EVALUATION OF MARGINAL FIT OF TWO TYPES OF GLASS CERAMICS

(IN VITRO STUDY)

Youssef Y. Ashour1, Samir I. Bakry2, Sanaa H. Abd elkadr2, Fayzal Elabbasy3.

1 Assistant Lecturer of Fixed Prosthodontics, Faculty of Dentistry, Pharos University.

2 Professor of Fixed Prosthodontics, Faculty of Dentistry, Alexandria University, Egypt.

3 Professor of Biomaterial Department, Faculty of Dentistry, Alexandria University, Egypt.

Email: Usfashour@gmail.com

Telephone: +201122956514

ABSTRACT

BACKGROUND: Veneered all-ceramic restorations are associated with a high incidence of

chipping and veneer delamination from the inner core. Monolithic all-ceramic crowns facilitate

the fabrication process and minimize residual stresses between core and veneer. A new

material, zirconia-reinforced lithium silicate (ZRL), Celtra Duo was recently introduced for

fabrication of monolithic anterior crowns to overcome the aesthetic drawbacks of traditional

zirconia and also to improve the strength of the lithium disilicate.

 Aim of the study: To evaluate the marginal fit of CAD/CAM: zirconia reinforced lithium di

silicat and to compare it with Lithium silicate glass-ceramic crowns.

Materials and Methods: Thirty monolithic ceramic specimens will be fabricated and divided

into THREE main groups; Group I: CAD/CAM ZLS Celtra Duo milled and polished, Group II

CAD /CAM ZLS Celtra Duo milled and glazed & Group III: CAD/CAM Lithium silicate

glass-ceramic (e.max CAD). For evaluation of the marginal fit 30 ceramic crown specimens

ten specimens from each material(N=10), subgroups Ia, IIa, IIIa will be fabricated according to

the manufacturers’ instructions and thermocycled to simulate one year clinical service.

Marginal fit will be measured for the same specimens by using CBCT (Ia, IIa, IIIa). For

evolution of marginal fit 30 ceramic crowns will be fabricated, ten crowns from each material

(N=10), subgroups Ia, IIa, IIIa.

Results: Will be tabulated and statistically analyzed.

INTRODUCTION

CAD/CAM systems are composed of three major parts: (1) a data acquisition unit,

which collects the data from the region of the prepared teeth and neighboring structures

and then converts them to virtual impressions (an optical impression is created at this

moment directly or indirectly); (2) software for designing virtual restorations anchored in

virtual impressions and setting up all the milling parameters; and (3) a computerized

milling device for manufacturing the restoration with solid blocks of the chosen restorative

material. The first two parts of the system play roles in the CAD phase, while the third is

responsible for the CAM phase (1).

The restoration parameters during the design also influence the marginal adaptation

of CAD/CAM all ceramic restorations; for example, the virtual configuration of the die

spacer between the tooth and the restorations is essential for the accuracy of the marginal

adaptation (2).

Marginal and internal fit can be influenced by several factors, starting from the

impression phase to the final cementation process. In general dental practice, impressions

using elastomer materials (polyether and vinyl polysiloxane) are a conventional procedure.

These can be made with monophase or multiphase consistencies as well as one- or two-

step technique (3, 4).

Although high quality impressions are achievable by these impression techniques

and workflows, several mistakes associated with the intraoral phase (subgingival

preparations, presence of blood, saliva, etc) or laboratory procedures (disinfection, pouring

the impression, transport, etc.) may occur, leading to inaccuracies (5, 6).

Several in vivo and in vitro quantitative evaluation fit methods of prostheses

developed to assess different conventional processes have been used to study the

CAD/CAM prostheses. Marginal fit was evaluated when prostheses were inserted in the

master cast using microphotography and light microscopy. Measurement with the silicone

replica of the misfit between the restoration and abutment. This replica is sectioned and

evaluated under light or electronic microscopy (7).

Measurement of the tooth–prosthetic interface after cementing or bonding dental

prostheses. The spacer is evaluated with light or electronic microscopy after sectioning.

