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J Am Dent Assoc, Vol 137, No 4, 514-522.
© 2006 American Dental Association
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RESEARCH

Fracture resistance of different partial-coverage ceramic molar restorations

An in vitro investigation



Christian F.J. Stappert, DDS, Dr med dent, Wael Att, DDS, Dr med dent, Thomas Gerds, Dr rer nat and Joerg R. Strub, DDS, Dr med dent, PhD


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Background. The authors conducted a study to evaluate the influence of preparation design on reliability and fracture resistance of press-ceramic posterior partial-coverage restorations (PCRs) under fatigue. They compared the results for PCRs fabricated of a new press ceramic (IPS e.max Press-VP 1989/4, Ivoclar-Vivadent, Schaan, Liechtenstein) with results for ceramic inlays and unprepared molars.

Methods. The authors randomly divided 96 human upper molars into six equal groups. Control group NP specimens remained unprepared. Control group IN specimens received a mesioocclusal-distal (MOD) inlay preparation. The test groups received PCR preparation designs based on group IN’s inlay design, with additional cuspal reduction that increased from group to group. The authors fabricated 16 ceramic inlays and 64 PCRs of IPS e.max Press and luted them adhesively. All specimens underwent masticatory fatigue loading (1.2 million cycles, 1.6 hertz, 98 newtons), 5,300 thermal cycles and observation for fracture patterns. Afterward, the authors loaded all surviving specimens until fracture.

Results. No fractures occurred during the exposure to the masticatory simulation. After undergoing loading in a universal testing machine, the groups showed no significant differences in fracture strength values (P = .6026). Thus, the different preparation designs of the PCRs demonstrated no significant influence on the restorations’ fracture resistance. The median failure loads ranged from 1,567 to 1,960 newtons.

Conclusion. All-ceramic PCRs for molars made of IPS e.max Press were shown to be fracture-resistant, results comparable with those of natural unprepared teeth.

Clinical Implications. When a posterior ceramic PCR is indicated, the clinician should perform a defect-oriented preparation that preserves tooth structure. Further clinical investigations are recommended to verify the authors’ in vitro results.

Key Words: Onlay; partial-coverage restoration; ceramic restoration; fracture strength; fatigue

Dental caries is one of the most common diseases; approximately 80 percent of the population in developed countries has experienced the condition.1,2 Researchers have investigated all-ceramic materials and suggested that, owing to their preferred optical and biological properties, they be used to restore the lost tooth structure.3 The increasing demands of patients for more esthetic restorations and the improvement in the physical properties of all-ceramic materials have made it possible to use them for fabrication of posterior restorations and potentially have led to a less invasive tooth preparation.

Many advantages can be gained by preserving as much of the healthy tooth structure as possible. This can be achieved best by using a defect-oriented tooth preparation. The general benefits of using inlay and onlay preparations are as follows:

– preserving healthy tooth structure3;
– facilitating superior periodontal health3–5;
– facilitating cementation without hydrolytic behavior6–8;
– preserving the pulp’s health3,9,10;
– preserving the tooth’s anatomical shape3,11;
– facilitating visual margin control3,12,13;
– facilitating easier performance of oral hygiene for the patient12,14,15;
– improving the reliability of tooth vitality testing.9,16–18

Long-term clinical studies with observation periods of up to 12 years showed that the survival rates of ceramic inlays and onlays range between 74 percent and 100 percent.3,13 The longevity of ceramic inlays and onlays depends on many factors (BoxGo).3,9,13,19 The literature, however, contains little information about the effect of preparation design on the reliability of ceramic inlays or partial-coverage restorations (PCRs).


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  		BOX Factors influencing the longevity of all-ceramic restorations.

