The use of composites as core material in root-filled teeth. I. In vitro study

To study how repeated temperature variations, "thermal stress", will influence the cement retention between composite materials--formed as cores--and gold crowns, extracted permanent teeth were prepared to core form. Stylized gold crowns were waxed up on the dentine preparations and cast, four for each of three chosen preparations, which in their turn were used to form cores in gold and three different composite materials. The attempt was to achieve four as similar test-pieces as possible for each core shape. Two new cements and four zinc phosphate cements were used as retaining medium for 48 test series. Four similar test-pieces have each time been subject to thermo-cycling during 12 hours in a specially designed apparatus. The temperature intervals were 0 degree C, 25 degrees C and 50 degrees C. Each series has then been loaded into a tensiometer and stressed until the cement retention was overcome. The test results showed that microfill composite cores were associated with least retention values. For the other materials: gold and macrofill composites (two paste system and single paste system), no differences were to be seen. The retentive ability for one (a zinc phosphate cement, normal- and fast-setting) of five different cements differed remarkably: about double the load was required to loosen the core from its crown than when any of the other cements was used.