Recently, the literature has reported other evaluation methods and processes for

developing CAD/CAM prosthesis. The silicon weight and density evaluation method.

Measurement by a triple scan protocol with a noncontact scanner and specific software to

perform a virtual 3D analysis. Internal and marginal adaptation measured by micro-CT

technology and without impression of cementation space (8).

In these quantitative assessments, two major methodological limitations are

emphasized by many authors. The first limitation is the number of measurement points.

Increasing the number of points on the entire periphery or volume of the joint tooth–

prosthesis would give an average assessment of pertinent adaptation. This is a real limitation

of these measurement protocols, since in the studies included, the number of measurement

points varied between 4 and 385 for conventional methods and up to more than 3500 points

for three-dimensional method. The second methodological limitation is related to the

geometric tracking system defining the limits of the marginal gap measured (9, 10).

Christenson (1971) agreed with the ADA specifications; others suggested modifying

it. Fransson et al., (1985) argued that the clinically acceptable marginal gap should be less

than 150 μm and 120 μm respectively. Additionally, Mclean and von Fraunhofer examined

the marginal fit of 1000 fixed restorations in-vivo using replica technique over a five years

period and indicated that a marginal gap less than 80 μm is difficult to detect under clinical

conditions (11, 12).

As technology evolved further, a new method of 3D fit assessment was described

and used in a study by Schaefer et al.,(2014) the prepared tooth and the fabricated

restorations were scanned using a self-calibrating structure-light scanner (Flex 3A, Otto

Vision Technology GmbH, Germany). Virtual crowns were superimposed upon the virtual

prepared tooth by computing all possible orientations using special software (Qualify 12,

Geomagic GmbH, Stuttgart, Germany). Colour-coded difference images allowed for semi-

qualitative information analyses of marginal and internal fit of the virtual crowns on the

virtual prepared surface and selecting the best fit position. The one inherent problem of all

non-contact optical digitizers is the accuracy and capability to actually capture the different

surfaces/materials such as highly reflective or translucent materials as all-ceramic crowns

(13, 14).

MATERIALS AND METHODS

Measurements of marginal fit with cone beam computed tomography (CBCT)

Test was done for group IA, IIA, IIIA.A highly accurate desktop computerized cone

beam tomography device was used for imaging the specimens. The master die was fixed to

its acrylic base in order to stabilize the die on the chin plate during imaging in the CBCT

device using orthogonal technique so that the x-ray beam was directed at right angle to the

long axis of the die.

Each crown was seated on the ceramic master die and stabilized during CBCT

imaging using a specially designed metal free device then placed on the plate of the CBCT

device to be scanned. During image acquisition, each specimen was automatically rotated

one step (0.9°) at a time through 180°. (Figure 1)

Figure 1: Metal-free holding device stabilizing the crown on the die during scanning.

A special software system was used for viewing the cross sectional images of the

specimens and making the necessary measurements, and the data were saved in DBM files

and sent to statistical analyses (J.Morita Veraviewepocs 3D R100, Irvine California 92618

USA. Telephone: 1-949-581-9600. Fax: 1-949-581-8811). (Figure 2,3)

Figure 2: Cross section of the each crown specimens

Figure 3: Cone beam CT

Absolute marginal discrepancy (AMD): The angular distance between the margin of

the crown and the finish line. The percentage of horizontally over-extended, properly

extended and under-extended margins were calculated for each group (15). The method

used is illustrated in (Figure 4).

Figure 4: Diagram for the marginal fit

The acrylic base made for the master die as shown in figure (1) to stabilize the

master die during imaging with the cone beam computed tomography the master die was

placed on the resting chin of the CBCT during imaging as shown in the figure (1).

RESULTS

Measurements of marginal fit with computed cone beam tomography results

Results of the marginal fit shown in table (1), figure (5) was found to be the highest

mean value with group CAD/CAM Celtra Duo (mill and glazed). 208.50±17.49 followed

by CAD/CAM Celtra Duo (mill and polished) 192.25±19.02 and finally the least marginal

fit with group Lithium Disilicate EMAX CAD/CAM 187.25±11.81.

