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COSMETIC DENTISTRY: Preparation Considerations for Posterior Restorations Utilizing Bonding Technology

October 1, 2001
by Steven J. Hill, DMD


Restorations that require cores of metal or other rigid materials can provide the strength and some of the beauty our patients demand but they do so by sacrificing tooth tissue. Advances in adhesive dentistry have produced bond strengths that have given us greater predictability when using esthetic restorative materials thereby allowing us to provide our patients the results they demand with a less invasive procedure.

Dentists have been reluctant to provide direct and indirect posterior esthetic restorations as both their preparation and their delivery is radically different to traditional restorations. Bond strengths to enamel are now such that if sufficient force is applied to the restored tooth, either the restoration will fail or the tooth structure will fail but not the bonded interface.1 Bond strengths to dentin are less predictable but adequate for good clinical performance. This allows us to change traditional retention and resistance form in our preparation and makes the bonding protocol essential for success. This is where the reluctance rises. Many of us have not received formal instruction in dentin hybridization and in truth, the final chapter on it probably has not been written. Without faith in the process, dentists will not adopt preparation principles that abandon retentive form.

The purpose of this article is to give clinicians an overview of preparation design principles that the author considers being essential to ensure success with conservative posterior bonded restorations. The readers will need to familiarize themselves with modern bonding protocols before attempting these restorations. Practice with anterior bonded restorations is recommended to increase your trust in the systems (isolation of the anterior is much simpler). Trust is only gained by repetition of clinical success.

G.V. Black’s legacy to dentistry was a protocol that, when followed, ensured a successful result with the materials of the day. It seems like some authors have suggested that by simply ignoring these principles, success with the new materials will be ensured. In truth, what we need are new protocols for the new materials that must be followed in strict sequence to ensure success. Just as we learned in dental school, with the proper diagnosis, preparation and delivery principles, a long lasting, functionally sound and esthetically pleasing restoration will be provided. With traditional cast restorations, compromising the protocol invited poor results. With the bonded restoration, compromise invites disaster.

INDICATIONS & CONTRAINDICATIONS

Thorough clinical diagnosis and planning are of utmost importance in determining which teeth might be best restored with a bonded restoration. After those considerations have been met, planning of the preparation can begin.

If complete isolation of the prepared teeth from all oral fluids cannot be achieved, adhesive dentistry is contraindicated. Similarly, if a clear enamel margin is not likely to be visually evident at the end of the preparation because of decay or previous old restorations, adhesive restorations are not indicated. Teeth that require extensive core build-up are to be considered carefully. Hybrid composite materials expand and contract 4-5 times more to heating and cooling than do porcelains or tooth structure. The treatment modality of choice in these situations may be a traditional cemented casting, a porcelain-fused-to-metal crown or a cored ceramic such as Procera. The sacrifice in tooth structure for use of these options is balanced by their inherent ability to withstand tensile forces.

All opposed teeth are subjected to forces from occlusion. These forces result in stress and for our purposes, we need to consider compressive, tensile and sheer stress.2 A fragile pane of glass can be walked upon if the underlying floor properly supports it. Glass, like our modern esthetic restorative materials, resists loads that generate compressive stress well but will fail under loads that generate tensile or sheer stress. The occlusal force necessary to cause a typical esthetic restoration to break under compressive stress is 5 to 10 times greater than the force required to break the same restoration under tensile or sheer stress (Figs. 1 & 2). When preparing teeth for restoration then, we need to plan for occlusal loads and provide underlying support so that tensile and sheer stresses are minimized.3

In addition to a healthy periodontium, a balanced occlusion is crucial. In the cuspid guided occlusal scheme,4 forces directed through posterior teeth during function are largely compressive (Fig. 3). It follows then that if the patient or the dentist is not prepared to balance an occlusion to cuspid rise and remove non-working side interferences, extensive adhesive restorations are not indicated.

