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J Dent Res 86(2):142-146, 2007
© 2007 International and American Associations for Dental Research

RESEARCH REPORT
Biomaterials & Bioengineering

Fracture of Porcelain-veneered Structures in Fatigue

B. Kim, Y. Zhang*, M. Pines, and V.P. Thompson

Department of Biomaterials and Biomimetics, New York University College of Dentistry, 345 East 24th Street, New York, NY 10010, USA

* corresponding author, yz21{at}nyu.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Porcelain-veneered crowns are widely used in modern dentistry, and their fracture remains problematic, especially in all-ceramic systems. We hypothesized that substructure properties have a significant effect on the longevity of porcelain-veneered crowns. Flat porcelain/metal or porcelain/ceramic structures were cemented to dentin-like composite, and a mouth-motion cyclic load of 200 N was delivered by means of a tungsten carbide spherical indenter (r = 3.18 mm), emulating occlusal loading on crowns supported by dentin. Findings indicated that porcelain on a low-hardness gold-infiltrated alloy was vulnerable to both occlusal surface contact damage and porcelain lower surface radial fracture, while porcelain on a higher-hardness palladium-silver alloy fractured chiefly from occlusal surface damage. The advantage of a high-modulus metal substructure was less pronounced. Fracture in the porcelain/zirconia system was limited to surface damage in the veneer layer, similar to that in the porcelain/palladium-silver system. Bulk fracture, observed in veneered alumina layers, was not found for zirconia.

KEY WORDS: porcelain-veneered structures • contact fatigue • fracture • ceramic • crowns


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Metal-ceramic crowns are the standard in dentistry and offer long-term structural performance, with porcelain fracture documented clinically at ~ 2% over 7 yrs (Coornaert et al., 1984) or ~ 3% to 4% at 10 yrs (Anderson et al., 1993; Creugers et al., 1994). All-ceramic crowns offer improved esthetics, biocompatibility, and inertness. Ceramics are inherently brittle and susceptible to premature failure, especially in repeated contact loading in moist environments (Lawn et al., 2001). The structural performance of all-ceramic systems remains less stable than that of metal-ceramic systems: Bulk fracture and porcelain cracking affect ~ 5% to 10% of single-unit all-ceramic prostheses by ~ 6 yrs (Kelly, 2004).

The continued search for strong and tough ceramic core materials has led to the integration of yttria-tetragonal zirconia polycrystals (Y-TZP) into restorative dentistry. Y-TZP offers high flexural strength and toughness, approximately twice that of alumina core materials. Clinical studies on the longevity of veneered Y-TZP crowns are not yet available, but anecdotal reports indicate that these zirconia cores are very fracture-resistant, but that fracture of the porcelain veneer is still problematic (Donovan, 2005).

We undertook this investigation to establish critical conditions for lifetime-limiting damage in porcelain-based crown-like layer structures. Controlled indentation fatigue testing with spheres on flat-layer specimens has been used to simulate the basic elements of occlusion on a restorative structure. Previous studies on single-cycle loading of porcelain/metal/polymer (Zhao et al., 2000, 2001, 2002) and porcelain/ceramic/polymer (Miranda et al., 2003) systems have identified and quantified several damage modes (Fig. 1Go): near-contact occlusal-surface fracture modes, including outer Hertzian cone cracks (Hertz, 1896) and median-radial cracks (Gurney and Hunt, 1967; Kim et al., 1999; Lee et al., 2000). The outer cone cracks initiate just outside the indenter contact area, where the maximum Hertzian field tensile stress occurs, and have an angle {alpha} ~ 22° relative to the specimen surface (Fig. 1Go). Quasi-plastic deformation occurs beneath the indenter, producing grain boundary microcracks, which coalesce and evolve into occlusal-surface median-radial cracks (Fischer-Cripps and Lawn, 1996a,b). Far-field veneer lower-surface radial cracks (Fig. 1aGo, porcelain/metal system) or cementation internal-surface radial cracks (Fig. 1bGo, porcelain/ceramic system) result from tensile stresses generated during loading from yield of metal substructure or tooth dentin support, respectively.


Figure 1
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  		Figure 1. Schematic diagram of crack geometry for cyclic contact loading with a tungsten carbide (WC) spherical indenter on porcelain-veneered layered structure cement bonded to dentin: (a) porcelain layer fused to metal coping, and (b) porcelain layer fused to ceramic core. Indenter radius r at load P, contact radius a, for number of cycles n, in water. Occlusal surface crack modes, outer cone cracks (O), inner cone cracks (I), and median-radial cracks (M). Porcelain veneer internal surface radial cracks (R), resulting from yield of metal coping (YM) and dentin support (YD); and cementation ceramic core internal surface radial cracks (R), owing to yield of dentin support (YD).

