INTRODUCTION
Dentistry as a whole is a very dynamic field, but when you get to restorative compounds, bonding agents, ceramics, and impression materials, the rate of change is astronomical. The goal of this article is to review and compare the newer products on the market. Keep in mind that by the time this presentation takes place, other newly released products will be available.
COMPOSITE RESINS
We started with four types of composite resins: conventional (or macrofilled), small particle, microfilled, and hybrid.(1) Today, the most common resins are hybrids and posteriors. The hybrids are the most promising and the ones changing the most. In a hybrid resin, one tries to combine the strength of a posterior resin and the polish of a microfilled. In a posterior resin, the main quality is its ability to withstand the occlusal forces with minimum wear. Few products have received the seal of approval from the ADA as posterior composites. The science of composite is now several years old and the big leaps in discovery are done, leaving us with differences between products that are minute. For example, the company 3M has two products on the market: Z100 and Z250, which are almost identical except for one type of resin that was removed from Z250 to decrease the shrinkage experienced during curing.(2) The differences between competing laboratories are not wide either and some of these differences are of little interest to veterinarians, such as color shades and esthetics. Veterinary dentistry is more concerned with handling, strength and durability.
One problem that does not seem to go away is the formation of micro fissures along the margin of restorations. No matter how good the resin is you still are faced with a minimum of 2% shrinkage during curing. With the advent of bonding agents, the shrinkage is transmitted to the tooth and results in stress to the dentin and the enamel. The stress is strong enough to cause crack lines in the enamel.(3) One way to reduce the stress is to apply a layer of low viscosity composite. Shrinkage is still present, but the low viscosity allows flow during curing, decreasing stress.
Another way is to use regular composite, but to use a pulse-delay technique to cure it. This technique consists in using a short, low intensity light exposure on the composite, wait three to five minutes while the composite restoration is finished and polished, and then complete the curing with full intensity of the light for the recommended time. The original pulse should be two to three seconds at 200 mW/cm2; the completion of the curing is done using 500 mW/cm2 for 30 to 40 seconds. By slowing the polymerization reaction, the stress formed as the cross- link network develops is compensated by the viscous flow of the composite.(4,5) The decrease in shrinkage varies from 10 to up to 34 percent. The slow curing results in better marginal adaptation, decreases the chance of leakage, and increases the potential for long-term clinical success.
BONDING AGENTS
A recent review listed twenty-six different products. The evolution of the adhesive went through successive generations. When we got to the fifth generation, the classification changed to types. Presently there are four types of agents on the market. Type 1 has etchant applied and washed off to remove the smear layer; primer and adhesive are then applied separately. Type 2 has etchant applied and washed off to remove smear layer; primer and adhesive are applied in a single solution. Type 3 has a self-etching primer applied to dissolve the smear layer and is not washed off; adhesive is applied separately. Type 4 has a self-etching primer and adhesive applied as a single solution to dissolve and treat the smear layer simultaneously. The advantage of the Types 3 and 4 (called SEP adhesives [Self-Etching Primer]) compared to the Types 1 and 2 is that the number of steps and bottles is reduced. Specifically, the SEP adhesives eliminate the critical damp dry step, encountered in the Types 1 and 2, that was never clearly defined and consistently executed.(6)
The more important question for veterinary dentists is: do they bond better? Comparison from product to product is difficult, it is literally like comparing apples to oranges. One product may bond very well to enamel but not so tightly to dentin or vice versa. Another may perform well on both dentin and enamel, but cost four times as much as the nearest competitor. Yet other products offer lack of post-operative sensitivity. If we eliminate esthetics, time to operate, and ease and concentrate on bond strength, then Type 3 products as a group seem to perform best.
IMPRESSION MATERIALS
Polyether and polyvinylsiloxane compounds dominate the markets because of their accuracy and their stability. They now come in wilder colours, but more importantly, the newer products possess enhanced hydrophilic properties. These products can now function in moist environments. Other advances in this area are: better mixing systems to minimize the presence of bubbles and heat-activated setting of the product. The compound sets only when placed into the oral cavity, eliminating those situations where the product hardens before the operator is ready to place it on the affected tooth.
