Key Terms

Acid–base reaction Chemical reaction between a compound containing replaceable hydrogen ions (acid) and a substance containing replaceable hydroxide ions (base) that yields a salt and water; for aqueous liquid and powder cements, the liquid is the acid, and the powder is the base.

Atraumatic restorative treatment (ART) Clinical procedure performed without dental burs, air/water spray, or anesthesia that consists of manual excavation of cavitated carious tissues and restoration of the tooth cavity with a fluoride-releasing cement.

Bioactivity Reactive potential of a material to form a layer of hydroxyapatite-like material on its surface in vivo.

Calcium phosphate cements Cement that is used for bone regeneration that consists of powder (di-, tri-, or tetra-calcium phosphates) that is mixed with an aqueous solution from which hydroxyapatite precipitates.

Cavity liner Material to coat the bottom of a cavity preparation to protect the pulp; a liner is applied in a thin layer.

Cement Substance that hardens from a viscous state to a solid union between two surfaces; for dental applications, cements act as a base, liner, filling material, root canal sealer, or adhesive to bind devices and prostheses to tooth structure or to each other.

Cement base Material used to protect the pulp in a cavity preparation by providing thermal insulation and sometimes a medicament, usually thicker and more distant from the pulp than a cavity liner.

Cement thickness Distance between an abutment tooth and cemented prosthesis, influenced by the design of the prosthesis and the viscosity of the cement during seating.

Compomer A term derived from the terms composite and ionomer ; a resin-based composite consisting of a silicate glass filler phase and a methacrylate-based matrix with carboxylic groups; also known as polyacid-modified glass-ionomer cement.

Endodontic sealer Material to prevent any transmittance of fluid or bacteria from the coronal to the apical region, used in conjunction with an obturation material such as gutta-percha.

Film thickness Thickness in μm of a cement 7 min after 150 N have been applied, which is the International Organization for Standardization (ISO) and American Dental Association (ADA) method for determining the ability of a cement to spread under pressure.

Glass-ionomer cement (GIC) Cement that hardens following an acid–base reaction between fluoroaluminosilicate glass powders and an aqueous-based polyacrylic solution; also known as polyalkenoate cement.

Hydraulic cement Cement that requires water to cause the material to set.

Luting agent A viscous cement-like material that fills a gap between two surfaces to prevent dislodgement. The term luting agent originally referred to nonadhesive agents only but now also includes adhesive agents.

Maturation Process of aqueous cement strengthening after setting.

Mineral trioxide aggregate (MTA) A tri- and di-calcium silicate-based cement used for vital pulp therapy and other endodontic indications.

Pulp capping A procedure to treat pulp that has been exposed through caries removal or trauma, by the application of a medicament; indirect pulp capping is the term used when the pulpal exposure is incipient, such as when dentinal tubules are visible in the cavity.

Resin cement A resin-based composite material used for attaching fixed prostheses or orthodontic brackets; these cements are less viscous than resin composite restorative materials.

Resin-modified glass-ionomer cement (hybrid ionomer cement) Cement that combines a glass powder and an aqueous solution of polymerizable groups attached to the polyacrylic acid.

Varnish Material applied to the floor of a cavity; a solution of natural gum, synthetic resins, or resins dissolved in volatile solvent, such as acetone, ether, or chloroform; material applied superficially to a tooth to release fluoride.

Zinc oxide eugenol (ZOE) cement Cement based on the reaction between zinc oxide and eugenol.

Zinc phosphate cement Substance formed by the reaction between zinc oxide powder and phosphoric acid liquid that can be used either as a base or as a luting agent.

Naturally occurring waxes and resins have been used since time immemorial for covering the surface of broken teeth and adhering replacement materials, including bone, animal teeth, wood, and early naturally occurring polymers. However, problems arose with many “natural” materials exposed to the warm, moist, and often acidic oral environment, leading to their gradual failure in vivo. Modern dentistry has evolved to achieve improved clinical results, which include a wide variety of cements created by various chemical reactions. Cements are used for many indications and may be needed for short-term (days to weeks), intermediate-term (weeks to months), and long-term (years) retention of dental appliances (devices).

History of Dental Cements

In the 1800s, zinc oxide (ZnO) powder of good purity was found to react with eugenol at room temperature to form a functional cement. The ZnO–eugenol material was used as an impression material and temporary restoration and was eventually found to be obtundent with the pulp and suitable as a root canal sealer. Zinc oxide was observed to react with phosphoric acid to yield a material stronger than ZnO–eugenol, capable of securing gold alloy restorations to tooth preparations. The high acidity of the ZnO–phosphate cement was unsuitable for pulpal contact but a good insulator for use as a base. In the mid-20th century, ZnO was also used with carboxylic acid to produce a cement that was lower in acidity and capable of bonding to tooth structure.

