Highlights
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Veneer chipping was the prominent fracture mode in clinically retrieved zirconia–ceramic and metal–ceramic fixed dental prostheses.
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All fracture initiated at the occlusal wear facets.
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Zirconia–ceramic exhibited a higher rate of veneer fracture with larger chip sizes relative to their metal–ceramic counterparts.
Abstract
Objectives
A recent 3-year randomized controlled trial (RCT) of tooth supported three- to five-unit zirconia–ceramic and metal–ceramic posterior fixed dental prostheses (FDPs) revealed that veneer chipping and fracture in zirconia–ceramic systems occurred more frequently than those in metal–ceramic systems . This study seeks to elucidate the underlying mechanisms responsible for the fracture phenomena observed in this RCT using a descriptive fractographic analysis.
Methods
Vinyl-polysiloxane impressions of 12 zirconia–ceramic and 6 metal–ceramic FDPs with veneer fractures were taken from the patients at the end of a mean observation of 40.3 ± 2.8 months. Epoxy replicas were produced from these impressions . All replicas were gold coated, and inspected under the optical microscope and scanning electron microscope (SEM) for descriptive fractography.
Results
Among the 12 zirconia–ceramic FDPs, 2 had small chippings, 9 had large chippings, and 1 exhibited delamination. Out of 6 metal–ceramic FDPs, 5 had small chippings and 1 had large chipping. Descriptive fractographic analysis based on SEM observations revealed that fracture initiated from the wear facet at the occlusal surface in all cases, irrespective of the type of restoration.
Significance
Zirconia–ceramic and metal–ceramic FDPs all fractured from microcracks that emanated from occlusal wear facets. The relatively low fracture toughness and high residual tensile stress in porcelain veneer of zirconia restorations may contribute to the higher chipping rate and larger chip size in zirconia–ceramic FDPs relative to their metal–ceramic counterparts. The low veneer/core interfacial fracture energy of porcelain-veneered zirconia may result in the occurrence of delamination in zirconia–ceramic FDPs.
1
Introduction
Metal–ceramic and zirconia–ceramic systems are widely used for fixed dental prostheses (FDPs) today. Metal–ceramic FDPs were introduced to restorative dentistry several decades ago. This material system has good load-bearing capacity and exhibits high survival rates. In addition, the reported rates for chipping of the veneering ceramic for metals were low. According to a 15-year clinical study by Walton, only 3% porcelain veneer and 3% metal framework fracture were reported . Yttria-tetragonal zirconia polycrystals (Y-TZP) is also widely used nowadays as an alternative material to metal for frameworks of posterior FDPs due to its high strength, good fracture toughness and excellent biocompatibility . Y-TZP was found to have a flexural strength of 900–1200 MPa and fracture toughness of 4–6 MPa √m . However, previous studies have reported high veneer chipping and fracture rate from 8 to 25% in all major brands of zirconia–ceramic FDPs .
Even though many clinical studies have been conducted on zirconia–ceramic and metal–ceramic FDPs, very few studies presented a direct comparison of these two restorations. In order to directly assess the clinical performance of zirconia–ceramic and metal–ceramic FDPs, patients in a clinical trial should be randomly assigned to either zirconia–ceramic or metal–ceramic restorations. In addition, the numbers of these two types of restorations should be comparable. Both criteria have been met in a 3-year randomized controlled trial (RCT) for three- to five-unit zirconia–ceramic and metal–ceramic posterior FDPs conducted by Sailer et al. .
In Sailer’s study, 38 metal–ceramic FDPs and 38 zirconia–ceramic were randomly placed in 59 patients who were generally and periodontally healthy with no bruxism. Out of these, 36 zirconia–ceramic and 31 metal–ceramic FDPs were examined at mean observation period of 40.3 ± 2.8 months. The results showed that both zirconia and metal frameworks had excellent survivability, but the veneer chipping and fracture in zirconia–ceramic systems occurred more frequently than those in metal–ceramic systems. Among the 31 metal–ceramic FDPs, 19.4% had veneer chipping. In contrast, the veneer chipping rate for 36 zirconia–ceramic FDPs was 33.4%. Also, for the zirconia–ceramic system, the rate of the chipping down to the framework was 5.6%, while the rate of restorations that needed to be replaced was 2.8%. Sailer’s study revealed that zirconia–ceramic FDPs underwent higher chipping rate and larger chipping size compared to their metal–ceramic counterparts after a 3-year observation period .
