Objectives: Functional occlusal loading can generate complicated stres
ses in the luting cement film between a restoration and its abutment.
Such stresses may lead to cement fractures, which can promote cement d
issolution and induce clinical problems. The purpose of this study is
to determine the function of the cement film under loading, and to cla
rify the relationship between the deformation of the restoration and t
he cement fractures. Methods: Eccentric loads were applied onto model
extension bridges which were cemented on brass dies with zinc phosphat
e cement. Strain measurements were recorded using of a strain gauge me
thod, ultimate strength by tensile tests, leakage by dye penetration t
ests and cement fractures by ultrasonic microscopic analyses. Addition
ally, the bridges which were just seated on their dies without cementa
tion were loaded, and strain measurements were recorded. Results: The
eases for linear strain increment with loading showed high tensile str
ength and minimal dye penetration. The cases with non-linear strain be
havior showed low tensile strength and considerable dye penetration. H
owever, no fracture was observed in the occlusal cement film in both c
ases. The cement fracture first occurred at the opposite marginal regi
on to the loading side. Conclusions: One of the significant functions
of the cement film is diminishing the deformation of restorations duri
ng loading. The deformation of the restoration affects initiation and
propagation of the cement fracture considerably. (C) 1998 Elsevier Sci
ence Ltd. All rights reserved.