Abstract: Objective: Consistent thickness and ideal form are important considerations for treatment with ceramic veneers, however conventional preparation technique allows for the risk of certain shortcomings relative to these factors. This article introduces a preparation method that creates superior abutments. Clinical Considerations: When preparing abutment teeth for ceramic veneers, it is desirable to remove as little tooth substrate as possible, both to preserve an enamel surface and respect minimally invasive principles. However, traditional veneer preparation techniques often make it difficult to achieve ideal form with a veneer of suitable thickness. Conclusion: Creating proper guide grooves on a tooth’s line angles permits the reliable creation of an ideal abutment form while removing only minimal amounts of tooth substrate. Clinical Relevance: This technique is aimed at improving the form of the abutment tooth.
Ceramic veneers are widely used to improve the esthetics and function of anterior teeth.1 The preparation of the underlying tooth has a significant influence on the fit and form of the final restoration.2 In addition, it is desirable for the shape of the prepared tooth to be a miniature version of the external shape of the final restoration.3 This allows the veneer to have a uniform thickness, which is critical for both esthetics and longevity. Different thicknesses of the veneer may lead to visible variations in color, and internal stresses in the restoration upon mastication are naturally concentrated in thinner areas, increasing the risk of veneer fracture.4
Preparation is conventionally carried out using guide grooves to indicate how much tooth substrate should be removed. Two standard approaches to the creation of these grooves are generally used. A common approach is to use a depth cut bur, which can create three guide grooves in a single pass. With this method, however, it is possible to neglect the curvature of the tooth surface, and different curvatures in different parts of the tooth may go unaccounted for. Preparing the tooth in this usual manner may make it more difficult to achieve the desired form with a uniform thickness of veneer. In extreme cases, it can even result in the exposure of dentin in some areas, negatively impacting longevity. Alternatively, specialized guide groove burs may be used to create the grooves. These burs are designed to automatically preserve the correct depth and naturally follow the contours of the tooth surface, making the preparation less technique-sensitive while improving the form of the abutment. Examples of such burs include Veneer Crown MI Preparation Y2 Kit (Horico Dental, horico.de/en/), shown in Figure 1, and PrepMarker Kit (Komet USA, kometusa.com).
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When the veneer is intended to cover multiple surfaces of a tooth, such as the facial and proximal surfaces, the use of these burs may not be sufficient to secure an ideal shape for the abutment. In such cases, the line angles between the surfaces must also be prepared. If the shape of the prepared tooth is to be a miniature of the final restoration, the ideal approach is to have the area around the line angle of the prepared tooth be located somewhat inward from the line angle of the final restoration.5 The techniques described above, however, are prone to the removal of relatively less substrate from the line angle than from the tooth surfaces. If the preparation is kept within the enamel on the surfaces to improve longevity, this may lead to an excess of substrate on the line angle, making it difficult to prepare a veneer with ideal form. This may also lead to the veneer being thin at the line angle, which risks increasing vulnerability to concentration of stress even more than does a thinner area within a surface.
This article, therefore, introduces a novel veneer preparation technique that incorporates guide grooves into the line angle to reflect the shape of the tooth. The effectiveness of this technique is demonstrated through digital analysis and clinical case studies. The application of the technique described is intended to improve the quality of ceramic veneers. This technique achieves a uniform thickness of the final restoration by shaping the preparation in line with the natural shape of the tooth at both the surfaces and line angles. Further, an ideal final abutment shape can be attained even in cases in which the shape of the tooth is changed, thus allowing the application of veneers to a broader range of cases. The technique is aimed at facilitating the placement of durable ceramic veneers and securing high patient satisfaction.
Digital In vitro Evaluation
Two models of each of three distinct types of teeth (tapered, ovoid, and square) were created from a resin disc (Aidite Temp A2 multilayer, Aidite, aidite.com) in a milling machine (CORiTEK 350i Loader PRO+, imes-icore, imes-icore.com), based on scans of natural teeth. These models were also used as the mockup for the desired final form. They were prepared by a single operator according to either the established method or the new approach described in this article. The teeth were prepared for a veneer covering 270 degrees: three surfaces of the tooth, including both proximal surfaces.
Model Creation With Conventional Veneer Preparation Method
The conventional preparation technique, in which guide grooves are added on the facial and incisal surfaces, was applied to the models.6 Three grooves were cut on the facial surface of the tooth at depths of 0.3 mm, 0.5 mm, and 0.7 mm at the cervical, central, and incisal regions of the tooth, respectively, using guide groove burs designed for each depth (FGAS 003, 005, 007, Horico Dental). Also, 1 mm guide grooves were cut into the incisal surface, using a 1 mm guide groove bur (FGAS 010, Horico Dental). Standard burs (FG 041 047, FG 199C 016, FG 165F 010, FG 119F 012, Horico Dental) were used to create a smooth surface guided by the grooves, and then to shape the proximal surfaces and refine the transition from the facial to the proximal surfaces. The surface was finished with a silicone point.
