By Randy S. Weiner, DMD, FAGD, FACD, FPFA
In Part 1 (March/April issue of Dental Equipment & Materials), we saw that there are many types of materials that can be used as a liner or a base. In this article, we will see that many of these same materials can also be used as a cement or luting agent. In some cases, it just depends on how the material is mixed. Of all the categories of materials that were mentioned in Part 1, only the varnishes cannot be used as a cementing medium.
Cements, as with liners and bases, also reduce microleakage by sealing the interface between the tooth and the restoration. Materials that are used as cements can serve two different purposes — retain a restoration or appliance in a fixed position in the mouth and/or act as a restorative filling material. In this case, it can be used either alone or in conjunction with other filling materials. As such, it is referred to as a base.
Generally, dental cements can be categorized as to the intent of the material. They can be either temporary or permanent cements. Temporary cements can also be referred to as short-term or weak, and permanent cements can also be considered final or hard cements.
In either situation, they should be characterized as having no adverse side effects on dental tissue, have low solubility, and be radiopaque. In addition, some suggest that temporary cement comes in a compact kit (for minimal waste), has controlled dispensing (doesn't continuously flow out of the container), and is easily mixed and cleaned up (both from instruments and restoration margins). In fact, all liners, bases, and cements should have these properties.
Those materials that can be used as a temporary cement include calcium hydroxide (CH), zinc oxide eugenol (ZOE), non-eugenol zinc oxide (non-ZOE), zinc polycarboxylate, and resin. Materials based on zinc oxide eugenol, zinc phosphate, zinc polycarboxylate (ZPC), glass ionomers (GI), or resin can be used as final cements.
Calcium hydroxide (Dycal by Dentsply Caulk and Nu-cap by GC America) is a material that generally is not thought of as a temporary cement. However, it has several advantages — it is easily removed from both resin and porcelain-fused to metal crowns, is slightly harder than ZOE-based products, and will not interfere with the use of resin-based final cements. Clinicians should be careful because if the CH comes in contact with moisture, the setting time is reduced.
Examples of zinc oxide (Type 1) temporary cement include Tempbond (Kerr, pictured at right) and Temrex (Temrex). The advantage of these materials is the effective marginal seal they provide, at least for a short time. According to some, the disadvantage of ZOE-based materials is that they interfere with the effectiveness of resin cements. Should this be true, clinicians should reevaluate whether or not they want to use these products if they are planning to use a resin cement as the final luting medium.
Instead of the ZOE-based cements, one can use the non-eugenol zinc oxide temporary cements. Nogenol (GC America) and RelyX NE (3M ESPE) are representatives of these materials. Non-ZOE products are similar to ZOE-based products except that the eugenol is replaced with carboxylic acids. Generally, the strength of the non-ZOE materials is stronger than the ZOE products. This may make it difficult to remove temporary crowns but it is also better suited for non-ideal preparations.
Zinc phosphate is a material that has a long history as a final cement. Zinc phosphate (ZOP) is a powder-liquid combination that initially is very acidic (pH of 3.5). Usually within one week, the pH rises to about 6.9. Zinc phosphate products (Fleck's Cement by Mizzy and Zinc Cement by Mission White) are exothermic. To reduce the heat produced, the material should be mixed on a cold, dry glass slab with small amounts of powder added at a time. Doing so will allow the operator to incorporate more powder into the liquid. The end result will be an increase in its properties. During cementation, the casting should be held in place under constant pressure until the cement is completely set.
Placement of a varnish over the margin of a restoration that was luted with ZOP will allow the material to mature and also develop an increased resistance to dissolution in the mouth. It is or has been taught that a varnish should be placed on the tooth prior to cementation with ZOP for pulpal protection, but doing so may reduce retention because ZOP adheres to the tooth structure mechanically, not chemically.
When using zinc phosphate for cementation of orthodontic brackets, the consistency of the mixture should be between that of a luting agent and a base.
