Q U I N T E S S E N C E I N T E R N AT I O N A L
RESTORATIVE DENTISTRY
Qian Du
Fractographic analysis of anterior bilayered ceramic crowns that failed by veneer chipping Qian Du, DDS, MD1/Michael V. Swain, BSc, PhD2/Ke Zhao, DDS, PhD3 Objective: To fractographically analyze the reasons for the chipping of veneering porcelain in clinically failed anterior lithium disilicate glass-ceramic (LDG) and glass-infiltrated alumina (GIA) crowns. Method and Materials: Five anterior bilayered ceramic crowns with clinical veneer chipping failure were retrieved, of which three were LDG crowns and two were GIA crowns. The fractured surfaces of the failed restorations were examined using stereomicroscopy and scanning electron microscopy (SEM). The principles of fractography were used to identify the location and dimensions of the critical crack and to estimate the stress at failure. Results: All five anterior crowns
failed by cohesive failure within the veneer on the labial surface. Fractography showed that the critical crack initiated at the incisal contact area and propagated gingivally. The estimated stresses at failure for veneer chipping were lower than the characteristic strength of the veneer materials. Conclusion: Within the limitations of this in-vivo study, the contact damage, fatigue, and processing flaws within the veneer are important reasons leading to chipping of veneering porcelain in anterior LDG and GIA crowns. (Quintessence Int 2014;45:369–376; doi: 10.3290/j.qi.a31536)
Key words: ceramic restoration, failure analysis, fractography
Bilayered ceramic restorations are potential substitutes for less esthetic metal-ceramic structures. Combining the strength of ceramic cores and the esthetics of veneering porcelain may generate a reliable and more esthetic restoration.1 However, the lower intrinsic stress of veneering porcelain may affect the longevity of the 1
Resident Doctor, Department of Prosthodontics, Guanghua School of Stomatology, Hospital of Stomatology, Sun Yat-sen University, Guangzhou, People’s Republic of China; and Guangdong Provincial Key Laboratory of Stomatology, Guangzhou, People’s Republic of China.
2
Professor, Faculty of Dentistry, The University of Sydney, Sydney, NSW 2010, Australia.
3
Professor, Department of Prosthodontics, Guanghua School of Stomatology, Hospital of Stomatology, Sun Yat-sen University, Guangzhou, People’s Republic of China; and Guangdong Provincial Key Laboratory of Stomatology, Guangzhou, People’s Republic of China.
Correspondence: Professor Ke Zhao, Department of Prosthodontics, Guanghua School of Stomatology, Hospital of Stomatology, Sun Yatsen University, Guangzhou 510055, China. Email: [email protected]. edu.cn
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restoration, despite the strong substrate, as the flexural resistance of a bilayered structure is dependent upon the veneering external layer of the structure.2 Cracks leading to fractures of veneering porcelain can severely compromise the esthetics and function of bilayered ceramic restorations, which has been a specific clinical problem with bilayered ceramic restorations. Chipping of veneering porcelain with zirconiabased ceramic restorations has been the research focus of many recent studies.3-7 The primary failure for glassceramic and alumina-ceramic crowns is usually caused by a radial crack, initiating from the cementation surface of the core and propagating through the entire crown, leading to bulk fracture.8 Chipping of veneering porcelain has also been detected in glass- and aluminabased ceramic crowns. A 5-year clinical evaluation showed that chipping within the porcelain was
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Table 1
A summary of clinically failed all-ceramic restorations Age (months)
Size of the critical crack (m)
Stress at failure (MPa)
Bruxism and biting with incisal edge
3.30 × 10-4
31.08
10
Biting on hard object
4.44 × 10-4
26.79
IPS Empress II/Eris
60
Biting on hard object
3.11 × 10-4
36.58
11
IPS Empress II/Eris
30
Biting with incisal edge on hard object
3.45 × 10-4
34.73
21
IPS Empress II/Eris
30
Biting with incisal edge on hard object
**
**
No.
Location*
Core/veneer material
1
11
VITA In-Ceram alumina/VM7
15
2
11
VITA In-Ceram alumina/VM7
3
11
4 5
Occlusion
*Tooth locations were recorded according to the FDI system. **The size of the critical crack for case 5 was obscured and could not be measured.
detected in three of 101 In-Ceram alumina crowns.9 A retrospective survival analysis of 261 lithium disilicate glass-ceramic (LDG) crowns showed that four crowns failed because of veneer chipping.10 Therefore, understanding the factors leading to chipping of veneering porcelain is important, not only for zirconia, but also for glass- and alumina-based ceramic crowns. Numerous reasons have been suggested for veneer chipping of zirconia-based ceramic crowns, such as areas of porosity,11 surface defects or improper support by the framework,12 overloading, and fatigue.13 However, all these reasons are based on laboratory experiments for all-ceramic structures. Laboratory testing can only offer a limited prediction of the expected in-vivo performance of tested restorations. Many intraoral variables are difficult to duplicate in vitro.14 Fractographic analysis of clinically failed all-ceramic restorations is gaining momentum for evaluating the in-vivo failure behavior of all-ceramic restorations. By examining fracture surfaces, failure mechanisms, fracture origins, and the probable causes of failure in retrieved clinical specimens can be analyzed.15-18 Characteristic fractographic markings, such as hackles, wake hackles, and arrest lines are used to trace a fracture back to the fracture origin.16,19 No current study has comprehensively evaluated the reasons for chipping of veneering porcelain in LDG and glass-infiltrated alumina (GIA) crowns. The aim of this study was to investigate the reasons for veneer
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chipping of clinically failed LDG and GIA crowns in anterior sites using fractographic analysis, which may suggest some solutions for preventing this common complication.
