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A Biocompatible Preparation Rationale Dean Mersky, DDS

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Because there are a thousand different ways to obtain proper tooth reduction, the question is often asked, which one is best? Having a well developed rationale will not only answer the question, but will also better serve the clinical needs of our patients.

Prep for Success, Preparation Rationale
The proper preparation rationale will provide guidance for tooth reduction parameters, and include much more than just making room for our preferred restorative material and margin. It will include why we choose certain burs, how to best use them, and the impact the decisions will have on our ability to:
  • Safely eliminate disease
  • Maintain strength and anatomic form
  • Provide retention and resistance
  • Allow for restorative material strength and aesthetics at the margin, axially and occlusally
  • Account for proper emergence profile
  • Preserve and manage tissue
  • Obtain an accurate impression
  • Facilitate properly formed, lasting and retrievable temporaries
  • Allow for proper restorative fabrication
  • Remove the least amount of tooth structure required for a biologically sound, comfortable and long lasting aesthetic solution
The foundation of our rationale is to always approach a tooth based on the best solution to satisfy the patient's clinical needs regardless of our personal preferences. The outcome of our efforts will be a completely Biocompatible Preparation.

Avoiding Square Pegs in Round Holes: the Pre-Preparation Exam
Always trying to use the same margin design and material in the same way would be the same as limiting our tool chest to only square pegs and then forcing them into round holes. We might be able to get them in the hole, but it won't provide the best fit. Clinically, we can avoid this kind of debacle by performing an open-minded "Pre-Preparation Exam."

As our starting point, the Pre-Preparation Exam gives us an opportunity to reacquaint ourselves with what we are likely to encounter during preparation with regards to:
  • Periodontal status
  • Supra- and subgingival tooth contours
  • Occlusion


The Periodontal Status will include assessment of the biotype, circumferential sulcus depth, attached gingiva, bone support, bleeding, pocketing and inflammation. A circumferential exam of equi- and subgingival tooth contours with an explorer will help us understand how to best blend margin type and placement with periodontal parameters, tooth configuration and available tooth structure, and reassess if a periodontal referral is warranted. For example, teeth with concavities or conical shapes might not lend themselves to porcelain butt margins, while teeth with divergent roots might be poor candidates for subgingival chamfers. This information will help us reconcile the most appropriate margin with the best possible restorative material. Studying the occlusion will give us clues on how much occlusal reduction will be needed to satisfy both function and material strength, if occlusal reduction will cause short axial walls, and whether there will be a need for more apical margin placement (subject to our perio evaluation).

In summary, the Pre-Preparation Exam helps to match our patients' periodontal status, tooth contours and occlusion to the best combination of restorative material, margin design, margin placement, and aesthetics. The exam helps assure that a patient will receive the best possible result.

Pre-Preparation Exam: Is Periodontal Status Bedrock or Sand?
I don't think anyone on Dentaltown demonstrates the periodontal aspect of a Pre-Preparation Exam better than Dr. Danny Melker. What makes Dr. Melker's insights so valuable? Instead of taking a late-stage reactive approach, he advises early and more predictable interceptive correction.

During our Pre-Preparation Exam we should look for early clues of attachment and bone loss that could affect preparation design, especially in areas of furcations and concavities. As Dr. Melker has shown, we can more predictably provide long lasting restorations and health by finding problems early in the disease process, and correcting them before final preparation. To help illustrate this, the top left photo of Fig. 2 shows what appears to be normal and healthy conditions with teeth prepared and temporized. Yet when uncovered we can see how the disease process is in its early- to mid-stages. Acknowledging this problem, restorative decisions were made to create a Biocompatible funnel prep in the bifurcation. The patient now has an easier-to-clean healthy solution that otherwise would have been a hopeless plaque trap ending in tooth loss.

