Fracture toughness of resin-based luting cements Lisa A. Knobloch, DDS, MS, a Ronald E. Kerby, DMD, b Robert Seghi, DDS, MS, b Jeffrey S. Berlin, c and Jeffrey S. Leec
College of Dentistry, The Ohio State University, Columbus, Ohio Statement o f p r o b l e m . The introduction of resin-modified glass ionomer cements has expanded the choices of luting cements available to the clinician; however, few independent studies are available on the fracture toughness of the currently available resin-modified glass ionomer luting agents compared with the composite cements. P u r p o s e . This investigation evaluated the relative fracture toughness (Kin) of 3 composite luting cements (Panavia 21, Enforce, and C&B Metabond), 3 resin-modified glass ionomer luting cements (Advance, Vitremer Luting, and Fuji Duet), and a conventional glass ionomer luting cement (Ketac-Cem) at 24-hour and 7-day storage times. Material and m e t h o d s . Km was determined by preparing minicompact test specimens (n = 8) with introduced precracks. Specimens were stored in distilled water at 37°C + 2°C until testing. Testing was pertbrmed on an Instron testing machine at a displacement rate of 0.5 mm/min. Results. ANOVA (P<.001) and REGW Multiple Range Test (P<.05) demonstrated significant differences among several of the cements tested. The mean fracture toughness values of C&B Metabond at 24 hours and Enforce at both 24 hours and 7 days were significantly greater than use any of the other cements tested. Conclusion. The resin-modified glass ionomer cements exhibited improved fracture toughness when compared with the conventional glass ionomer; however, they were still inferior to Enforce and C&B Metabond composite cements. (J Prosthet Dent 2000;83:204-9.)
The main cause o f failure o f cemented restorations is fracture o f the cement and microleakage 1 that can result f r o m p o o r c e m e n t a t i o n technique, i m p r o p e r prosthesis design, p o o r casting fit, malocclusion, excessive forces o f mastication, nonretentive tooth preparation, or weak c e m e n t s ) Because fracture o f the cement is the primary cause o f cementation failure, fracture toughness is a mechanical property that could help predict clinical performance.2, 3 Presented at the American Association for Dental ResearchMeeting, Orlando, Fla., 1997. Supported by The Ohio State University College of Dentistry. aAssistant Professor, Section of RestorativeDentistry, Prosthodontics, and Endodontics. bAssociate Professor, Section of Restorative Dentistry, Prosthodontics, and Endodontics. CDental Student. 204 THE JOURNAL OF PROSTHETIC DENTISTRY
The fracture toughness o f a material represents the ability, o f that material to resist crack propagationfi -6 Several m e t h o d s for determining fracture toughness have been r e p o r t e d in the literature; however, the m e t h o d o l o g y must m e e t the criteria for plane strain conditions to be a true measure o f fracture t o u g h ness. 5,7d3 Plane strain fracture toughness (KIc) is a measure o f the critical stress at the tip o f a flaw that allows propagation o f a crack under tension (Mode I).6 It is the lowest stress at which catastrophic crack propagation can occur. 4 Clinically, tensile failure is the most relevant for dental materials. 14 N u m e r o u s luting agents for cast restorations arc currently available. These luting agents are generally classified according to their cement m a t r i x ) 5 Decreased microleakage has been shown for the resin-based luting cements suggesting improved clinical durability; howVOLUME 83 NUMBER 2
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Table I. Luting cements tested Luting cement
Panavia 21 Enforce C&B Metabond Vitremer Luting Advance Fuji Due[ Ketac-Cem
Kuraray Co, LTD, Osaka, Japan Dentsply, Milford, Del. Parke[I, Farmington, N.Y. 3M, St Paul, Minn. Dentsply, Milford, Del. GC America, Chicago, III. ESPE, Norristown, Pa.
