Keywords
marginal fit - marginal accuracy - CAD/CAM - zirconia - lithium silicate
Introduction
Marginal accuracy of fixed prosthodontics is heavily researched as it determines their
clinical success.[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8] A maximum cement film thickness of 25 to 40 µm was identified as set by the American
Dental Association (ADA).[9] In spite of the absence of a clear evidence that a certain method of fabrication
provides a consistently superior marginal fit,[10] a gap of 25 to 40 µm is hard to achieve with the conventional fabrication processes
because of the various materials and clinical and laboratory procedures involved.
However, the increased popularity of computer-aided design and computer-aided manufacturing
(CAD/CAM) technologies and the development of novel microstructures of ceramic materials
have improved the fixed prosthodontics practice including the achievable marginal
gap.[11]
[12]
[13]
Aesthetics has increasingly become a great influence in choosing restorative materials
even in the posterior region. Lithium disilicate (LD) might be considered the most
attractive monolithic all-ceramic alternative for anterior restorations because of
its great esthetic combined with high strength. The high translucency of LD enable
the production of natural results even in cervical portion of the restoration where
in the conventional metal-ceramic restorations, a dark shadow is likely to be visible.[14] Translucent zirconia and zirconia-reinforced lithium silicate (ZLS) are relatively
new alternatives indicated for anterior restorations. Translucent zirconia was developed
with increased yttria content to up to 5 mol% to overcome the aesthetic disadvantage
of the material.[15] Sen and Isler[16] found that extra translucent zirconia produces comparable optical properties to
that of LD. Cho et al[17] showed that compared to LD, 5Y-ZP had 80% translucency at 0.8 mm thickness and 89%
at 1.5 mm thickness. Similarly, ZLS can produce satisfactory optical properties.[18] The material composed of lithium silicate as the main crystalline phase in a vitreous
matrix reinforced with 10% dissolved zirconia (ZrO2).[18] The highly dispersed ZrO2 content is responsible for the generation of significantly
more crystallization nuclei, which is supposed to present a higher ratio of the glass
phase when compared with the conventional LD.[19] However, little is known about the marginal fitting of these materials compared
to that of LD.
Several review papers on the marginal adaptation of fixed restorations showed that
it is inconsistent, variant, and directly affected by the experimental protocol employed
in investigating it.[20]
[21] These are caused by variations in study designs, measurement methods, and the adopted
definitions of the marginal fitting. Therefore, comparing marginal discrepancy values
across studies should be made with great caution. Instead, such comparisons could
be made for different crown systems in one investigation under standardized method.
To the authors' knowledge, there has been no previous research which compared the
marginal adaptation of the five crown systems in one study. Hence, this study aimed
to measure and compare marginal accuracy of five contemporary ceramic materials used
for anterior restorations. Our null hypothesis indicates no statistically significant
differences in their marginal gap.
Materials and Methods
Master Die and Crowns Fabrication
A master die (Nissin Dental Products INC, Kyoto, Japan) of maxillary central incisor
was prepared following guidelines for all-ceramic crown preparation with an axial/incisal
reduction of 1.5 mm and a chamfer of 1 mm width. The amount of tooth reduction was
controlled using an index of the same tooth before preparation. The master die was
marked with indentations placed external to the preparation finish line at mid-labial,
mid-palatal, mid-mesial, and mid-distal points to standardize gap measurement points
([Fig. 1]). Fifty impressions of the master die were made using silicone impression material
(3M ESPE, St. Paul, Minnesota, United States) and molded with epoxy resin die material
(Exakto-Form, Bredent, Germany) to produce 50 replicas of the master die.
Fig. 1 (A) Vertical gap measured. (B) The arrow shows the indentation on the die identifying the point of measurement.
The master die was sprayed with CEREC Optispray (Dentsply Sirona, Bensheim, Germany)
and scanned by Cerec inEos X5 (Sirona Dental Systems, Bensheim, Germany). Scanning
data were saved in STL (Standard Triangular Language) format to be used for the designing
of the all-ceramic crowns (inLab CAM SW16, Dentsply Sirona) starting with the biogeneric
design technique. The CAD system permits the adjustment of different parameters such
as restorative material thickness and cement space. Therefore, the minimum thickness
of the designed crown was set at 1 mm to correspond for the tooth preparation recommended
for anterior ceramic crown, and the cement space was set at 50 µm.[22] Then, the designed crown was milled from five dental ceramics ([Table 1]) using 5-axis milling machine (MC X5, Dentsply Sirona). Cercon xt and e.max ZirCAD
were dry milled while the e.max CAD, Vita Suprinity and Celtra Duo were wet milled.
