Non-Retentive, Adhesively Retained All-Ceramic Posterior Restoration

[DrAnil]

 

“Table-Top” Preparation Technique

When a tooth has been treatment planned for a full-coverage restoration, a traditional crown preparation is usually the procedure of choice. Kois states that cases that require cuspal protection but still maintain significant structural integrity in an axial dimension may be suited for an adhesively retained restoration.

The following steps show the progression of the “table-top” preparation for a typically encountered molar with a large 3-surface amalgam and that has been diagnosed with “cracked tooth syndrome.” For instructional purposes, in this article the steps are performed on a dentaform model. Understanding that this media represents the ideal scenario will give the reader a better visual of the steps involved using this particular preparation sequence. Whether to cover the cusp or not is a clinical decision. However, in their study, Krifka et al found that thin non-functional cusp walls of adhesively bonded restorations should be completely covered or reduced to avoid enamel cracks and marginal deficiency. As a general rule, cuspal coverage may be indicated where the remaining tooth structure is less than one-third to one-half of the intercuspal distance.

In addition to advocating the one-third/one-half rule, Christensen also warns the clinician to consider other factors. These include the presence of horizontal cracks in the tooth structure, a lack of supporting dentin under the cusp, the presence of a heavy occlusion, and highly discolored cusps in esthetically important areas. He also feels that patients who have a history of eating hard foods should not be left with an unprotected questionable cusp.

The minimal thickness of all-ceramic restorations should be at least 2 mm and follow the topography of the occlusal anatomy. The head length of a standard #330 carbide bur is 2 mm and can be used as a depth cutter in all areas of the tooth. The first step is to take a #330 carbide bur, place it in the central fossa, and drill down into the existing restoration until the hilt (where the head and the shank meet) is level with the occlusal surface of the restoration (Figure 1). From here, the bur is moved through the buccal groove until it cuts through the buccal wall of the tooth and then to the lingual side through the lingual groove (Figure 2). The bur is then moved from the central fossa to the mesial marginal ridge and back through the restoration to the distal marginal ridge. A cross-pattern depth cut of 2 mm can now be observed from the occlusal aspect (Figure 3).

The next step is to remove all of the cusps to the level of the cross-pattern occlusal depth cut. The #330 carbide bur or a #169L carbide bur can be used. Keeping the shank of the bur parallel with the pulpal floor, a cut is made under the entire mesial–buccal cusp starting at the buccal groove to the mesial marginal ridge depth cut (Figure 4 and Figure 5). The bur is then moved back to the buccal groove and directed to the distal marginal depth cut, which removes the entire distal–buccal cusp (Figure 6). The handpiece is then positioned on the lingual aspect of the tooth and both the mesial and distal–lingual cusps are removed (Figure 7).

At this point a minimum of 2 mm clearance has been provided for the ceramic material. If any of the existing restorative material still remains, it is then removed with a modified shoulder diamond bur. This removal will add to the final thickness of the all-ceramic restoration and also create an isthmus that will provide orientation as a positive seat insertion of the restoration (Figure 8). If there is no remaining restorative material an isthmus should still be provided for orientation purposes.

Interproximal Area

The interproximal areas become involved when there is an existing restoration, fractures, or caries. Using the same diamond bur, the restorative material is removed from the mesial and distal interproximal box areas. This step lowers the interproximal margins in a more cervical direction and also provides further orientation guidance for seating the restoration.

In cases where there is no interproximal restorative material present and the contact is still intact after the 2-mm reduction is completed, a decision must be made to either break the contact or leave it intact. From a laboratory standpoint or if a chairside digital scanner is being used to acquire the image of the preparation, breaking the contact will enhance the ability to locate the margin. A #7801 12-fluted finishing bur is used to break the interproximal contact (Figure 9). The bur is placed on either the buccal or lingual side and swept through the contact area. The thickness of this size bur provides sufficient room for impression material or a scanable view with a digital acquisition camera. The outer contour of the tooth will be flattened as the bur is passed through the interproximal space as irregularities in the anatomical form are eliminated. The flat margin enhances the margin tracing in a digital scenario as well as a traditional laboratory setting.

An end-cutting diamond bur is then used in the interproximal box area to eliminate any abrupt dimensional vertical/horizontal platform-to-wall transition changes (Figure 10). Sharp line angles tend to accumulate stress and should be avoided. Rounded internal line angles minimize stress concentrations.

An inverted cone diamond bur can be used to place further orientation grooves between the isthmus and the outer buccal and lingual occlusal tables (Figure 11). These added orientation grooves will help in the final seating of the restoration and contribute to the blending of the ceramic-to-tooth esthetic transition. The change from a flat occlusal platform to a varied platform helps create a chameleon or “contact lens” effect where the ceramic material accommodates the shade of the tooth.

As previously stated, the ceramic thickness is critical to the success of the restoration. A 2-mm flexible clearance tab can be placed over the occlusal surface of the prepared tooth and the patient instructed to close into centric occlusion. If sufficient reduction has been provided, the flexible tab should easily pull through. Any resistance encountered is then identified and corrected.

The last preparation step is to use a tapered finishing bur over the entire prepared tooth surface (Figure 12and Figure 13). This is done for two reasons. First, carbide finishing burs will produce a smoother surface compared to a diamond bur. A smooth, rounded prepared tooth surface reproduces better with all impression materials and die stones. Also, a smooth margin is easier to read on a digitally scanned virtual model (Figure 14 and Figure 15).

