Abrasion of Human Enamel by Different Dental Ceramics in vitro R.R. SEGHI, S.F. ROSENSTIEL', and P. BAUER School of Dentistry, University of California at Los Angeles, Los Angeles, California 90024; and IThe Ohio State University, College of Dentistry, Columbus, Ohio 43210
Manufacturers generally quote indentation hardness values when predicting the clinical wear potential of newly introduced ceramic restoratives. The objective of this study was to determine whether in vitro two-body wear correlated well with hardness. A modified polisher was used to abrade enamel cylinders against polished disks of commercially available dental porcelains and glass. Enamel loss after four h was measured with a micrometer. Five ceramic materials were tested, and enamel abrasion rates were correlated with Knoop hardness values. Dicor and Dicor coated with a shading porcelain were found to cause the lowest wear of enamel. These rates were statistically significantly lower than those obtained with Optec, the most abrasive material. These findings may be due to microstructural differences between the materials. Knoop hardness showed poor correlation with the results of the abrasive testing. J Dent Res 70(3):221-225, March, 1991
Introduction. Increased patient demand for esthetic dentistry has prompted renewed interest in all-ceramic dental restorations. Improved materials and innovative techniques have led many dentists to use all-ceramic crowns and inlays for the restoration of posterior occlusal surfaces. There is, however, considerable concern as to how these materials, formulated for improved strength, compare with respect to their tendency to abrade human enamel. Ideally, an esthetic restoration should wear at approximately the same rate as the enamel it replaces [reported by Lambrechts et al. (1987) to be about 20-40 Am per year]. In addition, the restoration should not increase the wear rate of an opposing enamel surface. Conventional feldspathic dental porcelain is generally considered to be more abrasive of enamel than other restorative materials, such as gold or amalgam. In a survey of members of the American Academy of Esthetic Dentistry, Christensen (1986) found "less wear on opposing teeth" to be the single most desirable need for change in posterior toothcolored crowns. Mahalick et al. (1971) reported enamel-porcelain wear, in vitro, to be 2.4 times greater than wear of enamel-acrylic resin and 17 times that of enamel-gold. Monasky and Taylor (1971) tested a variety of surface finishes of porcelain against tooth substance and concluded that the rate of tooth substance wear was a function of porcelain roughness. They recommended glazing or polishing porcelain to reduce enamel attrition. Ekfeldt and 0ilo (1988), using a bruxing subject, studied occlusal wear of porcelain, gold, and resin in vivo. They too found that enamel surfaces exhibited the greatest substance loss when opposed by feldspathic porcelain. These and other studies have led some clinicians (Rosenstiel et al., 1988; Wiley, 1989) to caution against the use of porcelain occlusal surfaces where rapid enamel attrition might be predicted, such as for a bruxer or complete-denture wearer having the porcelain opposed by natural teeth.
A ceramic restorative material that combines good strength without the disadvantage of increased enamel wear would be a significant addition to clinical dental practice. Traditionally, manufacturers quote indentation hardness values as a predictor of clinical wear, as reported by Tillitson et al. (1971). However, little has been written on the enamel wear characteristics of contemporary dental ceramics and whether wear rates can be predicted by indentation hardness. The objective of this study was to design a simple in vitro apparatus to measure the wear rates of human enamel against various commercially available dental ceramics, to assess the relationship between these wear rates and the measured hardness values of the ceramic materials, and to provide some scientific insight, where possible, into the observed wear phenomena.
Materials and methods. An apparatus was designed to produce continuous sliding contact between human enamel cylinders and ceramic disks by a slightly modified Minimet polisher and precision thinning attachment (Buehler Ltd., Lake Bluff, IL 60044). The stroke of the polishing machine was shortened to accommodate a smaller sample dimension, and a 3.5-mm hole, bored through the center of the adjusting screw, served as the enamel specimen holder. The essential parts of the apparatus are shown in Fig. 1. Cylinders of enamel and dentin, approximately 2 mm in diameter by 4 mm in length, were obtained from extracted human maxillary molar teeth in the following manner: The occlusal one-third of each lingual surface was ground by hand on a glass plate with 1000-grit silicon carbide slurry until a flat area of about 2-3 mm in diameter was obtained. The flat surface was cemented with cyanoacrylate resin to a glass slide, embedded in a methyl methacrylate resin by a 25-mm-diameter ring mold, removed from the glass slide, and polished to a 0.25-p~m diamond finish. Each tooth was drilled to a depth of about 4-5 mm with a 3.5-mm-inside-diameter diamond-tipped hollow-core drill bit (#2868A23, McMaster Carr, Los Angeles, CA), with copious amounts of water coolant. The flat Wear Machine Load
0 dng '
Bearing F
Received for publication March 16, 1990 Accepted for publication November 20, 1990
IMicroadjustible Specimen Holder
Ceramic Disc
Fig. 1-Schematic of the wear apparatus. D
=
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Dentin, E
=
Enamel. 221
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J Dent Res March 1991
TABLE 1 PRODUCT INFORMATION ID DI
Product Name
Dicor"'
DG
Dicor Shading Porcelain
GS
Glass Slide
VMK
OP
Vita VMK 68T"
Optec H.S.P.
Material Type Manufacturer Dentsply International, Inc. glass-ceramic York, PA 17405
Dentsply International, Inc.