M-10-P*/Bis-GMA-based composite UDMA*/Bis-GMA-based composite 4-META* + PMMA resin Resin-modified glass ionomer* Resin-modified glass ionomer* Resin-modified glass ionomer* Conventional glass ionomer
Polymerization Dual-cure polymerization Polymerization Polymerization + acid-base reaction Po[ymerization + acid-base reaction Polymerization + acid-base reaction Acid-base reaction
*Refer to text (Materialsand Methods for specific information.
ever, increased water sorptlon, inadequate compressive strength and significant plastic dcfbrmation have been reported as potential problems. 6A6-21 Subsequently, the use o f u r e t h a n e dimethacrylates22, 23 and incorporation o f filler particles 24-29 into these materials have been shown to improve properties such as fracture toughness and diametral tensile strength. Because several of these materials were recently introduced, longterm clinical data are not available. 2 The use o f glass ionomer cement as a restorative material has been limited to non-stress-bearing areas of the m o u t h because o f their low tensile strength and resistance to fracture. 3°-33 However, glass i o n o m e r cement as a luting agent has been increasing in popularity because o f fluoride release in an effort to prevent recurrent decay and lowest solubility.2,34, 3s In addition, the clinical success o f glass ionomer cement as a luting agent has been well documented.36, 37 In an 8-year follow-up study, 1230 restorations that were cemented with a glass ionomer luting agent were evaluated; only 1% o f the castings needed to be recementcd and no secondary caries was observed. 37 The introduction o f resin-modified glass ionomer cements has expanded the choices available to the clinician. 38 The modification o f the traditional glass ionomer chemistry with the addition of pendant methacrylate groups or polymerizable monomers has produced a material with the benefits o f the traditional glass ionomer cements such as chemical adhesion to calcified tissue, along with the benefits o f composite such as improved strength, fracture toughness and wear resistance.32, 39 Few independent studies are available on the fracture toughness o f the currently available resin-modified glass ionomer luting agents compared with the composite cements. The purpose o f this study was to compare the relative fracture toughness at 24 hours and 7 days after mixing o f 3 composite cements, 3 resin-modified glass ionomer cements, and 1 conventional glass ionomer cement. MATERIAL
Seven cements were tested in this study; product and manufacturer information is presented in Table I. FEBRUARY2000
Three composite cements were evaluated: Panavia 21, a Bis-GMA-based resin cement, which contains the adhesive m o n o m e r methylacryloxydecyldihydrogenphosphatc (M-10-P); Enforce, a Bis-GMA- and urethane dimethacrylate ( U D M A ) - b a s e d resin cement, which does not contain any adhesive chemicals; and C&B Metabond, a PMMA-based resin cement, which contains the adhesive monomer 4-trimellitic anhydride (4-META). Three resin-modified glass i o n o m e r cements were studied: Vitremer luting cement, a resinmodified glass ionomer cement that contains calcium aluminofluorosilicate glass powder and a resin ionomer liquid with pendant methacrylate groups off" the polycarboxylic acid chain; Advance, a resin-modified glass ionomer cement that contains calcium aluminofluorosilicate glass powder and the product of oxydipthalic dianhydride and H E M A in its resin matrix; and Fuji Duet, a resin-modified glass ionomer cement that contains calcium aluminofluorosilicate glass powder and a resin ionomer liquid with an admixture o f urethane dimethacrylate, H E M A , and polycarboxylic acid. Ketac-Cem conventional glass i o n o m c r cement was also studied. Manufacturers' specifications as to proper mixing time, paste-to-paste and powder-to-liquid ratios were carefully followed. Panavia composite cement is a 2-paste system supplied in a dispenser that expels the correct amount of catalyst and base. The Enforce composite ccment is also a 2-paste system that is dual cured; theretore, specimens were light cured using an 80-second exposure time. The Ketac-Cem glass ionomer cement is supplied in a preencapsulated form that was mixed in an amalgamator (ESPE Capmix, Germany) at high speed tbr 10 seconds. C&B Metabond, Advance, Fuji Duet, and Vitremer luting cement are supplied in a powder/liquid tbrm. Sixteen minicompact test specimens o f each cement type were prepared with a polytetrafluoroethylenc split-mold (Fig. 1) of which 8 specimens were used for 24-hour testing and 8 were used fbr 7-day testing. The relative fracture toughness (KIc) was determined from the minicompact specimens with an introduced precrack created by a razor blade as described by Kovarik et al6 in accordance with ASTM standard 399-83. 4o 205
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St Fig. 1. Polytetrafluorethylene split-mold assembly used in fabrication of minicompact test specimens. Split mold is assembled using guide screws designated by broken lines and razor blade is placed into notch following arrows. Specimen configuration produced from this assembly is labeled at top of diagram.
