ELSEVIER PII: SO300-5712(96)00047-4 Journal of Dentistry, Vol. 25, Nos 34, pp. 331-337, 1997 Copyright 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0300-5712/97 $17.001-0.00 ln vitro measurement of cuspal strain and displacement in composite restored teeth N. Meredith* and D. J. Setchellt *Department of Oral and Dental Science, Bristol Dental Hospital, Lower Maudlin Street, Bristol BSI 2LY, UK tDepaltment of Conservative Dentistry, Eastman Dental Institute, 256 Grays Inn Road, London WCIX 8LD, UK ABSTRACT 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 gaugesbonded to the buccal and lingual cusps of each specimen and cuspal displacement was recorded with a linear variable differential trans- former. 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 pm/m were recorded and a maximum cusp displacement of 14 pm 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<O.OOl). Conclusions: The irt vitro restor-ation 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 stressrelief procedure resulted in a decrease in cuspal strain and displacement of approximately 3040%. 0 1997 Elsevier Science Ltd. All rights reserved KEY WORDS: Composite, Strain, Bonded, Flexure, Dental J. Dent 1997; 25: 331-337 (Received 21 August 1995; accepted 21 May 1996) INTRODUCTION Recent developments in resin chemistry and filler materials have led to the production of dental compos- ites with improved physical and mechanical properties which may be considered suitable for the restoration of posterior teeth’. It has been reported, however, that the placement of adhesively bonded posterior composite restorations may result in post-operative sensitivity in up to 50% of cases2. Eick and Welch2 have suggested a number of possible causes including; stresses induced by polymerization contraction, marginal microleakage, occlusal loading and inadvertent acid etching of the Correspondence should be addressed to: Dr N. Meredith, Depart- ment of Oral and Dental Science, Bristol Dental Hospital, Lower Maudlin Street, Bristol BSI 2LY, UK. Tel.: +44 (0)1179 284335. Fax.: +44 (0)1179 253724.. dentine. Resin based dental composite materials under- go polymerization shrinkage on curing, and Goldman3 has shown that this may be as high as 3% by volume. The adverse effects of polymerization shrinkage have recently been reviewed by SSderholm4. It has been shown that the use of dentine bonding agents in com- bination with enamel acid etching may result in the formation of a strong bond between tooth and restora- tie?. Mechanical stresses produced by shrinkage of the composite restorative material which accompanies polymerization may be transmitted via such a bond to the surrounding tooth structure. Causton et aL6 reported that the placement of bonded composite restorations in extracted teeth resulted in a decrease in the distance between cusps measured using a dial gauge. It was concluded that this was as a result of the polymerization shrinkage that had occurred. Cuspal