Effects of cement bases on the stresses and deflections in composite restorations.

A model was used to investigate stresses and deflections in cement bases and in a composite restorative material induced in a posterior Class I restoration. Deflections in the composite were highest when it was supported by a base with a low modulus; however, the maximum tensile stress (12 MN/meter2) occurring in the composite restoration at the interface with the base was well below the maximum tensile strength of most composite materials (30 to 40 MN/ meter2). Tensile and shear stresses occurring in the cement bases were of sufficient magnitude to exceed the strength of the cement when a 1.5-mm layer of zinc phosphate cement (base consistency) was supported by a 0.5-mm layer of ZOE lining material. Lowest values of stress were observed when a 2-mm layer of a zinc polyacrylate base was used to support the composite restoration. An ideal situation would be to have a cement base with a modulus of elasticity equal to that of the composite material. Finally, the maximum stress and deflection values obtained here can be easily exceeded in a clinical situation when wider variations can be easily introduced, making the choice of a cement with the proper modulus even more critical.