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
a. History. 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.
b. Clinical Uses. Zinc phosphate cement is used both as an intermediate base and as a cementing medium.
(1) Intermediate base. A thick mix of zinc phosphate cement is used as an intermediate base beneath a permanent metallic restoration. This layer of cement protects the pulp from sudden temperature changes that may be transmitted by the metallic restoration.
(2) Cementing medium. Zinc phosphate cement is used to permanently cement crowns, inlays, and fixed partial dentures upon the remaining tooth structure. It is also used to hold splints, orthodontic appliances, and other appliances in place. This cement is used to cement facings to fixed partial dentures and certain types of artificial teeth to artificial denture bases. A creamy mix of cement is used to seat the restoration or appliance completely into place. The cementing medium does not cement two objects together. Instead, the cement holds the objects together by mechanical interlocking, filling the space between the irregularities of the tooth preparation and the cemented restoration.
c. Chemical Composition.
(1) Powder. The primary ingredients of zinc phosphate cement powder are zinc oxide and magnesium oxide.
(2) Liquid. The liquid used with the powder is phosphoric acid and water in the ratio of two parts acid to one part water. The solution may also contain aluminum phosphate and zinc phosphate. The water content of the liquid is critical and must be carefully controlled by the manufacturer to provide a satisfactory setting time. Liquids exposed in open bottles will absorb moisture from the air in high humidity. The liquids will lose moisture if humidity is low. Water gain hastens setting; water loss lengthens setting time. Liquid that has been left unstoppered for a long period, or is discolored, or is the last 25 percent portion remaining in the bottle should be discarded. Since the manufacture of zinc phosphate cement is a carefully controlled process, satisfactory results can seldom be achieved by mixing the powder of one brand of cement with the liquid of another.
PROPERTIES OF ZINC PHOSPHATE CEMENT
a. Advantages. Some advantages of zinc phosphate cement as a cementing medium are:
(1) Inconspicuous appearance.
(2) Speed and ease of usage.
(3) Sufficient flow to form a thin layer for the cementing of closely adapted crowns, fixed partial dentures, and inlays.
(4) Low thermal conductivity beneath a metallic restoration.
b. Disadvantages. Some disadvantages of zinc phosphate cement as a cementing medium are:
(1) Low crushing strength that varies between 12,000 and 19,000 psi.
(2) Slight solubility in mouth fluids.
(3) Opaque material not suitable for visible surfaces.
c. 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
a. 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.
b. 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.
c. 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.
(1) Lower the temperature of the glass mixing slab to between 65° and 75° F (18° to 24° C), if the glass mixing slab is not already cooled below the temperature at which moisture will condense on it.
(2) Blend the powder slowly.
(3) Mix the powder over a large area of the cool slab.
(4) Use a longer mixing time, within optimum limits.
PREPARATION AND USAGE OF ZINC PHOSPHATE CEMENT
a. 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.
b. 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.
c. 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.
d. 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.
e. Precautions. The following precautions should be observed.
(1) Prevent loss or gain of moisture in liquid cement by keeping bottles tightly stoppered.
(2) Dispense drops only when ready to mix.
(3) Use a cool, dry glass slab (65° to 75° F).
(4) Use the same brand of powder and liquid.
(5) Add increments of powder slowly.
(6) Use the maximum amount of powder to obtain the desired consistency. (To incorporate the most powder, the material should be mixed with a moderate circular motion over a large area of the slab, turning the spatula often.)
CHARACTERISTICS OF POLYCARBOXYLATE CEMENT
a. General. 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.
b. Clinical Uses. Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium.
c. Chemical Composition.
(1) Powder. The composition of polycarboxylate cement powder may vary slightly depending on manufacturers. It generally contains zinc oxide, 1 to 5 percent magnesium oxide, and 10 to 40 percent aluminum oxide or other reinforcing fillers. A small percentage of fluoride may be included.
(2) Liquid. Polycarboxylate cement liquid is approximately a 40 percent aqueous solution of polyacrylic acid copolymer with other organic acids such as itaconic acid. Due to its high molecular weight, the solution is rather thick (viscous).
d. Properties. The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength.
e. 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
a. 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.
b. 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.
c. 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.
d. 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.
e. Precautions. The following precautions should be observed.
(1) The interior of restorations and tooth surfaces must be free of saliva.
(2) The mix should be used while it is still glossy, before the onset of cobwebbing.
(3) The powder and liquid should be stored in stoppered containers under cool conditions. Loss of moisture from the liquid will lead to thickening.
CHARACTERISTICS OF GLASS IONOMER CEMENT
a. General. 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.
b. 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.
c. Chemical Composition.
(1) Powder. The composition of glass ionomer cement powder may vary slightly depending on the manufacturer. It generally contains a mixture of aluminosilicate glass with dry polymaleic acid.
(2) Liquid. Glass ionomer cement liquid consists of an aqueous solution containing an accelerator. (A chemical accelerator shortens the setting time.)
d. 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.
e. 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
a. 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.
b. 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.
c. 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).
d. 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.
e. Precautions. The following precautions should be observed.
(1) Do not insert glass ionomer cement as a ball of material into deep cavities or where the dentin is thin or where there is danger of pulpal involvement. In these cases, set in place a calcium hydroxide liner before inserting the glass ionomer cement.
(2) There may be an allergic reaction to the glass ionomer cement, in some cases.
(3) Upon contact with eyes, the powder may cause irritation due to foreign body reaction. Similarly, ingestion of the liquid may cause local irritation.
(4) All enamel, dentin, and metal surfaces must be clean and dry before use of glass ionomer cement.
(5) Do not overfill the crown. Brush a thin coat of glass ionomer cement on the internal crown surface and abutments.
(6) Bottles of liquid or powder should be tightly closed after use to prevent moisture contamination of the powder and evaporation of the liquid
CHARACTERISTICS OF ZINC OXIDE AND EUGENOL
a. General. 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.
b. 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.
c. 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
a. 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.
b. 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.
c. 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.
d. 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.
e. 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.