Dental cements are generally low strength materials prepared by mixing a powder with a liquid. These cements vary in their chemical composition, properties, and uses. Dental cements have lower heat conductivity than do metallic restorative materials. Dental cements, however, have the disadvantages of relatively low strength, varying degrees of solubility in mouth fluids, and setting shrinkage. As a group, they are more natural in appearance and are easier and faster to use. Although they are widely used in restorative dentistry, dental cements are considered to be among the least permanent of restorative materials. Four types of cement used in dentistry are zinc phosphate cement, polycarboxylate cement, glass ionomer cement, and zinc oxide and eugenol cement.
CHARACTERISTICS OF ZINC PHOSPHATE CEMENT
More than 100 years ago, a French architect proposed the use of zinc oxide as a stopping medium for carious teeth. Zinc phosphate cement has progressively advanced from the original wall plaster that induced its development over a century ago.
Clinical Uses. Zinc phosphate cement is used both as an intermediate base and as a cementing medium.
PROPERTIES OF ZINC PHOSPHATE CEMENT
Advantages. Some advantages of zinc phosphate cement as a cementing medium are:
Disadvantages. Some disadvantages of zinc phosphate cement as a cementing medium are:
Strength. The ratio of powder to liquid increases the strength of phosphate cements to a certain point. For this reason, the dental specialist must use as thick a mix as practical for the work being performed.
SETTING REACTIONS OF ZINC PHOSPHATE CEMENT
Chemical Reaction. The chemical reaction that takes place between the powder and liquid of setting phosphate cement produces heat. The amount of heat produced depends upon the rate of reaction, the size of the mix, and the amount of heat extracted by the mixing slab.
Powder to Liquid Ratio. The less powder used in ratio to the liquid, the longer the cement will take to harden. Good technique minimizes the rise in temperature and acidity of the setting cement that can injure the pulp. Generally, for increased strength, decreased shrinkage, and resistance to solubility, it is advisable to blend as much powder as possible to reach the desired consistencies.
Setting Time. The setting time of zinc phosphate cement is normally between 5 and 9 minutes. Four actions that may be taken to maintain and prolong the normal setting time are given below.
PREPARATION AND USE OF ZINC PHOSPHATE CEMENT
Equipment. The equipment required for mixing zinc phosphate cement consists of a glass mixing slab, a stainless steel spatula, and a matched set of powder and liquid. See figure 1-4.
Figure 1-4. Setup for mixing zinc phosphate cement.
Powder and Liquid. The first step in preparing a mix of zinc phosphate cement is to measure the desired amount of liquid and powder onto the surface of a clean, cool, dry, glass-mixing slab. The amount of each ingredient depends upon the amount and the consistency desired. Experience gained in usage of desired consistencies enables the dental specialist to estimate accurately the amount of powder used according to the number of drops of liquid dispensed. The estimated powder is placed on one end of the slab. The powder is divided into quarters. Then, the first quarter (only) is divided in half (into eighths), and the eighth portion (only) is further divided in half (into sixteenths). When the process is completed, a total of six portions of powder is readied for mixing. See figure 1-5. An additional small amount of powder is often placed on the corner of the slab for use if the estimated powder is insufficient for the desired mix. The liquid is dispensed with the dropper supplied by the manufacturer. The required number of drops of liquid is dispensed from the dropper bottle in accordance with the manufacturer's instructions. The drops are dispensed over a wide area. The close estimation of powder, according to type of mix and number of drops, and the small increments will enhance the slow blending of powder. This slows the setting reaction and enables the user to blend the maximum amount of powder to attain desired consistency. This will help obtain a cement with optimum physical properties.
Figure 1-5. The division of powder into standard portions.
Mixing. Mixing is done by the slow blending of the segments one at a time. This procedure aids in neutralizing the acid and achieving a smooth consistency. A considerable portion of the slab is used. Mixing is done with a moderate circular motion of the spatula blade held flat against the slab. The spatula should be rotated occasionally to blend the material that collects on the top of the blade. A good rule is to spatulate the first three segments for about 15 seconds. (See figure 1-5.) The next two segments should be spatulated for about 20 seconds. The final segment should be spatulated for 30 to 35 seconds. If this is done, the mixing time is not critical and completion of the mix will take about 1 1/2 minutes. It is important to reach the desired consistency by using more powder and not to allow a thinner mix to stiffen by crystallization.
Characteristics of Completed Mixes. When a mix is ready for use, it should be similar to the consistency of melted ice cream or liquid glue (adhesive rubber). When the spatula is placed on the slab and the spatula is raised to one inch, the mix will cling to the spatula in a thin thread (peak) for one or two seconds before it breaks and then gradually spreads.
Precautions. The following precautions should be observed.
CHARACTERISTICS OF POLYCARBOXYLATE CEMENT
The primary use of polycarboxylate cement is as a cementing medium of cast alloy and porcelain restorations. In addition, it can be used as a cavity liner, as a base under metallic restorations, or as a temporary restorative material.
Clinical Uses. Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium.
Properties. The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength.
Setting Reactions. Unlike zinc phosphate cement, the setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced. The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix.
PREPARATION AND USAGE OF POLYCARBOXYLATE CEMENT
Equipment. The equipment required for mixing polycarboxylate cement consists of a nonporous, polymer paper pad, a glass mixing slab, a stainless steel spatula, and a matched set of powder and liquid. See figure 1-6.