When the zinc oxide in zinc phosphate cement was replaced with a glass powder, a semitranslucent esthetic filling called silicate cement was created. The longevity of the silicate cement restorations was unimpressive (<4 years) because the silicate cements have high solubility. Gradually, the silicate cements lost anatomical contour and degraded at the margins. Despite the deficiency, the occurrence of recurrent caries adjacent to these cements was rare. Laboratory studies showed that fluoride was released from the glass in silicate cements. The impressive anticariogenic potential of the silicate cements highlighted the ability of fluoride ions (F−) to inhibit demineralization.

The lessons learned from silicate cement and zinc carboxylate cement led to the development of glass-ionomer cements (GICs), which contained a modified glass for the powder and polyacrylic acid for the liquid. Incorporating the polymerization technology of methacrylate resin into the liquid created dual-cured materials: acid–base and polymerization reactions. From this combination, resin-modified glass-ionomer cement (RMGI) evolved. Replacing the liquid component of RMGI with a water-free acidic polymerizable monomer created a light-curable single-component material called compomer, a term derived from the terms composite and ionomer . The goal of these two hybrid materials was to combine the strength of composite with the fluoride release and self-adhesion of glass ionomer in one material.

Around the mid-20th century, new attributes for dental cements were needed for use with gutta-percha points in endodontic therapy. These cements are denoted as endodontic sealers, which must cure in the moist root canal dentin. Furthermore, endodontic materials must ensure that oral bacteria cannot percolate through the root to the periapical bone; otherwise, serious infection persists or occurs. Esthetics are less important for endodontic cements than for supragingival indications. Several dental materials were adapted for endodontic use: zinc oxide–eugenol (ZOE), polyvinyl siloxane, epoxy resins, and hydraulic ceramics (tri-/dicalcium silicate). In the 1990s, the bioactivity and sealing ability of tri-/dicalcium silicate cements became recognized and appreciated in dentistry. This ceramic dental cement is now widely indicated for pulpal and periradicular contact, including endodontic indications.

Varnish has been used for pulpal protection, especially with amalgam in a cavity preparation. The use of varnish has declined with decreased amalgam use. However, surficial varnishes are popular, especially among pediatric dentists. These varnishes are placed on enamel to release fluoride to provide protection against caries and promote remineralization from acids in the oral cavity with calcium ions. Sealants based on resins are also used superficially on teeth to provide protection against caries.

In the 20th century, varnishes included silver ions, silver nitrate, and silver diammine fluoride solutions for arresting caries and reducing sensitivity. The hazard with silver compounds is imparting a black color to the caries, as discussed later.

Most current cements include polymers and ceramic powders. The remainder of this chapter is organized by general indications: pulp protection and enamel protection, fixed prosthodontic device cementation, endodontic restoratives, and sealing.

Indications for Dental Cements

Dental cements are classified according to their major component, as shown in Table 7-1 , which includes their general indications. Several formats are offered for dental cements: powder and liquid, two pastes, or single component. Biocompatibility, durability for the indication, and safety for the patient and dental practitioner are important general cement attributes, all with a “reasonable” price. Some cements achieve their biocompatibility by being mildly cytotoxic, whereas others are bioinert or bioactive, as described later. The biocompatibility of dental cements is discussed in general terms in this chapter. Principles of biocompatibility are discussed in Chapter 17, Biocompatibility .

Other characteristics are designed for a cement’s unique indications, such as viscosity. For pulpal protection or surficial tooth sealing, a fluid layer is usually suitable. A medium-­viscosity cement is used for cavity liner or cement-base indications. For adhering an appliance, a cement must have a sufficiently low viscosity to flow along the interfaces between the tooth tissue and a restorative. A low-viscosity cement is also needed to enable an endodontic sealer to fill complex root canal anatomy. Orthodontic bands require a low-viscosity cement, but orthodontic brackets require higher-viscosity cements to keep orthodontic brackets from drifting before the cement is cured. Base/liner materials should flow and set within, but not out of, the cavity preparation. Viscous cements are used for filling perforations, for root-end filling, or for covering pulps after a pulpotomy. Whether low or high viscosity, a low-contact-angle material is useful for wetting the surface of enamel or dentin or a prosthetic device.

