Effects of acid hydrolysis and mechanical polishing on surface residual stresses of low-fusing dental ceramics

Effects of acid hydrolysis and mechanical polishing on surface residual stresses of low-fusing dental ceramics Yaser M. Alkhiary, BDS, MScD,a Steven M. Morgano, DMD,b and Russell A. Giordano, DMD, DMScc Goldman School of Dental Medicine, Boston University, Boston, Mass Statement of problem. Cracks may arise in a ceramic restorative material over time, resulting in sudden fractures at stresses well below the yield stress. Purpose. This study evaluated by means of indentation technique the effects of acid hydrolysis and mechanical polishing on the surface residual stresses of low-fusing ceramic materials. Material and methods. A total of 64 ceramic bars were formed to produce 4 groups of 16 bars each for 4 ceramic materials (Duceram-LFC Dentin, Duceram-LFC Enamel, Finesse Dentin, and Finesse Enamel). Four surface-treatment groups (n⫽4) were then formed for each of the 4 materials. The 4 surface treatments were control (autoglaze), hydrolysis, glaze/polish, and polish/glaze. A Vickers indenter contacted the Duceram-LFC specimens with a 5-N load and the Finesse specimens with a 3-N load for 10 seconds. Scanning electron microscope (SEM) was used to study surface texture before and after hydrolysis and polishing. Differences in mean crack lengths were analyzed with 1-way analysis of variance and least significant difference test (␣⫽.05.) Results. SEM showed obvious surface flaws as a result of hydrolysis on Duceram-LFC Enamel and Dentin specimens. However, statistical analysis of the resulting crack lengths revealed no significant differences between values for the control groups (58.16 ⫾ 3.88) (53.53 ⫾ 2.67) and hydrolysis groups (57.11 ⫾ 4.09) (54.54 ⫾ 3.15) for Enamel (P⫽.081) and Dentin (P⫽.093) respectively. When comparing polished groups and nonpolished groups, the mean crack lengths were significantly shorter for polished specimens of Duceram-LFC Enamel (53.76 ⫾ 3.17), Finesse Enamel (40.56 ⫾ 3.31), and Finesse Dentin (39.76 ⫾ 3.81) porcelains compared with their control groups (58.16 ⫾ 3.88) (43.54 ⫾ 4.12) (41.19 ⫾ 3.47), respectively (P⬍.0001). The mean crack lengths were significantly longer for polished specimens of Duceram-LFC Dentin (59.16 ⫾ 3.52) porcelain compared with the control group (53.53 ⫾ 2.67) (P⬍.0001). Conclusion. Within the limitations of this study, hydrolysis did not improve surface residual stresses of Duceram-LFC and Finesse ceramic materials. Mechanical polishing improved surface residual stresses of all materials tested, except Duceram-LFC Dentin porcelain. (J Prosthet Dent 2003;90:133-42.)

CLINICAL IMPLICATIONS Polishing a low-fusing porcelain crown after occlusal adjustment may enhance the surface mechanical properties by inducing compressive stress on the adjusted surface. The manufacturer’s claim that strengthening occurs with hydrolysis of the surface of a low-fusing porcelain does not appear valid.

S

urface residual stress is an important factor in the comparison of the resistance to fracture of ceramic restorative materials because flaws or cracks may arise in a material or nucleate while in service, and sudden fractures can arise at stresses well below the yield stress. Vickers indentation has been used as a simple microprobe method to measure the stress in tempered glass surfaces.1 The crack lengths are induced by forcing the indenter into the material with a a

DScD candidate, Division of Biomaterials, Department of Restorative Sciences and Biomaterials. b Professor and Director, Division of Postdoctoral Prosthodontics, Department of Restorative Sciences and Biomaterials. c Associate Professor and Director, Division of Biomaterials, Department of Restorative Sciences and Biomaterials. AUGUST 2003

definite load application and then measuring the resulting median crack lengths, which initiate from the central deformation zone. Accuracy and reproducibility of cracklength values depend on the procedural approach, test parameters, and condition of the specimens.2 The basis of the indentation technique was described by Evans and Charles3 to create a series of cracks that formed under heavy loading in a brittle material around a Vickers diamond indenter. When viewed superiorly, the cracks appeared to emanate from each of the corners of the indentation. The lengths of the cracks, expressed by the surface dimension “c,” increased with an increasing indentation load. Al-Hiyasat4 reported that exposure to carbonated beverage accelerated enamel wear and decreased the THE JOURNAL OF PROSTHETIC DENTISTRY 133

