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Corresponding author: Dr Panagiotis Gakis, Department of Comprehensive Dentistry, University of Texas Health Science Center, 8210 Floyd Curl Drive San Antonio, TX 78229
Studies on the fit of heat-pressed anterior lithium disilicate veneers are sparse, and whether fit is affected by glaze firing or ceramic addition is unclear.
Purpose
The purpose of this in vitro study was to evaluate and compare the marginal fit of heat-pressed anterior lithium disilicate veneers (IPS e.max Press) fabricated with the staining or cutback technique by using 3-dimensional analysis.
Material and methods
Two groups of heat-pressed maxillary left central incisor lithium disilicate veneers were fabricated (n=10) differing only in core thickness and the fabrication process. The tooth preparation was standardized at 0.6 mm cervical and middle third and 0.7 mm incisal third, with 1.5 mm of incisal reduction. Group S (staining) was an anatomic contour veneer with 1 glaze firing. Group CB (cutback) had a cutback core of 0.6 mm on the cervical and middle third and 0.5 mm on the incisal areas for ceramic application with 3 firings (wash, incisal, and glaze firing). The amount of ceramic application was standardized, and all veneers had identical final dimensions. Marginal fit was evaluated at 2 stages: after pressing the copings (control) and after glaze firing or ceramic addition using the virtual replica technique and 3-dimensional analysis. The wax copings were invested, eliminated, and pressed with IPS e.max lithium disilicate high translucency ingots, and the overall marginal fit and change in marginal fit after firing were measured in the cervical, mesial, distal, and incisal areas. The Shapiro-Wilk test was used to evaluate normality. Repeated-measures ANOVAs were used to explore differences between the 2 groups (S and CB) as per time (before and after firing) for each location (cervical, mesial, distal, and incisal) (α=.05).
Results
A statistically significant change in marginal fit after firing was found for the mesial and distal areas of the CB group (P<.05). In all other areas of measurements, no statistically significant differences were found (P>.05). The mean ±standard deviation marginal fit of group S was 63 ±13 μm and 62 ±9 μm for group CB.
Conclusions
The results suggest that firing affects the marginal fit of heat-pressed anterior lithium disilicate veneers fabricated with the cutback technique but not for the staining technique.
Clinical Implications
The marginal fit of heat-pressed anterior lithium disilicate veneers was within clinically acceptable limits regardless of the fabrication technique. Firing significantly affected the marginal integrity of the cutback group. However, the difference was not clinically significant.
Ceramic laminate veneers are a predictable and conservative restorative treatment option for unesthetic anterior teeth, with high rates of long-term success (95.6% at 10 years).
As per the manufacturer’s recommendations, 2 techniques can be used to fabricate heat-pressed lithium disilicate veneers: the staining technique and the cutback technique. Three firings are usually required for the cutback technique: wash, incisal, and glazing and characterization firing, and only 1 firing is needed for the staining technique: a glazing and characterization firing.
the marginal fit of any dental restoration is essential to its long-term success. Fit values ranging from 50 to 300 μm have been reported for ceramic restorations.
Studies on feldspathic laminate veneers fabricated with either the refractory die technique or the platinum foil technique have reported a tendency for open margins at the cervicoproximal locations.
Research investigating the marginal fit of heat-pressed lithium disilicate veneers is sparse. Two in vivo and 1 in vitro studies investigated these veneers, with conflicting findings, but none used 3D analysis.
Other studies have evaluated the effect of different firing protocols at various fabrication stages on the marginal integrity of ceramic and metal-ceramic systems for complete coverage restorations.
However, the authors are unaware of studies that determined whether glaze firing or ceramic addition affects the marginal fit of lithium disilicate veneers.
The purpose of this study was to evaluate and compare the marginal fit of heat-pressed lithium disilicate veneers fabricated with the 2 commonly used techniques, staining and cutback, by using 3D analysis. The null hypotheses were that no difference would be found in the marginal fit of heat-pressed ceramic veneers fabricated with the staining and cutback technique and that glaze firing or ceramic addition would not affect the marginal fit of lithium disilicate veneers fabricated with these 2 techniques.