The fit of crowns onto replica dies was controlled by a stereomicroscope (Wild M3C,
Wild, Heerbrugg, Switzerland) with a magnification factor of 10. Cercon xt and e.max
ZirCAD crowns were sintered following the manufacturer's guidelines (in fire HTC speed,
Dentsply Sirona). The e.max CAD and Vita Suprinity crowns were crystallized in Programat
EP 3010 (Ivoclar Vivadent, Schaan, Liechtenstein). The crowns were glazed and secured
to epoxy resin dies with a droplet of temporary cement (RelyX Temp NE; 3M-ESPE) on
the incisal edge.[23]
Table 1
Details of the CAD/CAM materials evaluated in the study
|
Ceramic material
|
Composition
|
Manufacturer
|
|
e.max CAD
|
∼40 vol.% of ∼0.5 μm grain-size, lithium-metasilicate crystalline phase in a lithium
disilicate glass
|
Ivoclar Vivadent, Schaan, Liechtenstein
|
|
e.max zirCAD
|
Yttrium oxide- (yttria-) stabilized zirconium oxide (Y-TZP).
|
Ivoclar Vivadent, Schaan, Liechtenstein
|
|
Celtra Duo
|
Lithium silicate glass with 10% dissolved zirconia. It also contain diphosphorus pentoxide
to nucleate lithium metasilicate crystallization
|
Dentsply Sirona, Bensheim, Germany
|
|
Vita Suprinity
|
Lithium silicate glass with 10% dissolved zirconia. It also contain diphosphorus pentoxide
to nucleate lithium metasilicate crystallization
|
Vita Zahnfabrik, Bad Säckingen, Germany
|
|
Cercon xt
|
Yttrium oxide- (yttria-) stabilized zirconium oxide (Y-TZP).
|
Dentsply Sirona, Bensheim, Germany
|
Abbreviation: CAD/CAM, computer-aided design/computer-aided manufacturing.
Fit Measurement
Marginal accuracy of the restorations was evaluated by scanning electronic microscope
(SEM) (JSM-6610LV, JEOL, United States) with a magnification of 300 × . Fit evaluation
was made by measuring the vertical gap from the external crown margin to the opposite
preparation line ([Fig. 1]). Specimens were gold-coated by a Q15RS metallizer (Quorum Technologies, Sussex,
United Kingdom) before SEM examination. The measurements were taken by fixing the
specimens in a custom-made jig ([Fig. 2]) placed perpendicular to the optical axis of the microscope. The marginal fit was
measured for each crown at the predetermined four points.
Fig. 2 A custom-made jig to hold the crowns during marginal gap measurement.
Statistical Analysis
Data was analyzed using SPSS software 23.0 (SPSS, Chicago, Illinois, United States).
Shapiro–Wilk test confirmed the normal distribution of data. Descriptive statistics
(mean and standard deviation) for the marginal gap and maximum gap values were performed.
One-way analysis of variance was used to test differences between groups and Tukey
test was used for multiple comparisons between group combinations. A level of significance
at 95% was set for all statistics.
Results
Means, standard deviations, and the mean maximum gap values of different materials
are presented in [Table 2]. There was a statistically significant difference between groups (p < 0.5) in both mean marginal gaps and mean maximum gaps. The mean marginal gap of
the e.max crowns (49.2 µm) was statistically significantly greater than those of all
other groups (p < 0.05). However, when the e.max CAD group was excluded, the differences between
all other combinations were insignificant. Similar statistical results were obtained
for the mean maximum gap comparisons where the highest value was for the e.max CAD
(87.6 µm), which was statistically significantly greater than all other groups while
all other combinations showed no significant differences.
