15: Dental Biomaterials

CHAPTER 15 Dental Biomaterials

CHARACTERISTICS OF DENTAL MATERIALS

Understanding of mechanical, physical, electrical, surface, biological properties of dental materials enables clinician to determine proper use and care of dental materials.

image • See CD-ROM for Chapter Terms and weblinks.

Mechanical Properties

Mechanical properties include reactions of materials to application of external forces, such as the magnitude of biting forces.

A. Forces:

1. Tensile force: tends to pull object apart or elongate object.
2. Compressive force: downward application on object, compressing (pushing or crushing) structure (i.e., biting by teeth).
3. Shear force: rotation, twisting, or sliding of one portion of material by another portion.
4. Bending combination of ALL types of force.

B. Stress: reaction with which object resists external force (Figure 15-1). Example: stretching of orthodontic band causes stress because wire is being pulled in tension; can resist this external force up to point of fracture.

1. Stress = Force kg per mom/Area inches/meters.
2. Thus the smaller the area over which a force is applied, the larger the value of the stress.
3. Stress-related properties:

a. Ultimate strength: highest stress reached before fracture.
b. Fracture strength: point at which a material breaks.
c. Tensile strength: breaking strength when tested in tension.
d. Compression strength: breaking strength when tested in compression.

C. Strain: deformation; change in length or dimension of a dental material because of applied stress.

1. Deformation or change in length (inches per meter)/Original length (inches per meter). Example: dental waxes exhibit strain much MORE quickly than amalgam because of differences in stiffness.
2. The MORE stiff an object, the greater its ability to resist dimensional change.
3. Strain-related properties:

a. Elastic deformation: when material recovers after load is released or after material breaks.
b. Plastic deformation: permanent strain or change in shape of object that results when stressed beyond elastic limit.
c. Elongation: percentage of change in length up to point of fracture.
d. Ductility: ability of a material to withstand significant plastic or permanent deformation under stress before fracturing (e.g., gold), opposite of brittleness.
e. Brittleness: ability of material to fracture before undergoing significant amount of plastic or permanent deformation when placed under stress (e.g., glass fiber), opposite of ductility.
f. Malleability: ability of material to be compressed without fracturing; SIMILAR to ductility but specifically refers to compressive force applications.

D. Elasticity: permits material to be deformed by applied load and then assume original shape when load is removed (just like Gumby and Pokey). Examples: orthodontic wires resist fracture upon manipulation by clinician; adjustment of clasps on partial dentures; ability of fixed partial denture (dental bridge) to withstand forces of mastication.

1. Modulus of elasticity: measure of stiffness of material and ability to resist bending or change in shape.
2. Stress/strain curve: graphically shows change in dimension for given application and identifies important points such as elastic limit, proportional limit, and permanent deformation; used in dental research (Figure 15-2).
3. Elastic limit (proportional limit): maximum stress that material can withstand without being permanently deformed.
4. Inelasticity: permanent deformation as result of applied force.

E. Hardness: resistance of a solid to penetration. Example: clinician must instruct denture patients NOT to use abrasive cleansing materials, such as a paste with grit having higher hardness than denture, to prevent excessive abrasion of denture because of its low abrasion resistance.

1. Hardness tests:

a. Knoop: diamond indenter; ONLY one used in dentistry to produce value (Knoop hardness number [KHN]).
b. Vickers: square-based diamond point.
c. Rockwell: diamond cone and hardened steel ball indenter.
d. Brinell: steel ball (oldest test).

2. The higher the hardness number, the harder the material. Example: porcelain has high resistance to abrasion with high hardness number; conversely, it has ability to abrade opposing materials with lower hardness numbers such as tooth enamel and amalgams (Table 15-1).

image

Figure 15-1 Types of stress.

image

Figure 15-2 Diagram of the stress/strain curve.

Table 15-1 Hardness of restorative materials

Dental material KHN
Acrylic 20
Dentin 70
Calculus on teeth 86
Enamel 350
Porcelain 400-500

KHN, Knoop hardness number.