METHOD AND MATERIALS Three clinically failed anterior LDG crowns (IPS Empress II, Ivoclar Vivadent) and two anterior GIA crowns (InCeram, Vita) were collected. The materials and ages of the crowns, as well as the occlusal scheme of the patients, are shown in Table 1. The fragments were either provided by the patients, or successfully retrieved by the clinician without damage to the fractured surface. All of the restorations were ultrasonically cleaned with 70% ethanol, dried, and examined by stereomicroscopy (M205, Leica Microsystems) using low-power (50×) stereomagnification. A preliminary fracture surface map of the examined surfaces was drawn and used as a guide for the SEM analysis. After sputter-coating with a thin film of gold, the failure mode and characterization of the morphology, microstructure, and fractographic details of the fractured surfaces were analyzed using SEM (Quanta 200, FEI). Three observers (the current authors) examined the fractured crowns independently with various approaches, described in previous fractographic literature and referenced standards.15-18 After independent examination, all three observers discussed the findings and
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compared the SEM photos. Relevant features were identified and reexamined. The analysis started at the incisal margin of the chipped surface, moved towards the gingival section, and ended at the gingival margin of the crown. All recognizable features within the traveling path were photographed. Based on the microscopy findings, a final map was created on an overall view of the failed component, indicating the general direction of crack propagation, the area where the crack originated, and the sequence of crack events, allowing for further discussion about the possible reasons for failure. Figure 1 provides a diagram of the typical fracture surface features occurring in brittle materials.20 Crack origin is the center of the concentric semicircles, then the mirror region, the mist region, and the hackle region. Features of the fracture origin were determined, and both the depth (a) and half width (b) of the critical crack were measured. The size of the critical crack (C) was calculated using the following equation21: C = (ab)1/2 [1] The stress at failure (σf) was calculated using the following Griffin-Irwin equation21: KIC = Yσf (C1/2) [2] KIC is the fracture toughness of the material and Y is a geometric factor set at 1.24.
RESULTS The SEM images of the five failed bilayered ceramic crowns indicated that all specimens failed by cohesive failure within the veneer, with a thin layer of porcelain remaining on the framework. Crack origins of the five anterior all-ceramic crowns were found to be located at the contact area on the incisal edge. The fracture surface of the crown of case 1 was the most typical and was readily analyzed. This maxillary incisal crown failed by veneer chipping on the labial surface after only 15 months of intraoral function (Fig 2a). The main part of the crown remained on the tooth and was removed by a dentist, while the veneer chip was retrieved by the patient (Fig 2b). In Figs 2c and 2d, significant abrasion of the mandibular anterior
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Hackle Region Mist Region Mirror Region
C = (ab)1/2 Critical Flaw
a 2b
Fig 1 Diagram of the typical fracture surface features occurring in brittle materials.20
teeth could be seen, and most of the posterior teeth were restored with silver amalgam or metal-ceramic crowns. The major features were observed by SEM of the fracture surface of the veneer chip. Figure 3a shows the fracture surface with the major features highlighted. Clearly defined semicircular arrest lines were visible near the incisal contact area, indicating a slow process of damage accumulation with repeated cyclic loading (fatigue) during intraoral service. Hackle and wake hackle markings were recognizable on the concave side of the arrest lines (Fig 3b). These easily seen features provide a clear indication of the direction of crack propagation, which runs in an incisal to gingival direction. Indeed, the arrest lines are perpendicular to the crack propagation, while the origin of the crack is located on the concave side of the first arrest line, which means that it is located further up along the middle of the incisal edge (Fig 3a). Obvious contact damage and clusters of pores were detected in the veneer near the crack origin (Fig 3b). Figure 3c revealed hackles and arrest lines on the fracture surface of the veneer chip. With higher magnification, an obvious step or overlap crack was seen propagating through clusters of pores and was arrested in the veneer (Fig 3d). This overlap crack arises because the crack initiated from two independent flaws within the abrasion damage generated by repetitive contact of the teeth. Depth (a) and half width (b) of the critical crack in case 1 were further measured, as shown in Fig 3e. The KIC value of Vita VM7 porcelain was estimated to be
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a
c
Fig 3a Fracture surface of the main part of the In-Ceram alumina crown in case 1. Black arrows indicate arrest lines and white arrows indicate wake hackles (SEM, 60×).
b
2 mm
d
Fig 2a Intraoral image of the patient in case 1: an In-Ceram alumina crown (right maxillary central incisor) failed due to cohesive failure within the veneer on the labial surface. Fig 2b The main part of the crown remained on the tooth and was taken off for fractography without destroying the fractured surface (right). The veneer chip was recovered by the patient (left) (stereo, 10.4×).
Figs 2c and 2d On the occlusal surface of the dentition, serious abrasion of teeth could be seen. Most of the posterior teeth were restored with silver amalgam or metal-ceramic crowns.
Fig 3b Isolated small pores were located near the crack origin of case 1. Black arrows indicate hackles between arrest lines. Irregular edge chips due to contact damage are seen on the incisal edge (SEM, 500×).
0.33 mm 0.66 mm
Fig 3c Fracture surface of the veneer chip in case 1. Black arrows indicate arrest lines and white arrow indicates hackles (SEM, 50×).
