-->

Ceramic veneers preparation .. Part 4 "Special cases"

 Special Cases


Mandibular anterior teeth


From an esthetic standpoint, the mandibular veneer can provide an excellent result in most situations. However, its life expectancy can be drastically compromised unless the patient’s occlusion is favorable.

The usual problem with preparations for lower veneers is leaving enough tooth structure remaining after the horizontal and vertical reduction. A potentially weak point is at the inciso-labial junction, which must always be sufficiently reduced and rounded to allow the veneer to be thick enough in that area to have the strength to resist fracturing when placed under an occlusal load. Also, the incisal edge of the mandibular anterior teeth is usually the most visible part of the veneer so consider this fact when preparing the tooth. Sufficient incisal reduction is needed to ensure a normal incisal edge appearance for that patient.

One advantage of the mandibular veneer is that it is seldom necessary to go subgingivally, as with the maxillary veneer, because most people do not show the gingival margin of mandibular anterior teeth.

To reduce or not?

The most practical approach is to evaluate each patient, and indeed each tooth to be veneered, on the basis of:
  1. the thickness of the veneer needed for covering or reshaping,
  2. the degree of anticipated retention of the veneer, considering the receptivity of the tooth to the bonding agent and placement of the veneer, and
  3. recognition of how the increased thickness of the veneered tooth will change its appearance, structure, alignment, and function.



Ceramic veneers preparartion .. Part 3 " Guided prep. "

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.


Rehabilitation of a central incisor using pressed ceramic post & core

A patient came to my clinic with previously broken teeth but one of them was very controversial. 
Is it restorable or not?.  After a thorough diagnosis,  I decided to restore it with an all-ceramic post and core that can be used in combination with all-ceramic crowns to prevent the unesthetic metal display if a cast-metal post and core was used in such case.  

  • It is highly biocompatible 
  • increases the translucency of an all-ceramic restoration
  • with good modulus of elasticity.



Preoperative situation

Occlusal view

Diagnostic wax-up

Esthetic pre-evaluative temporaries

Caries removal 

Post space reparation immediately before taking impreesion

Pressed lithium disilicate post & core


Poreclain etchant application

Resin cement

Inserted under gentle pressure to allow escapement of excess cement 

Application of glycerin gel to complete light-curing

Postoperative x-ray

Esthetic crown lengthening to gain ferrule

To display the video of the procedure,, 

https://youtu.be/PTwpIppTDwI

To read the full explanation of the concept ,, click this link 

Esthetic crown lengthening




Temporization two days after the surgical procedure with the margins short of the gingiva

Two months recall after surgery

Intial teeth preparation then the final impression was delayed 

Tissues maturation .. 4 months recall

Preparation just before registering the final impression


Final restorations
Layered lithium disilicate crowns 

Absolute isolation to start the procedure of adhesive cementation 

Two different types of etchants; the red one is porcelain etchant for the ceramic core part
the blue is phophoric acid 37%

The phosphoric acid was applied for an extended time to achieve good etching for this part of sclerotic dentin.

Ethed substrate

Bonding agent application


Glycerin gel to complete light-curing


Final cementation after 6 months.



Finally it was survived.. New tooth, new life

Ceramic veneers preparartion .. Part 1

Veneers are partial extracoronal restorations usually indicated for anterior esthetic rehabilitations. The main purpose of a veneer is to cover the labial-proximal surfaces and potentially the incisal border of the anterior teeth in an attempt to correct color discrepancies, shape, texture, function, and position of the elements in the dental arch. There are several preparation designs for ceramic veneers, which vary according to the extent of the preparation. The use of different preparation techniques is dependent on several factors: the amount of remaining tooth structure, presence of previous restorations, length of the clinical crown, and presence of endodontic treatment.

Preparations for laminate veneers do, in fact, require great stringency and a great deal of training, as no rectification can be made once the procedure is completed. Experience is required for mastery of 0.3-0.5 mm reductions.


Enamel Vs Dentin


Tooth preparation for the laminate veneers is exacting. Using magnification loupes to facilitate careful preparation is tremendously helpful for accomplishing this step. It also helps to preserve as much enamel as possible as the bond strength of porcelain bonded to enamel is still superior when compared with the bond strength of porcelain bonded to dentin.

