Minimally Invasive Resin-Bonded Zirconia Veneers for the Treatment of Discolored Teeth: A Multidisciplinary Case Report by the First Committee of Junior Members of the Italian Dental Prosthesis and Oral Rehabilitation Society (SIPRO)

Abstract

Objectives: Among modern metal-free materials, zirconia, a high-performance ceramic material that can only be manufactured through CAM procedures, has certainly exponentially gained popularity thanks to its mechanical strength, biocompatibility, esthetic, and versatility. However, one of the main debates that has been raised in relation to zirconia is its usage as an adhesive material. The present case report describes the clinical outcome of a multidisciplinary case finalized with adhesive minimally invasive zirconia veneers for the treatment of discolored teeth after a 24-month follow-up. Methods: A 19-year-old female patient with discolored upper frontal teeth (first premolar to first premolar) negatively affecting her self-esteem and social life was visited by a prosthodontic specialist. The treatment plan included orthodontic treatment, soft and hard tissue management through surgical procedures, and, lastly, minimally invasive adhesive zirconia veneers. The zirconia veneers bonding was performed under a rubber dam by conditioning the dental substrate by sandblasting the enamel with 40-micron aluminum oxide, etching with orthophosphoric acid 37%, and using a proper adhesive system. Monolithic zirconia restorations were sandblasted with 70-micron aluminum oxide at 0.2 MPa, then cleaned with a specific cleaner, and treated with a primer. Results: At the last follow-up (24 months), neither biological nor mechanical complications were observed. The patient anecdotally reported being very satisfied with the functional and esthetic results obtained. Therefore, the case was considered successful. Conclusions: Within the limitations of the present case report, the reported case on the use of minimally invasive resin-bonded zirconia veneers for the treatment of discolored teeth showed excellent outcomes after a 24-month follow-up. The use of zirconia as an adhesive material seems to be emerging. However, more clinical studies are required to validate the procedure.

1. Introduction

In recent years, the field of fixed prosthodontics and dental veneers has radically changed, thanks to the evolution of adhesive procedures [1,2], the advent of novel materials [3,4], and the integration of cutting-edge digital technologies [5,6,7,8]. These changes together have led to the development of the so-called “minimally invasive dentistry”, significantly influencing the approach of restorative dentistry [9]. These advancements have not only optimized patient outcomes but have also changed the clinician’s approach, allowing for the maximum preservation of healthy tooth structures rather than excessive tooth reduction.

One of the key aspects of this change is represented by the evolution of adhesive procedures. Traditional fixed prosthodontics often require extensive tooth reduction and mechanical retention features, which inevitably lead to the removal of healthy dental structures [10,11]. However, with the evolution of adhesive dentistry, clinicians can now achieve reliable and durable bonds between tooth and restoration, which often requires minimal tooth reduction [9]. These procedures are based on principles of micromechanical and chemical bonding [1,2], allowing clinicians to restore teeth with improved precision and minimal invasiveness.

Another key aspect is represented by the introduction of novel materials, such as high-strength ceramics [12,13,14] and resin composites [15,16,17]. These materials offer improved esthetics, durability, and biocompatibility, resulting in the possibility to obtain minimally invasive restorations that closely mimic natural teeth. Furthermore, the integration of digital technologies, including CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing) systems [12,18] and 3D printing [19,20], has simplified the design and fabrication of these restorations, improving both the efficiency and accuracy of the treatment process.