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wear resistance of Duceram-LFC and Vita Mark II ceramics. Surface degradation can result in surface defects that could propagate cracks and lead to eventual failure of the material.5 The Duceram-LFC product was marketed as having greater strength compared with other types of dental porcelain. The manufacturer purports that a hydroxyl layer would regenerate in the mouth after its removal by mastication and that Duceram-LFC porcelain increased its flexural strength by more than 40% of its original strength after hydrolysis testing. The surface layer contained many OH-groups, built through alkali and hydroxyl exchange. This surface was claimed by the manufacturer to be more stable because it could “heal” surface flaws. Depending on the intensity of the hydrolytic testing and the time factor, the DuceramLFC surface displayed a growing Si-hydroxyl layer. The growth was more intense in the beginning and then slowed. After 10 cycles of repeated hydrolysis, the Sihydroxyl layer reached a maximal thickness of 3 ␮m. This result agreed with results from Risito et al.6 After repeated hydrolysis testing, the degraded amount from the original materials decreased in Duceram-LFC porcelain, whereas an increase was observed in other materials tested. The flexural strength of the Duceram-LFC porcelain also increased after the hydrolysis test. This result suggests very stable chemical properties at the surface of Duceram-LFC porcelain in the oral environment. Literature from Degussa purports hydrolysis can “heal” surface cracks because of the exchange of the alkali group from the ceramic and the hydroxyl group from acetic acid. Ion exchange involves the diffusion of ions from a higher concentration to a region of lower concentration, and replaces smaller diameter ions with larger ions at elevated temperatures. This process may create a residual compressive surface layer as demonstrated by Giordano et al7 when they compared the strengthening effect of the Tuf-Coat ion exchange system (formerly manufactured by GC America Inc, Alsip, Ill) with other surface treatments such as overglazing and polishing. Levy8 evaluated the effect of polishing with pumice and etching on the flexural strength of dental ceramics after air and vacuum glazing, and overglazing. He reported that polished, glazed specimens had higher strength values. Brackett et al9 tested the effects of autoglaze, overglaze, and autoglaze plus polish on the strength of 5 dental ceramics. Polishing was accomplished with a polishing kit (Shofu Polishing Kit; Shofu Dental Corp, San Marcos, Calif). The authors reported that the flexural strength of the specimens tested with an overglaze was significantly greater than that of specimens treated with autoglaze and those treated with autoglaze and polish. 134

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Fairhurst et al10 investigated the strengthening of feldspathic porcelain by analyzing the effects of various polishing and firing procedures on 4 groups (n ⫽ 50) of body porcelains (Jelenko, Ossining, NY). Group 1 was fired, glazed/no hold, and polished; group 2 was fired, polished, and glazed/no hold; group 3 was fired, polished and glazed/1 minute hold; and group 4 was fired, polished and not glazed. The specimens that were fired, polished to 1-␮m surface finish, and the nonglazed specimen were significantly higher in flexural strength compared with the other groups. The other 3 groups that received additional firing recorded a significant decrease in strength. The study concluded that self-glazing did not increase flexural strength and that some glazing techniques could be detrimental to the fracture properties of leucite-containing porcelains. Giordano et al11 reported that overglazing, grinding, and polishing all significantly increased the flexural strength of dental ceramics by 15% to 30%, and refiring of the ground and polished specimens decreased the flexural strength significantly from 11% to 18%. Refiring of the as-fired group did not affect the flexural strength. The flexural strength of the overglazed group was not significantly different from other groups. The purpose of this study was to evaluate the effects of acid hydrolysis and mechanical polishing on the surface residual stresses of Duceram-LFC and Finesse porcelains by use of the indentation technique method. Indentations and any surface modifications that occurred before and after hydrolysis and polishing tests were also observed by scanning electron microscopy (SEM) and subjectively evaluated. The hypotheses tested were that chemical hydrolysis induces residual tensile stresses on the surfaces of lowfusing dental ceramics (Duceram-LFC and Finesse), and that mechanical polishing induces residual compressive stress on the surfaces of low-fusing dental ceramics (Duceram-LFC and Finesse).