Hugh Courtright Chicago, IL 60638 Vita Zahnfabrik
feldspathic porcelain
TABLE 2 MEAN ENAMEL WEAR AND KNOOP HARDNESS VALUES OF VARIOUS CERAMIC MATERIALS KHN Enamel Wear n [kglmm2]t n Material (pm]t 379 (10) 25 DI 72 (19) 11 DG
10
81(20)
25
441 (18)
GS
10
105 (50)
25
443 (11)
VMK
10
130 (35)
25
436 (20)
OP
12
176 (58)
25
401 (12)
glass
Sackingen, Germany
feldspathic porcelain
Jeneric/Pentron, Inc. Wallington, CT 06492
leucite-reinforced glass
portion of the embedded tooth was again cemented to a glass
slide and sectioned on a slow-speed diamond wheel saw (Buehler Ltd.) equipped with a thin-section attachment (Buehler Ltd.). Each cylindrical specimen had approximately 1-2-mm thickness of enamel remaining. Some cylindrical specimens with sufficient enamel thickness could be used more than once, in which case the contact surface was re-polished by hand with the 0.25-gLm polishing paste. No attempt was made to categorize the teeth according to age, gender, or fluoride exposure. Commercially-available dental ceramics were formed into approximately 25-mm-diameter disks for both enamel abrasion and indentation hardness testing. The five ceramic materials tested are listed in Table 1. Glass slides (25.4 mm in diameter x 1.6 mm thick) were used as supplied by the manufacturer (Hugh Courtright, Chicago, IL) and were included in the study for purposes of comparison, with the presumption that the slides were largely free of the types of defects (surface roughness, pores, inhomogeneity of microstructure, non-parallel surfaces, etc.) that might occur when dental ceramics are hand-formed. The disks were fabricated by various methods that approximated their clinical application. Disks composed of material DI were cast in acrylic resin patterns and cerammed according to the manufacturer's recommendations. Half of these were ground on one side to remove the surface layer, and the DG material was applied according to the manufacturer's recommendation, with at least ten applications. Material VMK was condensed and fired on metal disks cast from Rexillium III (Rx Jeneric Gold Co., Wallingford, CT), and OP was formed in a polyvinyl siloxane mold, condensed with a plastic plunger into a large pellet, dried, and fired according to the manufacturer's recommendations for the furnace used (Starfire, J.M. Ney Co., Bloomfield, CT). Silicon carbide slurries on highspeed rotating metal disks were used to grind the surfaces of each material flat and approximately parallel. The test surface of each specimen was then hand-ground on a glass slab through a series of abrasives ending in a 1-pm alumina slurry. A final optical polish was achieved with 0.1-gm-diamond polishing paste for 90 min on an automatic polisher. Five specimens of each ceramic were fabricated. A ceramic specimen was attached to the center of the lower platen well of the polisher with thermoplastic epoxy resin (Crystalbond 509 Adhesive, Aremco Products Inc., Ossining, NY). The well was filled with distilled water, and a race bearing was put in place. The enamel specimen-holder was adjusted so that the end was approximately 10-20 gm from the surface of the disk and free from any contact. An enamel cylinder was placed in the holder, and the loading and drive arm of the polisher were activated. The enamel specimen was
tStandard deviations given in parentheses.
Vertical lines connect means that were not significantly different at alpha = 0.05.
held in contact with the ceramic surface with a 0.65-N load and traced a spiral pattern around the disk. The area of enamel contact on the ceramic specimen was approximately 10.0 mm2. The load and time interval used for this evaluation were selected based on prior experimental trials. These system parameters resulted in an amount of enamel loss that was measurable, but not severe enough to abrade through to the underlying dentin. The linear speed of the tracing was approximately 0.24 ms-1, and the length of each sliding stroke was approximately 75 mm. The apparatus was controlled by a digital timer, and each specimen was run for four h. The ceramic disk, enamel cylinder, and distilled water were changed after each period. Two tests were made with each specimen, with the ceramic being re-surfaced before the second test. A total of ten tests was made for each of the five ceramic materials. The cylinders were measured with a micrometer to an accuracy of 0.005 mm before and after each test period. The change in length was recorded as the difference between the two micrometer read-
ings. Variation in the wear rates of different enamel specimens was investigated by cycling ten enamel cylinders against a 15gm-grit diamond disk (Buehler Ltd.). Each specimen was run for four min with a 0.42-N load for this test and measured as before. A Buehler micro-indentation tester was used for Knoop hardness testing. The same ceramic specimens were tested in the central area, where no abrasion had occurred. Each sample was indented five times with a 4.9-N load. Mean enamel loss values were calculated for each experimental group, and differences between the means were tested for statistical significance with analysis of variance. Multiple comparisons were performed with Tukey's multiple range test (alpha = 0.05). This statistical analysis was also used for the hardness values.
Results. The mean wear rate of the ten control enamel specimens abraded by the 15-gum-diamond grit surface was 210 gim, with a standard deviation of 10 gLm. The results of the enamel abrasion test are summarized in Table 2. The number of specimens tested and the means and standard deviations of the amount of enamel loss in micrometers are reported. Vertical lines connect group means that are not significantly different. Mean differences less than 57 gm were not significant at the 0.05 confidence level. Material OP resulted in the most rapid enamel wear (176 gin), and material
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ABRASION OF ENAMEL BY DIFFERENT CERAMICS
Vol. 70 No. 3
Relationship Between Ceramic Hardness and Enamel Wear Rates
E
E m
400
z I
DU
DG
GS
VMK
OP
Ceramic Material
Fig. 2-Relationship between Knoop hardness values and loss of enamel for five ceramic materials. Refer to Table 1 for material ID codes. Vertical bars represent standard deviations of the means.
DI showed the least loss (72 Am). The standard deviation of the manufactured glass slides (50 gim) fell within the range of the dental ceramics (19 to 58 Am), suggesting that inconsistency of specimen preparation was not a problem with the other materials. The results of the Knoop hardness measurements are summarized in Table 2. The means and standard deviations of 25 measurements are reported for each ceramic group and for natural tooth enamel. Vertical lines connect group means that are not significantly different. Mean differences lower than 13 kg/mm2 were not significant at the 0.05 confidence level. Material DG had the hardest ceramic surface, while material DI was the softest and closest to human tooth enamel. A comparison between hardness and wear is given in Fig. 2. There was a poor correlation between the hardness of the ceramic material and its potential to abrade human enamel (r =
0.027).