Model Creation With Novel Veneer Preparation Technique
In addition to the conventional grooves on the facial surface and the incisal grooves, prepared as per the conventional technique, vertical grooves with a depth of 0.7 mm were cut along the line angles in the incisal portion of the tooth, again using the specialized bur (FGAS 007, Horico Dental). The depth of the groove, rather than being perpendicular to either the facial or proximal surfaces, is at 45 degrees to those surfaces, so that the deeper part of the groove approaches the center of the tooth. The facial and incisal surfaces were smoothed while the line angle guide grooves were preserved. The proximal surfaces were then shaped, and, finally, the transition was refined such that the line angle guide groove became the line angle of the prepared tooth (Figure 2 through Figure 5). The same burs as used for the conventional preparation were used for this preparation, and the abutment was finished with a silicone point.
Digital Analysis
Before and after preparation with both the conventional and novel techniques, the models were scanned (E3, 3Shape, 3shape.com), and the scans were overlaid in CAD software (Dental System, 3shape) to allow quantitative analysis of the changes in shape. The consistencies of the thicknesses of the preparations were compared, as shown in Figure 6. In the cervical region, an ideal consistency of thickness was obtained with either method (Figure 6, a-b). In the central region, the line angles of the outer surface and the conventionally prepared abutment were in the same location relative to the arch and did not have an ideal spatial relationship (Figure 6, c-d). In the incisal region, the conventional method had removed insufficient material at the line angle, and the line angle on the abutment was exterior to that on the final surface (Figure 6, e-f). With the new method, an ideal consistency of thickness was secured even in this region. Further, the abutment line angles were somewhat interior to the line angles on the model before preparation, achieving an ideal relationship.
The abutment should be a miniature version of the final form. This means that the abutment line angle should be somewhat interior to the line angle of the mockup, and the angle on the abutment should be somewhat nearer the center than the angle on the external surface. If these relative positions were reversed, the restorative material would become thin at the angle, risking problems with the form and strength of the final restoration. In these models, the model of the tooth before preparation serves the same role as the mockup in a clinical case. For Figure 6, in the b, d, and f graphics, the black line indicates the surface before preparation, the grey line shows the surface after preparation by the conventional preparation technique, and the purple line denotes the surface after preparation by the new preparation technique.
Case Reports
Case 1
A 40-year-old male patient presented with a principal complaint of esthetic defects due to deterioration and discoloration of composite resin restorations in the maxillary anterior teeth. Secondary caries and misalignment of the teeth were also diagnosed (Figure 7). After discussion with the patient it was decided to restore the four maxillary incisors with ceramic veneers.
First, the caries was removed and resin-based composite used to restore the teeth. The dental technician then created an esthetic and function diagnostic wax-up. In areas where the natural teeth extended beyond the surface of the planned restoration, the technician used this wax-up as a basis to create resin preparation jigs (proximal, incisal, and buccal) (Figure 8). In cases where reshaping and realignment are necessary, as in this case, thorough initial preparation makes it possible to achieve a result close to the ideal form (Figure 9). A wax-up of the final restoration shape was prepared in the laboratory and used as the basis for the creation of a mold in clear resin (EZ-COPY Clear Matrix, AMCO Inc, morimura-jpn.co.jp). The mold was placed over the prepared teeth, and flowable resin-based composite (Estelite® Flow Quick, Tokuyama Dental, tokuyama-us.com) was injected into it. Once full contact was confirmed, the resin-based composite was light-polymerized, the mold removed, and the mockup cleaned and polished (Figure 10). This mockup was used as a provisional restoration. The esthetic and functional quality were confirmed and intraoral adjustments made.
The preparation of the abutment teeth was carried out according to the method described above. Three guide grooves with depths of 0.3 mm, 0.5 mm, and 0.7 mm were created on the facial side using the specialized burs for the creation of horizontal guide grooves (Horico Dental). At the line angles, vertical guide grooves were created in the incisal half of the tooth, with the bur entering the material of the tooth at 45 degrees (Figure 11). The preparation was carried out so that the line angle guide groove was at the location of the line angle on the finished abutment. Through the use of a mockup and the new shaping technique, abutments can be created in line with the principle of minimal intervention, even in cases with misaligned teeth. Grooves extending 1 mm were created in the incisal edge, and the material was removed in line with the guide grooves to prepare the abutments (Figure 12).
The veneers were manufactured from pressable lithium-disilicate reinforced ceramic (IPS e.max® Press, Ivoclar, ivoclar.com). The site was isolated using a rubber dam, and the veneer placed using light-cure resin luting cement (Variolink® Esthetic LC, Ivoclar). The ideal form was achieved, and the patient was satisfied with the esthetic and functional results. Six months after placement, the veneers were functioning well, and the patient remained satisfied (Figure 13).
Case 2
A 25-year-old woman presented with a primary complaint of wanting to improve the symmetry of her maxillary incisors and reduce their size for esthetic reasons (Figure 14). After consultation with the patient, the clinician decided to achieve esthetic and functional improvement by reducing the size of the four maxillary incisors and placing ceramic veneers on them, and orthodontically realigning the mandibular teeth.
