Retrospective Long-Term Clinical Outcome of Feldspathic Ceramic Veneers

The purpose of this study was to evaluate the clinical outcome of feldspathic ceramic laminate veneers over a 7-year period using minimally invasive techniques, such as vertical preparation (without prosthetic finish line), or no preparation (no-prep). A total of 170 feldspathic ceramic veneers were cemented in the anterior region, including 70 maxillary and 100 mandibular veneers, after special conditioning of the teeth and restorations. The veneers were evaluated using the FDI World Dental Federation criteria evaluation kit after recalling all the patients between February and June 2021. In total, 14 feldspathic veneers failed and were replaced with lithium disilicate because of core fracture, and 10 cases of chipping occurred on the ceramic surface and were polished. The overall survival rate was 91.77% for up to 7 years of function, with a failure rate of 8.23%. In this retrospective survival analysis, the failures, including the fracture of veneers and dental hard tissue, occurred both in prep and no-prep teeth. No failures were observed in veneers with a maximum thickness of 0.5 mm compared to those with a maximum thickness of 1 mm, 1.5 mm, 2 mm, and 2.5 mm.


Introduction
All dental restorations are liable to failure; even in metal-ceramic prosthetic restorations, fractures can jeopardize function and esthetics [1].
Traditional metal-ceramic restorations have shown predictable strength, acceptable esthetic effects, and long-term survival in the oral environment [2]; however, a considerable amount of tooth structure must be removed during tooth preparation [3].
All-ceramic systems have been developed to achieve the most pleasing esthetic effect and expand treatment options for patients, causing a major shift in clinical workflow [4].
Abrasion and erosion are two increasingly common indications for dental treatment, and, if left untreated, may cause a loss of vertical dimension as well as diffuse and marked dentinal hypersensitivity [5,6]. To prevent these early-stage conditions, we must implement an appropriate treatment plan that preserves the dental hard tissue and ensures tooth vitality [7,8].
Several ceramic materials and fabrication methods are available for feldspathic porcelain veneers, including the platinum foil technique and the refractory die technique, which and received 170 feldspathic ceramic veneers (70 applied on maxillary teeth and 100 on mandibular teeth). The study was approved by the Institutional Review Board of "Vasile Goldis , " Western University of Arad. All patients agreed to participate in the study and gave their written informed consent. Two methods were used: a minimally invasive/biologically oriented preparation (prep) technique for 108 veneers, and a no-preparation (no-prep) approach for 62 veneers. The veneers were also divided in five groups, according to their maximum thickness: 0.5 mm, 1 mm, 1.5 mm, 2 mm, and 2.5 mm. The biologically oriented preparation technique is a vertical preparation technique without a finish line, used to create a new anatomic crown with a prosthetic emergence profile. We opted for one of these two treatment options after carefully assessing several factors: tooth attrition, malocclusion, dental anomalies, and esthetic alteration.
Vital teeth without endodontic treatment or major crown destruction were eligible to be included in the study. Patients who presented any parafunctional signs and symptoms of bruxism according to the International Classification of Sleep Disorders [46] were excluded from veneer treatment at the time of arrival. Patients that had severely malpositioned teeth were first referred for orthodontic treatment to allow a minimally invasive approach that avoided sacrificing healthy tooth structure in excess.
All patients were monitored periodically for at least 2 years after they received the veneers.
All restorations were made following a case set-up protocol. First, the patients underwent an esthetic and functional analysis (Figure 1), and impressions were taken. Subsequently, a diagnostic wax-up was made for the teeth that required treatment (Figure 2). Using a temporary resin material (Protemp 4, 3M ESPE, Seefeld, Germany) a direct mockup was made by transferring the wax-up into the mouth of each patient ( Figure 3). After 1-2 weeks of testing the provisionals, the decision regarding minimal or no preparation was made, based on the esthetic and functional preferences of each patient. In cases that presented enough space for the final restoration, with a minimal convexity of the teeth and a favorable color, the no-prep technique was applied ( Figure 4). The final impression was made with polyvinylsiloxane (Virtual 380, Ivoclar Vivadent, Schaan, Liechtenstein) using a single-impression double-mixing technique with a standard tray after the mock-up was removed from the teeth (Figure 4c). The color was recorded ( Figure 5) with a VITA classical shade guide and a polar_eyes cross-polarization filter (Bio-Emulation™, Freiburg im Breisgau, Germany) ( Figure 5b).           