Guided Preparation

A major change in the reduction principle appeared in the early 2000s. There was a step back from an invasive preparation to a more conservative reduction in order to obtain surely enameled preparations. The return to an enamel finish minimized the risk of failure.
This modification of the reduction principle follows the biomimetic concepts advocated by Magne: “The use of a mock-up as part of esthetic planning, where preparation is guided by the future and not by the present, supported this approach and validated this new approach.”

One of the very crucial issues in the production of ceramic veneers is to keep the maximum existing enamel of the tooth structure. In order to preserve the maximum amount of tooth enamel, the final tooth reduction should be designed according to the expected final outcome "guided prep". If not, the reduction of dental structures will not be the same within the space requirements for ceramic veneers.

The preparation design for ceramic veneers should allow for an optimal marginal adaptation of the definitive restorations and maximally resembling the ideal tooth morphology. Therefore, a diagnostic wax-up should be utilized as a reference for tooth reduction. The spatial orientation and architectural dimensions of the wax-up will be used to pre-design and validate the intended preparations for the teeth involved. This illustrates the importance of using correct wax-up techniques in creating the exact tooth shape desired. In such a treatment the most important element in the process is the wax-up. In order to transfer these data to the clinic, the dentist should be supplied with transparent templates and silicon indexes fabricated upon the wax-up by the laboratory technician, although these indexes can be easily made at the clinic by the dentist/assitant.

Control of reduction can be achieved by using (preparation guides). Silicone guides, fabricated over the wax-up, provide simple and indispensable tools for the control and reduction of enamel. Two guides should be fabricated: a vertical guide (sectioned in the buccolingual direction) for reduction control in cervico-incisal direction; and a horizontal guide for the mesiodistal reduction control. Using the vertical and horizontal silicone guides, it is possible to check the uniformity of the labial reduction.

Guided preparation means that the same considerations of the conventional preparation are going to be implemented but through the mock-up. The amount of tooth reduction should never be made according to the existing tooth surface but rather to the final volume of the restoration.

Thanks to the technique of Galip Gürel, proposed almost 20 years ago. It is a simple technique, which has great value for controlling the depth of the preparation in such special cases, and can be easily used. This technique using composite resin/bis-acrylic can simply be added to the facial surfaces of lingually positioned teeth (additive cases) with spot etching and bonding up to where the tooth needs to be buccally reoriented or filled by volume. 

It mimics the final outcome that we aim to restore with the ceramic veneers. When the mockup is still on the tooth, it is logical to use the depth cutter bur over that composite/bis-acrylic build-up, so that the true depth will be reached when the depth cutter is used and thus preserve the maximum enamel on the tooth surface. By doing this, we limit our depth cutter to go only as deep as our smile design dictates, resulting in an even more conservative tooth reduction. 

For example, let us assume that the tooth is tilted 0.2 mm lingually. If we do not use the technique explained above, then when we use the depth cutter of 0.3 mm we will end up with a 0.5 mm space that the ceramic veneers must fill. However, if we add the mock-up, and use the depth cutter over that volume, we will end up with the necessary reduction of only 0.1 mm, which will still provide the 0.3 mm of thickness for the final PLV. This way the enamel is being preserved. 

Aesthetic Pre-evaluative Temporary (APT) / Mock-up

The patient must validate the esthetic project. In order to fill the esthetically missing volume, materials such as composite or bis-acrylic mock-ups can be added to the facial surfaces of these lingually positioned teeth. This will help to create the correct esthetic placement of the tooth surface on the dental arch. This is a very simple way of visualizing their positions in the new smile design. This is done not only to perceive how the new smile will look but also to evaluate its occlusal compatibility. 

The position and alignment of teeth in the arch can significantly affect the appearance and balance of a smile. Rotated lingually or facially positioned or aligned tooth or teeth will disrupt the total harmony and balance. Poorly positioned or rotated teeth not only distort the shape of the arch but also interfere with their apparent relative proportion. Thus, if a dentist has difficulties in trying to perceive where and how the teeth should be aligned from the start, the mock-up overcomes this problem by using a simple silicon index that is prepared from the diagnostic wax-up.

When the position of the tooth is corrected with the mock-up, before the actual material preparation;
  • excessive healthy tooth reduction can be avoided 
  • a thicker layer of porcelain built up over the unnecessarily overprepared tooth (which can compromise the natural value and chroma) that will result in restoration with an artificial appearance is prevented.
By placing this customized index over the teeth, the dentist can visualize the teeth or a portion of a tooth that creates disharmony, either by a facial protrusion or unnatural axial inclination. The dentist can now trim down the protruding incisal edges, marginal ridges or axial inclinations until they can easily fit into the silicon index. By doing so,  overextended teeth towards the facial, or those that tend to stay out of the expected arch line, will be brought into their esthetically pleasing positions both vertically and horizontally. In this way, the facial limits of the ceramic veneers will be well defined even before beginning the actual preparation stage.

