Since the late 1950s, the metal ceramic crown system has remained a standard modality for rehabilitation of anterior dentition, thanks to their good mechanical properties and to somewhat satisfactory esthetic results, along with a clinically acceptable quality of their marginal and internal adaptation [
In the last 30 years, the growing patients’ demand for highly esthetic and naturally appearing restorations has led to the development of new all-ceramic materials, whose mechanical characteristics have been dramatically improved to provide suitable longevity and limitation of the technical problems. Furthermore, a great advancement in dental ceramics has been achieved since the introduction of high-strength tetragonal yttria-partially stabilized zirconia (Y-TZP) which became the most interesting polycrystalline ceramic available for dentistry mainly due to the transformation-toughening mechanism [
Porcelain compatibility is a concern on veneered zirconia restorations since recent studies on all-ceramic restorations reported a significant amount of porcelain chipping (15–62%), cracking (25–50%), delamination (less than 10.7%), and large fractures (3–33%) [
So, it is obvious that restoration of the anterior teeth with crowns that have a framework for porcelain support is further complicated by the requirement, generally placed on the veneering porcelain, to simulate a lifelike tooth appearance. This is particularly challenging for anterior restorations where a high level of translucency, especially in the incisal and middle third of the tooth, is required, while the presence of the framework in these areas is thought to be necessary to provide mechanical resistance to fracture.
Therefore, this study was conducted to evaluate the effect of two different incisal veneering porcelain thicknesses on the fracture resistance of the anterior all-ceramic CAD/CAM zirconia crown system when comparing it with the conventionally used metal ceramic crown system.
The materials used in this study were as follows.
The description of the crown materials is shown in Table All-ceramic crown materials Metal ceramic crown materials
Crown materials used in this study.
System | System | Metal ceramic |
---|---|---|
(1) Coping material | Zirkonzahn |
Nickel-chromium alloy art alloy BF |
Fabrication technique | CAD/CAM | Lost wax technique |
Composition | Main component is zirconium dioxide (ZrO2) + yttrium oxide (Y2O3 5%) |
Ni 62% |
(2) Veneering material | Porcelain for zirconia Ceramco®PFZ |
Porcelain for nickel-chromium alloy Ceramco 3 |
Fabrication technique | Layering technique | Layering technique |
Composition | Feldspathic porcelain containing no leucite | Feldspathic porcelain containing 0.30 volume fraction leucite |
Universal self-curing resin-based system with the light-curing option (Multilink N system pack; Ivoclar Vivadent ACT, Benderstr, Liechtenstein) was used for cementation of both groups in this study.
Metal die (made of art alloy BF) Doublident (Doublident® W+D dental, P.O.B 508, D-25305 Elmshorn. Reference: WD 5080C; duplicating addition curing silicon) Epoxy resin material (RenCast® Epoxy casting Resin, Klybeckstrasse 200, CH. 4057 BASEL, Switzerland) CAD/CAM Zirkonzahn ceramic system Thermal cycling machine (made by Biomaterial Department, Faculty of Dentistry, Alexandria University) Custom made by the load cyclic machine (made by Biomaterial Department, Faculty of Dentistry, Alexandria University) Universal testing machine (Comten Industries Inc, St. Petersburg Florida, USA. Model no 942 D 10-20)
A specially designed stainless steel metal master die was milled to simulate the maxillary central incisor crown prepared to receive metal ceramic and all-ceramic full coverage crowns (Figure
Stainless steel master die.
Twenty negative replicas of the prepared master die were made by the duplicating addition silicon material and filled with the epoxy resin material having the same elastic modulus of dentin to get twenty positive replicas of the prepared maxillary central incisor and then the reproduced dies were smoothly polished (Figure
Epoxy resin replicas of master die.
The specimens were divided into two main groups (Table Each group was subdivided into two subgroups according to the incisal veneering porcelain thickness.
Grouping of the specimens.
Groups | Group I | Group II | ||
---|---|---|---|---|
Crown material | CAD/CAM zirconia all-ceramic (0.5 mm coping thickness) | Nickel-chromium metal ceramic (0.5 mm coping thickness) | ||
Number of specimens | 10 | 10 | ||
Subgroups | Ia | Ib | IIa | IIb |
Number of specimens | 5 | 5 | 5 | 5 |
Incisal veneering porcelain thickness | 1.5 mm | 3.0 mm | 1.5 mm | 3.0 mm |
Copings of group I crowns were milled out from Zirkonzahn zirconia blanks in the following steps: optical impression of the specimens, designing of the coping using modeling software “Zirkonzahn modeler,” milling of zirconia blanks, and firing of the milled zirconia copings.