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Fig 3d Higher magnification of the area cycled in Fig 3c. An obvious cleavage crack (arrow) is seen propagating through clusters of pores and stopping in the veneer (SEM, 500×).
Fig 3e The depth (a, 0.33 mm) and width (2b, 0.66 mm) of a critical crack were measured (SEM, 90×).
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dcp
al wh
al
origin Fig 4 Schematic image shows the crack origin and general direction of crack propagation (dcp) for case 1 (wh, wake hackles; al, arrest lines).
Fig 5a “Wake hackle map” drawn for case 3. The critical crack originated at the incisal contact area (encircled) and propagated gingivally (arrows) (SEM, 90×).
0.7 MPa m1/2.21 The calculated stress at failure (σ) was 31.08 MPa, which was lower than the characteristic strength (σ0: 74.7 MPa)21 of Vita VM7 porcelain. The schematic image showed the crack origin and general direction of crack propagation for case 1 (Fig 4). Characteristic markings were also detected in other cases. Based on the microscope findings, a final map was then created for an overall view of the failed restorations, indicating the general crack propagation direction and the area where the crack originated. Figure 5a is the “wake hackle map” drawn for case 3, showing that the critical crack originated at the incisal contact area and propagated gingivally. Contact damage and isolated pores were also detected near the crack origin (Fig 5b). The calculated stresses at failure for the other cases are listed in Table 1.
DISCUSSION The reasons leading to veneer chipping of zirconiabased ceramic crowns have been the focus of recent studies. However, no study has comprehensively evaluated the reasons for veneer chipping of clinically failed LDG and GIA crowns. This present study presented preliminary fractographic evidence for LDG and GIA crowns in anterior sites that failed due to clinical veneer chipping, which may suggest some solutions for pre-
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Fig 5b Contact damage (black arrow) and isolated pores (white arrows) were detected near the crack origin.
venting this common complication for bilayered ceramic crowns. The first important observation is that all five anterior crowns presented cohesive failures within the veneer, with residual veneering porcelain remaining on the core. Chipping of the veneering porcelain can be due to either cohesive or adhesive failure. Cohesive failure is considered a chipping fracture within the veneering material, which is always associated with a layer of porcelain that remains on the framework.2 Adhesive failure always causes complete delamination of the veneering porcelain from the core.2 The results of the present study suggest that the bond strength between the veneer and core in LDG and GIA bilayered crowns is adequate. Consequently, it may be speculated that the veneering porcelain is the weakest link; thus, improving its strength and toughness could reduce the incidence of veneer chipping. The fracture origin of all five crowns was limited to a small area at the middle of the incisal edge, where obvious contact damage could be seen. An interesting finding of the current study is that three of the five patients bit with the incisal edge, and two patients remembered biting on hard chopsticks. In these situations, the applied load is acting over a relatively small contact point at the incisal edge, instead of having the stress applied to a larger surface area at the slopes of
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Fig 6a Under normal chewing cycles, the stress was applied over a larger surface area at the slopes of the functional cusps.
Fig 6b For patients biting with the incisal edge, the applied load is present over a small contact region of the incisal edge.
the functional cusps (Fig 6). This is exactly in line with the loading conditions for the generation of localized high stress. The pre-existing damage generated by the patient associated with their known bruxing behavior coupled with the localized contact by the hard object such as a chopstick, especially if the loading of the chopstick was not uniform along the tooth, would predispose to the formation of a vertical crack. These preexisting surface cracks associated with the patient bruxing behavior are clearly seen in Fig 3b. These small cracks provide the basis whereby upon a highly localized contact-induced stress, a larger crack forms from the merging of such cracks, but with a “cleavage step” marking where the cracks overlap.22 This crack was also seen propagating through clusters of pores and arrested in the veneer of case 1 (Fig 3e). With normal chewing cycles, the applied load is spread over a larger surface area at the slopes of the functional cusps, and such cracks cannot arise in such a condition. However, one possible exception of these cases is when a patient bites on an unexpectedly hard object, resulting in localized contact loading, which could lead to the generation of localized cracks.23 It could be inferred from the worn occlusal surface that this patient had strong bruxing habits (Fig 2), which means a highly stressed crown. The occlusal force on the anterior teeth is further
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increased with most of the posterior teeth restored with silver amalgam or metal-ceramic crowns. Therefore, some clinical conclusions could be obtained. Allceramic restorations using low-toughness veneering porcelains are not recommended for bruxers or patients that bite on the incisal edge of the front teeth. Multi-point occlusal contact is important when performing the occlusal adjustment, not only for centric occlusion but also for protrusive occlusion. Dentists should also advise patients not to bite on hard objects. Crack arrest lines and hackles are the markings of fatigue damage, indicating that failure took place in stages. This progression of failure involved cycles of growth and arrest. In this case, stresses at failure were calculated using the Griffin-Irwin equation. For a certain material, fracture toughness is a constant at the same temperature and loading speed. Therefore, fracture toughness of the veneer materials (VM7, VM9) were cited from the value obtained by Borba et al,21 where the VM7 and VM9 specimens were loaded at constant stress rates