Dental Cements Chapter 13

Dental Cements Few materials in dentistry are used as frequently as dental cements. Dental cements typically have multiple uses. The doctor will choose the type of cement to use according to the procedure and/or the purpose of placement. It is the responsibility of the dental auxiliary to know the particulars and the proper manipulation of each cement.

Uses of Dental Cements Pulpal protection Luting-cementation Restorations Surgical dressings

Pulpal Protection The bacterial effects of caries, the biological response to chemicals contained in restorative materials, and even the cutting of tooth structure may cause pulpal irritation. Pulpal irritation can also occur as the result of thermal conductivity of metal restorations placed over or near the pulp. It may also occur when the dentin remaining over the pulp is too thin to withstand compressive, tensile, and shearing stresses.

Cavity Varnish Acts as a protective barrier between preparation and restoration Natural copal or synthetic resins dissolved in a solvent such as alcohol or chloroform Applied in two or three layers to allow evaporation voids to be sealed Not used as often today because they tend to wash out at the margins

Liner/Low-Strength Base Calcium hydroxide is used as a liner/low-strength base in a cavity preparation. It is used when dentin no longer covers the pulp, also known as exposure or direct pulp cap. It stimulates reparative dentin formation. Calcium hydroxide has an alkaline pH between 9 and 11.

High-Strength Base Provides thermal insulation Provides support for restorations Cements used as a base are mixed to a secondary consistency, a thick putty-like consistency. In preparations with an estimated 2 mm or less dentin remaining, a base is often recommended.

Buildup A buildup, much like a high-strength base, provides mechanical support for a restorative material when an excessive amount of tooth structure is removed or missing. Placing a cement buildup reinforces the remaining tooth structure.

Luting Cementation Cements used for permanent or temporary luting of fixed prostheses, orthodontic bands, and pins and posts must have good wetability and flow to provide a thin film thickness. When the tooth structure and fixed prostheses are in intimate contact, a microscopic space exists. This is the tooth-restoration interface.

Luting The primary purpose of luting cement is to fill the interface. Cements mixed to primary consistency must have thin enough viscosity to be able to flow into a film thickness of 0.25 µm or less. If the viscosity of the cement is such that the prosthesis fails to regain intimate contact with the tooth, a thick layer of cement will be exposed at the margin.

Orthodontic Bands and Brackets Ortho bands are retained for months, even years. The cement must adhere tenaciously to the enamel and the orthodontic appliance to provide leverage for tooth movement. Demineralization of the tooth surface caused by the solubility of cement, with resultant leakage of bacteria between the band and the tooth surface, has been problematic. Cements that contain fluoride have helped to minimize the problem.

Restorations Because of their lower strength and wear resistance and higher solubility, cements are not frequently chosen as permanent restorations. The exception is a glass ionomer cement that is used at the cervical portion of a tooth. Provisional and intermediate restorations use dental cements in a secondary consistency, for their sedative effects.

Surgical Dressings As surgical dressings, cements are used to provide protection and support for the surgical site. They provide patient comfort and help control bleeding. They are mixed to a soft, putty-like consistency that hardens when placed over the tissue, forming a rigid covering. They may be chemical- or light-cured.

Properties of Dental Cements Properties differ from one cement to another. No cement is ideal for every situation. The clinician must consider both physical and biological properties when selecting cement for each individual procedure. The most important properties include strength, solubility, viscosity, biocompatibility, retention, esthetics, and manipulation.

Strength Cements are brittle materials with good compressive strength but limited tensile strength. The strongest cements are resin cements. The weakest are zinc oxide eugenols (ZOEs). Most cements combine a powder and liquid dispensed in a specific ratio.

Solubility Cements have a tendency toward dissolving in oral fluids, leading to microleakage. Most cements disintegrate in the oral environment over time. Resin cements are as close as possible to insoluble.

