Journal of Dental Research, Vol 80, 1574-1579, Copyright © 2001 by International & American Associations for Dental Research Online Journals
Evidence of a critical leucite particle size for microcracking in dental porcelains
J. R. Mackert Jr, S. W. Twiggs, C. M. Russell and A. L. Williams
Department of Oral Rehabilitation, School of Dentistry, Medical College of Georgia, Augusta 30912-1260, USA. rmackert@mail.mcg.edu
The leucite particles in dental porcelains are often partially encircled by
microcracks that are the result of the thermal expansion mismatch between
leucite and the surrounding glass matrix. Although the magnitude of the
stress at the particle-matrix interface is independent of the particle size
(Selsing, 1961), Davidge and Green (1968) showed experimentally that there
is a critical particle size below which microcracking is absent. The
critical particle size is explained by a Griffith-type energy balance
criterion: Below the critical size, the stress magnitude may be sufficient
to cause cracking, but there is insufficient strain energy for the creation
of the new surfaces of the microcrack. The purpose of the present study was
to determine whether the mean leucite particle size of a dental porcelain
influences the degree of microcracking in the porcelain. Microcrack
density, leucite particle surface area per unit volume, and leucite mean
volume-surface diameter, D3,2, were determined by quantitative stereology
on 10 specimens each of 6 dental porcelains and Component No. 1 of the
Weinstein et al. patent (US Patent 3,052,982, 1962). The fraction of
leucite particles with microcracks around them, f(mc), was estimated for
each porcelain from the microcrack density and the leucite surface area.
Using the equations of Selsing (1961) and Davidge and Green (1968), we
calculated the critical particle diameter, Dc, for leucite to be 4 microm.
The porcelains were partitioned according to whether their mean leucite
particle diameters, D3,2, fell above or below Dc, and their values of f(mc)
were analyzed by a permutation test with random re-sampling. The porcelains
with mean leucite particle diameters below Dc had a significantly lower
fraction of cracked particles compared with the porcelains with mean
leucite particle diameters above Dc (p < 0.05). This study provides
evidence that microcracking in dental porcelain can be minimized by a
reduction of the mean leucite particle diameter to less than 4 microm.