The statistical analysis revealed a significant difference between the test groups

where (p≤0.05). Upon further analysis the significant difference was found to be between

for CAD/CAM Celtra Duo (mill and glazed) and the Lithium Disilicate EMAX. There was

no significant difference between the other 2 pairs. The marginal gap values are presented

in (Table 1) and (Figure 5).

Table 1: Measurements of marginal fit with computed cone beam tomography in µm

Measurements of marginal fit with computed cone beam tomography

Group

CAD/CAM Celtra Duo

(mill and polished)

CAD/CAM Celtra Duo

(mill and glazed)

Lithium Disilicate EMAX

AMD- n- Min-Max- Mean±S.D- 95% CI of the mean- Median- IQR- KS test of normality

10160-212.5

192.25±19.02178.64-205.85

196.25a,b,c

182.50-210.00D=0.157, p=0.200

NS

10182.50-237.50208.50±17.49195.98-221.01

205.00a,b

195.00-222.50D=0.179,

p=0.200 NS

10175.00-215.00187.25±11.81178.80-195.69

186.25a,c

177.50-192.50D=0.192, p=0.200

NS

Test of significancep value

X2(KW)(df=2)=8.092p=0.017*

Post-hoc Multiple Comparison

CAD/CAM Celtra Duo(mill and polished)

Z=1.627p=0.311 NS

Z=1.207p=0.682 NS

CAD/CAM Celtra Duo(mill and glazed)

Z=2.834p=0.014*

Lithium Disilicate EMAX

n: NumberMin-Max: Minimum – maximumS.D.: Standard deviationCI: Confidence intervalIQR: Inter-quartile range (25th – 75th percentile)KW: Kruskal Wallis testdf: degree of freedom

Different superscript letters indicate Pair-wise statistical significance (using Dunn-Sidak method)* : statistically significant difference (p<0.05)NS: statistically no significant difference (p>0.05)

Figure 5: Measurements of marginal fit with computed cone beam tomography

DISCUSSION

Neves et al.,(16) and Roulet et al.,(17) stated that the main goal of a fixed restoration

is the close adaptation of the crown to the prepared tooth. A high marginal accuracy and an

adequate internal fit are the major determining factors for successful clinical performance.

There is still no standard protocol to evaluate the fit of indirect restorations. This lack of

standardization may compromise interpretation and comparison of data from different

studies and to provide guidelines for clinical practice. Thus, it is important to recognize the

limitations of the currently used techniques, as well as the kind of data that may obtained

with each method.

Kohorst et al. (18) and Preis et al. (19), showed that different method have been

described to evaluate the marginal gap of the crown microscopic methods, dye penetration

test, micro computed tomography also scanning electron microscope the best one is the

cone beam computed tomography this is what we used in the present study.amd showed

reliable results according to several studies.

 The aim of this vitro study was to compare marginal fit of and zirconia reinforced

lithium silicate to lithium disilicate crowns fabricated powder-free scanning with intraoral

video scanner (CEREC Omnicam), using cone beam computerized tomography. The

methodology followed in this study was first applied by Pelekanos et al. (20), using

computerized tomography to measure marginal gap as a radiographic method. According

to Pelekanos et al. (20), to discriminate the gap between two materials using radiograph

both should have the same coefficient of radiation absorption or similar radiodensity, that's

why the master die and crowns in our study were fabricated from lithium disilicate ceramic

and assessed good results. 

Lithium disilicate was selected in this study instead of other ceramic materials

because glass ceramic has the closest refractive index and optical properties to enamel and

dentin (1.55 for glass ceramic, 1.54 for dentin and 1.6 for enamel) which facilitated

scanning of the ceramic die during the study. Lithium disilicate has intermediate refractive

index between leucite based glass and zirconium due to its microstructure of random

interlocking plates shape of crystals rather than needle shape in leucite. Refractive index

explains the translucency and the reflectivity of light by the material which in turn affect

the scanning of this surface accurately and its x-ray scattering effect which will cause

radiographic distortion and image artifacts (21, 22).