CLINICAL STUDY

Age 59 YRS SCENARIO
Sex image Male image Female The patient is in the dental office because of a fractured tooth. Visual examination of tooth #18 reveals a fracture of the ML cusp. The remainder of the tooth holds a large MOD (mesio-occlusal-distal) amalgam restoration. After the dentist removes all of the old amalgam, some caries is discovered that will result in a near pulpal exposure. Patient also has amalgam restorations in teeth #2, #14, #19, and #31, and full gold crowns on teeth #3 and #30.
Chief Complaint “I bit down on a popcorn kernel and my tooth broke.”
Medical History
Pancreatitis last year
Past alcoholic, sober for 15 years

Current Medications None Social History
Animal groomer
Likes gardening

1. What mechanical properties were involved in the fracture of tooth #18?
2. The dentist decides to place a cavity liner before restoring this tooth. Explain what material would be appropriate and why.
3. How should this tooth be restored? Select an appropriate restorative material, and include indications and contraindications for different restorative materials.

1. Stress from biting on the popcorn kernel created both compressive and tensile forces on the tooth; smaller the area over which a force is applied, the larger the value of stress. In this instance, occlusal surface of #18, from contact with the kernel, magnified the stress. Second property involved was strain (reaction of tooth #18 to biting stress), which resulted in fracture of ML cusp.
2. Calcium hydroxide is appropriate material for lining the cavity. Advantages of material: (1) encourages recovery of the pulp by stimulation of secondary and reparative dentin; (2) protects pulp; (3) sufficiently strong. Mineral trioxide aggregate (MTA) may also be effective for lining the cavity.
3. Tooth #18 will probably require full-coverage restoration because of extensive loss of tooth structure. Because the tooth that opposes #18 has never been restored, full gold crown is indicated; gold would cause minimal abrasion of #15. Porcelain would be contraindicated in this circumstance because has high hardness number (KHN 400-500) compared with enamel (KHN 350) and would therefore result in the eventual abrasion of tooth #15.

Physical Properties

Physical properties depend primarily on the type of atoms and bonding that is present in a material. Allows clinician to understand how given material will withstand oral environment (i.e., temperature fluctuations, biting forces, moisture).

A. Thermal properties: reaction to temperature changes within oral cavity and subsequent expansion and contraction. Example: rapid change in oral temperature would occur when eating ice cream cone immediately before drinking cup of hot coffee and affect restorations by expansion.

1. Coefficient of thermal expansion: value or measurement of change in length per unit length for each degree of temperature change (change in volume in relationship to change in temperature).

a. The higher the coefficient of expansion, the greater the degree of contraction and expansion involved when material is exposed to temperature changes.
b. Because of differences in expansion and contraction rates of restorative materials and tooth structures, microleakage of oral fluids between the restoration and tooth can occur (Table 15-2).

B. Thermal conductivity: ability of dental material to transmit heat.

1. Enamel and dentin are poor thermal conductors, compared with gold alloys and amalgam.
2. Cements overlie and insulate inner pulp in areas of deep cavity preparations covered by metal restorations.

C. Electrical properties: generation of electrical currents through a variety of means.

1. Galvanism: small electrical currents that result from opposing of dissimilar metals across dental arches (Figure 15-3).

a. Have potential to create tooth hypersensitivity by irritating the pulp.
b. Roughness and pitting can also occur as result of galvanic action.
2. Corrosion: deterioration of metal by chemical or electrochemical reaction.

a. Can be result of adjacent restorations that are composed of dissimilar metals.
b. Also may result from chemical attack of metals by components in saliva and food; considered tarnish.

D. Color properties: give restorations and prosthetic appliances appearance of natural teeth and soft tissues (esthetics); important in selecting tooth-colored restorations and calibrating whitening procedures (digital machines available).

1. Hue: dominant color of object (i.e., red, yellow, blue).
2. Value: lightness or brightness of color; amount of gray present; MOST important color property.
3. Chroma: intensity of color.

E. Translucency: how light enters the tooth; affected in several ways:

1. Part of the light may be transmitted completely through the tooth (the MORE light that is transmitted through, the MORE translucent the material).
2. Part of the light may be reflected from the surface of the tooth and thus may NOT penetrate at all.
3. Part of the light may enter the tooth and then scatter and be absorbed (refraction).

a. Composites and ceramics (glass) can be made with varying degrees of translucency by adding opaquing agents to block light penetration; common problem with older anterior restorations, especially when photographed.
b. Anterior teeth can be very translucent at the incisal edge; extreme or excessive whitening procedures can actually increase this, creating the appearance of gray teeth.