Table 2
Mean marginal gap, standard deviation, and mean maximum gap of studied crowns
|
e.max CAD
|
e.max ZirCAD
|
Celtra Duo
|
Vita Suprinity
|
Cercon xt
|
|
Mean marginal gap
|
49.3a
|
17.7b
|
13.2b
|
15.3b
|
10.2b
|
|
Standard deviation
|
33.4
|
12.6
|
17.0
|
14.0
|
9.0
|
|
Mean maximum gap
|
87.6a
|
32.9b
|
29.0b
|
32.1b
|
21.7b
|
Note: Different superscript in the same row indicates significant difference.
Discussion
One spot with a large marginal discrepancy can determines the clinical risk of restoration.[24] Therefore, in addition to the mean marginal gap, the mean maximum gap values of
the five tested material were reported and compared. There was statistically significant
difference between the tested materials; hence, we rejected the null hypothesis.
Results showed that e.max ZirCAD and Cercon xt have a greater marginal precision compared
to LD (e.max CAD) and were well below the maximum cement space of clinical acceptability
identified by ADA.[9] Marginal gap values ranged between 0 and 75 μm were recorded for zirconia crowns[25]
[26] which is well within the acceptable range of 120 μm suggested by McLean and von
Fraunhofer.[27] This might be linked to the precision of the CAD/CAM system in milling zirconia
restorations, possibly because dental CAD/CAM systems were originally developed to
process polycrystalline materials.[28] A recent systematic review found that the performance of a specific CAD/CAM system
in terms of marginal adaptation is influenced by the type of restorative material.[11] Similarly, the two commercial examples of the ZLS (Vita Suprinity and Celtra Duo)
presented superior results compared to e.max CAD. A previous study[29] agrees with our results as Vita Suprinity had significantly lower marginal discrepancy
value (77 μm) than that of e.max CAD (130 μm). Though the mean marginal discrepancy
values reported[29] were noticeably higher than those of the current study (Vita Suprinity, 15.5 and
e.max CAD 49.2). On the contrary, Hasanzade et al[30] reported no significant difference between the two materials. Contradictions in
gaps reported be different studies are expected and attributed to variances in study
designs and protocols followed.
LD crowns showed the highest gap measured among the tested systems, which might influence
its clinical survival compared to other systems.[4] A wide range of mean marginal discrepancy values of e.max CAD crowns were reported
in previous studies with some of them being, according to McLean and von Fraunhofer,[27] clinically unacceptable: 87,[31] 147.56,[32] 63.73, 88.64,[33] 125.46 to 135.59,[34] and 132.2 μm.[35] However, as mentioned earlier, these variations across studies are expected. The
higher marginal discrepancy value of the e.max CAD crowns can be linked to ceramic
shrinkage at the margin during crystallization firing.[36] Additionally, Fraga et al[37] found that surface roughness and defects after milling LD were more than those observed
in zirconia.
Theoretically, the precision of the designing and milling produced by contemporary
CAD/CAM technology should produce a restoration with a marginal accuracy of zero discrepancy
all around the margin, but this is known to be practically impossible. Though our
SEM images showed a closed margin at several measurement points ([Fig. 3]), which were more frequent in the zirconia systems. Boitelle et al[12] in a systematic review suggested that the available CAD/CAM technology delivers
dental restorations with marginal discrepancy values of less than 80 µm, which is
confirmed by the current study. In fact, this study found that the average maximum
gap of all material except the e.max CAD were within the maximum range of the cement
thickness identified by the ADA.[9]
Fig. 3 Perfectly closed margins. (A) Celtra Duo, (B) Vita Suprinity, (C) e.max ZirCAD, and (D) Cercon xt.
All crown systems in this study showed a clinically acceptable mean marginal gap and
mean maximum gap of less than 120 μm.[27] The clinically acceptable marginal gap of fixed restorations has been a controversial
subject in the literature.[9]
[27]
[38]
[39] A value of 120 μm which was established in 1971[27] is the most commonly cited value for clinical acceptability. Though such value should
be revised as a marginal opening of 30 µm has been reported to encourage secondary
caries formation.[4]
In the current study, marginal fit was evaluated by measuring the vertical gap at
the margin which might be considered a limitation because the absolute marginal discrepancy
is the measurement that represents the total crown misfit at specific point, both
vertically and horizontally.[40] However, the vertical and horizontal measurements have different clinical implications.[30]
Conclusion
Within the limitations of this study, it can be concluded that the marginal accuracy
of LD crowns is clinically acceptable. Zirconia and ZLS materials can produce a greater
level of marginal accuracy compared to LD.
