Additional Restorative Materials
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TEMPORARY CROWNS
Acrylic resins are commonly used to fabricate a temporary crown, bridge, inlay or only to maintain occlusal and interproximal contacts. Methyl methacrylate (MMA), ethyl methacrylate, and ethylene imine resins have been used for provisional restorations. MMA consists of a liquid monomer that is mixed with the polymer polymethyl methacrylate. A benzoyl peroxide initiator is used. The temporary crown should be removed before polymerization is complete. If not removed the crown may become stuck, and the setting reaction is exothermic and may lead to pulpal tissue damage.
CEMENTS AND LINERS
Liners are very thin, around 5 microns, and mostly used under amalgam to protect the pulp by sealing dentin tubules without adding volume. They are too thin to provide thermal insulation and interfere with composite polymerization (should not be used under composite restorations). Cavity varnish/solution liners are organic solvent based and water insoluble, made from a natural gum like copal, rosin, or a synthetic resin. Copalite is the most common. Cavity varnish improves the marginal seal of the restoration. Suspension liners are water solvent based and water soluble, containing calcium hydroxide and occasionally ZOE suspended in a resin liquid. These suspensions are thicker than solution liners, around 20-25 microns. Both types of liners are radiolucent and can be confused with secondary caries radiographically.
Bases are generally placed underneath restorations, used to protect the pulp or to induce positive pulpal changes (indirect pulp cap procedure etc.). They are 1-2mm thick. Calcium hydroxide is the primary base under amalgam and composite. Zinc phosphate, zinc polycarboxylate, or glass ionomers are the primary selection for gold restorations. GICs are likely the most popular base.
Cements tend to be thinner, 15-25 microns, used to cement a restoration or appliance to teeth (orthodontic band, crown, inlay, only, veneers etc.). Whether you’ll need a liner, base or cement will depend on the treatment and the remaining dentin thickness. The wider the cement margin, the greater the cement loss. Resin luting cements are unfilled resins that bond to dentine. They obtain high compressive strength and low solubility, but are irritating to the pulp and leave a relatively high film thickness of more than 25 microns. They are the best choice for luting ceramic restorations.
Glass Ionomer cements are water based cements that are commonly selected because of their favorable characteristics. They have superior mechanical properties compared to zinc phosphate and polycarboxylate cements. Glass ionomer cements release fluoride, create micromechanical and chemical bonds to tooth structure, and exhibit high compressive strength, low solubility, and great compatibility with pulpal tissues. They are likely the least soluble cement. Glass ionomer cement is probably the most common, along with resin cements. Resin modified glass ionomers (RMGI) cements have properties similar to GI cements, but higher strength and lower solubility.
Zinc Polycarboxylate/Polyacrylate cements are less irritating and chemically bonds to the tooth structure. It is made by mixing zinc oxide powder and polyacrylic acid water solution. This material is used as luting agents for gold restorations and orthodontic appliances, and as a base under specific restorations. It can also be used as a liner, but needs two coats of cavity varnish to prevent irritation of the pulp. It can be used under composites. The cold slab technique is used to increase the powder to liquid ratio in order to improve physical properties, namely longer working time and shorter setting time after placement.
Zinc Phosphate, the oldest luting cement, is water based and is highly acidic (pH 3.5) when first mixed. It requires two coats of cavity varnish to protect the pulp. Once set the cement is a good thermal and electrical insulator, and strong (high compressive strength). It is made by mixing zinc oxide powder and phosphoric acid liquid. Zinc phosphate has a fast setting time and low solubility. The cold slab technique is used to increase the power to liquid ratio in order to improve the physical properties, lowering the viscosity and solubility, and increasing the strength of the final set. A thinner film thickness can be attained compared to RMGI, around 25 microns. Zinc phosphate cements have largely been replaced by more modern cements.
