These materials consist of tiny glass particles suspended in a resin (plastic) matrix. The size and amount of the glass particles give the composite filling materials different characteristics, such as strength and polishability. Composites are used in areas where the fillings are relatively small and there is plenty of tooth structure for support.
Composite fillings are less durable than amalgam if the filling is large, but comparable in durability if the filling is small to average size. Composite fillings in back teeth are significantly more difficult and time-consuming to place than amalgam fillings, therefore more expensive. Composite materials are most commonly placed directly into the tooth (like amalgam fillings), but can also be prefabricated and bonded into place indirectly (like a crown).
Amalgam is the most commonly used material for back teeth. It contains approximately 50% mercury and varying amounts of silver (30%), tin, zinc, and copper. It is the least costly and least time-consuming to place. It does not hold its shape over time, corrodes easily, and is expected to last 5-10 years. The controversy is that it contains mercury, a known neurotoxin (poison to the nervous system).
Galloy is a brand new material containing silver, tin, copper, indium, and gallium. It is meant to be mercury-free alternative to amalgam. Why is the American Dental Association developing & patenting this substitute for mercury amalgam if mercury amalgam is safe?
A direct composite is a special plastic material that bonds to tooth structure, is tooth colored, is more easily repairable, and requires less tooth structure to be trimmed away than any other material. It is expected to last 5-7 years, although small to moderate size fillings may last longer. Research has shown that it reinforces the tooth and makes it stronger. Cost and time to perform is about 50-75% more than amalgam. Composites are a petrochemical derivative and, as such, are a possible problem for the environmentally sensitive.
Indirect Composite Inlay/Onlay
This type of restoration is used when ideal fit and durability is desired, which is seldom achieved with a direct composite filling. Cost is approximately 2-3 times that of an amalgam filling and takes two visits.
Dental ceramics (sometimes referred to as dental porcelains) have come a long way! Until a few years ago, these materials were relatively weak (that’s why they required support from a metal substructure) and abrasive (causing wear on the opposing teeth). Today there are many different types of ceramic systems: Feldspathic, Leucite-reinforced, Polymer-reinforced, Zirconium-based–each with unique properties. From rebuilding broken teeth to replacing missing teeth (even in the back of the mouth), there is a ceramic to do the job. However, they are more difficult to use than conventionally cemented (non-bonded) crowns.
Picking the right ceramic for the job, proper tooth preparation, quality laboratory work, and meticulous cementation technique are all needed for a successful tooth restoration. It costs about the same as an indirect composite inlay/onlay and takes two visits. Most ceramic and resin-based materials contain metals in the form of oxides (such as aluminum) or even heavy metals (such as cobalt, barium or cadmium). These are usually added to give the amterials strength and improve their appearance. Sometimes they are added to make the restoration show up on x-rays. The number of materials that do not contain any of these products is very limited. However, the advantage of being oxide-free is lost when these are bonded to the tooth using an oxide-containing luting agent.
Because of gold’s long history, it is the standard against which other materials are judged. This type of restoration is used when maximum strength is desired and appearance is not a factor. Gold is almost never used in its pure form; rather gold is used as an alloy with other metal elements. It costs approximately three to four times more than an amalgam and takes 2 visits. There are many formulations of gold, varying from 1% to 99%. The other metals are added in order to give the gold strength and the ability to bond to porcelain (in the case of porcelain veneer fused to cover a gold crown). The most commonly added metals are palladium, silver, copper, and platinum.
The composition and amount of each metal in the alloy determines whether it is classified as a “high noble,” “noble,” or “base” metal. “Noble” metals are defined as gold, platinum and palladium. The most expensive gold alloys are “high noble” and they are defined as hving at least 60% noble metals and at least 40% gold. An alloy can still be called “noble” if it has at least 25% noble metal content. The cheapest materials fail even that test and are called “base” alloys–they have less than 25% noble metals. It is especially important for patients with metal sensitivities to avoid the base alloys since these usually contain toxic metals such as nickel and chromium;. But even the high noble materials can be incompatible for patients and even toxic; palladium, for example, is toxic.