In the group in which we applied minimally invasive techniques, the objective was to obtain space for the future restoration, removing the maximum tooth convexity and any existing undercuts ( Figure 6). The teeth were prepared using magnifying loupes (x4.3, Zeiss, Wetzlar, Germany) directly in the mock-up for reducing errors. As alternatives to full crowns, double veneers were used instead with a prepless technique ( Figure 7). In the group in which we applied minimally invasive techniques, the objective was to obtain space for the future restoration, removing the maximum tooth convexity and any existing undercuts ( Figure 6). The teeth were prepared using magnifying loupes (x4.3, Zeiss, Wetzlar, Germany) directly in the mock-up for reducing errors. As alternatives to full crowns, double veneers were used instead with a prepless technique ( Figure 7). Provisional restorations were prepared to maintain tooth position, function, and esthetics using a mock-up as described previously. Using a double-mixing technique, a fullarch, polyvinylsiloxane (ExpresXT, 3M ESPE, Seefeld, Germany) single impression was made for the opposing arch, and a type IV dental stone (GC Fujirock EP, Tokyo, Japan) Provisional restorations were prepared to maintain tooth position, function, and esthetics using a mock-up as described previously. Using a double-mixing technique, a fullarch, polyvinylsiloxane (ExpresXT, 3M ESPE, Seefeld, Germany) single impression was made for the opposing arch, and a type IV dental stone (GC Fujirock EP, Tokyo, Japan) was poured immediately. Provisional restorations were prepared to maintain tooth position, function, and esthetics using a mock-up as described previously. Using a double-mixing technique, a full-arch, polyvinylsiloxane (ExpresXT, 3M ESPE, Seefeld, Germany) single impression was made for the opposing arch, and a type IV dental stone (GC Fujirock EP, Tokyo, Japan) was poured immediately.
All restorations were made by the same dental technician following the manufacturer's instructions. Each restoration was placed on the tooth after removing the provisional mockup and assessed for proximal contacts, marginal adaptation, occlusal relationships, and shade matching. Veneers were then etched for 60 s with 3% to <7% hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar Vivadent, Schaan, Liechtenstein), washed and rinsed with water, and dried. Since etching with hydrofluoric acid leaves a significant amount of crystalline debris precipitate on the ceramic surface, the feldspathic restorations were also cleaned for 60 s using 36% orthophosphoric acid (  After cementation, periodic evaluations were performed every 6 to 12 months (during routine professional dental hygiene appointments) by two dentists who did not participate in the restorative procedure at the time of cementation (baseline). For the final reevaluation, after at least 7 years of function, all the patients were recalled between Febru- After cementation, periodic evaluations were performed every 6 to 12 months (during routine professional dental hygiene appointments) by two dentists who did not participate in the restorative procedure at the time of cementation (baseline). For the final reevaluation, after at least 7 years of function, all the patients were recalled between February and June 2021. The restorations were evaluated for apparent changes in their external structural integrity and marginal integrity. Secondary caries, fracture of the restoration/tooth, debonding of the indirect restoration, severe periodontal breakdown, or pulpal necrosis were considered absolute failures. A clinical qualitative evaluation was performed using the FDI World Dental Federation criteria [48,49] for assessing direct and indirect dental restorations considering three main indicators (a total of 16 items): biological (six items), functional (six items), and esthetic (four items). In our study, we were able to investigate 11 items (Table 1). Each veneer was clinically examined, and a score was given for each criterion (item) on a 5-point scale (e.g., 1, an excellent restoration; 5, a restoration that required replacement). Each examinator used an intraoral mirror, sharp explorer, and periodontal probe specifically designed for evaluating the FDI World Dental Federation criteria. All the veneers received a baseline score of 1 (excellent restoration) at the moment of cementation.  After cementation, periodic evaluations were performed every 6 to 12 months (during routine professional dental hygiene appointments) by two dentists who did not participate in the restorative procedure at the time of cementation (baseline). For the final re-evaluation, after at least 7 years of function, all the patients were recalled between February and June 2021.