Other factors also play a role in determining preparation options. Fig. 3 shows teeth with thin tissue, shallow sulcus, conical shapes and traumatic occlusion. The Pre-Preparation Exam would note that the thin biotype with shallow sulcus will not easily tolerate preparation for subgingival margins and retraction cord. The conical-shaped contours indicate that aggressive porcelain butt margins would unnecessarily weaken teeth that are already more brittle and fragile from occlusal trauma and erosion, and might not remain predictably bonded. At the same time, the abfractions need to be covered to prevent (the potential for) additional abrasion and erosion. Complicating matters further, the active occlusal trauma needs attention. The cuspid shows a great deal of incisal wear, probably from parafunction. The exam might conclude that posterior lateral interferences have accelerated occlusal wear and abfractions. To address this we might need extra occlusal reduction in select areas for lateral passageways that will help eliminate posterior occlusal interferences and root flexure.

A Pre-Preparation Exam for the above would help us recognize that biases for margin design, margin placement and preferred restorative material might not be in this patient's best interest. The most ideal approach would include periodontal correction followed by conservative tooth reduction and good occlusal clearance, all matched to a thin material that would be both kind to the tissue and aesthetic. Below is an example of a Captek crown with a metal collar over a feather margin. A Biocompatible Preparation matched to Captek's .275mm substructure and plaque reduction might be the perfect combination for the patient in Fig. 3.

Summary, Preparation Rationale
The Pre-Preparation Exam prepares for a successful outcome prior to tooth reduction. The exam helps you to better understand which preparation design and material selection will be best matched to the clinical conditions.

After accounting for periodontal considerations, margin placement and design, occlusal reduction needs and aesthetic demands, it is time to consider the type of burs to be used and their management. The right combination will help us remove the least amount of tooth structure and still provide a biologically sound, comfortable, and long lasting aesthetic solution. This is the essence of a Biological Preparation.

Prep for Success, Technique Rationale
I am privileged to visit many fine labs across the country where I am often asked to review cases. Anecdotally, I estimate that 50 percent of all lab cases have inadequate occlusal clearance, another 50 percent are over-tapered, and 30 to 40 percent are too short. Choosing the right burs for the situation and using them correctly is what I refer to as Technique Rationale. A sound Technique Rationale is crucial to fulfilling our Biological Preparation Rationale and the ten points listed above.

Although margin and material requirements will help us select the most appropriate burs, sometimes the burs we choose are less important than how we use them. To help us gain every advantage in their use it is best to consider both bur design and bur dimensions.

The Technique Rationale works for all margin placements and burs. Normally, my bur sequence would begin with the 1158 carbide (for gross reduction). For me, this was followed by a large modified chamfer, and then a smaller modified chamfer of the same shape, depending on the clinical and material requirements. I liked the 1158 because it gave predictable results. With a 1mm diameter and a 4mm cutting length, I used it like a ruler to measure depth cuts and axial height. Using it like a straight edge and protractor helped me to reduce in planes, maintain axial inclination, and keep axial walls parallel. A non cross-cut carbide, the 1158 has a rounded tip to help avoid tissue damage when approaching the free-gingival margin. However, this is an aggressive bur and might not be suitable for everyone. The same shape and dimensions in a diamond would be less aggressive and just as useful and predictable. I used the carbide to obtain 2 mm of occlusal reduction and 1 mm of axial reduction. Final preparation would net 1.5mm of axial reduction, and 2.5mm of occlusal reduction. Keep in mind, G.V. Black advocated about 2mm of reduction for an occlusal amalgam. I have found no problems with reducing 2.5mm, occlusally.

Diamonds were used to smooth irregularities and place the margin after gross reduction with the carbide. For Captek and other PFMs, I prefer the modified chamfer over end-cutters, shoulders and chamfers, which have a tendency to dive and ditch. I found bevel and feather margins had a tendency to wander and be uneven. The modified chamfer uses a semipointed end to provide an easy-to-place and predictable sloping chamfer that allowed me to place subgingival margins with less tissue trauma. However, different diamonds should be used if a porcelain butt, bevel or feather margin is better suited for the clinical situation. Regardless of the diamonds used, the key bur for me was my ruler and straight edge, the 1158 (see Fig. 4).

