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Metrology for Dental Materials

Summary:

Our goal is to provide measurement methods and reference materials to facilitate the rational design of polymeric dental materials, thus enabling improvements in their clinical performance. Specifically, clinically relevant methods for determining long-term performance of polymeric dental materials are needed to provide predictive information regarding the clinical longevity of new and existing materials, including materials with cariostatic and regenerative properties.

 

Description:
Additional Technical Details:

  

Description:

I. Overview:

The Center for Disease Control reports that over 40 % of children (ages 5 to 17) and most adults have dental caries (tooth decay). The annual US cost for treating tooth decay amounts to billions of dollars. Polymeric dental materials are increasingly used to treat tooth decay. Yet, most restorations must eventually be replaced due to secondary caries (recurrent decay at the tooth-composite interface), with posterior restorations have a limited clinical longevity (5-7 years). While research efforts have focused on developing new materials, current test methods do not predict the clinical outcomes and are not consistent among labs. To address these issues, we are developing and disseminating methods for evaluating critical aspects of dental materials with respect to the tooth-composite interface. Specifically, we focus on the underpinning measurement science to assist the development of robust (reproducible and accurate) testing methods and reference materials to support the dental industry.

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II. History and Partners:

NIST's involvement in dental materials dates back to 1919.  In 1928, the Paffenbarger Research Center (PRC) of the American Dental Association Foundation (ADAF) began working at NIST (then National Bureau of Standards, NBS) with the purpose of developing science-based standards for dentistry. The unique collaboration between NIST and PRC has afforded the dental profession the ability to participate in the development of science-based standards and new technologies that are relevant to the needs of the profession. Scientists at NIST and PRC have changed the profession of dentistry in fundamental ways through scientific advancements and discovery. Today, scientists continue the tradition of transferring scientific advances from the bench to the clinic.   

Much of this collaborative research has been funded by the National Institute of Dental and Craniofacial Research (NIDCR/NIH). NIST and NIDCR have had an interagency agreement since the 1960s to advance materials and measurement methods for improving oral health. Together, scientists at NIST and PRC, with the support of NIDCR, have developed numerous new technologies and established many scientifically based standards for dental materials, instruments, and therapies.

Today the NIST continues its close working relationships with NIDCR and the ADA Foundation, while also establishing closer ties with the Food and Drug Administration (FDA). The NIST dental research team is also actively involved in the consensus standards bodies, while working closely with dental industry and academic research laboratories. 

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Additional Technical Details:

I. Method Development

1. Shrinkage and Leakage Characterization

We have established an imaging-based method (e.g., X-ray microcomputed tomography, µCT) to quantify the spatial distribution of polymerization shrinkage and the resultant gaps that form between the material and tooth structure (an indicator of leakage). These methods demonstrated that for a given material, neither sample volume nor geometry (degree of constraint) affects the overall polymerization shrinkage, but both aspects do affect the magnitude of leakage.  

Further, leakage was quantified for the first time in terms of absolute area and percentage of the total composite-tooth interfacial area. An automated imaging process was developed to convert 3D leakage predictions into 2D leakage maps. Leakage areas predicted by image analysis of μCT images agreed well with those observed by dye penetration (Figure below). Leakage occurs in spatially non-uniform ways. 

 

Images of leakage visualization and prediction

Leakage visualization and prediction

 

µCT imaging was also applied to evaluate shrinkage and leakage for composites placed in extracted human teeth. Figure below shows a 2D μCT image slice that clearly distinguishes the composites (white) from dentin (dark gray) and enamel (light gray), and a 3D reconstruction of two restoratives in the same tooth. Polymerization shrinkage and leakage profiles in teeth were comparable to those in model cavities. 3D Leakage is presented in gray (Figure below, left). The corresponding shrinkage (ΔV) is measured as a function of sample depth (Figure below, right).

2D and 3D images of tooth with compositeAn image of 3D leakage prediction

2D μCT image, 3D reconstruction and 3D Leakage prediction

Sun, J. et. al. Dental Materials 2009, 25, 314-320.
Zeiger, D. N. et. al. Dental Materials 2009, 25, 1213-1220.
Sun, J.; et. al. Biomaterials 2009, 4457-4462.
Sun, J. R.; et. al. Dental Materials 2008, 24, 228-234.
 

2. Characterization of Stress Development

Continuum mechanics theory and finite element method were used to analyze the sensitivity and accuracy of an existing instrument for measuring polymerization stress. These analyses revealed that modifications in instrument configuration (i.e., aspect ratio and beam stiffness) are needed to optimize instrument sensitivity (Figure below). The reconstructed instrument showed the expected data trends as calculated by the beam equation. Current work focuses on extending the application of the tensometer to measure other material parameters.  