Material and methods
An Ivorine left maxillary central incisor (Viade Products Inc) was selected for the preparation die, and a silicone matrix (Lab Putty; Coltène) was fabricated before the preparation of the tooth. The tooth was prepared for a lithium disilicate labial veneer with no sharp angles or edges and a chamfer finishing line as per the manufacturer’s guidelines. A laminate veneer preparation kit (K0162 Lowe: Laminate Veneer Preparation System; Brasseler USA, Inc) was used, and the tooth was prepared to a uniform reduction of 1.5 mm of the incisal edge and a facial reduction of 0.6 mm in the cervical and middle third and 0.7 mm in the incisal third (Fig. 1). The incisal finish line was a butt joint, and the interproximal and cervical finish line was a light chamfer. The prepared Ivorine tooth die was duplicated 20 times with high-heat epoxy resin (Viade Products Inc) and was used during measuring procedures as the definitive resin die (Fig. 2). Twenty custom trays were also fabricated from a light-polymerized material (Triad TruTray VLC Custom Tray Material, Clear; Dentsply Sirona). Twenty impressions were made of the prepared Ivorine tooth die with heavy- (Aquasil Heavy Body, Regular Set; Dentsply Sirona) and light-body polyvinyl siloxane material (Aquasil Extra Light Viscosity [XLV], Regular Set; Dentsply Sirona). The 20 impressions were poured with Type IV dental stone (Resin Rock, Resin Fortified, Low Expansion Die Stone; Whip Mix Corp). The definitive stone casts were indexed with a pin system (Axiopin; SAM Dental) and a special stone for basing (Flowstone, Fluid Low-Expansion Base Stone; Whip Mix Corp) (Fig. 3). The stone fabrication dies were coated with 2 layers of a newly opened die spacer to 1 mm from the finish line (Tru-fit Die Spacer; Taub Products). Die spacer facilitated the seating of the veneer since only marginal fit was evaluated and not internal fit. Die hardener (Stone Die & Plaster Hardener; Taub Products) was applied on the dies to increase wear resistance and decrease abrasion. The stone fabrication dies were dipped in a wax pot (Hotty wax dipping pot; Renfert) to create a wax coping with even thickness and minimal shrinkage.
To determine sample size, a pilot study had been conducted with the same methodology, revealing that a sample size of 9 specimens per group would yield 80% power at α=.05. The staining group (S group) copings (n=10) were waxed to the anatomic contour with dimensions of 0.6 mm cervical and middle third and 0.7 mm incisal third. For the cutback group (CB group) (n=10), after the creation of anatomic-contour waxing, the incisal and middle thirds were cut back to create a coping with dimensions of 0.6 mm cervical and 0.5 mm in the middle and incisal third, meeting the minimum requirements of the manufacturer for a Press/CAD veneer. The silicone putty cutback matrix ensured adequate reduction. Each wax coping was sprued, invested with a phosphate-bonded investment (IPS PressVEST Speed; Ivoclar Vivadent AG), and pressed in a hot press furnace (Zubler Vario Press 300; Zubler USA) as per the manufacturer’s instructions. After pressing and cooling, the veneers were devested, and the reaction layer was removed. Sprues were removed with a silicon carbide separating disk (MF-2203SF; Pacific Abrasives, Inc) with irrigation. Each veneer was fitted to its respective definitive resin die. As the measuring technique was nondestructive, the same cores were used to apply the ceramic. The veneers were airborne-particle abraded with 100-μm Al2O3 for 10 seconds at 0.1 MPa. The surface was cleaned with a steam jet and subsequently dried. A feldspathic ceramic matched to the core (IPS e.max Ceram; Ivoclar Vivadent AG) was used to complete the morphology of the incisal veneer. Three firings were conducted, wash firing, incisal firing, and stain/glaze firing, as per the manufacturer’s instructions. The amount of ceramic applied was standardized by using a porcelain sampler (Portioner 5-1 -#4010-B; Smile line USA). The same procedures were followed for the staining technique with the following modifications. The silicone matrix was used to fabricate anatomic-contour veneers that were pressed with the same ingots. The difference with the CB group was that the glaze firing was conducted with different firing parameters, as specified by the manufacturer (Fig. 4).
Figure 4Completed veneers. A, Staining technique. B, Cutback technique
A dimensional inspection software program (Geomagic control software; 3D systems, Inc) was used to evaluate marginal fit. The individual definitive resin dies were scanned with a laboratory scanner (D900; 3Shape A/S) and standard tessellation language (STL) files were produced. Each veneer was placed on its respective definitive resin die, and the space between the die and the veneers was replicated with low-viscosity polyvinyl siloxane (Aquasil Extra Light Viscosity [XLV], Regular Set; Dentsply Sirona) (Fig. 5).