Table 15-2 Expansion of tooth and dental materials: linear thermal coefficients of expansion

Material Coefficient (×10-6/°C)
Tooth 11.4
Amalgam 25.0
Acrylic resin 81.0
Composite resin 35.0
Silicone impression material 210.0
image

Figure 15-3 Galvanism.

Surface Properties

Surface properties are associated with surface of dental materials and include surface tension and absorbability.

A. Solids: MOST materials in dentistry.

1. Have regular arrangement of atoms and molecules; crystalline or rigid at all temperatures below its melting point.
2. In contrast, amorphous structures have NO regularity or form; they possess many characteristics of solids but are noncrystalline; those such as glass do NOT possess a definite melting or freezing point. Example: dental waxes are amorphous and gradually soften without definite melting point.

B. Liquids: some dental materials are liquids at some point and then change to solids by cooling or chemical reactions.

1. Irregular arrangement of atoms and molecules.
2. Includes reversible hydrocolloid materials heated to liquefaction.

C. Attaching solid structures to each other: by mechanical bonding, cohesion, adhesion.

1. Mechanical bonding:

a. Does NOT require intimate attraction between atoms and molecules of the substances involved.
b. Examples:

(1) Dental cements with crown: liquid cement penetrates into irregularities on internal surface of crown and into porosities in tooth structure, thus creating locking mechanism.
(2) Acid etch technique for most enamel sealants and resin materials: minute pores are produced in enamel and dentin when acid is applied, allowing spaces for interlocking of materials.

2. Cohesion: force of attraction between like kinds of atoms and molecules within material, resulting in tenacious bond. Example: cohesive forces that hold cement together and prevent fracture within cement.
3. Adhesion: force of attraction between unlike atoms and molecules on two different surfaces when brought into contact.

a. MORE tenacious bond than mechanical bonding.

(1) Primary bond: chemical bond between atoms and molecules; example: dyeing cloth.
(2) Secondary bond (MOST common): van der Waals forces, or physical attraction between atoms and molecules that does NOT provide as strong a bond as chemical bonding. Example: materials with polyacrylic acid, such as glass ionomer cements and polycarboxylate cements.

b. Adhesion helps control the problem of micro-leakage around dental restorations.
c. Effectiveness of adhesion is determined as follows (Figure 15-4):

(1) Wetting: degree to which an adhesive will spread out on a surface.
(2) Contact angle: angle between the adhesive and surface that is measured to give a value for wetting; the larger the contact area, the BETTER the chance of adhesion.
(3) Surface energy: the higher the surface energy, the greater the attraction of atoms to the surface, resulting in better adhesion (e.g., metals have high surface energy).
d. Inhibition or failure of adhesion is affected by:

(1) Dirty or contaminated surfaces: result in reduced surface energy of adherent, thus reducing ability to attract other atoms and molecules.
(2) Viscosity: adhesives with high viscosity have high resistance to flow, are less likely to spread out on a surface; if viscosity is too low, though, may be hard to control placement of the adhesive.
(3) Shrinkage of adhesives during hardening: causes adhesive to pull away from tooth/restoration, thus compromising adhesion.

image

Figure 15-4 Wetting ability: the lower the contact angle, the greater the adhesion.

Biological Properties

Biological properties refer to the biological response of the human body to various materials and the continued examination of the host–foreign body response. Response of both patients and clinicians can involve allergy, microleakage, and toxicity factors.

• See Chapter 8, Microbiology and Immunology: allergic reactions, including rubber latex allergy.
A. Allergic response (immediate or delayed contact allergic reaction type):

1. Elicited in patients who are sensitized to nickel-containing materials; occurs infrequently.
2. Possibly also rubber latex allergy for both patient and clinician from rubber latex rubber dam and gloves; may be associated with placement of restorative materials; other restorative, rubber dam (silicone), and glove (vinyl) materials are available.
3. Higher percentage of women are sensitive or allergic to nickel and/or rubber latex.