A systematic fractographic analysis of the reasons for veneer fracture and for different clinical performances of zirconia–ceramic and metal–ceramic FDPs was not included in previous RCT studies. Descriptive fractography is an effective tool to identify the cause of failure and provide information about the loading conditions based on the fracture pattern recognition and analysis. Veneering ceramics of zirconia–ceramic and metal–ceramic FDPs are both brittle materials that have brittle fractures taking place with little or no plastic deformation. This nature helps brittle materials leave unequivocal patterns on fracture surfaces. The patterns are the direct consequence of crack perturbations during propagation . Certain types of fractures leave telltale fracture patterns for the determination of fracture origin, direction of crack propagation and stress state. Some early studies successfully determined failure causes for clinically used dental restorations by descriptive fractography . Then the systematic guide of this technique for clinically failed dental ceramics were established based on the American Society for Testing and Materials (ASTM) standard practice . In order to not disturb fracture surface after failure occurred, a replica technique for reproducing failed ceramic surfaces was introduced and its effectiveness for descriptive fractography was demonstrated by Scherrer et al. .
Accordingly, the purpose of this study was to determine the fracture origin, crack propagation path, and damage modes in veneer fractured zirconia–ceramic and metal–ceramic FDPs from Sailer’s study using the replica technique and the principles of descriptive fractography.
2
Materials and methods
2.1
Specimen source
Twelve zirconia–ceramic and 6 metal–ceramic FDPs with fracture from a previous randomized controlled trial (RCT) conducted by Sailer et al. were studied. The RCT included 59 patients (27 women and 32 men) that needed at least one FDP in the posterior region. All three- to five-unit posterior FDP sites were included and randomly assigned to either zirconia–ceramic or metal–ceramic restorations . All patients had good general and periodontal health with no obvious signs of bruxism. Informed consent was obtained and the requirements of the Helsinki Declaration were fulfilled. The conventional preparation criteria for abutments of metal–ceramic and zirconia restorations were followed .
The zirconia and metal frameworks were fabricated out of presintered zirconia blanks (Cercon base 30 or 38, Dentsply, PA, USA) and gold alloys (Degudent U, DeguDent, Hanau-Wolfgang, Germany), respectively. Two specially designed porcelain veneers were used for two different frameworks (veneering ceramic for zirconia: Cercon Ceram S, Ceramco, NH, USA; veneering ceramic for metal: Duceram Plus, Degudent, Hanau-Wolfgang, Germany). The porcelain firing protocols were provided by the manufacturer. In this case, both zirconia and metal FDPs were subjected to a so called fast cooling protocol, where the furnace was switched off after programed holding time at the elevated temperature. The furnace door was then opened and the FDP was left to cool. Details about the fabrication of FDPs have been published elsewhere .
After a mean period of 40.3 ± 2.8 months, fractured surfaces of 12 zirconia–ceramic and 6 metal–ceramic FDPs, which exhibited veneer chipping were cleaned with alcohol. After that, a low viscosity A-silicone impression material (President, Coltène Whaledent, Altstätten, Switzerland) was used to make impressions of FDPs surfaces . Two impressions were made for each fracture surface. The first impression was to clean the surface. The second impression mold was used to cast the epoxy resin (EpoFix, Struers, Birmensdorf, Switzerland) replicas for fractographic studies. The epoxy was poured at least 1 h after taking the impression mold.
2.2
Specimen preparation
All replicas were first cleaned using distilled water in an ultrasonic bath (Solid state/ultrasonic T-14B, L&R, NJ, USA). After cleaning, replicas were gold coated by the sputter coater (K650, Emitech, MA, USA).
2.3
Specimen examination
Specimens were inspected by stereomicroscope. Stereoptical microscope (SZX-ILLB100, Olympus, PA, USA) equipped with a camera port (Retiga 32-0013B-118, Qimaging, Vancouver, Canada) and dual gooseneck fiber optic lights (Kramer, Fostec, Siheung-daero Siheung-si Gyeonggi-do, Korea) were used to examine fracture surfaces to determine different types of fracture (chipping, delamination or bulk fracture). Then, in order to elucidate the fracture origin and path of crack propagation, a scanning electron microscope (S-3500N, Hitachi, Tokyo, Japan) was used to inspect fracture surfaces.
