Journal of Dental Research, Vol 75, 1700-1705, Copyright © 1996 by International & American Associations for Dental Research Online Journals
Maximum likelihood estimates for the lifetime of bonded dental prostheses
W. G. Rijk, M. Wood and V. P. Thompson
Department of Restorative Dentistry M/C 555, College of Dentistry, University of Illinois at Chicago 60612-7212, USA.
Clinical studies measuring the lifetime of dental prostheses produce
censored data when not all specimens have failed during the course of the
study. Such clinical data can be analyzed by the Weibull probability
distribution function. Algorithms are presented that provide the maximum
likelihood estimates of the distribution's parameters. These parameters are
the characteristic lifetime (time to failure for 63% of the specimens of
the total sample) and the Weibull or shape parameter. Two iterative methods
for solving the maximum likelihood equations are given. These mathematical
methods have been applied to the results from a retrospective clinical
investigation into the lifetime assessment of resin-bonded prostheses. This
study evaluated 164 resin-bonded prostheses (for 146 patients) placed
between January, 1980, and May, 1985. To date (April, 1995), 47 prostheses
(29%) have failed with a median time in service of 74 months (6.2 yr). For
the surviving prostheses, the median time in service is 123 months (10.3
yr) and still increasing. The maximum likelihood estimate of the
characteristic lifetime for these restorations is 255 months (21.3 yr).
Differences in the characteristic lifetime were observed between prostheses
placed anteriorly, 338 months (28 yr), and posteriorly, 207 months (17 yr).
Since there are no rigorous confidence intervals for deeply censored
samples, only provisional confidence bounds could be determined, which
substantiated the observed differences. The Weibull modulus value of 1.5
indicates that the probability of failure for resin-bonded prostheses
begins to decrease after 10 years in service.