MATERIAL AND METHODS Preparation of specimens for testing The 2 low-fusing ceramic materials used were Duceram-LFC, Dentin shade D3 and Enamel porcelain (Ducera Dental GmbH, Rosbach, Germany) and Finesse, Dentin shade C2 and clear Enamel porcelain (Dentsply, Burlington, NJ). Sixty-four test bars, approximately 24.0 ⫻ 3.0 ⫻ 2.0 mm, were fabricated, 16 for each material (Duceram-LFC Enamel, Duceram-LFC Dentin, Finesse Enamel, and Finesse Dentin). For each material, the specimens were randomly assigned to 4 groups, (n ⫽ 4) (control, hydrolysis, glaze/polish, and polish/glaze). All groups were autoglazed according to the manufacturer’s instructions (without addition of an overglaze). VOLUME 90 NUMBER 2

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Ceramic powder was mixed with deionized water and placed incrementally in a custom-made rectangular polyvinylsiloxane (Examix, GC America Inc, Alsip, Ill) mold (25.0 ⫻ 3.5 ⫻ 2.5 mm). The mix was packed into the mold against a gypsum block, and then vibrated for 2 to 4 seconds. Excess water was absorbed with tissue paper from the top of the mold while the gypsum block absorbed the excess water from the bottom of the mold. Excess ceramic material was removed with a carver from the top of the mold, and the mold was left on the bench for 2 hours to allow the ceramics to dry. The resulting green bars were transferred to a flat firing stand on the furnace (VITA Vacumat 200; Vita Zahnfabrik, Bad Sackingen, Germany) and fired according to the manufacturer’s instructions. The width of the specimens was chosen to match the diamond polishing wheel (Dialite; Brasseler USA, Savannah, Ga). The fired specimens were allowed to air-cool to room temperature, then ground flat on both sides on the variable-speed grinding machine (Ecomet III; Buehler Ltd, Lake Bluff, Ill) with a 70-␮m grit diamond disc. The opposing sides of the bars were flat and parallel. The specimens were cleaned in an ultrasonic cleaning machine with deionized water to ensure that all traces of grinding debris were removed.

Control group (autoglaze) The control (autoglaze) specimens were mounted on metal stubs with the use of carbon tape (05071-AP; SPI Supplies, West Chester, Pa), and sputter coated with gold for 60 seconds. All specimens were stored in a clean plastic container until the time of the microindentation test, which was accomplished within a maximum of 1 hour after sputter coating.

Hydrolysis group The hydrolysis group was placed in a glass-bottomed thimble, which was conditioned by storing it at 150° ⫾ 3°C to achieve a constant weight (nearest 0.1 mg), and weighed with a scale (Gram-Atic Balance; Mettler, Zurich, Switzerland). The thimble and the specimens were placed in the extraction apparatus with 4% acetic acid (acetic acid, 4% vol/vol solution in water of grade 3 as specified in ISO 3696).12 The specimens were refluxed continuously for 16 hours (reflux rate 15 to 20 min/ cycle). The temperature in the extraction area rose to 80°C during heat build-up and fell to approximately 60°C after the siphoning action of the extractor began. The specimens were washed profusely in the thimble with deionized water. The thimble and the specimens were again conditioned to a constant weight (nearest 0.1 mg) at 150° ⫾ 3°C. The bars were mounted on AUGUST 2003

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metal stubs, sputter coated with gold and stored in a clean plastic container until the time of microindentation test, which was accomplished within a maximum of 1 hour after sputter coating.