2.4
Descriptive fractographic analysis
The fracture origins were located by fracture pattern analysis. We determined fracture origins by recognizing characteristic fracture patterns on fracture surface and tracing back to fracture initiation sites. The following three fracture patterns were used extensively in the descriptive fractographic analysis of replicas of fractured dental restorations including wake hackles, twist hackles and arrest lines.
Wake hackles are trails extending from an irregularity at the crack front in the direction of crackling . Porcelain veneers for zirconia–ceramic and metal–ceramic restorations have a certain amount of pores introduced by the powder/water slurry technique for fabrication, which act as singularities . When an advancing crack encounters an elastic singularity such as pores in porcelain, the crack front may split at the object and sweep past it on both sides. As the two fronts pass the obstacle they are often on slightly different planes and create a “tail” between them. The tail may fade away quickly or persist for long distances. The local crack runs the same way as the tail fades and the singularity side of wake hackle is the origin side of this fracture .
Twist hackles are formed when the stress field changes . Veneering ceramics are glass reinforced with crystallites, which have coarse particles and agglomerates. Crack goes through coarse particles and agglomerates with local rotation in the axis of principal tension, so twist hackles are generated during the crack propagation. When the axis of principal stress is tilted, the crack is unable to rotate immediately, so it breaks into small, unconnected segments and leaves the “step”, twist hackle, between different segments. Usually the crack propagation is in the direction of fine to coarse hackles and the side with fine hackles is the side of fracture origin .
Arrest lines are formed when a crack slightly changes the propagation direction . During the crack propagation in dental restorations, the coarse grains make the crack hesitate or stop to redirect the path of crack propagation. When it hesitates or stops, the crack front profile becomes evident instantly in the form of arrest lines. The fracture origin would normally be on the concave side of an arrest line, and the convex direction is the direction of crack propagation .
These fracture patterns helped us not only to determine directions of crack propagation, but also to trace back to the fracture origins. From the different sites of fracture origins and the different stress fields specimens were involved in, we were able to elucidate sources and reasons for fracture.
2.5
Definitions of fracture modes
In order to facilitate ease of readership, we defined fractures that initiate from the occlusal surface but without exposing the framework as veneer chipping. Chipping area that involves less than one-fourth of the cusp was regarded as small chipping. Large chipping referred to the chipping area greater than one-fourth of the cusp. We designated fractures that extend to veneer/core interface as delamination.
2
Materials and methods
2.1
Specimen source
Twelve zirconia–ceramic and 6 metal–ceramic FDPs with fracture from a previous randomized controlled trial (RCT) conducted by Sailer et al. were studied. The RCT included 59 patients (27 women and 32 men) that needed at least one FDP in the posterior region. All three- to five-unit posterior FDP sites were included and randomly assigned to either zirconia–ceramic or metal–ceramic restorations . All patients had good general and periodontal health with no obvious signs of bruxism. Informed consent was obtained and the requirements of the Helsinki Declaration were fulfilled. The conventional preparation criteria for abutments of metal–ceramic and zirconia restorations were followed .
The zirconia and metal frameworks were fabricated out of presintered zirconia blanks (Cercon base 30 or 38, Dentsply, PA, USA) and gold alloys (Degudent U, DeguDent, Hanau-Wolfgang, Germany), respectively. Two specially designed porcelain veneers were used for two different frameworks (veneering ceramic for zirconia: Cercon Ceram S, Ceramco, NH, USA; veneering ceramic for metal: Duceram Plus, Degudent, Hanau-Wolfgang, Germany). The porcelain firing protocols were provided by the manufacturer. In this case, both zirconia and metal FDPs were subjected to a so called fast cooling protocol, where the furnace was switched off after programed holding time at the elevated temperature. The furnace door was then opened and the FDP was left to cool. Details about the fabrication of FDPs have been published elsewhere .