in artificial saliva at 37°C to allow water hydration. The stress at failure calculated by fractography is lower than the characteristic strength of the veneering porcelain, which may be explained by orally induced stress corrosion, under the influence of fatigue and the existence of flaws greater than those present during standard in-vitro tests. Chitmongkolsuk et al24 indicated that long-term repetitive low-level loading may cause veneer chipping at stress levels lower than the strength of the material. Additionally, flaws in dental ceramics increase the likelihood of fatigue failure of restorations. The smoothness and symmetry of the pores in Fig 3d indicate that they occurred as the result of trapped gas during processing. The conventional layering technique for veneer application is techniquesensitive. Defects like pores, voids, and inclusions are easily generated during the detailed processing, including powder preparation, consolidation, sintering, and aging. Taskonak et al25 also indicated that porosity existed in the veneer due to slurry preparation and the sintering of veneer powders. Quinn et al26 reported that a single smooth spherical pore is harmless. It is a simple stress concentrator with locally enhanced stress that is
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only two-times greater than the nominal stress; however, if a pore is next to a free surface or another pore, the stress concentration can be much greater and a crack can occur. The situation is even worse if the pores are located at the surface or near each other. In this situation, a sharp crack may easily occur. In the current cases, clusters of pores were detected close to the fracture origin, and these were in close proximity to each other or even touched each other (Fig 3d). Low-level loading with long-term repetition may cause stress concentrated at the pores, such that when a patient bit on an unexpectedly hard object, a cone crack could easily occur, leading to bulk chipping of the veneering materials. All of the details above indicate that flaws are easily generated in the veneer or core-veneer interface during the slurry preparation and sintering process of the layering technique for fabricating the veneer. Clinicians should know that the strength of ceramic materials provided by manufactures may not be relevant for ceramic restorations in the oral environment, as processing flaws, which act as stress concentration sites, and fatigue may largely compromise the strength of ceramic restorations. Recently, pressing technology has been recommended for veneered zirconia frameworks. The advantages of this system are simplicity, speed, and a lack of defects during manufacturing, while the application of the lost wax method provides special anatomical characterization, which is difficult to achieve using the layering technique.27 In addition, using higher toughness computer-aided design/computer-assisted manufacture (CAD/CAM) lithium disilicate veneers for restorations with a zirconia framework is considered to be a promising method of reducing the chipping rate of zirconia based bilayered restorations.28,29 However, these techniques have only been used for zirconiabased bilayered restorations in the laboratory, and further research is needed to apply these new veneer application techniques to other ceramic core materials.
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CONCLUSION Within the limitations of this in-vivo study, contact damage, fatigue, and processing flaws within the veneers of anterior lithium disilicate glass-ceramic and glass-infiltrated alumina crowns, are the principal causes for chipping of the veneering porcelain.
ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China under grant No. 81271175. We would like to thank the doctors from the Department of Prosthodontics, Hospital of Stomatology, Sun Yat-sen University who collected the clinically failed all-ceramic restorations, without which this study would not have been possible.
REFERENCES 1. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Part II: Zirconia veneering ceramics. Dent Mater 2006;22:857–863. 2. Zhao K, Pan Y, Guess PC, Zhang XP, Swain MV. Influence of veneer application on fracture behavior of lithium-disilicate-based ceramic crowns. Dent Mater 2012;28:653–660. 3. Vult von Steyern P, Carlson P, Nilner K. All-ceramic fixed partial dentures designed according to the DC-Zirkon technique: a 2-year clinical study. J Oral Rehabil 2005;32:180–187. 4. Raigrodski AJ, Chiche GJ, Potiket N, et al. The efficacy of posterior three-unit zirconium-oxide-based ceramic fixed partial dental prostheses: a prospective clinical pilot study. J Prosthet Dent 2006;96:237–244. 5. Tinschert J, Natt G, Mohrbotter N, Spiekermann H, Schulze KA. Lifetime of alumina- and zirconia ceramics used for crown and bridge restorations. J Biomed Mater Res B Appl Biomater 2007;80:317–321. 6. Sailer I, Feher A, Filse F. Prospective clinical study of zirconia posterior fixed partial dentures: 3-year follow up. Quintessence Int 2006;37:685–693. 7. Ortorp A, Kihl ML, Carlsson GE. A 5-year retrospective study of survival of zirconia single crowns fitted in a private clinical setting. J Dent 2012;40:527–530. 8. Rekow ED, Silva NR, Coelho PG, Zhang Y, Guess PC, Thompson VP. Performance of dental ceramics: challenges for improvements. J Dent Res 2011;90:937–952. 9. Kokubo Y, Tsumita M, Sakurai S, Suzuki Y, Tokiniwa Y, Fukushima S. Five-year clinical evaluation of In-Ceram crowns fabricated using GN-I (CAD/CAM) system. J Oral Rehabil 2011;38:601–607. 10. Valenti M, Valenti A. Retrospective survival analysis of 261 lithium disilicate crowns in a private general practice. Quintessence Int 2009;40:573–579. 11. Ohlmann B, Rammelsberg P, Schmitter M, Schwarz S, Gabbert O. All-ceramic inlay-retained fixed partial dentures: preliminary results from a clinical study. J Dent 2008;36:692–696. 12. Marchack BW, Futatsuki Y, Marchack CB, White SN. Customization of milled zirconia copings for all-ceramic crowns: a clinical report. J Prosthet Dent 2008;99:169–173. 13. Coelho PG, Silva NR, Bonfante EA, Guess PC, Rekow ED, Thompson VP. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dent Mater 2009;25:1122–1127. 14. Aboushelib MN, Feilzer AJ, Kleverlaan CJ. Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach. Dent Mater 2009;25:383–391.
