Dental cement is a specialized bonding agent designed to affix indirect restorations, such as crowns, bridges, inlays, and onlays, to the underlying tooth structure. Its purpose is to create a secure, durable seal that resists the constant forces of chewing and the harsh, moist environment of the mouth. The cement fills the minute space between the prepared tooth and the restoration, preventing micro-leakage and ensuring the longevity of the dental work. Proper application is paramount, relying heavily on correct material selection and a precise, controlled technique.
Selecting the Right Cement
The choice of dental cement is the first step in the application process, as it directly impacts the bonding mechanism and the overall durability of the restoration. Cements are broadly categorized into temporary and permanent types, each formulated for specific clinical needs. Temporary cements, often zinc oxide-eugenol based, are designed for short-term use, such as securing provisional crowns, offering a less aggressive bond that allows for easy removal once the final restoration is ready.
Permanent cements, conversely, are engineered for a long-lasting bond and are further divided into conventional and resin-based systems. Conventional cements, including glass ionomers and zinc phosphate, primarily rely on mechanical retention features built into the tooth preparation. Resin-based cements, which are polymer-based composite materials, offer superior strength and are used with restorations lacking strong mechanical retention, often requiring an adhesive bonding agent. Glass ionomer cements also offer the benefit of fluoride release, which can help prevent decay around the restoration margins.
Preparing the Tooth and Restoration
Achieving a lasting bond depends heavily on meticulous preparation of both the tooth surface and the internal fitting surface of the restoration. The process begins with the complete removal of any previous temporary cement, debris, or plaque from the prepared tooth, often achieved with a gentle scrub using non-fluoridated pumice. The tooth must then be isolated from saliva and blood, as moisture contamination is a common cause of cement failure, especially with resin-based materials.
For many advanced bonding agents, the prepared tooth surface requires specific chemical treatment to enhance adhesion. This can involve a conditioning step, such as acid etching with phosphoric acid to micro-roughen the enamel, followed by the application of a dentin primer or adhesive. The internal surface of the restoration, known as the intaglio surface, also needs preparation, which may include sandblasting for metal or zirconia, or treating ceramic with a silane coupling agent. This dual preparation ensures the cement can effectively adhere to both substrates, creating a unified and strong interface.
Mixing and Placement Techniques
Precise mixing of the cement is critical, as it determines the final physical properties, working time, and film thickness of the material. For conventional powder-liquid cements, the manufacturer’s recommended ratio must be strictly followed to ensure the correct consistency, typically a smooth, creamy texture for luting. Mixing is often performed rapidly on a cool glass slab to dissipate the heat generated by the chemical reaction, which helps to extend the working time.
Many modern cements come in pre-measured capsules or automix syringes, eliminating the variability of hand-mixing. Once mixed, the cement is applied directly to the internal surface of the restoration, ensuring the entire inner wall is coated without excessive bulk. The restoration is then swiftly and firmly seated onto the prepared tooth, applying steady, consistent pressure to ensure it is fully positioned before the cement begins its initial set. This seating pressure is necessary to displace the cement and achieve the thinnest possible film thickness, which is a requirement for a successful seal.
Curing and Finishing the Seal
After the restoration is fully seated, the cement enters its setting or curing phase, which varies based on the material’s chemistry. Chemically cured cements polymerize through a self-initiating chemical reaction, while light-cured cements require exposure to a specialized curing light. Dual-cure cements, common with resin materials, offer both light and chemical setting mechanisms, ensuring complete polymerization even in areas the light cannot penetrate, such as under thick or opaque restorations.
The excess cement, often called flash, must be removed promptly after the initial set, but before the material is fully hardened. Removing the flash when it is in a rubbery state prevents chipping the margins of the restoration and minimizes the risk of leaving hardened pieces that can irritate the gum tissue. A sharp explorer or scaler is typically used to clean the buccal and lingual surfaces, and dental floss is carefully threaded through the contact points to remove interproximal excess. Finally, the patient’s bite, or occlusion, is checked to ensure the restoration does not feel too high, preventing undue stress that could lead to fracture or premature cement failure.