Selection of Restorative Materials: Understanding Strength, Stress

Selection Of Restorative Materials

 

• A brittle material is highly susceptible to surface flaws and internal defects when tensile stresses are present.
• Because it cannot plastically deform to reduce stress concentrations and its tensile strength is far lower than its compressive strengths.
• The compressive strength of brittle materials is often reported even though most brittle materials rarely fall under compressive stresses.
• However, when tensile strength values are not available.
• Flexural strength values should be used since they reflect a tensile mode of fracture.
• When tensile strength, flexural strength, or fracture toughness data are not available the compressive strength can be useful for comparisons of the fracture resistance of a similar family of brittle materials such as groups of amalgams, composites, ceramics, or cement.
• The physical properties mentioned above have been obtained using specimen shapes and sizes that are quite different from those of tooth restorations.

So, material products must be selected intuitively on the basis of property comparisons.

• Furthermore, these expected stresses are multiplied by a “safety factor” to ensure that the structure will be able to withstand a certain amount of excess stress.
• However, the tensile strength values reported for restorative materials represent the mean stress values below which 50% of the test specimens have fractured and above which only 50% had survived.
• Because this is an unacceptable failure rate for restorative dentistry, the range of measured values should be known.
• From an ultraconservative viewpoint the lowest 5 or 10% of a strength distribution, not mean values, should be used to design a prosthesis intended to resist fracture at a high level of confidence.
• We can also use the 95% survival stress as determined from Weibull analysis of data
• It is unfortunate that the magnitudes of mastication forces are not known for individual patients to the extent that the dentist can predict the stresses that will be induced in dental restorations.
• However, patients who exhibit evidence of extreme bruxism represent a higher risk population for the fracture of brittle restorative materials.
• The knowledge of the relationships between the properties of restorative materials known to exhibit excellent long-term survival performance is reinforced by clinical experience.
• The best test of a successful restorative material is the test of time under actual clinical conditions.