 
In the last two decades, several new all-ceramic systems that offer good esthetics and simplified fabrication procedures have been introduced.12 Heat-pressing is a process that has been developed to overcome the inhomogeneities and porosity that occur during ceramming.20,21 The first heat-press ceramic material, IPS Empress (Ivoclar-Vivadent, Schaan, Liechtenstein), is a type of leucite-reinforced glass ceramic and has a flexural strength of 182 megapascals.21 The material is designed for the fabrication of inlays, onlays and veneers.20 Survival rates for IPS Empress inlays and onlays range from 96 percent at 4.5 years to 92 percent at eight years.22–26 Most failures were caused by bulk fracture. IPS Empress 2 (Ivoclar-Vivadent) is a lithium disilicate (2SiO2-Li2O) glass ceramic that does not bear any resemblance to the earlier leucite glass ceramic. The material has an average fracture strength of 350 MPa.27 Therefore, it is indicated not only for anterior three-unit fixed partial dentures, but also for restorations in the posterior region, which may include a first premolar as a pontic. However, no data are available regarding the performance of IPS Empress 2 inlays and onlays.

In 2005, an improved press ceramic material called IPS e.max Press (Ivoclar-Vivadent) was introduced for these indications. The IPS e.max Press material consists of a lithium disilicate pressed glass ceramic. The chemical basis of the material is the same as the chemical basis of IPS Empress 2 (2SiO2-Li2O), but properties are changed by a different firing process. Also, the framework can be veneered with a new type of sintered fluoroapatite porcelain. In comparison with IPS Empress 2, the two glass ceramic materials exhibit substantially improved physical properties and greater translucency.28

We conducted a study to evaluate the effect of preparation design on the longevity and fracture strength of ceramic PCRs fabricated of IPS e.max Press under fatigue in a mastication simulator, and to compare the data with those for ceramic inlays (IPS e.max Press) and unprepared teeth.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
We selected 96 caries-free human maxillary molars and cleaned them by scaling and stored them in 0.1 percent thymol solution at room temperature. For this investigation, we used only teeth that had been examined visually with a x 10 magnifying glass and had been found to be free from hypoplastic defects and cracks. After extraction, we stored the teeth no longer than three months. We randomly allocated the teeth to two control groups, no preparation (NP) and inlay preparation (IN) and four test groups of 16 samples each. The test groups (1C, 2C, 3C and 4C) received PCR preparation designs based on group IN’s inlay design with additional cuspal reduction that varied from group to group. We made sample holders for groups IN, 1C, 2C, 3C and 4C from small blocks of dental stone (GC FujiRock EP, type 4 dental stone, GC Europe, Leuven, Belgium) for the fabrication of models of the teeth before preparation.

We fixed two teeth inside each block using Formasil Xact (Heraeus Kulzer GmbH, Wehrheim, Germany). We numbered the teeth and carved the numbers on the blocks. Afterward, we took double-mixing technique impressions from the blocks containing the teeth using Dimension Garant L (3M ESPE, Seefeld, Germany), Permagum Putty Soft (3M ESPE) and perforated custom-made plastic trays (Minitrays, Hager & Werken GmbH, Duisburg, Germany). We then removed the impressions from the teeth and, one hour later, poured the master models using GC Fujirock EP type 4 dental stone (GC Europe). The models that we made before preparation were helpful for the fabrication of the final ceramic restorations to restore the original shape of the tooth.

Preparation of teeth. One clinician (W.A.) prepared teeth in groups IN, 1C, 2C, 3C and 4C according to a standardized preparation protocol, using a high-speed handpiece and diamond burs under water cooling. He conducted the primary preparation with 80-micrometer-grit preparation diamonds (837KR.314.012, 847KR.314.016, Komet Dental, Gebr. Brasseler, Lemgo, Germany) and carried out the finishing with finer diamonds (30- to 40-µm grain size, 8837KR.314.012, 8847KR.314.016, 8390.204.016, Komet Dental). The clinician prepared a 6-degree taper box form with rounded and soft internal line angles to represent a mesial-occlusal-distal (MOD) preparation for the inlay group IN, with a depth of 3 millimeters and an isthmus of 3 mm in width. The mesial and distal finishing line was 1 mm above the cementoenamel junction (Figure 1Go). The clinician prepared the teeth in the other groups in the same manner as those in group IN with the additional reduction of the mesiopalatal cusp for group 1C (Figure 2Go), both palatal cusps for group 2C (Figure 3Go), both palatal and distobuccal cusps for group 3C (Figure 4Go) and all cusps for group 4C (Figure 5Go). The clinician reduced the cusps by 2 mm with an angle of 45 degrees on the occlusal plane. He left the margins in an overlapping form without a bevel (Figures 2Go–5GoGoGo). All the margins of the different preparation groups remained in enamel.