Titanium is used when a gold alloy is not biocompatible; otherwise, the benefits, cost, and time to perform are the same as for a gold alloy, even though it is not a precious metal. It takes two visits.
Crown and Bridge Materials
A gold alloy is used when maximum strength is desired and appearance is not a factor. There are many formulations of gold, varying from 1% to 99%.
Titanium is used when maximum strength is desired, appearance is not a factor, and a gold alloy is not biocompatible. There are different purities of titanium, with grade-1 being the purest. This is used in joint replacement, dental implants, and bone pins. Cost is the same as for gold alloy.
Non-precious alloys are used when maximum strength is desired, appearance is not a factor, but cost is most important. Since it does not contain any gold, cost is less. There are two basic formulations, one that contains nickel and one that is nickel-free. The controversial issue is that nickel, beryllium, cobalt, chromium, and palladium may cause immune problems and/or toxicity.
Porcelain is used when appearance and wear resistance is the most important factor. It is much more fragile than metal and may break easily. Porcelain alone is not normally recommended for bridges.
Indirect composites are used when appearance is an important factor but when the risk of porcelain fractures and wearing down the other teeth is to be avoided. These are not quite as wear-resistant or esthetic as porcelain but very acceptable for normal situations.
One of the most difficult areas in dentistry today is the restoration of dental structures with biocompatible materials that are strong enough to withstand the forces of chewing (500-1000lbs pressure on molar teeth). Recent technology from Germany now offers a material that has overcome most of the pitfalls of present day products. Patients now have a choice of a material that is esthetic, strong, pure, biocompatible and capable of being used for single and long span dental bridgework. That material is called Zirconium oxide.
Zirconium oxide has the following superior characteristics that make it the most ideal material available:
* Excellent biological compatibility: absolutely bio-inert.
* Outstanding physical and mechanical qualities:
* Hardness (Vickers) 1200 HV
* Compressive Strength 2000 MPa
* Bending Strength 1000 MPa
* Modulus of Elasticity 210 GPa
* Tensile Strength 7 Mpavm
* Wear characteristics (Ring on disc) <0.002 mm 3/h
* Absolute corrosion resistance: Ringer’s solution 370C <0.01mg/ m2x24h
* Very small particle size: <0.6ym
* No glass phase for particle binding
* Extremely high density
* Porosity: 0%
* Purity (Zr/Hf/Y): 99.9%
* Translucence of the framework material makes excellent cosmetic results possible
* Equivalent fit to precision gold castings: edge opening 20-50 ym. Precludes the need to use adhesive cements.
* Zirconium oxide is manufactured and optimized industrially so that the material qualities remain unchanged through the complete pro duction chain.
* Optimal material for crowns: tasteless, radiopaque, no pulp irrita tion because there is no need to use adhesive cements and minimal invasive preparation by dentist.
Zirconium oxide forms the core of each crown and provides the cross-link that bridges the gap of missing teeth. The precision fit of the Zirconium core is derived from computer guided Swiss lathes that cut the form out of a solid Zirconium oxide block. The cutting instructions are obtained from a laser beam that reads 120 points per millimeter from the anatomy of a model of the prepared teeth. Once formed, new synthetic porcelain (99.9% pure) is baked on to the Zirconium core and then shaped like a tooth. Because of the extreme accuracy of the crown fit, the crowns can be cemented with biocompatible dental luting material. This avoids the use of an invasive procedure of etching the tooth with acid and injuring the pulp or nerve of the tooth. This latter procedure often times results in the pulp dying and necessitating root canal therapy.