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15. Lohbauer U, Amberger G, Quinn GD, Scherrer SS. Fractographic analysis of a dental zirconia framework: a case study on design issues. J Mech Behav Biomed Mater 2010;3:623–629. 16. Quinn JB, Quinn GD, Kelly JR, Scherrer SS. Fractographic analyses of three ceramic whole crown restoration failures. Dent Mater 2005;2:920–929. 17. Taskonak B, Mecholsky JJ, Anusavice KJ. Fracture surface analysis of clinically failed fixed partial dentures. J Dent Res 2006;85:277–281. 18. Scherrer SS, Quinn JB, Quinn GD, Wiskott HW. Fractographic ceramic failure analysis using the replica technique. Dent Mater 2007;23:1397–1404. 19. Scherrer SS, Quinn GD, Quinn JB. Fractographic failure analysis of a Procera (R) AllCeram crown using stereo and scanning electron microscopy. Dent Mater 2008;24:1107–1113. 20. Mecholsky JJ Jr. Quantitative fracture surface analysis of glass materials. In: Simmons CJ, El-Bayoumi O (eds). Experimental Techniques of Glass Science. Westerville, OH: American Ceramic Society 1993:483–520. 21. Borba M, de Araújo MD, Fukushima KA, et al. Effect of the microstructure on the lifetime of dental ceramics. Dent Mater 2011;27:710–721. 22. Swain MV, Lawn BR, Burns SJ. Cleavage step deformation in brittle solids. J Mater Sci 1974;9:175–183.
23. Aboushelib MN, de Jager N, Kleverlaan CJ, Feilzer AJ. Effect of loading method on the fracture mechanics of two layered all-ceramic restorative systems. Dent Mater 2007;23:952–959. 24. Chitmongkolsuk S, Heydecke G, Stappert C, Strub JR. Fracture strength of allceramic lithium disilicate and porcelain-fused-to-metal bridges for molar replacement after dynamic loading. Eur J Prosthodont Restor Dent 2002;10:15–22. 25. Taskonak B, Mecholsky JJ, Anusavice KJ. Fracture surface analysis of clinically failed fixed partial dentures. J Dent Res 2006;85:277–281. 26. Quinn GD, Hoffman K, Quinn JB. Strength and fracture origins of a feldspathic porcelain. Dent Mater 2012;28:502–511. 27. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Part 3: double veneer technique. J Prosthodont 2008;17:9–13. 28. Beuer F, Schweiger J, Eichberger M, Kappert HF, Gernet W, Edelhoff D. Highstrength CAD/CAM-fabricated veneering material sintered to zirconia copings: a new fabrication mode for all-ceramic restorations. Dent Mater 2009;25:121–128. 29. Schmitter M, Mueller D, Rues S. Chipping behaviour of all-ceramic crowns with zirconia framework and CAD/CAM manufactured veneer. J Dent 2012;40:154–162.
RESTORATIVE DENTISTRY
Qian Du
Fractographic analysis of anterior bilayered ceramic crowns that failed by veneer chipping Qian Du, DDS, MD1/Michael V. Swain, BSc, PhD2/Ke Zhao, DDS, PhD3 Objective: To fractographically analyze the reasons for the chipping of veneering porcelain in clinically failed anterior lithium disilicate glass-ceramic (LDG) and glass-infiltrated alumina (GIA) crowns. Method and Materials: Five anterior bilayered ceramic crowns with clinical veneer chipping failure were retrieved, of which three were LDG crowns and two were GIA crowns. The fractured surfaces of the failed restorations were examined using stereomicroscopy and scanning electron microscopy (SEM). The principles of fractography were used to identify the location and dimensions of the critical crack and to estimate the stress at failure. Results: All five anterior crowns
failed by cohesive failure within the veneer on the labial surface. Fractography showed that the critical crack initiated at the incisal contact area and propagated gingivally. The estimated stresses at failure for veneer chipping were lower than the characteristic strength of the veneer materials. Conclusion: Within the limitations of this in-vivo study, the contact damage, fatigue, and processing flaws within the veneer are important reasons leading to chipping of veneering porcelain in anterior LDG and GIA crowns. (Quintessence Int 2014;45:369–376; doi: 10.3290/j.qi.a31536)
Key words: ceramic restoration, failure analysis, fractography
Bilayered ceramic restorations are potential substitutes for less esthetic metal-ceramic structures. Combining the strength of ceramic cores and the esthetics of veneering porcelain may generate a reliable and more esthetic restoration.1 However, the lower intrinsic stress of veneering porcelain may affect the longevity of the 1
Resident Doctor, Department of Prosthodontics, Guanghua School of Stomatology, Hospital of Stomatology, Sun Yat-sen University, Guangzhou, People’s Republic of China; and Guangdong Provincial Key Laboratory of Stomatology, Guangzhou, People’s Republic of China.
2
Professor, Faculty of Dentistry, The University of Sydney, Sydney, NSW 2010, Australia.
3
Professor, Department of Prosthodontics, Guanghua School of Stomatology, Hospital of Stomatology, Sun Yat-sen University, Guangzhou, People’s Republic of China; and Guangdong Provincial Key Laboratory of Stomatology, Guangzhou, People’s Republic of China.