Discussion. Irreversible enamel loss due to attrition must be a concern to every dentist restoring teeth with dental ceramic materials. This is particularly true with newly introduced materials where levels of clinical wear have not been established. There are problems relating data from our study to the clinical setting. There are limitations as to precisely how the constant-contact two-body wear cycle used in this in vitro experiment models clinical tooth wear. Particulate debris of the abrasion process, especially from the enamel, may remain in place, leading to a three-body wear situation. Although this may be somewhat analogous to the clinical situation with bruxism patients exhibiting extensive occlusal wear, force levels and durations will differ considerably from those in our in vitro model. Delong and Douglas (1983), having considered the functional limitations of most in vitro wear-testing devices, designed and developed an "artificial mouth" for testing restorative materials. The artificial mouth simulates masticatory forces and durations, and these have been shown to correlate well with clinical occlusal contact wear (Delong et al., 1985; Sakaguchi et al., 1986). The results of this study are in good agreement with the results reported by Delong et al. (1989), in which the artificial mouth was used to study the wear of enamel when opposed by ceramic restorations. They found the glass-ceramic material (Dicor) to have approximately 60% of the abrasiveness of a metal-ceramic feldspathic porcelain (Cer-
223
amco II). This is comparable with the 55% less abrasion for material DI relative to VMK found in the present study. Previous investigations indicate that porcelain wears primarily by abrasion (Miller et al., 1975; Delong et al., 1986). The results of Delong et al. (1989) suggest that enamel opposed by ceramic materials exhibits similar abrasive wear characteristics. Regardless of the present system's shortcomings relative to the clinical situation, a comparison of the relative enamel wear that results may be useful for understanding the processes involved in abrasive-type wear. There is a tendency among manufacturers and clinicians to equate the hardness of a material with its potential to abrade the opposing natural dentition. For most materials, metals in particular, the wear resistance is believed to be directly proportional to the hardness, as expressed in Archard's theory (1953). This direct relationship has been verified experimentally by Babichev (1962) for elements where the hardness is controlled by the details of the interatomic forces. This relationship, however, does not appear to be valid for materials such as ceramics, in which the hardness is determined more by the microstructural inhomogeneities. Yust and Carignan (1985) conducted sliding wear experiments with a variety of ceramic materials. In each case, a ceramic cylinder was abraded against a ceramic plate. Similar pin-on-disk experiments are reported by Stachowiak and Stachowiak (1989) and Stachowiak et al (1989) with ceramic and metallic pins against ceramic plates. In each study, a poor correlation was found between ceramic wear rates and material hardness values. Similarly, the results of this investigation show an equally poor correlation between ceramic hardness and enamel wear, suggesting the existence of a more complex relationship. A theoretical analysis of ceramic wear by a hard material has been reported by Evans and Marshall (1980). They showed that an analysis based on the lateral cracking mechanism correlated reasonably well with published experimental data, with the volume removed being proportional to both the fracture toughness and hardness of the material. When ceramic slides against ceramic, wear does not usually occur by plastic deformation, as with metals, but by fracture (Buckley and Miyoshi, 1984; Fischer and Tomizawa, 1985). In this case, it stands to reason that the hardness (which is a measure of resistance to plastic deformation) plays a much smaller role, and that the wear resistance of the material is governed more by its fracture toughness (Fischer et al., 1989). Fracture toughness undoubtedly plays some role in the abrasive characteristics of ceramics opposed by tooth enamel, and further investigation is needed. Microstructural features-including porosity (Wu and Rice, 1985; Rice, 1985) and grain size (Wu et al., 1985; Cho et al., 1989)-have been implicated in the wear process. An understanding of microstructure might be useful in predicting the wear characteristics of new ceramic formulations. Permanent deformation, fracture, and wear mechanisms, although not fully understood, are different for crystalline and non-crystalline ceramics. The crystalline materials exhibit dislocations and twinning under indentation loads, while the non-crystalline ceramics indent primarily by densification and fracture. When microstructurally complex materials of both crystalline and non-crystalline phases are being compared, it is perhaps not surprising that wear and hardness are poorly correlated. Of interest, then, are the differences in the microstructures of the five ceramic test surfaces. Material VMK, like most dental ceramics designed for metal veneering, is formulated from a blend of two different frits (Weinstein et al., 1962). One contains leucite (K2O-Al203-4SiO2) crystals and provides the high coefficient of thermal expansion necessary for metal compatibility. The other is all glass, has a much lower thermal expansion coefficient, and also a low
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J Dent Res March 1991
al.
Fig. 3-Scanning electron micrographs of representative polished surfaces of ceramic specimens. Refer to Table 1 for material ID codes. Length of bar represents approximately 10 prm.