Powder and Liquid. Dispense the powder and liquid according to manufacturer's instructions (to achieve the desired consistency). Do not predispense and allow to sit. Loss of moisture will cause the liquid to thicken.
Mixing. Mixing is done by rapidly blending the powder and the liquid for 30 seconds on a polymer paper-mixing pad. Ensure that all the powder is incorporated into the mix. If extended working time is desired, mix on a cooled glass slab.
Figure 1-6. Setup for mixing polycarboxylate cement.
Characteristics of a Completed Mix. The correct cementing mix is more viscous than zinc phosphate cement. Because of its composition, the cementing mix flows adequately under pressure.
Precautions. The following precautions should be observed.
CHARACTERISTICS OF GLASS IONOMER CEMENT
The primary use of glass ionomer cement is for permanent cementing of inlays, crowns, bridges, and/or orthodontic band/brackets. In addition, it can be used as a cavity liner and as a base.
Clinical Uses. Glass ionomer cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium.
Properties. Glass ionomer cement is free from phosphoric acid and has very low solubility. It adheres chemically to enamel and dentin and, readily, to wet tooth structure, leaving minimal film thickness. It is well tolerated by the pulp and remains rigid under a load, exhibiting no creep. Glass ionomer possesses high compressive strength. It releases fluoride ions to tooth structure. It is simple to proportion, mix, apply, and clean up.
Setting Reactions. For glass ionomer cement as for other dental cements, the working time is reduced if a higher powder to liquid ratio has been used. Higher temperature shortens working time and lower temperature extends working time. Glass ionomer cement should always have a glossy appearance. When the surface becomes dull, the setting reaction has started, and the mix should be discarded. Exceeding the working time will result in loss of adhesion to enamel and dentin.
PREPARATION AND USAGE OF GLASS IONOMER CEMENT
Equipment. The equipment required for mixing glass ionomer cement consists of a polymer paper pad or a glass mixing slab, a stainless steel spatula, and a matched set of powder and liquid.
Powder and Liquid. The normal ratio is one level scoop of powder to two drops of liquid. For cement bases, more powder may be added to the mix to achieve a thicker consistency. For accurate proportioning, shake the powder bottle to fluff the powder. Then, fill the scoop that comes with the bottle of powder without packing the powder down. When removing the scoop from the bottle, slide it against the plastic lip in the neck of the bottle to scrape off excess powder. Be sure to hold the bottle of liquid vertically when dispensing drops of liquid so as to produce precise and uniform drops. See figure 1-7.
Mixing. All powder is incorporated into the liquid in two or three large increments. Each portion of the powder should be added to the liquid all at once. To extend the working time, mix the powder and the liquid on a cold and dry glass slab. At room temperature, the mix should be completed in about one minute (60 seconds).
Characteristics of a Completed Mix. The right consistency is obtained when the material breaks away from the spatula when it is raised one-half inch from the glass slab.
Figure 1-7. Dispensing uniform drops of glass ionomer liquid.
Precautions. The following precautions should be observed.
CHARACTERISTICS OF ZINC OXIDE AND EUGENOL
This material is used for many dental purposes ranging from temporary restorative material to pulp capping. The material is composed of a powder that is basically zinc oxide and a liquid that is called eugenol. Cavitec, a commercial preparation, is an example of zinc oxide and eugenol. Generally, however, a generic form is used in military dental clinics.
Chemical Composition. By National Bureau of Standards specifications, the powder must contain between 70 and 100 percent zinc oxide. The manufacturer may add hydrogenated resins to increase strength and zinc acetate to hasten the set. Eugenol is usually derived from oil of cloves. The oil of cloves contains more eugenol (82 percent) than do the oils of bay, orange, or cinnamon. Eugenol is an obtundent (pain-relieving agent). It is a clear liquid that gradually changes to amber when exposed to light.
Physical Properties. This material relieves pain, makes tissue less sensitive to pain, is slightly antiseptic, and is low in thermal conductivity. It provides a good marginal seal when placed in tooth cavities. The crushing strength (compression strength) of pure zinc oxide and eugenol is about 2,000 psi, which is low in comparison to other cements. The addition of hydrogenated resin increases the crushing strength to 5,000 psi.
CLINICAL USES OF ZINC OXIDE AND EUGENOL
Treatment Restoration. The most frequent use of zinc oxide and eugenol is as a treatment restoration. It helps prevent pulpal irritation when set in place for treatment of fractured teeth, lost restorations, advanced caries, or pulpitis. This dental material also exerts a palliative effect on the pulp.
Temporary Cementing Medium. Zinc oxide and eugenol is used as a temporary cementing medium for crowns, inlays, and fixed partial dentures. These fixed appliances may later be permanently cemented with zinc phosphate cement.
Intermediate Base. Zinc oxide and eugenol is used as an intermediate base. This material provides insulation between metallic restorations and vital tooth structure. Because of the low crushing strength, its use is sometimes contraindicated. The dental officer will often require that a zinc phosphate cement base be placed over the zinc oxide and eugenol to better support a metallic restoration.
Pulp Capping. This material is used in pulp capping for near and direct exposures of the pulp, but this use is declining. Calcium hydroxide is now preferred for pulp capping.
Surgical Packing or Dressing. This material is used as a surgical packing or dressing after certain periodontal surgical procedures. An example of this is the surgical dressing applied and adapted over the gingival area after a gingivectomy. This dressing protects the area and makes the tissue less sensitive.
|David L. Heiserman, Editor||
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Revised: June 06, 2015