Correspondence: Professor Ke Zhao, Department of Prosthodontics, Guanghua School of Stomatology, Hospital of Stomatology, Sun Yatsen University, Guangzhou 510055, China. Email: [email protected]. edu.cn
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restoration, despite the strong substrate, as the flexural resistance of a bilayered structure is dependent upon the veneering external layer of the structure.2 Cracks leading to fractures of veneering porcelain can severely compromise the esthetics and function of bilayered ceramic restorations, which has been a specific clinical problem with bilayered ceramic restorations. Chipping of veneering porcelain with zirconiabased ceramic restorations has been the research focus of many recent studies.3-7 The primary failure for glassceramic and alumina-ceramic crowns is usually caused by a radial crack, initiating from the cementation surface of the core and propagating through the entire crown, leading to bulk fracture.8 Chipping of veneering porcelain has also been detected in glass- and aluminabased ceramic crowns. A 5-year clinical evaluation showed that chipping within the porcelain was
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Table 1
A summary of clinically failed all-ceramic restorations Age (months)
Size of the critical crack (m)
Stress at failure (MPa)
Bruxism and biting with incisal edge
3.30 × 10-4
31.08
10
Biting on hard object
4.44 × 10-4
26.79
IPS Empress II/Eris
60
Biting on hard object
3.11 × 10-4
36.58
11
IPS Empress II/Eris
30
Biting with incisal edge on hard object
3.45 × 10-4
34.73
21
IPS Empress II/Eris
30
Biting with incisal edge on hard object
**
**
No.
Location*
Core/veneer material
1
11
VITA In-Ceram alumina/VM7
15
2
11
VITA In-Ceram alumina/VM7
3
11
4 5
Occlusion
*Tooth locations were recorded according to the FDI system. **The size of the critical crack for case 5 was obscured and could not be measured.
detected in three of 101 In-Ceram alumina crowns.9 A retrospective survival analysis of 261 lithium disilicate glass-ceramic (LDG) crowns showed that four crowns failed because of veneer chipping.10 Therefore, understanding the factors leading to chipping of veneering porcelain is important, not only for zirconia, but also for glass- and alumina-based ceramic crowns. Numerous reasons have been suggested for veneer chipping of zirconia-based ceramic crowns, such as areas of porosity,11 surface defects or improper support by the framework,12 overloading, and fatigue.13 However, all these reasons are based on laboratory experiments for all-ceramic structures. Laboratory testing can only offer a limited prediction of the expected in-vivo performance of tested restorations. Many intraoral variables are difficult to duplicate in vitro.14 Fractographic analysis of clinically failed all-ceramic restorations is gaining momentum for evaluating the in-vivo failure behavior of all-ceramic restorations. By examining fracture surfaces, failure mechanisms, fracture origins, and the probable causes of failure in retrieved clinical specimens can be analyzed.15-18 Characteristic fractographic markings, such as hackles, wake hackles, and arrest lines are used to trace a fracture back to the fracture origin.16,19 No current study has comprehensively evaluated the reasons for chipping of veneering porcelain in LDG and glass-infiltrated alumina (GIA) crowns. The aim of this study was to investigate the reasons for veneer
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chipping of clinically failed LDG and GIA crowns in anterior sites using fractographic analysis, which may suggest some solutions for preventing this common complication.
METHOD AND MATERIALS Three clinically failed anterior LDG crowns (IPS Empress II, Ivoclar Vivadent) and two anterior GIA crowns (InCeram, Vita) were collected. The materials and ages of the crowns, as well as the occlusal scheme of the patients, are shown in Table 1. The fragments were either provided by the patients, or successfully retrieved by the clinician without damage to the fractured surface. All of the restorations were ultrasonically cleaned with 70% ethanol, dried, and examined by stereomicroscopy (M205, Leica Microsystems) using low-power (50×) stereomagnification. A preliminary fracture surface map of the examined surfaces was drawn and used as a guide for the SEM analysis. After sputter-coating with a thin film of gold, the failure mode and characterization of the morphology, microstructure, and fractographic details of the fractured surfaces were analyzed using SEM (Quanta 200, FEI). Three observers (the current authors) examined the fractured crowns independently with various approaches, described in previous fractographic literature and referenced standards.15-18 After independent examination, all three observers discussed the findings and
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compared the SEM photos. Relevant features were identified and reexamined. The analysis started at the incisal margin of the chipped surface, moved towards the gingival section, and ended at the gingival margin of the crown. All recognizable features within the traveling path were photographed. Based on the microscopy findings, a final map was created on an overall view of the failed component, indicating the general direction of crack propagation, the area where the crack originated, and the sequence of crack events, allowing for further discussion about the possible reasons for failure. Figure 1 provides a diagram of the typical fracture surface features occurring in brittle materials.20 Crack origin is the center of the concentric semicircles, then the mirror region, the mist region, and the hackle region. Features of the fracture origin were determined, and both the depth (a) and half width (b) of the critical crack were measured. The size of the critical crack (C) was calculated using the following equation21: C = (ab)1/2 [1] The stress at failure (σf) was calculated using the following Griffin-Irwin equation21: KIC = Yσf (C1/2) [2] KIC is the fracture toughness of the material and Y is a geometric factor set at 1.24.
