Journal of Dental Research, Vol 71, 1613-1618, Copyright © 1992 by International & American Associations for Dental Research Online Journals
A semi-empirical model for prediction of how post-retained crowns will fail under compressive loading
C. A. Mitchell, J. F. Orr and J. G. Kennedy
Department of Restorative Dentistry, Queen's University of Belfast, School of Dentistry, Royal Victoria Hospital, United Kingdom.
The aim of this in vitro study was to determine the relative contributions
to retention of the different components of the post-retained crown and the
mode of failure when the crown was loaded at 50 degrees to the vertical
(i.e., in an apical direction). Failure was initiated in all the specimens
as a rotation of the crown about a point on the labial surface of the root
at the crown-root interface with pull-out of the post. A semi-empirical
model was developed to explain this mode of failure. From experimental
results, it was noted that the retention provided by the post was the most
significant factor in the equation. However, large variation in the maximum
forces recorded from pull-out tests on 40 posts suggested that the
glass-ionomer cement was acting in the manner of a brittle composite
material whose failure may be attributed to defects in the structure
causing stress concentrations. The probability of survival at each ultimate
strength recorded experimentally was plotted for comparison with the curve
generated by the derived Weibull function. Close agreement was noted. Thus,
for any given force applied compressively at 50 degrees to the vertical to
a post-retained crown of known dimensions, it is possible, by use of the
described equation, to calculate the force, P, applied to the post. This
value of P may then be inserted into the Weibull equation to give the
probability of the crown failing under that applied force.
