Authors
Conservative Department, Specialized dental center, Babylon health directory, Babylon, Iraq.
Abstract
Keywords
All-ceramic systems can be used as alternatives to metal ceramic systems in terms of esthetic restorative materials for crowns and fixed partial dentures (FPDs). Increasing expectations of high esthetics, biocompatibility and strength have led to numerous improvements in ceramic restoration. Recently developed yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and transformation toughening have resulted in a breakthrough in the field of esthetic full-veneer restorations (Zirkonzahn; Zirkonzahn GmbH, Bruneck, Italy). As a result, high-strength zirconia is now being used in the production of fixed partial prostheses, even in load-bearing areas (1-3). On the other hand strong and tough ceramic materials that can be used alone without underlying metal frame are available, such as lithium disilicate-reinforced glass ceramic (IPS e.max CAD; Ivoclar Vivadent, Schaan, Liechtenstein) (3).
IPS e.max CAD (Ivoclar Vivadent) was introduced in 2005 as an improved ceramic material designed for CAD/CAM processing. It consists of a lithium disilicate- reinforced glass ceramic, but its physical properties and translucency are improved (4).
Prettau zirconia is extra translucent zirconia that when used in conjuction with the specific Prettau colouring technique it give 100% aesthetic full-zirconia restorations. It is particularly suitable in cases of heavy occlusion (bruxism). The Zirkonzahn system is a direct ceramic machining system that uses a scanning and machining process. The milled specimen is 20% enlarged to compensate for shrinkage after the sintering process at 1600°C according to manufacture instructions.
Zirconia or alumina-based materials have many advantages such as biocompatibility, low bacterial adhesion, traditional cementation and perfect mechanical properties. However, high durability of dental restoration is not only the result of mechanical properties. Marginal gaps can cause carious teeth, cement resolution, defected margins, subgingival microflora changes, periodontal destruction and periapical lesions resulting in pulpal effect. For these reasons, marginal fit plays a significant role in the longevity of restorations (5,6).
The introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) technology in dentistry enabled dentists to use new treatment modalities and changed the design and application limits of all-ceramic restoration as the demand for esthetics in the posterior region of the mouth has increased (7). CAD/CAM machining for construction of dental restorations are gaining popularity and are clinically proven (8).
Types of finish lines and ceramic manufacturing technique are the factors that have been investigated for all-ceramic crowns. Heavy chamfers and rounded shoulder finish lines have been advocated for all-ceramic crowns (9-11).
The aim of this study was to evaluate effect of two gingival finishing lines (90° shoulder and deep chamfer) on marginal fit of two types of full anatomic all-ceramic crowns, zirconia crowns (Zikonzhan) and glass ceramic crowns IPS e-max CAD milled with CAD/CAM system.
Materials and Methods
Two dentoform teeth of maxillary left first molar were fixed on the dentoform maxillary dental arch. In order to achieve standardized tooth preparation, a modified surveyor with a suspending arm was used to control the handpiece orientation during tooth preparation in which the movable table of the surveyor was adapted to secure the dentoform maxillary dental arch in such a way, so that the long axis of the ivorine tooth was parallel to the bur to ensure the same angle of convergence for all preparations (Fig. 1).
For deep chamfer finishing line preparation, a diamond bur No. (S 6856) (Komet, Germany) was used for axial reduction of (0.8mm) and a bur No. (8856) (Komet, Germany) for smoothening and finishing the gingival finishing line. For shoulder finishing line preparation, a diamond bur No. (8847KR) (Komet, Germany) was used for axial reduction of (1.2mm) and a bur No.(8372P) (Komet, Germany) for smoothening and finishing the gingival finishing line. The limit of the finishing line was (0.5) mm coronal to the cemento-enamel junction following the cervical anatomy of the tooth. Preparation of occlusal surface was standardized with a height of 4 mm using a disk bur on a straight handpiece adapted to the surveyor (Fig. 2).