RESULTS The SEM images of the five failed bilayered ceramic crowns indicated that all specimens failed by cohesive failure within the veneer, with a thin layer of porcelain remaining on the framework. Crack origins of the five anterior all-ceramic crowns were found to be located at the contact area on the incisal edge. The fracture surface of the crown of case 1 was the most typical and was readily analyzed. This maxillary incisal crown failed by veneer chipping on the labial surface after only 15 months of intraoral function (Fig 2a). The main part of the crown remained on the tooth and was removed by a dentist, while the veneer chip was retrieved by the patient (Fig 2b). In Figs 2c and 2d, significant abrasion of the mandibular anterior
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Hackle Region Mist Region Mirror Region
C = (ab)1/2 Critical Flaw
a 2b
Fig 1 Diagram of the typical fracture surface features occurring in brittle materials.20
teeth could be seen, and most of the posterior teeth were restored with silver amalgam or metal-ceramic crowns. The major features were observed by SEM of the fracture surface of the veneer chip. Figure 3a shows the fracture surface with the major features highlighted. Clearly defined semicircular arrest lines were visible near the incisal contact area, indicating a slow process of damage accumulation with repeated cyclic loading (fatigue) during intraoral service. Hackle and wake hackle markings were recognizable on the concave side of the arrest lines (Fig 3b). These easily seen features provide a clear indication of the direction of crack propagation, which runs in an incisal to gingival direction. Indeed, the arrest lines are perpendicular to the crack propagation, while the origin of the crack is located on the concave side of the first arrest line, which means that it is located further up along the middle of the incisal edge (Fig 3a). Obvious contact damage and clusters of pores were detected in the veneer near the crack origin (Fig 3b). Figure 3c revealed hackles and arrest lines on the fracture surface of the veneer chip. With higher magnification, an obvious step or overlap crack was seen propagating through clusters of pores and was arrested in the veneer (Fig 3d). This overlap crack arises because the crack initiated from two independent flaws within the abrasion damage generated by repetitive contact of the teeth. Depth (a) and half width (b) of the critical crack in case 1 were further measured, as shown in Fig 3e. The KIC value of Vita VM7 porcelain was estimated to be
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a
c
Fig 3a Fracture surface of the main part of the In-Ceram alumina crown in case 1. Black arrows indicate arrest lines and white arrows indicate wake hackles (SEM, 60×).
b
2 mm
d
Fig 2a Intraoral image of the patient in case 1: an In-Ceram alumina crown (right maxillary central incisor) failed due to cohesive failure within the veneer on the labial surface. Fig 2b The main part of the crown remained on the tooth and was taken off for fractography without destroying the fractured surface (right). The veneer chip was recovered by the patient (left) (stereo, 10.4×).
Figs 2c and 2d On the occlusal surface of the dentition, serious abrasion of teeth could be seen. Most of the posterior teeth were restored with silver amalgam or metal-ceramic crowns.
Fig 3b Isolated small pores were located near the crack origin of case 1. Black arrows indicate hackles between arrest lines. Irregular edge chips due to contact damage are seen on the incisal edge (SEM, 500×).
0.33 mm 0.66 mm
Fig 3c Fracture surface of the veneer chip in case 1. Black arrows indicate arrest lines and white arrow indicates hackles (SEM, 50×).
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Fig 3d Higher magnification of the area cycled in Fig 3c. An obvious cleavage crack (arrow) is seen propagating through clusters of pores and stopping in the veneer (SEM, 500×).
Fig 3e The depth (a, 0.33 mm) and width (2b, 0.66 mm) of a critical crack were measured (SEM, 90×).
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dcp
al wh
al
origin Fig 4 Schematic image shows the crack origin and general direction of crack propagation (dcp) for case 1 (wh, wake hackles; al, arrest lines).
Fig 5a “Wake hackle map” drawn for case 3. The critical crack originated at the incisal contact area (encircled) and propagated gingivally (arrows) (SEM, 90×).
0.7 MPa m1/2.21 The calculated stress at failure (σ) was 31.08 MPa, which was lower than the characteristic strength (σ0: 74.7 MPa)21 of Vita VM7 porcelain. The schematic image showed the crack origin and general direction of crack propagation for case 1 (Fig 4). Characteristic markings were also detected in other cases. Based on the microscope findings, a final map was then created for an overall view of the failed restorations, indicating the general crack propagation direction and the area where the crack originated. Figure 5a is the “wake hackle map” drawn for case 3, showing that the critical crack originated at the incisal contact area and propagated gingivally. Contact damage and isolated pores were also detected near the crack origin (Fig 5b). The calculated stresses at failure for the other cases are listed in Table 1.
DISCUSSION The reasons leading to veneer chipping of zirconiabased ceramic crowns have been the focus of recent studies. However, no study has comprehensively evaluated the reasons for veneer chipping of clinically failed LDG and GIA crowns. This present study presented preliminary fractographic evidence for LDG and GIA crowns in anterior sites that failed due to clinical veneer chipping, which may suggest some solutions for pre-
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Fig 5b Contact damage (black arrow) and isolated pores (white arrows) were detected near the crack origin.
venting this common complication for bilayered ceramic crowns. The first important observation is that all five anterior crowns presented cohesive failures within the veneer, with residual veneering porcelain remaining on the core. Chipping of the veneering porcelain can be due to either cohesive or adhesive failure. Cohesive failure is considered a chipping fracture within the veneering material, which is always associated with a layer of porcelain that remains on the framework.2 Adhesive failure always causes complete delamination of the veneering porcelain from the core.2 The results of the present study suggest that the bond strength between the veneer and core in LDG and GIA bilayered crowns is adequate. Consequently, it may be speculated that the veneering porcelain is the weakest link; thus, improving its strength and toughness could reduce the incidence of veneer chipping. The fracture origin of all five crowns was limited to a small area at the middle of the incisal edge, where obvious contact damage could be seen. An interesting finding of the current study is that three of the five patients bit with the incisal edge, and two patients remembered biting on hard chopsticks. In these situations, the applied load is acting over a relatively small contact point at the incisal edge, instead of having the stress applied to a larger surface area at the slopes of
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Fig 6a Under normal chewing cycles, the stress was applied over a larger surface area at the slopes of the functional cusps.