Among modern metal-free materials [21,22], zirconia, a high-performance ceramic material that can only be manufactured through CAM procedures, has certainly exponentially gained popularity thanks to its mechanical strength, biocompatibility, esthetic, and versatility [13]. However, despite its optimal documented properties, one of the main debates that has been raised in relation to zirconia is its usage as an adhesive material. As observed and highlighted in a review on this topic by Zarone et al. [13], zirconia cannot be etched with hydrofluoric acid at temperatures, times, and concentrations readily available to dental practitioners [23,24,25]. Moreover, zirconia also lacks a glass phase, and, therefore, silica-to-silane bonding is not effective [26]. Therefore, as highlighted by the review’s authors, its usage as an adhesive material should be limited to cases where proper retention is not guaranteed. Different methods and techniques have been investigated over the years to improve the adhesive bonding interface between zirconia and tooth substrate [27,28,29]. To date, the most recognized procedure includes the usage of alumina air-particle-abrasion at 0.2 MPa as a pre-treatment, followed by the application of MDP monomer-based primers to optimize the adhesive bond interface between resin cement and zirconia [30,31].

However, while different in vitro articles tested and investigated the above-mentioned procedure [32,33,34], in vivo articles are currently limited [35,36,37,38], and more research on this topic is required.

Therefore, the aims of the present article are: (1) to report the outcomes of minimally invasive resin-bonded zirconia veneers for the treatment of discolored teeth after a follow-up period of 24 months and (2) to provide an insight discussion on possible advantages in comparison to other metal-free materials.

2. Case Report

The present case report was established following the CARE Checklist (https://www.care-statement.org/checklist, accessed on 15 October 2024).

A 19-year-old female patient was referred to a prosthodontic specialty practice (S.B.) by a general dentist to improve her smile’s esthetics. The medical history was unremarkable. The patient reported taking tetracycline during childhood. At the initial clinical examination (Figure 1), the patient presented with frontal teeth (first premolar left to first premolar right) with a high degree of discoloration and an excessive gingival display.

The patient reported that the negative esthetic of the teeth strongly affected her self-esteem and social activity.

After the clinical examination, the diagnosis was the following: second-class malocclusion with increased overbite; linguo-inclination of the lower incisors; endo-inclination of the upper incisors; altered torque of the upper posterior sectors; teeth discoloration (first premolar left to first premolar right) due to previous intake of tetracycline; and excessive gingival display associated with altered passive eruption (APE). Initially, a first attempt to treat the teeth discoloration was made with an external teeth whitening.

Due to the insufficient result obtained, the multidisciplinary treatment plan was the following: initial orthodontic treatment followed by clinical crown lengthening to treat APE. Subsequently, minimally invasive adhesive monolithic zirconia veneers to improve dental morphology and improve color, covering dental discoloration.

After the orthodontic treatment was completed (Figure 2), a digital wax-up was performed, choosing an ideal tooth shape for the patient’s new smile, which was tested through a mock-up with bis-acryl resin (Luxatemp Star DMG) (Figure 3).

Following project evaluation, the patient proceeded to the surgical phase (Figure 4). The flap was designed by creating submarginal parabolic incisions, starting from the angular lines of the teeth and crossing at the level of the interdental papillae. The correct placement of the primary incision is based on the probing depth and the amount of keratinized tissue available. The flap was elevated with a full-thickness approach, and the osteotomy and the osteoplasty were executed. The flap was finally sutured with vertical mattress sutures.

After 6 months, the prosthodontic phase was carried out. A guided preparation was carried out through mock-up in an attempt to minimize the sacrifice of healthy dental tissue (Figure 5).

A vertical finish line preparation was adopted, which allows greater freedom of change in dental morphology for the final restoration at the level of the cervical area of the dental element (Figure 6).

Using the double retraction cord technique, a dental impression was then acquired using a polyvinylsiloxane material.

The zirconia block chosen for the restorations was 3D Pro-ZIR ^® produced by AIDITE ^® (AIDITE, Kaistrasse 18, 40221 Duesseldorf Germany) color A1, which is a layerless natural shade gradient zirconia (1050 MPa cervical layer, 700 MPa incisal layer) with 57% translucency in the incisal area and 43% translucency in the cervical one. After the digital project was executed, pre-sintered zirconia restorations were milled with a dental laboratory milling machine (PrograMill PM7, Ivoclar Vivadent) and sintered, glazed, and stained by the technician (Figure 7).

