In vitro abrasion resistance and hardness of glass-ionomer cements

In vitro abrasion resistance and hardness of glass-ionomer cements H. Forss L. Seppfi R. Lappalainen Faculty of Dentistry University of Kuopio P.O.B. 6 SF-70211 Kuopio, Finland

Received January 17, 1990 Accepted July 2, 1990 Dent Mater 7:36-39, January, 1991

Abstract-The aim of this study was to compare the abrasion resistance and surface hardness of four glass-ionomer cements. The effects of hydration and dehydration on wear resistance were also studied. A composite material, enamel, and dentin were used as controls. For wear testing, the specimens were abraded on abrasion discs under water. All glass ionomers showed greater wear than composite and enamel, but less wear than dentin. Ketac-Fil showed the highest and Ketac-Silver the lowest wear resistance. Hydration or dehydration of the specimens did not significantly influence the wear rate of conventional glass ionomers, but the wear resistance of Ketac-Silver was increased due to dehydration. Ketac-Fil had the highest and Ketac-Silver the lowest hardness rating of the glass ionomers. The cermet material did not show abrasion resistance better than that of the conventional glass ionomers, as has previously been suggested.

he ability to resist mechanical wear is an important requirement of a material designed for use as a posterior dental filling. Recently, glass-ionomer cements, generally accepted as filling materials for class III and V cavities (for a review, see McLean and Gasser, 1985; Walls, 1986; Knibbs, 1988), have also been recommended as an alternative material for posterior occlusal fillings in low-stress-bearing areas and for restoration of early caries lesions, especially in the deciduous dentition (McLean, 1988). In the laboratory studies that have evaluated the wear resistance of glass-ionomer cements, conventional glass ionomers have generally shown more wear than resin composites (Smales and Joyce, 1978; Moore et al., 1984). In the studies by Engelsmann et al. (1988) and Walls et al. (1988), a conventional glass-ionomer cement showed greater loss of anatomical form than did amalgam in deciduous dentition. The cermet c e m e n t s - t h a t is, the glass ionomers with silver ions sintered to the glass particles-were developed with a view to improving some undesirable properties, e.g., the low wear resistance of glass ionomers (McLean and Gasser, 1985). In some studies, cermet cements have clearly shown less wear than conventional glass ionomers (Moore et al., 1985; McKinney et al., 1988). However, in the study by Walls et al. (1987), comparing cermet cements and conventional glass ionomers, a cermet cement was similarly susceptible to two-body abrasion with an element of fatigue, but was more susceptible to three-body abrasion. Due to a prolonged setting reaction, dehydration or hydration of glass ionomers after the initial setting has been reported to influence their mechanical strength, surface h a r d n e s s , and w e a r r e s i s t a n c e (Causton, 1981; Mount and Makin-

T

36 FORSS et al,/ABRASION RESISTANCE OF GLASS IONOMERS

son, 1982; McKinney et al., 1987, 1988). Since there is little information about the relative wear of different types of glass ionomers, the present study was undertaken to compare the surface hardness and the abrasion r e s i s t a n c e of four commercially available glass-ionomer cements, including a cermet material. The effects of hydration and dehydration after the initial setting reaction on wear resistance were also studied. MATERIALS AND METHODS The materials studied are shown in the Table. Nine cylindrical specimens (diameter, 25 ram; thickness, 4 mm) were prepared from each material according to the manufacturers' instructions. The dental nurses and dentists preparing the specimens were experienced in handling glass-ionomer cements. The freshly mixed material was packed into the silicon rubber mould provided with the test apparatus. Each specimen of the encapsulated materials (Ketac-Fil, Ketac-Silver, Fujicap) was prepared by use of 12 capsules, which were mixed simultaneously in highspeed amalgamators (Silamat; Ivoclar, Liechtenstein) for 10 s. The hand-mixed material (Chemf'fl) was spatulated simultaneously by four dental nurses. The powder:liquid ratio was measured by use of the scoop and dropper bottle provided by the manufacturer. For each material, three of the specimens were covered with a protective varnish (Odus Dental Varnish; Odus Dental AG, Ziirich, Switzerland) and stored in 100% humidity, and six of the specimens were left uncovered. Of those left uncovered, three were stored dry, and three were allowed to set for four rain before being stored in distilled water. After the preparation, all specimens were stored as mentioned at room temperature (22° C) for 14 days.

