Effect of different core design made of computer-aided design/computer-aided manufacturing system and veneering technique on the fracture resistance of zirconia crowns: A laboratory study.

BACKGROUND: It is unclear how the different core designs made of computer-aided design/computer-aided manufacturing (CAD/CAM) system and veneering techniques affect the fracture resistance of endodontically treated teeth. AIM: The aim of this in vitro study is to investigate the effect of different core designs made of CAD/CAM system and veneering techniques on the fracture resistance of zirconia ceramic crowns.
MATERIALS AND METHODS: Two types of zirconia copings were designed; the first one with circumferential 0.5-mm chamfer and the second one with circumferential 1-mm deep chamfer. The core specimens (in subgroups) were veneered anatomically with either a layering technique (hand-layer) or with press-on technique resulting in four test groups (n = 12). All crowns were then cemented using self-adhesive resin cement. After that, all specimens were loaded in a universal testing machine until fractured. STATISTICAL ANALYSIS: Data were then analyzed with two-way analysis of variance (ANOVA) (α =0.05).
RESULTS: Mean (standard deviation) failure loads for groups ranged from 2412.7 N (±624.6) to 3020.1 N (±1099.8). Two-way ANOVA revealed no statistically significant differences among groups (P > 0.05). Almost all groups showed cohesive failure in the veneering ceramic.
CONCLUSIONS: Within the limitations of this laboratory study, neither the core design nor the veneering technique affected the fracture resistance of all-ceramic crowns significantly.

INTRODUCTION

The disadvantages of the metal-ceramic restorations are their esthetics[1] and biological compatibility.[2] All-ceramic restorations have become more widely used due to their high esthetic appearance and their excellent biological compatibility.[3] During the last decade, the use of all-ceramic restorations with zirconia cores produced by computer-aided design/computer-aided manufacturing (CAD/CAM) has become a routine procedure in dentistry. All-ceramic restorations with zirconia cores have the advantages of superior esthetics, high mechanical strength, and biocompatible properties.[45] The high-strength oxide ceramic materials (such as zirconia) have been improved to be used either in anterior or posterior area.[6] The remarkable strength characteristics (as a result of the tetragonal-to-monoclinic structural transformation reinforcement) of yttrium-stabilized polycrystalline tetragonal zirconia make it an attractive core material.[78] Different techniques can be used for veneering the zirconia frameworks such as layering technique, pressable technique, or a combination of both. Although the dental laboratory procedures involve the sintering of several dental porcelain layers on the zirconia coping substrate achieve excellent fracture resistance[9] and esthetic results,[10] this procedure is time-consuming, and several studies have investigated an alternative approach of more rapidly hot pressing the ceramic onto the zirconia coping.[1112] Chipping of the veneering ceramic in zirconia-supported restorations has been reported to be one of the most common clinical failure types.[13141516] However, it is unknown whether the veneering method of the zirconia crown coping will affect the fracture strength of the all-ceramic crowns.

Therefore, the aim of this study was to evaluate the effect of different core designs made of CAD/CAM system and veneering techniques on fracture resistance of all-ceramic crowns. The null hypothesis was neither the core design nor the veneering technique would affect the fracture resistance of all-ceramic crowns.

MATERIALS AND METHODS

An artificial acrylic resin tooth (Prosthetic Restoration Jaw model, Nissin Dental Products Inc., Kyoto, Japan) corresponding to the maxillary first molar was prepared to accommodate an all-ceramic restoration. It received a full-crown preparation accounting for 2-mm occlusal clearance and 1.5-mm axial reduction. A circumferential chamfer finish line of 1 mm in depth (which was mesial and distal 1 mm more coronal than the facial and lingual surfaces and which were cervical to the cementoenamel junction [CEJ]) and a convergence angle of 6° were prepared using a custom-made parallelometer. The tooth was duplicated using a silicon impression mold (Deguform, Degudent, Hanau-Wolfgang, Germany) and afterward cast in a Co-Cr alloy (Wironit, Bego, Bremen, Germany). According to this procedure, 48 identical metal specimens were fabricated. The prepared replicas were laser scanned (Scanner S600 ARTI, Zirkonzahn GmbH, Bruneck, Italy) after application of scanning spray (Zirko Scanspray, Zirkonzahn GmbH, Bruneck, Italy). Thereafter, zirconia frameworks were designed (Zirkonzahn Modellier, Zirkonzahn GmbH, Bruneck, Italy) using one of the following coping designs:

Circumferential regular 0.5-mm chamfer [Figure 1]

Figure 1

Snapshot of three-dimensional image of a zirconia coping with a circumferential 0.5-mm chamfer finish line

Circumferential 1-mm deep chamfer [Figure 2].

