Tech Talk: Cements

By Mary Yakas, THE DENTAL ADVISOR

A continually evolving area, and one of confusion for many dental professionals is that of what type of cement to use with different ceramic and metal materials. We test the bond of both cements and the substrates at THE DENTAL ADVISOR, and often find that it is misuse of the product that leads to failure. It is critical to understand the differences in how products work in order for them to perform well clinically.

Cement choices
All permanent cements can classified into one of two broad categories:

  • Traditional cements: (zinc phosphate, zinc polycarboxylate, glass ionomer, and resin-modified glass ionomer) are essentially “gap fillers” relying on micromechanical retention provided by the luting agent. Resin- modified glass ionomers (RMGI) are most commonly used in this category because they offer slightly better strength and adhesion, easy cleanup, and lower solubility.
  • Resin cements: (self adhesive, adhesive resin and esthetic resin) have gained popularity mainly because they address the shortcomings of luting cements; they exhibit high bond strength to tooth structure, higher esthetics, and the lowest solubility of the available cements. They rely on both micromechanical retention and chemical bonding.

There are many types of cement to choose from and often there is more than one viable option.

An easy place to start in the decision-making process is by looking at the strength of the ceramic, as well as taking into account the retentiveness of the preparation.

Generally, when esthetics is of high concern, low- to medium-strength glass ceramics (feldspathic, leucite- reinforced, lithium disilicate) are selected. A benefit of using high-strength cements (adhesive- or esthetic resin) is that they will add strength to the entire restoration. When using a high-strength ceramic (zirconia) with a retentive preparation, a low-strength cement such as a self-adhesive resin cement or RMGI can be used because it is not necessary to rely on the cement for additional strength.

Resin cements
Resin cements exhibit high bond strength to tooth structure, excellent esthetics, and the lowest solubility of the available cements. Currently, resin cements can be classified into three categories:

  1. Self-adhesive resin cements: No separate etching or priming of teeth or restorations
  2. Adhesive resin cements: Bonding to teeth based on self-etch primer, primer not required for restorative substrate
  3. Esthetic resin cements: Bonding to teeth based on total-etch adhesive, restorative substrate requires primer

Self-adhesive resin cements
Self-adhesive resin cements are easy to use and provide low to medium bond strengths (4-16 MPa) to tooth structure. They do not require the use of a bonding agent on the tooth; however, some bonding agents recommended by manufacturers can be compatible with self-adhesive resin cements. Self-adhesive resin cements have a lower incidence of sensitivity than adhesive or traditional crown and bridge cements.

Advantages:

  • Easy to use (no etch or primer required)
  • Less technique sensitivity
  • Low postoperative sensitivity
  • Easy cleanup
  • Dual-cured

Disadvantages:

  • Can have a shade shift over-time
  • Isolation from moisture/contaminants
  • required

Adhesive resin cements
Adhesive resin cements provide medium-high bond strengths and are an excellent choice when retention is of concern with all restorative materials. They differ from self-adhesive resin cements because they require the use of a separate bonding agent on the tooth. One trend we are seeing from manufacturers is the bundling of their adhesive resin cements and universal bonding agents into prepackaged kits. With universal bonding agents included in kit, the clinician has the

option for total-, self-, or selective-etching of the tooth. Indications for adhesive resin cements include all-ceramic crowns/bridges and inlays/onlays, high-strength ceramic (zirconia) crowns/bridges, Maryland bridges, and posts (metal and fiber).

Advantages:

  • Can use with all restorative materials
  • Can be used when retention is of concern
  • Common shades: universal, translucent, and opaque
  • Dual-cured

Disadvantages:

  • Can have a shade shift over time
  • Clean up can be difficult
  • Isolation from moisture/contaminants required

Esthetic resin cements
Esthetic resin cements generally contain an option for dual-cured or light-cured only polymerization. However, a few esthetic resin cement kits include light-cured only cements suitable for thin, all-ceramic restorations such as veneers and inlays/onlays. Light-cured

cements are preferred due to their color stability and increased working time. Light-cured resin cements do not contain an amine catalyst (found in many dual-cured resin cements), so there is less chance of shade shift over time that can result from oxidation of the amine catalyst.

These cements typically require etching the tooth with phosphoric acid, followed by priming of the restoration and application of resin cement. Most manufacturers provide a variety of shades and corresponding try-in pastes, which makes them ideal for esthetic restorations.

Advantages:

  • Highly esthetic
  • Light-cured or dual-cured
  • Longer working time with LC cement
  • Color stable
  • Highest bond strength to enamel and dentin with appropriate bonding agent
  • Available in a variety of shades with corresponding try-in pastes

Disadvantages:

  • Most technique sensitive
  • Isolation from moisture/ contaminants required

Glass ionomers
Since their introduction in the 1970s, numerous modifications have been made to glass ionomers making them suitable for an array of modern-day clinical applications, including use as a luting agent. Glass ionomers are derived from an acid-base reaction between a basic glass powder and an acidic water-soluble polymer. During this hardening reaction, significant amounts of fluoride ions are released. Unlike resin cements, glass ionomer cements are hydrophilic and include water in their formulation, resulting in a moisture tolerant material that directly adheres to tooth structure by a chemical bond. Additionally, because glass ionomers set by an acid-base reaction, minimal shrinkage occurs. They are classified as either conventional glass ionomer or resin-modified glass ionomer, which have resin added to the formula to improve physical properties.

Both cements chemically bond to enamel and dentin, release fluoride, have a coefficient of thermal expansion similar to that of tooth structure, exhibit hydrophilic properties, and are available in powder-liquid, paste-paste, and encapsulated formulas.

Resin-modified glass ionomer cement
Resin-modified glass ionomers (RMGI) were developed in the 1980s to overcome the high solubility of conventional glass ionomers. The addition of resin to the glass ionomer formulation provides slightly greater bond strengths, releases a similar amount of fluoride, and allows for the material to harden when light cured. Although they offer lower strength than resin cements, their adhesion is adequate for metal, metal-ceramic and high-strength ceramic restorations (zirconia) as well as for metal and composite fiber posts. RMGI cements are a great option in cases where moisture control is an issue or when you need fluoride release.

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