Figure 1
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  		Figure 1. Preparation of group IN (teeth with mesio-occlusal-distal inlay preparation).

 

Figure 2
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  		Figure 2. Preparation of group 1C (teeth with mesio-occlusal-distal inlay preparation with reduction of the mesiopalatal cusp).

 

Figure 3
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  		Figure 3. Preparation of group 2C (teeth with mesio-occlusal-distal inlay preparation with reduction of both palatal cusps).

 

Figure 4
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  		Figure 4. Preparation of group 3C (teeth with mesio-occlusal-distal inlay preparation with reduction of both palatal and distobuccal cusps).

 

Figure 5
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  		Figure 5. Preparation of group 4C (teeth with mesio-occlusal-distal inlay preparation with reduction of all cusps).

 
Fabrication of the ceramic inlays and onlays. We took impressions of the prepared teeth using the same impression materials and technique. After setting, we removed the impressions and, one hour later, poured the master models using dental stone (GC Fujirock type 4). We sent all models except those in group NP, before and after preparation, to the manufacturer (Ivoclar-Vivadent) for fabrication of the ceramic inlays and PCRs. By the use of die hardener (Margidur, DUS Dental-U, Richmond, British Columbia), the dental technician strengthened critical line angles along the preparation margin and enhanced their mechanical resistance. He applied die spacer (Purargent 20 milliliters, DUS Dental-U) to the cavity surfaces (approximately 10 µm) at a distance of 1.5 mm from the marginal areas. First, before preparation, the dental technician made full wax-ups of the inlays and the PCRs as a guide, using the silicone keys from the models. Then, he attached a 3-mm round wax sprue to each wax-up at an angle of approximately 45 degrees. We invested wax patterns in Empress 2 Speed investment material (Ivoclar Vivadent). The preheating cycle was carried out in a preheating furnace (Type 5636, KaVo Dental GmbH, Biberach, Germany), at a temperature of 850 C and with a holding time of 60 minutes. Subsequently, we transferred the molds to the EP 500/V2.9 ceramic furnace (Ivoclar Vivadent) and pressure-filled them with IPS e.max Press VP1989/4 ingot material, using a temperature of 915 C and a holding time of 20 minutes. After divestment and separation of the restoration, we carried out two glaze-firing cycles in the Programat P100 furnace. For this purpose, we applied and fired C27688 Empress 2 glazing material (Ivoclar Vivadent), using a firing temperature of 770 (± a standard deviation of 10 C) and a holding time of six minutes. After fabricating the restorations, we placed them on the prepared teeth to check the fit using a Bite Checker (GC Europe, Leuven, Belgium). We removed interferences on the internal aspects of the ceramic restorations under water cooling, using a high-speed angled handpiece and fine-grain diamonds. Finally, we sandblasted the fitting surfaces of the restorations with Al2O3 (type 100) at 1 bar pressure.

Cementation of the restorations. We etched all test teeth (enamel for 60 seconds, dentin for 15 seconds) with 37 percent phosphoric acid (Total Etch, Ivoclar Vivadent) and conditioned them with Syntac Primer (Ivoclar Vivadent) for 15 seconds, Syntac Adhesive for 10 seconds and Heliobond (Ivoclar Vivadent). We etched the inner surfaces of the 2SiO2-Li2O ceramic restorations with 4.9 percent hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar-Vivadent) for 20 seconds and then silanized them with Monobond S (Ivoclar-Vivadent). We luted the ceramic inlays and onlays using Variolink II dual-polymerizing resin composite (Ivoclar-Vivadent). We light-polymerized the specimens with a light intensity of at least 650 milliwatts/square centimeter (Elipar FreeLight 2, 3M ESPE) in increments from the buccal to the cervical and palatal aspects for at least 40 seconds for each increment. After polymerizing, the excess resin composite was removed with a scalpel (BD Bard-Parker Special Surgeon’s Blades no. 15C scalpel, Becton-Dickinson, Franklin Lakes, N.J.) and fine polishing discs (Sof-Lex, 3M ESPE). Additionally, we cleaned all specimens using a nonfluoridated prophylaxis paste and a rotary brush.