Images of tensometer instrumentation and results

Chiang M.Y.M.; et. al. Dent Mater (2011), doi:10.1016/j.dental.2011.05.006

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3. Bacterial challenge and their effects on tooth and polymeric materials bacterial challenge

The interactions of dental restorations with oral bacteria can also affect their clinical performance. Our work on biofilm-material interactions includes efforts to develop methods for simulating the oral environment through bacterial challenge and to quantify the effects of biofilms and dental polymers on one another.  

Our foundational work in this area focused on sample preparation and control materials. Results revealed that even seemingly insignificant aspects of the protocol for fabricating polymer films can alter surface hydrophobicity, surface chemistry, and therefore the resultant initial bacterial colonization, as seen below for Streptococcus mutans, a commonly studied oral bacterium that contributes significantly to tooth decay. Bioactive materials that reduce biofilm growth were developed to use as positive controls during our experiments and methods development. These materials include concepts such as dental composites with silver nanoparticles and dimethacrylate dental monomers with quaternary ammonium functionalities.   

 

Images of S. mutans colonization on polymer films

S. mutans colonization on polymer films

Ongoing studies include the development of methods to quantify biofilms, such as chemical and mechanical properties, biochemical processes, and pathogenicity via tooth demineralization/erosion. For instance, measures of biochemical processes in biofilms were used to quantify the effects of material properties of dental polymers, such as degree of conversion, on oral biofilms. Further studies will explore the effects of the biofilm on the material integrity and performance.

 

Cheng, Y.J. et. al. JBMR-B, 2011.
Zeiger, D. N. et. al.  Langmuir2010.

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4. Modeling of material factors involved in development of secondary caries

Our technical objective is to develop a multidisciplinary computational model to investigate effects of synergistic interactions between environmental factors and material degradation on the failure of restorations that would lead to secondary caries. Specific aims of the proposed physicochemical model include: 1) To develop a computational framework that couples the oral environment and the material degradation in the failure of polymeric-based restorations. 2) To implement cyclic mechanical/chemical attacks pertinent to oral environmental conditions in a computational model, such that dominant diffusion paths of migrating species leading to the second caries can be identified. 

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5. Combinatorial Approach

In support of the material characterization and cell-material interaction work, we have developed various two-dimensional (2D) combinatorial platforms for rapidly screening the properties of dental polymers/composites.

Illustrations of combinatorial methods to characterize dental polymers

Simon, Jr. C.G. and Lin-Gibson, Advanced Materials 2011.
Lin-Gibson, S et. al.  Acta Biomaterialia 2009.
Lin, N.  et. al. CCHTS 2009.
Lin, N. J. and Lin-Gibson, S. Biomaterials 2009.
Lin, N. J. et. al., Dental Materials 2007.

II. Development of Reference Materials  

 

1. Biomimetic approaches to generate tooth-like materials  

 
Test methods that involve the use of tooth generally have large standard deviations, in part due to large variability of the tooth specimen. A consistent tooth-mimic material of known properties would greatly improve the reproducibility of intra- and interlaboratory comparisons for a given test. An ongoing research effort aims to determine the chemical and physical mechanisms of CaP biomineralization, thereby affording acellular fabrication of bone/tooth like materials. Current emphasis is on determining the role of polymeric chaperone molecules in mediating intrafibrillar mineralization within the collagen fibril. Further efforts will focus on the assembly of higher order structures.  
 

Images of CaP mineralization on collagen

2. Development of reference materials

An ongoing need in materials testing is for reference materials with a known response to serve as controls. Using such reference materials would allow operators to identify any sources of discrepancy in measured data and better compare their data. NIST continues to evaluate the need for reference materials that can be used for these purposes.    
 

Standards Activities  

 
NIST actively participates in the Standards Committee for Dental Products (SCDP), an ANSI accredited organization that develops and approves standards for dental products within the U.S. NIST also works with ISO in method development efforts.
 

Logos of the standards organizations ADA and ISO

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Major Accomplishments:

  • The Center for Disease Control reports that over 40 % of children (ages 5 to 17) and most adults have dental caries (tooth decay). The annual US cost for treating tooth decay amounts to billions of dollars.
  • Standard materials and clinically relevant measurement methods developed at NIST will enable and accelerate the improvement of polymer-based dental materials, the fastest-growing segment of the dental materials market.
  • With a program in dental materials dating back to 1919, collaborations with the American Dental Association (ADA) since 1928, and collaborations with NIH since 1964, NIST has a long history of impact in addressing measurement needs for dental materials.
  • Customers include the National Institute of Dental and Craniofacial Research (NIDCR), the ADA Foundation, the Food and Drug Administration (FDA), the dental materials industry, and academic research laboratories. 
Photo of tooth slice labeled 
Photo of schematic of interphase region in tooth

Start Date:

April 1, 2007

End Date:

ongoing

Lead Organizational Unit:

mml

Customers/Contributors/Collaborators:

Associated Products:

Contact

Sheng Lin-Gibson
(Polymers Division)
301-975-6765
slgibson@nist.gov