Polished groups A custom-polishing protocol was used. A slow-speed hand piece (KaVo Model K9; KaVo America, Lake Zurich, Ill) was connected to the computer-numerically– controlled (CNC) lathe saddle (Compact 5 CNC; Emco Maier Corp, Columbus, Ohio) and run at a speed of 20,000 rpm. A digital tachometer (20904-011; VWR International Inc, West Chester, Pa) was used to measure the speed. The “glaze/polish” and “polish/glaze” groups were mounted on metal stubs with the use of carbon tape. The entire assembly was then placed in a Jacobs-style chuck and secured in a clamp at the end of one side of a lever arm. Dead weights were positioned in a weight holder at the appropriate distance from the fulcrum to produce loads of 0.6 N for a coarse wheel, 1 N for a medium wheel and 1.3 N for a fine wheel. The corresponding upward polishing loads exerted by the dead weights were confirmed on a universal testing machine (model 4202; Instron Corp, Canton, Mass). The CNC lathe was programmed to move back and forth along its horizontal plane over the bar at a feed rate (work-piece speed) of 499 mm/min. over a distance of 25 mm. The width of the diamond polishing wheel (Dialite; Brasseler USA) was equal to the specimen’s width. The cycle was repeated twice (52 seconds) for a coarse wheel, 4 times (104 seconds) for a medium wheel, and 6 times (156 seconds) for a fine wheel.

Microcrack formation Surface residual stresses were determined by conventional microindentation crack technique as described by Marshall and Lawn.1 To determine the amount of force and time needed to measure surface residual stresses, a pilot study was performed on each material. The results indicated that for Duceram-LFC porcelain, 5-N for 10 seconds was necessary to induce measurable radial cracks and that 3 N was adequate for Finesse porcelain. The standard deviation derived from the same pilot study was used in a power analysis test to calculate a sample size at 95% confidence level. According to the power analysis test, 80 microindentations were needed to obtain approximately 20 measurable microindentation cracks, which were directly measured immediately after indentation with the microhardness tester (Micromet 2003; Buehler Ltd, Lake Bluff, Ill). For each microindentation, 4 cracks were produced; therefore, there were a total of 80 cracks produced by the 20 microindentations. Sputter coating of the specimens with gold before inducing the cracks enhanced crack visibility and readability from the microhardness tester. 135

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Fig. 1. SEM picture at original magnification ⫻600 for valid microindentation crack, where 57.5 ␮m is “c” (crack length measured from the center of indentation), and 24.4 ␮m is “a” (half diagonal of indentation).

The dried, gold-sputtered specimens were placed in the microhardness tester. A 5-N load was applied for 10 seconds to create the required cracks for the DuceramLFC porcelain. A 3-N load was applied for 10 seconds to create the required cracks for the Finesse porcelain. A minimum distance of 250 ␮m was maintained between 2 neighboring indentations to eliminate any possibility of crack interference. All indentations were made in air. The only microindentation cracks chosen were those that possessed 4 radial cracks formed with a c/a ratio ⬎ 2 (“c” is the distance from the center of the indentation to the tip of the crack, and “a” is the distance from the center of the indentation to the corner of the diamond) (Fig. 1). Any microindentations with evidence of crack chipping or measurable crack branching were rejected. The SEM was also used to study the surface texture before and after hydrolysis for all materials.

Data analysis The crack measurement data were statistically evaluated by use of a 1-way analysis of variance (ANOVA), and least significant difference test was conducted where indicated, at P⬍.05. Because the experiments were not designed to evaluate any differences among any of the various ceramic materials, the 1-way ANOVA was considered appropriate. 136

SEM analysis All indented bars were evaluated under SEM (XL20; Phillips Electronic Instruments, Mahwah, NJ) at original magnification ⫻600 to observe the indentations and any surface modifications that occurred before and after hydrolysis and polishing tests.

RESULTS Figures 2 and 3 display the SEM pictures at original magnification ⫻600 of representative specimens from the various groups of Duceram-LFC and Finesse porcelain. Figures 4 and 5 display the mean crack lengths in micrometers for all groups.

Duceram-LFC Enamel porcelain ANOVA indicated significant differences in crack lengths among the means of the 4 surface-treatment groups (P⬍.0001) (Table I). There was no significant difference between the means for the control (58.16 ⫾ 3.88) and hydrolysis groups (57.11 ⫾ 4.09) (P⫽.081). There were, however, significant differences between the means for the control and glaze/polish groups (55.2 ⫾ 4.02) (P⬍.0001) and the control and the polish/ glaze groups (53.76 ⫾ 3.17) (P⬍.0001). The crack lengths were longest for the control group and progressively shorter for the others. VOLUME 90 NUMBER 2

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Fig. 2. SEM pictures at original magnification ⫻600 of control and hydrolysis surfaces of Duceram-LFC (Enamel and Dentin) and Finesse (Enamel and Dentin) porcelains.