Diseased Tooth Removal
The first step after the Pre-Preparation Exam is to use the 1158 to remove diseased or redundant tooth structure. At this stage, we should also place needed build-ups to assure proper strength and form, and fill undercuts that are too large to be prepped away. I would leave small undercuts until the very last step to avoid possible over-tapering.

Occlusal Reduction
I suggest a change in thinking when addressing occlusal reduction. Instead of occlusal reduction, or clearance, I suggest thinking in terms of occlusal requirements, first for the clinical situation, and second for the material to be used in the final occlusal scheme. Keeping focused on clinical and material requirements during occlusal reduction helps the lab provide consistently stronger and longer lasting restorations.

Many believe optimum clearance is 1.5mm; however this number varies. For example, a Captek occlusal metal island only requires .6mm of thickness, while some zirconia crowns require almost 2mm. There is also the need to factor in a material's cement space. Captek might have 30 microns of occlusal cement, while milled restorations might have up to 200 microns of occlusal cement. Additionally, preparation clearance can be lost to super eruption, condylar seating, or both. This means the best way to allow for the proper occlusal thickness is: material thickness + cement thickness + .5mm interim occlusal clearance loss (during temporization). Proper reduction also accounts for occlusal grooves, making anatomic reduction mandatory.

After having removed decay, old fillings, redundant tooth structure, and calculated material thickness requirements, I would notch the buccal cusp arms 2mm. I would then sink the 1158 half-way (2mm) into the central groove and slide across its entire mesial-distal length. I would then notch the lingual cusp arms 2mm. The marginal ridges are left intact until the last step of gross reduction. The photo shows a 335 (2 mm cutting length) being used for buccal cusp notching, and an amalgam removing bur being used for removing cusp arms (Fig. 5).

After the occlusal depth cuts, lay the bur lengthwise in the buccal and then the lingual cusp-notches with the bur's tip at the base of central groove's depth. Maintaining all incline angles, slide the 1158 across the buccal occlusal surface and then the lingual occlusal surface. An amalgam removing carbide with the same dimensions as an 1158 is used in the photo. The result is a wedge-shaped 2mm reduction of the occlusal surface.

Axial Reduction
After the occlusal reduction, make two or three mesial and distal buccal axial wall depth cuts for bicuspids (three or four for molars), all to the depth of the 1158's 1mm diameter (Fig. 7 shows an amalgam removing bur). All depth cuts extend from the free-gingival margin occlusally without changing the axial inclination while still parallel to the two or three buccal or facial planes. Connect the depth cuts by sweeping the 1158 counterclockwise, beginning from the occlusal and working apically while remaining parallel to the buccal or facial plane angles. Buccal and facial gross reduction should extend into the interproximal areas parallel to the tooth without breaking the interproximal contacts (Fig. 7).

Lingual reduction is always a challenge. It is difficult to see, as water and saliva get in the way, and sometimes we have to fight with the tongue. It's no wonder the lingual is considered the most challenging area of our preps. You can avoid these problems by first setting a non-spinning 1158 on the buccal aspect of the axial wall. Then, without changing the angle, lift the bur and slide it down the lingual wall until it touches the tissue. Raise the bur slightly above the tissue without changing the angle and make an axial wall depth cut, watching the occlusal for a 1mm depth of cut. Make a couple more lingual depth cuts and carefully connect them, using the same technique used on the buccal (Fig. 8, showing an amalgam removal bur).

It is much easier to see the interproximal areas once the buccal and lingual walls are reduced. Extending into but not through the interproximal contacts leaves about 1mm of remaining tooth structure. Pass the 1158 through the contact parallel to the tooth, beginning at the top half of the contact, extending just short of the tissue peak (Fig. 9).

Using the 1158 as a ruler, the gross reduction provides 2mm of occlusal reduction and 1mm of axial reduction. Using the carbide as a straight edge and protractor, the axial inclination remains unaltered, the buccal and facial planes remain intact with parallel axial walls (Fig 10).