The more the bonding surfaces were located in dentin, the lower the success rate..


This could be explained by the difference in the elastic modulus between the substrate and the ceramic. The lower this ratio, the better the fracture resistance. This explains why bonding to enamel (70 GPa) is preferable to dentin (18 GPa) because the difference in the elastic modulus is lower with ceramic (between 65 GPa for feldspathic and 95 GPa for IPS e.max).

Dentistry

To ensure the bond strength of the resin composite to the tooth surface it is necessary to reduce the enamel. Due to its poor retention capacity, the aprismatic top surface of mature teeth that have not been prepared must be removed, or at least roughened. Careful attention is of the utmost importance in order to obtain successful results and solid bonding. However, Rigidity in the tooth is due to the amount of enamel thickness, and thus,


The more enamel that is removed, the more the tooth flexes.


As the anatomy and the shade of the restoration are directly affected by the preparation design, very precise planning is necessary. The ceramic material allows for the reproduction of natural light transmission (i.e. refraction, reflection, translucency) and an esthetic result can be hindered by insufficient tooth preparation in relation to the soft tissue parameters and the restoration.


Enamel removal

The shape of the preparations also influences the appearance and longevity of a restoration. In order to manipulate light and to establish a depth of translucency and space for the incisal effects is desired.


Without reduction, the teeth will be larger and more labially positioned (in lingually inclined teeth, this may be an advantage). Failure to remove proximal enamel can result in the finish line placed too far labially and encroachment on the embrasure areas, resulting in exposure of unsightly porcelain margins that may be difficult to finish. Proceeding without preparation will lead to not only distinct overcontouring at cervical and proximal tooth surfaces, but also to higher clinical failure rates as a result of gingival inflammation and secondary caries due to an increase in microbial plaque accumulation.

The continuing evolution of bonding processes to dental structures makes possible the implementation of more conservative restorative techniques and enables the use of minimally invasive preparations. 
In teeth with slight or no discoloration, for example, ceramic veneers with thicknesses ranging from 0.3 to 0.7 mm may be used.  Extremely thin ceramic veneers emerged, requiring minimal tooth reduction that is limited to merely smoothing sharp angles and eliminating undercuts. Teeth indicated to receive this type of restoration usually have favorable characteristics such as a good insertion axis and adequate space for the restoration. These preparations have very specific indications, where reshaping or correction of tooth volume is necessary by adding material. 

NB. It should be noted that ceramic veneers are of limited use in situations requiring major correction.


When to consider reduction?
  1. Remove convexities and provide a path for insertion in those situations where either the incisal or the interproximal areas are to be included in the veneer.
  2. Provide space for adequate opaquing or heavier coloring like in cases of a dark substrate.
  3. Provide a definite seat to help to position the veneer during placement.
  4. Prepare a receptive enamel surface for etching and bonding the veneer.
  5. Allow for a smoother transition from the veneer to the tooth surface, enabling the patient to more easily keep it plaque‐free.

Contraindications to reduction:
  1. The size of the pulp. If young individuals or others with large pulps require laminating, consider an alternative to enamel reduction, especially if there is any indication of irreversible sensitivity by reducing the thickness of the enamel.
  2. The patient’s psychological state and feelings about tooth reduction or veneers. If the patient is apprehensive and unsure, then it is wise to do no reduction. Then if the patient becomes dissatisfied with the veneer or the slightly overbuilt look, the option will be removing the porcelain veneer and repolishing the enamel, thus returning the patient to a semblance of their pre‐veneer state.

If reducing,, how much?

As a general principle, the enamel should be reduced just as much as necessary to facilitate the placement of an esthetic restoration. Ideally, one would like to remove the same amount of enamel that will eventually be replaced by the veneer and bonding composite resin. Decisions about reduction need to take into account the relative position of the tooth in the arch. For example, in treating a crowded or rotated tooth or a tooth in a labial version, it may be advantageous to first bring the offending tooth into alignment with the rest of the arch by reducing its labial contour through cosmetic contouring. Thus, The use of mock‐ups, followed by a wax model, esthetic pre‐evaluative temporaries, and silicone index, provides the best esthetic, phonetic, and functional assessment of necessary tooth preparation for veneers.

