Spring 2005 Table of Contents
     
Academic, Government, Corporate Alliance Produces Findings Destined to Extend the Life of Ceramic Crowns
 

Top: Fracture patterns in dental ceramics, showing competing failure modes (radial cracks, ring cracks, yield). Right: Typical all-ceramic crown failure at 10 years. Courtesy of Dr. Kenneth Malament.
While all-ceramic dental crowns are popular because of their aesthetics and biocompatibility, they have not performed as well as hoped, failing at rates of approximately three percent each year despite considerable efforts to improve the materials. However, a major collaborative effort is underway that promises to change the situation.

With support from NIDCR, universities, and the corporate sector, basic and clinical scientists are working together to extend the life of ceramic crowns. The scientists are affiliated with NYUCD, the National Institute of Standards and Technology, Oklahoma State University, Princeton University, the University of Maryland (College Park and Baltimore), and the University of Medicine and Dentistry of New Jersey (UMDNJ). Their corporate partners, who include Corning, Dentsply Ceramco, Ivoclar, Nobel Biocare, 3M/Espe, Refractron, and Vita, provide all materials at no cost. In addition, Marotta Dental Studios and Jurim Dental Laboratories fabricate specimens at below-market costs.

Total NIDCR funding for the first five years of the project was $3.7 million. Total funding for the current five years (2002-2007) is $5.9 million plus $100,000 to train a minority PhD student.

"Our team's focus," explains principal investigator Dr. Dianne Rekow, Professor of Basic Science and Craniofacial Biology and Director of Translational Research, is to characterize damage modes and failure mechanisms in clinically relevant, layered crown-adhesive-tooth systems and thereby provide guidelines for the design of next-generation dental crowns." The grant's co-principal investigator is Dr. Van P. Thompson, Professor and Chairman of the Department of Biomaterials and Biomimetics and Acting Chairman of the Department of Cariology and Operative Dentistry.

Clinical implications of their findings are outlined below:

  • Sandblasting severely damages ceramics. Laboratory procedures can reduce material strength by 20 to 30 percent. The effect is not immediately evident but manifests itself after about 100 cycles, particularly when sharp, tough particles are used for indentation.
  • The cement and tooth supporting structure (dentin, foundation restoration or endodontic post) play important roles in the clinical survival of all-ceramic, full coverage crowns. Stiffer supporting structures enhance survival.
  • Failure modes of crowns with alumina cores are different from those with zirconia cores. With alumina cores, radial fracture from the adhesive interface of the core is most likely, leading to bulk fracture of the crowns. With zirconia cores, quasiplastic yield of the zirconia at the veneer interfaces raises stress in the veneer, creating fractures in the veneer only manifest as chipped porcelain.
  • The team has discovered a failure mode previously undetected. In water, inner cone cracks develop from the surface beneath the indenter (opposing cusp tip), then trap water and drive the crack through the veneer, resulting in failure/fracture.
Implications for lab testing are as follows:
  • Hydroscopic expansion of testing substrates (emulating dentin) and resin-based adhesives creates sufficient stress to spontaneously (with NO load applied) fracture structures. Consequently, all specimens must be preconditioned for a least seven to 30 days (depending on the material). This mechanism may account for early failures in all-ceramic crowns on teeth resin buildups or with thick luting cement.
  • Geometry of the test specimen substantially influences crack initiation load and propagation rates and patterns. Flat samples behave differently from hemispheres of the same material.
"What is so exciting," says Dr. Rekow, "is that these findings will culminate in guidelines for improved product development and subsequent significant benefits for patients, dentists, and industry. And collaboration is the key to making it all possible."