MATERIAL SELECTION

There are a wide variety of esthetic materials available today. Generally the more translucent ones are more brittle. Most will serve the purpose of restoring molar teeth well if there is sufficient tooth structure left to support these materials once they are bonded in place. Some of these restorative materials may arguably be better than others, but there exists enough fracture resistance in well-condensed feldspathic porcelain to predictably restore the posterior dentition. The reader may choose from cast, stacked or pressed ceramics or from the variety of polyglass materials available. It is imperative that careful attention is paid to case selection and tooth preparation. If resin cements are employed one must ensure that they are utilized appropriately.

PREPARATION DESIGN

Successful restoration of the dentition is the focus of much of our early training in school but clinical experience is vital to the decision making process. The decision to cover cusps needs to be made by the competent practitioner given the specific circumstances presented to him at the time.

If, in the opinion of the dentist, sufficient tooth structure remains after tooth preparation to withstand occlusal forces, an inlay preparation is indicated. If the restoration must serve to protect tooth cusps, then an onlay preparation may be considered. This applies to both direct and indirect restorations. One tremendous advantage that direct composite restoration have over traditional amalgam restoration stems from the divergence of the prepared walls of the tooth. Undercuts for retention are not necessary so that the remaining tooth structure is supported by itself and is much less prone to fatigue fracture.

The esthetic bonded restoration performs well under compressive loads so it follows that we must provide adequate thickness of material especially under occlusal contact points. Identifying these points before preparation begins is as helpful as it is simple. Mark the occlusal contacts with articulating paper as your first step. In the case of the inlay preparation, the contacts we would be concerned about would be found in the central fossae and on the proximal marginal ridges. Most authors agree that a minimum thickness of 1.5 mm of restorative material is sufficient. Most laboratory technicians confide that they almost never receive this minimum standard.

To get around this problem, I use any small carbide bur that I have verified its head length with a Boley gauge (Fig. 4) to be 1.5 mm (different manufacturers make different head sizes). I first make a vertical cut into the appropriate occlusal contacts that have previously been identified with articulating ribbon to the depth of the bur. I then mark the cuts with a quick spray of Occlude. Depth reduction has been completed when the color on the floor of the preparation is gone.

Inadequate reduction is rarely a problem in the interproximal box as either previous amalgam restorations or caries dictates sufficient depth. In the case where the proximal contact was not involved but simply undermined by the existing restoration, breaking the contact and leaving a light to medium chamfer is to be avoided. As the interproximal contour will be restored, this leaves the forces of occlusion to result in sheer stress of the restoration leaving it prone to breakage (Fig. 5). The floor of the ‘box’ should be a minimum 1.5 mm wide mesiodistally with a butt joint and blend into the flaring walls. Ninety-degree angles anywhere in the preparation are to be avoided.

The walls are prepared to diverge by 10-15 with special attention paid to the proximal wall (Fig. 6). Stress areas inside the restoration are most prone to occur where the internal form of the restoration is generated around sharp 90 (or more) angles. These areas can develop microfractures during fabrication and fail (i.e. break) in the lab or (most distressingly) during seating.

Extension of the preparation follows the fissures. This extension can be very minimal with direct techniques and might utilize the air abrasion technology. The fissures are cleaned and inspected for caries and appropriately restored.

When the decision is made to cover the cusps, an onlay preparation begins where the inlay preparation left off. Additional depth cuts are placed where the articulating ribbon has identified the functional cusps and these are marked with Occlude (Fig 7-8). A coarse football diamond is used to join the depth cuts. Under areas of load, a concave or flat surface is provided in order to convert the forces of occlusion into compressive stress wherever possible. The lingual incline of the facial cusps of maxillary posterior teeth are not loaded during function in the cuspid protected occlusion so a thinning of the restoration can occur up to the cusp tip. This will leave the facial aspect of the tooth virtually untouched thereby preserving the esthetics. In Fig. 9a, the distobuccal cusp was lost leaving only the mesiobuccal. The preparation is smoothed over with fine diamonds or a white stone (note that in the example a core build-up was performed and the disto-occlusal restoration on 15 was replaced at the same time). The form of the final preparation is best seen in the prepared die (Fig. 9b).

Depending upon the extension of the undercut of the previous restoration, often the flaring of the mesial proximal wall will involve the facial cusp tips. If it does, the prep is finished with a butt margin parallel to the enamel rod orientation and the laboratory is instructed to use incisal shade porcelains to better blend the color. This is easier prescribed than fabricated and a discussion with your ceramist is well advised before beginning.