All specimens in the present study were designed using software (CEREC 3D, V4.2

Sirona, Germany) applying biogeneric individual design and milled with one milling

machine (CEREC MC XL, Sirona, Germany) using size 12S step bur and 12S cylinder

pointer burs. A new set of burs was used for each group milling to eliminate the effect of

milling machine on the accuracy of the crowns.

The mean value of the present study regarding marginal gap was found to be the

lowest marginal gap was Lithium Disilicate EMAX while the highest for CAD/CAM

Celtra Duo (mill and glazed). However, Azarbal et al. (23), and Zimmermann et al. (24),

showed that the overall mean difference in marginal gap between the zirconia based

ceramic, hybrid ceramic and crystallized lithium-disilicate copings was statistically

significant (p<0.01). The overall mean difference in marginal gap before and after firing

(pre-crystallized and crystallized lithium-disilicate copings) showed an average of 62

microns increase in marginal gap after firing.

Crystallization firing has a significant effect on the marginal gap of lithium disilicate

(IPS e-max CAD- Ivoclar Vivadent) CAD/CAM crowns fabricated with CEREC (Sirona)

system (23).

Brenes et al. (25) and Holmes et al. (26, 27), claimed that absolute marginal

discrepancy as the angular combination of the marginal gap and extension error (over-

extension or underextension); it is the combination of the vertical marginal discrepancy

and horizontal marginal discrepancy. This agreed with the present study assessed that the

absolute marginal discrepancy is considered the best alternative measurement to the

marginal gap since its always the largest error at the margin and reflects the total crown

misfit at that point.

Taha et al. (28), claimed that the difference between marginal gaps values of material

significant increase after cementation significant difference was found between hybrid

ceramic crowns higher than the Celtra Duo due to different thermocylcing techniques and

cementaion procerdure used. While the present study showed that the marginal gap is least

for emax CAD than the celtra duo without cementation.

Preis et al. (19), showed that the as monolithic crown the lithium disilicate is the

proven better than the zirconia reinforced lithium silicate crowns by using mix of thermo

cycling and mechanical loading and the interface is investigated by scanning electron

microscope. Also, it agreed with the present study that showed least marginal gap for the

lithium disilicate crowns rather than the zircona reinforced lithium silicate.

Tinschert et al. (29), stated that increased the thickness of the margins to avoid the

irregularity induced by milling a brittle material like zirconia based ceramics by using

special parameters, and they suggested manual adjustment under light microscopy.

However, the present assessed that special parameters used for the Celtra Duo and the

emax lithium disilicate crown sufficient regular margin thickness. Therefore Marginal

accuracy is important to reduce plaque accumulation and secondary cavities. Nawafleh

et al. (30), suggested the goal of 25-40 μm for marginal fit while nowadays 75-160 μm are

considered clinically successful.

Wittneben et al. (31), demonstrated that the difference of fit between CAD/CAM

restorations is directly related to the gap parameters from the computer design and also

related to the intrinsic properties of the CAD/CAM system; the die spacer should be

uniform and facilitate agreed with the present study by using the same parameters for the

material used Celtra Duo and emax CAD several studies have looked at the effect of die

spacer on the retention and physical properties of crowns.

e Silva et al.,(4), Seelbach et al.,(32) and Ueda et al.,(33) concerning the marginal fit of

CAD/CAM-generated restorations based on intraoral scanning, some in vitro studies

demonstrated better marginal precision than restorations produced with conventional

impressions. This is supported by Syrek et al. (34), Ahrberg et al.,(35) Boeddinghaus et al.,

(36) Pradíes et al.,(37) and Zarauz et al.,(38) the findings of several vivo studies. In contrast,

there are also in vitro studies, that show no significant differences regarding marginal

accuracy.

CONCLUSION

Lithium disilicate provided better marginal fit than polished and glazed Celtra Duo.

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