Cements must harden (set) within a reasonable time that depends on the indication, ranging from minutes to days. Strength requirements vary from a compressive strength of 35 MPa for pulp capping and base materials to 50 MPa for aqueous cements for the adhesion of devices. The strength should be appropriate for the indication and increase during maturation. For instance, the tri-/dicalcium silicate cements mature over about 1 month. All cements undergo a chemical reaction in vivo. Varnishes set by evaporation of the solvent to form a coherent protective layer. Two-part cements begin setting/hardening when the components are mixed. Single-component polymer-based cements set through polymerization activated by visible light. Dual-cured resin cements use both chemical and light-curing mechanisms to maximize polymerization. The hydraulic bioactive cements set by reaction with water that is either included in two-part systems or absorbed from the tooth. The exception is the nonsetting materials used with primary teeth after a pulpectomy, where a cement should not interfere with exfoliation.

Long-term retention of devices is important for crowns and bridges. However, temporary cements are sometimes needed, for instance, when transient devices are placed during prosthetic treatments. Dental cements are crucial for orthodontic brackets and bands, but the durability of these materials can be less than other cements because orthodontic treatments usually last about 2 years. Sealants and cements to reduce sensitivity (by occluding exposed dentinal tubules) are not expected to last long term.

Performance Criteria for Dental Cements

Dental cements must solidify into a biocompatible mass in the oral cavity and function over time in vivo by resisting hydrolysis (degradation by water). Properties of dental cements vary, but those that are often compared include the mixing time, working time, setting time, film thickness, compressive strength, and vulnerability to acid. Requirements for dental cements (base/liner and luting materials) have been codified in International Organization for Standardization (ISO) Standards 9917-1 and 9917-2 (Dentistry−Water-Based Cements) and ISO 6876:2012 (Root Canal Sealing Materials), where the test methods for these properties are described. These standards apply to water-based luting cement, bases, linings, restorations, core buildup materials, and root canal sealing. See Table 7-2 for the requirements. Except for root canal sealers, cements should set in less than 8 minutes (min) and resist acid erosion.

Solubility and Disintegration of Cements

Cements are often placed supragingivally and exposed to a variety of acids produced by microorganisms or as components of foods and drinks. ISO 9917-1 describes the erosion test of impinging a 0.1-M solution of lactic acid/sodium lactate (pH = 2.74) on water-based cements. The depth of erosion after 24 hours (h) is measured as mm/h. The results show that the rate of erosion decreased from conventional GIC to hybrid GIC to compomer. The solubility procedure described in ISO 4049 for dental composites uses the weight (mg) and volume (mm ³ ) of the specimen before immersion in water and the weight of the specimen after 5 days of immersion, and then desiccation. The solubility is calculated by dividing the weight changes by the volume. ISO 6876 for endodontic sealers measures the weight of the specimen before 24 h of immersion in water, and the weight percentage of residue is reported.

Testing conditions have been shown to significantly influence the solubility, such as varying the pH, the electrolytic concentration of the storage media, the soaking or impingement of a test medium on the cement, the powder-to-liquid (P/L) ratio of the cement, and the testing duration. For example, lowering the pH and P/L ratio will generally increase solubility. Delaying exposure to a storage medium decreases solubility. Some cements show no solubility (by weight) when concurrent dissolution and water sorption occur. Therefore poor correlation has been found for solubility data from the acidic-erosion and passive-weight-loss tests. In vitro data show that cement solubility differs among categories of materials and products. For the cements that rely on water to set, the solubility increases from glass ionomer to zinc phosphate and polycarboxylate cements. The solubilities between zinc polycarboxylate cement and zinc phosphate cement were not significantly different when mixed as recommended. However, a reduction in the powder-to-liquid ratio for zinc polycarboxylate reduced the viscosity and produced a cement that disintegrated rapidly. Zinc phosphate cements have a relatively low solubility in water; however, in vivo disintegration occurs with the lactic, acetic, and citric acids present in foods.

With solubility being inversely proportional to the resin content, resin cements are the least soluble of all dental cements, especially hydrophobic resins. The low solubility of resin cement makes this material suitable for cementing all-ceramic or resin-based restorations, where more cement may be exposed compared with metal or metal-ceramic restorations. Polymerizing monomers in glass-ionomer cements (RMGIs) have reduced the solubility compared with GIC.

The durability of some cements has been tested by placing small specimens of the cements in intraoral appliances that can be removed from the mouth to measure the loss of material. In one study, cements were inserted in tiny wells placed in the proximal surfaces of cast crown restorations. These crowns were cemented with temporary ZOE cement. After 1 year, the crowns were removed, and the cement loss was measured. GIC degraded the least, followed by zinc polycarboxylate cement, zinc phosphate cement, and zinc polycarboxylate cement mixed in a lower P/L ratio.