Ceramic veneers preparartion .. Part 2 " Conventional preparation "


Conventional preparation

I’d like to talk about a preparation that follows three main stages applied to four main areas of the tooth.

Three main stages to get the final veneer preparation: 
1. Cutting
2. Refinement 
3. Finishing & polishing

 

Four main areas: 

1. Incisal edge

2. Cervical margin 

3. Labial/buccal surface

4. Proximal surfaces.


Each tooth surface (incasal, cervical, labial/buccal, mesial or distal) will pass by these stages of preparation. So, let's get started..



Cutting 


An essential question for the dental practitioner; how to control your hand during teeth preparation for ceramic veneer?, or in other words, what is the penetration depth of the bur through the tooth surface? and The keypoint here is the Depth Cutters.

As we all know, the first step for the actual teeth preparation is to obtain a sufficient depth with the help of depth cutters. However, the depth cutters always reduce the same amount of tooth that their grid depth indicates. In other words, no matter what the position of the tooth is, or whether they are lingually or buccally inclined, the depth cutter will always remove the same amount of tooth structure that will not necessarily dictate the true depth of the restoration. For the inexperienced dentist, who religiously follows systematic preparation techniques for ceramic, the improper application of these depth cutters can be extremely destructive.

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


Vertical Vs Horizontal depth cuts

Preparation of vestibular grooves in the vertical direction with rounded-ended conical diamond burs, respecting the axial inclinations (cervical, middle and incisal) to preserve the convergence of the buccal surface. The depth of each groove is defined according to each case and based on the diagnostic wax-up. In this way, one can check the amount of wear with a millimeter probe by comparing the prepared side with the intact. At this point, it is essential to have knowledge about the thickness of the labial enamel of a natural tooth, so that the completion of the preparation is performed, having also the preservation of enamel as a guide whenever possible. Then, joining the labial grooves with a conical diamond bur, the larger diameter to prevent deepening of the guide grooves and forming an uneven surface.


Horizontal “ depth cuts “  giving us more controlled cutting. A special three-tier diamond depth cutter comes in different thicknesses 0.3, 0.5, or 0.8 is used to cut across the labial surface then mark the depth cuts with a pencil or a marker then the cutting is completed respecting the triple angulation of the labial surface to the base of the depth cuts until reaching the markings. 


The depth cutter can only penetrate until the non-cutting shaft is flush with the tooth surface. The shape on the right represents the surface after the depth cutter is used at only one angle. Note that the only true depth is gained in the area of B (middle third, black dots), to reach the necessary depth in points A (gingival third, green dots) and C (incisal third, red dots), the bur should be used in three different angulation. 






Homogenization of the grooves is done respecting the buccal convexity. Whatever the preparations to be performed, it is essential that the margins are properly defined (cervical and proximal) so they can be easily identified by the technician. This will allow a good fit of the veneers, better stability, and higher adhesion values.




Even if there is no need for thickness it is essential to use airborne-particle abrasion on the enamel surface in order to optimize bonding and remove the aprismatic enamel layer.

The preparation design consists of the preparation of different tooth surfaces:

  1. Incisal edge preparation.
  2. Cervial margin preparation
  3. Labial/buccal surface "Triple angulation"
  4. Creation of a proximal slide.

1. Incisal edge preparation:

There are two basic techniques for the placement of the incisal finish line:
  1. The first terminates the prepared facial surface at the incisal edge. There is no incisal reduction or prep of the lingual surface and it can be in the form of a window or intra-enamel preparation or the feathered incisal preparation.
  2. In the second technique, the incisal edge is slightly reduced and the porcelain overlaps the incisal edge "Butt-joint"
The two techniques were used equally and both provided clinically acceptable results.

Butt-joint margin
Instead of a chamfer margin that may weaken the final restoration (see Extensive preparation), a flat shoulder should be used at the incisal edge. However, if the tooth exhibits the length desired for the ceramic veneers, a reduction will be required which will establish a thickness of 2.0 mm in the buccal palatal direction of the incisal margin. Subsequently, a flat incisal surface that emerges perpendicular to the angle of the final emergence profile in the veneers should create the delicate incisal edge configuration where the incisal edge grooves are connected.