sintering temperature, allowing the material to be fired below the melting range of the metal substructure. A composite structure is formed when the restoration is fired (Barreiro et al., 1989). Material OP also has a glass/leucite composite microstructure with a leucite content of about 55% to 75% by weight, estimated to be 40% greater than that of a conventional metalceramic porcelain (Katz, 1989). The increased leucite content has been claimed to improve physical properties, but may also account for the high relative enamel wear found in this study. This was despite OP's relatively low indentation hardness value. However, the Knoop indentation was much larger than the crystal size (about 5 Atm) of the composite (Katz, 1989). It is not known how the glass matrix compares in hardness with the leucite crystals, or how the indentation microhardness of the leucite differs with orientation of the indenter to the crystal. Further microhardness analysis may be helpful in understanding the wear characteristics of this material. OP is recommended by its manufacturer as being suitable for the fabrication of posterior crowns and fixed partial dentures without a metal substrate-situations where sliding contact may occur. Opposing tooth wear might well be a clinical problem under these conditions and should be carefully monitored. Material DI had the lowest hardness value as well as the least enamel wear of the materials tested. It too is a glass/ crystalline composite, with tetrasilicic-mica as the crystalline phase (Adair, 1984). It is one of a group of glass-ceramics described as having excellent machining properties due to the ease with which mica can be cleaved and the random arrangement of the crystals in the material (Grossman, 1972). This may also account for its low abrasiveness. Glass-ceramics have been suggested as a coating for sliding metal parts in industrial
machine tools, since they have been shown to impart wear resistance and decrease the coefficient of friction (Sen et al., 1989). In practice, DI is often coated with a colored glass layer (DG) for improved esthetic properties. DG was found to be statistically significantly harder than the other ceramics, but had low relative abrasiveness. The exact composition of DG is proprietary, but is stated by the manufacturer to be a feldspathic glass with low crystalline opacifier and low pigment content. Lack of a significant crystalline phase or porosity may account for its low relative abrasiveness despite a high hardness value. Its abrasiveness was similar to that of the glass microscope slides. Another possible explanation for the differences in enamel abrasion might be attributable to differences in surface finish of the ceramics. As mentioned previously, Monasky and Taylor (1971) concluded that enamel wear was greatest with rough porcelain surfaces. At the start of the four-hour cycle, all specimens had similar optical finishes (0.1-gm diamond); however, there might have been surface differences due to voids in some materials. 0ilo (1988) measured internal defects in the fractured surfaces of dental ceramics. He found DI to have significantly fewer and smaller voids than representative metalceramic porcelains. Lack of voids may, in part, account for the low abrasiveness of DI, compared with those of the metalceramic VMK and OP. For this reason, scanning electron micrographs were made of our test specimens for qualitative evaluation. This qualitative assessment is presented in Fig. 3. Material OP showed the most noticeable surface variation, with the two micrographs labeled OP in Fig. 3 representing fields of views at the two extremes of this variation. The other materials were more consistent, and only one field is given as representative. Materials DI and GS appeared to show the fewest voids. No obvious correlation between voids and enamel wear rates was noticed, with material DG showing voids, but with abrasiveness similar to that of material GS. However, further quantitative assessment might be justified. It is also likely that changes in surface topography occurred during the four-hour test with abrasion of the ceramic. Although not quantified, the DI material showed the most surface degradation at the end of the four-hour period, and all specimens showed some breakdown. What is not known is whether the abrasiveness of some or all of these materials changed during the test. Possibly, harder materials such as DG and GS were more resistant to surface scratching, retaining their polished surfaces for longer periods of time, with less breakdown. Stachowiak and Stachowiak (1989) have reported that wear rates reached a steady state after 1/2 h of testing, while Levy and Jee (1988), using the washer-on-disk wear test described in ASTM standard D-3702-78, found that constant wear rates of ceramic coatings occurred after one h. It is clear that the physical factors and relationships that govern the wear of ceramics and tooth structures are complex, and no simple theory or model can adequately describe this phenomenon. Further investigation is needed to clarify the mechanisms involved and optimize the future design of materials with regard to this characteristic.
Acknowledgments. We thank Ram Alkaly and Bob Jones for their assistance with this study.
REFERENCES ADAIR, P.J. (1984): Glass-ceramic Dental Products. U.S. Patent No. 4,431,420, Feb. 14.
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ARCHARD, J.F. (1953): Contact and Rubbing of Flat Surfaces, J Apple Physiol 24:981-988. BABICHEV, M.A. (1962): Investigation of Abrasive Wear of Metals by the Brinnell Method. In: Friction and Wear in Machinery, Vol. 14, M.M. Kruschov, Ed., ASME Translations, New York: American Society of Mechanical Engineering, pp. 1-29. BARREIRO, M.M.; RIESGO, O.; and VICENTE, E.E. (1989): Phase Identification in Dental Porcelains for Ceramo-metallic Restorations, Dent Mater 5:51-57. BUCKLEY, D.H. and MIYOSHI, K. (1984): Friction and Wear of Ceramics, Wear 100:333-353. CHO, S.