Using an x-ray microscope developed at Lawrence Livermore National Laboratory for the Department of Energy, researchers at the UC San Francisco School of Dentistry are probing the intricate structure of dentin, the porous material that lies under the hard enamel of teeth.
An x-ray tomographic microscope (XTM), invented by a Lawrence Livermore scientist to analyze ceramic components used in jet engines, allows the dental researchers to observe structures in the dentin as small as two micrometers -- about the size of a human cell.
Atomic force microscopy (AFM), another technology used at UCSF and Lawrence Livermore, provides an even closer view. Researchers test the strength and stiffness of dentin by pushing the very atoms and molecules apart and using the device to observe how they respond.
Researchers want to understand the structure and properties of dentin in order to find methods and materials that will create a tighter, more permanent bond between the tooth and the plastic-based fillings now used to repair most cavities.
Metal alloys create a more durable restoration than newer plastic-based and ceramic materials used for filling tooth cavities, but the ability to match the color of these materials to the natural color of a tooth makes the newer materials much more popular choices.
"We want to make the polymer (plastic-based) and ceramic materials as strong and long-lasting as metal," explains Sally J. Marshall, PhD, UCSF professor of restorative dentistry. She and Grayson W. Marshall Jr., DDS, MPH, PhD, UCSF professor of restorative dentistry, led the UCSF research team working with John H. Kinney, PhD, senior research scientist at Lawrence Livermore National Laboratory.
The team of researchers recently reported on the progress of their study at the annual scientific meeting of the American Association for Dental Research. The study is funded by a grant from the National Institute for Dental Research. In the first stage of the study, the researchers examined the way dentin is structured and how a bond forms between the natural material of the tooth and the plastic filling.
Strong bonds form easily between restorative materials and the surface enamel of teeth because about 97 percent of the hard surface material is mineral. But because tooth decay eats through the enamel and into the underlying layer of the tooth, a bond must be formed with the dentin.
Because dentin contains a great deal of moisture and organic tissue -- only about 50 percent of dentin is mineral -- bonds are more difficult to form. By using the powerful microscopes to observe the bonding process, the UCSF researchers now can offer a better explanation of the reasons bonds succeed and fail.
When dentists prepare a tooth for bonding a filling, they use an acid to demineralize the surface of the cavity. This process removes the mineral material from the dentin, leaving a framework of collagen tissue and tiny tubules that run from the center of the tooth toward the surface.
When a plastic-based filling is placed in the cavity it is in a liquid form, allowing the material to penetrate the exposed framework before hardening. The UCSF researchers study how quickly and efficiently different acids work in the demineralization process and how various materials used to form bonds withstand the constant stress placed on teeth.
AFM is unique in its ability to provide a high resolution image in any environment, including wet or acid solutions. As a result, the UCSF researchers were the among the first to describe why the polymer bonding process works best when the dentin remains moist. In the bonding process the liquid filling penetrates moist dentin more easily than drier dentin -- much as a moist sponge more quickly absorbs water than a dried sponge, Sally Marshall explains.
In the next stage of the study, the research team is examining how tooth decay progresses differently in different areas of dentin and how age affects the process of tooth disease.