Zinc phosphate

Zinc phosphate is the oldest luting cement (introduced in the 1800s), and has been used with a high degree of success for metal, metal-ceramic, and porcelain restorations; it is the standard to which other cements are compared. It is the classical AB cement, being supplied as a separate powder/liquid system—the powder approximately 90% zinc oxide (ZnO) and the liquid approximately 67% buffered phosphoric acid. Aluminum (1%–3%) in the liquid is needed for the cement-forming reaction, and water (∼33%) partially controls the reaction rate. The liquid bottle should remain closed unless dispensing to prevent water loss by evaporation, and batches of powder and liquid are matched by the manufacturer, so items should not be interchanged. Many modifications have been tried with no significant improvement in properties ; silicate was added to provide a more translucent material for luting porcelain jacket crowns .

Zinc phosphate should be mixed on a cool, dry, glass slab to slow the exothermic reaction, allowing maximum powder to be brought into the mix while controlling the viscosity. Powder should be incorporated into the liquid over 60 to 90 seconds in several small increments, by spreading the mix over a broad area with a metal spatula. The correct mix consistency for optimal strength and to allow complete seating of the restoration is important—strength is linear to powder/liquid ratio, but viscosity also increases . It should be fluid, yet string about 2 to 3 cm when lifting the spatula from the mix . The restoration should be seated within 3 to 5 minutes with firm, steady pressure, which should be maintained several minutes until the initial set has occurred .

Zinc phosphate functions by nonadhesive bonding, quickly reaching maximal physical properties within 24 hours. Compressive strength is very high, tensile strength low compared with other available cements. The set material is brittle and stiff, having a high elastic modulus. Early solubility is high, but falls rapidly as the cement ages, yet can be significant, especially in an acid environment . Minimal exposure to oral fluids is necessary (ie, well-fitting restoration margins are required) and caution is recommended for use in patients who have a very acidic diet, or who have acid reflux problems. At cost per unit dose, zinc phosphate is the least expensive luting agent.

The pH of zinc phosphate is very low (less than 4) at 1 hour after delivery, but reaches neutrality by 48 hours. Its use in not recommended for deep preparations, or if pulpal irritation is a concern (because of low pH and hydraulic seating pressure). Some have recommended use of a cavity varnish or calcium hydroxide liquid over the preparation before cementation, if less than 1 mm of dentin remains between the pulp and cement . Use of a resin-based sealer is not recommended because of a marked reduction in retention . Because of early strength and acceptable physical properties, extremely low cost, and lack of technique sensitivity, zinc phosphate remains a good clinical choice for luting metal, well-fitting metal-ceramic restorations, long-span fixed partial dentures, and cast dowel (post) cores .

Polycarboxylate

Zinc polycarboxylate cement was introduced in 1968 by Smith as the first luting cement that would adhere to tooth structure. As a hybrid of zinc phosphate, the AB cement powder is mostly zinc oxide (10% magnesium oxide) and the liquid a 30% to 43% solution of high molecular weight poly(alkenoic acids). When hand-mixed in correct proportions, the mix is somewhat thicker than zinc phosphate because of the viscous nature of the organic acids, but it is pseudo-plastic, and flows readily upon seating of the restoration because of shear thinning . The cement should appear glossy when used; if dull, it may be too thick to allow proper seating of the restoration . Working time (4–6 minutes) can be prolonged by cooling the mixing slab, and seating of the restoration should be with prolonged, firm pressure, as with zinc phosphate.

Polycarboxylate cement reaches early end strength, but values cited in the literature are quite variable, depending on testing conditions and parameters . In general, the compressive strength for polycarboxylate is one half to two thirds that of zinc phosphate, and the tensile strength one third more. The modulus of elasticity (stiffness) is approximately one third of that for zinc phosphate, giving a material that can display significant plastic deformation upon loading . For that reason, it is not recommended for long-span fixed partial dentures, or where subjected to high functional stress . Many feel that cement modulus of elasticity should match that of dentin; therefore zinc phosphate and polycarboxylate deviate from ideal by being too stiff and too flexible respectively. Solubility is comparable to zinc phosphate, with acidic conditions greatly increasing the erosion of the cement .

Two desirable properties of this luting agent are a degree of adhesion to the preparation and favorable biocompatibility. Chemical adhesion to tooth occurs through interaction of free carboxylic groups to calcium , so bonding is best to enamel, and requires a clean, uncontaminated surface . Adhesion to tooth should not be an excuse to overlook primary retention and resistance factors in the preparation and the importance of a well-fitting restoration, because interfacial adhesive failures can occur at the cement-metal interface . Although more acidic than zinc phosphate when mixed, polycarboxylate pH rises rapidly, and penetration of the large organic acid molecules into dentin tubules is minimal . Therefore the pulpal response is mild; biocompatibility is considered to be excellent . Encapsulated polycarboxylate (Duralon, 3M ESPE, St. Paul, Minnesota) is now available, which simplifies mixing while improving consistency.

