Other Prosthodontics Materials
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INVESTING AND CASTING
After a wax pattern is made of a die cast, a sprue is attached to the wax pattern and the assembly is placed into a casting ring and invested. The sprue pin diameter should be equal to or greater than the thickest portion of the wax or plastic pattern. A 12-gauge sprue is used for small premolar patterns, otherwise a 10-gauge sprue is sufficient for most patterns. The sprue is attached to the point of greatest bulk at a 45° angle to allow the alloy to freely flow. A shrink back porosity occurs when turbulence in the flow of molten metal causes a shrinkage void, or suck back porosity. It can happen if the sprue is attached to a thin area or if the sprue is too small. Low investment permeability and insufficient wind-up of the casting machine can also cause a shrink back porosity.
A dental investment is a refractory material used to surround the wax pattern forming a mold into which the alloy is cast after removing the wax or plastic. Some investment materials can also be used for pressing ceramic. Heat is used to remove the wax pattern (burnout) leaving a negative reproduction of the pattern (lost wax technique). The casting machine then injects a melted alloy into the investment space.
Investing materials
Gypsum-bonded investments contain 55-75% silica, 25-45% gypsum binder (calcium sulfate hemihydrate), and 5% modifiers. They are more commonly used in low heat investments (under 1100°C, or 2100°F), like casting gold alloys or silver-palladium alloys for dowel cores. Dowel cores don’t require as much expansion as crowns, and even though they are cast using alloys that usually require high temperatures for expansion (silver-palladium). Gypsum-bonded investments cannot be used for casting alloys that require higher temperatures, like titanium crowns or copings, chromium-cobalt alloys, Type IV gold, or the coping for metal-ceramic crowns. Gypsum-bonded investments are not common in modern labs and are mostly used for high precision gold castings such as the Tucker technique.
All molten alloys shrink upon solidification, gold by approximately 1.5%, and nickel chromium alloys by as much as 2.4%. Gypsum-bonded investments expand to compensate for the shrinkage of metal during cooling. Three controllable processes can affect the extent of material expansion:
- Setting expansion (exposed to air). Caused by crystal growth, but restricted by the metal investment ring. Causes expansion of about 0.4%.
- Hygroscopic expansion (exposed to water). Allowing the investment to set in the presence of water allows for the replacement of water used by the hydration process, maintaining the space between growing crystals. Causes expansion of 1.2-2.2%.
- Thermal expansion (exposed to heat). When the investment is heated in the burnout oven.
Phosphate-bonded investments contain 80% quartz and/or cristobalite, 20% MgO and a monoammonium phosphate binder. They are more commonly used in high heat investments (over 1100°C, or 2100°F), like silver-palladium, gold-platinum, and nickel-chromium alloys. Type IV gold alloys require heating above 2100°F so phosphate-bonded investments are used. All “pressing” investments are phosphate bonded. Special liquid-containing colloidal silica increases both the strength and setting expansion, while decreasing surface roughness. Most phosphate-bonded investments are strong enough for a ringless technique.
Silica-bonded investments are used for casting base metal alloys used in frameworks for dental prostheses. They have a silica binder. Magnesium phosphate reacts with primary ammonium phosphate to produce magnesium ammonium phosphate which provides strength. At higher temperatures silicophosphates form to provide the greatest strength. Silica-bonded investments are rarely used.
IMPLANT MATERIALS
The ultimate goal of an implant is to maintain a stable, long term direct attachment to bone (osseointegration) in order to provide support for some type of a prosthesis. Osseointegration has been observed using many materials but is most often used to describe the reaction of bone to titanium. After implant placement, intramembranous osteogenesis results in the formation of woven bone, followed by the formation of parallel-fibered lamellar bone. New bone can be observed on the implant surface around 1 week after installation. Bone remodeling starts at between 6 and 12 weeks and continues throughout life.
Surface modifications that increase the implant surface area accelerates the osseointegration process of titanium implants, for example sandblasting and subsequent acid-etching to increase micro-roughness. Hydrophilic chemical surface modifications will result in increased osseointegration speed. Many implant designs have been proposed, including press-fit cylinders and plates, but self tapping screw type implants are by far the most commonly used.
- Subperiosteal implants lie between the soft tissue and cortical bone.
- Transosteal implants extend through the entire width of the jaw bone. These two are not common in prosthodontics.