Once the manufacturing of the veneers was completed, the clinician (S.B.) performed the adhesive cementation following the manufacturer’s instructions.

The conditioning of the dental substrate was carried out following the traditional procedures for cementing adhesive restorations (Figure 8): isolation with a rubber dam, conditioning of the dental substrate by sandblasting the enamel with 40-micron aluminum oxide, etching with orthophosphoric acid 37%, and use of a proper adhesive system (2-step Clearfill adhesive system SE BOND 2, Kuraray Noritake, 65795 Hattersheim, Germany). Monolithic zirconia restorations were sandblasted with 70-micron aluminum oxide at 0.2 MPa, then cleaned with a specific cleaner (KATANA cleaner, Kuraray Noritake, 65795 Hattersheim, Germany), and treated with a primer (Clearfill CERAMIC primer, Kuraray Noritake, 65795 Hattersheim, Germany). The definitive adhesive cementation was executed (PANAVIA SA Cement Universal, Kuraray Noritake, 65795 Hattersheim, Germany).

After that, the rubber dam and cement excess were removed.

Initially, the patient was asked to return for a follow-up after 1 week and 1 and 6 months (Figure 9) to verify the proper integration of the monolithic zirconia veneers with the soft tissues and to check that no functional issue emerged during the normal functional activity of the patient.

Then, the patient was asked to return for a follow-up 12 and 24 months (Figure 10) after the adhesive cementation, and a teeth cleaning was performed on each occasion.

3. Results

At the last follow-up (24 months), neither biological nor mechanical complications were observed. The patient anecdotally reported being very satisfied with the functional and esthetic results obtained. Therefore, the treatment was considered successful. An excellent integration of the restorations with the soft tissues appearing pink and healthy was observed. A new architecture of the soft tissues was obtained, with a harmonious and natural progression. The dental elements had a new shape, and the discoloration was completely removed. Even though the zirconia veneers used in the present report were monolithic material, they were rich in incisal characteristics and transparencies with a natural and biomimetic appearance, allowing the obtainment of an optimal esthetic. Through the use of a color imaging spectrophotometer (Spectroshade Micro II, Oxnard, CA, USA), it was possible to detect the transition from an initial color D2 (VITA classical A1-D4^® color scale) at the incisal level and C4 at the cervical level to a final color B1 at the incisal level and A1 at the cervical level (Figure 11).

Data from the Spectroshade analysis showed a high level of translucency in the incisal area and opacity in the cervical area, which reflected the inside properties of the zirconia block adopted. There were no failures such as debonding or secondary caries, and no superficial marginal discoloration was observed. The final medium thickness of the veneers was 0.6/0.7 mm (Figure 12).

4. Discussion

The first aim of the present case report was to report the outcomes of minimally invasive resin-bonded zirconia veneers for the treatment of discolored teeth after a follow-up period of 24 months. The patient initially presented with a high level of dyschromia in all the frontal upper teeth due to a previous intake of tetracycline during childhood. Following the patient’s request to change the esthetic of her smile, a multidisciplinary approach was adopted, including orthodontic and periodontic procedures to create the optimal condition to prosthetically improve the shape and shade of the teeth. Lastly, the case was completed by performing adhesive rehabilitation with minimally invasive preparation and monolithic zirconia veneers of the frontal upper area (first premolar to first premolar).

