Liners, Bases, and Cements in Clinical Dentistry A Review and Update

Liners, bases, and cements used in clinical dentistry are an important part of restorative and prosthodontic care and are constantly being improved. However, there is some degree of confusion in terminology. For example, the term “lining cement” may be used even though the material is not intended to be used as a cement, or “cement” may be used in reference to certain glass ionomer materials, when in fact the product is not intended for use as a cement. This confusion may be related to suggestions that dentists should reevaluate the liners and bases being used.¹ A study published in 1996 indicated that many dental schools could not agree on when to use each type of material.²

This article will review the clinical applications for liners, bases, and cements and the different types of materials that are available. Examples of products and manufacturers in each category are provided, but these are not intended to represent a comprehensive list of available products.

CLINICAL USES

It was generally held that the toxic effects of dental materials were responsible for pulpal inflammation. More recently, adverse pulpal reactions are believed to be caused by the influx of bacteria and their toxins into the dentin.^3,4 This can lead to pulpal inflammation and possibly necrosis of pulpal tissue.⁵ Therefore, the clinician should attempt to reduce or eliminate microleakage, which would result in reduction and possibly elimination of postoperative sensitivity.

It has been proposed that secondary caries, marginal discoloration, and marginal gap/fracture account for the majority of failed restorations.⁶ These failures occur at the interface between the restoration and the cavity preparation. Therefore, improving the seal at this interface will reduce the need for replacement of restorations.

One means of reducing the effects of microleakage is to prevent the development of caries via the use of appropriate restorative materials. The choice of which material to use is in part determined by the clinical situation, including the amount of remaining tooth structure. In terms of pulpal health, it is more beneficial to conserve tooth structure when possible than to remove that same tooth structure and replace it with a restorative material.⁷ Meryon demonstrated that a 0.5-mm thickness of dentin reduces toxicity of a material by 75%, and if that thickness is increased to 1.0 mm, a reduction of 91% is seen.⁸ This study used materials that were known to have toxic effects. When these materials came in contact with the test culture cells, a reduction in the number of viable cells was seen. However, when a layer of dentin (dentin powder) was placed between the test materials and the cells, an increase in the number of viable cells was seen compared to the controls. The same holds true with respect to the smear layer. An intact smear layer helps occlude the dentinal tubules and therefore provides a barrier to bacterial invasion. Since removal of the smear layer is taught (with respect to resin bonding), one must consider protection of the pulp.

Currently, there are a number of methods for preparing a cavity, including use of a bur, air abrasives, lasers, and hand instruments. The method by which tooth structure is removed does not affect microleakage.⁹

The available materials that can be used as either a liner, a base, or a cement can be divided into 7 categories: varnishes, calcium hydroxide, zinc oxide eugenol/noneugenol zinc oxide, zinc phosphate, zinc polycarboxylate, glass ionomer, and resin. In different situations, a material can be considered a liner, a base, or a cement (see Table).

Many clinicians use more than one material beneath a restoration. However, caution is needed because certain materials are not compatible with each other. For example, Yang and Chan demonstrated that varnishes can reduce the surface hardness of glass ionomers.¹⁰

Following is a definition of liners, bases, and cements, and a discussion of how the 7 categories of materials are used.

LINERS

Liners are materials that are placed as a thin coating (usually 0.5 mm) on the surface of a cavity preparation. Although they provide a barrier to chemical irritants, they are not used for thermal insulation or to add bulk to a cavity preparation.¹¹ Furthermore, these materials do not have sufficient hardness or strength to be used alone in a deep cavity.¹² Of the categories listed above, varnishes, calcium hydroxide, glass ionomers, and resins can be used as liners. Zinc eugenol, zinc phosphate, and zinc polycarboxylates are generally not used as liners.