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Fig. 3. SEM pictures at original magnification ⫻600 of glaze/polish and polish/glaze surfaces of Duceram-LFC (Enamel and Dentin) and Finesse (Enamel and Dentin) porcelains.

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Fig. 4. All groups of Duceram-LFC porcelain under 5 N for 10 seconds.

Fig. 5. All groups of Finesse porcelain under 3 N for 10 seconds.

Table I. One-way ANOVA for Duceram-LFC Enamel porcelain Source

Between groups Within groups Total

Sum of squares

df

Table II. One-way ANOVA for Duceram-LFC Dentin porcelain

Mean square

F ratio

P value

21.213

⬍.0001

922.619

3

307.540

4581.196 5503.815

316 319

14.497

Duceram-LFC Dentin porcelain ANOVA indicated significant differences in crack lengths among the means of the 4 surface-treatment groups (P⬍.0001) (Table II). There was no significant difAUGUST 2003

Source

Between groups Within groups Total

Sum of squares

df

Mean square

F ratio

P value

34.372

⬍.0001

1490.450

3

496.817

4567.481 6057.931

316 319

14.454

ference between the means for the control (53.53 ⫾ 2.67) and hydrolysis groups (54.54 ⫾ 3.15) (P⫽.093). There were, however, significant differences between the mean for the control and glaze/polish groups (56.64 ⫾ 5.32) (P⬍.0001) and the control and the polish/glaze groups 139

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Table III. One-way ANOVA for Finesse Enamel porcelain Source

Between groups Within groups Total

Sum of squares

df

Mean square

F ratio

P value

19.427

⬍.0001

1000.248

3

333.416

5423.259 6423.507

316 319

17.162

(59.16 ⫾ 3.52) (P⬍.0001). The crack lengths were shortest for the control group and progressively longer for the others. This result is the reverse of the findings for DuceramLFC Enamel porcelain.

Finesse enamel porcelain ANOVA indicated significant differences in crack lengths among the mean values of the 4 surface-treatment groups (P⬍.0001) (Table III). There was no significant difference between the means for the control (43.54 ⫾ 4.12) and hydrolysis group (44.76 ⫾ 4.19) (P⫽.063), as well as between the glaze/polish group (40.89 ⫾ 4.81) and polish/glaze groups (40.56 ⫾ 4.19)(P⫽.603). There were, however, significant differences among the mean values for control, glaze/polish, and polish/glaze groups and among means for hydrolysis, glaze/polish, and polish/glaze groups. The crack lengths were shortest in both the glaze/polish and polish/glaze groups (P⬍.0001).

Finesse dentin porcelain ANOVA indicated significant differences in crack lengths among the means of the 4 surface-treatment groups (P⬍.0001) (Table IV). There was no significant difference between the means for the control (41.19 ⫾ 3.47) and hydrolysis groups (42.29 ⫾ 3.18) (P⫽.055), as well as between the glaze/polish group (39.82 ⫾ 3.96) and polish/ glaze groups (39.76 ⫾ 3.81) (P⫽.925). There were, however, significant differences among the means for control, glaze/polish, and polish/glaze groups and among means for hydrolysis, glaze/polish, and polish/glaze group. The crack lengths were shortest in both the glaze/polish and polish/glaze groups (P⬍.05).