Prior to being tested, the test speclinens were embedded in stone plaster, and a flat brass button was fLxed to the center point of the stone plaster surface. This facilitated the exact measurement of the loss of height during the wear test. Before being tested, the specimen ends which were to be abraded were ground flat under water with silicon carbide discs (1200-grit). For the controls, the same silicon rubber moulds were used. Several pieces of human enamel or dentin from extracted teeth were tightly embedded mosaic-like in self-curing acrylic to form a specimen with the same diameter and height as the test specimens. In addition, one specimen was prepared from Silux Plus. The control specimens were ground fiat as above and stored in 100% humidity before the wear test. Wear testing was performed with use of the apparatus (Scandimatic 3430; Hans P. Tempelmann, Hagen, FRG) previously used for wear testing of dental materials (Lappalainen et al., 1989). During each test cycle, the specimen holder of the apparatus with three test specimens moved back and forth against the rotating silicon carbide disc (diameter, 200 mm; 150 rpm; load 306 g/test specimen). The t e s t cycles were performed under running water at room temperature (22°C). The placement of the specimens in the specimen holder was changed every 30 min, and the process was repeated six times for each specimen. The rate of abrasion was constant during each period. The distance between the upper end of the brass button and the center point of the abraded surface was measured after every 30minute period, by means of a micrometer (NSK Digitrix II; Japan Micrometer Mfg. Co., Osaka, Japan), and the loss of height was calculated. The mean wear rate of the group was expressed as the mean loss of height of the three specimens within the group during each period. For hardness tests, three specimens (diameter, 10 ram; thickness, 1.5 mm) were prepared from each test material. The specimens were prepared in cylindrical brass moulds resting on a glass plate. The freshly mixed cement was inserted into the mould, covered with a celluloid strip

(3M C o m p a n y , MN, USA), and pressed with the help of a glass microscope slide. After the strip was removed, each specimen was covered with protective varnish and stored in 100% humidity for one week at room temperature (22°). Before the hardness tests, the specimen surface was ground with a 1200-grit silicon carbide paper under running water for removal of its outermost layers. The hardness of the materials was determined at room t e m p e r a t u r e (22°C) by means of a Leitz hardness tester (Ernst Leitz GmbH, Wetzlar, FRG) with a Vickers diamond and 200-g load. Three indentations were made in each specimen, and the mean of nine indentations for each material was calculated. In controls, the hardness was determined after the abrasion cycles. Nine indentations were made in each specimen. Data were analyzed by one-way analysis of variance for detection of significant differences, and Newman-Keuls multiple comparison tests were used for pair-wise comparisons. RESULTS

The mean amount of wear for the materials is shown in Table 2. All

glass-ionomer materials showed greater wear than composite and enamel, but less wear than dentin. For specimens stored in 100% humidity, differences in the wear rate between the glass ionomers were significant statistically, with KetacFil showing the highest wear resistance (p<0.05). The cermet cement (Ketac-Silver) showed significantly greater wear than did conventional glass ionomers (p < 0.05). Hydration or dehydration of the specimens did not significantly influence the rate of wear of conventional glass ionomers, but the wear of Ketac-Silver was decreased due to dehydration (Table 2). Ketac-Silver also had the lowest hardness rating of all glass ionomers (Table 3). Each of the conventional glass ionomers was significantly harder than Ketac-Silver (p<0.05). Ketac-Fil showed a hardness rating significantly higher than that of the rest of the conventional glass ionomers (p < 0.05). DISCUSSION