B. Microleakage:

1. Inability of materials to adequately seal margins by adhesion, allows leakage of saliva, dental biofilm (dental plaque), and by-products into the area, which can lead to secondary caries, pulpitis, pain, necrosis.
2. LESS potential with polyacrylic acid, currently ONLY material able to adhere sufficiently to avoid microleakage.

C. Toxic effects of materials: can occur during and after restoration of carious tooth, may subject pulp to injury.

1. Includes microleakage, thermal shock, galvanism, chemical irritation.
2. Can be minimized or eliminated by taking proper steps (e.g., using insulating bases, tooth isolation with rubber dam).

IMPRESSION MATERIALS

Function of impression and replication materials is to accurately record hard and soft tissues in the oral cavity. Impressions produce negative reproduction, which then can be used by clinician for construction of restorations to replace missing tooth structure and for fabrication of preventive devices. Vary in accuracy and ease of use, especially in removal from the mouth, such as around tooth undercuts, portion of tooth that lies between its height of contour and attached gingiva. Replication materials to preserve the impressions are discussed in next section.

A. Classification of impression materials:

1. Inelastic materials: rigid impression materials are restricted to applications in which NO undercuts exist.

a. Compound (modeling plastic): tray compound and impression compound.
b. Zinc oxide–eugenol (ZOE) impression paste: LESS use, replaced by rubber impression materials.
c. Impression plaster: LESS use because of rigidity and ease of fracture; used primarily to mount casts on articulator.

2. Elastomeric materials: aqueous elastomeric materials (hydrocolloid): largely replaced by BOTH alginate and rubber base impression materials:

a. Hydrocolloid is a suspension of fine particles dispersed in water to produce a viscous solution.
b. Irreversible hydrocolloid, such as alginate, and reversible hydrocolloid, such as agar-agar.

3. Nonaqueous elastomeric materials (rubber):

a. Polysulfide (mercaptan rubber).
b. Silicone rubber; BOTH condensation and addition (polyvinylsiloxane) types.
c. Polyether.

B. Composition and chemistry:

1. Compound (modeling plastic): 40% resin, 7% wax, 3% organic acid, 50% filler, coloring agent.

a. Thermoplastic (material will reversibly soften on heating and harden on cooling).
b. Tray compound is in shape of a tray; impression compound is in form of sticks and cones.

2. ZOE impression paste: supplied as two-paste system: zinc oxide, eugenol, oils (such as oil of cloves), resin, additives.
3. Irreversible hydrocolloid (alginate) (Table 15-3).
4. Reversible hydrocolloid (agar) (Table 15-4).
5. Polysulfide: supplied as two-paste system:

a. Base: 80% organic polymer-containing reactive mercaptan groups and 20% reinforcing agents.
b. Catalyst: MOST often is lead dioxide.

6. Silicone rubber:

a. Condensation type: supplied as base and accelerator; available in light, regular, heavy-bodied consistency and as putty:

(1) Base: silicone liquid, dimethylsiloxane.
(2) Accelerator: tin organic ester suspension and alkyl silicate.

b. Addition: type (polyvinylsiloxane): supplied as two-paste or putty system, with silicone and silane hydrogens.

7. Polyether: supplied as BOTH base and catalyst; base is polyether and catalyst is sulfonic acid ester.

C. Characteristics, properties, manipulation of each:

1. Compound:

a. Flow is 85% at 113° F and 6% at mouth temperature (98.6° F); hardens in mouth.
b. Tray compound is stiffer and has less flow than impression compound.
c. Thermoplastic; does NOT involve chemical change.
d. LOW thermal conductivity.

2. ZOE:

a. Soft-set ZOE is tougher than and NOT as brittle as hard-set ZOE.
b. BOTH types are classified as rigid; CANNOT be used to record undercut areas.
c. Setting time that is shortened by presence of water, high humidity, high temperatures.
d. Accurate; has low dimensional change during setting of ∼0.1%.
e. Adheres to tray compound or acrylic tray material, eliminating need for tray adhesive.

3. Irreversible hydrocolloid (alginate):

a. Suspension of molecules (colloid) in some type of dispersion medium (water); a liquid colloid is a sol.

