Securing a dental crown requires a precise material, often called a luting agent, to permanently attach the fabricated crown to the prepared tooth structure. This material acts as a durable interface, filling the microscopic gap between the restoration and the tooth surface to create a sealed, unified unit.
The Essential Role of Dental Cement
Securing a crown requires a material that meets specific biological and physical standards to ensure the restoration’s longevity. One fundamental requirement is low film thickness, meaning the cement must be fluid enough to flow into the tiny space, often less than 25 to 40 micrometers, allowing the crown to fully seat against the tooth preparation. If the film is too thick, the crown will sit too high, disrupting the patient’s bite.
The cement must demonstrate biocompatibility, meaning it cannot irritate the dental pulp or surrounding tissues. It also needs high resistance to solubility, preventing it from dissolving over time when exposed to the moisture and acids of the oral environment. This resistance maintains the marginal seal and prevents microleakage, which could lead to decay under the crown.
The functional difference between materials is based on their retention mechanism: luting or adhesion. Luting agents fill the microscopic space and rely primarily on the mechanical fit and geometry of the prepared tooth to hold the crown in place. Adhesive or bonding cements achieve a chemical bond to the tooth structure and the crown material, offering greater strength and retention regardless of tooth shape.
Conventional Cements: Relying on Mechanical Fit
Conventional cements represent the traditional approach, where the crown’s retention is provided by the shape of the prepared tooth. The cement acts mainly as a hydraulic seal and space filler, relying on the surface roughness of the crown and the tooth to lock the restoration in place. These materials are used with restorations that already possess good mechanical retention, such as full-metal crowns or porcelain-fused-to-metal (PFM) crowns.
Zinc Phosphate cement has been used in dentistry for over a century. It is formed by mixing zinc oxide powder with an acidic liquid containing phosphoric acid, resulting in a material with high compressive strength. The initial acidity of the cement can cause temporary irritation to the pulp, often necessitating a protective liner on the prepared tooth. This cement does not chemically bond to the tooth structure, relying entirely on mechanical interlocking.
Traditional Glass Ionomer (GI) cement achieves a slight chemical bond to the tooth structure through the chelation of polyacrylic acid molecules to calcium ions in the dentin and enamel. A primary advantage is its ability to release fluoride ions over time, which helps prevent secondary decay around the crown margin. However, traditional GI cements have lower tensile strength and are susceptible to moisture contamination during the initial setting phase. These conventional choices are still widely used, particularly for metal-based restorations where aesthetics are not a concern and the tooth preparation is retentive.
Resin-Based Cements: Achieving Chemical Bonding
Modern restorative dentistry requires materials that provide a chemical connection, achieved through resin-based cements. These cements are necessary for high-strength ceramic restorations, such as those made from lithium disilicate or zirconia, which require bonding to reinforce the material and compensate for the lack of mechanical retention. Resin chemistry led to the development of hybrid materials that blend the properties of conventional cements with the strength and insolubility of resin.
Resin-Modified Glass Ionomer (RMGI) cement is a popular hybrid option. It combines the fluoride-releasing capability and chemical adhesion of traditional GI with the improved physical properties of resin. The resin components enhance the material’s strength, reduce its solubility, and make it less sensitive to moisture during placement. RMGI is frequently selected for luting metal-based and zirconia crowns when a straightforward, moisture-tolerant procedure is desired.
For maximum bond strength and superior aesthetics, especially with thin, translucent restorations like veneers or all-ceramic crowns, Pure Resin Cements are the material of choice. These methacrylate-based cements achieve retention through microscopic mechanical interlocking and chemical bonding to both the tooth and the restoration. The procedure often requires multiple steps, including etching the tooth surface with phosphoric acid and applying separate priming and bonding agents to prepare the dentin and enamel.
Self-adhesive resin cements simplify this process by incorporating the etching and bonding agents directly into the cement paste, streamlining the procedure into a single step. While self-adhesive systems are easier to use and less technique-sensitive, traditional multi-step resin cements offer the highest bond strength. This strength is essential when the underlying tooth preparation is short or when bonding to enamel is required for maximum retention. Selection depends on the specific crown material, available tooth structure, and aesthetic outcomes needed.