Specimens were maintained in distilled water at 37°C + 2°C and then fractured under wet conditions on a universal testing machine (Instron, Canton, Mass.) at 24 hours and 7 days in tensile mode using a displacement rate o f 0.5 m m / m i n (Fig. 2). Measurements o f specimen dimensions were made with a traveling microscope (60×) (Nikon Measurescope MM- 1 1 , Tokyo, Japan). The value for f>acture toughness (KIc) was then calculated for each specimen using the following equation40: (KIC) = P c • f ( a / w ) BW 0.5 where: Pc = maximum load before crack advance; f ( a / w ) = function o f a and w (ASTM 399)35; B = specimen thickness; W = dimension from the unnotched edge o f the specimen to a plane centerline o f the loading holes (m). A 2-way analysis o f variance (ANOVA), followed by the Ryan-Einot-Gabriel-Welsh (REGW) Multiple Range Test, 41 was perfbrmed on all data. RESULTS The fracture toughness results at both 24 hours and 7 days are presented in Figure 3 in which groups that are labeled with the same letter are not significantly different from one another. The ANOVA (P<.001) and R E G W Multiple Range Test (P<.05) indicated significant differences between several o f the cements tested for both 24-hour and 7-day storage times (Table II). The mean fracture toughness of C&B Metabond at 24 hours and Enforce at both 24 hours and 7 days was significantly greater than Panavia, a composite cement, 206
Fig. 2. Fracture toughness testing apparatus. Specimen is secured in Instron testing machine with guide pins placed through specimen holes. Force is applied in direction of arrows.
as well as all resin-modified and conventional glass ionomer cements tested. In addition, the fracture toughness o f the resin-modified glass ionomer cements was at least 4 times greater than that of the conventional glass ionomer, but was still significantly less than that o f the composite cements C&B Metabond and Enforce. The mean fracture toughness ofKetac-Cem, a conventional glass ionomer cement was one fourth that of any o f the other cements tested at both 24-hour and 7-day storage times, with the exception o f Vitremer luting cement at 24 hours. Finally, the 24-hour mean f}acture toughness o f the C&B Metabond resin cement was significantly higher than that at 7 days. Conversely, VOLUME 83 NUMBER 2
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Table II. Analysis of variance procedure for fracture toughness values
6 1 6 98
2.138 0.058 0.184 0.028
75.55 2.04 6.52
<.001 .157 <.001
Cement Time Cement*time Error
0.8J 0.6 0.4
i Panavia 21 i
the 24-hour fracture toughness o f Vitremer luting, a resin-modified glass ionomer cement was significantly less than that at 7 days.
Fig. 3. Mean fracture toughness (Kic) for cements tested in MPa • m<. Groups that are labeled with same letter are not significantly different from one another.