Varnish

A varnish is defined as natural gum (copal or resin) dissolved in an organic solvent such as acetone, chloroform, or ether.¹³ After the dentin in the cavity preparation is covered with a varnish, the solvent evaporates, leaving the solute as a thin layer or film. The theory behind a varnish (example: Copalite/Cooley & Cooley) is that it seals the dentinal tubules, thus reducing the effects of micro-leakage. When amalgam is first placed, the tooth/amalgam interface is not microscopically sealed. Eventually the varnish dissolves and is replaced with the corrosion products of the amalgam.¹⁴

There are several fluoride-containing varnishes available (examples: Duraphat, Colgate Oral Pharmaceuticals; Dura-flor, Pharmascience Inc; Fluor Protector, Ivoclar Vivadent). Although the FDA has approved these products both as cavity liners and for the treatment of sensitive teeth, they are not approved for caries prevention.¹⁵ However, in Europe, fluoride varnishes have been used in caries prevention programs/studies since 1968. The results showed a reduction in caries ranging from 18% to 77%.¹⁶ In order for these varnishes to receive approval for use as a method of caries prevention, the manufacturer would need to submit clinical studies to the FDA, in which case the varnish would be classified as a drug.

Calcium Hydroxide

It is generally believed that calcium hydroxide (CH) is ideal for direct pulp capping since it accelerates the formation of reparative dentin. There are 2 reasons for this: first, since the material is basic (pH of 11), it serves as an irritant stimulating the formation of reparative dentin; and second, the therapeutic affect of CH may be due to its ability to extract growth

factors from the dentin matrix.¹⁷ The result is the formation of a dentin bridge, which allows pulpal repair. However, these concepts are challenged by Schuurs et al,¹⁸ who concluded that although CH causes the formation of a dentin bridge, this seal does not last. Eventually the pulp will undergo necrosis as a result of microleakage.

Due to the fact that CH has a basic pH, it is not generally supportive of bacterial growth. When the base and catalyst portions of CH were tested separately, only the catalyst component was shown to have any antibacterial effect.¹⁹ In addition, since bacterial byproducts are acidic, CH will directly counteract this acidity and effectively neutralize these byproducts. It for this reason that CH is placed under zinc phosphate—to help reduce the acidity of the zinc phosphate. CH is available in chemical- cured forms (example: Dycal, DENTSPLY Caulk) and light-cured forms (example: Prisma VLC Dycal, DENTSPLY Caulk) .

Glass Ionomer

This material has been available for more than 30 years. A current version of glass ionomer (GI) is the resin-modified glass ionomer (example: Vitrebond, 3M ESPE). There are 2 clinical benefits attributed to GIs: first is their ability to ionically bond to tooth structure (between the carboxylate groups in the GI and the calcium ions in the enamel and dentin)20; and second, they release fluoride. The ability of fluoride to inhibit the formation of secondary caries is established. Prati et al²¹ demonstrated that when GI is used as a liner, a reduction in the consequences of microleakage is seen. They attributed this to its antimicrobial properties. This benefit, along with its ability to adhere to and seal the dentin,²² has made this material popular as a cavity liner.

GIs should not be used as pulp-capping agents. Unlike calcium hydroxide, GI does not promote the formation of dentin bridges. In fact, in a clinical study, GI was found in the pulp chamber, which triggered a persistent inflammatory response and appeared to prevent the formation of dentin bridges.²³

One restorative procedure that is often used clinically is known as the “sandwich technique.”²⁴ In this technique, the lining materials are brought to the cavosurface margin. There are 2 advantages to this technique when using GI; first, released fluoride has a beneficial effect on the tooth structure at the margin of the restoration. Donly et al,²⁵ have shown that when this technique is used, there is less recurrent caries at the margin of restorations. Second, the fluoride that is released can be subsequently replaced with externally delivered fluoride. This can be via gel, mouthrinse, or toothpaste, with the gel being most effective.²⁶ GIs, both conventional and resin modified, are available in both chemical- cured formulations (example: Ketacbond, 3M ESPE) and light-cured formulations (example: Vitrebond, 3M ESPE). The light-cured products have been shown to provide a better seal.²⁷ Hand-mixed forms (examples: Vitrebond and Ketacbond) and encapsulated/cartridge-dispensed forms (example: Fuji Lining LC, GC America) are also available.

GIs have certain disadvantages. They are extremely sensitive to moisture. A study by Cattani-Lorente et al,²⁸ demonstrated that when GI comes in contact with water, there is a decrease in its physical properties. In addition, resin-modified GIs expand after coming in contact with water.

