Journal of Oral Rehabilitation, 1993. Volume 20. pages 133-146 Dual-cure luting composites. Part I: filler particle distribution S. INOKOSHI*, G. WILLEMS, B. VAN MEERBEEK?, P. LAMBRECHTS, M. BRAEMS, and G. VANHERLE Department of Operative Dentistry and Dental Materials, Catholic University of Leuven, "Visiting Professor from Tokyo Medical and Dental University, Tokyo; ?Aspirant National Fund of Scientific Research for Belgium and SOrofacial Morphology and Function, Faculty of Medicine, Rijksuniversituir Centrum Antwerpen Summary Fourteen dual-cure luting composites were analyzed for their filler particle shape, predominant and maximum filler size, and filler weight in function of their clinical use. Polished surfaces were etched with an argon ion beam and studied by means of scanning electron microscopy. The type of filler particles, either inorganic or prepolymerized, could clearly be recognized. Their shapes were angular, rounded or spherical, depending on the product. The maximum filler size varied extremely from less than 1 pm to 250 ym. A particle-size distribution analyser disclosed a bell-shaped filler-size distribution. The predominant filler size for all the products was much smaller than the maximum filler size. The filler weight varied from 36 to 77%. After ion etching, some products showed small areas with a low degree of filler loading. A classification of the luting composites based on the maximum filler size is proposed. Since the particle size varies widely within the group of products analyzed, a standard specification for luting composites is urgently needed. Introduction The success of tooth-coloured inlays, either ceramic or composite resin, greatly depends on the effectiveness of the adhesive luting cement, which must fix the restoration to tooth structure, compensate for the fragility of the restored tooth, and guarantee the final marginal adaptation. A dual-cure luting composite is the material of choice for these kind of inlay restorations. It ensures adequate curing in deeper areas (Nathanson & Hassan, 1987) and shortens the setting time (Albers, 1989; Lambrechts et al., 1991). The wear resistance of such a luting composite, its physical and mechanical pro- perties, and the degree of the fitting of the inlay. are determined mainly by the filler type, the particle-size distribution, and the filler content. Although many adhesive luting composites are on the market or under development, little information about these characteristics, however, is available. Recently, argon-ion-beam etching has been found useful in disclosing the sub- structure of dental restorative composites by the differential removal of material Correspondence: Prof Dr G. Vanherle, Department of Operative Dentistry. Catholic University of Leuven, Kapucijncnvoer 7, B -300 Lcuvcn, Belgium. 133
Transcript
Journal of Oral Rehabilitation, 1993. Volume 20. pages 133-146
Dual-cure luting composites. Part I: filler particle distribution
S . I N O K O S H I * , G . W I L L E M S , B . V A N M E E R B E E K ? , P. L A M B R E C H T S , M . B R A E M S , and G . V A N H E R L E Department of Operative Dentistry and Dental Materials, Catholic University of Leuven, "Visiting Professor from Tokyo Medical and Dental University, Tokyo; ?Aspirant National Fund of Scientific Research for Belgium and SOrofacial Morphology and Function, Faculty of Medicine, Rijksuniversituir Centrum Antwerpen
Summary Fourteen dual-cure luting composites were analyzed for their filler particle shape, predominant and maximum filler size, and filler weight in function of their clinical use. Polished surfaces were etched with an argon ion beam and studied by means of scanning electron microscopy.
The type of filler particles, either inorganic or prepolymerized, could clearly be recognized. Their shapes were angular, rounded or spherical, depending on the product. The maximum filler size varied extremely from less than 1 pm to 250 ym. A particle-size distribution analyser disclosed a bell-shaped filler-size distribution. The predominant filler size for all the products was much smaller than the maximum filler size. The filler weight varied from 36 to 77%. After ion etching, some products showed small areas with a low degree of filler loading. A classification of the luting composites based on the maximum filler size is proposed. Since the particle size varies widely within the group of products analyzed, a standard specification for luting composites is urgently needed.
Introduction The success of tooth-coloured inlays, either ceramic or composite resin, greatly depends on the effectiveness of the adhesive luting cement, which must fix the restoration to tooth structure, compensate for the fragility of the restored tooth, and guarantee the final marginal adaptation. A dual-cure luting composite is the material of choice for these kind of inlay restorations. It ensures adequate curing in deeper areas (Nathanson & Hassan, 1987) and shortens the setting time (Albers, 1989; Lambrechts et al., 1991).
