Quantitative Analysis of Vascular Canals in Vertebral Endplate

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1 Quantitative Analysis of Vascular Canals in Vertebral Endplate Kristine Tan 1, Won C. Bae, PhD 1, Tomonori Yamaguchi, MS 1,2, Kelli Xu, BS 1, Iris Shieh, BS 1, Jade He, BS 1, Robert L. Sah, MD, ScD 1, Nozomu Inoue, MD, PhD 2, Koichi Masuda, MD 1. 1 University of California, San Diego, San Diego, CA, USA, 2 Doshisha University, Kyoto, Japan. Disclosures: K. Tan: None. W.C. Bae: None. T. Yamaguchi: None. K. Xu: None. I. Shieh: None. J. He: None. R.L. Sah: None. N. Inoue: None. K. Masuda: None. Introduction: Degenerative disc disease of the lumbar spine is associated with low back pain [1], and is an active target of therapeutics. Nutrition to intervertebral discs (IVD) is supplied via vascular canals in the bony endplate. Abnormal changes to vascular canals may lead to inhibition of nutrition and disc degeneration. Morphology of vascular canals has not been studied in detail. The purpose of this study is to describe techniques used to determine quantitative morphology of vascular canals in human vertebral endplates, and report preliminary results. Methods: Lumbar Spines: Lumbar spines (including L1 to L5) from cadavers (n=9, 59±11 yo, mean±sd; 2 females, 7 males) were obtained from a tissue bank within ~48 hours of death, and imaged using a 3-T magnetic resonance imaging (MRI) scanner to obtain T2-weighted images in mid-sagittal plane (FOV=16 to 20 cm, slice thickness=3 mm, TR=2000 ms, TE= 90 ms, image matrix= ). All intervertebral discs were graded using the Pfirrmann grading scheme [2]. Discs with grades 2 to 4 were used for this study. Endplate Samples: After MRI, disco-vertebral cores, 5 mm in diameter, were obtained from superior surfaces of L3 and L5, at anterior, central, and posterior regions (3 per level). A total of 54 cores were harvested. Micro-Computed Tomography (μct): The cores were imaged using μct (Shimadzu SMX 160CTS) at 2 µm resolution and 67 kv. Reconstruction was performed using Bone3D software. Analysis of Quantitative Morphology: Three-dimensional (3D) models representing the canals were obtained using Mimics software (Figure 1). Linear canal segments were isolated (a total of 595 canals) and their angle (relative to the endplate s surface normal vector), volume, and mean diameter were determined using a custom Matlab program. Based on the angle, the canals were categorized as horizontal (angle of 60 to 90 deg), oblique (30 to 60 deg) or vertical (0 to 30 deg). Statistics: Initial tests found no significant effect of level, so it was not considered further. ANOVA and posthoc Tukey test were used to determine the effects of region and canal orientation on canal volume and diameter. In addition, chi-square test was used to determine association of region and orientation, as well as MRI grade and orientation. Results: Mean canal diameter (μm) (Figure 2) varied significantly with region (p<0.001), but not orientation (p=0.19). The posterior region had canals with the smallest average diameter (~50 μm) relative to anterior (posthoc p<0.001) and central (posthoc p<0.001) regions which averaged ~60 μm. Average canal volume did not vary significantly with region or orientation (each p>0.5). Chi-square test revealed that the distribution of different canal orientations was significantly associated with region (p<0.01; Figure 3). The distribution of orientations by disc MRI grade was not significant (chi-square p=0.3). Discussion: A technique to determine quantitative morphology of vascular canals within vertebral endplate was described. Using a high resolution micro CT scanner, it was feasible to segment small vascular canals, demonstrating a complex 3D canal network consisting of vertical, oblique and horizontal canals. Vertical canals directly connect the bone marrow in the vertebral body to the IVD while horizontal canals interconnect the vertical canals and allow lateral distribution of nutrients across the endplate. This study showed that out of the three canal orientations, all three sample regions of the endplate had horizontal canals in the greatest number, suggesting an important contribution to nutrient transport within the entire endplate. In the center region, the smaller in number, but larger diameter, vertical canals indicate that the function of the vertical canals is to go through the endplate and connect the bone marrow to the IVD. This data is consistent with previous studies that found the center region of the endplate to be more porous than surrounding peripheral regions [3]. The mean diameter of the canals, ~50 to 60 μm, is also consistent with those seen on endplate surface on scanning electron micrographs [4]. In future studies, comparison of μct models with other reference measures, such as 3D histology [5] would be useful for further validation of our results. In addition, additional measures of interest, including porosity and density of canals will be analyzed. Significance: The determination of the quantitative structure of vascular canals in vertebral endplates is important for a better understanding of the etiology of disc degeneration. Acknowledgments: NIH NIAMS, General Electric, LifeSharing, Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C) , UCSD PRIME program, Julia Brown Research Scholarship

2 References: 1. Vanharanta+ Spine Pfirrmann+ Spine Nachemson+ Acta Orthop Scand Benneker+ Spine Jadin+ Biomaterials 2007.

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5 ORS 2014 Annual Meeting Poster No: 0188

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