Dental Cements: An Update
Derek W. Jones, PhD, FIM, C.Chem. FRSC(UK), FBSE
The proliferation of cement materials on the market in the past few years can be confusing for the practicing dentist. You point out that you may have as many as 10 different types of cement in your dental office. Perhaps, this is as it should be, as no one material has all of the properties required for every clinical application.
Three International Standards Organization (ISO) classifications have been specified for the use of dental cements: I) luting applications; II) restorative applications; and III) liner or base applications. Some types of cements are supplied in more than one of these three classifications. Selection of cements for particular applications requires knowledge of the chemistry and physical properties of the particular cement type.
The following are examples of the use of various types of cement.
Cement Systems Principally Used For Luting Comprise: Zinc Phosphate, Zinc
Polycarboxylate, Polyalkenoate (Glass Ionomer) Resin Cement, (especially orthodontic
appliances) Resin Filled cement, Resin Ionomer Cement (resin modified ionomer).
Table I lists the names of some commonly used dental cements.
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Only those cements which make use of polyacrylic acid, such as zinc
polycarboxylate and glass polyalkenoate (glass ionomer), are capable of providing the
carboxylate groups that have chemical adhesion to tooth structure.
Several cements are now being supplied in capsules containing the preproportioned powder and liquid. This system is very convenient and ensures consistent powder/liquid ratios. However, it does take away the flexibility of being able to vary the consistency of the mixed cement to suit specific applications. It also does not provide the latitude for controlling the amount of mixed cement. In addition, small changes in the time of mechanical mixing with the amalgamator can significantly influence the setting of the cement.
A large number of factors may influence the performance of luting cements. Cementing errors occur due to a range of possible influences such as: incorporating too much or too little powder; premature exposure of the cement to moisture; delay between completion of the mix and seating; room or mixing slab temperature is too high; surface contamination of casting or preparation.
To avoid premature setting of the cement, apply the cement first to the appliance at room temperature and then to the preparation at mouth temperature. The elevated mouth temperature accelerates the setting of the cement. Cementing problems can occur such as: seating difficulty; loosening of appliances; excessive solubility and disintegration; or too short a working time.
One method which can aid in the seating of appliances during cementation, is the use of vibration, as well as stress. Careful attention to the above details are required in order to avoid errors in cementation. Another consideration for larger bridge structures is the question of working time. The zinc phosphate cements have relatively longer setting time especially when mixed on a cool glass slab.
Interestingly, an evaluation of zinc phosphate cements found them to give a lower film thickness at three minutes compared to earlier times of testing at one, 1.5, two, and 2.5 minutes. The simple explanation being, that the particle size of the zinc oxide powder is reduced with time during the reaction with the acid, and the viscosity changes due to the setting reaction. This allows for a thinner film at three minutes.
I always remind my students, with tongue in cheek, that when all else fails, they should read the instructions. It is extremely important to read the instructions for all the dental materials we use. Keep the instructions with the particular batch of material. From time-to-time, manufacturers may make changes to the instructions, thus with each new package you should read the instructions again to be on the safe side. Follow the manufacturers instructions for storage and mixing of the material.
Always dispense the powder first, before the liquid, to minimize the loss of water due to evaporation. Always hold the bottle or vial upright to ensure consistent sized drops when dispensing liquid. Be sure to fluff up the powder in the bottle prior to using the dispensing measuring scoop. Remember, for many cements it can be beneficial to cool the glass slab in the refrigerator prior to mixing, to slow down the reaction rate and increase the working time.
There may not be a simple answer to this question, since it very much depends on the type of dental practice you operate. For the permanent cementation of crowns, glass ionomer cements may be the most popular with polycarboxylate and zinc phosphate placed second and third. For bridges, glass ionomer and zinc phosphate are among the most commonly used. The greater rigidity of these cements and lower creep, is particularly beneficial in long span bridges. Conclusion The dentist of 1998 has an excellent range of dental cements available to apply to a wide range of clinical situations. Dentists can rest assured that the zinc phosphate and zinc oxide-eugenol cements which were in use during dental school days are still valuable and very necessary materials in the armamentarium of a busy general dental practice.
Dr. Jones is professor of biomaterials, at Dalhousie University, Halifax, Nova Scotia. The author has no declared financial interest in any company manufacturing the types of products mentioned in this article.
I invite readers to send me questions about clinical problems experienced in everyday practice. I will seek answers to these questions from recognized Canadian experts. You can send me your questions by e-mail, fax or regular mail. I look forward to hearing from you.