Development of a clinically validated bulk failure test for ceramic crowns.
BACKGROUND
Traditional testing of ceramic crowns creates a stress state and damage modes that differ greatly from those seen clinically. There is a need to develop and communicate an in vitro testing protocol that is clinically valid.


OBJECTIVE
The purpose of this study was to develop an in vitro failure test for ceramic single-unit prostheses that duplicates the failure mechanism and stress state observed in clinically failed prostheses. This article first compares characteristics of traditional load-to-failure tests of ceramic crowns with the growing body of evidence regarding failure origins and stress states at failure from the examination of clinically failed crowns, finite element analysis (FEA), and data from clinical studies. Based on this analysis, an experimental technique was systematically developed and test materials were identified to recreate key aspects of clinical failure in vitro.


METHODS
One potential dentin analog material (an epoxy filled with woven glass fibers; NEMA grade G10) was evaluated for elastic modulus in blunt contact and for bond strength to resin cement as compared to hydrated dentin. Two bases with different elastic moduli (nickel chrome and resin-based composite) were tested for influence on failure loads. The influence of water during storage and loading (both monotonic and cyclic) was examined. Loading piston materials (G10, aluminum, stainless steel) and piston designs were varied to eliminate Hertzian cracking and to improve performance. Testing was extended from a monolayer ceramic (leucite-filled glass) to a bilayer ceramic system (glass-infiltrated alumina). The influence of cyclic rate on mean failure loads was examined (2 Hz, 10 Hz, 20 Hz) with the extremes compared statistically (t test; α=.05).


RESULTS
Failure loads were highly influenced by base elastic modulus (t test; P<.001). Cyclic loading while in water significantly decreased mean failure loads (1-way ANOVA; P=.003) versus wet storage/dry cycling (350 N vs. 1270 N). G10 was not significantly different from hydrated dentin in terms of blunt contact elastic behavior or resin cement bond strength. Testing was successful with the bilayered ceramic, and the cycling rate altered mean failure loads only slightly (approximately 5%).


CONCLUSIONS
Test methods and materials were developed to validly simulate many aspects of clinical failure.
Kelly
J Robert
JR
Department of Reconstructive Dentistry and Center for Biomaterials, University of Connecticut Health Center, Farmington, CT 06030-1615, USA. Kelly@nso1.uchc.edu
Rungruanganunt
Patchnee
P
Hunter
Ben
B
Vailati
Francesca
F
D000704:Analysis of Variance; D003442:Crowns; D001840:Dental Bonding / Q000379:methods; D003776:Dental Porcelain; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis / Q000379:methods*; D055119:Elastic Modulus; D006801:Humans; D008422:Materials Testing / Q000379:methods*; D019279:Resin Cements / Q000737:chemistry; D014093:Tooth, Artificial
D019279:Resin Cements; D003776:Dental Porcelain