The solubilities of endodontic sealers based on ZOE and early formulations of glass ionomer were near the maximum allowed in ISO specifications (3%). ZOE-based root canal sealers have been used successfully for decades in endodontics, despite their high solubility. Sealers with epoxy or polyvinyl siloxane (PVS) bases have significantly lower solubility among endodontic sealers. The tri-/dicalcium silicate cements do not meet this limit after 24 hours; however, after longer times, these materials become less soluble. Immediately after setting, the tri-/dicalcium silicates have calcium hydroxide on the surface, which is beneficial for bioactivity but appears as solubility. Nevertheless, the tricalcium silicate sealers are highly successful clinically.

The variety of dental cements encompasses many material categories, and the properties vary widely, as discussed herein.

Fluoride-Releasing Materials

The anticariogenic potential of silicate cement confirms the ability of F ^– to inhibit demineralization and lead to the development of several fluoride-containing restorative materials and cements: conventional glass ionomers, resin-modified glass ionomers, polyacid-modified composites (compomers), and fluoridated varnishes. For the fluorosilicate glass-based materials, fluoride is first dissolved in the matrix from the glass during the acid–base reaction, and then released to the oral cavity. For fluoridated varnishes, the fillers incorporated in the material dissolve partially upon contact with saliva, and then F ^– is released from the varnishes. The rate of fluoride release depends on the cement matrix, porosity, and nature of the fluoridated filler (i.e., type, amount, particle, and size). Figure 7-1 illustrates the typical behavior of fluoride release from dental materials.

Typical fluoride-release profiles. A, Release rate versus time; the spike reflects fluoride recharging, and the blue dotted line represents the fluoride release where no recharging has occurred. B, Cumulative release over time; the diamonds are experimental data of (A) without recharging, and the curve is the best-fit curve of Y (total release) = a + b • (time) ^{1/2} . C, The same data in (B) plotted against the square root of time, revealing a linear relationship. The effect of recharging is not shown in (B) or (C) .

In vitro data have shown that fluoride release from GIC remains detectable for years, although the rate of release is reduced by a factor of 10 within the first few months ( Figure 7-1, A ). The release rate and duration depend on the cement matrix; porosity; and the fluoridated filler’s type, amount, silanation, and particle size. Figure 7-1, B & C illustrate the typical behavior of fluoride release from dental materials.

Pulp Protection

Tooth enamel is a translucent crystalline bioceramic but is vulnerable to acids and the repeated stress of a lifetime of chewing. The underlying dentin may be exposed to the oral cavity when the enamel is worn, chemically eroded by acidic foods or cariogenic bacteria, mechanically abraded by food or opposing teeth, fractured, or prepared for restoration. Depending on the remaining dentin thickness, the exposed dentinal tubules provide a communication path to the pulp for acids and microbiological insults. The dentin must be covered to minimize further damage. When metallic material is used to restore the tooth, discomfort may arise from hot or cold nutrition or galvanic contact with metal restoratives on opposing dentition. During restorative procedures, the acidic components of some dental cements may irritate the pulp before setting. Therefore pulp protectants are required when little protective dentin is left after a cavity preparation.

Pulp protectants for cavity preparations approximating the pulp are denoted as pulp-capping materials, varnishes, cavity liners, or base materials. Pulp-capping materials are in direct contact with the pulp, and their biocompatibility is crucial to the pulp’s healing. Varnishes as very thin resin layers used to plug and block the dentinal tubules exposed in a cavity preparation. Cavity liners are thin layers and are used for indirect pulp capping when the remaining dentin is less than 1 mm and are more protective than varnishes. Base materials are placed in thicker layers than liners and are commonly placed farther from the pulp. The base material is needed for insulation from the thermal, chemical, or galvanic irritations that may occur with restorative materials.

Cements for Luting

The first zinc oxide–based cement for cementing fixed prostheses was not adhesive to the tooth or metal prostheses. The cement functioned by filling the microscopic space between the device and the tooth and hardening to a high cohesive mass, which prevented dislodgement ( Figure 7-4 ). The mechanical retention is obtained by the combination of the microscopically rough tooth and prosthesis surfaces, along with the rigid cement that fills the gaps. This mechanical retention is referred to as luting, where zinc phosphate is the dominating cement. The degree of cement retentiveness for a given restoration relies on many factors: prepared tooth form, height, number of abutments, accuracy of superstructure fit, distribution of the cement, and occlusion. Dental prostheses that are mechanically retained include the following: (1) fixed prosthodontics made of metal, ceramic-veneered metal, or ceramic; (2) temporary restorations; (3) pins and posts used for retention of restorations; and (4) implant superstructures. As discussed in Chapter 6, Luting Agents , luting traditionally refers to the use of nonadhesive materials for retaining prostheses, and bonding refers to the use of adhesive bonding materials. The distinction has faded in the literature; the use of adhesive bonding medium is also referred to as luting.