Ten CAD/CAM zirconia copings were veneered with Ceramco®PFZ using the layering technique. To standardize the specimen’s size, two extra specimens were milled to the full anatomical contour of maxillary central incisors, and the only difference was in the incisal edge thickness. One of the specimen had 2 mm incisal edge thickness (0.5 mm core, 1.5 mm veneer), and the other had 3.5 mm incisal edge thickness (0.5 mm core, 3 mm veneer); the thickness of both was checked using the Iwanson gauge. Two impressions were made for the full contoured crowns from duplicating addition silicon with the aid of a metal ring to reproduce two silicon molds (mold #1 and mold #2) which were then split along the long axis to be used as guidance during building of the porcelain veneering, and then all the specimens were fired according to the manufacturer’s instructions (Figures
Making impression of full contoured CAD/CAM zirconia crown: (a) CAD/CAM zirconia coping inside the metal ring; (b) metal ring filled with the duplicating material; (c) split mold adapted on lined zirconia coping; (d) application of body porcelain and condensation.
Checking thickness of the finished CAD/CAM zirconia all-ceramic crown: (a) all-ceramic crown of 2 mm incisal thickness; (b) all-ceramic crown of 3.5 mm incisal thickness.
For standardization of the metal coping thickness and contour with that of CAD/CAM zirconia coping, duplicating addition silicon was used to make an impression for one of the zirconia copings; then, hard stone type IV was poured, and the stone die (Figure
Poured stone die.
Vacuum formed resin template.
Coping former.
Ten resin copings were sprued, invested using phosphate-bonded investment, and then placed inside the preheating furnace where burn out was done followed by casting of the nickel-chromium alloy according to the manufacturer’s instructions. After divesting the metal copings, the sprues were cut and the copings were sandblasted and then finished and ultrasonically cleaned to get rid of all investment traces. Finally, the metal copings were seated onto their corresponding epoxy resin dies and checked for thickness and proper fit.
Ten nickel-chromium copings were veneered with Ceramco 3 using the layering technique, and as means of standardization, the split silicon molds that were used for fabrication of zirconia all-ceramic crowns were used again for fabrication of metal ceramic crowns with the same technique. After checking the metal ceramic crowns thickness and fit, the crowns were finally finished and glazed according to the manufacturer’s instructions.
Self-curing luting resin with the light-curing option (Multilink N system pack) was used for luting all of the crowns according to manufacturer’s instructions, and the restoration was seated in place and fixed by finger pressure for 15 seconds and then under static load of 5 kg for 10 minutes [
Seating the crown after cementation (a) under finger pressure and (b) under static load of 5 kg.
Specimens inside the thermal cycling machine.
Cycling loads in this study were corresponding to 6 months of clinical service. Accordingly, samples were exposed to 120,000 mechanical cycles [
A rubber sheet of 0.5 mm thickness was inserted between the metal stylus and the crown to prevent sharp contacts, to distribute the applied force equally and to represent the consistency of the food substance [
Schematic representation of load at an angle of 135° to the root long axis (45° to the horizontal plane).
Loading stylus applying load onto the palatal surface of the crown 135° to the long axis of the tooth during the cyclic loading test.
Specimens subjected to cyclic load. A, motor drive connected to an oval-shaped acrylic block with a metal rod; B, the metal arm fixed to the base of the machine from the one end and the other end carrying a metal block 49 N; C, four custom-made copper molds, carrying the specimens; D, four metal stylus, each one ended with 7 mm × 2 mm diameter beveled metal rod.
Loading stylus applying load onto the palatal surface of the crown 135° to the long axis of the tooth during the fracture resistance test.
The load was applied on each specimen until catastrophic failure occurred. Catastrophic failure was defined as exhibition of visible cracks and events of chipping or fracture [ Mode I: visible cracks in the crown. Mode II: veneer chipping. Mode III: bulk fracture of the crown.
Data were collected and revised and coded and fed to statistical software IBM SPSS version 20. The given graphs were constructed using Microsoft excel software.
All statistical analysis was done using two-tailed tests and an alpha error of 0.05. Descriptive statistics included the mean with standard deviation and percent to describe the scale and categorical data, respectively Analysis of numeric data included the independent sample Analysis of categorical data included the Mont Carlo exact test and Fisher’s exact test
Most of the recommendations for a clinically relevant in vitro load-to-fracture test for ceramic restorations described by Kelly [
After thermal and load cycling, no signs of fracture were detected in the specimens of each group. Signs of fracture were demonstrated by the specimens of each group only in the fracture resistance test, and the load at which fracture occurred for each specimen was recorded in newton.
The results revealed that there was a significant difference in the fracture resistance between the two incisal veneering thicknesses in both groups. For CAD/CAM zirconia all-ceramic group, there was a significant difference at (
Bar chart showing fracture resistance of the two incisal veneering thicknesses in CAD/CAM zirconia all-ceramic groups.
For the metal ceramic group, a significant difference was also reported at
Bar chart showing fracture resistance of two incisal veneering thicknesses in the nickel-chromium metal ceramic group.