METALS
Nothing is really new on this front. New alloys appear regularly, but they remain a mixture of gold, palladium, platinum, silver, copper, zinc, indium, tin, gallium, cobalt, iron, ruthenium, iridium, and rhenium. Platinum, palladium, copper, tin, gallium and iron are used as strengtheners. Platinum and palladium decrease the coefficient of thermal expansion of the alloy. Copper, zinc, and indium increase the coefficient of thermal expansion of the alloy. Zinc is primarily used as an oxygen scavenger to prevent the formation of gas bubbles in the casting. Ruthenium, iridium, and rhenium are grain refiners. For veterinary work, gold alone is much too soft and strengtheners such as palladium are needed. Auto-cure or chemical cure cements are best for bonding metal crowns. Use of dual-cure cements often results in lower strength bonding due to improper curing.(7)
CERAMICS
Ceramics are not used much in veterinary dentistry but things are changing. There is a definite trend in this field: feldspathic porcelains are being replaced by metal free sintered aluminum oxide ceramics. Let’s quickly review ceramics. They are non-metallic compounds comprised of metal oxides, borides, carbides, nitrides or complex mixtures of these. They are very strong, but unfortunately, they are also extremely brittle and will fail after minor flexure. On the good side, they show adequate strength for dental works, they can be finished in a manner that inhibits leakage, they are resistant to corrosion, they do not create allergic or toxic reactions, they have the best esthetics, and they are the most biocompatible of all dental restorative materials. Feldspathic porcelains have a low flexural strength measuring 60 to 70 MPa. Porcelain Fused to Metal (PFM) has a flexural strength of 300 to 500 MPa. The newer products vary in fabrication techniques and in strength. Castable glass-ceramic made of leucite (OPC™, IPS Empress™) or mica (Dicor®) has the best translucency of all but lack flexural strength, ranging from 120 to 180 MPa.
Ceramic core products made of alumina (PJC), slipcast (In-Ceram®), zirconia (In-Ceram® Zirconia), or pressed (Procera™) are more promising for veterinary dentists as they show flexural strengths ranging from 450 to 750 Mpa.(8) The zirconia-based compounds now even reach 800 MPa, becoming comparable to softer metal alloys. The previous products require laboratory help, the following two are ceramic machining systems that allow the operator to create the crown in office. Celay® is a system that uses a replica of the crown to machine the restoration. CEREC® is a computer driven system that uses a tri dimensional picture to precision machine the restoration. Both systems start with a block of ceramic material made of glass ceramic, feldspar-based ceramic, or even the stronger zirconia ceramic. The last two systems do not offer crowns as translucent as laboratory made ones, but they cut down the number of preparation steps and the time of fabrication considerably. The other advantages are that the product is as strong as the strongest ceramic, the fit is as tight as it would be coming back from a laboratory, and the amount of tooth reduction necessary is less that what is needed for a PFM restoration. Use of composite resin cements on all-ceramic restorations improves their success rate. It is recommended to acid-etch the ceramic followed by a silane treatment before bonding it with dual-cure cement.
REFERENCES
1. Phillips RW. Skinner’s science of dental materials, 9th ed. Philadelphia: WB Saunders Co, 1991; 221-232.
2. Miller M. Product evaluation: Bisfil 2B. J Am Acad of Cosm Dent 1992; 7: 13-15.
3. Suh BI, et al. The effect of the pulse-delay cure technique on residual strain in composite. Comp Cont Ed in Dent 1999; 20: 2-12.
4. Rueggeberg F. Contemporary issues in photo curing. Comp Cont Ed in Dent 1999; 20:S4-S15.
5. Suh BI. Controlling and understanding the polymerization shrinkage-induced stresses in light cured composites. Comp Cont Ed 1999; 20: S34-S41.
6. Clinical Research Associates. Enamel-dentin adhesives, self-etching primers (SEP). CRA Newsletter 2000; 24, 11: 1-3.
7. Mc Comb D. Adhesive Luting cements—classes, criteria, and usage. Comp Cont Ed in Dent 1996; 17: 759-773.
8. Giordano RA. Dental ceramic restorative systems. Comp Cont Ed in Dent 1996; 17: 779-794.