Preparation of the test specimens. The abutment mobility is a decisive factor in the evaluation of fracture strength.29 Thus, to imitate the physiological tooth mobility, we covered all roots of the selected teeth with an artificial periodontal membrane made of a 0.25 mm–thick layer of gum resin (Anti-Rutsch-Lack, Wenko-Wenselaar GmbH, Hilden, Germany).28,30 Before fixing teeth in the sample holders, we covered each tooth coronally with wax 2 mm short of the cementoenamel junction and then dipped it once in the gum resin to make it conform to the biological width. We fixed each tooth at 90 degrees vertical into a silicone mold. We attached a sample holder to the prepared silicone mold with the tooth in place. Then we mixed a chemically polymerizing polyester resin (Technovit 4000, Heraeus Kulzer GmbH, Wehrheim, Germany) and poured it into the sample holder. After the resin had set, we removed the silicone and cleaned the specimens.

Tests. All specimens of each group underwent fatigue in a computer-controlled mastication simulator (type N6C41/N6W26, Willytech, Munich, Germany) at 1.2 million mechanical cycles.28,31 The applied load was 98 newtons (10 kilograms) and the thermocycling was 5 C to 55 C for 60 seconds each with an intermediate pause of 12 seconds, maintained by the thermostatically controlled liquid circulator (Haake, Karlsruhe, Germany) for 5,300 cycles. We applied the load vertically onto the center of the occlusal surface of the restoration using a ceramic antagonist (Steatite, Hoechst Ceram Tec, Wunsiedel, Germany) of 6 mm in diameter.28,31,32 We applied a force profile in the shape of an ellipsoidal curve with a vertical movement of 6 mm, a horizontal movement of 0.5 mm and a cycle frequency of 1.6 megahertz.

During dynamic loading, we examined all samples twice per day. We recorded a fracture of a tooth or of the porcelain as a failure. Finally, we loaded all samples that survived the masticatory simulation until fracture using a universal testing machine (Z010/TN2S, Zwick GmbH, Ulm, Germany). We placed a 1 mm–thick layer of tin foil over the occlusal surface of the teeth to achieve a homogenous stress distribution. We applied a perpendicular load onto the center of the occlusal surface of each sample, under a stroke control of 2 mm/minute. We calculated and evaluated the loading values by means of the Xpert V 7.1 software (Zwicktest Xpert Software, Zwick).

We used robust statistical methods that relied solely on the ranking of the observed failure load values. In particular, we used the Kruskal-Wallis test analysis of variance to compare the failure loads among the six groups. In addition, we performed pairwise Wilcoxon rank sum tests combined with the Bonferroni-Holm method to adjust P values. The level of significance was 5 percent (P = .05).


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
All the specimens survived 1,200,000 cycles of dynamic loading and thermocycling in the masticatory simulation, resulting in a 100 percent survival rate value for the test and control groups. After the load-to-fracture testing, none of the specimens demonstrated a fracture strength value below 800 N. The smallest value occurred in test group 4C (852.4 N), whereas we observed the highest value in the control group NP (3,616.0 N) (Table 1Go). For each group, we computed statistics for load-to-fracture values after masticatory fatigue (Figure 6Go).