In spite of the wide variety of sophisticated methods designed for wear-testing of dental materials, none has proved superior in simulating oral

TABLE 1 MATERIALS USED IN THE STUDY Material Chemfil II

Fujicap II Ketac-Fil Ketac-Silver Silux Plus

Manufacturer DeTrey, Dentsply, Konstanz, FRG G-C Dental Industrial Corp., Tokyo, Japan Espe GmbH Seefeld, FRG Espe GmbH Seefeld, FRG 3M Company St Paul, MN, USA

Batch No. 881184 880257 070491

0044 0058 0126 7C3B

TABLE 2 MEAN WEAR RATE FOR MATERIALS

Wear (l~m/30 rain) 100% Humidity Air Material Mean S.D. Mean S.D. Chernfil II 59.0 13.9 49.6 14.1 Fujicap II 56.2 22.0 57.3 17.0 Ketac-Fil 34.1 9.7 41.3 16.1 Ketac-Silver 76.0 20.2 62.1 18.9 Silux Plus 18.2 5.3 Enamel 23.7 11.9 Dentin 136.0 50.0 After the initial setting, the specimens were either stored in 100% humidity allowed to dry.

Water Mean 44.2 54.4 28.6 83.4

S.D. 24.3 12.8 14.3 32.6

or in water, or were

Dental Materials~January 1991 37

TABLE 3 MEAN VALUES FOR VICKERS MICROHARDNESSNUMBER OF THE MATERIALS

Material Chemfll II Fujicap II Ketac-Fil Ketac-Silver Silux Plus Enamel

Dentin

Mean 51.1 73.5 89.9 39.2 40.6 293.9 57.2

S.D. 2.3 9.8 7.3 6.9 1.4 17.1 2.6

A 200-gram load was used.

conditions (for review, see McCabe, 1985; Sulong and Aziz, 1990). Therefore, we considered that less sophisticated methods, such as used in the present study, can be applied to compare and rank the abrasion resistance of materials with r a t h e r similar composition. In the present study, glass-ionomer materials showed less abrasion resistance than did a microfilled composite material, which is in line with previous findings (Smales and Joyce, 1978; Moore et al., 1984). However, the finding that the silver cermet cement was clearly less abrasion-resistant than conventional glass ionomers disagrees with the findings of Moore et al. (1985) and McKinney et al. (1988), and deserves further consideration. In general, clinical wear of a material is a combination of several types of wear mechanisms (McCabe and Smith, 1981). When one is considering the results of in vitro studies, it is important to know which type of wear is involved. In the present study, the test method with a fine abrasive disc used as a counterpart provides information about the two-body abrasive wear of the tested materials, whereas in the studies of Moore et al. (1985) and McKinney et al. (1988), with a smooth, hard slider used as a counterpart, a different type of wear is tested. According to McKinney et al. (1988), the incorporation of silver provides a lubricating effect which contributes to the better abrasion resistance of silver cermet material. However, in the presence of flue abrasive particles, the behavior of cermet cement seems to be different. Our findings confm~a the results of Walls et al. (1987), who used a three-body wear test with abrasive slurries. In their study, the cermet cement showed less abrasion resistance than did conventional glass ionomers.

Furthermore, in i n vitro abrasion studies, the amount of abrasion depends on the force exerted on the specimen and the grit size of the abrasive paper, although this relationship is not linear (McCabe and Smith, 1981; Harrison and Moores, 1985). With the finer abrasives, such as used in the present study, the type of wear may differ from that in the studies with coarser a b r a s i v e s - f atigue wear may also be involved (McCabe and Smith, 1981). In addition, the force exerted on the specimens in the present study (306 g/test specimen = 0.006 MPa) was considerably lower than that applied in previous studies of glass ionomers. The ranking order of the materials might have been different if greater forces had been used. Dehydration of glass ionomers has been reported to result in microscopic cracking within the material (Subrata and Davidson, 1989). In the present study, hydration or dehydration had little effect on the wear resistance of conventional glass ionomers, although a tendency of inc r e a s e d r a t e of w e a r due to dehydration was found for Ketac-Fil. The finding that dehydration increased the wear resistance of Ketac-Silver is in accordance with that of McKinney et al. (1988). The wear resistance and hardness of the materials were not directly proportional, which agrees with previous findings ( H a r r i s o n and Draughn, 1976; Lappalainen et al., 1989). However, the fact that KetacSilver also showed the lowest hardness of all materials further supports findings that Ketac-Silver used in the present study does not show better physical properties than conventional glass ionomers. In the studies by McKinney et al. (1987, 1988), the hardness values of a cermet cement and conventional glass