Resin

Of the materials discussed here, resins are the most recent additions to the clinician’s armamentarium. They are very versatile (generally being of high compressive and tensile strength), possess low solubility, and are available in different viscosities and different shades.

When resins are used as a cavity liner, it is important to remember that it is the dentin bonding agent (examples: Clearfil SE Bond, Kuraray America; Excite, Ivoclar Vivadent) that comes into contact with the dentin. There are different types of dentin bonding agents, and their performance differs.²⁹ These resins are not recommended for direct pulp capping since, like glass ionomers, they do not promote the formation of dentin bridges. In fact, there is a persistent mild inflammatory pulpal response associated with resins when they are used as a direct pulp-capping agent.30 Studies do confirm, however, that adhesives placed below amalgam restorations reduce microleakage,^31,32 thus supporting the current trend toward this practice of using resin as a liner. However, the clinician must consider the logistics of using adhesive resin liners. Lining cavities with copal varnish is faster and less technique-sensitive than using adhesive resin, and resins cost more and have a limited shelf life.

In an attempt to overcome the polymerization shrinkage associated with traditional composite resin, a new material—modified hybrid resin—has been developed. A resin with a reduced filler load, it is referred to as a flowable composite (examples: Unifil Flow, GC America; Tetric Flow, Ivoclar Vivadent). The better flow and reduced modulus of elasticity of these materials theoretically reduce microleakage by increasing adaptation and by forming a stress-absorbing layer.³³ The result is a decrease in gap formation at the flowable-resin/tooth interface, which will ultimately lead to a decrease in secondary caries and pulpal inflammation and a longer lasting restoration.³⁴ Although there is increased adaptation of the resin to the cavity preparation, the material has a reduced filler content. This leads to an increase in polymerization shrinkage. The net result, however, is a reduction in gingival microleakage when flowable resin is used, compared to a conventional composite resin.³⁵

It has been observed that some adhesives do not bond well to dentin in deep cavity preparations. This makes them more susceptible to polymerization shrinkage stress that develops in deep cavities.³⁶ Since the bond strength to dentin near the pulp chamber is low, the polymerization shrinkage that the resin undergoes can cause a gap to form. This was the conclusion reached by Gordan et al, who also showed that the weakest bond was at the flowable-resin/tooth interface and dentin near the pulp chamber.³⁷

One study comparing a resin-modified GI, a flowable composite, and a dentin bonding agent concluded that the resin-modified GI was associated with less microleakage than the other materials.³⁸

BASES

Bases can be considered as restorative substitutes for the dentin that was removed by caries and/or the cavity preparation. They act as a barrier against chemical irritation, provide thermal insulation, and can resist the condensation forces on a tooth when placing a restoration. Also, the clinician can shape and contour base materials after placement into the cavity preparation.¹¹ Varnishes and calcium hydroxide materials are not in this category.

Zinc Oxide Eugenol

Zinc oxide eugenol (ZOE) (example: Intermediate Re-storative Material, DENTS-PLY Caulk) materials provide an excellent seal of the cavity preparation. The ability of ZOE to reduce postoperative sensitivity is most likely due to⁴¹A disadvantage of this material is its inability to withstand the forces of condensation immediately after placement. The clinician should allow approximately 24 hours to pass prior to placing amalgam above a ZOE base.

Zinc Oxyphosphate

Zinc oxyphosphate (ZOP) is a powder/liquid combination that is an ideal base material since it can provide thermal insulation and will allow the condensation of amalgam several minutes after placement. The material is acidic when placed (pH of approximately 3.5), but rises to a pH of 6.9 after a week. Mixing of the material should be performed on a cold glass slab. This promotes cooling of the exothermic reaction that occurs when the powder and liquid are combined and allows the clinician to incorporate more powder with the liquid, thus increasing the physical properties.

