In vitro measurement of cuspal strain and displacement in composite restored teeth.

OBJECTIVES: This study was carried out to measure changes in cuspal strain and displacement occurring during placement and polymerization of bonded composite restorations in extracted human teeth in vitro.
METHODS: Strains were measured using electrical resistance strain gauges bonded to the buccal and lingual cusps of each specimen and cuspal displacement was recorded with a linear variable differential transformer. Mesio-occluso-distal cavities of two types were prepared in lower molar teeth. Following enamel acid etching and application of a dentine adhesion promoter, specimens were restored incrementally with a light curing posterior composite material.
RESULTS: It was shown that the shrinkage of a composite material during polymerization generated stresses which resulted in tensile strains on the tooth surface. Strains of up to 882 microns/m were recorded and a maximum cusp displacement of 14 microns was also measured. These strains were reduced but not eliminated by preparation and restoration a mesio-distal slot running the full length and depth of the restoration. Statistical analysis revealed significantly higher strains and displacement produced on the buccal cusps of teeth that had a reduced cusp width (P < 0.001).
CONCLUSIONS: The in vitro restoration of posterior teeth with a bonded composite material generates polymerization stresses which can be recorded as tensile strains and displacements on the tooth surface. Strains measured during composite placement were greater when the remaining cusp width was less. A stress relief procedure resulted in a decrease in cuspal strain and displacement of approximately 30-40%.