Dental Materials Properties
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Direct answer
Compressive strength, tensile strength, elastic modulus, hardness, creep, solubility, water sorption and thermal behaviour are the properties that decide which material survives which cavity. Amalgam is strong in compression (with specification minimums of 80 MPa at one hour) but weak in tension and thermally conductive, so deep cavities need an insulating base; composite matches enamel's appearance but shrinks on curing and expands thermally about three times more than tooth structure; glass ionomer bonds chemically and releases fluoride yet fractures under occlusal load. Choosing a material is matching property profiles to the mechanical, thermal and biological environment of the cavity.
What you must remember
- Strength types: compressive, tensile (measured indirectly by the diametral test in brittle materials), shear and flexural; amalgam and glass ionomer are far stronger in compression than tension.
- Elastic modulus is stiffness: dentine roughly 15–20 GPa, enamel higher; fibre-reinforced posts are chosen because their modulus approximates dentine, while steel is about ten times stiffer and concentrates stress.
- Hardness is measured by Knoop, Vickers, Brinell or Rockwell tests; enamel Knoop hardness of around 300–350 KHN anchors the scale, and a polished restoration surface should end up smooth enough not to abrade the opponent.
- Creep: slow deformation under constant load; ADA limits amalgam creep to 3% and high-copper alloys achieve well under 1%, which is why their margins hold.
- Dimensional stability: amalgam sets with minimal net change; zinc-containing amalgam contaminated by saliva undergoes delayed expansion (up to about 4%) from hydrogen gas; composite polymerises with 2–3% volumetric shrinkage.
- Thermal properties: amalgam conducts heat rapidly to the pulp (insulate with a base in deep cavities); composite and glass ionomer are insulators; coefficient of thermal expansion of composite is roughly 3 times that of tooth, driving marginal microleakage with hot-and-cold cycling.
- Solubility and sorption: glass ionomer and zinc phosphate dissolve slowly in oral fluid; composite absorbs water and stains; a luting cement needs low film thickness and low solubility.
- Electrochemical behaviour: dissimilar metals in contact (amalgam touching a gold crown) create galvanic currents — the "shock" patients report; tarnish is surface discolouration, corrosion is actual degradation.
- Biocompatibility covers pulpal irritation, allergy (nickel, resin monomers) and mercury hygiene handling for amalgam.
Reasoning through a deep Class II with numbers
A mesio-occlusal cavity in a lower first molar, caries just short of the pulp, patient wants the cheapest durable option: start the material decision from the property profile. Amalgam gives high compressive strength and low creep with a high-copper alloy, and its 80 MPa one-hour specification strength allows same-day carving and loading; but its thermal conductivity demands insulation over deep walls, and its poor tensile strength argues for capping the weakened distal cusp. Composite avoids the insulation problem and bonds the walls, but two numbers stand against it: 2–3% shrinkage acting on a high C-factor box, and thermal expansion around three times the tooth's — both challenge the gingival margin where moisture control is hardest. Glass ionomer is the material of the gingival seat — chemical adhesion and fluoride release — but its low fracture toughness excludes it from bearing the whole occlusal load. The reasoning converges on the classic laminate answer: glass ionomer base or gingival liner, bonded composite or amalgam body, cuspal coverage if the distal cusp is weak — property-driven choices, not habit.
How examiners probe properties
The favourite discrimination is strength versus stiffness: candidates write "composite is stronger than glass ionomer" and lose the mark because the intended answer is about modulus and brittleness, not a single strength number. A second stock question asks why a patient reports an electric shock after a new amalgam is placed beside an old gold inlay — galvanism from dissimilar metals in contact through saliva — with the practical management of polishing or separating the restorations. The third trap is delayed expansion: zinc plus moisture produces hydrogen, and the restoration swells and becomes sensitive; students blame a high point and grind the surface, diagnosing the wrong property. Quote the specification number (80 MPa at one hour) and the 3% creep limit, and the examiner knows the numbers have actually been studied.
Frequently asked questions
Which strength is most relevant to an amalgam restoration?
Compressive strength, since occlusal load is mainly compressive; tensile and transverse strengths are its weak points, explaining isthmus and marginal ridge fractures.
Why do fibre posts cause fewer root fractures than steel posts?
Their elastic modulus is close to dentine's, so stress distributes along the root instead of concentrating at the post tip as it does with far stiffer steel.
What is the clinical meaning of creep in amalgam?
Slow flow under sustained load causes marginal ditching; high-copper alloys with creep below 1% resist this, within the ADA 3% ceiling.
What causes delayed expansion of amalgam?
Moisture contamination of a zinc-containing mix produces hydrogen during setting; the restoration expands over days to weeks, causing pain and proximal pressure.
What is the difference between tarnish and corrosion?
Tarnish is surface discolouration from deposits or sulphide films; corrosion is electrochemical degradation of the material itself, which weakens margins and releases products.