The prepared dentoform teeth were used as patterns for the construction of master metal dies. The dentoform teeth were sprued, invested, burned out and casted using type III hard non- precious dental casting alloy. After casting, each metal die was smoothed with a rubber wheel and polished with pumice in a lathe brush followed by rouge to gain smooth polished surface, so that no interference with seating of the all crowns could occur later (Fig. 3). For each preparation type, 16 impressions were taken with a polyvinylsiloxane impression material (3M ESPE, USA), and poured using type IV dental stone (Zhermack, Italy) to get 16 stone die models.
The whole number of 32 teeth with two different gingival finishing lines (sixteen with 90º shoulder finishing line and sixteen with deep chamfer finishing line) were utilized to construct sixteen zirconium full anatomy (Zirkon zahan) and sixteen glass ceramic full anatomy (IPS e-max CAD) CAD/CAM crowns using Zirkonzahan M5-speed CAD/CAM machine.
Measurement of marginal gap
Four indentations on the margin of the die at the midpoint of different surfaces (mesial, distal, buccal and palatal) were selected to measure the marginal gaps along vertical planes by using a stereo-microscope (12-15).
A screw loaded holding device following (16-18) was used during measurements in order to maintain a seating pressure of (13.4N) between the all-ceramic crown and the master metal die during measurements calculation for this purpose.
The marginal gap of the crown was determined by measuring the vertical marginal gap between the margin of the die and the margin of the crown, the measurement were made on four different points on each surface (two on each side of the indentation that means left and right), 1st point will be at the edge of indentation while the 2nd one at (1mm) from the previous one (19). This was achieved by using a stereo microscope provided with a digital camera in the eye lens and connected with the computer (Fig. 4).
The microscope was adjusted its calibration to 0.001mm (1um) at magnification 160X and the measurements were done by placing the sample on the microscope stage, which was adjusted until the image of the marginal area was displayed clearly on the computer monitor, and the digital image of the gaps were then captured. The image was treated with program (Image J), which was used to measure the vertical marginal gap between the crown margin and metal master die finishing line. The program (Image J) was used to measure the value in pixels mark by drawing a line between the finishing line on the die and the crown margin line. All digital readings were recorded and converted to (um) by a magnification factor (The length of a microscopic image divided by the object length) (15).
Sixteen measurements were continued for all the four surfaces (mesial, palatal, distal, and buccal) of each sample. All measurements were achieved by the same investigator (19,20), for accuracy purposes and to get a precise measures. Repeated measures were made to decrease the possibility of errors (13). The marginal discrepancy value of each crown was the arithmetic mean of these 16 measurements on the four surfaces.
The collected data were analyzed statistically using ANOVA (analysis of variance) test and student t-test. ANOVA test was performed to find if there is any statistical difference among the four groups. Since there are two variables included in this study (type of finishing line, and crown material type) and for each variable not more than two groups of comparison are found, Student t-test was selected to test the difference between these two corresponding groups.
Figure 1. Crown preparation on dental surveyor.
Figure 2. Diamond disk bur and straight handpiece adapted to the modified surveyor for occlusal reduction.
Figure 3. Finished master metal die. (A) Shoulder finishing line. (B) Chamfer finishing line.
Figure 4. Digital image was captured during the measurement at four positions, (A) Crown. (B) Marginal gap. (C) Metal die.
Results
The lowest mean of vertical marginal gap was scored in Group I (zirconia chamfer group) (73.55µm), while the highest mean was recorded by Group IV (Ceramic Shoulder group) (162.34µm) (Table 1) and (Fig. 5).
ANOVA test results showed that there was statistically significant difference among groups (P<0.05) (Table 2).
Shoulder finishing line produced significantly greater marginal gap with zirconia crowns (group I) compared to chamfer finishing line (group II) (Table 3) and (Fig. 6A and 6B). Regarding ceramic groups (III and IV), shoulder finishing line produced greater marginal gap with ceramic crowns in comparison to chamfer finishing lines but statistically the difference was not significant (Table 4) and (Fig. 6C and 6D).
Regarding chamfer finishing line, ceramic crowns produced significantly greater marginal gap (group III) in comparison to zirconia crowns (group I) (Table 5) and (Fig 6A and 6C). Similarly, with shoulder finishing line, t-test results revealed that ceramic crowns (group IV) produced significantly greater marginal gap compared to zirconia crowns (group II) (Table 6) and (Fig 6B and 6D).