- Endosteal implants are the most common. Thread designs and surface treatments vary greatly and new designs are often promoted. The larger the surface area of the implant the better the bone-implant stability. Most Implants come in different lengths and diameters to accommodate different areas of the mouth.
The implant abutment is the component that attaches to the implant to support or retain a prosthesis. They are commonly prefabricated to fit a particular implant system. The original external hex abutment has mostly been replaced with internal connection that can either be retained by screws or cemented like traditional prostheses. The Morse taper design is a mechanically locking friction fit connection.
DENTURE MATERIALS
Acrylic resin or polymethyl methacrylate (PMMA) is the material of choice for denture bases. Acrylic resin powder contains:
- polymer beads.
- a benzoyl peroxide initiator.
- pigments, dyes, opacifiers, and organic fibers.
The acrylic monomer liquid contains:
- methylmethacrylate (MMA).
- hydroquinone (inhibits polymerization during storage).
- Plasticizers.
Heat-cured resins utilize heat to decompose the benzoyl peroxide initiator, initiating polymerization. About 3-7% shrinkage can be expected when using heat-polymerized resin. They have less residual monomer and higher molecular weight than self-cured resins, making them stronger and more resistant to discolouration. If acrylic is heated too quickly, vaporized liquid becomes trapped as gas bubbles. Porosities are most commonly due to insufficient pressure on the flask during processing. Acrylic resin used for denture repairs should be under 20-30psi during processing to help eliminate porosities, which will most likely form in the thickest parts of the denture. Porosities can also occur if the packing acrylic mix is too plastic (stringy/sandy), which allows the liquid to vaporize and does not allow sufficient pressure during the closure of the flask.
Self-cured resins use a tertiary amine or dimethyl-p-toluidine to initiate the polymerisation reaction. About 0.2% shrinkage can be expected when using auto-polymerized resin. They are generally selected for denture repair to prevent heat induced distortions. Excessive shrinkage may occur if too much liquid monomer is added to the polymer powder. Acrylic resins can expand when immersed in water and distort when dried out. The greater the molecular weight of the polymer, the better the polymerization and harder the resin.
Denture teeth are generally constructed from either porcelain, acrylic or composite resin.
- Porcelain teeth exhibit the best hardness and resistance to wear. They are, however, difficult to change or adjust, and do not strongly bond to the base.
- Acrylic teeth are the softest and wear easily. They are very easy to work with ( change and adjust) and bond very well to the acrylic denture base.
- Composite-filled resin teeth maintain physical properties between acrylic and porcelain teeth, and bond well to the denture base. They can easily be adjusted.
CEMENTS
The ideal cement material would have a low film thickness, long working time, short setting time, low pulpal irritation, and very low solubility and microleakage, leading to a high prosthesis retention rate. Cements increase the frictional resistance between two surfaces, the tooth and the restoration, preventing them from sliding past each other. The cement film thickness is dependent on the powder-liquid ratio, powder particle size, and seating pressure.
Zinc phosphate is one of the oldest luting agents, made by mixing zinc oxide powder and phosphoric acid. For a long time it was the gold standard cement with its long record of success. Zinc phosphate cements maintain high compressive strength and good film thickness. After mixing it has a very low pH (3.5) which can cause pulpal irritation leading to post cementation sensitivity. Because of the low initial pH at least two layers of varnish are applied before cementation. The acidity neutralizes after 2 days. Zinc phosphate does not bond chemically to tooth structure. The frozen slab technique can be used to improve the physical properties of the cement, extending the working time.
Zinc polycarboxylate cement (aka polyacrylate cement) does chemically adhere to the tooth and restoration. Bonding is due to a chelation reaction between the carboxyl groups of the cement and calcium in the tooth structure. Zinc polycarboxylate forms a stronger bond to enamel since there is a higher proportion of calcium in tooth enamel (higher inorganic mineralized content) compared to dentine or cementum. Polycarboxylate cement is formed when the polyacrylic acid reacts with zinc ions. The solubility, pH and tensile strength of polycarboxylate cements are very similar to zinc phosphate cements. Zinc polycarboxylate is much kinder to the pulp than zinc phosphate, but it has a shorter working time, lower compressive strength, higher viscosity, and requires an additional tooth conditioning step.