**  However, this can not be easily implemented especially in cases that have a protrusion (subtractive cases) or some interferences that prevent the insertion of the mock-up. In such cases, we have to take the decision with the patient about some initial reduction (aesthetic pre-recontouring) that we have to implement to allow the mock-up insertion. The patient signs a consent and then we can start the treatment.

The ability of the dentist to reshape the tooth to conform or to enhance nature's given contours is truly an art form that is essential to esthetics. One of the most important avenues that require this artistic procedure is the aesthetic pre-recontouring (APR). In order to dictate the final outcome of a pleasant smile and to obtain an adequately equal tooth reduction for the technician for laminate veneers build-up, an APR should be considered before the actual material preparation.

Aesthetic Pre-recontouring (APR)

Teeth that are to be treated with ceramic veneers may exhibit positions on the dental arch different from what is considered to be pleasing. The final esthetic and functional form determines the actual tooth preparation and the entire restoration buildup we seek to achieve. In the restoration process, factors like occlusion, function, interproximal position of the adjacent teeth and their contact zone, the size of the pulp and the hard and soft tissues, along with the age of the patient, are all very important. 

Tooth preparation that has been done without prior evaluation and planning is doomed negatively to affect the final result. Teeth are single objects that are part of a whole - the dental arch. In the actual material preparation (AMP) the required amount of enamel, and sometimes dentin, must be removed to provide enough space for veneers build-up. However, before doing so, any minor or major problems must be evaluated and misalignments corrected. In other words, an APR has to be made in order to put these things into order and to obtain a pleasing symmetry and balance in the arch. This will depend on how many teeth are to be restored for the new smile design and should definitely be decided before the actual material preparation starts.

When the individual tooth position or its alignment needs to be altered, the basic principle of APR is to put the partially protruding axially misaligned or rotated teeth into proper alignment on the arch, having the imaginary finished ceramic veneers designs in the dentist's mind before starting the actual material preparation.

Protruding teeth or facially slanted teeth, even without color alterations, must be reduced more to allow adequate space, so that the finished restorations will not be overbulked. Occasionally with the tooth in the lingual version, very little preparation is necessary. Most of the time, preparation of the lingually tilted tooth may be limited to creating a finish line on the proximal and the gingival regions and/or the removal of surface luster on the facial enamel. This strategy is especially indicated to improve the appearance of cone-shaped lateral incisors.

Mock-up for exact facial reduction

The major advantage of using the mock-up (mock-up) is to ensure the final outcome is accepted by both the dentist and the patient. The exact facial thickness can be double-checked with the help of a silicone index. As the mock-up (APT) now mimics the final outcome, the teeth can be prepared very precisely through it being that they represent the final contours of the actual restorations. 

The mock-up's facial thickness and the use of depth cutters through it will dictate the necessary facial reduction. In doing so, the dentist will avoid the unnecessary loss of enamel associated with excessive tooth preparation and be able to supply the ideal preparation depth and volume for the ceramic veneers production. 

The rotated tooth may need both APR and APT in advance. When discussing such a condition, it is possible that the mesial portion of the tooth may be buccally rotated whereas the distal portion is lingually positioned. If a combination of the rules mentioned earlier is applied, it will be very easy to visualize the tilt that will transform the tooth into its normal position as it should be in a pleasant smile. 

** An interesting clinical trick that always facilitate this step is to use a transparent preparation guide (vacuum-formed tray) with light-body impression inside it. All the interferences will be easily shown through the colored material, then these areas can be trimmed down, after that the silicone index loaded by the bisacrylic material can be pasvely inserted.

It is obviously necessary to 
make a thorough, deeper preparation of the protruding prominent area so that the restoration can be made with the proper physiologic contour. This can be accomplished by first contouring any of the surfaces that extend buccally. The buccally positioned mesial part must be ground down enabling the dentist to visualize whether the necessary preliminary reduction has been achieved or not

The facially protruded central is trimmed down with a fissure diamond bur. So, the silicone index can passively sit on the unprepared teeth.

If the preparation is limited to one or two teeth, then the lingualy positioned teeth can be buccaly contoured with composite mock-up. (b) when multiple teeth are involved, a flowable composite can be placed on the teeth with the help of a transparent template, thus creating the APT.

Once the silicone index is properly seated over the facially reduced teeth, it confirms that the mock-up is successfully finished. Once this is established, the transparent template, which has actually been prepared to build up our provisionals, can easily be seated into its original position without touching the reduced facial surface of the tooth, which was protruding before. 

When this is achieved, the teeth are spot etched and an adhesive is applied to the surface area and light-cured. Then the template/silicone index is loaded with a small portion of the flowable composite or acrylic resin and seated over the teeth, and light-cured. This way the mock-up is firmly seated on the teeth that are about to be prepared. This now resembles the finished surface and volume of the final restorations. 