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  		TABLE 1 Load-to-fracture test results, in newtons.*

 

Figure 6
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  		Figure 6. Box plots of the load-to-failure test results, in newtons (N). NP: Natural teeth with no preparation. IN: Mesio-occlusal-distal (MOD) inlay preparation. 1C: MOD preparation with reduction of the mesiopalatal cusp. 2C: MOD preparation with reduction of both palatal cusps. 3C: MOD preparation with reduction of both palatal and distobuccal cusps. 4C: MOD preparation with reduction of all cusps.

 
Comparisons of the fracture strength values showed no significant differences among all groups (Table 2Go) (P = .6026). Thus, the different preparation designs of the PCRs demonstrated no significant influence on the fracture resistance of the restorations.


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  		TABLE 2 Results of Kruskal-Wallis test (analysis of variance), pairwise Wilcoxon rank sum tests with P values adjusted by the Bonferroni-Holm method.*

 
All specimens of the control group NP fractured in a homogenous manner. A coronal fracture was initiated below the impact position of the testing head. We noticed no root fractures. The majority of specimens in groups IN through 4C fractured without debonding of the ceramic restorations (Figure 7Go). The fracture initiated in the ceramic below the contact position of the testing head, starting at the cement interface and extending toward the occlusal surface (Figure 8Go). Two, four, three, two and three specimens of the groups IN, 1C, 2C, 3C and 4C, respectively, demonstrated fractures extending into the tooth structure.


Figure 7
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  		Figure 7. Fracture pattern of group IN (teeth with mesio-occlusal-distal inlay preparation).

 

Figure 8
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  		Figure 8. Fracture pattern of group 4C (teeth with mesio-occlusal-distal inlay preparation with reduction of all cusps).

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
We used extracted human teeth in this investigation because their bonding characteristics, thermal conductivity, modulus of elasticity and strength represent the clinical situation better than would metal, plastic or animal teeth. We stored the extracted teeth in 0.1 percent thymol solution to prevent them from drying out and thereby turning brittle. In addition, the storage solution we used inhibits microbial activity.33 Tooth mobility has been demonstrated to be a decisive factor in the evaluation of fracture strength.29,34 In our study, the roots of the teeth were covered with a thin layer of gum resin (Anti-Rutsch-Lack). This layer simulated the periodontal ligament during masticatory simulation and load-to-failure testing. This method resulted in an artificial tooth mobility of 100 ± 30 µm in the horizontal direction and 65 ± 21 µm in the vertical direction under a force of 5 N.30 These values are similar to the physiological tooth mobility reported in the literature.35 Natural tooth mobility is not always considered in the testing of fracture resistance of ceramic inlays and onlays.36–39

The objective of our masticatory simulation, as already indicated, was to introduce a comparable cycle fatigue component for all specimens. The parameters we used for the mastication simulator were related to the physiological values found in the literature.40 A cycle loading force of 49 N was used in several in vitro studies.28,31,40 Research has shown that occlusal forces in the posterior dentition can exceed functional loading forces of 49 N during mastication or swallowing.41 Therefore, we applied a cycle loading force of 98 N. Clinical studies have shown that humans have an average of 250,000 masticatory cycles per year.42,43 To simulate a service time of five years, researchers28,41,44 have performed 1,200,000 masticatory cycles. In our investigation, all specimens survived fatigue in the mastication simulator. This is not unusual. In two further in vitro studies, all ceramic inlays and onlays resisted masticatory fatigue loading.45,46 Owing to differences in test parameters and materials, further comparisons with our investigation are not admissible.

Data from load-to-fracture tests of ceramic PCRs using natural teeth as abutments after masticatory fatigue are not available in the literature. Despite different preparation designs, our statistical analysis demonstrated no significant differences in the load-to-fracture values between the test and control groups. PCR extension did not demonstrate a significant influence on the fracture resistance of the specimen. Similar results were demonstrated by van Dijken and colleagues3 in a clinical investigation in which they found no differences between different prepared dentin-enamel–bonded ceramic coverage restorations with regard to failure rate and failure type. In our study, all specimens had fracture strength values of more than 800 N. These values exceeded the maximum biting force of approximately 725 N for posterior single teeth reported in the literature.32,47 The maximum fracture strength results of our study were higher than those of inlays composed of Vitadur N (Vita Zahnfabrik, Bad Säckingen, Germany) (2,680 N) and of Ceramco II (Dentsply Ceramco, York, Pa.) (1,662 N) luted on natural teeth and unprepared natural teeth (3,547 N), as reported in a previous in vitro investigation.36 Deviations in reported load-to-fracture resistance under in vitro conditions may be caused by different types of ceramics, different test methods and different preparation designs.