According to ADA Specification No. 96, packages of ZOP contain 20% more liquid than is necessary to combine with the powder. This is because some of the liquid will evaporate during use. This specification applies to zinc phosphate, zinc polycarboxylate, and GI together since they all are water-based. This is important for the clinician to consider. Since the water can evaporate, these materials can become viscous, leading to difficulty in seating crowns. Furthermore, loss of water will result in a decrease in the pH of the liquid, making the cement less biocompatible. Examples of ZOP are Zinc Cement (Mission White) and Fleck’s Cement (Mizzy).

Zinc Polycarboxylate

A material that is comparable to ZOP is zinc polycarboxylate (ZPC). The important difference is the liquid component. The liquid in ZPC is polyacrylic acid, which is quite viscous. Zinc polycarboxylate adheres to the tooth via an interaction be-tween the carboxylic acid and the calcium in the dentin. Polyacrylic acid has a very low pH (1.7), but the pH approaches neutrality upon mixing with the powder. This cannot harm the tooth since the relatively large size of the polyacrylic acid molecule and/or its ability to combine with protein prevents it from diffusing into dentin tubules. Durelon and Durelon Maxicaps (3M ESPE) and Hybond zinc polycarboxylate (Shofu) are examples of this product.

Glass Ionomer and Resin

GIs and resins are generally not used as bases. They do, however, meet a number of the characteristics of a base; eg, they can be shaped and contoured and provide a chemical barrier. When used under another restorative material, they are usually employed as a core buildup.

There are reports in the literature that advocate the use of these materials under a final restoration.^42,43 Two studies involved teeth with large carious lesions. The first  study involved complete removal of the caries, followed by filling the cavity with either a GI or a composite resin.⁴² At a later visit, the tooth was prepared, leaving the original material as a base. The results suggested that using either a GI or a composite resin as a base under amalgam was clinically acceptable. The second  study involved partial removal of caries, then the placement of GI over calcium hydroxide.⁴² Six to 12 months later, the filling material was removed, and the dentin was found to be hard and dry. This technique resulted in a reduction in cultivable microorganisms, thought to be due to sealing the remaining caries from extrinsic substrate.

Examples of GI products that can be used for this purpose include Ketac-Silver (3M ESPE) and Fuji II LC Core Material (GC America). Paracore (Coltene/Whaledent) and Luxacore (Zenith/DMG) are examples of resin products.

CEMENTS

Materials that are considered cements can be used for 2 different purposes; the first is to retain restorations or appliances in a fixed position in the mouth44. The other is as a restorative filling material, used either alone or with other materials. In this situation, the material would be referred to as a base.

As with liners and bases, cements also reduce microleakage by sealing the interface between the tooth and the restoration. Dental cements can be categorized as either temporary (short-term) or permanent (final). They should (1) have no adverse effects on the pulp tissue, (2) be of low solubility, (3) have high compressive and tensile strength, and (4) be radiopaque.

It has been suggested that temporary cements be available in compact kits so waste is minimal, dispensing is controlled, and mixing is easy. They should flow but not drip, be easily cleaned from both instruments and margins, and have a quick set.45 These suggestions also apply to other types of materials discussed in this article.

Temporary cements

Calcium hydroxide, zinc oxide eugenol/nonzinc oxide eugenol, zinc polycarboxylate, and resins can be used as temporary cements.

Calcium Hydroxide

Although not usually thought of as a temporary cement, calcium hydroxide, as cited above, is not harmful to the tooth. It is harder than ZOE-based temporary cements and is easily removed from the margins of resin and PFM crowns. Also, since it is not eugenol-based, it will not interfere with the final resin cements (see below). It is available in dentin and ivory shades.

Zinc Oxide Eugenol/Noneugenol Zinc Oxide

ZOE-based materials are known to provide an effective seal at the tooth/restoration interface. ZOE has been shown to have adverse effects on resin-based products.^46,47 However, others were not able to confirm this.⁴⁸ Tempbond (Kerr) and Embonte (Cadco) are 2 examples of a ZOE temporary cement.