The two advantages of this application are that;

  1. The patient will immediately see the final outcome even before we start treating the case.
  2. Because the preparation will be executed through the partially bonded temporaries it will be a very conservative preparation with no removal of unnecessary enamel with the depth cutter. 

APR of Gingiva

The APR in some cases is not limited to the hard teeth tissue. It can also be applied to minor gingival alterations. Biologic parameters permitting gingival contouring to achieve proper height can be accomplished with a diode laser. While doing that, the zenith points can also be changed, especially in the diastema cases. When minor gingival tissue remodeling is done with the diode laser surgery, no post-op apical migration of the tissue is witnessed.

** Whenever possible the geometric principles of tooth preparation for porcelain veneers are followed to maximize their strength. However, in some certain cases, the tooth or teeth may be lingually positioned in the arch;
  • For those teeth that are extremely lingually inclined, orthodontic intervention is a must.
  • For the patients who do not want to receive orthodontic treatment with teeth that are only slightly inclined to the lingual, an aesthetic treatment is possible. The lingual inclination of the tooth can be more than the depth of our depth cutter. Therefore, the amount of composite/bis-acrylic mock-up added to that surface properly to align its position over the dental arch might be thicker than our intended reduction (e.g. more than 0.5 mm). In such cases, after we prep the tooth with our predecided depth cutter, we will still see some composite over the tooth.
    In this situation, the area that 
    was prepped with the depth cutter relative to where the facial surface of the ceramic veneers will be is actually in a position deeper than the grid depth of the bur. This is why we should still see the mock-up material from underneath the prepped area of the depth cutter. If this is the case, and the dentist wants the finished ceramic veneers to have maximum contact with the enamel surface, he/she should go ahead and remove the remaining mock-up material from the surface and slightly roughen the enamel surface to remove the surface luster (aprismatic layer) for improved bonding, even though the result will be a veneer displaying greater thickness. This should be discussed with the lab, informing them that the thickness of the veneer will be thicker in that area. On such occasions, the most important issue is to be able to visualize the final outcome.

Mock-up for incisal Preparation

The use of a mock-up (APT) is not limited to preserving and exacting the final facial volume but is also used to determine the exact incisal length and the necessary amount of reduction of the incisal edge. Reduction during the preparation should also be done through the mock-up to exact the prepared incisal edge position. 

In the restoration process, it is important that the functional incisal edge has been properly contoured. When restoring the lingually inclined tooth, an overly thick incisal edge must be avoided. In order to reduce the faciolingual dimension of the incisal part of the tooth, the enamel must be prepared to the lingual edge of the incisal surface, if permitted by the occlusion. If the lingual areas take part in the functional contacts while engaging in protrusive movements, then no alteration can be introduced. However, if slight reductions of the incisal edge on the lingual surface of the tooth will not affect the anterior guidance, then this portion can be slightly modified within the limits of the enamel to prevent the excessive thickness of the final incisal outcome.

The previously explained APR and APT (mock-up) techniques enable these treatments to be accomplished with very little effort and the utmost precision. However, utilizing them sounds like a long and time-consuming procedure, it is not, and it is extremely beneficial to the final outcome as nothing is left to chance. Everything is controlled and the dentist very accurately dictates the result.

Preparation sequence 

  1. Preparation depth of the bur through the mock-up

    Four essential rules:
    No cervical groove
    Incisal groove 2 mm from the free edge 
    0.5 mm in the buccal (at the coronal two thirds)
    1.5- 2 mm at the free edge

  2.  Homogenization of the grooves is done respecting the buccal convexity. 

  3. Creation of the butt-joint at the incisal edge through the mock-up.

    At this point the remaining mock-up is removed to complete the procedure. After removing the mock-up, there are two options;

    Marks are visible on the teeth (mock-up thickness less than 0.5 mm)
    No marks are visible on the teeth (mock-up thickness greater than 0.5 mm).

  4. Positioning of the margins "cervical and proximal":
    Peripheral delimitations of the preparation with spherical diamond burs surrounding the entire labial surface of the tooth without disruption of the proximal contact and without subgingival extension. 
    * Supragingival cervical (0.5 mm) and proximal (mid-thickness of the contact point) margins are recommended in normal circumstances. 

  5. Triple angulation of the buccal convexity.

  6. Creation of a proximal slide.

  7. Retraction of gingival tissues.

  8. Refinement:
    repeat on all the prepared surfaces and margins using diamond points similar to those mentioned above already employed except with fine and extra-fine grits.

  9. Finishing & polishing:
    Using abrasive silicone rubbers and discs with decreasing granulation. Every angle and corner should be uniform, with rounded lines, to improve the adaptation of the resin cement and laboratory build-up.

** It is crucial to review the basic principles of veneers preparation in the previously shared post 👇
 Ceramic veneers preparartion .. Part 2 " Conventional preparation "

** The next post will discuss veneers preparation for some special situation that need utmost care. Then I'll share video demonstration for all the preparation procedure,, Stay tuned 

Regards ..

To get the PDF file click this link Ceramic Veneers Preparation