For many years, I believed I had mastered tooth preparation since I did it day in and day out in my practice. A busy schedule adds constraints that create a process where patterns develop. These patterns result in our preparations moving from being individualized, based on the demands of the clinical situation, to the repetition of a certain style of prep.
We discuss tooth preparation by type – full coverage or partial coverage – and commonly by reduction – minimal vs. aggressive. These broad categories do describe tooth preparation, but they also limit our thinking.
Each time I sit down to prepare a tooth, I follow a decision tree based on the present condition of the tooth and the desired outcome. These choices then determine margin placement and reduction. After the prep is complete, I give it a name and place it in a category, only because I need to for record-keeping and insurance companies.
There are risks and benefits to both minimal and aggressive styles of tooth preparation. The concept behind minimal prep is to conserve as much of the natural tooth as possible. This should be part of our thought process every time we pick up a handpiece.
Minimal prep veneers can be "no prep" and minimal reduction preparations (0.3 mm gingival third, 0.5 mm middle third, 0.7 mm incisal third, no incisal reduction, and no interproximal reduction). This design is the perfect solution in many clinical situations. It allows us to conserve tooth structure, use supragingival margins, and bond exclusively to enamel.
Along with these benefits come some challenges, including limited ability to change tooth color, risk of bulky labial contour, and challenging fabrication of provisionals and the final restorations. Often we have patients who present with imperfections in the enamel or undersized teeth, but have beautiful natural tooth color. These are perfect clinical indications for minimal reduction and partial coverage.
At the other extreme are extensive veneer preparations (0.8 mm gingival third, 1.0 mm middle third, 1.2 mm incisal third, 2.5 mm incisal reduction, and prepared through the contacts to the lingual line angle). The first point to know with this type of prep is that your ceramist will love you. The more tooth reduction we give the ceramist, the more control that person has regarding the final esthetics. Aggressive preps increase our ability to change the contour and color and make provisional fabrication much more predictable.
One challenge is that the veneers are now being bonded primarily to dentin. Dentin bonding is something routine today, and it adds a clinical factor we need to evaluate that impacts longevity. In clinical cases where we are changing the final tooth color more than two shades on a classic shade guide, or have discolored teeth and the goal is to stay with partial coverage, this amount of reduction is a necessity.
Incisal edge reduction allows the addition of incisal effects, such as translucency and dentinal lobes, to be added to the restoration. Additionally, incisal reduction is needed if we are moving the position of the tooth either labially or lingually in the arch. Preparing through to the lingual side of the contact facilitates altering tooth contour, closing black triangles, or correcting a tooth that is rotated.
In between minimal and extensive preparation, we can create an unlimited number of preparation styles. For me, tooth preparation always begins with treatment planning. Begin by determining the final position, including incisal edge position, labial and lingual surfaces, and alteration to tooth alignment and rotation. The addition of length to incisors will reduce the amount of incisal edge reduction required. This planning can be communicated to the technician and a diagnostic wax-up produced.
From this wax-up, a bis-acryl mock-up of the proposed tooth position over the unprepared teeth facilitates accurate, conservative reduction. The mock-up is accomplished using a silicone matrix made on the provisional model (I recommend using a stone duplicate of the wax-up), and spot-etch the labial surface of the teeth to be prepared. Fill the silicone matrix with bis-acryl and seat over the teeth. When fully set, remove the matrix and evaluate the mock-up. The material should be extremely thin over the gingival tissue and easy to separate and remove.
If the material is thick over the tissue, it usually indicates the matrix flexed due to pressure when seating and the proportions of the mock-up will not duplicate the wax-up. If the mock-up is accurate, you can now place depth cuts into the acrylic, thereby knowing you have adequate room for porcelain without removing unnecessary tooth structure.
Commonly we want to alter the rotation or AP position of teeth in the arch, without creating unnatural tooth proportions. This can cause gingival issues, as well as feel uncomfortable to the patient. Another factor to consider when using restorative dentistry to correct alignment issues is the impact on the pulp. It may be necessary to perform endodontic therapy prior to tooth preparation. Repositioning teeth in the final wax-up can either reduce or increase the amount of reduction, depending on the direction of movement relative to the current tooth position, and can become structurally compromised.
As with incisal edge reduction, we can use a wax-up and a mock-up as a guide during preparation to preserve tooth structure. The challenge when we face these types of alterations is seating the matrix. In these situations, I will use a copyplast matrix and reduce the necessary tooth surfaces until I can seat the matrix completely. Having the ceramist prepare and mark a model of the teeth that you can follow will facilitate this process.
When restoring anterior teeth, it is routine to alter tooth contour and attempt to close gingival embrasures. This necessitates giving the ceramist plenty of running room to create the proper emergence profile from the margin, without which we will create hygiene challenges. The margin of the preparation needs to be placed as far subgingivally as possible without violating the biology, and to the lingual side of the contact to allow the space to be closed in porcelain.