Figure 2

Snapshot of three-dimensional image of a zirconia coping with a circumferential 1-mm chamfer finish line

Twenty-four frameworks were produced for every design by milling CAD/CAM partially sintered zirconia blocks (Zirkonzahn GmbH, Bruneck, Italy) in the milling unit (Milling Unit M1, Zirkonzahn GmbH, Bruneck, Italy) by an experienced dental technician. The 20% shrinkage of the zirconia block after sintering was compensated for through the CAD software by milling the frameworks in a 20% enlarged state. All milling burs were exchanged with new ones after each group milling procedure for standardization purposes. Then, the frameworks were sintered at 1600°C for 10 h according to the manufacturer instructions with a special furnace (Zirkonofen 600, Zirkonzahn GmbH, Bruneck, Italy). The frameworks fit to the dies were controlled by probing and visual inspection. Frameworks in subgroups (two subgroups of 12 specimens each) were either anatomically veneered with a layering technique (IPS e.max Ceram, Ivoclar Vivadent, Schaan, Liechtenstein) or with press-on technique (IPS e.max ZirPress, Ivoclar Vivadent, Schaan, Liechtenstein). For the subgroup veneered with press-on technique, the veneer form designed with the coping was duplicated using the castable wax (Renfert GmbH, Hilzingen, Germany). The duplicated wax replicas and sprues were attached to each coping, which were then embedded in the investment. The wax was burnt out and the final prosthesis acquired by filling the space with a pressable glass-ceramic ingot (IPS e.max ZirPress, Ivoclar Vivadent, Schaan, Liechtenstein). Firing was performed in a proper ceramic furnace (Programat EP 3000, Ivoclar Vivadent, Schaan, Liechtenstein). After recovery of the restorations, they were finished and glazed according to the manufacturer's instructions. After the veneering process, the previously described measurements were performed on every veneered restoration. Then, veneered ceramic materials were fused to zirconia framework by a developed glass-ceramic (IPS e.max CAD Crystall/Connect, Ivoclar Vivadent, Schaan, Liechtenstein) that turns into liquid when vibrated (Ivomix, Ivoclar Vivadent, Schaan, Liechtenstein). Fused materials were then sintered in the furnace (Programat P300, Ivoclar Vivadent, Schaan, Liechtenstein), and the crystalline intermediate (metasilicate) stage of the ceramic was converted into disilicate phase after crystallization firing. Finally, four examination groups were defined (n = 12) which differed in substructure design and veneering technique as follows:

CRL: Circumferential regular 0.5-mm chamfer (with layering technique)

CRP: Circumferential regular 0.5-mm chamfer (with press-on technique)

CCL: Circumferential 1-mm deep chamfer with 1-mm thickness zirconia collar (with layering technique)

CCP: Circumferential 1-mm deep chamfer (with press-on technique).

The standardization of the thickness of the veneering ceramic was accomplished with a silicone index made from an impression of the waxed desired anatomy, which was used to guide veneer contour and anatomy in all crowns. The final thickness of the entire restoration was 1.5 ± 0.1 mm. All crowns were airborne-particle abraded for 20 s with 50-μm alumina particles (Aluminum Oxide Abrasive, Heraeus Kulzer, Hanau, Germany) at 0.25 MPa and ultrasonically cleaned in 99% isopropanol (Saher Alreef Co., Riyadh, KSA) for 3 min.[1718] Before bonding, abutments were cleaned using a rotary brush and a slurry of fine pumice and thereafter with water spray. All crowns were cemented using self-adhesive resin cement (RelyX Unicem, 3M ESPE, Seefeld, Germany). Each crown was held in position for 7 min under a 3-kg load using custom-made device. The cemented crowns were mounted in acrylic resin blocks (Idofast Unipol, Unidesa-Odi, Madrid, Spain) up to a level 2 mm below the CEJ and with its long axis perpendicular to horizon using a custom-made surveyor. Then, all teeth of groups were kept in physiological saline in an incubator at 37°C for 30 days before the mechanical testing. Materials used are listed in Table 1. All the specimens were tested with a universal testing machine (Instron Corp., Canton, Massachusetts, USA) until were fractured. The crosshead speed was 1 mm/min along the long axis of the tooth. A 0.5-mm tin foil was placed between the loading element and the specimens to reduce large stresses. The failure load was recorded when the force-versus-time graph showed a sudden dip which indicated ceramic fracture. All fractures were assessed using a stereomicroscope (MP 320, Carl Zeiss) at ×20 magnification for failure mode analysis. Fracture modes were defined as (a) adhesive failure between the zirconia framework and veneering ceramic, (b) cohesive failure in the veneering ceramic, (c) cohesive failure in the zirconia framework, and (d) mixed failure when a combination of cohesive and adhesive failures occurred. Data were explored for normality using Anderson–Darling test, which showed that data were normally distributed. Among the four groups, fracture load data were analyzed with two-way analysis of variance (ANOVA) using with SPSS 18.0 (SPSS version 18.0 for Windows; SPSS Inc, Chicago, Illinois, USA).