Table 1. Descriptive statistics (mean, standard deviation, minimum and maximum) of vertical marginal gap for the different groups measured in micrometers.
|
Groups |
N |
Mean |
SD |
Min |
Max |
|
Group I (Zirconium Chamfer) |
8 |
73.55 |
15.56 |
56.31 |
103.5 |
|
Group II (Zirconium Shoulder) |
8 |
92.60 |
12.965 |
69.69 |
108.5 |
|
Group III (Ceramic Chamfer) |
8 |
151.45 |
33.7625 |
100.10 |
201.6 |
|
Group IV (Ceramic Shoulder) |
8 |
162.34 |
31.06375 |
108.30 |
201.3 |
Table 2. One way-ANOVA test among the four groups.
|
|
Sums of Squares |
Degree of Freedom |
Means of Squares |
F |
P value Sig |
|
Between groups |
45520 |
3 |
15170 |
-- |
-- |
|
Within groups |
19750 |
28 |
705.4 |
-- |
-- |
|
Total |
65270 |
31 |
-- |
21.51 |
0.0233 (S) |
S: Statistically significant difference.
Table 3. Student t-test between Zirconium groups with different finishing lines (Group I and Group II).
|
Mean Difference |
Degree of Freedom |
T |
P Value |
Summary of Significant |
95% CI of differences |
|
|
Lower |
Upper |
|||||
|
19.06 ± 7.160 |
14 |
2.030 |
0.0186 (S) |
P < 0.05 |
3.700 |
34.41 |
S: Statistically significant difference.
Table 4. Student t-test between ceramic groups with different finishing lines (Group III and Group IV).
|
Mean Difference |
Degree of Freedom |
T |
P Value |
Summary of Significant |
95% CI of differences |
|
|
Lower |
Upper |
|||||
|
10.89 ± 17.36 |
14 |
0.627 |
0.5407 (NS) |
No significant |
-26.35 |
48.13 |
NS: Statistically no significant difference.
Table 5. Student t-test between chamfer groups with different crown material (Group I and Group III).
|
Mean Difference |
Degree of Freedom |
T |
P Value |
Summary of Significant |
95% CI of differences |
|
|
Lower |
Upper |
|||||
|
-77.90 ± 13.03 |
14 |
5.981 |
0.00008 (VHS) |
P < 0.001 |
-105.8 |
-49.96 |
VHS: Statistically very highly significant difference.
Table 6. Student t-test between shoulder groups with different crown material (Group II and Group IV).
|
Mean Difference |
Degree of Freedom |
T |
P Value |
Summary of Significant |
95% CI of differences |
|
|
Lower |
Upper |
|||||
|
-69.73 ± 13.53 |
14 |
5.154 |
0.0001 (VHS) |
P < 0.001 |
-98.75 |
-40.71 |
VHS: Statistically very highly significant difference.
Figure 5. Bar-chart showing the mean values of the vertical marginal gap of the four groups.
|
B |
|
A |
Figure 6. Marginal gap differences among the different groups.
(A) zirconium chamfer (B) zirconium shoulder (C) ceramic chamfer (D) ceramic shoulder.
Discussion
For full ceramic crowns, a marginal gap between 1 and 165 μm is considered acceptable (21,22). However, some authors suggested that 100- 150 μm is acceptable for various restorations (23,24). On the other hand, 120 μm as the maximum acceptable marginal gap value based on the results of a five-year study consisting of 1,000 restorations was recommended by Mclean and Fraunhofer (25). In this study, mean marginal gap values for groups were below 120 μm, except for the ceramic groups, which also had an acceptable borderline value (120.1±43.1 μm).
Many previous studies reported the mean marginal gap for full ceramic crowns created by CAD/CAM between 23-74 μm (5,26,27). Our study, Zirkonzahan zirconia crown on chamfer finishing line created by CAD/CAM (group I) had similar mean marginal gap value, but the other groups had a higher values. The difference in sintering procedures of zirconia crown and ceramic crown created by CAD/CAM could be the reason for difference in the mean marginal gap values among the four groups. Therefore, this may decrease the precision of marginal adaptation and explain the results of the present study.