* Another interesting diamond point that I would like to use for incisal preparation is the wheel-shaped, but you have to be careful while using to protect the neighbouring sound teeth.



Another important aspect to be aware of is the condition of the palatal surface of the tooth that is about to be prepared. The finish line on the lingual aspect of the restoration should not end on a wear facet and it is also preferable not to place the finish line on the central contact points. In the case of a worn incisal edge, a 2 mm reduction from the desired length of the finished veneer is required. If the already existing tooth length is 1.5 mm shorter than the expected final outcome then in order to achieve a 2.0 mm incisal reduction, only 0.5 mm of preparation will be sufficient.
** The fracture potential of the final restoration can be avoided by the rounding of the incisal line angles that reduce the internal stress.

The butt joint preparation is simpler and consumes less time than the tooth preparation with palatal chamfer, and the master cast can be used for a precise palatal finish line. The path of insertion is from the facial. Therefore, only butt joint incisal preparations will allow such a buccal-palatal path of insertion.

Advantages:
  • the porcelain is stronger and produces a positive seat during the cementation process.
  • The proper seating of the veneer is made possible by the vertical stop that the incisal overlap provides.
  • The esthetic characteristics of the porcelain veneer are more easily handled and controlled by the dental technician to improve the translucency of the veneer.
  • it reduces stress concentration within the veneer by distributing the occlusal load over a wider surface.

The incisal involvement offers many advantages above all by the possibility of incorporating incisal characterizations such as translucency, evidence of the spindles, and of the opacity of the halo to the restorations, mimicking its naturality.


Window Design
A window design prepared entirely into the enamel was resistant to axial stress.




Advantage
It is considered the most conservative preparation design or window preparation is the treatment of choice when strength is the first priority.

Disadvantage
It may result in an aesthetic problem due to the fact that the preparation finishes in the most translucent area of the tooth. This is indicated in cases of substrates with the desired value without chromatic alteration in the incisal region.



2. Gingival margin preparation:

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. A spherical diamond point, used at a 45-degree angle to the surface, so that only half the diameter of the point promotes the reduction. 





NB. Preparation of the subgingival cervical margin and initial finishing of the preparation in indicated situations with rounded-ended diamond burs. It is worth to emphasize the importance of gingival tissue protection during preparation, in order to prevent post-traumatic gingival re-cession, especially in thin marginal tissue.

The fabrication of the ceramic veneers is directly affected by the placement of the finishing line. Smooth margins that are fully exposed and readily cleansable generally provide the best results. The dentist has to determine where to place the finishing line where the margins of the restoration can be kept clean by the patient and properly finished. In addition, finish lines must be placed so that they can be duplicated by the impression, without tearing or deforming the impression when it is removed past them.
With no cement line or margin visible, the porcelain blends in with the underlying composite resin in a harmonious finish. Whenever possible, the finish lines should be placed in the enamel following the contour of the soft tissue from mesioproximal to distoproximal.


 

Supragingival Margin:

Placement of the gingival margin supragingivally or coronally frees the gingival margin. This has many advantages such as:

  1. Eliminating the chances of injury to the gingival tissue
  2. Decreasing the risks of undue exposure of the dentin in the cervical region
  3. Obtaining crisp clear margins
  4. offering easier access to the finishing and polishing stages with easily accessible margins. 
  5. Impressions are easier to make.
  6. During the try-in and bonding stages, proper isolation of the operative field is easier, so moisture control and the chances of contamination during adhesive procedures are reduced.
  7. Postoperatively it eliminates the possibility of impingement on biological widths by an inadvertent overextension of the preparation
  8. Making it possible for the patient to perform meticulous hygiene in this critical region.
  9. Allowing the dentist to evaluate marginal integrity during the follow-up and maintenance visits.
  10. Increasing the likelihood that the restoration will end on enamel and this increased area of enamel is extremely important for stronger adhesion and less microleakage in the future.