Table 1

Materials used for restorative procedures

Material	Company	Batch number
Zirconia blocks	Zirkonzahn GmbH, Bruneck, Italy	ZB6143N
Press-on ceramic	IPS e.max ZirPress, Ivoclar Vivadent, Schaan, Liechtenstein	N63066
Layering ceramic	IPS e.max Ceram, Ivoclar Vivadent, Schaan, Liechtenstein	S28053
Self-adhesive resin cement	RelyX Unicem, 3M/ESPE, Seefeld, Germany	618486
NiCr alloy	Bellabond plus, Bego, Bremen, Germany	731810612

RESULTS

The mean fracture loads of four groups and standard deviations are presented in Table 2. They ranged from 2412.7 ± 624.6 N to 3020.1 ± 1099.8 N. The highest mean fracture load was recorded for CRP and the lowest one was recorded for CCL. Two-way ANOVA revealed no statistically significant differences among groups (P > 0.05). There was no statistically significant interaction between the coping design and veneering technique factors (P = 0.332).

Table 2

Fracture loads in (means±standard deviations)

Group	Fracture loads in N
CRL	2950.7±1125
CRP	3020.1±1099.8
CCL	2412.7±624.6
CCP	3001±704.8

CRL: Circumferential regular 0.5 mm chamfer (with layering technique), CRP: Circumferential regular 0.5 mm chamfer (with press-on technique), CCL: Circumferential 1 mm chamfer with 1 mm thickness zirconia collar (with layering technique), CCP: Circumferential 1 mm chamfer (with press-on technique)

In the recent study, almost all groups showed cohesive failure in the veneering ceramic which extended from loading point to the buccal margin, lingual margin, or proximal margin. There were no coping fractures during testing.

DISCUSSION

The present study investigated the influence of different core design made of CAD/CAM system and veneering technique on fracture resistance of all-ceramic crowns. Compared with clinical studies, laboratory experiments are less expensive, easier to reproduce, and less exposed to influences that are at best known only qualitatively. Model materials and testing conditions were chosen carefully to imitate clinical reality as faithfully as possible. Preparation of teeth and dimensions of the framework and veneering were performed according to the manufacturers' recommendations using custom-made parallelometer. With regard to crown configuration, crowns were manufactured by one dental laboratory technician with the same procedure using the same materials. The crowns may be considered as having a clinically relevant shape, and at the same time, the core and veneer thickness were identical for all crowns in each subgroup, which facilitates comparisons. Fracture strength test method is one of the most common techniques used to evaluate the mechanical properties of the loaded specimens and rely on subjecting the specimens to high stresses, leading to controlled failure at the site of stress concentration.[19] These tests are conducted under controlled laboratory conditions and on well-standardized specimens, leading to better understanding of the variables under investigation.

The null hypothesis that neither the core design nor the veneering technique would affect the fracture resistance of all-ceramic crowns was accepted (P = 0.298, P = 0.221, respectively). No significant differences were observed between groups which mean that the coping design and veneering technique did not reduce fracture strength. This can be explained by the fact that the zirconia core shows less deformation because of its higher elastic modulus, less stress is induced in the ceramic shoulder and an overload of the zirconia core inevitably results in fracture of the core. Furthermore, the horizontal extension of zirconia framework in the finish line region will not increase the fracture resistance of the crowned teeth.[20] A previous study reported that the zirconia ceramic crown should not be recommended if high translucency is required.[21] Therefore, zirconia crowns with 0.5-mm chamfer may be considered a clinical option for improving esthetics without reducing strength.

The mean fracture strength of the zirconia ceramic crowns in this study exceeded 2000 N in all groups, which is higher than the reported posterior masticatory force of 300 N to 600 N.[2223] Therefore, zirconia ceramic crowns are unlikely to fracture. The fracture resistance results observed in the present study can be compared well to the previous laboratory studies.[2425] As a zirconia core shows less deformation because of its higher elastic modulus; therefore, less stress is induced in the zirconia core and an overload of the zirconia core inevitably results in fracture of the veneering ceramic. Almost all groups showed cohesive failure in the veneering ceramic which extended from loading point to the buccal margin, lingual margin, or proximal margin. There were no coping fractures during testing. This can be explained by the providing solid support of zirconia framework for the brittle veneering ceramic which creates conditions for mainly compressive forces during loading. Another explanation could be that the thick layers of veneering ceramic on frameworks with low thermal diffusivity such as zirconia generate high subsurface residual tensile stresses which are more likely to result in fractures of the veneering ceramic.

This study tested a veneered zirconia crown cemented on a maxillary molar; however, both loading stresses and loading direction differ considerably in the anterior region of the mouth. Dynamic loading, temperature effects, and the oral environment effects were not included in the present study and might be considered as a shortcoming of the present study. Furthermore, the final crown configuration used in this experiment was also carefully modified using silicon index taken before tooth preparation to simulate posterior teeth. However, crowns might not be completely identical to one another. These are limitations of the study.

CONCLUSIONS

Within the limitations of this laboratory study, neither the core design nor the veneering technique affected the fracture resistance of all-ceramic crowns.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.