-J., HOCKEY, B.J.; LAWN, B.R.; and BENNISON, S.J. (1989): Grain-size and R-curve Effects in the Abrasive Wear of Alumina, JAm Ceram Soc 72:1249-1252. CHRISTENSEN, G.J. (1986): The Use of Porcelain-fused-to-metal Restorations in Current Dental Practice: A Survey, JProsthet Dent 56:1-3. DELONG, R. and DOUGLAS, W.H. (1983): Development of an Artificial Oral Environment for the Testing of Dental Restoratives: Biaxial Force and Movement Control, J Dent Res 62:32-36. DELONG, R.; DOUGLAS, W.H.; SAKAGUCHI, R.L.; and PINTADO, M.R. (1986): The Wear of Dental Porcelain in an Artificial Mouth, Dent Mater 2:214-219. DELONG, R.; SAKAGUCHI, R.L.; and DOUGLAS, W.H. (1985): The Wear of Dental Amalgam in an Artificial Mouth: A Clinical Correlation, Dent Mater 1:238-242. DELONG, R.; SASIK, C.; PINTADO, M.R.; and DOUGLAS, W.H. (1989): The Wear of Enamel when Opposed by Ceramic Systems, Dent Mater 5:266-271. EKFELDT, A. and 0ILO, G. (1988): Occlusal Contact Wear of Prosthodontic Materials, Acta Odontol Scand 46:159-169. EVANS, A.G. and MARSHALL, D.B. (1980): Wear Mechanisms in Ceramics. In: Fundamentals of Friction and Wear of Materials, D.A. Rigney, Ed., Metals Park, OH: American Society for Metals, pp. 439-452. FISCHER, T.E.; ANDERSON, M.P.; and JAHANMIR, S. (1989): Influence of Fracture Toughness on the Wear Resistance of Yttriadoped Zirconium Oxide, JAm Ceram Soc 72:252-257. FISCHER, T.E. and TOMIZAWA, H. (1985): Interaction of Microfracture and Tribochemistry in the Friction and Wear of Silicon Nitride, Wear 105:29-45. GROSSMAN, D.G. (1972): Machinable Glass-ceramics Based on Tetrasilicic Mica, JAm Ceram Soc 55:446-449. KATZ, S. (1989): High Strength Feldspathic Porcelains Containing Crystalline Leucite, U.S. Patent No. 4,798,536, Jan. 17. LAMBRECHTS, P.; BRAEM, M.; and VANHERLE, G. (1987):
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Evaluation of Clinical Performance for Posterior Composite Resin and Dental Adhesives, Oper Dent 12:53-78. LEVY, A.V. and JEE, N. (1988): Unlubricated Sliding Wear of Ceramic Materials, Wear 121:363-380. MAHALICK, J.A.; KNAP, F.J.; and WEITER, E.J. (1971): Occlusal Wear in Prosthodontics, JAm Dent Assoc 82:154-159. MILLER, G.R.; POWERS, J.M.; and LUDEMA, K.C. (1975): Frictional Behavior and Surface Failure of Dental Feldspathic Porcelain, Wear 31:307-316. MONASKY, G.E. and TAYLOR, D.F. (1971): Studies on the Wear of Porcelain, Enamel, and Gold, J Prosthet Dent 25:299-306. 0ILO, G. (1988): Flexural Strength and Internal Defects in some Dental Porcelains, Acta Odontol Scand 46:313-322. RICE, R.W. (1985): Micromechanics of Microstructural Aspects of Ceramic Wear, Ceram Eng Sci Proc 6:940-958. ROSENSTIEL, S.F.; LAND, M.L.; and FUJIMOTO, J. (1988): Contemporary Fixed Prosthodontics, St. Louis: The C.V. Mosby Co., p. 301. SAKAGUCHI, R.L.; DOUGLAS, W.H.; DELONG, R.; and PINTADO, M.R. (1986): The Wear of a Posterior Composite in an Artificial Mouth: a Clinical Correlation, Dent Mater 2:235-240. SEN, R.; DUTTA, S.; DAS, S.K.; and BASU, S.K. (1989): Evaluation of a Glass-ceramic Coating for Machine Tool Slides, Wear 130:249-260. STACHOWIAK, G.W. and STACHOWIAK, G.B. (1989): Unlubricated Friction and Wear Behavior of Toughened Zirconia Ceramics, Wear 132:151-171. STACHOWIAK, G.W.; STACHOWIAK, G.B.; and BATCHELOR, A.W. (1989): Metallic Film Transfer during Metal-ceramic Unlubricated Sliding, Wear 132:361-381. TILLITSON, E.W.; CRAIG, R.G.; and PAYTON, F.A. (1971): Friction and Wear of Restorative Dental Materials, J Dent Res 50:149-154. WEINSTEIN, M.; KATZ, S.; and WEINSTEIN, A.B. (1962): Fused Porcelain-to-metal Teeth. U.S. Patent No. 3,052,982, Sept. 11. WILEY, M.G. (1989): Effects of Porcelain on Occluding Surfaces of Restored Teeth, J Prosthet Dent 61:133-137. WU, C.CM. and RICE, R.W. (1985): Porosity Dependence of Wear and Other Mechanical Properties on Fine-grain A1203 and B4C, Ceram Eng Sci Proc 6:977-994. WU, C.CM.; RICE, R.W.; JOHNSON, D.; and PLATT, B.A. (1985): Grain Size Dependence of Wear in Ceramics, Ceram Eng Sci Proc 6:995-1011. YUST, C.S. and CARIGNAN, F.J. (1985): Observations on the Sliding Wear of Ceramics, ASLE Trans 28:245-252.
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Manufacturers generally quote indentation hardness values when predicting the clinical wear potential of newly introduced ceramic restoratives. The objective of this study was to determine whether in vitro two-body wear correlated well with hardness. A modified polisher was used to abrade enamel cylinders against polished disks of commercially available dental porcelains and glass. Enamel loss after four h was measured with a micrometer. Five ceramic materials were tested, and enamel abrasion rates were correlated with Knoop hardness values. Dicor and Dicor coated with a shading porcelain were found to cause the lowest wear of enamel. These rates were statistically significantly lower than those obtained with Optec, the most abrasive material. These findings may be due to microstructural differences between the materials. Knoop hardness showed poor correlation with the results of the abrasive testing. J Dent Res 70(3):221-225, March, 1991
Introduction. Increased patient demand for esthetic dentistry has prompted renewed interest in all-ceramic dental restorations. Improved materials and innovative techniques have led many dentists to use all-ceramic crowns and inlays for the restoration of posterior occlusal surfaces. There is, however, considerable concern as to how these materials, formulated for improved strength, compare with respect to their tendency to abrade human enamel. Ideally, an esthetic restoration should wear at approximately the same rate as the enamel it replaces [reported by Lambrechts et al. (1987) to be about 20-40 Am per year]. In addition, the restoration should not increase the wear rate of an opposing enamel surface. Conventional feldspathic dental porcelain is generally considered to be more abrasive of enamel than other restorative materials, such as gold or amalgam. In a survey of members of the American Academy of Esthetic Dentistry, Christensen (1986) found "less wear on opposing teeth" to be the single most desirable need for change in posterior toothcolored crowns. Mahalick et al. (1971) reported enamel-porcelain wear, in vitro, to be 2.4 times greater than wear of enamel-acrylic resin and 17 times that of enamel-gold. Monasky and Taylor (1971) tested a variety of surface finishes of porcelain against tooth substance and concluded that the rate of tooth substance wear was a function of porcelain roughness. They recommended glazing or polishing porcelain to reduce enamel attrition. Ekfeldt and 0ilo (1988), using a bruxing subject, studied occlusal wear of porcelain, gold, and resin in vivo. They too found that enamel surfaces exhibited the greatest substance loss when opposed by feldspathic porcelain. These and other studies have led some clinicians (Rosenstiel et al., 1988; Wiley, 1989) to caution against the use of porcelain occlusal surfaces where rapid enamel attrition might be predicted, such as for a bruxer or complete-denture wearer having the porcelain opposed by natural teeth.