Color change is another common goal we aim to accomplish with anterior restorations. Often patients present with teeth that vary in color, and they want to create a more attractive, uniform appearance. In other clinical situations, they may have discoloration from antibiotics or older dental restorations that they are interested in correcting.
When the alteration expected is small – less than two shades – we have great flexibility in tooth preparation. Minimal reduction and a variety of materials are able to exquisitely accomplish this goal. Margins can be left equigingival, making accurate impression-taking easier, as well as postoperative oral hygiene. When the color change desired begins to differ from the existing color by more than two shades, the predictability of results decreases, and we have to alter the preparation to accomplish our goals.
The first design feature is to place the margins subgingivally. As part of the initial reduction, place the margin equigingival. The equigingival margin will now become the guide to where the final margin is placed in the sulcus. Having measured the sulcus depth prior to tooth preparation, place an initial size "0" retraction cord so that the top of the cord is 1.5 mm from the base of the sulcus.
As an example, if the original facial sulcus depth was 3 mm, I will place a size "0" cord and with a periodontal probe make sure the top of the cord is 1.5 mm from my current margin. Now, the margin is dropped to the top of the cord. The position of the first cord has allowed us to precisely place the margin within the sulcus, as well as protect the tissue so that we cause minimal tissue trauma during preparation.
In order to avoid shadowing of the original tooth color, if we are using partial coverage, the interproximal margin line must be placed to the lingual line angle of both contacts. This allows the technician to have control over the visual effect of the porcelain beyond the height of contour.
The final color of an all-porcelain restoration is created by the combination of the underlying tooth, the resin cement, and the porcelain. The color and light properties of all three layers play a role in the final visual perception. When the underlying tooth is dark, we can begin to see the transition line at the incisal and the interproximal between where we have solid porcelain.
To minimize this effect, we need to gradually increase the thickness of the porcelain as we approach the edge of the preparation. For severely discolored teeth, the most predictable results may still depend on full-coverage preparation and use of a restoration with a core that is then layered with porcelain.
The existing occlusal parameters and any proposed changes must be considered during tooth preparation. For patients with low functional risk, where we are accepting their existing occlusion, the concerns are minimal but still present. When planning for veneers, I mark the patient's existing intercuspal position prior to beginning the prep. I want to plan for the final intercuspal stops to be on the porcelain or on the tooth, not at the interface between the two. The location of the stops may necessitate additional incisal reduction to move the margin.
One consideration is the choice between full coverage and veneers. Many dentists simply feel more confident in these cases doing a full-coverage preparation. In either situation, care must be taken to create an exquisitely refined final occlusion. Paying close attention to excursive movements, including protrusive guidance, edge to edge, and crossover, is critical after seating the final restorations.
One of the parameters that I pay special attention to is fremitus. Anterior teeth that move during protrusive guidance are at higher risk of failure with veneers, especially if the porcelain is bonded to dentin. Another consideration for patients with extreme wear is bonding to secondary dentin and retention of the restoration if the lingual is not prepared.
Structural considerations include old restorations, endodontic therapy, and adequate tooth structure to retain the restoration. One of the advantages of bonding is decreased reliance on traditional retention and resistance form. Many practitioners are restoring anterior teeth with very minimal remaining tooth structure using all porcelain that can be bonded. The quality of the remaining tooth structure, the amount of bonded surface to dentin, and the functional load the patient places on the teeth all have to be considered in these situations.
When using full-coverage restorations and planning for cementation, we need to have adequate ferrule as well as adequate wall height to create retention form. Ferrule is the amount of natural tooth remaining. Our ability to bond posts, as well as the final restoration has minimized these numbers. Failure occurs because of repetitive loading. Ferrule requires buccal and lingual walls of natural tooth as the interproximal does not play a role in structural longevity.
At a minimum, plan to have 1.5 mm of ferrule and understand that the seal will fail at some point when the magic number of loads occurs. Previous restorations traditionally have been incorporated into the new restorative process. I still have a preference for placing my margins on sound tooth structure. I also remove all previous restorative materials so as not to bank the success of the new restoration on the bond of an old composite, or the lack of decay underneath.
The benefit of careful, well-planned tooth preparation includes predictably reaching the desired clinical outcome. This predictability means no compromises on the final esthetic results, long-term success, and not having to bring patients back to alter a preparation after they are already in provisionals. Work through all of the parameters of treatment planning and involve your ceramist in this process. Create a plan for preparing the teeth to effectively move from the present condition of the teeth to the planned outcome. The final preparations may be veneers or full coverage, minimal or extensive, but they will be appropriate for that patient to achieve the desired results.
More by Lee Ann Brady, DMD:
- Choosing the right all-ceramic material
- The value of occlusal adjustment
- One patient, so many appliance designs! What should I use?
Lee Ann Brady, DMD, is a nationally recognized educator, lecturer, and writer. She is the clinical director of the Pankey Institute. Dr. Brady is president of leeannbrady.com, which offers continuing education workshops, seminars, and online content. She is the clinical editor of the Seattle Study Club Journal and a guest faculty member for The Pankey Institute. Reach her via email at [email protected].