Calcium hydroxide has an alkaline pH between 9 and 11. This alkali stimulates secondary dentin when in direct contact with the pulp, providing a barrier between pulp and restoration. It has some antimicrobial and thermal insulating properties and provides minimal strength to support the forces of condensation. Under amalgam restorations calcium hydroxide paste slowly leaches out over time.

Mechanical luting with traditional cements was the only means of accomplishing cementation until dentin bonding with acid-etch systems was developed in the mid-1960s, leading to today’s adhesive dentistry. Successful cementation ensures predictable retention for the indirect restoration. The restorative material itself may be the primary determinant for the cement selection. When choosing a luting/bonding agent, the clinician must consider which of the following indirect restorations are being used:

Other considerations include the amount of mechanical retention from the form of the preparation and the patient’s parafunctional habits (i.e., bruxing). In order for the cement to be successful, the tooth preparation must have adequate retention and resistance form. While bonding cements can enhance retention of the restoration, they cannot overcome the negative influence of an over-tapered, non-retentive preparation.

A luting agent is a material with low viscosity that, placed between tooth structure and a prosthesis, sets and firmly attaches the prosthesis to the tooth structure. New-generation luting cements with adhesive capability are now successfully used to improve the clinical success of fixed prosthodontics.

Cements used for permanent or temporary luting (i.e., the cementing together of two components) of fixed prostheses, orthodontic bands and brackets, and pins and posts must have good wettability and flow to provide a thin film thickness. When the tooth structure and the fixed prostheses are in intimate contact, a microscopic space exists—the tooth/restoration interface. This space is often created in the dental laboratory to allow room for the cement. Without this interface and because of the thickness of the cement, the restoration may be prevented from seating completely. CAD/CAM restorations allow the cement space to be milled into the restoration. The primary purpose of both luting and bonding cements is to fill this space (Figure 14-3). Cements mixed to primary consistency must have low-enough viscosity to be able to flow into a film thickness of 25 μm (micrometer, or micron) or less. If the viscosity of the cement is such that the prosthesis fails to regain intimate contact with the tooth, a thick layer of cement will be exposed at the margin. This may also occur if the margins do not fit precisely before luting. Exposure of cement to oral fluids at the margins results in dissolution of the cement. As cement is lost at the margins, bacterial plaque can accumulate beneath the crown where the patient cannot clean, resulting in tooth sensitivity and/or recurrent caries. Bonding cements have the ability to chemically bond the prosthesis to the tooth. The chemical bond enhances retention by reducing the separation created by the tooth/prosthesis interface.

As temporary/provisional and intermediate restorations, dental cements are mixed to secondary consistency. The choice of cement for this type of restoration is largely based on the particular clinical situation. Provisional restorations are used for emergency situations when appointment scheduling does not allow sufficient time to place a permanent restoration. Provisional restorations are placed when a tooth is symptomatic or when deep caries removal is required. By placing a sedative provisional restoration, the dentist is able to evaluate the response of the pulp before reappointment for a permanent restoration. Provisional and intermediate restorations are used to restore teeth awaiting treatment such as inlays, between endodontic appointments, or when extensive treatment plans require several weeks or months of coverage before treatment can be completed.

See Chapter 18 (Provisional Restorations) for a discussion of provisional luting and intracoronal cement provisionals.

The strongest cements are resin cements, and the weakest is zinc oxide eugenol. Cements used for permanent luting and high-strength bases need good compressive, tensile and flexural strength. As can be seen in Table 14-2, resin-based cement is high in mechanical strength and fracture toughness, and polycarboxylate cement is low in both. Most cements consist of a combination of powder and liquid; their ratio determines many of their properties. The strength of cement is controlled primarily by the amount of powder used in the prepared mix. In general, an increase in the powder-to-liquid ratio increases the strength of the cement. However, excessive powder or liquid can weaken the cement.

For primary consistency, also known as luting consistency, cements should be able to be mixed thin—to about the consistency of honey.

In many dental applications, chemical adhesion and mechanical adhesion occur together. We now have a much better understanding of the surface characteristics of enamel and dentin and the requirements needed to obtain good bonds to them. The most recent dental cements, using resin adhesive technologies, strive to achieve a micromechanical and chemical bond between the tooth and the restoration. Several things may weaken the strength of the adhesion between two materials, including differences in the coefficient of thermal expansion of the two materials, dimensional changes that occur during setting of the adhesive agent, and contamination of the substrates by water and saliva or by residual enamel and dentin debris, known as the “smear layer.”

There are three possible reasons for failure of an adhesive. Structural failure occurs as a result of internal failure of the substrate material; the tooth itself breaks away from the restoration, leaving the restoration intact. Adhesive failure occurs when the substrate separates from the adhesive layer; the restoration is dislodged from the tooth and/or recurrent caries forms. Cohesive failure occurs when there is internal failure of the adhesive layer; this may also result in dislodging of the restoration with the cement layer still present in the restoration and on the tooth substrate (Figure 14-7).

Failure of the adhesive will result in failure of the restoration if the direct or indirect restoration breaks away from the tooth structure altogether, or if leakage occurring between the tooth structure and restoration or prosthesis, results in deterioration of the margins and the formation of secondary caries. The acid-etch bonding system is the basis of today’s adhesive dentistry. In this process hydroxyapatite is removed from the tooth structure with the etchant to create micropores, and the resin monomers of the cement fill the micropores to create resin tags that micromechanically lock the substrate to the restoration (Figure 14-8).

Even skilled hand mixing may incorporate air that is thought to lead to the reduction of bond strength between the restoration and the tooth substrate. If hand mixing is chosen, the amount of time taken to mix the cement must be carefully monitored: too much time and the cement may be too viscous; too little time and the cement may not be mixed thoroughly (Figure 14-9).

Cements, like composite restorative materials, come in self-cure, light-cure, and dual-cure delivery systems.

Advantages and disadvantages of each delivery system are listed in Table 14-3.