A ceramic restorative material that combines good strength without the disadvantage of increased enamel wear would be a significant addition to clinical dental practice. Traditionally, manufacturers quote indentation hardness values as a predictor of clinical wear, as reported by Tillitson et al. (1971). However, little has been written on the enamel wear characteristics of contemporary dental ceramics and whether wear rates can be predicted by indentation hardness. The objective of this study was to design a simple in vitro apparatus to measure the wear rates of human enamel against various commercially available dental ceramics, to assess the relationship between these wear rates and the measured hardness values of the ceramic materials, and to provide some scientific insight, where possible, into the observed wear phenomena.
Materials and methods. An apparatus was designed to produce continuous sliding contact between human enamel cylinders and ceramic disks by a slightly modified Minimet polisher and precision thinning attachment (Buehler Ltd., Lake Bluff, IL 60044). The stroke of the polishing machine was shortened to accommodate a smaller sample dimension, and a 3.5-mm hole, bored through the center of the adjusting screw, served as the enamel specimen holder. The essential parts of the apparatus are shown in Fig. 1. Cylinders of enamel and dentin, approximately 2 mm in diameter by 4 mm in length, were obtained from extracted human maxillary molar teeth in the following manner: The occlusal one-third of each lingual surface was ground by hand on a glass plate with 1000-grit silicon carbide slurry until a flat area of about 2-3 mm in diameter was obtained. The flat surface was cemented with cyanoacrylate resin to a glass slide, embedded in a methyl methacrylate resin by a 25-mm-diameter ring mold, removed from the glass slide, and polished to a 0.25-p~m diamond finish. Each tooth was drilled to a depth of about 4-5 mm with a 3.5-mm-inside-diameter diamond-tipped hollow-core drill bit (#2868A23, McMaster Carr, Los Angeles, CA), with copious amounts of water coolant. The flat Wear Machine Load
0 dng '
Bearing F
Received for publication March 16, 1990 Accepted for publication November 20, 1990
IMicroadjustible Specimen Holder
Ceramic Disc
Fig. 1-Schematic of the wear apparatus. D
=
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Dentin, E
=
Enamel. 221
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J Dent Res March 1991
TABLE 1 PRODUCT INFORMATION ID DI
Product Name
Dicor"'
DG
Dicor Shading Porcelain
GS
Glass Slide
VMK
OP
Vita VMK 68T"
Optec H.S.P.
Material Type Manufacturer Dentsply International, Inc. glass-ceramic York, PA 17405
Dentsply International, Inc.
Hugh Courtright Chicago, IL 60638 Vita Zahnfabrik
feldspathic porcelain
TABLE 2 MEAN ENAMEL WEAR AND KNOOP HARDNESS VALUES OF VARIOUS CERAMIC MATERIALS KHN Enamel Wear n [kglmm2]t n Material (pm]t 379 (10) 25 DI 72 (19) 11 DG
10
81(20)
25
441 (18)
GS
10
105 (50)
25
443 (11)
VMK
10
130 (35)
25
436 (20)
OP
12
176 (58)
25
401 (12)
glass
Sackingen, Germany
feldspathic porcelain
Jeneric/Pentron, Inc. Wallington, CT 06492
leucite-reinforced glass
portion of the embedded tooth was again cemented to a glass
slide and sectioned on a slow-speed diamond wheel saw (Buehler Ltd.) equipped with a thin-section attachment (Buehler Ltd.). Each cylindrical specimen had approximately 1-2-mm thickness of enamel remaining. Some cylindrical specimens with sufficient enamel thickness could be used more than once, in which case the contact surface was re-polished by hand with the 0.25-gLm polishing paste. No attempt was made to categorize the teeth according to age, gender, or fluoride exposure. Commercially-available dental ceramics were formed into approximately 25-mm-diameter disks for both enamel abrasion and indentation hardness testing. The five ceramic materials tested are listed in Table 1. Glass slides (25.4 mm in diameter x 1.6 mm thick) were used as supplied by the manufacturer (Hugh Courtright, Chicago, IL) and were included in the study for purposes of comparison, with the presumption that the slides were largely free of the types of defects (surface roughness, pores, inhomogeneity of microstructure, non-parallel surfaces, etc.) that might occur when dental ceramics are hand-formed. The disks were fabricated by various methods that approximated their clinical application. Disks composed of material DI were cast in acrylic resin patterns and cerammed according to the manufacturer's recommendations. Half of these were ground on one side to remove the surface layer, and the DG material was applied according to the manufacturer's recommendation, with at least ten applications. Material VMK was condensed and fired on metal disks cast from Rexillium III (Rx Jeneric Gold Co., Wallingford, CT), and OP was formed in a polyvinyl siloxane mold, condensed with a plastic plunger into a large pellet, dried, and fired according to the manufacturer's recommendations for the furnace used (Starfire, J.M. Ney Co., Bloomfield, CT). Silicon carbide slurries on highspeed rotating metal disks were used to grind the surfaces of each material flat and approximately parallel. The test surface of each specimen was then hand-ground on a glass slab through a series of abrasives ending in a 1-pm alumina slurry. A final optical polish was achieved with 0.1-gm-diamond polishing paste for 90 min on an automatic polisher. Five specimens of each ceramic were fabricated. A ceramic specimen was attached to the center of the lower platen well of the polisher with thermoplastic epoxy resin (Crystalbond 509 Adhesive, Aremco Products Inc., Ossining, NY). The well was filled with distilled water, and a race bearing was put in place. The enamel specimen-holder was adjusted so that the end was approximately 10-20 gm from the surface of the disk and free from any contact. An enamel cylinder was placed in the holder, and the loading and drive arm of the polisher were activated. The enamel specimen was
tStandard deviations given in parentheses.
