The dimensional accuracy and stability of composite inlays.

The dimensional accuracy and stability of two composite inlay materials was studied by assessing polymerisation shrinkage, thermal expansion and water sorption. Impression taking, die accuracy and stability and wear of the material were also considered. The mean coefficients of thermal expansion (20 to 65 degrees C) determined using a differential dilatometer were: Clearfil 21 X 10-6 and Isosit 31 X 10-6. A metal die similar to that used by Taylor in 1931 to assess casting shrinkage of gold inlays was used to assess polymerisation shrinkage. Isosit had a mean protrusion value of 0.321 mm providing a value for linear polymerisation shrinkage of 0.9%. In contrast Clearfil specimens had a mean protrusion of 0.108 mm providing a value for linear polymerisation shrinkage of 0.3%. The expansion due to water sorption at seven days was very small and did not offset the polymerisation shrinkage. However, these must be interpreted in light of the results of long term immersion studies. The total dimensional change of an Isosit specimen is -1.06%. For an inlay of 5 mm in diameter the linear dimensional change would be 54 micrometers, providing a cement space of 27 micrometers. The total dimensional change of a Clearfil specimen is -0.53%. For an inlay of 5 mm in diameter the linear dimensional change would be 27 micrometers, providing a cement space of 13.5 micrometers.