Advantages of ET zirconium high performance ceramic compared with other full ceramics
Zirconium oxide ceramic primarily stands out due to its high crack resistance. Crack resistance is the resistance with which the material counteracts the spreading of cracks. If a material is stressed, it usually comes to excessively high tension within a defect area (pores, surface deficiencies, cavities) or it cracks. While with metals under high tension in the area of cracks, plastic deformation appears and the top of the tension can be reduced by rounding the cracks; in ceramics due to missing plastic deformation possibility the cracks continue to grow. The unusual feature of zirconium oxide ceramic in comparison with other ceramics is that at the appearance of a high-tension area a transformation of the crystal structure can take place. This process is also accompanied by a volume expansion. By this volume increase it builds wedges in the crack and therefore it reduces the continuation of the crack. While the critical tensile strength (KIC) e.g. of Dicor, Vita Mark II and Empress is in the area of 1-2.5 Mpam-1/2, zirconium oxide shows values in the range of 10 Mpam-1/2. Even In-Ceram (glass infiltrated Al203 ceramic) and Procera aluminum oxide (pure Al203 ceramic) show values less then 5 Mpam-1/2.
In connection with the tensile strength there also stands the characteristic of bending strengths. While conventional glass ceramics show results of 100-200 Mpa and aluminum oxide ceramics lie in the area of 400-600 Mpa, zirconium oxide reaches a bending strength of over 1000 Mpa.
Because of the high tensile strengths exhibited in test results, it is now possible to fabricate posterior bridges with zirconium oxide. Further decisive advantages of zirconium oxide are its high resistance to corrosion; stability to hydrolysis and its high biocompatibility in comparison with other ceramics makes this material ideal for restorative dentistry.
In medicine, zirconium oxide is being used more and more as the material of choice especially for hip prosthesis. For years there has existed substantial clinical tests and examinations which confirm the high quality of zirconium oxide.
Dentures are usually made from acrylic, stainless steel, and chromium-cobalt, but can be made of nylon, a gold alloy, or titanium. Most pink-colored acrylics and vinyls contain cadmium, which is considered toxic and/or immune reactive. The alternative is to use cadmium-free pink or clear materials. Metals are used to increase rigidity and increase retention of the prosthesis in the mouth during function. If metals are not used, the opposite is true, which is not desirable from a functional perspective.
Since all direct fillings (composites) and most indirect restorations (inlays, onlays, and crowns) being placed today use a process called bonding, it’s good to have a basic knowledge of how this process works. While there are a myriad of variations in this process, here are the basic steps:
Step 1–Prepare the tooth surface using a mild acid solution. This creates a “honeycomb” in the top layer of tooth.
Step 2–Paint a liquid resin-bonding agent on the tooth. It flows and “locks” into the honeycomb created in Step 1 (technically forming a hybrid layer that is part-tooth and part dental-resin). This layer is “cured” (hardened using a photo-chemical reaction) with a visible light source.
Step 3–Place luting cement if an indirect restoration (e.g., crown) is being used. Essentially, this material is a more liquid form of white filling material. It bonds to both the tooth and the pre-fabricated ceramic restoration and fills the gaps between them. The surface of the first layer of cured bonding agent is highly reactive and is easily bonded to with today’s composite filling materials.
Since the 1960s, alloy-porcelain combinations, known to the dentist as bonded restorations have been available. These porcelain-covered metal castings combine the strength of a metallic superstructure with the aesthetic appearance of dental porcelain, creating the illusion that the restorations are real teeth. Alloys have been developed to which dental porcelains form durable retentive bonds, and many of these are now based on nickel-chromium. These metal frameworks are so rigid that they can be bonded via composites to the backs of acid etched teeth, thus eliminating the need for cutting down sound teeth, figure 1. Just as etching dental enamel creates retentive ‘chasms’, these nickel-chromium alloys can be electrolytically etched to produce features that allow the formation of mechanical bonds with resin-based composite cements.
Internal view of a dental bridge bonded via a resin-based cement to the backs of acid-etched teeth.
The oxides that form on these alloys can also be used to promote chemical links to cements via bifunctional primers, thus eliminating the challenge of producing a uniformly etched surface.
Key Words : dentures affordable dentures partial dentures same day dentures cheap dentures permanent dentures