Zinc oxide-eugenol (ZOE) cements are known to have a therapeutic effect on the pulp, but have decreased in popularity because of the lack of strength and lack of compatibility with composite restorative materials (inhibits polymerization). It can be used as a temporary cement or used as a sedative or temporary filling material. Zinc oxide + rosin + zinc acetate + accelerator + eugenol liquid. Polymethylmethacrylate reinforced ZOE (IRM) can be used as a sedative/temporary filling material to reduce pulpal inflammation. Patients can be allergic to the eugenol component (oil of cloves).
INDIRECT RESTORATIONS
Some complex Class I and Class II restorations in mouth will benefit from an indirect restorative technique. Generally a higher quality restoration (improved strength) can be made outside the mouth, then cemented in place. Fabricating an indirect composite resin restoration allows controlled buildup, moisture control and proper extra-oral curing, eliminating the downfalls of dimensional changes when curing inside the mouth. Feldspathic porcelain looks good but is too fragile to bear the force needed in posterior teeth. Cast ceramic and milled ceramic (CAD/CAM) are superior and can benefit from resin bonding for an even stronger final result. Porcelain can be adhesively bonded and repaired using a combination of sandblasting (air abrasion), hydrofluoric acid etch, silane coupler and resin bonding agents.
Onlays can permanently restore teeth and reinforce tooth structure, and are more conservative than conventional crowns. Their limitation used to be retention, but with modern adhesive cements many dentists no longer place conventional crowns, opting for more conservative onlays and inlays. Cases where a cast gold inlay would be indicated:
- The restoration of large lesions.
- A restoration when optimum contour and proximal contact is required.
- Restoration of structurally compromised endodontically treated teeth.
- Restoration in a patient who likes the gold tooth look.
- The restoration of a tooth as an abutment for the RDP to create ideal planes, rest seats and undercuts.
Inlays will not seat if there is an undercut present. For a Class II inlay a dovetail/occlusal lock is required to prevent proximal dislodgement. The cavosurface margins are beveled to 40 degrees to improve marginal adaptation (burnish). Composite inlays should be seated in preps with divergent walls. Indirect inlays are losing popularity due to the increased cost, and simultaneous improvements in direct restorative materials.
GOLD
Noble metals are very resistant to corrosion and do not oxidize during casting. They include gold, silver, palladium and platinum. Karat refers to the amount of pure gold in an alloy. 24 karat is 100%, 12 karat is 50%. Fineness is a similar measure but out of 1000. 1000 fineness is 100% (gold foil restorations), 500 is 50%. 999.999 (six nines fine) is the purest gold ever produced.
Gold inlay and onlays are still used because of their high success rate and low tooth wear, especially in bruxer cases. Tooth preparation is a little more complicated and patients generally do not appreciate the appearance of gold teeth any more. The cost of gold is high, and gold is a good thermal conductor/poor insulator. Because of the cost and softness of gold, it is usually mixed with other metals to form an alloy. Each component in the alloy adds different properties of the materials
- Gold (Au) – the most ductile. Increases the resistance to tarnish and corrosion.
- Copper (Cu) – hardening effects. Adds an orange color.
- Silver (Ag) – the second most ductile. Offsets the color shift from copper. Reduces melting temperature.
- Zinc (Zn) – oxygen scavenger. Increases fluidity, decreases the surface tension, increases castability. Reduces the melting point.
- Palladium (Pd) – increases hardness, raising melting point, and adds strength. Strong whitening effect.
- Platinum (Pt) – raises the melting point. Third most ductile. Increases tensile strength. Reduces tarnish and corrosion. Active in age hardening.
There are 4 classifications of high gold (>75% gold) alloys.
- ADA Type I with the highest noble metal content of 83%. Used for small inlays, high ductility, easy burnishing. Yield Strength (MPa) – <140.
- ADA Type II with >78% noble metal content is used for larger inlays and onlays, and can also be burnished. Yield Strength (MPa) – 140-200.
- ADA Type III with >75% noble metal content is used for onlays and crowns. It is capable of being heat-treated. Yield Strength (MPa) – 201-340.
- ADA Type IV with >75% noble metals are used for bridges and RPDs. It is the hardest high gold alloy and can be heat treated. Yield Strength (MPa) – >340.