Restrictions to enamel is a necessity for marginal tooth preparations and bonded restorations as exposure of the dentin margins may reduce bond strengths and increase the chance of microleakage. When the preparation margins are completely located in the enamel, microleakage is minimal or none at the tooth-luting agent interface and negligible in the resin/porcelain interface. It is always better to finish the cervical margin on enamel since more microleakage has been found at the luting composite/tooth interface when the cervical preparation margin was located in the dentin.


Subgingival Margin:

The reaction of the gingival tissues largely depends on the cervical extension of the restoration in regard to the location of the gingival margin. Generally speaking, the major etiological factor in periodontitis is the subgingival placement of a restoration.
In a majority of the cases, it is best to place the subgingival extension of the intrasuclar margins at about half the width of the crevice depth;
  • to create a buffer zone between the epithelial attachment and the bur, to prevent encroaching of the epithelial attachment of the biologic width if the preparation was extended deeper than the desired depth.
  • to leave enough space for the gingival cord placement. 
The deeper the restoration margin resides in the gingival sulcus, the greater the chance of inflammatory response and that such tissues can bleed upon probing.

 

Disadvantage:

The difficulty of visually following the cervical margins so that even the experienced restorative dentist can miss marginal defects.

Advantage:

It allows the technician to preserve the existing height of the papilla as well as to make certain that all interproximal spaces and/or diastemas will be closed while permitting control over the emergence profiles.


* non or minor tooth color changes (such as one or two shades lighter), there is no need to prep the tooth subgingivally. In other words, once the teeth are free from discoloration or presenting "discreet color alteration", the gingival margin must not be placed in the gingival sulcus.

* It is not only the color but the nature of the lip-line as well that determines whether the ceramic veneers marginal placement will be supragingival, equigingival or subgingival. If there is gingival exposure while smiling, subgingival placement may be necessary in order to achieve desirable esthetic results, but to a certain extent, it depends on the esthetic expectations of the patient.

It is still recommended that the placement be supragingival whenever possible.


Chamfer Preparation

It is almost impossible to finish accurately fitting ceramic veneers without being overcontoured over a knife-edge preparation at the cervical area. Therefore, irrespective of its placement (sub or supragingivally), a chamfer is preferred for all the gingival margins.
  • In comparison with the 90-degree shoulder, the chamfer finish preserves more natural tooth structure.
  • Owing to the gradual color transition between restoration and tooth substrate, thereby avoiding the sudden delineation between tooth and crown, it is also a better option esthetically.
  • Very distinct and visible finish line by removing of the serrated overhanging enamel prisms.
To avoid periodontal inflammatory responses and future gingival recession, improving the gingival architecture, survey lines and points of concavity with a chamfered margin while carefully controlling the anatomical contours in the cervical third, is of the utmost importance. The rounded tip of the fissure diamond bur will enable the cervical margin to be initiated slightly above the gingival level.


3. Triple angulation:

To provide a natural healthy look for the incisor that mimics its true convex nature, a uniform removal of the substrate is essential and can be achieved through the use of the bur, keeping it at three different angles.

One of the main characteristics of the veneers is biological preservation. This is possible when the buccal contour is preserved during preparation, by following the three inclinations of the natural tooth; the cervical third, middle third and incisal third. So, the veneer can be inserted in a rotational movement allowing preservation of the enamel and access to cervical and proximal undercuts areas.

It is important to note that:

* in the vertical dimension by using three angles of the bur

* in the horizontal dimension by following the tooth morphology.

Otherwise, one plane facial reduction may come too close to the pulp. When the diamond is swept in the mesiodistal direction, a gentle convex surface in the gingival and incisal 1/3rds is obtained.





Tooth preparation without respecting the facial convexity. Such straight preparation can result in irreversible pulp damage.

If the incisal or the cervical third is not prepared deeply enough, the final restoration may be overcontoured in this area.