The wear resistance of such a luting composite, its physical and mechanical pro- perties, and the degree of the fitting of the inlay. are determined mainly by the filler type, the particle-size distribution, and the filler content. Although many adhesive luting composites are on the market or under development, little information about these characteristics, however, is available.
Recently, argon-ion-beam etching has been found useful in disclosing the sub- structure of dental restorative composites by the differential removal of material
Correspondence: Prof Dr G. Vanherle, Department of Operative Dentistry. Catholic University of Leuven, Kapucijncnvoer 7, B -300 Lcuvcn, Belgium.
133
134 S. Inokoshi et al.
components (Hosoda et al., 1989; Inai, 1990; Inokoshi et al., 1990) In the present study, polished surfaces of 14 dual-cure luting composites were etched with an argon- ion beam to enable the shape and size of the filler particles to be determined by means of scanning electron microscopy. The particle-size distributions of several products were determined with a laser diffraction particle size analyzer (Willems et al., 1992).
Materials and methods Twelve commercially available and two experimental dual-cure luting composites were studied (Table 1). All were mixed according to the manufacturer’s instructions and were placed in translucent cylindrical moulds of 5mm in diameter and 3mm in height. The top and bottom surfaces were then light-cured for 60s each. Three specimens were prepared for each product. The cured composite was embedded in a self-curing acrylic resin* and ground and polished using wet silicon carbide papers (600, 1200 and 4000 grit size) and diamond pastes (3 pm and 1 pm grit size). The polished surfaces were then etched with an argon-ion beam for 10min using an argon ion milling apparatus?. The aperture of the original specimen holder was enlarged from
Table 1. Investigated dual-cure luting composites
Product name Batch number Manufacturer
Exp. Cerec-Coltbe Duo Cement
Exp. 3M Luting Composite Resin
Choice Porcelain Adhesive
Brilliant Duo Cement
Dicor MGC Luting Composite
Gcera I1
Heliolin k
Microfill Pontic C
Mirage FLC
Optec Dual Cure Luting Cement
Palfique Inlay Resin Cement
Porcelite Dual Cure Ccment
Vita Cerec Duo Cement
Vivadent Dual Cement
Base CE-101189-6 Cat. CE-101189-5 A 220A B 220B Base 089150 Cat. 089100 Base 201190-32 Cat. 201190-32 Base 081090 Cat. 092090 Base 1121UB Cat. 1121UC Base 260597 Cat. 160063 Base 034 Cat. 036 Base C100290 Cat. D100890 A 010980 B 010990 RCAlOl RCB601 Base 02060 Cat. 02276 Base 290191-04 Cat. 290191-04 Base 260566 Cat. 260556
Co l the AG, Altstatten, Switzerland
3M, St-Paul, MN, U S A .
Bisco, Downers Grove, IL, U.S.A.
Coltkne AG, Altstatten, Switzerland
Caulk-Dentsply, Milford, DE, U.S.A.
GC, Tokyo, Japan
Vivadent Ets., Schaan, Liechtenstein
Kulzer, Wehrheim, Germany
Chameleon, Kansas City, KA, USA
Jeneric/Pentron, Wallingford, CT, U.S.A.
Tokuyama Soda, Tokuyama, Japan
Kerr, Romulus, MI, U.S.A.
Vita Zahnfabrik, Bad Backingen, Germany
Vivadent, Shaan, Liechtenstein
Vita Cerec Duo Cement is a commercial version of the experimental Ccrcc-Colttne Duo Cement.
* Technovit 4001. Kulzer, Wehrheim, Germany. t DuoMill Type 600. Gatan Inc., Warrendale. PA, U.S.A
Filler characterization of dual-cure luting composites 135
3.0mm to 10-0mm to establish a circular etched surface with a diameter of 5.0mm. Two ion guns, which were placed on either side of the specimen, were adjusted to an angle of 20" with the polished surface. The accelerating voltage was set to 5 kV and total gun current to 1mA. During etching, the specimen stage was continuously rotated at the rate of twice a minute to ensure uniform etching. The etched surfaces were sputter-coated with gold and examined under a scanning electron microscope*.