Compared to zinc phosphate, zinc polycarboxylate (Livcarbo by GC America and PreservePlus by Healex) has more of a chemical adhesion to the tooth than mechanical. With zinc polycarboxylate (ZPC), the weak link is between the metal and the ZPC, whereas with ZOP, the weak link is at the ZOP/tooth junction.
This material has more of a rubbery consistency and, therefore, care should be taken when removing it from the margins. It is best if the ZPC is fully set, otherwise it may tear away from under the restoration and cause a gap to form. Once the ZPC appears dull, not glossy, the mixture should be discarded as it has started to set. Although not as exothermic as ZOP, a cold glass slab will allow the working time to be extended.
A very popular material used for cementation is the glass ionomer (GI). They are available in both powder-liquid varieties and in premeasured capsules. Examples include Fuji Plus (GC America) and CX-Plus Glass Ionomer Cement (Shofu).
To increase the chance of a successful cementation when using GIs, it is best if the tooth is clean and dry, and the consistency of the mixture allows a thorough covering of the tooth surface and does not interfere with seating of the casting. This is true for all cements. Using a glass ionomer cement with orthodontic appliances is ideal because they will minimize the decalcification of enamel due to the fluoride content. As mentioned in Part 1, glass ionomers are moisture-sensitive. Therefore, extreme care should be taken to prevent moisture (saliva) from contacting the GI cement until after it is fully set. A varnish, such as Fuji Varnish (GC America), a glass ionomer sealant, can be applied to the margin. Doing so will allow the cement to mature while being protected from moisture.
In contrast to the use of a varnish to seal GI cements, a study by Irie and Suzuki showed that the bond strength of GIs increased after coming in contact with water prior to polishing the margin. They also concluded that marginal seal was improved due to hygroscopic expansion.
The most recent cements available to the clinician are resin-based. They are marketed for both metallic and nonmetallic restorations. When used in esthetic situations, clinicians must consider the finished surface at the margin — the smaller the particle size, the greater the polishability of the cement. A rough surface will tend to stain more easily and may also cause gingival irritation.
Resin cements can be either self-cured (Parapost cement by Coltene-Whaledent), light-cured (RelyX Veneer cement by 3M ESPE), or a combination of both (Linkmax by GC America, see below). The light-cured varieties are not suited for metallic crowns, since the light does not pass through the metal to the cement. Some manufacturers suggest light-curing the extra cement at the margins to hold the restoration in place, while the remainder of the cement sets. Resin cements are best suited for nonideal preparations because of the high compressive and tensile strengths that they have.
There are a number of practicing dentists who have many years of experience with the older materials. These dentists may be asking, "Why is there a need to change materials?" or "Why have new ones when the older ones work just fine?" If something works, don't fix it. To back this philosophy, Jokstad and Mjor showed with a 10-year study that the prognosis of abutment teeth restored with fixed prostheses are just as good when an older type cement is used compared to a new one.
For many dentists, post-cementation sensitivity is a problem. There are two procedures that are recommended in an attempt to prevent this. The first, according to Leinfelder, is the placement of a hybridization material on any exposed dentin prior to cementation. He also writes that the application of excessive pressure during cementation causes the odontoblastic processes to deform. Leinfelder suggests placing the casting in position using a gentle force along with vibration or oscillation until it is seated. Wilson concluded that the amount of force required to seat a casting is cement-specific, and that a venting procedure is effective in reducing the amount of force needed to seat the casting.
Summary
As with liners and bases, there are several different types of cements available to the clinician. Within each category, there are a number of products. The makers of these products each claim that their product is superior to all the others. It is therefore the responsibility of the clinician to stay abreast of current research on this topic.
Editor's Note: References available upon request.
Dr. Weiner received his DMD degree from Tufts University in 1986. He is a Fellow of the Academy of General Dentistry, the American College of Dentists, and the Pierre Fauchard Academy. He has written many articles and presented numerous lectures on the topic of liners, bases, and cements. He maintains a private practice on family and cosmetic dentistry in Millis, Mass. He can be reached by e-mail at [email protected].
