Dental Cements and Their Clinical Considerations

Zinc Phosphate

Zinc phosphate cement, which was introduced in the 1880s, has been used for more than a century. Because of its lengthy clinical history of success, despite its inability to adhere to tooth structure and inferior physical properties when compared with contemporary dental cements, zinc phosphate is considered the standard with which other cements are compared.⁵ It remains a useful luting agent for many indirect restorations. Generally, zinc phosphate cement is produced in a powder/liquid system in which the powder is approximately 90% zinc oxide (ZnO), and the liquid contains 67% buffered phosphoric acid, aluminum, and water. Mixing zinc phosphate cement is a technique-sensitive procedure that requires the use of a cool glass slab on which to incrementally add the powder to the liquid.^6,7

Zinc Polycarboxylate

In the late 1960s, zinc polycarboxylate cement was developed.⁸ This was the first dental cement with the ability to adhere to the tooth structure. The powder in zinc polycarboxylate cement, which is zinc oxide and 10% magnesium oxide, is mixed with a 30% to 40% solution of poly(alkanoic acid).⁹ Because Zinc polycarboxylate cement may undergo plastic deformation under dynamic loading, its use is limited to single-unit restorations and short-span bridges. It has also been used as a provisional cement when maximum retention is needed in situations involving nonretentive preparations.⁷

Glass Ionomers

The development of glass-ionomer cements pioneered bonding to tooth structure. Regarding their composition, the powder usually contains a calcium aluminosilicate cement and fluoride, and the liquid is composed of a mix of different organic acids. Chemical adhesion to the tooth structure is achieved through chelation with calcium and phosphate ions.²

Glass-ionomer cements are considered to be anticariogenic because they release fluoride. Their fluoride-releasing properties are dependent on pH, and a greater amount of fluoride release occurs in environments with a lower pH. However, the amount of fluoride released deteriorates over a very short period of time, and the significance of its impact is not well established in the literature.¹⁰

To improve upon the physical properties of conventional glass-ionomer cements, resin particles were later incorporated. The resulting resin-modified glass-ionomer cements undergo three reactions: an acid-based reaction, chemical polymerization, and photopolymerization. When compared with glass-ionomer cements, resin-modified glass-ionomer cements provide improved physical properties, adhesion to tooth structure, and handling.

Resin-based cements were introduced in the 1980s, and since then, they have rapidly improved and gained attention.^11,12 Because these adhesive cements bond to the tooth structure, their use is optimized in minimally retentive or nonretentive preparations. Resin-based cements are further classified based on their curing modes and bonding protocols. These cements can be self-, light-, or dual-cure materials, and although resin cements require bonding protocols involving multiple steps, self-adhesive resin cements require no additional steps.¹² Regarding their formulation, resin-based cements are essentially composed of dimethacrylate monomers, inorganic fillers (60% to 70% by weight), polymerization initiators, and silica. Bonding to enamel is achieved by acid etching it so that the resin cement can micromechanically interlock with its surface. Bonding to dentin is more complex and technique sensitive because partial removal of the smear layer and formation of the hybrid layer is necessary to achieve optimal outcomes.⁶