(1) Liquid sol is changed into a gel by chemical means, which is an irreversible process.
(2) Normal-set alginate sets in not less than 2 minutes, not more than 4½ minutes (mixing time, 1 minute).
(3) Fast-set alginate sets in 1 to 2 minutes (mixing time, 30 to 45 seconds).
(4) Water/powder (W/P) ratio affects setting time; thinner mixes increase time required for material to set; increased powder decreases setting time.

b. Has 1.5% permanent deformation.

(1) Increased strength if thick rather than thin mixes are used.
(2) Results in increased tear and compressive strength if impression is left in mouth until fully set.
(3) Less accurate impression material than agar/rubber impression materials.
(4) Loses accuracy (dimensional change) if stored for MORE than 10 minutes in air because of syneresis (loss of water by evaporation and by exuding fluid); wrap in moist paper towel or keep in 100% humidity and pour as soon as removed.
(5) If stored in contact with water, will absorb water (imbibition), leading to dimensional change.

c. Procedure for use:

(1) Mixing:

(a) Fluff alginate powder container or package; measure appropriate amount of cool water for required number of scoops of alginate; place water in plastic bowl.
(b) Add alginate powder to water in bowl, thus helping to eliminate entrapped air in final mix.
(c) Stir powder and water vigorously to wet powder completely, strop (wipe) mix against side of the bowl for 60 seconds to homogenize and remove bubbles.
(d) Visually inspect mixture for a creamy, thick consistency.

(2) Filling and seating the tray:

(a) Spray tray with adhesive, then fill with mixture by spatula, from posterior regions forward.
(b) Smooth alginate surface with moistened finger; precoat the occlusal and other anatomical areas (e.g., vaulted palate, frenums) for BETTER impression.
(c) Seat tray in a posterior to anterior direction.
(d) Border mold by pulling lips and cheeks over tray and hold tray in place until alginate sets.

(3) Removing and pouring impression:

(a) Lift lips and cheeks away with fingers to break seal.
(b) Grasp handle and pull tray away from teeth with quick (thrust or snap) motion for less distortion.
(c) Wash impression under cool water to eliminate saliva and blood.
(d) Spray impression with disinfectant and seal in plastic bag for 10 minutes.
(e) Rinse thoroughly and pour impression as soon as possible or store in 100% humidity, 15 minutes maximum.

4. Reversible hydrocolloid (agar): sol is changed into gel by physical (cooling) reaction and therefore is reversible; gives highly accurate impression; MUST be poured immediately to avoid dimensional changes.
5. Polysulfide: provides MOST reproduction of surface detail.

a. Demonstrates shrinkage of impression material during first 24 hours; therefore models and dies SHOULD be poured within 1 hour.
b. Highly compatible with model plaster and stone; long shelf-life, improved by refrigeration.

6. Silicone rubber:

a. Condensation type:

(1) LESS working and setting time than polysulfides.
(2) LESS viscosity than polysulfides, making them easier to mix.
(3) Greater dimension change than polysulfides; reproduces fine details.
(4) Compatible with model plaster and stone.
(5) Known to cause allergic reactions.

b. Addition (polyvinylsiloxane) type:

(1) Low dimension change of 0.1%; does NOT require immediate pouring, can be delayed for up to 7 days.
(2) Lowest level of permanent deformation of ALL elastomeric impression materials on removal from mouth.
(3) MORE stiff than polysulfides and condensation silicones; makes removal from undercuts MORE difficult; NOT as stiff as polyethers.
(4) Causes less tissue reaction than condensation silicones.
(5) Very hydrophobic material, can cause problems with trapped air bubbles on the surface when pouring gypsum casts or dies; surfactants added to make more hydrophilic.

7. Polyether:

a. LESS working time than any of the rubber impression materials.
b. Permanent deformation greater than that of polysulfides, but NOT as low as that of silicones.
c. High stiffness and low flexibility, thus can cause problems during removal from mouth; trays must be made short of tissue undercuts, NOT for removable appliances.
d. Lower dimension change than any other rubber material, EXCEPT for polyvinylsiloxane.
e. Can cause tissue irritation because of aromatic sulfonic acid ester catalyst.

D. Clinical uses:

1. Compound:

a. Tray compound: primary impression for dentures; another impression material, such as rubber base, put into tray to make secondary or corrective impression.
b. Impression compounds: border molding for denture impressions.

