A28    Discuss the clinical significance of properties such as modulus of elasticity, tensile strength, compressive strength and flexural strength.

Modulus of elasticity

High modulus means that the material is rigid and does not deform significantly under occlusal load. Low deformation decreases the cusp deflections and reduces the risk of cusp fractures. At the same time, an impression material with high modulus is difficult to remove and increases the risk of fracturing the gypsum cast during impression removal.

Tensile strength

The tensile strength is measured as the maximal load a material can carry when it is stretched. As a general rule, ceramic materials have much lower tensile strength than compressive strength.

Compressive strength

Compressive failures are not common. A restoration that fail under occlusal load rather fails due to flexure and the introduction of tensile stresses.

Flexural strength

Dental constructions flex during usage. During flexure, tensile stresses are induced on one side, and compressive stresses on the other side. These stresses are oriented parallel to the bend surface. Since the tensile strength is often lower than the compressive strength, the failure often starts on the surface in tension.

The thicker the base is, the more it will deform in microns (conclusion drawn from Hooke's Law). Increased deformation means increased flexing of the filling material. The more the filling material flexes, the higher risk for fracture of material A. Material B will flex easier, but it will not break in contrast to Material A. The prognosis of Material A increases as the base thickness decreases and the modulus of elasticity of the base increases. The latter become particularly important if the base thickness increases.