Vertical lines connect means that were not significantly different at alpha = 0.05.
held in contact with the ceramic surface with a 0.65-N load and traced a spiral pattern around the disk. The area of enamel contact on the ceramic specimen was approximately 10.0 mm2. The load and time interval used for this evaluation were selected based on prior experimental trials. These system parameters resulted in an amount of enamel loss that was measurable, but not severe enough to abrade through to the underlying dentin. The linear speed of the tracing was approximately 0.24 ms-1, and the length of each sliding stroke was approximately 75 mm. The apparatus was controlled by a digital timer, and each specimen was run for four h. The ceramic disk, enamel cylinder, and distilled water were changed after each period. Two tests were made with each specimen, with the ceramic being re-surfaced before the second test. A total of ten tests was made for each of the five ceramic materials. The cylinders were measured with a micrometer to an accuracy of 0.005 mm before and after each test period. The change in length was recorded as the difference between the two micrometer read-
ings. Variation in the wear rates of different enamel specimens was investigated by cycling ten enamel cylinders against a 15gm-grit diamond disk (Buehler Ltd.). Each specimen was run for four min with a 0.42-N load for this test and measured as before. A Buehler micro-indentation tester was used for Knoop hardness testing. The same ceramic specimens were tested in the central area, where no abrasion had occurred. Each sample was indented five times with a 4.9-N load. Mean enamel loss values were calculated for each experimental group, and differences between the means were tested for statistical significance with analysis of variance. Multiple comparisons were performed with Tukey's multiple range test (alpha = 0.05). This statistical analysis was also used for the hardness values.
Results. The mean wear rate of the ten control enamel specimens abraded by the 15-gum-diamond grit surface was 210 gim, with a standard deviation of 10 gLm. The results of the enamel abrasion test are summarized in Table 2. The number of specimens tested and the means and standard deviations of the amount of enamel loss in micrometers are reported. Vertical lines connect group means that are not significantly different. Mean differences less than 57 gm were not significant at the 0.05 confidence level. Material OP resulted in the most rapid enamel wear (176 gin), and material
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ABRASION OF ENAMEL BY DIFFERENT CERAMICS
Vol. 70 No. 3
Relationship Between Ceramic Hardness and Enamel Wear Rates
E
E m
400
z I
DU
DG
GS
VMK
OP
Ceramic Material
Fig. 2-Relationship between Knoop hardness values and loss of enamel for five ceramic materials. Refer to Table 1 for material ID codes. Vertical bars represent standard deviations of the means.
DI showed the least loss (72 Am). The standard deviation of the manufactured glass slides (50 gim) fell within the range of the dental ceramics (19 to 58 Am), suggesting that inconsistency of specimen preparation was not a problem with the other materials. The results of the Knoop hardness measurements are summarized in Table 2. The means and standard deviations of 25 measurements are reported for each ceramic group and for natural tooth enamel. Vertical lines connect group means that are not significantly different. Mean differences lower than 13 kg/mm2 were not significant at the 0.05 confidence level. Material DG had the hardest ceramic surface, while material DI was the softest and closest to human tooth enamel. A comparison between hardness and wear is given in Fig. 2. There was a poor correlation between the hardness of the ceramic material and its potential to abrade human enamel (r =
0.027).
Discussion. Irreversible enamel loss due to attrition must be a concern to every dentist restoring teeth with dental ceramic materials. This is particularly true with newly introduced materials where levels of clinical wear have not been established. There are problems relating data from our study to the clinical setting. There are limitations as to precisely how the constant-contact two-body wear cycle used in this in vitro experiment models clinical tooth wear. Particulate debris of the abrasion process, especially from the enamel, may remain in place, leading to a three-body wear situation. Although this may be somewhat analogous to the clinical situation with bruxism patients exhibiting extensive occlusal wear, force levels and durations will differ considerably from those in our in vitro model. Delong and Douglas (1983), having considered the functional limitations of most in vitro wear-testing devices, designed and developed an "artificial mouth" for testing restorative materials. The artificial mouth simulates masticatory forces and durations, and these have been shown to correlate well with clinical occlusal contact wear (Delong et al., 1985; Sakaguchi et al., 1986). The results of this study are in good agreement with the results reported by Delong et al. (1989), in which the artificial mouth was used to study the wear of enamel when opposed by ceramic restorations. They found the glass-ceramic material (Dicor) to have approximately 60% of the abrasiveness of a metal-ceramic feldspathic porcelain (Cer-
223
amco II). This is comparable with the 55% less abrasion for material DI relative to VMK found in the present study. Previous investigations indicate that porcelain wears primarily by abrasion (Miller et al., 1975; Delong et al., 1986). The results of Delong et al. (1989) suggest that enamel opposed by ceramic materials exhibits similar abrasive wear characteristics. Regardless of the present system's shortcomings relative to the clinical situation, a comparison of the relative enamel wear that results may be useful for understanding the processes involved in abrasive-type wear. There is a tendency among manufacturers and clinicians to equate the hardness of a material with its potential to abrade the opposing natural dentition. For most materials, metals in particular, the wear resistance is believed to be directly proportional to the hardness, as expressed in Archard's theory (1953). This direct relationship has been verified experimentally by Babichev (1962) for elements where the hardness is controlled by the details of the interatomic forces. This relationship, however, does not appear to be valid for materials such as ceramics, in which the hardness is determined more by the microstructural inhomogeneities. Yust and Carignan (1985) conducted sliding wear experiments with a variety of ceramic materials. In each case, a ceramic cylinder was abraded against a ceramic plate. Similar pin-on-disk experiments are reported by Stachowiak and Stachowiak (1989) and Stachowiak et al (1989) with ceramic and metallic pins against ceramic plates. In each study, a poor correlation was found between ceramic wear rates and material hardness values. Similarly, the results of this investigation show an equally poor correlation between ceramic hardness and enamel wear, suggesting the existence of a more complex relationship. A theoretical analysis of ceramic wear by a hard material has been reported by Evans and Marshall (1980). They showed that an analysis based on the lateral cracking mechanism correlated reasonably well with published experimental data, with the volume removed being proportional to both the fracture toughness and hardness of the material. When ceramic slides against ceramic, wear does not usually occur by plastic deformation, as with metals, but by fracture (Buckley and Miyoshi, 1984; Fischer and Tomizawa, 1985). In this case, it stands to reason that the hardness (which is a measure of resistance to plastic deformation) plays a much smaller role, and that the wear resistance of the material is governed more by its fracture toughness (Fischer et al., 1989). Fracture toughness undoubtedly plays some role in the abrasive characteristics of ceramics opposed by tooth enamel, and further investigation is needed. Microstructural features-including porosity (Wu and Rice, 1985; Rice, 1985) and grain size (Wu et al., 1985; Cho et al., 1989)-have been implicated in the wear process. An understanding of microstructure might be useful in predicting the wear characteristics of new ceramic formulations. Permanent deformation, fracture, and wear mechanisms, although not fully understood, are different for crystalline and non-crystalline ceramics. The crystalline materials exhibit dislocations and twinning under indentation loads, while the non-crystalline ceramics indent primarily by densification and fracture. When microstructurally complex materials of both crystalline and non-crystalline phases are being compared, it is perhaps not surprising that wear and hardness are poorly correlated. Of interest, then, are the differences in the microstructures of the five ceramic test surfaces. Material VMK, like most dental ceramics designed for metal veneering, is formulated from a blend of two different frits (Weinstein et al., 1962). One contains leucite (K2O-Al203-4SiO2) crystals and provides the high coefficient of thermal expansion necessary for metal compatibility. The other is all glass, has a much lower thermal expansion coefficient, and also a low
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SEGHI et
J Dent Res March 1991
al.