DISCUSSION Several methods have been reported in the literature for determining fracture toughness. 7 12 Methods have included the 3-point bend test, 7 double torsion m e t h o d , 8 indentation hardness, 9 single e d g e - n o t c h beam,l°, 11 and the compact test specimen. 39 A review o f the literature revealed that fracture toughness values for the same material varied depending on the testing m e t h o d used. 4,12 To obtain a true measure o f fracture toughness, it is essential that thc testing method meet the criteria tbr plane strain conditions. 5 To satisfy plane strain conditions from a theoretical standpoint, the thickness o f the specimen would have to be infinitely thick. 5 The effect of specimen size is important because fracture mechanics theory is based on assumptions o f specimen geometry. 13 Theoretically, a thicker specimen would allow minimal plastic flow adjacent to the crack tip, thus, tcsting the matcrial in true plane strain conditions. 13 However, a specimen that is too thick presents a question regarding the clinical correlation due to the bulk of composite tested and the depth of polymerization limitations that exist with these materials. The research design o f this study followed the recommendations of Kovarik et al, s who demonstrated that plane strain conditions are satisfied as long as the specimen thickness was not less than 1.6 mm. The recommended specimen thickness o f 2.0 mm was used in this study. In practice, these cements are used at a thickness o f approximately 0.05 mm and the clinical correlation is unknown. White et a116 suggested that C&B Metabond not be used as a luting agent for fixed prosthodontics because o f inadequate compressive strength properties due to its plastic behavior. C&B Metabond exhibited significant plastic deformation that has been n o t e d as a potential failure mechanism of luting agents.17,18 It is an unfilled PMMA-based cement that contains long flexible chains o f high molecular weight, which tend to lead to higher fracture toughness values when compared with highly cross-linked brittle materials such as FEBRUARY 2000
the composite and glass ionomer. Plastic deformation delays the onset o f brittle fracture, resulting in higher fracture toughness values. 19 The 7-day fracture toughness o f C&B Metabond was significantly lower than that found at 24 hours. This could be attributed to increased water sorption o f this unfilled resin that contains hydrophilic monomers such as 4-META. This may result in hydrolytic degradation and plasticization within the cement matrix. Plasticization o f a material that occurs as a result o f water absorption may lead to increased toughness; however, subsequent degradation o f the matrix may reverse this effect and enhance crack propagation. 6 The 7-day fracture toughness o f Vitremer luting cement was significantly higher than the 24-hour results. The fracture toughness o f Vitremer luting cement is initially low because o f its relatively high glass ionomer composition and minimal polymerization reaction. Vitremer luting cement contains a novel water-soluble microencapsulated initiator/accelerator system that tends to dissolve over time, resulting in a continued free-radical cure. Enforce exhibited a relative fracture toughness that was significantly higher than Panavia, despite the fact that b o t h Panavia and Enforce are Bis-GMA-based composite cements, which contain high-volume fractions o f inorganic fillers. Panavia is primarily based on M-10-P, whereas Enforce is primarily based on UDMAs. The higher fracture toughness o f Enforce could be due to the presence o f U D M A in Enforce, which have been shown to increase the toughness of composite materials. 22,23 The fracture toughness o f Ketac-Cem was one fburth that o f any of the other cements tested at both 24-hour and 7-day storage times. Ketac-Cem is a conventional glass ionomer cement that contains no covalent cross-linldng and is therefore more likely to undergo water degradation. Conventional glass ionomers are susceptible to water dehydration and crazing during 207
