A working guide for dentists
Many factors contribute to achieving an optimal restoration, including selecting the right cement. The type of restoration (i.e., veneers vs. crowns), the restorative material used, tooth preparation design, necessary bond strength, clinical implications (inflamed soft tissue, isolation, etc.), retentive vs. non-retentive implications and the desired esthetic outcome all determine what cement will work best.
Depending on the clinical requirements and material selection, dentists may place restorations using either conventional or adhesive cementation techniques. Conventional cementation combines preparation design, such as retention/resistance form to lute restorations to the underlying tooth structure. Adhesive cementation, on the other hand, produces a micromechanical and chemical bond between the tooth structure and the restoration. Dentists therefore need a thorough understanding of not only the restorative material used in the fabrication of a dental restoration, but also the cementation options and protocols for predictable clinical outcomes.
Conventional cements lute restorations with underlying tooth structure by creating a hardened cement layer between the restoration and the tooth. Conventional cements most notably include zinc polycarboxylate, glass ionomers, resin-modified glass inomers and zinc phosphate cements. These materials provide limited-to-no chemical bond with the tooth structure; hence, retentive preparation designs should be taken into consideration. Clinicians use these cements to lute high-strength ceramics and metal-based restorations. The physical and mechanical properties of cements varies, depending on their chemistry. The zinc polycarboxylate cement consists of metal oxides and polyacrylic acid. The dry mixture is mostly used as a powder, which is mixed with water for processing. The complicated setting reaction takes place by the reaction of metal oxides with the polyacrylic acid. The comparatively high solubility of the cement and mild pulpal reaction is a substantial disadvantage. Contrary, glass-ionomer cements consists of fluoroaluminosilicate glass and liquid containing polyacrylic, itaconic acid and water. They demonstrate the advantage of being able to release fluoride ions. Setting reaction takes place with the help of an acid-base reaction.
Clinical experiences with glass-ionomer cements have been gathered for more than 20 years, however it produces low retention rates and hence is indicated for retentive tooth preparations. The resin-reinforced glass-ionomer cements were formed by replacing part of the polyacrylic acid in conventional glass ionomer cements with hydrophilic methacrylate monomers. This group of luting agents includes a number of hybrid cements, the physical and clinical properties of which vary strongly, depending on the composition of the individual components. Their adhesion to the tooth structure is often weak, and when applied to moist dentin, produce little post-cementation thermal sensitivity.
Adhesive resin cements
Adhesive resin cements are superior options for all-ceramic restorations. They are methacrylate-based and consist of monomers and inorganic filler particles. Their setting reaction is based on a cross-linking of the polymer chains, which is initiated chemically (self-cure resin cements) and/or by light (dual-cure or light-cure resin cements) and provides chemical bonding between the tooth and the indirect restoration. These resin cements demonstrate high mechanical properties (greater wear resistance and resistance to the oral environment) and offer outstanding aesthetics given the wide choice of shades.
Generally, adhesive resin cements require the tooth preparation to be etched and rinsed, and then conditioned with an adhesive bonding agent and cured. Etching removes the smear layer and demineralizes the tooth surface, whereas the bonding agent forms an interpenetrating network with free collagen fibers (the hybrid layer). Adhesive resin cements have significantly superior mechanical properties; however, when proper isolation can’t be achieved, conventional cements are recommended.
Adhesive cements are further classified according to the following curing options:
- Self-cure resin cements (chemical cure).
- Dual-cure resin cements (chemical and light cure).
- Light-cure resin cements (light cure only).
Dual-cure and light-cure resin cements require light energy for complete polymerization of cement. However, restorations made of metals, metal alloys and opaque ceramics, such as traditional Zirconia oxide, are impervious to light, contraindicating the use of dual-cure and light-cure resin cements for cementation. As such, self-cure resin cements are indicated in these clinical conditions.
By comparison, the cementation of highly esthetic restorations, such as veneers in the anterior region, requires materials that ensure long-term color stability for high quality esthetics. One option is to use amine-free resin cements.
Self-adhesive resin cements
Self-adhesive resin cements combine some of the advantages of resin cements with the convenience of conventional cements, including moderate level bond strengths. These cements don’t require the application of conditioners or bonding agents on the prepared tooth surface, making the system easy to use and error-prone. However, given the comparatively lower bond strength and mechanical properties, these cements are not highly recommended for low strength glass ceramics.
Preparation design and restorative materials
The preparation design and restorative material largely determine what type of cement is used. Preparation design is significant in cement selection. Depending on the inclination/taper of the axial wall of the prepared tooth in relation to the longitudinal axis of tooth, the preparation design is generally classified as retentive or non-retentive. Depending on the height of the prepared tooth (>4mm), the preparation design is classified as a short preparation. Retentive preparations feature an inclination/taper between 4 to 8 degrees, with a longitudinal axis of tooth. This feature provides additional mechanical retention, which facilitates the cementation of a restoration, either using adhesive or conventional cementation techniques.
While adhesive cementation is known to provide a strong bond and good marginal seal, the luting forces of conventional cement are sufficient to lute restorations with retentive tooth prep design due to additional mechanical retention achieved by the tooth preparation design. At the same time, non-retentive preparations feature an inclination/taper in excess of 8 degrees and lack retentive features. Given their limited or no-chemical bonding properties, conventional cements are not ideal for this clinical scenario. Rather, adhesive resin cements are recommended due to their ability to chemically bond with the restoration and tooth.
The restorative material used also determines cement selection. Indirect restoratives include:
- Metal and metal-based (metal alloys and porcelain-fused-to-metal restorations).
- Intermediate-strength ceramic (feldspathic, leucite-reinforced and fluoroapatite restorations).
- High-strength ceramic (lithium disilicate, alumina and zirconia restorations).
- Indirect composite (reinforced composites)
Intermediate strength glass ceramics, such as those with a high glassy content, obtain additional support from adhesive bonding and must be adhesively cemented using resin cements. Therefore, feldspathic, leucite-reinforced and fluorapatite ceramic restorations should be cemented with adhesive resin cements.
Speaking the language
Because different cements are suited to different clinical situations, dentists require a thorough understanding of their attributes and limitations – including restoration type (veneer, crown, inlay, onlay), restorative material (low or high strength, or opaque), tooth prep (retentive or non-retentive) and conditions such as isolation – in order to select the material best suited to their practice.
Adhesive resin cements are recommended for all clinical situations, except when an ideal isolation cannot be maintained. Conventional cements are considered highly versatile. And light-cured resin cements are indicated for thin veneers due to their high color stability, while self-cure resin cements are ideal for opaque restorations, such as metals or opaque thick zirconia restorations.
A clinician looking to achieve an intermediate bond strength and ease-of-use can use self-adhesive resin cements. In fact, the presence of MDP in selective self-adhesive resin cements has eliminated the need for a primer application on zirconia restorations. As such, these cements are the product of choice for cementing zirconia restorations with retentive prep design.
Regardless of what type of cement the practice relies on, it is essential for clinicians and their dental staff to read the instructions for use to ensure the most predictable, successful clinical outcome.
Editor’s note: Efficiency in Group Practice would like to thank Ivoclar Vivadent for its assistance with this article.