by its corresponding disk geometry to account for differences in disk sizes between levels. A one-way ANOVA was used to evaluate differences between disk level and CEP geometry. CEP thickness measurements were evaluated using a two-way ANOVA with repeated measures, where the factors were disk level and anterior-posterior disk location. A posthoc Bonferonni test was performed when significance was detected. Linear regression analysis was performed to compare CEP geometry with T2 mapping values and subject age.ResultsDirectly measured whole disk and CEP geometry for each level are provided in the Table. The mean CEP thickness in the mid-sagittal plane across all disk levels and locations was 0.50 ± 0.20 mm. No significant differences were found in CEP thickness with respect to the superior or inferior endplate or disk level (p > 0.2). There was no significant difference in CEP A-P width, lateral width, or circumference area with respect to superior/inferior location (p > 0.7) or disk level (p > 0.6) except between L1-L2 and L4-L5 (p < 0.05) (Fig. 2A-C). L1-L2 A-P width, lateral width, and circumference area were larger than in L4-L5 by 14, 18, and 28%, respectively.CEP thickness varied significantly with respect to anterior-posterior location for every level (p < 0.005) (Fig. 2D). The minimum thickness was at the center of the disk (superior = 0.39 ± 0.10 mm, inferior = 0.38 ± 0.11 mm). The thickness at the center was 19.9 and 42.4% less than that of the ± ¼ and ± ¾ A-P locations.In the linear regression analysis, no correlation was found between CEP geometries and T2 mapping or age.ConclusionThis study noninvasively visualized the CEP morphology and quantified CEP circumference, area, A-P width, lateral width, and thickness using an MRI 3D FLASH sequence. The measured disk geometries in this study agree well with previous literature.5,6CEP geometries were not linearly correlated with age or T2 mapping, which is correlated with disk degenerative grade. The lack of correlation between CEP geometries and either age or T2 mapping as a marker of degeneration was not completely expected, since it has been discussed that CEP thickness decreases with degeneration.1–3 Additional data and increased sample size may provide more insight into the potential role of the CEP in the degenerative cascade.Acknowledgment NIH grant R01 AR050052.I confirm having declared any potential conflict of interest for all authors listed on this abstractYesDisclosure of InterestNone declaredUrban JPG. Arthritis Research Therapy 2003;5:120Roberts S. Spine 1989;14:166Moore RJ. European Spine Journal 2000;9:92Bernick S, Spine 1982;7:97O'Connell GD. Spine 2007:32:328Elliott DM. Spine 2004;29:713