Medium gold alloys contain 25-75% noble metals, and low gold alloys contain less than 25% gold or other noble metals. The retention of a gold restoration relies on the cement but most importantly the tooth preparation, with long (minimum 3mm) near parallel axial walls (2-5 degrees per wall).
Burnishing occurs after the crown is cemented. A cast gold restoration usually ends with a beveled cavosurface margin. If more volume is needed at the margin a chamfer is used. Once seated, if the new cast crown does not fit, the most common reason is residual temporary cement left over from the temporary restoration. The second most likely problematic area is the proximal contact. If left high, the patient will likely come back complaining of pain and temperature sensitivity. It may even cause mobility.
Base metal alloys, for example cobalt-chromium, are also known as non-precious metal alloys and contain corrosion resistance. The surface metal corrodes, but the oxide layer that forms creates an outer film that passively protects the remaining material. Base metals are less resistant to corrosion than noble metals, but have great strength, relatively low cost and low density. They can be harder to cast and finish compared to noble metals.
Ductility refers to the metal’s ability to be easily worked into the desired shape, that is to form a wire from a material. It requires plastic deformation before failing (fracture). Ductility is dependent on plasticity and tensile strength. It is presented in percent elongation, a higher value means the metal is more ductile. As the temperature is increased, ductility is decreased.
Malleability is the ability of a material to be hammered/compressed in a thin sheet without failure. Plasticity dictates malleability (not tensile strength). As the temperature increases, malleability increases.
Capping the compromised functional cusp is a good idea. Shoeing the functional cusp is never recommended.
GOLD FOIL
BLEACHING
Hydrogen peroxide is the most common active ingredient used in bleaching procedures. It works by oxidizing and removing interprismatic organic matter within enamel and dentin. Only the chroma and value (not hue) of the teeth are altered. Carbamide peroxide, also known as urea peroxide, is an organic white crystalline compound formed by combining hydrogen peroxide and urea. The stable complex will decompose in contact with water to release hydrogen peroxide in roughly a 3:1 ratio. Bleaching agents with 10% carbamide peroxide will release 3.5% hydrogen, 35% carbamide peroxide will release 12% hydrogen peroxide.
Patients looking for brighter teeth may benefit from vital bleaching. This can be done in the office with a light activated solution (the light itself doesn’t do much), ~35% peroxide in 4-10 minute cycles, or home bleaching kit containing around 10% hydrogen peroxide or 10-30 % carbamide peroxide. A stronger solution requires less contact time. 10% carbamide peroxide is used overnight (8-10 hours). The stronger the solution, the more likely there will be adverse effects. Dentin sensitivity and gingival irritation are the most common side effects. Dentin sensitivity can vary from very mild to extreme. Bleaching toothpastes contain the correct active ingredient but a low concentration, so likely won’t have much of an effect. They are good for maintaining color after a course of vital bleaching. Bleaching should be performed before restoring a tooth (restorations will remain unaffected after treatment).
Deep intrinsic stains (hypomineralization, tetracycline staining etc.) do not respond readily to bleaching. Other treatment options may need to be considered: micro-abrasion (pumice plus acid), direct composite restorations, direct composite veneers, indirect porcelain veneers or full coverage (crowns).
A tooth that has experienced pulpal trauma or received root canal treatment can appear discolored. Walking bleaching/non-vital bleaching refers to bleaching material placed inside the crown and sealed in for a period of time. Sodium perborate and 30% aqueous hydrogen peroxide is commonly used. Sodium perborate will decompose to form sodium metaborate and hydrogen peroxide when it is in contact with water. Most of the time, carbamide peroxide is used in a 37% concentration for internal bleaching. The mixture is left for 3-7 days and the tooth color is reassessed. Often the tooth is slightly “overbleached” to compensate for a rebound in value. Once the desired brightness is attained the bleach is removed and the tooth restored. Bleaching can affect composite polymerization and it may be prudent to wait a week before the final restoration.