When the products contained particles larger than 5 pm, the maximum filler size of the 20 largest particles was measured using an incident light measuring microscope (Lambrechts et al., 1984).
The polished surface of the experimental Cerec-Colthe Duo Cement was observed under SEM prior to argon-ion beam etching.
In order to measure the filler weight, 0.5 g of the resin paste was weighed (PL200133, accuracy = 0.001 gt) . The paste was dissolved by adding 2.0 ml of acetonet, stirred, and centrifuged for 30min at 3000rpm. The solution was carefully discarded without jeopardizing the precipitated filler, and 2-0ml of acetone was again added. After centrifugation, the solution was discarded and the precipitate was dried at 37°C for 12 h. The weighing container with the precipitate was then weighed. The filler weight was determined by calculating the weight difference before and after extracting the monomer resin by centrifugation. This procedure was repeated five times for each paste, and the filler content of base and catalyst pastes were averaged
The obtained filler powders were analysed with a laser diffraction particle-size analyserD to observe the filler distribution and to determine the most predominant particle size by volume. If 30% of the filler sizes of the luting composite powder was less than 3pm, a refractive index of the particles was required by the apparatus to allow it to determine the particle-size distribution more accurately by creating a specific optical model. A refractive index of 1.550 (based on manufacturer's information), was used for barium-glass particles and for those of unknown composition, and an index of 1.525 (based on manufacturer's information) for feldspatic particles. The accuracy of the device was confirmed using spherical Sioz powder particles of known diameter (mean diameter = 3.2 pm, standard deviation = 0-013) and known refractive index (1-49)l.
Results
Filler-particle shape Filler particles, either inorganic or pre-polymerized, were clearly recognized under SEM after argon-ion etching. The filler shape was either angular, rounded, or spherical, depending on the product (Table 2). However, neither pre-polymerized nor inorganic small particles were observed in Heliolink. Although the manufacturer claims that Heliolink is homogeneously microfilled, these particles were not detectable under SEM, and only globular structures were observed under a 5920 time magnification (Fig. 1). Argon-ion etching of Palfique Inlay Resin Cement revealed inorganic spherical particles without pre-polymerized complexes (Fig. 2). Ten of the products showed
:F XL20, Philips, Eindhoven. The Netherlands. t Mettler Instrument AG, Greifensee, Switzerland. $ Pro analysi, Merk, Darmstadt, Gemany. I Coulter LS 130, Coulter Corp., Hialeah. U.S.A. 7 Spherical silica SP-32H, lot. 2046. Tokuyama Soda, Tokuyama, Japan.
136 S. Inokoshi et al.
Table 2. Fillcr type, filler size, and filler contents of luting composites
Heliolink Palfiquc IRC Exp. Cercc Vita Cerec Duo Porcelite Dua. CC Brilliant Duo C. Dicor MGC Exp. 3M LCR Miragc FLC GCera I1 Optcc DC Lut. C . Microf. Pont. C. Vivadent Dua. C.
Choicc Porc. A .
Microfiller Sphcrical In. Angular In. Angular In. Angular In Angular In. Angular In. Rounded I n . Angular In. Angular In . Angular In. Angular In. PPF-Roundcd Rounded I n . Angular I n .
Composition of fillers is based on manufacturcr’s information; Ba(A1)B = Barium (aluminum) borosilicatc glass: Ba = Barium: Sr = Strontium; YbF3 = Ytterbium trifluoride; In. =Inorganic filler; PFS. = Predominant filler sizc by volumc determined by a particle size analyser: Max. = Maximum sizc range: w/w% = mean filler contcnt by wcight.
Fig. 1. Heliolink ( ~ 5 9 2 0 ) . The ion-beam ctchcd surface reveals a globular structure similar to that of a prepolymcrizcd filler. Field width = 10btm. Corner inscrtion ( ~ 4 0 0 ) . Field width = 70pm.
angular shaped filler particles (Figs 3-S,7 and 8). The experimental 3M lutingcomposite was composed of rounded inorganic fillers (Fig. 6). Dual Cement contained two different kinds of fillers, rounded pre-polymerized fillers and small rounded inorganic fillers (Fig. 9A). High magnification ( X S 9 2 0 ) of the pre-polymerized filler showed a globular structure (Fig. 9B).