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the initial setting reaction. 33 The resultant microcracks would act to initiate and facilitate crack propagation within the cement matrix during testing. The incorporation of filler particles into the composite matrix has been shown to increase fracture toughness.16,24, 25 Johnson et al 19 found that a composite that contains approximately 80% filler by weight, of which 10% is microfine filler, would provide optimal fracture toughness and diametral tensile strength. This can be attributed to the mechanisms of "crack bowing'3,26, 27 and microcrack-induced toughening.I], 2g The filler particks cause the propagating crack to bow between the particles, causing an increase in fracture energy and a resultant increase in fracture toughness. 16 In addition, the presence of filler particles may also help improve toughness by increasing the fracture surface area.16, 29 The significantly higher fracture toughness of resinmodified glass ionomer luting agents when compared with the conventional glass ionomer cement is in agreement with a previous study. 39 Conventional chemically cured glass ionomer cements have been limited to non-stress-bearing areas of the mouth because of poor tensile strength, fracture resistance, and wear.J4,3°, 39 Modification of the conventional glass ionomer chemistry by the grafting of pendent methacrylate groups has been shown to produce a material with the benefits of composite such as increased compressive strength, improved fracture toughness, and lower solubility in water, gl The improved fracture toughness values o f these resin-modified glass ionomers over the conventional glass ionomer could expand the clinical use of these materials. 39 CONCLUSIONS Within the limits of this study, the tbllowing conclusions were drawn: 1. The highest mean fracture toughness values were fimnd with En~brce composite cement and the PMMAbased cement C&B Metabond at 24 hours. 2. The resin-modified glass ionomer cements had mean fracture toughness values between the resin luting cements and the conventional glass ionomer cement; however, the resin-modified glass ionomer cements exhibited an increase in fracture toughness fi:om 24 hours to 7 days. 3. The En~brce composite cement exhibited a significant decrease in fracture toughness from 24 hours to 7 days. 4. The conventional glass ionomer cement was significantly lower in fracture toughness when compared with the resin-based luting cements. REFERENCES 1. Wise MD. Failure in the restored dentition: management and treatment. Chicago: Quintessence; 1995. p. 24-5,89. 208
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2. Rosenstie] SF, Land MF, Crispin BJ. Dental luting agents: a review of the current literature. J Prosthet Dent 1998;80:280-301. 3. Muel[er HJ. Fracture toughness and fractography of dental cements, lining, build-up, and filling materials. Scanning Microsc 1990;4:297-307. 4. Kovarik RE, Fairhurst CW. Effect of Griffith precracks on measurement of composite fracture toughness. Dent Mater 1993;9:222-8. 5. Kovarik RE, Ergle JW, Fairhurst CW. Effects of specimen geometry on the measurement of fracture toughness. Dent Mater 1991;7:166-9. 6. Ferracane JL, Berge HX. Fracture toughness of experimental dental composites aged in ethanol. J Dent Res 1995;74:1418-23. 7. Roberts JC, Powers JM, Craig RG. Fracture toughness of composite and unfilled restorative resins. J Dent Res 1977;56:748-53. 8. Truong VT, Tyas M]. Prediction of in vivo wear in posterior composite resins: a fracture mechanics approach. Dent Mater 1988;4:318-27. 9. Ferracane JL. Indentation fracture toughness testing and crack propagation mode in dental composites. Mater Res Symp Proc 1989;110:619-24. 10. Lloyd CH, Adamson M. The development of fracture toughness and fracture strength in posterior restorative materials. Dent Mater 1987;3:22531. 11. Kim KH, Park JH, Imai Y, Kishi T. Microfracture mechanisms of dental resin composites containing spherically-shaped filler particles. J Dent Res 1994;73:499-504 12. Fujishima A, Ferracane JL. Comparison of four modes of fracture toughness testing for dental composites. Dent Mater 1996;12:38-43. 13. Ferracane JL, Antonio RC, Matsumoto H. Variables affecting the fracture toughness of dental composites. J Dent Res 1987;66:1140-5. 14. Uctasli S, Harrington E, Wilson HJ. The fracture resistance of dental materials. J Oral Rehab 1995;22:877-86. 15. O'Brien WJ. Dental materials: properties and selection. Chicago: Quintessence; 1989. p. 215. 