 
Recent studies unveiled inner cone cracks observed only in cyclic loading in water (Fig. 1Go) (Kim et al., 1999; Zhang et al., 2005a,b). These cracks form inside the expanding contact and quickly propagate downward, at a relatively steep angle compared with their outer counterparts (Fig. 1Go). For brittle ceramics like porcelain, cone cracks, both outer and inner, are the predominant damage modes. This study utilized mouth-motion fatigue on crown-like layer structures to compare porcelain/metal layers with porcelain/ceramic layers, both cemented on resin-based dental composite blocks, simulating dentin support, with regard to fracture (cracking) and deformation (yield) mechanisms.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Material Systems
Flat porcelain/metal and porcelain/ceramic bilayers, with dimensions of 10 x 10 x 1.2 mm, were fabricated according to manufacturers’ specifications for indentation fatigue-testing (Fig. 1Go). Properties of the relevant materials are shown in the TableGo. The porcelain/metal systems selected were clinically well-documented porcelain veneers on gold-infiltrated (P/Au) alloy copings (Captek, 10 x 10 x 0.5 mm, Precious Chemicals USA Inc., Longwood, FL, USA) and palladium-silver (P/Pd) alloy plates (IPS d.SIGN 59, 10 x 10 x 0.5 mm, Ivoclar Vivadent Inc., Amherst, NY, USA). These were supplied by the respective manufacturers. The P/Au coping consisted of a rigid scaffold core (0.35 mm thick) that was infiltrated with high Au (98%) to form an Au/core/Au sandwich structure, leaving inner and outer surfaces with a thin (~ 75 µm) layer of Au. The metal copings were masked with a ~ 60-µm-thin opaque porcelain layer and then covered with a thick translucent body porcelain layer (d.SIGN, Ivoclar). The total thickness of the porcelain veneer was 0.7 mm. The porcelain surface was polished with successive grits to 1 µm finish. It was expected that the P/Au and P/Pd bilayers would possess a slight residual compressive stress in the porcelain veneer, due to its relatively low coefficient of thermal expansion (CTE) compared with that of the alloys. The lower surface of the metal substructure was ground with 600-grit SiC abrasive paper and cemented (Variolink II, Ivoclar-Vivadent, Schaan, Liechtenstein) to a composite support block (Z100, 12 x 12 x 4 mm, 3M/ESPE, St. Paul, MN, USA). The Z100 blocks, having an elastic modulus similar to that of dentin (TableGo), were all incubated in water over 2 wks before cementation, to allow for hydroscopic expansion. After cementation, the bonded structures were aged in water for 10 days before being fatigue-tested.


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  		Table. Materials Used in This Study
 
The all-ceramic system was comprised of porcelain on a Y-TZP core (P/Zr). Fully sintered CAD/CAM Y-TZP plates ({phi} 20 x 0.5 mm) (LAVA, 3M, St. Paul, MN, USA) were veneered with LAVA porcelain by the manufacturer. The lower surface of the zirconia core was roughened with 600-grit SiC abrasive paper and cemented (Rely X, ARC, St. Paul, MN, USA) to a Z100 block. The porcelain surface was polished to a 1 µm finish. The final dimension of the P/Zr specimens was {phi} 20 x 1.2 mm. The cemented structures were aged 10 days in water before being tested.

Fatigue Tests
Fatigue loading was delivered with a spherical tungsten carbide (WC) indenter (r = 3.18 mm) and a mouth-motion simulator (Elf 3300, EnduraTEC Division of Bose, Minnetonka, MN, USA) in water with a controlled stroke profile: maximum load (biting force), Pm = 200 N; loading and unloading rates, 1000 N/sec; and a chewing frequency ~ 1.5 Hz. Each load cycle consisted of the indenter coming into contact with the specimen, loading to a maximum, holding for 0.2 sec, unloading, and lifting off (0.5 mm), completely unloading the specimen. Fatigue tests were interrupted after a prescribed number of loading cycles, and the specimen was inspected for damage, from the top surface, with the use of a 3D polarized specular reflection microscope (Edge R400, Micro Science Technologies, Marina Del Rey, CA, USA), focusing from the occlusal-surface down into the translucent interior. The specimen was then indexed and re-introduced to the mouth-motion machine for continued testing. Fatigue tests were stopped at 10,000, 15,000, 25,000, 50,000, 75,000, 100,000 cycles, etc. A minimum of 3 specimens was used for each material system for the prescribed loading cycles. We sectioned selected specimens to evaluate the extent of subsurface damage (Mikosza and Lawn, 1971).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Damage sustained in the P/Au and P/Pd systems is shown in Fig. 2Go. In all cases, outer cone cracks extending from the surface formed within several cycles. Inner cone cracks became visible at ~ 103 cycles. For the P/Au system, inner cone cracks penetrated both body and opaque porcelain layers, reaching the porcelain/metal interface at ~ 10,000-15,000 cycles (Fig. 2aGo). These through-veneer inner cone cracks were relatively fine, similar to hairline cracks observed clinically. At this stage, the P/Au systems were still functional. At around 20,000–25,000 cycles, radial cracks initiated at the lower surface of the porcelain veneer (Fig. 2bGo). These radial cracks were oriented normal to the plate surface and propagated rapidly upward (approaching the occlusal surface) and sideward (toward the specimen edges). The onset of radial cracks was defined as failure.


