Biomechanics & Properties
Define force, stress, and strain in the context of restorative materials.
Force is any push or pull on matter; stress is the internal reaction within a material to applied force that can cause distortion; strain is the change produced within the material as a result of stress.
List three materials classified as direct restorative materials.
Amalgam, composite resins, glass ionomers (also temporary restorative materials, tooth-whitening products).
What metals commonly compose an amalgam alloy powder?
Silver (strength), tin (workability and strength), copper (strength and corrosion resistance), and often zinc (to suppress oxidation).
Define microleakage and name one primary cause.
Microleakage is the passage of bacteria, fluids, molecules, or ions between a restoration and tooth structure. Primary cause: inadequate adhesion or improper sealing at restoration margins.
Which of the following terms is used for the mixing of mercury and alloy powder to form a mass of amalgam needed to restore the tooth?
Amalgamation
Trituration
Name and briefly describe three types of mechanical stress dental materials must resist.
Tensile stress (pulling/stretching), compressive stress (pushing together), and shear stress (sliding forces causing breakdown along planes).
What are the primary clinical indications for choosing dental amalgam?
Primary/permanent teeth in stress-bearing posterior areas, small-to-moderate cavities in posterior teeth, foundation for cast restorations, when moisture control or cost is an issue, or poor patient hygiene.
Describe the purpose of high-copper alloys in dental amalgams.
High-copper alloys reduce the formation of the weaker, corrosive gamma-2 phase, increasing strength, reducing corrosion, and improving clinical performance.
What are the main clinical contraindications for choosing composite resin in posterior restorations?
Poor patient oral hygiene, high occlusal stress areas where esthetics are not needed, and when cost is a limiting factor (composites may be more technique-sensitive and time-consuming).
Why is the small end of the amalgam carrier filled and transferred first?
To extend to the smaller portions of the preparation first.
Explain why hardness and corrosion resistance are clinically important properties for metallic restorations.
Hardness resists indentation, scratching, and abrasion preserving occlusion and anatomy; corrosion resistance prevents surface degradation and discoloration, reduces ion release, and extends longevity of restorations.
Outline the basic steps from capsule mixing to final occlusion check when placing an amalgam restoration.
Triturate sealed capsule in amalgamator, place mix in amalgam well, load carrier, carry and place increments into preparation, condense each increment to pack and remove excess mercury, carve anatomy with hand instruments, smooth with burnisher, have patient bite on articulating paper and adjust for final occlusion.
What is Galloy and what unique property causes handling/clinical concerns?
Galloy is a nonmercury alloy composed of gallium, indium, and tin. Gallium liquefies at about 86°F, making it liquid near body temperature and sensitive to moisture, causing corrosion and expansion concerns.
Compare intermediate restorative material (IRM) vs provisional restorative materials in typical use cases.
IRM (reinforced zinc-oxide eugenol) is for short-term restorations, emergency restorations, primary teeth and has sedative pulp effect. Provisional restorative materials (acrylic resins) cover larger tooth areas for longer periods (e.g., temporaries for crown preparations), fabricated using impression/tray and cemented with temporary cement.
The most common noble metals used for crowns and bridges are
gold, palladium, and platinum
Describe how thermal expansion mismatch between a restorative and tooth structure can lead to clinical failure; include mitigation strategies.
If the coefficient of thermal expansion of the restoration differs substantially from dentin/enamel, cyclic heating/cooling causes differential expansion/contraction leading to marginal gaps, microleakage, and restoration failure. Mitigation: choose materials with closer thermal coefficients, use adhesives/bonding agents to buffer stress, design restorations to reduce thin margins.
Describe the composition of composite resins and the role of the coupling agent.
Composite resin composition: organic resin matrix (e.g., dimethacrylate/BIS-GMA monomers), inorganic fillers (quartz, glass, silica), coupling agent (organosilane), pigments, and additives (initiator, accelerator, stabilizers). Coupling agent chemically bonds filler particles to the resin matrix: silane bonds to filler; organic end bonds to resin, improving mechanical strength and wear resistance.
Explain how filler particle size and distribution influence composite mechanical properties and polish ability.
Larger filler particles (macrofilled) increase strength and wear resistance but yield rougher, duller surfaces; smaller particles (microfilled) allow higher polishability and esthetics but lower strength. Hybrids combine particle-size ranges to balance polishability and strength.
Outline the key steps and rationale for in-office power (light-accelerated) tooth whitening.
Isolate soft tissues with resin-based barrier, apply high-concentration hydrogen peroxide gel (25–38%), expose to light source for short intervals (6–15 minutes) to accelerate peroxide breakdown and penetration. Rationale: faster oxidation of chromogens; careful isolation prevents soft-tissue irritation; monitor sensitivity.
The most common type of ceramic used in dentistry is
porcelain.
Compare and contrast ductility and malleability and give an example of how each property affects clinical handling or restoration longevity.
Ductility is the ability to be drawn or stretched without fracturing (affects tensile deformation under load); malleability is the ability to be hammered or compressed into thin sheets (affects adaptation and contouring of cast or restorative metals). Example: A ductile alloy may endure tensile strain without cracking at connector areas of a bridge; a malleable matrix alloy facilitates seating and burnishing of a crown margin.
Explain polymerization (curing) mechanisms for composites and list factors that influence light-cured composite final conversion.
Polymerization is conversion from pliable monomer to hardened polymer network via auto-curing (chemical reaction) or light-curing (photo-initiated). Factors: manufacturer's recommended exposure time and intensity, incremental thickness, shade (darker shades require more energy), filler load, and distance/angulation of curing light.
Discuss the chemical basis of fluoride release from glass ionomers and its clinical implications.
Glass ionomers are formed through an acid–base reaction between fluoroaluminosilicate glass particles and polyacrylic acid (ion-crosslinked polymers). Fluoride ions are released from the glass matrix over time, providing caries-inhibiting effects, useful especially in primary teeth, nonstress areas, sealants, and as core build-up/base materials.
For an extensively destroyed tooth to receive an indirect restoration (crown), compare indications for Type III vs Type IV gold-noble alloys and the reasons for choosing one over the other.
Type III (hard) alloy is acceptable for inlays, full crowns, and bridge abutments—used where strength and moderate workability are needed. Type IV (extra-hard) is designed for crowns, bridges, and removable partial denture frameworks where maximum strength, rigidity, and resistance to deformation under load are required. Choose Type IV when higher connector strength and fatigue resistance are critical (e.g., long-span prostheses or clasping frameworks).
When working with light-cured composites, the darker the shade of the material, the _______ the curing time will be.
longer