Minimal Preparation Veneers
The risks are worth the reward
Zachary S. Sisler, DDS
When utilizing minimal preparation veneers to rehabilitate a smile, a dentist needs to be able to design like an architect and analyze like an engineer. The architect of a smile needs to thoroughly understand the parameters of smile design in order to blend the pink and white esthetics to make a smile appear effortless within the frame of the lips and face1; the engineer needs to manage the structural tolerances of restorations that are 0.3 mm to 0.5 mm in thickness in relation to function in order to achieve a long-term, predictable result for the patient.2 Using minimal preparation techniques yields many advantages, such as conservation of enamel, an inherently strong enamel bond, natural shading from underlying tooth structure, and more. Although minimal preparation veneers are beautiful and conservative, they can present clinical challenges that a dentist needs to understand prior to placing a diamond bur against the teeth. These challenges include handling issues, fractures, shade discrepancies, preparation design, margination, and functional stability.3 This article examines three critical components of restoration with minimal preparation veneers that can pose challenges to the clinician but, when handled appropriately, yield unquestionably esthetic results. By focusing on the preparation design, the gingival contours, and the underlying functional requirements, the result can be a smile that is not only beautiful but also able to endure the test of time.
Minimal preparation is advantageous, lending itself to greater conservation of tooth structure and remaining enamel for bond strength; however, it can be challenging to work in such finite spaces and achieve three distinct planes of reduction on the facial surface of the teeth as well as extend into the embrasure and interproximal areas.4When designing a preparation, the goal is to minimize the unnecessary reduction of excess tooth structure, and this can be accomplished thorough proper case selection and treatment planning.5 Carefully planned reduction leads to an increase in the amount of retained enamel, which improves both the strength of the bond and the overall strength of the restoration—two elements that are crucial when placing minimal thickness restorations.6
During the preparation appointment, the preparation's design and dimensions need to be meticulously monitored. This can be accomplished with the use of preparation guides that are fabricated from a diagnostic wax-up. This also aids the clinician in preventing the teeth from becoming under- or overprepared.7
Although thin restorations can appear sleek and natural, it is still paramount to design a restoration that facilitates proper cleaning at the gingival and interproximal margins. One of the advantages of a minimal preparation is that it allows for the margins of the restoration to be kept equigingival or, in certain circumstances, supragingival. The challenge is in preventing the gingival margin of the restoration from becoming overcontoured. If overcontoured, the patient can experience gingival inflammation and hygiene issues.8 The same problem could exist interproximally in certain minimal preparation cases where the proximal contact remains intact. The restoration needs to be designed and fabricated in such a manner as to not create overhanging porcelain that could prevent interproximal cleaning with floss or impinge upon the papilla.
From the laboratory perspective, waxing and pressing ceramics require a minimum thickness of 0.3 mm at the margins, so the technician in the case that follows used a microscope and a fine diamond-impregnated wheel to very slowly finish the margins to make the edges feather-thin. The goal is for it to be almost invisible in the mouth, with no visible demarcations—similar to the edges of a contact lens. Significant amounts of time may be necessary in order to avoid unnatural-appearing contours. The emergence profile must be at zero from the gingival contour to flow to the height of contour, which is approximately one-third of the gingival area and then goes to the incisal. Of course, the rest of the contour must be fit into the matrix that has been made from the approved provisional.
Minimal preparation veneers can be fabricated out of pressed lithium disilicate; however, the tolerances of this material can be tested to the maximum if put under the intense functional stresses of parafunctional activity.9 The use of pressed porcelain techniques offers the advantage of inherently higher strength values but also presents the challenge of ensuring that the occlusion is dialed in to ensure a stable functional environment for such a thin restoration. Case selection should be driven by proper treatment planning that includes photography and mounted casts to ensure that this is achievable prior to preparation. Being able to visualize the teeth in a functional esthetic matrix provides confidence that a restoration of minimal thickness is not being placed in harm's way because of an unprotected occlusal scheme.10
A 31-year-old, female patient presented to the office unhappy with the esthetics of her upper front teeth (Figure 1 and Figure 2). She reported that her teeth were stained and always appeared yellow in photographs. She also felt that her smile was too narrow and was disappointed with the chipping of her upper central and lateral incisors. She displayed tense masseter muscles and reported a history of clenching both at night and during the day. Her temporomandibular joints were tested using the bimanual manipulation technique and no signs of tension or tenderness were found in the area. Using Doppler auscultation, it was determined that a lateral pole reducing click was exhibited by both joints in translation. The periodontal charting displayed a moderate gingival biotype with bleeding upon probing.
The patient expressed a desire to enhance her smile through the use of porcelain veneers but wanted her treatment to remain conservative without aggressive tooth preparation. It was explained that porcelain veneers can be completed very conservatively and only require 0.3 mm to 0.5 mm of thickness in order to produce a new, more esthetic and brighter smile. To help the patient visualize the potential changes, composite filling material was placed over the unprepared tooth structure, which demonstrated how the smile could be made to appear more uniform with a relatively minimal thickness of material. Through this visualization process, the patient was able to achieve a better understanding of the direction of the proposed enhancements that would be established through a diagnostic wax-up later in the treatment process.