DISCUSSION The word hydrolysis comes from the Greek prefix “hydro,” meaning “water,” and the Greek word “lysis,” meaning “a loosening”. In this study a hydrolysis technique was used with a 4% acetic acid reagent, which, in theory, may react and split the silicon-oxygen bond in the ceramic material. The hydroxyl group (OH⫺) from the acetic acid could split the silicon-oxygen bond on the ceramic surface. This breakage in the silicon bond will 140

Table IV. One-way ANOVA for Finesse Dentin porcelain Source

Between groups Within groups Total

Sum of squares

df

Mean square

F ratio

P value

9.013

⬍.0001

353.606

3

117.869

4132.743 4486.349

316 319

13.078

create the surface flaw. The intensity of these surface flaws depends on many factors, including the pH of the acetic acid, the availability of active silicon-oxygen bonds, and the time of contact between the acid and the ceramic surface. Porosity of the ceramic surface also plays a role in this reaction because porosity allows the acid to penetrate the ceramic surface and allows the reaction to continue inside the ceramic specimens. Therefore, the amount and the size of surface porosities may act as a reservoir for the acid to remain and induce surface flaws, which could explain why most of these surface flaws were related to surface voids. In general, surface flaws were the weakest points on the ceramic surface. Cracks started from these flaws, which may lead to future ceramic failure. It was expected that statistically significant differences in crack lengths for Duceram-LFC porcelain (Enamel and Dentin) would be observed after hydrolysis. SEM pictures displayed obvious surface flaws that developed after hydrolysis. However, statistical analysis indicated no significant difference in the crack lengths between control and hydrolysis groups. Duceram-LFC porcelain (Enamel and Dentin) and Finesse porcelain (Enamel and Dentin) surfaces were smooth and clear of cracks and flaws after glazing-then-polishing and polishing-then-glazing procedures. Mean crack lengths were approximately 7% shorter for the polished Duceram-LFC Enamel and Finesse Enamel specimens, and 3% shorter for Finesse Dentin specimens. Nevertheless, mean crack lengths were approximately 10% longer for polished Duceram-LFC Dentin specimens. Polishing procedures may have removed defects created during fabrication. Furthermore, polishing procedures probably produced surface compressive stresses because heat was generated at the polishing surface as a result of friction between the polishing wheel and the ceramic surface. Polishing may have also overheated the surface layer and assisted in producing plastic deformation. This process can also generate thermal mismatch between the outer and inner layers of the ceramic specimens that may lead to the development of tensile stresses in the inner layer and desirable compressive stresses on the outer layer. All the previously mentioned mechanisms during the polishing procedure probably worked together to induce residual compressive stresses on the ceramic surVOLUME 90 NUMBER 2

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faces tested. However, one question remains: why did Duceram-LFC Enamel porcelain behave differently from Duceram-LFC Dentin porcelain? In this study the Duceram-LFC Enamel porcelain behaved differently from Duceram-LFC Dentin porcelain, whereas Finesse Enamel and Dentin porcelains behaved similarly. One possible explanation for this difference in behavior is related to the microstructure of the Duceram-LFC material. Duceram-LFC porcelain is a leucite-free ceramic manufactured from a single-phase glassy material. The main difference between Enamel and Dentin porcelains is the presence of metallic oxides (color frits) in Dentin materials. These powders are added in small quantities to obtain the delicate shades necessary to imitate the color of natural teeth. They are prepared by adding the metallic oxides to a fine glass, fusing the mixture in a furnace, and regrinding the resulting material to form a powder. In general, the metallic pigments constitute a small percentage of the porcelain mixture, although these pigments may affect the composition of Duceram-LFC Dentin porcelain. A second possible explanation is related to the grain size of Duceram-LFC Enamel porcelain and Dentin porcelain. Dentin porcelain could have a larger grain size than the Enamel porcelain. Induced cracks may travel a further distance to reach the boundary of Dentin grain particles compared to shorter distances in Enamel porcelain if it has a smaller grain size. Compressive residual stress develops outside the grain particle itself, so the magnitude of compressive stress developed could be the same for both Enamel and Dentin porcelains, but because of differences between grain sizes, different crack lengths would develop with the Vickers indenter. Underfiring of the porcelain may also be a reason for the different behavior of Duceram-LFC Dentin porcelain. The firing temperature (660°C) used in this study was according to the manufacturer’s recommendations. If the assumption of underfiring is correct then the Duceram-LFC Dentin porcelain would not be completely sintered. When the indenter contacted the surface, the resulting crack tip would travel longer because of less resistance to crack propagation as compared with fully sintered ceramic. DuceramLFC Dentin porcelain is the only group that showed longer crack lengths with polished specimens. Perhaps polishing of a partially sintered surface does not induce surface compressive stress when compared with the fully sintered ceramic. There are limitations to the use of microindentation cracks as a measure of surface residual stresses. Creation and measurement of microindentation cracks are technique sensitive, and accurate results depend on many factors. The nature of the material to be tested, including its smoothness and integrity (voids or contaminaAUGUST 2003