2. ZOE impression paste: cementing media; also is used for surgical dressings (after periodontal surgery), temporary restorations, root canal filling material, bite registration paste, impression material for dentures.
3. Irreversible hydrocolloid:

a. Diagnostic (study) cast for educational purposes, fabrication of fluoride trays and mouth protectors, orthodontic appliance construction.
b. Preliminary cast for development of complete and partial dentures (to make trays and diagnoses).

4. Reversible hydrocolloid: SAME uses as irreversible hydrocolloid but produces MORE accurate detail.
5. Polysulfide:

a. Inlay and crown impressions.
b. Development of fixed partial dentures (bridge) and secondary (final) impressions for dentures.

6. Silicone:

a. Polyvinylsiloxane (addition silicone): same uses as polysulfide.
b. Condensation silicone: used for inlay and crown impressions as well as fixed partial dentures.

7. Polyether: SAME uses as condensation silicone.

Table 15-3 Composition of irreversible hydrocolloid alginate impression material

Ingredient Percentage (%) Function
Diatomaceous earth 60 Filler
Calcium sulfate 16 Reacts with potassium alginate to create gelation
Potassium alginate 15 Reacts with calcium sulfate to create gelation
Zinc oxide 4 Filler
Potassium titanium fluoride 3 Accelerates setting of gypsum
Trisodium phosphate 2 Retardant

Table 15-4 Composition of reversible hydrocolloid

Ingredient Percentage (%) Function
Agar-agar 8-15 Substance is extracted from a type of seaweed whose fibrils form a colloid that results in a partially rigid but elastic gel
Water 80-85 Main component that occupies the spaces between the agar-agar fibrils
Borax Trace amounts Increases the strength of the gel (also can retard the setting of gypsum; requires the addition of an accelerator)
Potassium sulfate 2 Accelerates the setting of gypsum

Replication Materials

Impression plaster and stone are examples of replication materials that are used to produce positive reproduction of impressions. Standard precautions for lab discussed later.

A. Composition: gypsum is calcium sulfate hemihydrate (CaSO4 • 2 H2O).
B. Types:

1. Model plaster (type II): heating gypsum under atmospheric pressure results in a porous, irregularly shaped material.
2. Dental stone (type III): heating gypsum under steam pressure results in hemihydrate particles that are less porous and MORE uniform in shape.
3. High-strength dental (die) stone (type IV [low expansion], V [high expansion]): dehydrating gypsum in 30% solution of calcium chloride results in LEAST porous hemihydrate particles.

C. Characteristics and properties:

1. Hemihydrate: physical differences in hemihydrate particles determine manipulative properties for mixing particles and use of each.

a. Mixing (Table 15-5):

(1) Amount of water needed depends on size, shape, porosity of the particles.
(2) Porous, irregularly shaped hemihydrate crystals require MORE water to facilitate wetting and mixing.
(3) High-strength stone requires LEAST amount of water.
(4) Too much water prolongs setting time and creates weaker cast or die.
(5) Too little water results in increased expansion.
b. Setting:

(1) When mixed with water, forms hard substance.
(2) Produces heat (exothermic reaction) through chemical reaction of hemihydrate with water.
(3) Allow 45 to 60 minutes to set before removing impression from the cast or die.

2. Accelerators and retarders: more practical method of controlling setting time than varying W/P ratio (added by manufacturer).

a. Accelerator is potassium sulfate (salt solution); reduces setting time from ∼10 minutes to 4 minutes.
b. Retarder is borax; increases setting time (blood, saliva, agar, alginate also retard setting).
c. Impressions thoroughly rinsed and disinfected using infection control protocol (remove traces of blood, saliva).

3. Temperature and humidity:

a. Higher water temperatures (during mixing) decrease setting times.
b. High humidity results in decreased setting time.

