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
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:
Implications for lab testing are as follows:
- 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
- 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.
"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."
- 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.