Other studies on Zirkonzahan (2,3) found higher marginal gap values on both chamfer and shoulder finishing lines compared to this study. In addition, they found smaller marginal gap values for ceramic group on both chamfer and shoulder finishing lines compared to our study. These differences between the present and previous studies may be attributed to the different measurement techniques and reference locations used, the subjective nature of the measurements, and the use of different blank materials.
Effect of finishing line type
In the current study, the marginal gap of zirconia crowns on chamfer finishing line was lower than on shoulder finishing line and this may be due to (a) the difference in the depth of the preparation (0.8mm for chamfer and 1.2 mm for shoulder); (b) the design of finishing line, which was more round angle between the axial and gingival seat for chamfer finishing line that enable more accurate seat for the crown than with shoulder finishing line (90°) with slight round angle lead to incomplete seat of the crown and increase the vertical marginal gap; and (c) it may be due to the accuracy of scanner detection that is being influenced by differences in depth preparation (19,28,29). Other possible causes may be the limited precision of the milling process with increased variation in preparation depth and this was parallel with other study30 which observed that increasing difference in preparation depth would lead to increased marginal gap.
Our finding that related to the superiority of chamfer finishing line is consistent with the results obtained by Karatasli et al (2) and Pera et al (31). However, it disagrees with other studies (18,28,32) which preferred the shoulder finishing line as they suggested that all-ceramic crowns should be supported by shoulder preparation to resist extensive loading whenever it may be expected, moreover our findings disagrees with other previous studies (1,3,10,33) which reported that the type of finish line design did not influence the marginal adaptation of all-ceramic crowns.
For ceramic groups, similar to the zirconia groups, it was observed that the vertical marginal gap of ceramic crown on chamfer finishing line was lower than on shoulder finishing line. The same causes discussed above for zirconia groups could be attributed to this result.
Effect of crowns material type
In current study the zirconia (Zirkonzahan) groups showed lower marginal gap than ceramic (IPS e-max CAD) groups on both chamfer and shoulder finishing lines, this may be due to difference in microstructures of the two materials and different fabrication techniques, so the marginal gap may be affected by the preparation design, milling process, size and type of milling burs which was carbide burs for zirconia and diamond burs for ceramic blocks and material conditions during the milling procedure in CAD/CAM system in which the zirconia blanks milled in dry field and ceramic blocks in wet field this was in agreement with Park and Lee (29) who stated that in the case of CAD/CAM, the scanning, software design, milling process would influence the marginal adaptation.
The above findings were in agreement with previous studies (18,34), which stated that zirconia crown fabricated with CAD/CAM system show better marginal adaptation than other tested groups. Other studies2,35 in which they compared the marginal gap of crowns fabricated with CAD/CAM system by using different materials found that the best results were obtained with ceramic group using CAD/CAM system than with zirconia and this disagrees with present study and this may be attributed to the differences in fabrication process for the tested materials.
In the present study, zirconia (Zirkonzahan) crowns sintering might not affect on the marginal gap because of the high strength of zirconia (33). However, this finding is not in agreement with Balkaya et al. (36). Who found in their study that after firing there was an increase in marginal gap.
For ceramic crowns, the marginal gap would be increased with firing and this was in agreement with two previous studies (33,36), which explained that the ceramic was not strong as zirconia (400Mpa compared with 900Mpa), so the firing would affect the marginal gap. The glaze of ceramic crown would not affect the marginal gap of ceramic (36,37) which reported that the glaze had no significant effect on the marginal gap.
Conclusion
The findings of this study showed that for zirconia crowns, chamfer finish line significantly reduced the marginal gaps compared to shoulder finish line, also, for glass ceramic crowns, chamfer preparation produced better marginal fit, and no statistical significant differences were found between both finish line designs. Regardless the type of finish line, zirconia crowns produced significantly better marginal fitness than glass ceramic crowns.
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