The patient’s desired shape was determined using a diagnostic wax-up. After initial preparation using a preparation jig created as in the first case described above, a direct mockup was placed, again in the same manner as was done in the first case (Figure 15 through Figure 17). The esthetic and functional quality of the mockup was confirmed while orthodontic treatment was carried out on the mandibular teeth.
In order to change the shape of the teeth, the proximal surfaces were shaped across three surfaces using the new preparation technique with additional grooves on the line angle. The position of the line angle is particularly important when making the teeth smaller, as the line angle must not be too close to the desired final surface or the veneer will be thin and weak at this point. Therefore, the guide groove on the line angle is critical (Figure 18). Impressions were taken of the abutments with gingival retraction cords in the sulci (Figure 19).
Ceramic veneers were fabricated from pressable lithium-disilicate reinforced ceramic (IPS e.max Press) and placed under split rubber dam isolation using light-cure resin luting cement (Variolink Esthetic LC). The results matched the patient’s desires for the shape of her teeth, and she was satisfied with the esthetic and functional results (Figure 20).
Discussion
This novel preparation technique modifies existing technique through the use of additional guide grooves at the line angles, making it easier to accurately create the desired abutment shape in these areas. Digital analysis confirmed that the prepared abutment was a miniature version of the ideal final shape established before treatment. In the clinical cases, the patients were very satisfied with the esthetic and functional results obtained.
It has long been recognized that a uniform thickness of the restoration must be obtained when placing a veneer.7 In the clinic, it has frequently been observed that failure to do so can lead to protrusion of the restoration at the line angle in the incisal half/coronal region of the tooth.8 The authors’ digital analysis of the preparation of abutment teeth under the conventional preparation technique showed that less tooth substrate is removed at the line angle than necessary, leading to this esthetic shortfall. While this issue may be addressed by removing more tooth substrate across the whole face of the tooth in the incisal area, doing so may remove all of the enamel in some regions, exposing underlying dentin. It is well-established that indirect restorations have superior adhesion to enamel than to dentin, and, moreover, the principles of minimal intervention discourage such indiscriminate removal of healthy enamel.
The primary advantage of this new preparation technique is that it secures a more uniform thickness of the restorations. With the conventional technique, excessive tooth substrate often remained in the vicinity of the line angle. If the thickness of the veneer was preserved, the surface would protrude more than desired, impairing esthetics.9 Conversely, if the veneer is made thinner at that point to secure the desired esthetics, there is a risk that the restoration will be weaker and vulnerable to damage.10-12 This technique therefore makes it easier to create veneers with high esthetic qualities and good strength. Because any additional removal of tooth substrate is limited to the area in which it is necessary, this approach also supports the principles of minimally invasive dentistry. In the two case reports presented, this method was also shown to be effective when the restoration dictates that the shape of the tooth must be modified. These cases were particularly difficult to treat effectively using the established preparation method.
Conclusion
Creating proper guide grooves on the line angles permits the creation of an ideal abutment form while removing only minimal amounts of tooth substrate. The novel veneer preparation technique described enables the placement of esthetic veneers with an ideal form without removing excessive amounts of tooth substrate.
DISCLOSURE
The authors declare no conflicts of interest.
ABOUT THE AUTHORS
Yuichiro Yoshiki, DDS
Director, Y’s Dental Clinic, Private Practice, Nagoya, Japan; Researcher, Department of Operative Dentistry, Aichi Gakuin University School of Dentistry, Nagoya, Japan
Miyuki Oshika, DDS
Assistant Professor, Department of Operative Dentistry, Aichi Gakuin University School of Dentistry, Nagoya, Japan
Yuji Tsuzuki, RDT
Director, Ray Dental Labor, Private Dental Laboratory, Kyoto, Japan
Akimasa Tsujimoto, DDS, PhD
Professor and Chair, Department of Operative Dentistry, Aichi Gakuin University School of Dentistry, Nagoya, Japan; Adjunct Associate Professor, Department of Operative Dentistry, University of Iowa College of Dentistry, Iowa City, Iowa; Visiting Associate Professor, Department of General Dentistry, Creighton University School of Dentistry, Omaha, Nebraska
References
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10. Blunck U, Fischer S, Hajtó J, et al. Ceramic laminate veneers: effect of preparation design and ceramic thickness on fracture resistance and marginal quality in vitro. Clin Oral Investig. 2020;24(8):2745-2754.
11. Gonzalez-Martin O, Avila-Ortiz G, Torres-Muñoz A, et al. Ultrathin ceramic veneers in the aesthetic zone: a 36-month retrospective case series. Int J Prosthodont. 2021;34(5):567-577.
12. Haak R, Siegner J, Ziebolz D, et al. OCT evaluation of the internal adaptation of ceramic veneers depending on preparation design and ceramic thickness. Dent Mater. 2021;37(3):423-431.
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