Diamond Refinement
The first diamond is used only to smooth the irregularities left by the carbide and is matched to the clinical needs of the situation. If appropriate, use the first diamond to refine the margin to the level of the free-gingival crest with a full shelf. Then, smooth the axial walls in planes without changing the angles or removing undercuts, and smooth the wedge-shaped occlusal table. A tapered diamond is tilted away from the axial walls so as not to allow its taper to alter the axial inclination. It is not used to create draw. This diamond removed approximately .25mm of tooth structure in all areas (Fig 11, a chamfer was used in the photo).

I would use a smaller version of the first diamond to refine and finish the preparation, if going sub gingival. To assure proper draw, place the diamond's shaft parallel to the axial inclination. Allow the diamond's taper to create draw naturally without "leaning" the bur. I would also use this diamond to eliminate remaining undercuts, further smooth the occlusal table, and round sharp edges. Because the diameter and tip are smaller than the first diamond it is able to extend the margin subgingival with less likelihood of injuring the tissue (Fig 12, a chamfer was used in the photo).

The final diamond removed an additional .5mm of tooth structure, producing a predictably consistent total reduction of about 1.5mm axially, and about 2.5mm occlusally. The axial wall taper would be about 10 to 15 degrees, and the occlusal reduction was anatomical. A final polishing of the prep is a great way to help the lab provide the best possible restoration.

Summary
Performing a pre-preparation exam helps to reacquaint us with available tooth structure, root form, concavities, furcations, emergence profile, occlusion, and periodontal needs. Assessing these factors will help us decide which margin design will work best, where the margin should be placed, and how much reduction we will need in designing the most Biocompatible Preparation for the best suited material(s). Using the right burs for the job in the right way will help prevent under-reduction and over-tapering. Performing gross reduction with a straight rounded-tip carbide or diamond of known dimensions will help assure proper reduction, axial height, axial inclination and parallel walls. A generalized Technique Rationale is:
  1. Notch the buccal and lingual cusp arms 2mm
  2. Drop 2mm into the central groove, moving mesial to distal, leaving marginal ridges untouched
  3. Cut an occlusal wedge by moving the bur mesial to distal with the bur tip in the base of the central groove and the bur length in the base of the cusp notch
  4. Create 1mm axial depth cuts parallel to each axial plane, 2-3 for bicuspids, 3-4 for molars
  5. Connect the depth cuts by working counterclockwise from the occlusal to the gingival margin
  6. Use the buccal axial wall to establish the lingual axial wall reduction angle and depth cuts to prevent over-tapering
  7. Make parallel passes vertically through the interproximal contact extending near but not into the tissue
The final touches are completed when the best diamond choice is used to refine gross reduction, margin design and margin placement. If a subgingival margin is required, a larger diamond is used first to establish an equigingival shelf so that a smaller diamond can safely refine the margin subgingival. When the objective is an equigingival or supragingival margin it is possible to use a single diamond. As a last step, polishing the prep will help the lab assure the best fit possible.

Our patients are best served when we discard biases and match the most Biocompatible Preparation with the most appropriate restorative material. Doing so will help us provide the excellent results our patients deserve

Author’s Bio
Dr. Dean Mersky, a 1976 graduate of the University of Detroit, School of Dentistry, practiced in Manhattan Beach, California, for 26 years. Dr. Mersky's dental practice was concentrated in the areas of treatment of the TMJ, dental reconstruction, and cosmetic dentistry. His expertise with Captek restorations comes from the almost 2,000 Captek crowns and bridges that he has placed, and the many different uses and clinical trials he has contributed to. Dr. Mersky, an accredited member of the American Association of Dental Aesthetics and the Northeast Gnathological Society, is the vice president of sales & marketing for Dentalle Inc. Dr. Mersky can be reached at (215) 348-2079, or dmersky@dentalle.com.
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