Filler characterization of dual-cure luting composites 137
Fig. 2. Palfique Resin Inlay Cement (X5920). Homogeneous spherical particles of about 0.2 to 0.3 vm can clearly be detected. Field width = 1 0 p . Corner insertion ( x ~ O O ) , Field width = 70ym.
Fig. 3. Porcelite Dual Cure Cement ( x 1480). Small angular particlcs of less than 1 lun up to 3 pm are densely packed. Field width = 40 pm. Corner inscrtion (XJOO), Field width = 70 pm.
Maximum filler size The maximum filler size extremely varied from less than 1 pm to 250 ym. (Table 2). Ten of the 14 products showed a maximum filler size below 25pm. Although Optec Dual Cure Luting Cement was composed of very small fillers, some large particles of 35-58ym could be observed scattered on the surface (Fig. 7). Microfil Pontic C contained a high amount of angular fillers larger than 10ym. Its maximum size was comparable to that of Optec Dual Cure Luting Cement. (Fig. 8). The largest particles
138 S. Inokoshi et al.
Fig. 4. Brilliant Duo Cement ( X 1480). Angular particles of less than 1 pm up to 10 pm are densely packcd. Field width = 40 pm. Corner insertion (X400). Field width = 70 pm.
Fig. 5. Dicor MGC Luting Composite (x740). Angular particles of different size are densely packed. Field width = 80 pm. Corner insertion (X400), Field width = 70 pm.
of Choice Porcelain Adhesive were nearly 250km. The maximum filler size of the pre-polymerized filler of Dual Cement was 58-87 ym.
Particle size distribution und predominant filler size Since very fine filler partides tend to agglomerate during the particle-size-distribution measurement, Heliolink, Palfique IRC, the experimental Cerec-Coltene Duo Cement, Vita Cerec Duo Cement, and Porcelite Dual Cure Cement were excluded from analysis. Such unintentional agglomeration of submicron fillers has been reported for Palfique
Filler characterization of dual-cure luting composites 139
Fig. 6. Experimental 3M Luting Composite (X740). Rounded filler particles of different size are densely packed. Field width = 80 pm. Corner insertion ( x ~ O O ) , Field width = 70 km.
Fig. 7. Optec Dual Cure (~400) . An angular macrofiller is observed among the small particles. Field width = 147 wm.
IRC (Nitta, Yamada & Hosoda, 1990). All the products analyzed had a monomodal bell-shaped distribution, except for Optec Dual Cure Luting Cement, which had a bimodal distribution (Fig. 10). The predominant filler size was always much smaller than the maximum filler size (Table 2).
Filler weight The filler weight content is listed in Table 2 for the different products analysed.
140 S. Inokoshi et al.
Fig. 8. Microfil Pontic C (X400). Angular fillers of more than lOpm are very common in this product. Field width = 147 urn.
Whereas Heliolink showed a very low amount of filler particles (36w/w%), the filler weight content of the other products varied from 58 to 77%.
A dditional findings Argon-ion etching sometimes disclosed smooth, dark areas (Fig. 11A). High magni- fication of those areas (Fig. 11B) showed a globular surface structure similar to that of Heliolink (Fig. 1) or to a pre-polymerized filler particle, as found in Dual Cement (Fig. 9A). However, these dark areas were rather concave in contrast to the protruded pre-polymerized filler. Such areas were frequently found in the experimental Cerec- C o l t h e Duo Cement and in Porcelite Dual Cure Cement and occasionally in the other resins.
Chestnut-like crystals could be frequently observed on a non-etched polished surface of the experimental Cerec-Coltene Duo Cement (Fig. 12). When the catalyst and base pastes were separately examined under an optical microscope by placing them between two glass plates, the crystals were observed scattered in the catalyst paste and not in the base paste. Their maximum diameter was measured to be about 30-40 km. However, Vita Cerec Duo cement, the commercial version of the cement, did not contain these crystals.