Glass ionomer cements also form a chemical bond with enamel and dentine, and can absorb and release fluoride (anticariogenic). These cements exhibit a coefficient of thermal expansion similar to that of dentine, with high compressive strength and low solubility, but can be prone to moisture contamination and desiccation. Once set the pH can be relatively low, though not as low as zinc phosphate. Glass ionomers are physically superior to zinc polycarboxylate and zinc phosphate. Resin-modified glass ionomer cements have increased strength and lower solubility, while maintaining all the properties of regular glass ionomer cements.
Resin cements were developed from total-etch and self-etch dentin adhesive systems. They are essentially unfilled resins that form a strong micromechanical bond to dentine and enamel, and can chemically bond to porcelain through a silane-coupling agent. Resin cements exhibit high compressive strength and low solubility. Teeth that will receive a resin-bonded restoration need to be cleaned and etched with 37% phosphoric acid before employing a dentine bonding agent.
Zinc oxide-eugenol cements are reinforced with ethoxybenzoic acid, alumina, or polymers to form EBA cement (e.g SuperEBA from Bosworth). They are compatible with pulpal tissues but have a lower compressive strength compared to zinc phosphate. Zinc oxide-eugenol cements will interfere with composite resin bonding.
A die is constructed with 20–40 μm of relief to allow room for the cement. Prolonged sensitivity to heat, cold and pressure after the cementation of a crown or fixed bridge is usually related to occlusal trauma.
PROVISIONAL RESTORATIVE MATERIALS
Following crown or bridge preparations, the fabrication of a provisional restoration accomplishes the following:
- Protects the pulp between appointments.
- Prevents tooth movement.
- Maintains occlusal function.
- Helps with hygiene.
- Improves esthetics between appointments.
Many materials are used to fabricate provisional restorations, including:
- Polymethyl methacrylates.
- Polyethyl methacrylates.
- Polyvinyl methacrylates.
- Bis-acryl composite resins.
- Visible light-cured urethane dimethacrylates.
Restorations can be prefabricated using anatomic metal crown forms or clear celluloid/polycarbonate shells, often used for single tooth restorations. They can also be custom made using an impression (alginate, elastomeric impression material) of a diagnostic cast wax pattern, used for single or multi-unit restorations.
SOFT TISSUE RETRACTION
Soft tissue displacement in the gingival sulcus is achieved through a combination of mechanical and chemical means. Impression cord (retraction cord) is placed in the sulcus to push the soft tissues away from the tooth to aid in soft tissue protection during preparation and accurate capture during impressions. They come in different sizes and surface textures, and are usually impregnated with a solution to help attain hemostasis and moisture control. Common hemostatic agents include:
- Epinephrine – works by causing vasoconstriction. Be wary of cardiac patients.
- Aluminum chloride – causes minor tissue loss.
- Ferric sulfate – good hemostatic agent but discolors the tissues.
- Zinc chloride – causes excessive tissue destruction and is no longer used.
- Electrosurgery – can be used for soft tissue modification as well as cauterizing the area.
- Potassium aluminum sulfate (alum) – causes minor tissue loss. Used in patients with hypertension to minimize physiological changes.
Electrosurgery is often used to remove a thin layer of crevicular gingival tissue in order to access to the cavosurface margins of preparations, while also helping with coagulation and hemostasis. It works by passing a small electrical current through the gingival tissues, causing cells to dessicate (scorch), cauterizing the wound. Electrosurgery is a more radical means of soft tissue retraction that causes delayed healing, and is contraindicated in patients using electric medical devices like cardiac pacemakers, transcutaneous electric nerve stimulation (TENS) units, insulin pumps etc. It is not recommended in patients with impared immune function or impared healing potential (e.g. uncontrolled type II diabetes).
Areas where the attached gingiva is thin or where underlying dehiscence is suspected should be avoided. Plastic instruments are preferred over metal to prevent unwanted burning/tissue destruction. Rapid, single, light strokes are used, spaced by 5 second intervals. The operator should also avoid contact with metal restorations or tooth structure, which may cause irreversible pulpal damage. Tissue drag indicates the electrical current being used is too low.
Antisialogogues reduce salivary secretions and can be used to control moisture during preparations and impressions. Medications such as atropine, dicyclomine, glycopyrrolate and methantheline are examples of antisialogogues that can be cautiously used. Anticholinergic drugs should not be used in patients with glaucoma, especially narrow-angle glaucoma. A rapid increase in intraocular pressure may result in blindness. Anticholinergics are also used with caution when dealing with patients suffering from heart disease or urinary retention.