Generally, the fracture strength of a natural tooth is compromised after an onset of caries, causing lesions, or after tooth preparation.3,38 Irrespective of the different preparation design, we found that IPS e.max Press PCRs, along with adhesive cementation, supported the residual tooth structure, applied a good distribution of stresses generated by masticatory fatigue and enabled the prepared tooth to reach the fracture strength of unprepared molars. The patterns of fracture recorded in all groups that received restorations were representative of patterns in ceramic inlays and PCRs. In these groups, we found fractures mostly in the ceramic material, with the exception of 14 specimens (17.5 percent) in which the fracture pattern extended into the tooth structure. In general, it is preferred that if the restoration fails, it do so without compromising the remaining tooth structure.48

In our study, the crack initiation for almost all restored specimens started at the cement interface and extended to the occlusal aspect of the restoration. This is in agreement with data from a previous investigation,49 which reported that crack initiation started at the cement interface. When a ceramic layer is uniformly supported by, and bonded to, a less stiff material, high tensile stresses develop in the ceramic at its interface with the cement, directly below the loaded area. These interfacial stresses arise from different stress or strain behaviors of ceramic, cement and dentin on the basis of discrepancies in modulus of elasticity. Therefore, researchers have studied whether cracks usually initiate at the interface level, thus leading to a subsequent total failure of the restoration.49,50 These so-called "radial cracks" are known to be a severe failure mode in monolithic ceramics.51,52 In this study, we had limited opportunities to obtain detailed information about the integrity of the bond strength between the ceramic and enamel or the microstructural behavior of the ceramic material. Ideally, testing a higher number of specimens would increase the statistical power of the investigation. High costs of dental restoration specimens and machine running time restrict these types of protocols.

The observed crack initiation and failure pattern provide strong evidence that the in vitro test model we used in this study provides clinically relevant information. Even though IPS e.max Press demonstrated a high potential for fracture-resistant inlays and PCRs based on the described preparation designs, additional clinical studies should be conducted.


   CONCLUSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
Within the limits of this investigation, we can conclude the following:

– All-ceramic PCRs for molars made of IPS e.max Press are fracture-resistant, showing results comparable with those of natural unprepared teeth.
– A defect-oriented tooth preparation in the posterior region for the restoration of a compromised tooth with a partial-coverage ceramic restoration is justifiable.


   FOOTNOTES
 

Dr. Stappert is a senior lecturer, Department of Prosthodontics, Albert-Ludwigs-University, Freiburg, Germany, and visiting research professor, Department of Biomaterials and Biomimetics, Department of Implant Dentistry, New York University College of Dentistry, Arnold and Marie Schwartz Hall of Dental Sciences, 345 E. 24th St., Room 803, New York, N.Y. 10010, e-mail "christian.stappert{at}nyu.edu". Address reprint requests to Dr. Stappert.


Dr. Att is an assistant professor, Department of Prosthodontics, Albert-Ludwigs-University, Freiburg, Germany.


Dr. Gerds is a statistician, Department of Prosthodontics and Institute of Medical Biometrics and Medical Informatics, Albert-Ludwigs-University, Freiburg, Germany.


Dr. Strub is a professor and the chairman, Department of Prosthodontics, Albert-Ludwigs-University, Freiburg, Germany.


The authors thank Mr. Hans-Peter Foser, master dental technician, Ivoclar-Vivadent, Schaan, Liechtenstein, for his efforts in the fabrication of the ceramic restorations used in this study.


   REFERENCES
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 

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