Make Mine a Double

TACC advanced computing resources help visualize, test strength of newly-discovered double spinal endplate

A double layer endplate is made up of a porous, thin top layer (red) and thicker second layer (green). The double and single layer endplates (orange) that were examined had a very similar overall thickness.

Detailed study of human anatomy requires a sharp eye, and sharp tools to match. Revealing internal structures—whether for Leonardo Da Vinci's 15th century notebooks or today's microscope slides—can require some serious slicing and dicing.

Recently, researchers at The University of California, San Francisco (UCSF) have confirmed the existence of a tissue that has historically been overlooked, unnoticed, or inadvertently sliced up in the dissection or sampling process: double layer vertebral endplates.

Endplates cap the top and bottom of each vertebral body in the spine, providing support and nutrition to the intervertebral disc sandwiched between them.

"This feature has a mystical quality because there are only a handful of studies that have reported seeing it, and they have reported it only anecdotally since its structure and function are difficult to study with traditional sectioning techniques," said Aaron Fields, a post-doctoral researcher in the Laboratory for Orthopaedic Bioengineering at UCSF.

The findings appeared in the journal Spine in October 2012.

Everyone has vertebral endplates. These thin layers of bone are essential to the health of intervertebral discs, the tissues that link the backbones together and cause major damage if they degenerate or slip out of place. But individuals with a double endplate may have a structural advantage that may help sustain the endplate's function in case of damage.

Researchers noticed the second layer of bone after harvesting small cores from the vertebral bodies of six cadaver spines and scanning them with a micro-CT scanner. Like a medical CT scanner, a micro-CT scanner uses x-rays to create cross-sectional images of a specimen's internal structure without destroying the specimen. The scans can then be layered to create a 3D-image of the structure. The high resolution of the scans (about 10 micrometers for this study) allowed for exceptionally thin images of the endplate cores to be collected and reconstructed into highly detailed digital renderings that brought the double endplate structure into view, said Andrew Burghardt, the technical director of the Quantitative Micro-Imaging Facility at UCSF.

"We were looking to see how different endplate structures affect disc health and we immediately recognized the double endplate structure and thought it would be a good place to start because of its potential to more effectively balance conflicting endplate functions," said Fields.

The endplate must balance two opposing functions: to manage weight distribution and also provide nutrient and waste transport. To do so, the structure must be strong to evenly distribute loads on the disc and adjacent vertebrae, yet porous so nutrients from the vertebrae can reach disc cells and waste can be cleared away. It is a balancing act that the double endplate seems better equipped to accomplish, with the first layer providing increased permeability and the second layer lending structural support.

(From left to right) A single layer endplate, the top layer of a double endplate, the second layer of a double endplate.

In the samples, the presence of double layer endplates was associated with healthier adjacent disc tissue. The first layer of the double endplate was about half as thick as a single endplate and much more porous, making it more permeable than a single endplate. It is a feature that likely enhances nutrient transport, said Fields.

Despite its delicate nature, the first layer was found to be just as strong as a single endplate because of the reinforcement provided by the second, more substantial layer of bone in the double endplate.

"Having a redundant layer of bone means that when there's damage in the first layer, some of the load can be redistributed to the second layer and proper function is maintained, much in the same way that structural redundancies in a bridge or a tunnel might permit safe function when a part of the structure fails," said Fields.

Instead of stressing the physical endplates to find their compressive strength, Fields used the micro-CT data to perform high-resolution finite element simulations—virtual "stress tests" that reveal the weak parts of a structure. The simulations also allowed the researchers to also carry out permeability experiments with the endplates.

"The big advantage of this is that we can acquire a very high resolution, three dimensional image and it's completely non-destructive, so we don't destroy the sample that we're looking at," said Burghardt.

Creating high-resolution models required some serious hardware and software for analysis. To accomplish the task, the researchers turned to the Texas Advanced Computing Center (TACC). By remotely accessing the processing power of the "Ranger" supercomputer, and the "Spur" visualization system, the California-based researchers were able to make their models come to life while using resources halfway across the country.

"These large finite element models contain hundreds of millions of degrees of freedom so they're far too large for conventional computer systems to handle," said Fields. "Without the resources at TACC, this research wouldn't have been possible."

Aaron Fields, post-doctoral researcher in the Orthopaedic Bioengineering Laboratory at UCSF

The confirmation of the double endplate variant and the benefits that it may offer for disc health naturally raises a question in most who hear about the research: Do I have double endplates?

It is hard to estimate the prevalence of double endplates in the general population based on the small size of the study; two of the six spines examined had double endplates. But if you do have this structure, early findings indicate that it is likely your kids will too.

The high heritability of disc degeneration—up to 75 percent according to one study—along with all-or-nothing expression of the double endplates in the lumbar spine indicates endplate layers are likely to be inherited features like hair or eye color. Still, there is a possibility that the trait could be acquired during development, said Fields.

The discovery of the double endplate spinal structure could have an impact on personalized spinal treatments of the future, says Fields, with spinal implants being designed to take advantage of a patient's unique spinal anatomy.

"Regenerative therapies for restoring the proper anatomy and function of the disk might depend on these different endplate phenotypes," said Fields. "Another area is understanding how these different endplate features might be better imaged so we better select patients for treatments."

According to the research, double endplates appear to have a distinct MRI scanning signature that could be used clinically to identify those with and without the double structure, said Fields.

"I don't think we would start screening the general population for this, but it may be that as we're exploring personalized treatments for degenerative disc disease, the double endplate could be a factor that makes one particular treatment more effective than another," said Fields.

Monica Kortsha, Science and Technology Writer
January 24, 2013

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The Texas Advanced Computing Center (TACC) at The University of Texas at Austin is one of the leading centers of computational excellence in the United States. The center's mission is to enable discoveries that advance science and society through the application of advanced computing technologies. To fulfill this mission, TACC identifies, evaluates, deploys, and supports powerful computing, visualization, and storage systems and software. TACC's staff experts help researchers and educators use these technologies effectively, and conduct research and development to make these technologies more powerful, more reliable, and easier to use. TACC staff also help encourage, educate, and train the next generation of researchers, empowering them to make discoveries that change the world.