Discussion Argon-ion bombardment has been used by metallurgists for etching metal surfaces in order to disclose their structure. Already in 1962, Boyde and Stewart used argon-ion- beam etching to erode dental tissues and found it useful for accentuating differences in structural composition at the surface of mineralized tissues. Recently, argon ion- beam-etching has been used to disclose the subsurface damaged layer in dental restorative composites (Hosoda er al., 1989; Inai, 1990) and the interfacial structure between an adhesive resin and dentine (Inokoshi et al., 1990) Bohm et al. (1991) reported ion-beam slope cutting as a non-mechanical method for SEM sample prepar-
Filler characterization of dual-cure luting composites 141
Fig. 9. (A) Dual Cement (X400). Rounded, dark, prepolymerizcd particles. and rounded. inorganic, bright particles of ytterbium ttifluoride are observed. Field width = 147 pm. (B) Dual Cement (~5920) . High magnification of the boundary between a pre-polymerized filler and the resin matrix shows that the pre-polymerized filler slightly protrudes from the matrix phase. A globular surface structure of the filler is apparent. Numerous small particles are densely packcd in the matrix phase. Field width = 10 vm.
ation of dental materials. During ion etching, accelerated argon ions hit and remove atoms at the specimen surface. The resistance of the material components against removal by argon-ion-beam etching is related to the composition of the material. The resin component of a polished composite resin surface is removed much faster, which exposes and accentuates the filler particles (Hosoda et al., 1989: Inai, 1990).
In the present study, argon-ion-beam etching not only provided a view of the filler distribution but also clearly disclosed fillers down to the submicron level (Fig. 2, 9A and 11B), It revealed the globular structure of the pre-polymerized filler surface,
Fig. 10. Filler particle distribution of Microfil Pontic C (top) and Optec Dual Cure Luting Cement (bottom). Microfil Pontic C shows a typical monomodal distribution, while a bimodal distribution is apparent of Optec Dual Cure Luting Cement. Although their maximum particle size is comparable, Optec Dual Cure Luting Cement contains much more smaller particles.
which suggests that microfillers had agglomerated (Fig. 9B). It also revealed a non- homogeneous distribution of particles in some products (Fig. 11). Argon-ion-beam etching is, therefore, a useful alternative for the morphological investigation of the filler particles of luting composites.
The laser-diffraction-particle-size analyzer gave quantitative information on the particle-size distribution. Although the particle sizer failed to detect the largest particles observed under the microscopes, it clearly differentiated Optec Dual Cure Luting Cement and Microfil Pontic C, for which the maximum particle size was comparable but for which the particle distribution patterns were completely different (Table 2). Although the manufacturer claims that Optec Dual Cure Luting Cement has a trimodal particle distribution, only two peaks could be clearly recognized in the particle dis- tribution (Fig. 10). The third peak might be microfillers, which were undetectable by the particle sizer.
Classifications of restorative composites were proposed by Lutz et al. (1983), Lutz & Phillips (1983), and Hosoda, Yamada & Inokoshi (1990). According to their
Filler characterization of dual-cure luting composites 143
Fig. 11. (A) (~740) . The experimental Cerec-Colthe Duo cement. Smooth and dark areas appeared after ion etching. Field width=80pm. (B) (X5920). High power magnification of the boundary area reveals aglobular structure similar to the etched surface of a pre-polymerized filler. It is slightly concave in contrast to the protruded surface of a pre-polymerized filler. Field width = 10 ym.
criteria, it is difficult to classify luting composites because their high content of small particles requires that many of them would be classified as hybrid resins or heavily filled resins. A classification of restorative composites based on particle size was introduced by Van Noort & Davis (1984), Craig (1985) and Leinfelder (1991).
Based on the maximum particle size, the 14 dual-cure luting composites investigated can be placed in five groups (Fig. 13). Luting composites containing filler particles with sizes less than 0.1 pm are defined as micro-particle filled luting composite, sizes between 0.1 pm and 1 pm as XXfine-particle filled, sizes between 1 pm and 5 pm as Xfine-particle filled, sizes between 5 pm and 25 pm as fine-particle filled, and, finally,
144 S. Inokoshi et al.
Fig. 12. ( X 1980). Experimental Cerec-Coltene Duo Cement. A chestnut-like crystal appeared on the polished, unetched surface. Field width = 35 pm.