The replica was then scanned with the same laboratory scanner, and a new STL file was generated. The 2 STL files were uploaded to the dimensional inspection software program and were converted to point cloud, and N-point alignment was executed. Then, best fit alignment with zero tolerance was selected twice. The alignment was assessed by calculating the root mean square of the palatal surface, as this area remained unchanged between the 2 STL files. An alignment was considered successful if the mean square estimate was ≤12 μm. After the removal of the internal surfaces, 3 values for each area (cervical, mesial, distal, and incisal) and an overall marginal fit was obtained per specimen, averaging 200 000 points of measurements. Any negative values were excluded from the calculations because the actual marginal openings could not be less than 0. The mean of these 3 values was calculated to produce an overall value for the mesial, distal, cervical, and incisal areas for each specimen. This measuring protocol was conducted before and after the firing of each specimen, with each being self-controlled (Fig. 6).
Figure 5Creation of silicone replica cement gap. A, Definitive resin die scanned before cementation of veneer. B, Die coated with PVS adhesive and space filled with light-body PVS. C, Veneer removed leaving PVS replica adhered to abutment. Abutment with PVS replica scanned. PVS, polyvinyl siloxane.
Figure 6Geomagic alignment and measuring protocol. Step 1 – Import STLs into Geomagic software program from Figure 4A and 4C. Step 2 – Select palate and perform best fit alignment with zero tolerance. Step 3 – After alignment complete everything outside preparation deleted. Internal areas also selected and deleted in order to measure only marginal discrepancies. Step 4 – Measurements performed. Three arbitrary points selected per surface (incisal, mesial, distal, and cervical). Each point of measurement mean of several thousand points in radius selected in software program.
The data were analyzed by using a statistical software program (IBM SPSS Statistics, v25.0; IBM Corp). Repeated measures ANOVAs were used to explore differences between the 2 groups (S and CB) as per time (before and after firing) for each location (cervical, mesial, distal, and incisal) (α=.05). In the presence of a significant interaction, further analysis of simple main effects was conducted to detect differences between the 2 groups at each level of time. Data distribution was tested by using the Mauchly sphericity test. The Kolmogorov-Smirnov test was used to verify the normality of the residuals.
Results
The overall mean ±standard deviation marginal opening for CB group was 57 ±15 μm before firing and 62 ±9 μm after firing. For group S, the overall opening was 61 ±6 μm before firing and 63 ±13 μm after firing. The difference between the specimens of the 2 groups was not statistically significant (P=.425). Differences within the groups as per the firing cycle (P=.299) and the interaction effect (P=.672) were also not statistically significantly different as shown in Table 1. Further analysis of the change in marginal fit as a function of firing procedures and measurement locations showed that the S group marginal openings were not statistically significant in all locations either before or after firing. However, group CB showed a decrease in marginal opening in both the mesial (P=.006) and distal areas (P=.029) after firing.
Table 1Marginal fit (mean ±standard deviation) as per group (S and CB) for each time (before and after firing) and location (μm)
The purpose of this study was to evaluate and compare the marginal fit of heat-pressed lithium disilicate veneers fabricated with the staining or the cutback technique. The first null hypothesis was not rejected because the veneers produced with these 2 techniques showed no significant differences in all measurement areas. The mean overall marginal opening for the staining and cutback techniques was 63 μm and 62 μm (P=.425), respectively (Table 1).
These results conflict with those of previous studies on the marginal fit of heat-pressed lithium disilicate veneers. Aboushelib et al
reported a marginal gap of 295 μm. These higher values may be because a different methodology and different measuring methods were used compared with the present study. More specifically, Aboushelib et al
used an impression replica technique. Both these methods provide limited information on the marginal fit, as a small number of measurements can be obtained. However, the results of the present study are consistent with those of studies for heat-pressed lithium disilicate crowns. In a critical review of IPS e.max margins, the mean marginal fit varied from 31 μm to 138 μm.
used the triple-scan protocol to evaluate the 2-dimensional and 3D marginal fit of pressed and computer-assisted design/computer-assisted manufacturing–generated lithium disilicate crowns. They reported that the mean marginal fit of these crowns was 48 μm. However, the crowns were not cemented on the die, possibly affecting the marginal discrepancy.
Most of the studies investigating marginal discrepancies in crowns use definitive dies made either from metal or acrylic resin.
In the present study, the definitive die was duplicated with highly filled epoxy resin. The advantage of duplicating the die with the acrylic resin is the lack of wear of the die during the measurements, as each die was used on only 1 specimen and acted as its own control, minimizing the effect of variability as duplicated dies are not identical. A natural tooth was not used in the present study as it would have been difficult to standardize the tooth preparation and the restorations.