A common error is to place the incisal third too far labially in the final restoration. This is frequently due to insufficient tooth reduction. The preparation must reproduce the natural convexity of a maxillary central incisor and provide for a minimum reduction thickness of 0.7 at the junction of the middle and incisal thirds of the tooth. Insufficient tooth reduction, in terms of leaving a sharp line angle and not rounding off the incisal labial area, is the leading cause of an over tapered preparation or produces opaque show through, or overcontouring of the incisal third of the restoration. 
This will not only influence the external facial form of the tooth but the light reflection as well. Light reflects off the tooth and travels through it at the incisal. If the edge has a rounded surface at the incisal-labial, that reflected light is being diffused and yields an ideal transition of the shade from the incisal color to body color.


4. Creation of a proximal slide:

This stage is characterized as one of the most conflicting points during preparations for a fixed prosthesis, performing reduction to enable insertion axis and elimination of retentions. It is essential to understand the fact that the majority of cases of ceramic veneers with or without preparation, presents a passive fit in the labial-palatal direction, and the supposed interproximal retentions are overlooked when veneers “fit in” without resistance. In this way, it is possible to carry out more conservative interproximal preparations, restricted mainly to the cervical region even if retentions exist when viewed pragmatically in an inciso-cervical sense.


The creation of a proximal slide allows placement of veneers when the proximal anatomy is challenging, as well as creation of proper emergence profile.

The preparation for the proximal surfaces must be thoroughly planned. The aim must be to place the margins beyond the visible area and to preserve the contact area. Esthetics should be carefully considered when placing the proximal "stop-line", providing that the teeth are free of proximal restorations. It is very important to go beyond the visible area that can be viewed from the front or side, especially when tooth color is very different from that of the laminate veneers.

Destroying the contact areas in order to create the margins is unnecessary, but, in some cases, the preparation margin may be extended further in a lingual direction. The margin can be drawn back even more in the lingual direction if the natural contact area has already been lost due to:

  • a diastema
  • restoring a broken angle
  • to encompass a proximal composite.

A tapered diamond point of smaller diameter is used to extend the margins from both the mesial and distal aspects until they are out of the visible area. Performing the proximal preparation with a diamond bur of smaller diameter. The protection of the adjacent teeth with a metal matrix is imperative.



The proximal extension of the preparation is directly associated with the static and dynamic area of visibility and preexisting restorations. The observation of the preparation in a static way, merely by labial, gives a false impression that all visible areas of the substrate were included in the preparation. Thus, the dynamic view of the preparation laterally enables an actual verification on whether or not to proximally extend the preparation.

In order to be accurate and to achieve the best results esthetically and functionally, the dentist should always be able to visualize the three-dimensional form. For the biologic integration of laminate veneers into the area, one should have a better understanding of the placement and design of the proximal finishing line.

The dynamic visibility area is the entire region of the non-prepared tooth and which remains visible regardless of the angle of observation. See that, when observing the tooth from an oblique view areas of unprepared enamel can be seen. The preparation of this area is essential to the success of the esthetics restorations, especially in darkened teeth. Even non-discolored teeth, however, also require this care, to ensure long-lasting veneers. In the preparation of these regions, the same fine and extra-fine grit diamond points are employed. The aim is to extend the preparation until the margins are fully hidden, whatever the position of observation is. Note that within the subcontact proximal areas it is necessary to displace the preparation margins to a considerable extent towards the palatal. To minimize the chances of inadvertently reaching the proximal surface of the adjacent tooth, strips of a metallic matrix are recommended for protection. It is also interesting at this stage to work with diamond points slightly angled with respect to the long axis of the tooth so that only a small part of the active tip is in contact with the tooth structure.


After this initial preparation, it is not possible to view the proximal margin of the preparation, confirming that the dynamic visibility area was adequately hidden.



To facilitate the placement of interproximal extensions, the margin of the porcelain veneer should be hidden within the embrasure area. The proper interproximal extension will provide additional stability and retention, due to the wraparound effect.

** Yet, it should be considered that placing the finish line in the proximal contact area creates a higher risk of interdental decay, especially with patients with compromised dental hygiene. Another factor to consider in the placement of the interproximal margins is the size of the interdental space. If there is an unsightly gap that needs to be closed, the exact placement of the interproximal margin will vary depending upon the size of the space.

The larger the space, the further mesiolingually or distolingually the margin will need to be extended. Otherwise, the resultant contact areas will be bulky and potential food traps.

