The material informally called “dental glue” that secures a crown to a tooth is professionally known as dental cement or a luting agent. Its primary function is to create a durable, thin interface between the crown and the prepared tooth structure. The cement layer holds the restoration in place and provides a seal against the margin, preventing the ingress of bacteria, saliva, and debris that could lead to sensitivity or decay. An ideal cement must have a very low film thickness to ensure the crown seats completely and accurately, maintaining the intended fit.
Temporary Versus Permanent Dental Cements
Dentists use two broad categories of cement based on the required duration: temporary and permanent. Temporary cements are designed for short-term use, typically securing a provisional crown while the patient waits for the final restoration. These materials are formulated for easy removal, allowing the dentist to separate the temporary crown without causing damage. Temporary cements often contain zinc oxide, sometimes mixed with eugenol, an oil-based component that can have a soothing effect on a sensitive tooth.
Permanent cements are engineered for maximum bond strength and long-term stability, ensuring the final crown remains fixed for many years. Their composition is significantly more robust to withstand the constant forces of chewing and the harsh environment of the mouth. They are designed to be insoluble in oral fluids and provide a lasting marginal seal against microleakage. The choice between temporary and permanent cement is determined by the stage of the restorative process.
Detailed Types of Permanent Crown Adhesives
The most significant distinction among permanent crown adhesives lies in their chemical composition and the mechanism they use to adhere. Understanding these differences explains why a dentist chooses one type of cement over another for a specific restoration.
Resin Cements
Resin cements are polymer-based materials known for offering the highest bond strength and superior aesthetics. Their retention mechanism is micromechanical, penetrating the microscopic pores and irregularities of the prepared tooth and the crown material to physically interlock upon setting. For the strongest bond, these cements often require a separate adhesive or bonding agent applied to the tooth, followed by pretreatment of the crown surface, such as etching or priming. Resin cements are the preferred choice for all-ceramic restorations and crowns made from materials like lithium disilicate, which lack the inherent strength of metal and require the cement to contribute to reinforcement.
Glass Ionomer and Resin-Modified Glass Ionomer Cements
Glass Ionomer (GI) cements are unique because they form a chemical bond with the calcium ions in the tooth structure. This material is composed of fluoroaluminosilicate glass powder mixed with polyacrylic acid. A beneficial property of both GI and its improved version, Resin-Modified Glass Ionomer (RMGI), is their ability to release fluoride over time, which helps prevent recurrent decay at the crown margin. RMGI cements contain a resin component that improves physical properties, such as strength and resistance to moisture, while retaining the fluoride-releasing benefit. These are widely used for routine cementation of metal-based and porcelain-fused-to-metal crowns due to their ease of use and moderate technique sensitivity.
Zinc Phosphate Cement
Zinc phosphate cement is a traditional luting agent, having been used in dentistry for over a century. It is an acid-base reaction product of zinc oxide powder and a liquid phosphoric acid solution. This material relies primarily on mechanical retention, acting as a simple filler that hardens to physically block the crown from being dislodged. Because it does not chemically bond to the tooth, its use is typically limited to restorations that have excellent retention form from the tooth preparation alone, such as full-metal crowns.
Clinical Factors Dictating Material Selection
The dentist’s decision regarding which permanent cement to use connects the science of the materials to the specific clinical needs of the patient and the crown. A primary consideration is the crown material, as certain crowns require a specific cement to maximize their strength. For instance, a thin, all-ceramic crown benefits from the high bond strength and reinforcement provided by a resin cement, while a robust full-metal crown can be secured reliably with a less complex cement like RMGI.
The geometry of the prepared tooth is also a major factor, particularly the amount of remaining tooth structure and the height and taper of the preparation. If the tooth preparation is too short or overly tapered, providing minimal mechanical retention, a high-strength adhesive resin cement is often chosen to compensate for the lack of physical grip. Conversely, a well-prepared tooth with high walls and minimal taper provides excellent inherent retention, allowing for the use of a more conventional cement.
Another consideration is the need for absolute moisture control during cementation. Resin cements are highly sensitive to contamination from saliva or blood, and the inability to maintain a dry environment may lead the dentist to choose a moisture-tolerant cement like RMGI. Finally, for sensitive teeth, a less acidic cement, such as a self-adhesive resin or RMGI, may be preferred over traditional, more acidic options like zinc phosphate, to minimize post-operative pain.