Placement

The placement of definitive crowns or retainers is the last important area to be evaluated for the prevention of tooth hypersensitivity. The margins of the casting should be checked on the die and intra-orally for accuracy of fit. Several liquid marking materials are available that can be applied to the inside of the casting. These films are water-soluble and easily cleaned off (fig. 7). The marking material will indicate very small areas of rubbing that prevent a crown from fully seating (fig. 8). These areas can be adjusted with a high-speed bur. The film is reapplied as needed until the casting is judged to be seated completely. All castings need some adjustment due to the fact that the casting procedure will produce shrinkage and there are dimensional changes with all die materials. The use of 50-micron aluminum oxide to air abrade the internal surface of the casting will clean it thoroughly and enhance retention of the restoration. Air abrading will also remove the glaze on a porcelain facial margin to provide a better adhesion of the luting agent to the porcelain.

The occlusion should be carefully evaluated intra-orally and when all adjustments have been completed, the prosthesis should be re-polished and placed in an ultrasonic unit for five minutes. The author uses a mixture of pumice and chlorhexidine gluconate applied to the entire tooth preparation to remove any remnants of the temporary cement or other debris. Particular attention should be paid to the finish line. Following rinsing with warm water, the teeth should be left in a moist condition. Salivary contamination should be avoided prior to cementation. The teeth should be dried immediately before the crown being cemented is seated on the tooth.

The author rarely uses local anesthesia to perform these placement procedures. This has the added advantage of allowing the patient to retain proprioception while evaluating the occlusion with the definitive prosthesis or prostheses.

There are many products available for the cementation of definitive fixed prostheses. It is recommended that the clinician check with technical support provided by each company regarding questions about their product.

Currently, the clinician can choose between adhesive and conventional luting agents. Some of the all-ceramic systems require the use of a resin-luting agent to "bond" the restoration to the tooth. This may result in hypersensitivity following placement of the restoration because the dentin was etched with acid as part of the seating process. Zinc phosphate cement develops a low pH during the setting process which can contribute to hypersensitivity.

The post cementation sensitivity associated with glass ionomer cement has been found to be the result of improper manipulation of the material and not the cement itself. Proper manipulation of any luting agent is important so that the restoration is allowed to seat completely with the appropriate film thickness for the material used. All luting agents that contain acrylic resin shrink when they set. The bond to the restoration is greater than the bond to the dentin and results in uniaxial shrinkage of the material. This creates gaps at the dentin-restoration interface if the film thickness is too thin. This
permits contamination with bacteria and oral fluids. The leakage and bacterial contamination is thought to be a major cause of post cementation sensitivity when resin-luting agents are used.

It is impossible to prevent all hypersensitivity when teeth are prepared to receive some type of full or partial coverage restoration. If attention is paid to each of the steps involved from diagnosis to placement, tooth hypersensitivity can be greatly minimized and, in the majority of cases, eliminated. It is important to remember that even though we as clinicians are meticulous in our preparation, impression, provisional and placement procedures, these are traumatic events to the tooth.