Fig 6b For patients biting with the incisal edge, the applied load is present over a small contact region of the incisal edge.
the functional cusps (Fig 6). This is exactly in line with the loading conditions for the generation of localized high stress. The pre-existing damage generated by the patient associated with their known bruxing behavior coupled with the localized contact by the hard object such as a chopstick, especially if the loading of the chopstick was not uniform along the tooth, would predispose to the formation of a vertical crack. These preexisting surface cracks associated with the patient bruxing behavior are clearly seen in Fig 3b. These small cracks provide the basis whereby upon a highly localized contact-induced stress, a larger crack forms from the merging of such cracks, but with a “cleavage step” marking where the cracks overlap.22 This crack was also seen propagating through clusters of pores and arrested in the veneer of case 1 (Fig 3e). With normal chewing cycles, the applied load is spread over a larger surface area at the slopes of the functional cusps, and such cracks cannot arise in such a condition. However, one possible exception of these cases is when a patient bites on an unexpectedly hard object, resulting in localized contact loading, which could lead to the generation of localized cracks.23 It could be inferred from the worn occlusal surface that this patient had strong bruxing habits (Fig 2), which means a highly stressed crown. The occlusal force on the anterior teeth is further
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increased with most of the posterior teeth restored with silver amalgam or metal-ceramic crowns. Therefore, some clinical conclusions could be obtained. Allceramic restorations using low-toughness veneering porcelains are not recommended for bruxers or patients that bite on the incisal edge of the front teeth. Multi-point occlusal contact is important when performing the occlusal adjustment, not only for centric occlusion but also for protrusive occlusion. Dentists should also advise patients not to bite on hard objects. Crack arrest lines and hackles are the markings of fatigue damage, indicating that failure took place in stages. This progression of failure involved cycles of growth and arrest. In this case, stresses at failure were calculated using the Griffin-Irwin equation. For a certain material, fracture toughness is a constant at the same temperature and loading speed. Therefore, fracture toughness of the veneer materials (VM7, VM9) were cited from the value obtained by Borba et al,21 where the VM7 and VM9 specimens were loaded at constant stress rates in artificial saliva at 37°C to allow water hydration. The stress at failure calculated by fractography is lower than the characteristic strength of the veneering porcelain, which may be explained by orally induced stress corrosion, under the influence of fatigue and the existence of flaws greater than those present during standard in-vitro tests. Chitmongkolsuk et al24 indicated that long-term repetitive low-level loading may cause veneer chipping at stress levels lower than the strength of the material. Additionally, flaws in dental ceramics increase the likelihood of fatigue failure of restorations. The smoothness and symmetry of the pores in Fig 3d indicate that they occurred as the result of trapped gas during processing. The conventional layering technique for veneer application is techniquesensitive. Defects like pores, voids, and inclusions are easily generated during the detailed processing, including powder preparation, consolidation, sintering, and aging. Taskonak et al25 also indicated that porosity existed in the veneer due to slurry preparation and the sintering of veneer powders. Quinn et al26 reported that a single smooth spherical pore is harmless. It is a simple stress concentrator with locally enhanced stress that is
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only two-times greater than the nominal stress; however, if a pore is next to a free surface or another pore, the stress concentration can be much greater and a crack can occur. The situation is even worse if the pores are located at the surface or near each other. In this situation, a sharp crack may easily occur. In the current cases, clusters of pores were detected close to the fracture origin, and these were in close proximity to each other or even touched each other (Fig 3d). Low-level loading with long-term repetition may cause stress concentrated at the pores, such that when a patient bit on an unexpectedly hard object, a cone crack could easily occur, leading to bulk chipping of the veneering materials. All of the details above indicate that flaws are easily generated in the veneer or core-veneer interface during the slurry preparation and sintering process of the layering technique for fabricating the veneer. Clinicians should know that the strength of ceramic materials provided by manufactures may not be relevant for ceramic restorations in the oral environment, as processing flaws, which act as stress concentration sites, and fatigue may largely compromise the strength of ceramic restorations. Recently, pressing technology has been recommended for veneered zirconia frameworks. The advantages of this system are simplicity, speed, and a lack of defects during manufacturing, while the application of the lost wax method provides special anatomical characterization, which is difficult to achieve using the layering technique.27 In addition, using higher toughness computer-aided design/computer-assisted manufacture (CAD/CAM) lithium disilicate veneers for restorations with a zirconia framework is considered to be a promising method of reducing the chipping rate of zirconia based bilayered restorations.28,29 However, these techniques have only been used for zirconiabased bilayered restorations in the laboratory, and further research is needed to apply these new veneer application techniques to other ceramic core materials.
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CONCLUSION Within the limitations of this in-vivo study, contact damage, fatigue, and processing flaws within the veneers of anterior lithium disilicate glass-ceramic and glass-infiltrated alumina crowns, are the principal causes for chipping of the veneering porcelain.
ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China under grant No. 81271175. We would like to thank the doctors from the Department of Prosthodontics, Hospital of Stomatology, Sun Yat-sen University who collected the clinically failed all-ceramic restorations, without which this study would not have been possible.
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