Glass-ionomer (glass polyalkenoate)

Glass-ionomer cement, introduced in 1969 by Wilson and Kent, was originally know as ASPA (aluminosilicate polyacrylic acid) based on the main constituents of the AB cement. It was developed from the desire to have a luting agent with the fluoride release/translucency of dental silicate cement and the adhesion to tooth of polycarboxylate cement. The International Standards Organization officially uses the name “glass polyalkenoate cement,” with the term “glass-ionomer” considered as generic, and covering a larger group of cements with similar compositions .

The powder is usually a calcium aluminosilicate glass (some mixtures replace calcium with strontium or lanthanum), and contains fluoride to help control cement formation and modify properties. The liquid is dilute poly(alkenoic acids)(poly(acrylic acid), itaconic acid, maleic acid, plus other minor organic acids, although the acid components may be dried and combined with the powdered glass, to be later mixed with water or dilute tartaric acid solution . Dispensing should be exactly to manufacturer instructions, because too much powder will reduce the working time while increasing viscosity; too little powder will significantly reduce physical properties. Most manufacturers offer encapsulated ingredients for machine mixing, which simplifies and expedites mixing. Mount recommended periodically checking the quality of glass-ionomer mixing by using a loss-of-gloss technique to insure consistent, predictable results. Typically, mixing time is ten seconds at 3000 cycles/min; either too long or too short of a mixing cycle can affect working time and physical properties markedly .

Working time for glass-ionomer cement is shorter than that for zinc phosphate or polycarboxylate (∼2–3 1/2 min.). The material should have a glossy surface when the restoration is placed, and should flow easily to allow complete seating without the firm, sustained pressure required for the previously discussed luting agents. A dull finish on the surface of the excess occurs rapidly with the material achieving a sudden “snap set” . Because of the snap set, quickness must be exercised to insure complete seating for all restorations. Prior to cementation, the tooth surface should be clean and dry but not dehydrated, with the smear layer retained. To reduce potential postoperative sensitivity, the use of a resin-based sealer, which also enhances retention, has been recommended .

When to remove the excess and how to expose the glass-ionomer to the oral environment has been somewhat confusing. Water balance is important in the newly placed cement, because it contains water and releases water during the setting process. As with any AB cement, contamination by saliva must be avoided for several minutes to prevent loss of material by erosion caused by early solubility. Wilson and Nicholson recommended temporary protection with a varnish after bulk removal, because some of the ions are still in soluble form while the matrix is forming. Curtis and colleagues found that leaving excess glass-ionomer cement expressed during restoration seating undisturbed for 10 minutes prevents any significant erosion in a wet field; in contrast, keeping the exposed cement dry for too long risks possible dehydration and microcracking. Applying petroleum jelly to the exposed glass-ionomer cement margin after bulk removal has been suggested as a simple solution to maintain water balance . Mount discussed this dilemma, and commented that newer glass-ionomer luting cements are fast-setting and have relatively high resistance to water within 5 minutes, so that it is unnecessary to use a waterproof varnish or resin sealer to cover the exposed cement, as previously recommended. Dehydration remains a problem, so isolation from the oral environment for longer than 10 minutes is not recommended.

Although glass-ionomer luting cement has a snap set, the chemistry of the entire setting reaction is quite complex, and has been divided into at least four overlapping stages, which begin at mixing and take several months to reach completion . Patients should limit heavy functional stress on restorations luted with glass-ionomer cements for several days to allow physical properties of the cement to fully develop. Laboratory tests have reported compressive strength and crown retention results higher than zinc phosphate; microleakage studies give variable comparisons, and antibacterial properties are considered slightly better . Shelf life may be an issue, because viscosity has been shown to increase after 24 months . The modulus of elasticity for glass-ionomer is less than that for zinc phosphate, and may be a concern for usage with multiple abutment or long-span, fixed partial dentures, or in areas of excessive masticatory stress .

Chemical adhesion to tooth structure by chelation with calcium and phosphate ions in dentin and enamel, good translucency, and slow, long-term fluoride-release–enhancing cariostatic potential are all factors that have made glass-ionomer an extremely popular definitive luting agent . Fluoride release has been shown to be pH-dependent (being greater at lower pH values), plus glass-ionomer cement displays fluoride uptake (fluoride recharge) when exposed to topical fluoride. Selection of this material as a luting agent may be an important issue for the patient who has high caries potential .

Early concerns and reports of post-cementation sensitivity when using glass-ionomer cements have largely been dismissed as being multifactorial in origin , although studies concerning pulpal reactions to these materials are highly variable in results . As with all AB cements, the dentist should take care to insure that at least 1 mm of sound dentin surrounds the pulp for any preparation, avoid desiccation and bacterial contamination, and use proper cementation technique to optimize pulpal health .