Viscosity The consistency of mixed cement is the measure of its ability to flow under pressure. This is particularly important in the case of cement used for luting because it determines the dentist’s ability to seat the indirect restoration properly. Primary consistency is mixed thin, about the thickness of honey. Secondary consistency is mixed to a putty-like state.

Biocompatibility Many cements are a combination of a powder of zinc oxide or powdered glass and an acid. The pH of the acid both at placement and after complete setting is a matter of concern. Careful attention to powder-to-liquid ratios, dispensing technique, and mixing recommendations can minimize this concern.

Retention Retention of indirect restorations is accomplished by adhesion. Adhesion is the bonding of dissimilar materials by the attractive forces of atoms or molecules. Mechanical adhesion is based on the interlocking of one material with another. Makes the restoration highly retentive and resistant to microleakage

Esthetics Cements are available in a variety of shades and opacities for luting porcelain veneers, ceramic or composite inlays, and porcelain full crowns. A shade is chosen that approximates the color of the restoration. Sometimes a masking shade is used to cover discolorations or densities.

Manipulation It is important that cements be mixed to their appropriate consistency in accordance with manufacturers’ recommendations by measuring ratios with meticulous attention to detail. Cements that are mishandled may lead to difficulties in seating or retaining the restoration or may promote pulp sensitivity. The setting of cements may be initiated by three means: chemical, light-activated, or a combination of chemical and light-activated (dual).

Mixing Cements may be hand-mixed or may come in pre-dosed capsules and syringes. Some cements have been packaged in automixing cartridges. Working time and setting time are considerations in the selection of cement and mixing mechanism. The dental assistant is responsible for delivering the cement at the proper consistency within the appropriate working time.

Loading the Restoration The dental assistant may be responsible for filling the crown with a luting cement before passing it to the dentist. Once mixed, the cement should be gathered in one location with the spatula. Wipe the blade of the spatula against the margin of the restoration. Cover the walls with a thin, even coating of cement.

Removal of Excess Cement Excess cement must be removed from the surface of the restoration or the tooth surface. Some cements may be wiped clean with a 2 × 2 gauze immediately after placement. Others must be fully set before removal. Read ALL instructions completely to use the recommended technique.

Cleanup The removal of cement from instruments before it sets allows easier cleanup. Clean instruments and equipment that come in contact with cement as soon as possible with gauze squares that are saturated in water or alcohol, in keeping with manufacturers’ instructions. Disinfect or sterilize as recommended.

Zinc Oxide Eugenol (ZOE) ZOE cements have been used widely for many years. They are available in powder/liquid and paste/paste systems. ZOE has a neutral pH of 7, so it is friendly to the tissue. ZOE has low strength and high solubility. It acts as a sedative to the pulp.

Zinc Phosphate The oldest dental cement Not widely used today Available in a powder/liquid system Can be mixed only on a cool glass slab pH is an acidic 4.2 Causes pulp irritation Exhibits high solubility

Zinc Polycarboxylate First cements developed with an adhesive bond Used primarily for the final cementation of an indirect restoration Powder/liquid system High viscosity High solubility Low strength Short working time

Glass Ionomer Introduced in 1969 Originally developed for esthetic restoration of anterior teeth (light-cure) Used for permanent luting agents (self-cure) Uses chemical adhesion Has low to moderate strength Includes a fluoride ion Produces postoperative sensitivity

Hybrid Ionomer Cements Similar to glass ionomer Modified with additional resin (light-cure) Improved bond strength, compressive strength, and tensile strength Insoluble Includes a fluoride ion Not recommended for ceramic restoration

Resin-Based Cements Resin cements are basically modified composites used to bond ceramic indirect restorations, conventional crowns, and bridges, and to indirectly bond orthodontic brackets. Light-cured Dual-cured Self-cured

Summary No single cement satisfies all dental purposes. Cements are chosen for their properties in each situation. Proper manipulation of each material can enhance the success of the restoration.