Esthetic properties
The restorations were evaluated for apparent changes in their external structural integrity and marginal integrity. Secondary caries, fracture of the restoration/tooth, debonding of the indirect restoration, severe periodontal breakdown, or pulpal necrosis were considered absolute failures. A clinical qualitative evaluation was performed using the FDI World Dental Federation criteria [48,49] for assessing direct and indirect dental restorations considering three main indicators (a total of 16 items): biological (six items), functional (six items), and esthetic (four items). In our study, we were able to investigate 11 items (Table 1). Each veneer was clinically examined, and a score was given for each criterion (item) on a 5-point scale (e.g., 1, an excellent restoration; 5, a restoration that required replacement). Each examinator used an intraoral mirror, sharp explorer, and periodontal probe specifically designed for evaluating the FDI World Dental Federation criteria. All the veneers received a baseline score of 1 (excellent restoration) at the moment of cementation.

Statistical Analysis
Collected data were statistically analyzed using the SPSS Statistics platform (SPSS Inc. Released 2008. SPSS Statistics for Windows, Version 17.0. Chicago, IL, USA: SPSS Inc.). Nonparametric statistical tests (Mann-Whitney U, Kruskal-Wallis H, Wilcoxon Signed Ranks) were used to analyse the categorical data sets. The survival rate of the indirect restorations was calculated using the Log Rank (Mantel-Cox) test. The tests were considered statistically significant for p < 0.05. The feldspathic ceramic veneers were analyzed starting at baseline and ending when the clinician determined that an irreparable failure of the restoration occurred. When a veneer failed, it was replaced with a new lithium disilicate restoration.

Comparison between the Prep and No-Prep Technique
No statistically significant differences were found between the maximum thickness (mm) of the veneers used in the no-prep and prep method (Mann-Whitney U Test, p = 0.814).
The Mann-Whitney U test results for comparing the prep and no-prep methods for each FDI criterion after a 7-year period showed that the scores for the no-prep method were significantly higher than the scores for the prep method for C3 (p = 0.038), C6 and C10 (p < 0.001), and for C8 (p = 0.001), respectively.
Using the Mann-Whitney U test, we further refined the results, comparing the maximum veneer thickness groups in pairs for each FDI criterion after a 7-year period ( Table 2). For the C1, C4, C6, C8, and C10 criteria the scores for the veneers with a maximum thickness of 0.5 mm were significantly lower than those for the veneers with a maximum thickness of 1 mm.
With the exception of the C9 criterion, the scores for the 0.5 mm group were significantly lower than those for the 1.5 mm group.
With the exception of the C7 criterion, the scores for the 0.5 mm group were significantly lower than those for the 2 mm group and the 2.5 mm group.
For the C7, C8, and C10 criteria the scores for the 1 mm group were significantly lower than those for the 1.5 mm group.
For the C3, C4, C8, C9, and C10 criteria the scores for the 1 mm group were significantly lower than those for the 2 mm group.
For the C4 criterion the scores for the 1 mm group were significantly lower than those for the 2.5 mm group.
For the C3, C6, and C10 criteria the scores for the 1.5 mm group were significantly lower than those for the 2 mm group.
No statistically significant differences were found for any score when comparing the 1.5 mm and 2.5 mm groups, and the 2 mm and 2.5 mm groups.

Comparison between the Baseline Scores and the Scores after 7 Years
The Wilcoxon Signed Ranks test results (p-values) after comparing the baseline scores (T1) and the scores after 7 years (T7) for each FDI criterion are presented in Table 3.
For the C1, C3, C4, C6, C8, and C10 criteria, the scores after 7 years for the no-prep method are significantly higher than the baseline scores.
For the C1, C3, C4, C6, C7, C8, C9, and C10 criteria, the scores after 7 years for the prep method are significantly higher than the baseline scores.
All the feldspathic ceramic veneers were rated clinically excellent for color match and translucency even after 7 years of function (score 1).
The functional properties on marginal adaptation were also very good (score 1). Regarding the biological properties, postoperative sensitivity and tooth vitality were rated as excellent (score 1) for the no-prep method, while the adjacent mucosa was rated as excellent (score 1) for both methods. No secondary caries or pulpal necrosis was observed in the natural teeth. C1-C11-each criterion is explained in Table 1; T1-the baseline score; T7-the score after 7 years; N-no-prep; P-prep; * p < 0.05.