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tion) can affect the results. The alignment and calibration of the machine used, as well as the contact angle of the indenter, load, and time of applied load are additional important considerations. A pilot study is mandatory to ensure acceptable cracks that can be found and mapped under SEM. Carefully planned pilot studies were performed to ensure accurate and reliable measurements. Another limitation of this study is related to the polishing method. Although Dialite wheels are commonly used by dental laboratory technicians and dentists to polish the surfaces of dental ceramic materials, polishing is accomplished by hand. To ensure standardization of the polishing procedure in this study, a highly controlled polishing protocol was used that may not entirely mimic clinical conditions.

CONCLUSIONS Within the limitations of this in vitro study, the following conclusions were drawn: 1. No statistically significant differences in mean crack lengths were found between the control and hydrolysis groups for Duceram-LFC and Finesse ceramic materials. 2. Although there was no statistically significant difference in mean crack lengths recorded between the control and hydrolysis groups of Duceram-LFC Enamel and Dentin porcelains, SEM showed surface flaws and microcracks after hydrolysis. 3. Statistical analysis indicated that polishing procedures decreased mean crack lengths on the surfaces of all materials tested, except Duceram-LFC Dentin porcelain. 4. No statistically significant difference in mean crack lengths was observed between Finesse Enamel glaze/ polish and polish/glaze groups. 5. No statistically significant difference was observed in mean crack lengths between Finesse Dentin glaze/ polish and polish/glaze groups. REFERENCES 1. Marshall DB, Lawn RB. An indentation technique for measuring stresses in tempered glass surfaces. J Am Ceram Soc 1977;60:86-7. 2. Scherrer SS, Denry IL, Wiskott HW. Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic. Dent Mater 1998;14:246-55. 3. Evans A, Charles E. Fracture toughness determinations by indentation. J Am Ceram Soc 1976;55:371-2. 4. al-Hiyasat AS, Saunders WP, Sharkey SW, Smith GM. The effect of a carbonated beverage on the wear of human enamel and dental ceramics. J Prosthodont 1998;7:2-12. 5. Oilo G. Flexural strength and internal defects of some dental porcelains. Acta Odontol Scand 1988;46:313-22. 6. Risito C, Luthy H, Loeffel O, Scharer P. [The chemical solubility and stability of low-melting dental porcelains]. Schweiz Monatsschr Zahnmed 1995;105:611-6. German.

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7. Giordano RA 2nd, Campbell S, Pober R. Flexural strength of feldspathic porcelain treated with ion exchange, overglaze, and polishing. J Prosthet Dent 1994;71:468-72. 8. Levy H. [Effect of laboratory finishing techniques and the mechanical properties of dental ceramic]. Inf Dent 1987;69:1039-45. French. 9. Brackett SE, Leary JM, Turner KA, Jordan RD. An evaluation of porcelain strength and the effect of surface treatment. J Prosthet Dent 1989;61:44651. 10. Fairhurst CW, Lockwood PE, Ringle RD, Thompson WO. The effect of glaze on porcelain strength. Dent Mater 1992;8:203-7. 11. Giordano R, Cima M, Pober R. Effect of surface finish on the flexural strength of feldspathic and aluminous dental ceramics. Int J Prosthodont 1995;8:311-9. 12. International Organization for Standardization. Dental Ceramic ISO Standard. 1995.

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Reprint requests to: DR STEVEN M MORGANO BOSTON UNIVERSITY SCHOOL OF DENTAL MEDICINE 100 EAST NEWTON STREET, ROOM G219 BOSTON, MA 02118 TEL: 617-638-5429 E-MAIL: [email protected] Copyright © 2003 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2003/$30.00 ⫹ 0 doi:10.1016/S0022-3913(03)00277-4

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