For those who use a resin cement as a final cement and want to be confident that the temporary cement used will not interfere with final luting, noneugenol zinc oxide temporary cements are available. Although these products (examples: Nogenol, GC America; Temrex TNE, Temrex ) do not have a sedative effect on the tooth, they are compatible with resin-based provisional restorations. In fact, since they have been shown to be stronger than  ZOE temporary cement, it is recommended that these products be used with preparations where retention is low.⁴⁸

Zinc Polycarboxylate

There are 2 zinc polycarboxylate temporary cements (examples: Shofu’s Hybond zinc polycarboxylate temporary cement soft kit and Ultradent’s Ultratemp) that are available. Both are eugenol-free. The Hybond product contains fluoride.

Resin

Provilink (Ivoclar Vivadent) and Sensitemp (Sultan Chemists) are examples of provisional resin cement. This type of temporary cement is ideal for anterior restorations due to its aesthetic qualities. Since it is a material that is bonded to the tooth, it has excellent retention properties. In fact, occasionally removal of the temporary restoration proves to be difficult. One disadvantage of this material is evident after removal of the temporary restoration. The clinician may find brown stains on the tooth.⁴⁵ This stain is a potential problem when placing all-ceramic restorations. Pumicing the preparation prior to cementation should remove the stain. Another problem that the clinician may encounter with all resin cements is due to the shade of the material. Since these materials are tooth colored, it might appear to the clinician that all excess cement has been removed from the margins prior to patient dismissal. The clinician must be very careful when removing excess cement. This is especially true with certain tooth morphology; for example, in subgingival regions in proximity to furcations and other areas of tooth concavity.⁴⁰

Final Cements

Zinc Oxide Eugenol

ZOE, Type II, is used for permanent or final cementation. This material has an inorganic filler added to the powder and ortho-ethoxybenzoic acid added to the liquid. It appears that this type of cement is not particularly popular, since there are not many brands available. Also, a review of the literature did not identify any studies in which a ZOE final cement was comparatively evaluated. Fynal (DENTSPLY Caulk) is an example of this kind of cement.

Zinc Oxyphosphate/Zinc Polycarboxylate

The liquids in both of these mixtures are acids. Therefore, these products are self-etching. These cements may demineralize dentin, but this does not occur in a uniform manner. ZOP causes more demineralization than does ZPC.50 Both of these materials have a long history of clinical use in dentistry. In fact, a version of ZOP was introduced in the late 1800s and was known as Ames’ black copper cement. It contained copper (cupric oxide 97%) and was found to be germicidal. This product was eventually discontinued, but was reintroduced as Doc’s Best Red Copper Zinc Phosphate Cement (Cooley & Cooley). This currently available product contains 7% cupric oxide. Unlike other ZOP cements, this product has a brown color, which makes it unsuitable for all-ceramic restorations.

ZPC has a rubbery consistency when setting. Therefore, it is best to remove all excess from the margins only when it is fully set, or the material may be pulled from under the casting. When using ZPC, the clinician should be aware that the cement-casting interface is where failure may occur (ZPC chemically adheres to the tooth). This is in contrast to ZOP, where the cement-tooth interface is where failure can occur.

Glass Ionomer

GI cement is available in both hand-mixed forms (examples: Fuji I Glass Ionomer Cement, GC America; CX-Plus GlasIonomer cement, Shofu)  and encapsulated/cartridge forms (examples: FujiCem, GC America; VivaglassCem, Ivoclar Vivadent). These are self-cured formulations.

One disadvantage of using GI as a cement or luting agent is its sensitivity to moisture. In fact, Mojon et al⁵¹ suggest that when using GIs, the involved teeth should be protected from contamination from saliva for as long as 15 minutes after mixing. In contrast, the resin-modified GIs (RMGIs) are hydrophilic (water-absorbing). This presents both an advantage and a disadvantage. The advantage is that the material will expand after coming in contact with moisture, filling any marginal gaps. The disadvantage is that this same expansion can cause all-ceramic crowns to fracture. Lastly, it has been suggested that RMGIs are ideal luting agents for patients who have a high caries index. (Fluoride is released from RMGI.)52 Nevertheless, it might be difficult or impossible to remove a post cemented with this material.

Resin

Resin-based cements can be grouped into categories based on how they are cured. They can be either self- or chemical-cured (examples: Parapost Cemen