Luting agents

History

One of the first dental adhesives was conceived by the Mayan Indians to attach semiprecious stones to cavities on the facial surface of anterior teeth. A mixture of bone powder and fruit juice served to etch both the tooth and the stone. Crystals that formed propagated into the etched surface to provide micromechanical retention for the stones.¹ In the 1850s the only existing cement was zinc oxide and eugenol,² mixed using zinc oxide-based powder and eugenol liquid. The cement had an obtunding effect on pulp but its disadvantages, including a high film thickness, have limited its use. Fifty years ago the selection of cement was relatively easy because there was still little to choose from and gold was the predominant indirect restorative material. Zinc phosphate, a combination of zinc oxide powder and phosphoric acid, was one of the oldest (1879) and most widely used cements. It has the advantages of high compressive strength and a thin film thickness of less than 25 microns. It was acceptable for placement of mechanically retentive metal-constructed restorations but its solubility, low strength, minimal adhesion, and poor esthetics limit its use currently. Zinc polycarboxylate was developed by Dennis Smith in 1968 and was one of the first chemically adhesive formulas (adheres primarily to enamel and to a lesser degree to dentin). This cement is formed when zinc oxide powder is mixed with polyacrylic acid. The advantages of zinc polycarboxylate are its kindness to pulp tissue and its ability to bond to tooth structure. Zinc polycarboxylate may plastically deform resulting in failure after a few years.³ It has a short working time and greater solubility than other cements.⁴ Glass ionomer cements are considered hybrids of silicate cements and polycarboxylate cements. They consist of fluoroaluminosilicate glass and a liquid containing polyacrylic acid, itaconic acid, and water. The development of glass-ionomer cements was first announced by Wilson and Kent. All these cements have two main advantages: they are inexpensive and their chemistry and mode of use are well understood. Appropriate use can still provide excellent results. In 1994 resin modified glass ionomers were introduced, formed by replacing part of the polyacrylic acid in conventional glass ionomer cements with hydrophilic methacrylate monomers.⁵ The mechanical properties of all glass ionomers increase with time, which possibly contributes to their clinical success.⁶ They can be used with metal and with high strength core materials. Composite resin cements are modified restorative materials and because of this exhibit high strength, excellent adhesion, and minimal solubility and are esthetically pleasing. These attributes are advantageous in their use with weak esthetic restorations such as glass ceramics and indirect composite resins and to increase the retention of a restoration. They also vary in composition (paste-paste, single paste, or powder-liquid).

Luting agent requirements

A luting agent is dental cement that is used to attach indirect restorations to prepared teeth.⁷ Luting agents may be classified as definitive or provisional. It has been stated that “no available product satisfies the requirements for an ideal luting agent and comprehensive patient care requires several materials . . . . the best choice is not always easy.”⁵ Luting agents are intended to maintain an indirect restoration in place for a specific period of time and fill the space at the interface between a tooth and the restoration. Their basic requirements include: not deleterious to tooth or oral tissues, adequate working time to place the restoration, enough flow to seat the restoration completely, able to resist functional forces, radiopacity, and should be insoluble to maintain an intact seal.⁸

Classification

Most cements are created by combining a powder that can release cations into an acid solution (base) and a liquid (acid). The resulting cations react with acid anions to form a salt. These materials are categorized as AB (acid-base) cements. Resins are formed by the polymerization of macromolecules.⁹ Cements have been categorized based on their main ingredient (i.e., zinc phosphate, zinc silicophosphate, zinc oxide-eugenol, zinc polyacrylate, glass-ionomer, and resin),¹⁰ or by matrix type (i.e., phosphate, phenolate, polycarboxylate, resin, and resin-modified glass-ionomer).¹¹ Cements can be considered active or passive. Active cements such as compomers and composite resin cements involve an interaction with dentin by forming a hybrid layer with the dental material. Cements that mechanically interlock with rough surfaces on tooth structure and the internal aspect of the restoration are considered passive. Although glass ionomers form intermediate layers on dentin they do not bond to materials etched by hydrofluoric acid or treated with silane and are considered passive.

Definitive (Nonprovisional) luting agents

Glass-ionomer cements

Glass-ionomer cements (glass polyalkenoate cement). The term glass-ionomer is considered generic and includes a larger group of cements with similar compositions⁹ that have been very popular due to ease of use, good flow properties, adhesion to tooth structure and base metals, fluoride release, sufficient strength, and moderate cost. Its primary indication is for metal and porcelain fused to metal restorations. The setting reaction for glass ionomers cements is an acid-base reaction with the calcium aluminosilicate (sometimes replaced by strontium or lanthanum) glass reacting with polyalkenoic acids, polyacrylic acid, itaconic acid or maleic acid to develop a hydrogel. The material experiences a sudden set but continues to mature over several months prior to completion.⁹ The length of time necessary for a complete set of glass ionomers and its moderate modulus of elasticity may limit its use to single units and fixed partial dentures with a limited span. Post cementation is also not recommended as any vibration would diminish its mechanical retention.¹² To reduce potential postoperative sensitivity, the use of a resin-based sealer, which also enhances retention, has been recommended.¹³ Contamination by saliva should be avoided for several minutes to prevent loss of material by erosion caused by early solubility. However, the newer glass ionomer luting cements are fast setting and after 5 minutes are resistant to any water challenge.¹⁴ The bond to tooth structure is significantly reduced when the tooth is excessively dried, which also contributes to postcementation thermal sensitivity.¹⁵

Resin-modified glass-ionomer

Resin-modified glass ionomer (resin-modified glass polyalkenoate) was developed in the 1980s to increase the physical properties and minimize water dissolution of glass ionomers. To accomplish this, water soluble or polymerizable resins were added to regular glass ionomers to create a new classification, resin modified glass ionomer cement (RMGI). These materials consist of hydroxymethyl methacrylate and ethylene glycol dimethacrylate and glycidyl methacrylate and Bisphenol A epoxy (Bis-GMA). RMGIs are considered dual hybrids because the setting is both a glass-ionomer reaction and chemical or light activation of the resin component. The initial setting reaction is caused by polymerization of the resin followed by the acid-base reaction that forms a polysalt hydrogel matrix that strengthens the previously formed polymer matrix.⁹ The change in composition makes RMGI less susceptible to early erosion during setting, less soluble, and creates higher compressive and tensile strengths than unmodified glass-ionomer luting cement. In addition, when mixed properly and applied to moist dentin little postcementation sensitivity will result.¹⁵

Resin luting agents

Resin luting agents