The extensive butt margin of the functional cusp (the palatal, in this example) typically does not blend as well with the supporting tooth structure (Fig. 10) as the non-functional cusp (Fig. 11). This is rarely a problem for the patient but confirmation of this is always advisable before final impressions are taken. It more often is a concern for patients when mandibular posterior teeth are prepared because of the high visibility of the lower functional cusps. If it is a concern, then an esthetic modification can be provided. A medium chamfer margin (0.3-0.4 mm) is extended down the tooth facially and the laboratory is instructed to use more translucent material in order to draw the existing tooth color into the restoration. With careful planning of the occlusal loads, this extension of the restoration can be kept thin.

SHADE SELECTION

We use our best science to restore a tooth to function, but it takes more than that to restore its ‘looks’. We study dental esthetics in order to reduce the visual impact that our restoration has on the tooth. ‘A picture speaks a thousand words’ and nowhere is this more true than with communication to the ceramist who will create the restoration. Accurate color mapping is a time consuming process that this author avoids.

A preoperative photograph of the tooth is taken with an appropriate shade tab in the frame and a postoperative photograph is taken as well with a shade tab estimating the color of the prepared tooth. Prints are easier to view than slides and although the printing process may not replicate the colors exactly, having the shade tabs in the frame allows for some correction. This holds true for both traditional and digital image processing. There are a wide variety of cameras capable of providing us appropriate information and the author suggests using one that you are comfortable with.

PROVISIONALS

Providing predictable temporary restorations for teeth prepared this way is challenging. Inlays can be quickly temporized using Fermit-N). The complete lack of parallelism anywhere in the onlay preparation however, does not allow for use of tradition temporary cements such as Nogenol. The author recommends using Seal&Protect (Dentsply) on the dentin followed by free-hand placement of composite filling material to roughly restore the contours of the tooth or use of a self-cure, bis-acryl temporary material in a pre-prep sectional alginate. After curing, the contacts can be cleared carefully with floss and the occlusion adjusted. Seal&Protect is marketed as a self-etching dentinal adhesive to decrease dentinal sensitivity and the author finds that its bond strength is ideal for temporization. Additionally, use of this product results in near-zero intra-operative sensitivity.

Fermit-N removal from inlay preps is straightforward. Onlay removal entails cutting a cross-shaped groove, under local anaesthetic, onto the surface of the provisional onlay and the pieces fractured off with a plastic-forming instrument. The residue from the dentin-bonding agent is scrubbed off with 50u pumice and the final restoration is tried in for fit and cementation.

CONCLUSIONS

Our patients today are increasingly more demanding of their esthetic tooth restoration. Metal free restorations provide the results for these patients and they can be more conservative of natural tooth structure when appropriately done. The care and attention that was required in the past to achieve good results is still demanded of us, but we need to adopt new preparation protocols to ensure clinical success with the new esthetic materials.OH

Dr. Hill is an Accredited Member of the American Academy of Cosmetic Dentistry. He is co-founder and current president of the Western Canadian Academy of Cosmetic Dentistry. Dr. Hill is Oral Health’s contributing consultant, Cosmetic Dentistry.

Dr. Hill wishes to acknowledge Dr J. Eubank of Plano, TX, for sharing his insight and experience with all-ceramic restorations and Mr. William Varda of Anvarda Laboratory, Vancouver, BC for the restoration featured here in this article.

Oral Health welcomes this original article.

REFERENCES

1.Jorgensen KD., Asmussen, E., and Shimokobe, H. Enamel damage caused by contracting restorative resins. Scand.J.Dent.Res.83:120-122, 1975.

2.O’Brien, WJ. Dental Materials: Properties and Selection. Quint.1989.

3.Eubanks, J. Interview after lecture series at LSU 1997.

4.Fox, CW., Abrams, BL.,and Doukoudakis, A. Principles of Anterior Guidance: Development and Clinical Applications. J.Cranio. Pract.vol2, no.1, p.24-29.