Diagram of the suggested mechanism through which a dental cement provides mechanical retention of a gold crown. The cement penetrates irregularities in the tooth structure and the casting. Upon hardening, these retentive sites aid in retaining the cast crown in place. The enlargement shows a fracture through the cement, resulting in dislodgment of the crown.

When a device is seated without a luting agent, point contacts are made between the tooth and the appliance ( Figure 7-5 A & B ). The cement is used to fill the areas that are not in contact. The cement prevents oral fluid from flowing between the surfaces, thereby preventing bacterial invasion. Luting of a device to a tooth requires a cement of low viscosity and low contact angle to the tooth and the appliance to form a continuous film ( Figure 7-5, C ), without the formation of voids at interface ( Figure 7-5, D ). Voids impair the cement’s ability to support and retain a prosthesis.

Schematic illustration of the abutment–prosthesis interfaces. A, Irregular surface morphology of the two surfaces to be bonded. B, Two surfaces pressed against each other without an intermediate layer. Note the small number of points of contact illustrated by the circles. C, Continuous interface when a third material, either cement or adhesive, is used as the intermediate layer. D, Voids generated as a result of the inability of the intermediate layer to wet the surfaces completely.

Porosity in powder-liquid cements is created from incorporating air while mixing powder into liquid. One study with luting agents showed that the method of mixing had no influence on the formation of small pores, but hand mixing yielded more large pores than triturator mixing. Porosity measurement of hand-mixed powder-to-liquid cements showed that polycarboxylate cement had the highest porosity, followed by zinc phosphate and GICs. Higher viscosity generally leads to more pores during mixing. If air bubbles are entrapped, masticatory forces may fracture weak appliances such as veneers. When a void-free layer is formed and hardens, one surface is wedged against another to resist the shearing stresses that might dislodge the appliance.

To choose a cement for attaching a fixed prosthesis, the key characteristics are the working time, setting time, consistency, ease of removal of excess material, and adhesion. Any cement for attachment must flow under pressure and wet (spread over) the tooth and prosthetic surfaces. The internationally accepted requirements for luting cements are shown in Tables 7-2 . Luting cements are required to have a film thickness less than 25 μm when tested according to American National Standards Institute (ANSI)/American Dental Association (ADA) Specification No. 96 and ISO 9917-1 or 9917-2. For endodontic sealers, which are discussed later, the maximum film thickness is 50 μm per ISO 6876. The film thickness essentially reflects the consistency of the cement mixture and is affected by the size of particles and the P/L ratio. The ISO film-thickness test method requires freshly mixed cement to be placed between two optically flat surfaces, and a 150-N vertical load is applied. Seven minutes later, the thickness of the film is measured. Such samples should appear to create continuous films.

Table 7-2

ISO Requirements

ISO 9917-1 Water-Based Cements—Part 1: Powder/Liquid Acid-Base Cements
Cement Type	Zn PHOSPHATE		Zn POLYCARBOXYLATE			GLASS IONOMER
	Luting	Liner or Base	Luting		Liner or Base	Luting	Liner or Base
Maximum mixing time (min)	2.5	2	2.5		2	1.5	1.5
Final setting time (min)	8	6	8		6	8	6
Maximum film thickness (μm)	25	nr	25		nr	25	nr
Minimum compressive strength (MPa)	50
Maximum acid erosion (mm)	0.3		0.4			0.17
ISO 9917-2 Water-Based Cements—Part 2: Resin-Modified Cements
Cement Use		Luting	Liner or Base			Restorative
Minimum working time (min)		1.5	1.5			1.5
Minimum setting time (min)		8	6			6
Maximum film thickness (μm)		25	nr
Minimum flexural strength (MPa)		10	10			25
ISO 6876 Root Canal Sealing Materials/ADA 57 Endodontic Sealing Materials
Property				Requirement
Flow (mm)				>17
Working time				To be stated by manufacturer in minutes or hours
Setting time				To be stated by manufacturer in minutes or hours
Maximum film thickness (μm)				50
Solubility (%)				<3
Radiopacity (mm of equivalent Al)				>3
Dimensional stability (ADA 57 only)				<0.1% expansion, less than 1% shrinkage

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. 15: Dental Amalgams
2. 8: Impression Materials
3. Structure and Properties of Cast Dental Alloys
4. Dental Polymers
5. Physical and Chemical Properties of Solids
6. Casting Investments and Casting Procedures

Stay updated, free dental videos. Join our Telegram channel

Join