Secondly, coarse diamonds produce a thick, uneven smear layer, whereas carbide burs produce a thin, even smear layer. The significance in the different smear layers is pertinent when a self-etching adhesive is to be used. Yiu et al found higher bond strengths were achieved with a self-etching adhesive when it was applied on dentin surfaces that had been prepared with carbide burs. There was less penetration of the milder acids contained in self-etching adhesives through the thicker, more uneven smear layers produced by diamond burs. The thicker smear layers also had more of a buffering or neutralizing effect on the milder acids. Barros et al found in their study that carbide burs leave a surface that is more conducive to bonding than diamond burs.

After this step, gingival retraction is initiated wherever necessary. This can be done with either a non-impregnated retraction cord or a diaode laser. If using the traditional two-appointment method, the exposed dentinal tubules should be sealed prior to the impression step or digital scanning. A one-step, two-step, or three-step method of applying a dentin adhesive can be used. It is imperative that the oxygen-inhibited layer be removed by applying a water-soluble clear gel over the resin-coated prepared tooth and light-polymerized. It is recommended that pumicing of the sealed surface be completed prior to taking the impression. If an in-office one-appointment milling method (eg, CEREC, Sirona Dental Systems, Inc, http://www.sirona.com or E4D, D4D Technologies, http://www.e4dsky.com) is used, then the sealing step is omitted and the prepared tooth can be digitally scanned.

Provisional Restorations

Provisional restorations are only necessary for the two-appointment method. In these cases, since the dentinal tubules have already been sealed, postoperative sensitivity is not an issue. The purpose of the provisional at this point is to maintain the positions of the adjacent and opposing teeth. Composite material can be placed using either a free-hand technique or a vacuum-formed stent from a preoperative model. Retention is gained from the interproximal undercuts from the adjacent teeth and by extending the provisional material into the undercut area below the buccal and lingual margins.

Ceramic Preparation for Bonding

 

The bonding mechanism of a resin to a ceramic surface is a combination of the effects of micromechanical interlocking and chemical bonding. There are numerous articles addressing the various methods of conditioning the intaglio ceramic surface for bonding purposes. Micromechanical interlocking is created by acid or sand-air abrasion or roughening the surface with a diamond bur. Each of these methods creates microporosities and increases the surface area. However, overuse of each of these methods can create surface flaws leading to crack initiation. Strong micromechanical bonds are formed as the resin flows and interlocks into the porosities.

The bond strength of the chemical bond between the resin and the ceramic is affected by the silane coupling agent. Hydrofluoric acid has been the most commonly used acid to obtain microporosities through etching of glass surfaces. It is imperative to seek out the manufacturer’s recommendations for bonding resin to their ceramic. This information would also include the type of acid they recommend as well as the concentration and the etching time.

The alternative is phosphoric acid in combination with a ceramic primer. The phosphoric acid is applied for 5 seconds and rinsed with water. The primer is then applied with a microbrush to the intaglio surface and can be immediately air-dried.

Insertion

The insertion steps differ somewhat depending on whether a one- or two-appointment procedure was used. Both techniques require strict isolation either with a rubber dam or some other system that ensures a dry field. For the one-appointment technique, the manufacturer’s recommended protocol for a three-step, two-step, or one-step dentinal adhesive system should be followed. For the two-appointment technique, after removal of the provisional restoration, the prepared surfaces should be lightly air-abraded to ensure a clean bonding surface. The recommended steps for whatever adhesive system is chosen are followed. The occlusion should only be checked after the restoration has been bonded to the tooth. Any adjustments should be finished with a series of diamond-impregnated points and polished with a bristle brush and diamond paste.

Clinical Case

This clinical case shows some of the preparation steps previously illustrated. The patient presented with a chipped distal–buccal cusp of the lower right first molar. There was a large 3-surface restoration present. The tooth was thermal sensitive and produced pain upon chewing, suggesting a “cracked tooth” problem. Diagnostic tests with a bite stick elicited discomfort from both the distal–buccal and distal–lingual cusps (Figure 16).

Using a #330 carbide bur, a depth cut to the hilt of the bur was placed buccal–lingually and mesial–distally, creating a cross pattern (Figure 17). On a horizontal plane, the same bur was used to connect the mesial and buccal depth cuts, which resulted in removal of the mesial cusp (Figure 18). The same steps were followed to remove the distal cusp (Figure 19). The lingual cusps were then reduced in the similar fashion (Figure 20). A flat-end diamond bur was then used to remove the decay in the mesial area and the remaining restoration and decay in the distal portion. A #7801 finishing bur was used to break the contact and flatten the mesial margin surface (Figure 21). The last step was to use a finishing bur (#7686) over the entire preparation to smooth any sharp angles and reduce the thickness of the smear layer (Figure 22). The tooth was scanned (Figure 23) and a milled lithium-disilicate restoration was fabricated. The pre-sintered restoration was tried in for fit and to check the occlusion. Surface stains were applied (Figure 24) and the crown was then removed. A spray-on glaze was applied. The crown was secured onto a crystallization pin with object putty and placed in a two-cycle porcelain furnace for final crystallization. Upon cooling, the restoration was bonded into place (Figure 25). In a open-mouth view, the unprepared axial walls of the tooth did not create any visual objection (Figure 26).

Conclusion

The non-retentive, all-ceramic posterior restoration is a viable option in specific situations depending on the location, esthetics, and occlusal habits that may be present. Its advantages include elimination of the axial portion of the traditional crown preparation, which provides for a more conservative approach to restoring posterior teeth. Case selection is vital to the success of the technique. Using the previously described step-by-step preparation technique will ensure proper occlusal reduction, preservation of enamel, and supra-gingival margins where possible, and improve margin definition and decrease the amount of time the preparation bur is in contact with the tooth.