Veneer restorations are recommended for different esthetic reasons as a minimally invasive treatment option [39,40]. This prosthetic approach was decided due to the high esthetic demand of the patient and the low results obtained with previous external whitening, making direct composite veneers a non-optimal treatment. In the event that the external whitening was sufficient to restore the natural color of the teeth, direct composite veneers could have represented a non-invasive and economical treatment option to restore the teeth shape. An alternative treatment plan could have been represented by the use of a zirconia ceramic material instead of a monolithic one. Zirconia ceramic is known to offer an increased esthetic, but it requires a more invasive preparation to allow enough thickness for both the zirconia and the layered ceramic. In the present case, monolithic zirconia was preferred to keep the preparation as minimally invasive as possible. Thanks to the adoption of modern zirconia with a natural and biomimetic appearance, an optimal final esthetic was obtained, as documented in the present report. Furthermore, an alternative treatment plan could have been the traditional full crowns, but since the teeth were healthy and due to the invasiveness of the preparation required in the case of full crowns, it was initially not considered by the authors.

In recent years, preparation techniques have evolved, leading to a minimally invasive approach to preserve a higher quantity of sound tissue [10,11]. In the present report, a guided preparation through a mock-up based on the final wax-up was performed, allowing the preparation of only the area with an insufficient thickness [41,42]. Furthermore, monolithic zirconia was used as the final material for the veneers. In this specific clinical case, the use of feldspathic ceramic or glass ceramic can be very challenging due to the altered color of the substrate, which would require an aggressive preparation to obtain enough space to reach a correct esthetic [43,44]; indeed, these materials are less opaque and more translucent than zirconia. Based on different types of zirconia, it has been documented that approximately 25% of incident light passed through this material, making it particularly suitable in cases where a dark abutment is present, such as metal posts or high-level discolored teeth such as in the present case report [13,45,46,47].

The patient was then followed up to 24 months after the adhesive cementation. At the last follow-up, the patient anecdotally reported being very satisfied with the treatment, and no biological or mechanical complications were observed. Therefore, the treatment was considered successful. However, the results should be taken with caution, and no final conclusions can be drawn. The main limitation of the present article is inherent to the single case report presented. A single report is insufficient to validate a technique from a scientific standpoint. However, case reports are important in presenting novel techniques. The successful results described in the present article encourage us to continue the research on this topic, including clinical studies with larger sample sizes and longer follow-up and randomized control trials comparing zirconia veneers with different cementation modalities and with alternative materials.

Currently, there is a lack of clinical reports in the literature on the clinical performance of zirconia used in cases of monolithic veneers, and the only available data are from case reports or case series. Silva et al. [48] reported the treatment of three patients with zirconia veneers with a follow-up of 5 years. Precise esthetic treatment steps were performed, both in the teeth and in the veneer’s internal surface, and the authors observed a survival rate of 100% after five years of follow-up, with only one marginal discoloration observed in the maxillary left lateral incisor of one patient. In agreement, Souza R. et al. [49] reported the one-year follow-up of monolithic zirconia veneers and found excellent results in both the aesthetic and functional aspects. Furthermore, Luna-Domínguez et al. [35] described a clinical case of full mouth rehabilitation (posterior and anterior teeth) treated with zirconia veneers through a fully digital workflow. The authors concluded that the workflow adopted, including zirconia bonding procedures, demonstrates a reliable workflow for the minimally invasive treatment of the patients. Therefore, the result obtained in the present article is in agreement with those reported in the literature. However, the evidence from the case report is limited, and, as highlighted in all the reports currently available, more prospective studies are required to confirm the procedure.

The second aim of this article was to provide an insightful discussion on the possible advantages of using zirconia as a material for veneers in comparison to other metal-free materials. As highlighted in a Cochrane review [50], no evidence is currently available on what material is most suitable when employed in indirect veneer restorations. Composite, feldspathic ceramic, and glass ceramic are some of the most commonly adopted materials for this type of rehabilitation. Different articles have reported the survival rate of ceramic laminate veneers in the medium to long-term follow-up period [51,52,53,54]. In accordance with them, the survival rate of ceramic laminate veneers is reported to range between 80 and 96% after an observational period of 10–21 years [52,53,54]. The main reasons for failure are due to the fracture of the ceramic and the marginal defects [52,53,54,55]. In recent years, thanks to the adoption of digital technology in daily clinical and laboratory practice [56,57], zirconia has gained more and more popularity among modern “metal-free” materials. As previously stated, there is currently a lack of clinical research comparing zirconia veneers with other materials. A very recently published systematic review and meta-analysis by Klein et al. [58] aimed to analyze the survival and complication rates of feldspathic, leucite-reinforced, lithium disilicate, and zirconia ceramic laminate veneers. In regard to zirconia veneers, the authors concluded that not enough data are currently available and, therefore, the long-term performance of this material employed for veneers remains uncertain. The authors concluded that all the other materials analyzed in the systematic review and meta-analysis are viable options for veneers, exhibiting high survival rates over time. However, lithium disilicate seems to outperform the other materials by presenting lower rates of biological and mechanical complications.