Figure 2
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  		Figure 2. Optical micrographs illustrating damage sustained in d.SIGN porcelain layers (both translucent and opaque) fused to metal copings, cement-bonded to dental composite support (Z100) following P = 200 N cyclic loading with a sphere indenter (r = 3.18 mm) in water. Specimens were sectioned through the indentation site and polished for cross-sectional examination. d.SIGN porcelains on gold alloy coping after (a) n = 15,000, and (b) n = 25,000. d.SIGN porcelains on palladium-silver alloy coping following (c) n = 50,000, and (d) n = 400,000. Note inner cone cracks (I) and radial cracks (R).

 
For the P/Pd system, inner cone cracks reached the interface at ~ 25,000–50,000 cycles (Fig. 2cGo), a notable delay compared with the P/Au system (Fig. 2aGo). In addition, no radial cracking was observed at the lower surface of the porcelain veneer, even after 400,000 loading cycles (Fig. 2dGo). At this stage, damage was still confined to the region surrounding the contact, and the system remained functional. No radial cracks were observed in the P/Pd system until the maximum fatigue load increased to ~ 600 N.

Fig. 3aGo shows damage sustained in the P/Zr system following 550,000 loading cycles. Outer cone cracks formed after several cycles, and inner cone cracks formed at ~ 103 cycles. Although inner cone cracks reached the veneer/core interface at ~ 50,000–100,000 cycles, they remained contained after 550,000 cycles—neither penetrating the Y-TZP core nor propagating along the porcelain/Y-TZP interface (Fig. 3aGo). There was no evidence of radial cracking at the lower surface of either the porcelain veneer or the Y-TZP core. In fact, no radial cracks were observed even after prolonged fatigue-loading at 600 N. These findings were similar to the fracture development observed in the P/Pd system (Fig. 2dGo), but differed from the veneered alumina, where both contact-induced occlusal surface fracture (outer and inner cone cracking) and flexure-induced cementation surface bulk fracture (radial cracking) were evident (Stappert et al., unpublished observations). The radial cracking initiated at the lower surface of the alumina core and propagated into the porcelain veneer in 90% of the specimens tested in the fatigue load range between ~ 180 and 230 N (Stappert et al., unpublished observations).


Figure 3
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  		Figure 3. Cross-section of optical micrographs illustrating damage modes in (a) LAVA porcelain fused to LAVA zirconia core (n = 550,000). Thickness of the ceramic core is ~ 0.5 mm. Note inner cone cracks (I). Scanning electron micrograph of (b) a clinically fractured metal-ceramic crown abutment after 16 yrs in service, showing fracture initiated within the occlusal contact area, and (c) a similar failure in a 19-year-old bilayer ceramic crown (Cerestore, Coors, Boulder, CO, USA). In both (b) and (c), the crack extends to the gingival margin in these lingual views. SEM images were supplied by Susanne Scherrer and were prepared from clinical polyvinyl siloxane impressions of the crowns, which were cast with epoxy resin. The resulting replica samples were sputter-coated and examined in the SEM.

 
In all cases, no delamination between the porcelain veneer and metal copings or ceramic cores was observed after this prolonged uni-axial fatigue.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
We have examined fracture and deformation modes in flat crown-like layered systems consisting of porcelain veneers fused onto selected metal alloy copings or ceramic cores, cemented to dentin-like substrates for repeated contact loading in a wet environment. In all cases, outer cone cracks formed within several contact cycles, indicating that, at a maximum 200 N fatigue loading, the near-contact local stresses exceeded the elastic limit of porcelain. The outer cone cracks, however, did not penetrate deep into the material under the 200 N load. Inner cone cracks formed within the contact region and became significant typically at ~ 103 cycles. Inner cone crack propagation was chiefly driven by hydraulic pumping, a result of water entrapment in the crack and the associated stress build-up under loading (Chai and Lawn, 2005; Zhang et al., 2005a,b). Although inner cone cracks formed much later than outer cone cracks, they quickly outgrew their outer counterparts and penetrated deep, eventually terminating at the porcelain/substructure interface—neither propagating into the metal copings or ceramic cores nor extending along the veneer/substructure interface. This is because cracks are unlikely to propagate from a low-modulus, low-toughness ceramic to a high-modulus, high-toughness ceramic (the reverse is not true) (Kim et al., 2006).