Survival Time Analysis
Out of 170 veneers, 156 veneers (91.77%) survived after 7 years of function, with a failure rate of 8.23%. A total of 14 feldspathic veneers completely failed due to core fracture and were replaced with lithium disilicate restorations.
The failures were both on prep and no-prep teeth, but no failures were observed for the veneers with a thickness of 0.5 mm ( Figure 10 and Table 4). No statistically significant differences were found between the time-dependent survival curves both for the prep and no-prep methods (Log Rank (Mantel-Cox), p = 0.247), and for the five thickness groups (Log Rank (Mantel-Cox), p = 0.065). We observed that the veneers with a thickness higher than 1.5-2 mm were more susceptible to fracture (Figure 11b); the fractures implicated both the veneers and dental hard tissue. Minimal chipping was observed on the ceramic surface of 10 veneers, in cases where the space for the ceramic was under 1 mm (Figure 12a). These surfaces were finished and polished without replacing the veneers (Figure 12b). We observed that the veneers with a thickness higher than 1.5-2 mm were more susceptible to fracture (Figure 11b); the fractures implicated both the veneers and dental hard tissue. Minimal chipping was observed on the ceramic surface of 10 veneers, in cases where the space for the ceramic was under 1 mm (Figure 12a). These surfaces were finished and polished without replacing the veneers (Figure 12b).

Discussion
In this retrospective study, we evaluated the clinical outcome of feldspathic veneers placed in our private practice for up to 7 years. Feldspathic veneers had a survival rate of 91.77%, which is comparable with the results of previous studies [12,35,50]. These studies report several clinical outcomes [12,35], with an overall estimated cumulative survival We observed that the veneers with a thickness higher than 1.5-2 mm were more susceptible to fracture ( Figure 11b); the fractures implicated both the veneers and dental hard tissue. Minimal chipping was observed on the ceramic surface of 10 veneers, in cases where the space for the ceramic was under 1 mm (Figure 12a). These surfaces were finished and polished without replacing the veneers (Figure 12b).

Discussion
In this retrospective study, we evaluated the clinical outcome of feldspathic veneers placed in our private practice for up to 7 years. Feldspathic veneers had a survival rate of 91.77%, which is comparable with the results of previous studies [12,35,50]. These studies