It is useful to divide this surface into two regions:

  1. The gingivoproximal area, which extends gingivally from the interdental contact area
  2. The interproximal contact area, which is located in the incisal two thirds of the proximal surface.

Gingivoproximal Preparation

The interproximal embrasure is an important esthetic factor that assures harmony in the dental composition that is formed from the cervical point of contact, the interdental papilla and the proximal wall of the adjacent teeth. When preparing the area for veneers, the prognosis of the papilla, periodontal tissues, and its marginal integrity is directly proportional to careful planning and delicate preparation.

In veneers preparation, maintenance of the gingival embrasure space is only possible when sufficient tooth preparation has been done. Sometimes overpreparation of the tooth may be indicated to avoid any overcontouring.

Although it is generally underestimated, the interproximal tooth structure which is gingival to the contact area of the adjacent tooth is very important. This area is not visible from a direct frontal view and so it is left either underprepared or totally unprepared. However, when observed from obliquely, it is visible and it is therefore essential that this area be meticulously prepared. This is even more critical in cases where the unprepared tooth structure and the final restoration differ significantly in terms of color.

Gingivoproximal margin preparation should begin after the gingival preparation is finished. The round-end tapered fissure diamond is held at an almost 60-degree angle, and follows the gingival prep towards the palatal, from both the mesial and then the distal.


When viewed from the mesiofacial aspect, a concavity is created with the ultra-fine tip of the bur in the shape of an interproximal elbow or dogleg form. Depending on the facial convexity of the incisors, their depth can alter. It is usually kept supragingival.


As mentioned previously, the type of margin is preferably a chamfer form. The question can be, "what kind of chamfer should be used?" or, in other words, "what should be the depth of that margin?".


When significant shading differences exist between the tooth and the restoration, and distinct color alteration is necessary, in order to prevent the margin from being visible, the proximal margin must be extended palatally and halfway into the faciolingual dimension of the contact area.
It is possible to extend the proximal margin palatally in the gingivoproximal area until the unprepared tooth surface is no longer visible. In this way, the esthetics is not harmed by unsightly tooth angles that remain visible. The color and visibility of the teeth in that area will determine the amount of palatal extension in the gingivoproximal area. It is important to consider the visibility from different angles in order to determine the margin finish line in the gingivoproximal area. After the preparation is completed, esthetic problems may arise if the gingivoproximal area of the tooth structure is visible from any angle.

Adjusting the color of ceramic veneers with the help of different colored resin cement, at the time of bonding, is not suggested. However, in real life, these problems may occur owing to a lack of proper color communication with the lab.

If the chamfer is kept too deep then the replacing porcelain will be thicker than it should be and the thicker porcelain will allow less of a color change. Therefore, color-adjustment options will be limited when a heavy chamfer is used. Any attempt to alter the color with the help of resin cement will be unsuccessful. Although a thin chamfer may produce improved color control, it is impractical because its thin porcelain margin makes it vulnerable in the dental laboratory or even in the dentist's chair. Thus;
  • Light chamfer without internal line angles allows preservation of the enamel and thereby prevents marginal microleakage.
  • However, a medium chamfer that allows color adjustments and yet has a porcelain margin of adequate thickness. Especially in cases of dark discoloration and thick gingiva, the margins will still be placed on enamel.
They can be placed subgingival and more palatally in cases that display severe discoloration or diastema closure, or in cases where it is followed by a reduction of the gingival level, or when periodontal disease or periodontal surgery has taken place.


Interproximal Preparation
Proximal reduction is simply an extension of facial reduction. Using the same round-end tapered fissure diamond bur, the gingivoproximal reduction is continued by uprighting the angle of the bur vertically into the proximal area, making sure to maintain adequate reduction, especially at the line angle.



To correct an uneven finish line, ensure that the diamond is parallel with the long axis of the tooth and parallel to the mid-line. Even though this seems to be a minor detail, it is a vitally important one. It not only helps the dental technician to have enough space for ceramic veneers build-up but guides him/her to the correct esthetic incisal line by mounting the cast into the articulator, perpendicular to the correctly placed mid-line.


