Dental Materials
Dental Materials tests the physical and mechanical properties of restorative and impression materials, their handling, and their clinical failure modes. Expect calculations of stress and strain, comparisons of elastic modulus and thermal expansion, and questions on setting reactions and polymerization shrinkage.
Core rules
- Stress equals force divided by area; strain is deformation per original length. Elastic modulus is the slope of the stress-strain curve in the elastic region.
- Amalgam is strong in compression but weak in tension; the gamma-2 phase (Sn-Hg) is the least desirable and is reduced by high-copper formulations.
- Composite resins polymerize by free-radical addition and shrink on setting; shrinkage can cause marginal leakage and cuspal flexure.
- Glass ionomer cements bond chemically to tooth structure and release fluoride, but are weaker and more moisture-sensitive than composites.
- Impression materials: alginate is hydrocolloid and dimensionally unstable if stored wet or delayed; elastomers are more stable.
- Thermal expansion mismatch between restorative material and tooth can contribute to microleakage.
Worked example
A force of 200 N is applied over an area of 2 cm squared. Convert area to 200 mm squared. Stress = 200 N divided by 200 mm squared = 1 N/mm squared = 1 MPa. If the same force acted on 1 cm squared, the stress would double to 2 MPa, showing that stress depends on area rather than force alone.
Traps
- Do not compare materials by strength alone; match the property to the direction of loading.
- Mixing ratio errors and moisture contamination are common causes of failed cements and impressions.
- Polymerization shrinkage is not the same as thermal expansion; both can affect the margin but by different mechanisms.