38 FORSS et aL/ABRASION RESISTANCE OF GLASS IONOMERS

ionomers were of the same order. The hardness of the composite was in line with previous findings (Lappalainen et al., 1989). It should be remembered that the results of an in vitro study must always be interpreted with caution. Abrasion resistance is only one of the properties influencing the longevity of fillings. For instance, erosive processes contribute to the wear of glass-ionomer materials (McKinney et al., 1987, 1988; Roulet and Wglti, 1984). However, it seems that clinical studies are urgently needed to determine whether silver cermet cements are in fact more wear-resist a n t t h a n c o n v e n t i o n a l glass ionomers, as is generally believed. ACKNOWLEDGMENT We are grateful to Ms. Hanna Eskelinen for skillful technical assistance. REFERENCES CAUSTON,B.E. (1981): The Physico-mechanical Consequences of Exposing Glass Ionomer Cements to Water During Setting, Biomaterials 2: 112115. ENGELSMANN, U.; KOCHER, T.; and ALBERS, H.-K. (1988): Vergleichende Langzeituntersuchung fiber die Ffillungsmaterialien Ketac Fil und Amalgam an Milchz~ihnen,Dtsch Zahndrztl Z 43: 291-294. HARRISON, A. and DRAUGHN, R.A. (1976): Abrasive Wear, Tensile Strength, and Hardness of Dental Composite Resins-Is There a Relationship?, J Prosthet Dent 36: 395--398. HARRISON,A. and MOORES,G.E. (1985): Influence of Abrasive Particle Size and Contact Stress on the Wear Rate of Dental Restorative Materials, Dent Mater 1:15-18. KNIBBS,P.J. (1988): Glass Ionomer Cement: 10 Years of Clinical Use, J Oral Rehabil 15: 103-115. LAPPALAINEN, R.; YLI-URPO, A.; and SEPP/~, L. (1989): Wear of Dental Restorative and Prosthetic Materials in vitro, Dent Mater 5: 35-37. MCCABE, J.F. (1985): In vitro Wear Testing of Composite Resins. In: Posterior Composite Resin Dental Restorative Materials, G. Vanherle and D.G. Smith, Eds., St. Paul, MN: 3M Company, pp. 319-330. MCCABE, J.F. and SMITH, B.H. (1981): A Method for Measuring the Wear of Restorative Materials in vitro, Br Dent J 151: 123-126. McKINNEY,J.E.; ANTONUCCI,J.M.; and

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SULONG, M.Z.A.M. and AzIz, R.A. (1990): Wear of Materials Used in Dentistry: A Review of the Literature, J Prosthet Dent 63: 342-349. WALLS, A.W.G. (1986): Glass Polyalkenoate (Glass-Ionomer) Cements: A Review, J Dent 14: 231-246. WALLS, A.W.G.; ADAMSON, J.; MCCAPE, J.F.; and MURRAY,J.J. (1987): The Properties of a Glass Polyalkenoate (Ionomer) Cement Incorporating Sintered Metallic Particles, Dent Mater 3: 113-116. WALLS, A.W.G.; MURRAY, J.J.; and MCCABE, J.F. (1988): The Use of Glass Polyalkenoate (Ionomer) Cements in the Deciduous Dentition, Br Dent J 165: 13-17.

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