The possible advantages of zirconia in comparison to composite, feldspathic ceramic, and glass ceramic may be related to its mechanical properties [13]. In detail, zirconia is documented to have higher fracture toughness and resistance to wear compared to composite and glass ceramics. This property makes the zirconia less likely to chip or crack under stress compared to the other materials [13,59,60]. This characteristic is particularly valuable for patients who may be prone to bruxism or clenching, where additional stress can be placed on the veneers due to parafunctional activity. Furthermore, given its strength, zirconia can be made thinner while maintaining structural integrity [59,60,61]. This property allows for minimal preparation of the tooth structure while preserving more of the natural tooth when the veneers are placed. The ability to create thin yet strong veneers can lead to a less invasive procedure with biological benefits for the patients. This allows zirconia to be used in a more conservative approach compared to the other materials. In addition, zirconia’s resistance to wear and erosion represents another advantage. In environments with high acidity or abrasive forces (like parafunctional activities), zirconia tends to resist better than ceramics [45]. This property can contribute to the longevity of the restoration and potentially reduce the need for future replacements or repairs. Furthermore, since its introduction when zirconia was considered a non-esthetic prosthesis, this material has evolved today, offering a high level of translucency [62,63] and, therefore, is now commonly adopted even in esthetic areas. However, despite all the mechanical advantages reported above, zirconia employment as a material for indirect veneer rehabilitation is currently limited. The main reason is that the use of zirconia as an adhesive material is currently the object of debate [13]. In the case of veneers, in order to perform minimally invasive preparations, the preparation design is commonly a non-retentive preparation, and, therefore, the reason why the restoration remains in place is totally dependent on the adhesive bond between the restoration and the tooth structure. In the case of zirconia, the traditional bonding procedures are not effective due to the impossibility of etching the zirconia and the lack of glass phase inside the material [13,23,24,25,26]. For these reasons, novel procedures with dedicated MDP monomer-based primers were developed [30,31]. However, in vivo, articles on the application of zirconia as an adhesive material in non-retentive scenarios are currently limited. The most notable data are present from studies that analyzed the outcomes of single-retained zirconia resin-bonded fixed dental prostheses (RBFDP). Sasse et al. followed 30 [36] and 42 [37] anterior single retained zirconia RBFDPs, respectively, for 3 and 6 years and found a survival rate of 100% in both of the studies. Furthermore, Kern et al. [38] reported the outcome of 180 zirconia cantilever RBFDPs followed up to 10 years and found a survival rate of 98.2% after the mean observational period that was considered.

To conclude, the use of zirconia as an adhesive material is a developing field with promising applications in various contexts. The present case report contributes to the growing body of reports currently available in the literature, providing insights into the potential benefits and challenges of zirconia in this role. Further randomized trials are required to explore the efficacy and optimize the use of zirconia in adhesive applications.

5. Conclusions

Within the limitations of the present case report, the reported case on the use of minimally invasive resin-bonded zirconia veneers for the treatment of discolored teeth showed excellent outcomes after a 24-month follow-up. The use of zirconia as an adhesive material seems to be emerging. However, more clinical studies are required to validate the procedure.