0.04 0.1 0.3 1 2 3 5 1 0 50 100 pm I
1
i t i i ' i " t t Hdidink Palfique IRC Exp. Cerec-Cc. DC Dicor MGC Choice Porc. A.
V i Cerec Duo I Mirage FLC Porcelke Dua. CC. Exp. 3M LCR Microffl Pont. C.
Vivadent DIM. C. I GCemi' Optec DC Lut. C. Brilliant Duo C.
Micro XXFine XFine Fine Macro
Fig. 13. Classification of luting composites. 14 products are arranged on a logarithmic scale in function of their maximum filler size.
luting composites containing filler particles with sizes larger than 25 pm are defined as macro-particle filled luting composites.
Heliolink was classed as a micro-particle filled luting composite, since neither inorganic small fillers nor pre-polymerized fillers were detected. The globular structure observed seems to be clustered microfillers. Palfique Resin Inlay Cement was classed as a XXfine-particle filled luting composite. The filler weight content was rather high in spite of its small particle size.
Vita Cerec Duo Cement, the experimental Cerec-Coltene Duo Cement and Porcelite Dual Cure Cement were classified as Xfine-particle filled luting composites. With regard to their filler particle shape and size distribution, the experimental Cerec- Coltkne Duo Cement and its commercial version, Via Cerec Duo Cement, were identical. The experimental Cerec-Coltene Duo Cement is reported to be wear resistant even after 6 months of clinical service (Van Meerbeek et al., 1992).
Brilliant Duo Cement, the experimental 3M Luting Composite, Dicor MGC, Mirage FLC, and GCera I1 were all placed in the same group as fine-particle filled luting composites. In these resins, a combination of filler particles of different sizes is apparent. Their particle size permits a film thickness under 25 pm, which is the upper
Filler characterization of dual-cure luting composites 145
limit of film thickness for dental luting cements (American Dental Association, 1973). The experimental 3 M Luting Composite contains rounded inorganic fillers similar to those of P50 (Hosoda et al . , 1990). Van Meerbee et al. (1992) reported similar wear behaviour for the composite and for the heat-treated P50 inlays, Poor wear resistance was reported for Dicor MGC by Isenberg et al. (1991). Further research is needed to confirm the clinical performance.
Optec and Mirofil Pontic C contain macrofillers larger than 25pm, which may prevent adequate seating of a good-fitting restoration. Poor wear resistance has been reported for Microfil Pontic C (Essig et al., 1991; Van Meerbeek et al., 1992). Choice Porcelain Adhesive contains particles much larger than 100pm, so its use a luting agent for inlays is not recommended. These luting composites were ranked as macro- particle filled.
Dual cement is unique in being a combination of pre-polymerized and inorganic fillers. The rounded inorganic fillers, which could be observed as bright spots under SEM, are probably of ytterbium trifluoride, which was added to the luting cement for its radiopacity. Although good wear resistance has been reported after 1 year of clinical service (Essig et al., 1991), the large pre-polymerized particles might prevent optimal seating of restorations. This cement was classed as a macro-particle filled luting composite.
Levine (1989) reported a great diversity in film thickness for a group of chemically cured luting resins. A similar result might be expected with the dual-cure luting composites because of the wide variation in their filler sizes.
Since some additional material seems to be removed by ion-beam etching in the smooth, dark areas, a higher amount of organic material less resistant to etching had probably been present in these zones. Due to agglomeration of microfillers or an inadequate mixture of material components, less filled areas might unintentionally have been produced during manufacturing. Such non-homogeneous distribution of filler particles is undesirable, since luting composites must have a high wear resistence.
The chestnut-like crystals, which exclusively appeared in the catalyst paste of the experimental Cerec-Coltkne Duo Cement, might be caused by a certain incompatibility between the organic components. Their presence is possibly related to the shelf life of the product (Lambrechts & Vanherle, 1983). Since the crystals are not observed in the commercial version of the cement, Vita Cerec Duo Cement, the manufacturer seems to have improved the catalyst paste.
It is concluded that the filler particle size of dual-cure luting composites varies widely, so that a specification for luting composites is urgently required. In order to develop criteria for selecting the appropriate dual-cure luting composite in the clinical situation, further research shouid be done on more clinically related properties, such as working time, film thickness, and consistency.
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