Discussion
In this retrospective study, we evaluated the clinical outcome of feldspathic veneers placed in our private practice for up to 7 years. Feldspathic veneers had a survival rate of 91.77%, which is comparable with the results of previous studies [12,35,50]. These studies report several clinical outcomes [12,35], with an overall estimated cumulative survival rate between 87-89% [12], while the most frequent complication was fracture/chipping. Compared to prior studies reporting high survival rates of feldspathic and porcelain veneers (1), our study considered secondary caries and endodontic complications absolute failures in terms of the long-term success of such restorations. Most of the failures were observed in the first 6 months, after cementation. Regarding the outcome of the vertical preparation group, similar results were observed in previous studies, in which the vertical preparation method was considered a promising alternative to horizontal preparation [50].
Regarding the esthetic objectives, we found similarities with other studies [35], which aimed to use a preparation method that could fix an anatomical crown with a prosthetic emergency profile and integrate it harmoniously with the teeth already present in the anterior region of the dental arches.
When using minimally invasive or no-prep techniques, the vertical dimension of occlusion (VDO) can be safely increased for up to 5 mm in the anterior region without causing pain or discomfort [51]; the patient can rapidly adapt to the new VDO within 2 weeks, if the temporomandibular joints are healthy and the articular disks are properly aligned. Utilizing feldspathic ceramic restorations in cases of dental attrition is an efficient way to increase the VDO.
The accuracy of the new VDO can be verified with the phonetic test by analyzing the pronunciation of words, particularly the use of the "s" consonant [52], as well as using provisional restorations [28], which aid both in restoring the physiognomic function and maintaining an optimal position for the final restorations. Direct mock-ups can also be used to test the modified VDO.
Although the materials used in all-ceramic restorations have been recommended for fabricating inlays/onlays [53][54][55] and single anterior and posterior crowns [56], these ceramic materials are not strong enough in the posterior region without adequate enamel support. One recent study [57] demonstrated that lithium disilicate overlays can increase the VDO using minimally invasive techniques, and show a 32-month survival rate of 97.7%. The average amount of dental tissue removed was less than 1 mm (0.98 mm in nonfunctional cusps, 0.88 mm in functional cusps, and 0.57 mm in the central fossa). The study concluded that lithium disilicate posterior overlays provided a complication-free treatment option, with an excellent survival rate, and the material allowed for conservative restorations with minimum thickness if the adhesive cementation technique was taken into consideration.
Generally, higher tensile and shear stress occurs when there are large areas of unsupported feldspathic ceramic (as in cases of diastema closure and teeth with chipping [12] or fracture), because these materials are too weak when the ceramic material must be extended more than 2 mm beyond the surface of the tooth [58]. We also observed that in spaces larger than 1.5 mm, the ceramic is susceptible to cracks (Figure 10b).
The final color of the tooth is affected by the thickness of the restoration, substrate color, ceramic color, and cement shade [59,60]. Feldspathic ceramic made on a refractory cast can restore tooth shape and color effectively [60]; however, it cannot be used to mask a darkened dental substrate [24,61], because these types of restorations are indicated in cases that require only a slight color change. In all cases of tooth discoloration or when a color change is desired, feldspathic ceramic restorations are not indicated. In our study, the restorations in both groups (the vertical preparation group and no-prep group) were performed by the same technician using the refractory die technique [18].
A photoshop assessment and functional evaluation, as well as a wax-up and mock-up, were used during pre-prosthetic planning. The esthetic properties of such restorations depend on the technician's ability to properly replicate the anatomy, color, and translucency into the restoration; therefore, a strong collaboration between the patient, dentist, and technician is crucial [10,14].
An advantage to feldspathic ceramics is the absence of a core material, which allows increased space for characterization in the middle and incisal thirds; some of these materials showed excellent antibacterial performance owing to the 30% nanosized percentage of Ag ions [62]. With this technique, it is possible to minimize the tooth preparation and avoid over-contouring of the future restoration; the veneers can be aligned with the surface of the enamel, on the same level as the dental hard tissue [40]. After adhesive cementation, feldspathic ceramics allow for much easier finishing and polishing of the veneer margins than other ceramic materials.
The minimally invasive adhesive treatment is limited only to vital teeth without discoloration. The preservation of tooth structure and the remaining enamel should provide sufficient resistance strength, even in the presence of reduced thickness of the ceramic feldspathic material.
In a quality assessment of dental treatments using en-face optical coherence tomography, Sinescu et al. [63] found microleakage at prosthetic interfaces and material defects in several types of prosthetic restorations. Future studies should assess long-term veneers and their related complications, using optical coherence tomography (OCT) to analyse the marginal closure in more detail. Another technique that can be taken into consideration is the use of an intraoral scanner to make the refractory model. These digital impressions seem to be a viable alternative of analog technique [64].

Conclusions
The results obtained in this retrospective study showed that the use of feldspathic ceramic veneers in a private general practice, using minimally invasive preparation methods, achieved an overall success rate of 91.77% over a period of 7 years, on vital teeth without major crown destruction. The failures, including the fracture of the veneers and dental hard tissue, occurred both in prep and no-prep teeth. No failures were observed in the veneers with a maximum thickness of 0.5 mm, compared to those with a maximum thickness of 1 mm, 1.5 mm, 2 mm, and 2.5 mm. No statistically significant differences were found between the time-dependent survival curves for the two methods, and for the five thickness groups.
No secondary caries or pulpal necrosis were observed. The postoperative sensitivity and tooth vitality were rated as excellent for the no-prep method. All the feldspathic ceramic veneers were rated clinically excellent for color match, translucency, marginal adaptation, and adjacent mucosa, both for the prep and the no-prep method.  Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.
Data Availability Statement: Additional data supporting the reported results can be requested from the corresponding authors.

Conflicts of Interest:
The authors did not have any commercial interest in any of the materials used in this study.