To break the contact or not?

The esthetic and functional demands of the specific tooth involved will determine the design of the preparation of the proximal walls.

To keep the contact

Under normal circumstances, this preparation should stop just short of breaking the contact. The proximal margin on teeth that are free from color alteration or with slight discoloration should be hidden by the preparation made far enough into the contact area so that the proximal wall will end 0.25 mm facial to the actual contact area.
Throughout the placement of the proximal margin, the dentist must continually check for any portion of darkened tooth structure beyond the preparation that will still be visible after the restoration is complete, by frequently checking from different viewpoints.



Preferably, the interproximal margin should stay short of the contact area. Whenever possible, it is best to preserve the contact area, as it is an anatomical feature that is difficult to reproduce.

** In some instances, the dentist may only be preparing a single tooth or two teeth and must therefore pay extra attention not to scratch the tooth next to it, which will stay intact. One of the ways of protecting the proximal surface of the adjacent tooth is to place a metal matrix band in between. However, sometimes, poor placement of this metal band may injure the papilla.

** When the actual treatment is in progress and the teeth are not periodontally stable, then it becomes a challenge to keep them intact and to prevent displacement of the teeth between the preparation and placement sessions. Consequently, the try-in procedures are simplified and save clinical adjustments of the contact areas where the fine ceramics are intricately placed.


To open the contact

Certain clinical circumstances, such as:
  • closing a diastema or changing the shape or position of a group of teeth, may require some specific preparation of the interproximal areas in order to allow the technician greater freedom in alteration of the form or position.
  • the existence of caries, defects or preexisting composite fillings. In such cases, it is important that after a thorough elimination of carious dentin, the weakened residual enamel thickness be evaluated. 
At times it may be necessary to full-slice through the contact from the facial to palatal to open a diastema and to give freedom to the dental technician.

If for some reason the preparation must finish on the proximal contact area, instead of extending the preparation more palatally, which will cause the loss of intact enamel in the contact area, it can be stripped to achieve a clear interproximal margin.

A diamond stip should be used to create a definite clear marginal edge.


At the restoration dentin interface, the connection of resin cement to dentin may be the weakest link. Bonding ceramic veneers onto a composite filling considerably increases the risk of failure. When the preparation margin is located within an existing filling, numerous failures have been reported. The authors believe in the incorporation of all the existing composite fillings and caries cavities into the preparation as much as possible.


Finally, The depth of the interproximal preparation can be classified as short, medium and long wrapping (Magne).





Refinement


During preparation, gingival displacement is achieved by delicately inserting the braided retraction cords into the sulcus so as to avoid any bleeding. Supragingival preparations do not need special attention. However, if the preparation is subgingival, it necessitates the displacement of the gingiva, to avoid injury.
It may also be beneficial to use non-medicated deflection cords instead of the medicated ones as they may be the cause of a secondary gingival recession. While doing this, the coronal preparation limit of the gingival contour of the tooth can be recorded, before the gingival displacement.

NB. If the dentist wants to be certain of the exact placement of the cervical margin in order not to make a deeper preparation than that is necessary, he/she should draw a line at the present location of the gingival cavo-surface margin with a sharpened pencil or a water-insoluble pen before placing the tissue displacement cords into the sulcus.

At this time, we start to refine all the preparation surfaces using diamond points similar to those mentioned above already employed except with fine and extra-fine grits.

Before and after refinement




Finishing and polishing


A facial reduction of 0.5 mm with chamfered margins and 1.5-2 mm of incisal reduction are preferable considering the conservative nature of porcelain laminate veneers preparation. It is not difficult to ensure adequate uniform thickness that is necessary for structural integrity when under stress when there is an emphasis on conservative tooth reduction in this preparation phase. Higher stress at the surface and interface of the restoration may occur when the restoration is too thin or has a poor internal fit.

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.

One of the most important tasks is the manual instrumentation of the cervical margins. Often even after finishing and polishing, the region has small irregularities and sharp angles, related to the angle of enamel rods and the form of diamond points used for preparation. To eliminate these irregularities, the best alternative is using a manual cutting instrument. 




To get the PDF click this link Ceramic Veneers Preparation