D. Types and uses of gypsum:

1. Impression gypsum (type I): mount casts on articulators or as interocclusal record.
2. Model plaster (type II): weakest; lowest compressive strength because of excess water, higher W/P ratio; NOT used intraorally; makes study casts (models) for documentation:

a. To permit clinicians to examine conditions in the mouth from all views; allows study of relationships between adjacent and opposing teeth and permits dentist to examine, measure, and analyze without discomfort to the patient.
b. Beginning of treatment and progress of involved or long-term treatment; for drawing, completing diagnostic waxing, or performing proposed treatment on models.
c. To illustrate existing conditions to the patient during case presentation.
d. To examine occlusal relationships and chart existing factors such as wear facets, open contacts, rotated teeth, and tissue recession.
e. To demonstrate individualized homecare procedures; allows practice of techniques on models.
f. Forensic cases.

3. Dental stone (type III):

a. Used to make casts of impressions for fabrication of dentures and to produce study casts.
b. Harder, stronger, MORE durable than model plaster (type II); greater strength result of lower W/P ratio.

4. High-strength dental (die) stone (type IV [low expansion] or V [high expansion]):

a. Type IV: used as die material on which wax patterns of inlays, crowns, and other castings are produced.

(1) Four times greater compression strength than model plaster (type II) because excess water has been minimized.
(2) Lowest w/p ratio.

b. Type V: used as die material with higher expansion to compensate for greater shrinkage that occurs in higher-melting alloys.

E. Procedure for use:

1. Dispensing: weigh powder and vacuum mix; water should be measured in graduated cylinder to get consistent results; dimensional accuracy increased when gypsum is weighed and water is measured.
2. Mixing: performed in a rubber bowl, using a stiff metal spatula, with room-temperature water.

a. Place water in bowl first and then add powder, sift into water slowly.
b. Spatulate vigorously for 30 to 60 seconds to break up clumps and enhance wetting of powder, strop (wipe) against side of the bowl.
c. Vibrate to remove remaining air bubbles; presence of air bubbles results in porosities in cast, reducing strength.
d. When complete, mix looks very glossy on the surface and has smooth, creamy texture (if sandy, grainy, or watery, incorrect W/P ration has been used and mixing is incomplete); can be BEST accomplished using vacuum mixer technique.

3. Pouring:

a. Surface of the impression is dried, excess water removed.
b. Plaster or stone is dripped into one corner of the impression and gently vibrated to slowly move the mix around the arch and express any air to coat impression.
c. Second increment is added and vibrated; excessive vibration SHOULD be avoided because it creates air bubbles in the material.
d. Process is repeated until entire surface of impression is coated and teeth imprints are completely filled; larger increments can be added to fill the impression completely.
e. One-pour technique involves initial mix of plaster or stone that is large enough to fill impression and form base; two-pour technique involves second mix of plaster or stone, used to form a base after first pour is set.

F. Cast trimming: excess gypsum is removed with lab knife after loss of gloss.

Table 15-5 Water/powder (W/P) ratios of gypsum materials∗

Gypsum Water (mL/100 g powder)
Model plaster (type II) 37-50
Dental stone (type III) 28-32
High-strength dental stone (type IV-V) 19-24

∗ Recommended W/P ratios vary among products and manufacturers.

RESTORATIONS

A restoration restores or replaces lost tooth structure, teeth, or oral tissues. Includes any filling, inlay, crown, bridge, partial denture, or complete denture.

DIRECT RESTORATIVE MATERIALS

Direct restorative materials can be placed directly in or on a prepared tooth. Advantageous because of immediate results and timesaving benefits. Can be either temporary or permanent restorations.

• See Chapter 11, Clinical Treatment: enamel (pit and fissure) sealants.

Temporary Restoratives

Many procedures cannot be completed in one appointment because of need for lab work or because of complications of infections. In these instances, temporary (provisional) restoration that is placed over preparation is indicated. Materials such as acrylics, stainless steel, composites, or cements are used in preparation to maintain relationship in occlusion until permanent restoration can be placed.

A. Acrylics with compositions similar to denture bases.
B. Stainless steel or aluminum:

1. Stiff enough to bear occlusal forces when cemented to prepared tooth with temporary cement.
2. Easy to cut and adjust; has poor esthetic qualities.

C. Composite-type material:

1. Of intermediate rigidity, somewhat flexible after light curing.
2. Flexibility allows easy cutting and removal when permanent restoration is ready for insertion.

D. ZOE (type III) (Fynal):

1. Designed for use as temporary filling when mixed to base or puttylike consistency.
2. Protects the pulp; reduces pulpal inflammation.