This finding is in accordance with Swain [
The results revealed that there was a significant difference (
Bar chart showing fracture resistance of the two studied groups.
Comparison between modes of fracture of total specimens in each incisal veneering thickness.
Mode of fracture | Incisal veneering thickness | MCP | |||
---|---|---|---|---|---|
1.5 mm | 3.0 mm | ||||
No. | % | No. | % | ||
Crack in the crown (Mode I) | 5 | 50.0 | 2 | 20.0 | 0.364 |
Veneer chipping (Mode II) | 4 | 40.0 | 6 | 60.0 | |
Bulk fracture (Mode III) | 1 | 10.0 | 2 | 20.0 |
MCP:
This finding is similar with other studies conducted by Silva et al. [
Mode II (veneer chipping) under stereomicroscope: (a) all-ceramic; (b) metal ceramic.
Regarding the failure load values in this study, both groups demonstrated failure load values around 1500 N (Table
Comparison between the two studied groups according to the fracture resistance in each incisal veneering thickness and in both thicknesses.
Incisal veneering thickness | Fracture resistance | Group |
|
|
|
---|---|---|---|---|---|
Group I | Group II | ||||
1.5 mm | Minimum | 1339.0 | 1707.0 | 6.7 | 0.000 |
Maximum | 1535.0 | 2100.0 | |||
Mean | 1428.5 | 1940.5 | |||
SD | 72.2 | 153.7 | |||
|
|||||
3.0 mm | Minimum | 1230.0 | 1562.6 | 10.0 | 0.000 |
Maximum | 1316.0 | 1770.0 | |||
Mean | 1278.2 | 1667.9 | |||
SD | 33.3 | 80.2 | |||
|
|||||
Total | Minimum | 1230.0 | 1562.6 | 7.0 | 0.000 |
Maximum | 1535.0 | 2100.0 | |||
Mean | 1353.4 | 1804.2 | |||
SD | 95.3 | 184.4 |
According to Waltimo and Könönen study, the mean maximum incisive force of the anterior teeth was 263 N for men and 243 N for women [
It was found that there was no significant difference in the modes of fracture demonstrated by the two groups in the two incisal veneering thicknesses since 50% of the total specimens demonstrated Mode II (veneer chipping). However, 35% demonstrated Mode I (visible crack), and only 15% demonstrated Mode III (bulk fracture) (Figure
Bar chart showing the mode of fracture of each veneering thickness in both groups.
Veneer chipping demonstrated by 50% of total specimens (Figure
Mode I (visible crack) detected under stereomicroscope: (a) all-ceramic; (b) metal ceramic.
The chip size in the current study was much greater with zirconia crowns, creating more unacceptable defects; the surfaces where the veneering porcelain were delaminated from the core appeared smooth with no residual porcelain detected, and these observations were similar to previous studies [
Visible crack demonstrated by 35% of total specimens (Figure
Bulk fracture demonstrated by 15% of total specimens and only by the CAD/CAM zirconia all-ceramic specimens (Figure
Mode III (bulk fracture) in all-ceramic specimen.
Observations in this study and other studies revealed that fracture or chipping of veneering porcelains can be either a fracture of the porcelain itself or a fracture originating from the interfaces between the coping and porcelain [
Poor wettability of feldspathic porcelain is also another material specific factor for veneering chipping and is influenced by some processing parameters such as the roughness of the core surface and the atmosphere in which the feldspathic veneer was fused on the dental zirconia core. Such factors have a huge effect on the bond strength between zirconia and the overlying porcelain [
Regarding the fracture pattern, in the present study, the fracture pattern was extending from the lingual to the labial surface, and this finding is in consistent with Geminiani et al. [
Two-way analysis of variance (ANOVA) test of significance for comparing the mean fracture load by groups, incisal veneering thickness, and modes of fracture.
Source |
|
|
---|---|---|
Group | 75.7 | 0.000 |
Thickness | 14.6 | 0.003 |
Modes of fracture | 1.2 | 0.344 |
Group |
0.5 | 0.494 |
Group |
0.2 | 0.634 |
Thickness |
0.3 | 0.737 |
The results revealed that there was no interaction between the tested crown material and the veneering thickness (
Increased numbers of specimens could have reduced the influence of data variations on the statistical outcome. Furthermore, as with any in vitro study, it remains unclear as to what extent the results may be different in a clinical setting. Higher numbers of loading cycles may be required to represent longer service time.
Scanning electron microscopic (SEM) investigation of the initiation and propagation direction of the cracks and failures would have been beneficial in studying the association between the defect at the loading point and the crack or fracture lines. Use of finite element analysis (FEA) also would be helpful in investigations of stress distribution to evaluate the mechanical behavior of restorations.
The current study [
The experimental, observational, simulation, and compiled data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Dr. Sanaa Hussein, Dr. Fayza Alabbasy, and Dr. Amir Azer are acknowledged for their guidance and support.