Diagnosis and Treatment Planning
After the comprehensive exam, a set of facebow-mounted casts in centric relation were used to analyze the occlusion for treatment planning purposes.11 In addition, it was determined that the patient needed a thorough dental prophylaxis to help achieve healthy gingival tissues. Due to her young age, it was important to ensure a balanced and stable occlusion to prevent further degradation or potential fractures later in life. The treatment plan consisted of an equilibration coinciding with centric relation. The occlusal scheme would entail stable, equal intensity stops on all teeth with anterior guidance in harmony with the envelope of function. A full coverage restoration would be used to replace the crown that was in place on tooth No. 4. Teeth Nos. 5, 12, and 13 would be treated with onlay porcelain veneers because of the mesial-occlusal-distal restorations that were in place; teeth Nos. 6 through 11 would receive minimal-preparation porcelain veneers. After a discussion with the laboratory regarding porcelain material selection, the decision was made to use pressed lithium disilicate with a cutback technique and microlayering of feldspathic porcelain. This would provide a strong yet beautiful restoration. With the advantage that lithium disilicate can be pressed as thin as 0.3 mm, clinicians have the option to use it without cutback and layering, but with this patient's esthetic demands, the cutback and layering option would offer more benefit.
Silicone preparation guides were fabricated from the diagnostic wax-up to aid in the preparation design and to help visualize the thickness of the final restorative material. Prior to preparation, the equilibration was performed, removing interferences through minor enameloplasty. This allowed for the preparations to be very conservative because the functional requirements would be on natural lingual enamel tooth structure.
A provisional silicone matrix was used to apply a bis-acryl transfer of the diagnostic wax-up to the unprepared teeth (Figure 3). The depth cuts for the preparations were made into the bis-acryl material in an attempt to minimize the possibility of over-reduction of tooth structure and practice responsible esthetics. The initial depth cuts were made to 0.5 mm on the facial surfaces in three distinct planes of reduction: the gingival one-third, the middle one-third, and the incisal one-third. This was followed by a 1.5-mm incisal reduction to allow adequate depth for translucency to be built into the final porcelain restorations. Next, the depth cuts into the bis-acryl were marked with a graphite pencil, and the bis-acryl was prepared away, acting like a guide to help visualize the thickness of the final restorations (Figure 4). Once the excess bis-acryl was removed and the teeth were prepared, facial, lingual, and incisal silicone matrices were tried in the mouth to serve as a secondary check after the initial reduction (Figure 5 and Figure 6). This allows the clinician to visualize that proper clearances were created for the laboratory and serves as another way to minimize the possibility of over-reduction. Once the clearances were established, the preparations still needed to be refined interproximally because of decalcified enamel that was discovered. This necessitated that the interproximal areas be prepared through the proximal contacts on the mesial and distal aspects of the teeth in order to ensure that the margins would be on sound enamel tooth structure (Figure 7). The preparation shade was recorded, which is of the utmost importance to the laboratory because minimal thickness restorations will rely on the underlying natural tooth structure for their final shade to mimic that of natural enamel.12
After a 1-week evaluation to test the esthetics, phonetics, and function of the provisional restorations, an impression was taken along with a facebow record to communicate the final tooth shape, shade, and form to the laboratory. The information sent to the laboratory included a detailed prescription as well as photographs of the unprepared teeth, the prepared teeth, the approved provisional restorations (Figure 8 and Figure 9), and the shade tabs.
The final restorations were fabricated using pressed lithium disilicate (Amber® Press HT, HASS BIO America, Inc.). The technician used a cutback technique using the matrix from the dentist's provisional model and characterized with an internal stain. The restorations were then baked at 720° for a very short amount of time. The technician then layered with veneering porcelain (GC Initial® LiSi, GC America) on the cutback areas to create more depth to the incisal edge effects and improve translucency (Figure 10). After glazing, the surface area often looks unnatural, almost glassy. To better mimic the natural surface texture of teeth, the technician gently abraded the surface of the restoration with a diamond wheel and hand polished using Zircon-Brite (Dental Ventures of America, Inc.) and a bristle brush.
The restorations were pre-etched at the laboratory using 9.5% hydrofluoric acid (Ceram-Etch, Gresco Products, Inc.); the technician suggests 20 to 25 seconds for lithium disilicate because any more than that will result in over-etching. After try-in, they were cleaned (Ivoclean, Ivoclar Vivadent), silanated (Monobond Plus®, Ivoclar Vivadent), loaded with esthetic luting composite (Variolink® Esthetic LC [neutral shade], Ivoclar Vivadent), and placed to the side in an orange light filtering box to prevent any curing of the cement. The neutral shade was selected after using a cement simulator corresponding to the same cement shade.
In the mouth, the teeth were isolated with a latex-free rubber dam using a split dam technique then etched and disinfected (Consepsis™, Ultradent Products, Inc.). A bonding agent (Adhese Universal, Ivoclar Vivadent) was applied to the teeth, air-thinned (Warm Air Tooth Dryer, A-dec), and light cured. Using a small suction tip, the restorations were placed onto the teeth starting with teeth Nos. 8 and 9 and moving posteriorly. This method allowed for easier handling of the thin veneers without the risks of dropping them or applying too much pressure upon placement. Once all of the veneers were in place and the gross excess cement was removed from the margins, the restorations were light cured using a tack-and-wave technique. Any remaining excess cement was then removed, and the teeth were cleaned and polished (Figure 11 through Figure 13). The occlusion was checked meticulously with articulating paper to ensure stable, equal intensity contacts and posterior disclusion by way of anterior guidance.13
A minimal preparation restoration requires diligence in the preparation design. In addition, the creation of a smooth, undetectable contour that transitions from the surface of the restoration to the tooth in the gingival and interproximal areas is absolutely critical. The last key piece to the puzzle is the creation of a stable occlusion to ensure long-term functionality of the new minimal thickness restorations. In the end, minimal preparation veneers can present challenges, but when done properly, the risks are worth the reward.
About the Author
Zachary S. Sisler, DDS
Associate Faculty Member
The Dawson Academy
Shoji Suruga, CDT, AAACD
Bay View Dental Laboratory