Permanent Restorations

Permanent restorations are composed of dental materials whose properties allow them to function in the oral cavity for a greater length of service than that of temporary restorations. Include both esthetic and amalgam restorations.

• See Chapter 11, Clinical Treatment: caries classification and dental charting.

Esthetic Restorations

Esthetic (tooth-colored) restorations include ceramics, porcelains, composites.

A. Ceramics: compounds formed by union of metallic oxides and minerals.

1. Include glass, fine crystal, gypsum.
2. Glass ceramics are used MAINLY as reinforcing agents and fillers for dental composites and routinely as coatings and veneers to improve esthetics of metallic dental restorations; also in several dental cements and temporary restorations.
3. Generally considered very brittle materials; CANNOT be bent or deformed without cracking or breaking.
4. Characterized by high melting points and low thermal and electrical conductivity.
5. Inert, NOT chemically reactive, biocompatible.

B. Porcelains: specific type of ceramic that is used extensively in dentistry; fabricated by dental technician.

1. Chosen because of esthetic qualities; matches adjacent tooth structure in color, intensity, translucence.
2. Unique ability to transmit light when illuminated; gives natural and vital appearance.
3. Contains three main components: quartz, feldspar (75% to 80%), kaolin clay (aluminum silicate).
4. High hardness number (Mohs 6 to 7, KHN 400-500); may cause rapid abrasion of opposing enamel, which has lower hardness number (Mohs 5 to 6; KHN 350).
5. Used in restorative dentistry to produce denture teeth, jacket crowns, porcelain-fused-to-metal (PFM) crowns and bridgework, veneers, inlays.
6. Sensitive to acidic preparations, which can cause pitting and roughening; thus MUST use neutral sodium fluoride therapy, also NOT stannous because of staining properties; radiolucent.

C. Composites: used in ALL classes of restorations, as veneers to cover teeth stained by drugs and chemicals such as fluoride or tetracyclines, to fill spaces between teeth (diastemas), to enhance contour of misshapen teeth.

1. Composed of three major constituents:

a. Organic resin matrix: monomers called dimethylacrylates (bis-GMA).
b. Inorganic filler: quartz, glasses, colloidal silica particles.
c. Coupling agent: chemically bonds filler to resin matrix (silane).

2. Composite materials are classified by particle amount and/or size:

a. Amount of filler resin:

(1) The greater the amount of filler content, the stronger the restoration.
(2) The greater the amount of filler content, the LESS wearing away of the restoration.

b. Size of filler particles:

(1) Smaller size of the filler particles: MORE polishable restoration.
(2) Larger size of the filler particles: MORE difficult to achieve smoothly polished finish; however, larger filler particles result in stronger restoration able to tolerate MORE abrasion.

3. Polymerization: chemical reaction that converts small, individual monomer molecules into long, giant polymer molecules.

a. Consists of activated two containers of composite paste: one contains a benzoyl peroxide initiator, other contains tertiary amine activator.
b. When mixed, amine reacts with benzoyl peroxide and initiates polymerization.

(1) Visible light-activated resins: supplied as a single paste that contains both the photo initiator and amine activator.
(2) Dual-activated resins: contain activation chemicals for both chemical and lightactivated resins; when two pastes are mixed, slow chemical activation begins, then exposure to curing light results in rapid photoactivation.

4. Polymerization shrinkage: contraction that accompanies polymerization reaction.

a. MOST problematic property; even with acid etching of enamel and bonding agents, stresses from shrinkage can exceed strength of bond between restoration and tooth structure, resulting in marginal leakage.
b. Two techniques for minimizing shrinkage:

(1) Insertion and polymerization of composite in layers.
/>

Related posts:

  1. 10: Medical and Dental Emergencies
  2. 7: Nutrition
  3. 8: Microbiology and Immunology
  4. 14: Pain Management
  5. 6: General and Oral Pathology
  6. 3: Anatomy, Biochemistry, and Physiology

Stay updated, free dental videos. Join our Telegram channel
Tags:
Jan 1, 2015 | Posted by in Dental Hygiene | Comments Off on 15: Dental Biomaterials

VIDEdental - Online dental courses
Get VIDEdental app for watching clinical videos