Fig. 3-Scanning electron micrographs of representative polished surfaces of ceramic specimens. Refer to Table 1 for material ID codes. Length of bar represents approximately 10 prm.
sintering temperature, allowing the material to be fired below the melting range of the metal substructure. A composite structure is formed when the restoration is fired (Barreiro et al., 1989). Material OP also has a glass/leucite composite microstructure with a leucite content of about 55% to 75% by weight, estimated to be 40% greater than that of a conventional metalceramic porcelain (Katz, 1989). The increased leucite content has been claimed to improve physical properties, but may also account for the high relative enamel wear found in this study. This was despite OP's relatively low indentation hardness value. However, the Knoop indentation was much larger than the crystal size (about 5 Atm) of the composite (Katz, 1989). It is not known how the glass matrix compares in hardness with the leucite crystals, or how the indentation microhardness of the leucite differs with orientation of the indenter to the crystal. Further microhardness analysis may be helpful in understanding the wear characteristics of this material. OP is recommended by its manufacturer as being suitable for the fabrication of posterior crowns and fixed partial dentures without a metal substrate-situations where sliding contact may occur. Opposing tooth wear might well be a clinical problem under these conditions and should be carefully monitored. Material DI had the lowest hardness value as well as the least enamel wear of the materials tested. It too is a glass/ crystalline composite, with tetrasilicic-mica as the crystalline phase (Adair, 1984). It is one of a group of glass-ceramics described as having excellent machining properties due to the ease with which mica can be cleaved and the random arrangement of the crystals in the material (Grossman, 1972). This may also account for its low abrasiveness. Glass-ceramics have been suggested as a coating for sliding metal parts in industrial
machine tools, since they have been shown to impart wear resistance and decrease the coefficient of friction (Sen et al., 1989). In practice, DI is often coated with a colored glass layer (DG) for improved esthetic properties. DG was found to be statistically significantly harder than the other ceramics, but had low relative abrasiveness. The exact composition of DG is proprietary, but is stated by the manufacturer to be a feldspathic glass with low crystalline opacifier and low pigment content. Lack of a significant crystalline phase or porosity may account for its low relative abrasiveness despite a high hardness value. Its abrasiveness was similar to that of the glass microscope slides. Another possible explanation for the differences in enamel abrasion might be attributable to differences in surface finish of the ceramics. As mentioned previously, Monasky and Taylor (1971) concluded that enamel wear was greatest with rough porcelain surfaces. At the start of the four-hour cycle, all specimens had similar optical finishes (0.1-gm diamond); however, there might have been surface differences due to voids in some materials. 0ilo (1988) measured internal defects in the fractured surfaces of dental ceramics. He found DI to have significantly fewer and smaller voids than representative metalceramic porcelains. Lack of voids may, in part, account for the low abrasiveness of DI, compared with those of the metalceramic VMK and OP. For this reason, scanning electron micrographs were made of our test specimens for qualitative evaluation. This qualitative assessment is presented in Fig. 3. Material OP showed the most noticeable surface variation, with the two micrographs labeled OP in Fig. 3 representing fields of views at the two extremes of this variation. The other materials were more consistent, and only one field is given as representative. Materials DI and GS appeared to show the fewest voids. No obvious correlation between voids and enamel wear rates was noticed, with material DG showing voids, but with abrasiveness similar to that of material GS. However, further quantitative assessment might be justified. It is also likely that changes in surface topography occurred during the four-hour test with abrasion of the ceramic. Although not quantified, the DI material showed the most surface degradation at the end of the four-hour period, and all specimens showed some breakdown. What is not known is whether the abrasiveness of some or all of these materials changed during the test. Possibly, harder materials such as DG and GS were more resistant to surface scratching, retaining their polished surfaces for longer periods of time, with less breakdown. Stachowiak and Stachowiak (1989) have reported that wear rates reached a steady state after 1/2 h of testing, while Levy and Jee (1988), using the washer-on-disk wear test described in ASTM standard D-3702-78, found that constant wear rates of ceramic coatings occurred after one h. It is clear that the physical factors and relationships that govern the wear of ceramics and tooth structures are complex, and no simple theory or model can adequately describe this phenomenon. Further investigation is needed to clarify the mechanisms involved and optimize the future design of materials with regard to this characteristic.
Acknowledgments. We thank Ram Alkaly and Bob Jones for their assistance with this study.
REFERENCES ADAIR, P.J. (1984): Glass-ceramic Dental Products. U.S. Patent No. 4,431,420, Feb. 14.
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