A dental crown acts as a protective cap placed over a damaged tooth, restoring its shape, size, strength, and appearance. Dentists rely on a specialized adhesive known as dental cement to secure this restoration. This material must be biocompatible, withstand intense forces from chewing, and resist dissolution in the moist environment of the mouth. The selection of this cement is a precise clinical decision, as its properties directly influence the crown’s longevity and performance. The cement forms a thin layer, referred to as a luting agent, that fills the microscopic gap between the prepared tooth structure and the inside surface of the crown.
The Difference Between Temporary and Permanent Cements
Dental adhesives are divided into temporary and permanent cements based on their intended lifespan and function. Temporary cements, often formulated with zinc oxide eugenol or non-eugenol compounds, are designed for short-term use. They are deliberately weaker to allow for easy removal of the provisional crown or for the final crown during a trial period. Temporary cements are used while the permanent crown is fabricated, sealing the prepared tooth and minimizing sensitivity until the final restoration is ready.
In contrast, permanent cements are engineered for durability, strength, and long-term stability, creating a definitive seal. Their formulations withstand constant pressure from biting, resist oral fluids, and prevent microleakage that could lead to recurrent decay. The goal is to secure the crown for many years, making the crown and the tooth function as a single, cohesive unit.
The Main Types of Permanent Dental Cements
The modern dentist selects from a few primary classes of permanent cements, each offering different bonding mechanisms and properties.
Resin Cements
Resin cements are highly valued for their superior bond strength and ability to chemically adhere to both the tooth structure and certain crown materials. These polymer-based cements often require a separate bonding agent and a meticulous, moisture-controlled environment. They are typically the preferred choice for esthetic restorations like all-ceramic or zirconia crowns, where their strength and excellent color stability are beneficial.
Glass Ionomer (GIC) and Resin-Modified Glass Ionomer (RMGI) Cements
GIC and RMGI cements achieve a chemical bond with the tooth structure and are unique because they release fluoride into the surrounding tooth, which may help prevent decay at the crown margins. RMGI, which incorporates a resin component, offers greater strength and lower solubility than traditional GIC, making it a reliable option for many routine crown cementations.
Traditional Cements
Traditional cements, such as Zinc Phosphate and Zinc Polycarboxylate, are older materials that still have specific applications. Zinc Phosphate provides mechanical retention by physically filling the space between the crown and the tooth, relying on the crown’s precise fit and the tooth’s shape for stability. These traditional cements are sometimes used for metal or porcelain-fused-to-metal crowns that have strong built-in retention, as they lack a true chemical bond.
Factors Influencing Cement Selection
A dentist’s choice of permanent cement is a highly individualized decision based on several specific clinical variables.
The material of the crown is a major factor. High-strength resin cements are often mandated for bonding glass ceramic crowns, like lithium disilicate, to enhance the crown’s overall fracture resistance. Conversely, full-metal crowns with excellent mechanical retention may be adequately secured with more traditional cements.
The amount of remaining natural tooth structure, or retention form, also directs the decision. If the prepared tooth is short or lacks parallel walls, a high-strength adhesive resin cement is necessary to chemically bond the crown and compensate for the poor mechanical retention. The ability to maintain a dry, moisture-free field is paramount; if the tooth cannot be isolated, a moisture-tolerant material like RMGI may be selected over a moisture-sensitive resin cement.
Aesthetic considerations play a role, especially for front teeth, where translucent or tooth-colored resin cements are chosen to avoid showing a dark line through the ceramic restoration. The patient’s clinical history, such as a high risk of decay, might favor an RMGI cement because of its fluoride-releasing property.