Several measures can be proposed to suppress inner cone crack propagation:

  1. Design porcelain veneers with improved fracture toughness, since toughness provides the resistance to crack propagation.
  2. Promote a residual compressive stress in the veneer layer by selecting an appropriate CTE combination between the veneer and substructure, since brittle materials are most vulnerable to tensile stress. From this point of view, all three systems, P/Au, P/Pd, and P/Zr, should display a slight compressive stress in the porcelain veneers, with the P/Au possessing a higher compressive stress than P/Pd, owing to a larger CTE mismatch between the porcelain and Au alloy coping (TableGo). The higher compressive stress in the P/Au system should result in a larger number of cycles for inner cone cracks to reach the interface, compared with the P/Pd system, which is contradictory to the experimental findings. This may be attributed to the low-hardness and low-modulus Au alloy, which may facilitate flexure of the porcelain veneer, exposing the lower half of the veneer layer to tension, accelerating the propagation of inner cone cracks once they reach halfway through the veneer layer.
  3. Utilize a high-hardness and high-modulus substructure material. The importance of a hard, stiff substructure is further demonstrated by fatigue data of the P/Zr system, where the hard, stiff ceramic core effectively prevents the flexure of porcelain veneer, resulting in delay of inner cone cracks intersecting the veneer/core interface.

In the metal-ceramic systems, far-field radial fractures at the lower surface of the porcelain veneer could occur with yield of the metal substructure, facilitating flexure of the overlying porcelain veneer. Yield of the metal substructure is primarily determined by its hardness (yield strength of metal is ~ 1/3 of its hardness value). A high-modulus metal can have counterproductive effects on the radial fracture in porcelain: It will shield the compliant dentin support and thus hinder further radial cracking; however, it will also increase the flexure of the metal substructure, and hence facilitate radial cracking (Zhao et al., 2002). Therefore, a hard metal substructure can prevent radial fracture at the lower surface of a porcelain veneer. In the all-ceramic systems, the hard, stiff ceramic core provides support to the veneer, and with yield of the tooth dentin support, the tensile stresses are now transferred to the lower surface of the core. With dentin support yield, the strength of the ceramic core becomes most critical, since strength is the resistance to crack initiation. This explains why the hard, stiff, but relatively low-strength, alumina core is vulnerable to cementation surface radial cracking. Radial cracks, both at the lower surface of a porcelain veneer and at the ceramic core, are oriented normal to the plate surface and are susceptible to flexural tensile stresses generated during function. Hence, once initiated, radial cracks propagate sideward and upward, ultimately leading to failure (Kelly, 1999).

We acknowledge that dental crowns have a complex geometry. Thus, the flat specimens utilized in the present study have limitations in demonstrating other fracture modes, such as margin fracture. Preliminary studies of single-cycle loading on curved structures have shown that, once the radial cracks form, they have a strong tendency to propagate to the margin (Qasim et al., 2005, 2006), similar to those observed clinically (Figs. 3b, 3cGo; images supplied by Susanne Scherrer). In addition, occlusion involves enamel-porcelain or porcelain-porcelain antagonistic contacts at various loads and locations. The current contact fatigue tests were performed with hard spherical indenters, which provided a worst-case occlusal scenario (while preserving the indenter), while not applying lateral loads, as would be experienced clinically.

As to the design of veneered crowns, in the case of porcelain/metal crowns, high-hardness metal cores have better resistance to yield. A thick porcelain layer (~ 1 mm) offers necessary protection to the metal substructure from yield. A hard, stiff ceramic core clearly has an advantage in preventing radial fracture of porcelain. However, this makes the ceramic cores vulnerable to cementation internal surface radial fracture, particularly when the total thickness is thin, leading to so-called bulk fracture. Hence, the stronger Y-TZP cores have better resistance to bulk fracture than do their alumina counterparts.


   ACKNOWLEDGMENTS
 
This work was supported by the New York University Research Challenge Fund and by National Institute of Dental and Craniofacial Research grant PO1 DE10976. Valuable discussions with Drs. Brian R. Lawn and Susanne Scherrer are appreciated.

Received April 12, 2006; Last revision August 3, 2006; Accepted October 6, 2006


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TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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