Curing Modes

Self-cure or "chemically cured" resin cements (eg, PANAVIA^™ 21, Kuraray America; C&B^™ Cement, BISCO, Inc.; Super-Bond C&B, Sun Medical) are composed of two pastes and rely on an etchant, self-etch system, or primer for adhesion. These cements are recommended for use under metal-based restorations in situations when retention and resistance forms are compromised. Light-cure resin cements (eg, RelyX^™ Veneer Cement, 3M; Variolink^® Veneer, Ivoclar Vivadent; Choice^™ 2 Light-Cured Veneer Cement, BISCO, Inc.) provide on demand curing and have excellent color stability. However, to ensure complete polymerization, these cements are only indicated for use under thin ceramic restorations, such as veneers, inlays, and onlays. Regarding dual-cure resin cements (eg, RelyX Ultimate^™, 3M; PANAVIA^™ V5, Kuraray America; EstreCem II^®, Tokuyama Dental), although they are formulated to polymerize without the use of a curing light, to ensure maximum cure and optimize strength, light activation of the accessible areas is required to initiate the polymerization process, which is then completed by a self-polymerizing catalyst.^13,14 Among other applications, these cements are beneficial for luting restorations that attenuate the passage of light. The color stability of dual-cure cements over time is a concern because the color can darken due to the presence of tertiary amine components and the photoinitiator camphorquinone; however, the development of newer photoinitiators (eg, Ivocerin, Ivoclar Vivadent; RAP Technology^™, Tokuyama Dental; Lucirin TPO^®, BASF) and chemistries has resulted in dual-cure cements with promising color stability.¹⁵ Research has demonstrated that delaying the light-curing of dual-cure cements clinically by 2 minutes to 5 minutes can reduce polymerization stress without jeopardizing the degree of conversion.¹⁶

Self-Adhesive Resins

Self-adhesive resin cements (eg, RelyX^™ Unicem 2, 3M; G-CEM ONE^™, GC America; SpeedCEM^® Plus, Ivoclar Vivadent) differ from adhesive cements in that they incorporate functional acidic monomers. When placing indirect ceramic restorations, these cements do not require any additional bonding steps. The low pH of a self-adhesive resin cement promotes demineralization of the dentin and facilitates penetration of the cement¹⁷; however, the pH level then increases as a result of the cement's interaction with the hydroxyapatite. Self-adhesive resin cements are initially hydrophilic, which is important for wetting the dentin surface, but once in contact with the dentin, they become hydrophobic.¹⁸ Understanding the chemical complexity of these cements is essential to using them appropriately. In addition, it should be noted that self-adhesive resin cements are contraindicated for substrates other than enamel or dentin. The chemical process will not be initiated when the cement contacts other substrates, such as metal, amalgam, or composite.¹²

Numerous studies have been conducted to examine the various properties of the different classifications of luting agents.^19-22 According to the research, resin cements have demonstrated the highest strengths, followed by self-adhesive resin cements, which demonstrate comparably reduced strengths because the addition of the acidic monomers influences their physical properties. The conventional water-based cements provide the least strength. To maximize the performance of luting agents, it is critical to follow the manufacturers' instructions regarding application and storage.

There is no one ideal cement for every clinical situation. Light-cure resin cements should only be used for thin ceramic restorations (eg, veneers, inlays, onlays) where full light polymerization is achievable and color stability is critical for an esthetic outcome.¹¹ Self-cure and dual-cure resin cements are indicated for full-coverage ceramic restorations with no to limited light attenuation in situations where retention and resistance are minimal. When retention and resistance forms are adequate, self-adhesive resin cements are suitable.¹² Resin-modified glass-ionomer cements are indicated with cast metal restorations (eg, gold and porcelain-fused-to-metal crowns, Maryland bridges) when preparations demonstrate appropriate retention and resistance forms. When there is a lack of retention and resistance form, these restorations can be cemented with self-cure resin cements accompanied by metal primers.²³ In addition, self-cure resin cements can provide high strength for the cementation of endodontic posts. The selection of a cement to retain an implant-supported crown is dependent on the retrievability needed by the clinician. A weaker cement should be used in situations where subsequent removal of the crown is expected.²

The success of indirect restorations is multifactorial, and the selection of a proper luting agent is just one of these factors. The shape and height of the prepared tooth, the substrate, and the type of prosthesis, as well as the prosthetic material, must be considered during selection. Other patient-related factors, such as occlusion, oral hygiene, and parafunctional habits, are also critical to the longevity of a fixed prosthesis. With so many factors affecting success, it is essential for clinicians to acquire sufficient knowledge about the different types of cements available to appropriately guide their clinical decisions.

Tariq A. Alsahafi, BDS, MS
Laboratory Manager
Dr. Sulaiman’s Biomaterials Laboratory
Adams School of Dentistry
University of North Carolina at Chapel Hill
Chapel Hill, North Carolina