The change in the marginal fit of each veneer was also evaluated at different stages in the fabrication process. The first set of measurements was obtained after the core fabrication of each group (baseline). The core thickness used met the minimum requirements from the manufacturer. No significant differences were found between the 2 groups in the 4 areas of measurements. These results are consistent with those of Farid et al,
who evaluated the effect of core thickness and fabrication stages on the marginal fit of heat-pressed lithium disilicate crowns. They reported no significant differences in the core fit of 0.8 mm and 1.5 mm before the ceramic application and the glaze firing. In the CB group, the amount of ceramic applied was controlled to manage the potential effects of a nonuniform mass application on marginal distortion. No significant differences were found between the 2 groups in the 4 areas of measurements. This was consistent with the study of Cho et al.
However, the study accepted the second null hypothesis for the S group. The glaze and stain firing had no significant effect on the marginal fit in all 4 areas of measurements (Table 1). Other studies reported similar results, that glaze firing does not affect marginal integrity.
For the CB group, the second null hypothesis was rejected as a significant difference was found on the interproximal areas. No significant difference was noted on the cervical (P=.639) or incisal areas (P=.463). This is also consistent with the study of Cho et al,
who reported that during the glazing and characterization stage, there was a decrease in marginal opening. A possible explanation is that glaze firing for the CB group has different parameters than the one that is used for the staining technique. These results are not in agreement with the findings from studies evaluating the marginal fit of feldspathic veneers.
reported that after firing, feldspathic veneers had interproximal openings 2 to 4 times greater than the cervical and occlusal areas. As a result, a better marginal fit was achieved with the heat-pressed lithium disilicate veneers compared with feldspathic labial veneers. However, tooth preparation, indications, and fabrication process are different in these 2 types of restorations, limiting direct comparison.
In the present study, the virtual replica technique was used along with the Geomagic software program. A low-viscosity polyvinyl siloxane impression material was used because a high-viscosity replica material could prevent correct adaptation of the restorations.
The overall marginal fit was calculated by averaging approximately 200 000 data points of measurements. Moreover, the mean change in marginal fit on the cervical, mesial, distal, and incisal areas was calculated by measuring data points on a radius of 0.5 mm. Some negative values were obtained as a result of the best-fit alignment process. The software program uses an algorithm that minimizes the mesh distance error between each corresponding data point and spreads error evenly. The larger the discrepancies between the data sets, the higher the chances of acquiring clinically irrelevant outcomes. In the present study, the maximum allowable alignment error had a root mean square estimate of ≤12 μm for each specimen analyzed. This protocol was described and validated by Zeller et al.
Clinicians can be confident that carefully fabricated lithium disilicate veneers fabricated with either technique will meet existing marginal adaptation standards. However, limitations of the study included that the prepared Ivorine tooth and duplicate definitive resin die were not identical and the mean overall marginal fit value was not absolute if compared with the Ivorine die. However, the results give some indication of the possible marginal adaptation when precision 3D measurement techniques are used. Moreover, with 3D analysis, it is difficult to describe the marginal discrepancies as a vertical or horizontal misfit. Finally, the study evaluated the effect of firing only on veneers fabricated from specific IPS e.max ingots. Future studies are recommended to investigate and compare the 3D marginal and internal fit of milled and heat-pressed lithium disilicate veneers.
Conclusions
Based on the findings of this in vitro study, the following conclusions were drawn:
1.
No difference was found in the marginal fit between the cutback and the staining technique after firing.
2.
A statistically significant (P≤.05) change was found in the marginal fit of the interproximal areas of the heat-pressed lithium disilicate veneers fabricated with the cutback technique during ceramic addition.
3.
Heat-pressed lithium disilicate veneers fabricated either with the cutback or the staining technique demonstrated marginal fit within the clinically acceptable limits for ceramic restorations.
Acknowledgments
The authors thank Ivoclar Vivadent AG for providing materials and Dr Despoina Bombolaki for her valuable comments on this manuscript.
References
Layton D.
Clarke M.
Walton T.
A systematic review and meta-analysis of the survival of feldspathic porcelain veneers over 5 and 10 years.
Supported in part by a Stanley D. Tylman Research Grant from the American Academy of Fixed Prosthodontics, and Research and Graduate studies, Texas A&M University, College of Dentistry, grant #576390-00025.
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