INTRODUCTION: Understanding structure-function relationships in the human endplate morphology has key implications when related to diagnosing and treating degenerative disc disease. Our previous unpublished data has shown concentrations of potential mineralized particles present in the cartilaginous tissue on micro-Computed Tomography of the cartilaginous endplate (CEP) of lumbar human spines of varied degeneration grades. We hypothesize that discs with higher degeneration grades will exhibit larger presence of calcification. The purpose of this study is to develop an approach to quantify the degree of calcification of the human lumbar spine CEP with disc degeneration and report the preliminary results.

METHODS: Lumbar spines were harvested from two donors (61M/76F). Clinical 1.5T MRI with T2-weighted sequences was used to evaluate the disc quality according to the Pfirrmann grading system. Six 4 mm diameter cylindrical core endplate samples were collected with a biopsy punch from either the superior or inferior surface of different lumbar vertebral levels, on the central region of the disc (within the footprint of the nucleus pulposus). These endplate core samples were embedded in polymethylmethacrylate mold, sectioned, ground and polished to a mirror finish before being stained using a combination of toluidine blue and basic fuchsin. Microscopic evaluation of each slide was done to create a 10x magnification mosaic image representation of each CEP, as shown in Figure 1.  Based on these images, the ratio of all individual calcification surface areas to the entire CEP surface area for each sample was calculated and named calcification area ratio (Figure 2). A Spearman Rho analysis between area ratio and disc grade provided correlation data between these two variables.

RESULTS: The grades of the six discs ranged from 1 to 5 on the Pfirrmann classification system, lacking a grade 4 disc. The ratio of calcification surface areas to the CEP surface area increased with advancing disc degeneration grades. For example, Figure 2 shows that a grade 2 disc will have a ratio of 0.011 compared to a grade 5 disc that shows a ratio of 0.015. The Grade 1 endplate in Figure 1 shows a total calcification surface area of 25,768.2 µm² to a total CEP surface area of 2,728,418.3 µm² resulting in a ratio of 0.009. The calcification area ratio showed a positive correlation with disc grade as the Spearman Rho correlation produced an R-value of 0.971, with a p-value of p < 0.001, which shows its significance.

DISCUSSION: The data supports our hypothesis in human endplates: as the disc degeneration grade increased, the level of calcification in the CEP also increased. Some limitations exist: a) as this was a pilot study, there was a relatively small sample size. b) The technique of preparation also limits the available size of the region of interest contained in each slice. In the end, this work was able to reproduce preliminary evidence of increased calcification on the human lumbar spine cartilaginous endplate, implying a higher disc degeneration status. Further work to explore this topic further on a larger scale with a larger sample size is warranted.