After a mean period of 40.3 ± 2.8 months, fractured surfaces of 12 zirconia–ceramic and 6 metal–ceramic FDPs, which exhibited veneer chipping were cleaned with alcohol. After that, a low viscosity A-silicone impression material (President, Coltène Whaledent, Altstätten, Switzerland) was used to make impressions of FDPs surfaces . Two impressions were made for each fracture surface. The first impression was to clean the surface. The second impression mold was used to cast the epoxy resin (EpoFix, Struers, Birmensdorf, Switzerland) replicas for fractographic studies. The epoxy was poured at least 1 h after taking the impression mold.
2.2
Specimen preparation
All replicas were first cleaned using distilled water in an ultrasonic bath (Solid state/ultrasonic T-14B, L&R, NJ, USA). After cleaning, replicas were gold coated by the sputter coater (K650, Emitech, MA, USA).
2.3
Specimen examination
Specimens were inspected by stereomicroscope. Stereoptical microscope (SZX-ILLB100, Olympus, PA, USA) equipped with a camera port (Retiga 32-0013B-118, Qimaging, Vancouver, Canada) and dual gooseneck fiber optic lights (Kramer, Fostec, Siheung-daero Siheung-si Gyeonggi-do, Korea) were used to examine fracture surfaces to determine different types of fracture (chipping, delamination or bulk fracture). Then, in order to elucidate the fracture origin and path of crack propagation, a scanning electron microscope (S-3500N, Hitachi, Tokyo, Japan) was used to inspect fracture surfaces.
2.4
Descriptive fractographic analysis
The fracture origins were located by fracture pattern analysis. We determined fracture origins by recognizing characteristic fracture patterns on fracture surface and tracing back to fracture initiation sites. The following three fracture patterns were used extensively in the descriptive fractographic analysis of replicas of fractured dental restorations including wake hackles, twist hackles and arrest lines.
Wake hackles are trails extending from an irregularity at the crack front in the direction of crackling . Porcelain veneers for zirconia–ceramic and metal–ceramic restorations have a certain amount of pores introduced by the powder/water slurry technique for fabrication, which act as singularities . When an advancing crack encounters an elastic singularity such as pores in porcelain, the crack front may split at the object and sweep past it on both sides. As the two fronts pass the obstacle they are often on slightly different planes and create a “tail” between them. The tail may fade away quickly or persist for long distances. The local crack runs the same way as the tail fades and the singularity side of wake hackle is the origin side of this fracture .
Twist hackles are formed when the stress field changes . Veneering ceramics are glass reinforced with crystallites, which have coarse particles and agglomerates. Crack goes through coarse particles and agglomerates with local rotation in the axis of principal tension, so twist hackles are generated during the crack propagation. When the axis of principal stress is tilted, the crack is unable to rotate immediately, so it breaks into small, unconnected segments and leaves the “step”, twist hackle, between different segments. Usually the crack propagation is in the direction of fine to coarse hackles and the side with fine hackles is the side of fracture origin .
Arrest lines are formed when a crack slightly changes the propagation direction . During the crack propagation in dental restorations, the coarse grains make the crack hesitate or stop to redirect the path of crack propagation. When it hesitates or stops, the crack front profile becomes evident instantly in the form of arrest lines. The fracture origin would normally be on the concave side of an arrest line, and the convex direction is the direction of crack propagation .
These fracture patterns helped us not only to determine directions of crack propagation, but also to trace back to the fracture origins. From the different sites of fracture origins and the different stress fields specimens were involved in, we were able to elucidate sources and reasons for fracture.
2.5
Definitions of fracture modes
In order to facilitate ease of readership, we defined fractures that initiate from the occlusal surface but without exposing the framework as veneer chipping. Chipping area that involves less than one-fourth of the cusp was regarded as small chipping. Large chipping referred to the chipping area greater than one-fourth of the cusp. We designated fractures that extend to veneer/core interface as delamination.
3
Results
The incidence of different types of fracture was summarized based on the stereomicroscope examination ( Table 1 ). Then, the scanning electron microscope (SEM) was used to discern the fracture origin and direction of crack propagation in fractured FDPs. In the interest of briefness, we selected only representative cases from each of the fracture modes. Detailed stereomicroscopic and SEM images of selective small and large chipping and delamination on the zirconia–ceramic and metal–ceramic FDPs are shown below.