16. White SN, Yu Z. Physical properties of fixed prosthodontic, resin composite luting agents. Int J Prosthodont 1993;6:384-9. 17. Paddon JM, Wilson AD. Stress relaxation studies on dental materials. I, Dental Cements. ] Dent Res 1976;4:183-9. 18. McLean JW. The future of restorative materials. J Prosthet Dent 1979; 42:154-8. 19. Johnson WW, Dhuru VB, Brantley WA. Composite microfiller content and its effect on fracture toughness and diametral tensile strength. Dent Mater 1993;9:95-8. 20. Tjan AH, Dunn JR, Grant BE. Marginal leakage of cast gold crowns luted with an adhesive resin cement. J Prosthet Dent 1992;67:11-20. 21. Yoshida K, Tanagawa M, Atsuta M. In-vitro solubility of three types of resin and conventional luting cements. I Oral Rehab 1998;25:285-91. 22. Ruyter IE. Monomer systems and polymerizations. In: Vanherle G, Smith DC, editors. Posterior composite resin dental restorative materials. Utrecht, The Netherlands: Peter Szulc Co; 1985. p. 109-35. 23. Cook WD, Mahendra M. Influence of chemical structure on the fracture behavior of dimethacrylate composite resins. Biomaterials 1990;11:272-6. 24. Pilliar RM, Vowles R, Williams DF. The effect of environmental aging on the fracture toughness of dental composites. J Dent Res 1987;66:722-6. 25. Lloyd CH, lannetta RV. The fracture toughness of dental composites. L The development of strength and fracture toughness. J Oral Rehab 1982;9:5566. 26. Young RJ, Beaumont PW. Failure of brittle polymers by slow crack growth; Ill. Effect of composition upon the fracture of silica particle-filled epoxy resin composites. J Mater Sci 1977;12:684-93. 27. Lange FF. The interaction of a crack front with a second phase dispersion. Phil Mag 1970;22:983-92. 28. Fu Y, Evans AG. Some effects of microcracks on the mechanical properties of brittle solid. Acta Meta[I 1985;33:1515-23. 29. Lange FF, Radford KC. Fracture energy of an epoxy composite system. J Mater Sci 1971;6:1197-203. 30. Goldman M. Fracture properties of composite and glass-ionomer dental restorative materials. J Biomed Mater Res 1985;19:771-83. 31. Mathis RS, Ferracane JL. Properties of a glass-ionomer/resin-composite hybrid material. Dent Mater 1989;5:355-8. 32. Wilson AD, Hill RG, Warrens CP, Lewis BG. The influence of polyacid molecular weight on some properties of glass-ionomer cements. J Dent Res 1989;68:89-94. 33. McLean JW, Wilson AD. The clinical development of the glass ionomer cement. III. The erosion lesion. Aust Dent J 1977;22:190-5. 34. Finger W. Evaluation of glass ionomer luting cements. Scand J Dent Res 1983;91:143-9. 35. Canay S, Hersek N, Akca K, Ciftci Y. The effect of weight loss of liquid on VOLUME 83 NUMBER 2
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36. 37. 38. 39. 40.
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the diametral tensile strengths of various kinds of luting cements. Int Dent J 1996;46:52-6. Brackett WW, Metz JE. Performance of a glass ionomer luting cement over 5 years in a general practice. J Prosthet Dent 1992;67:59-61. Metz JE, 8rackett WW. Performance of a glass ionomer luting cement over 8 years in a general practice. J Prosthet Dent 1994;71:13-5. Wu JC, Wilson PR. Resin luting cements for full coverage restorations. Aust Prosthod J 1994;8:55-63. Kovarik RE, Muncy MV. Fracture toughness of resin-modified glass ionomers. Am J Dent 1995;8:145-8. ASTM Standard E-399-83. Standard test method for plane-strain fracture toughness of metallic materials. In: 1992 Annual book of ASTM standards. Philadelphia: ASTM; 1992. p. 676-706. Welsch RE. Stepwise multiple comparison procedures. J Am Stat Assoc 1977;72:566-775.
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A 5-year clinical evaluation o f ceramic inlays (Cerec) cemented w i t h a dual-cured or chemically cured resin com-
posite luting cement Sj6gren G, Molin M, van Dijken JWV. Acta Odontol Scand 1999;56:263-7. Purpose. Presently, the Cerec ceramic inlay system is the only available chairside C A D / C A M system available for the direct manufacture of ceramic inlays. The aim of this study was to evaluate Cerec ceramic Class II restorations, cemented with either a dual-cured or a 2-component chemically cured resin composite after 5 years. Material and methods. Sixty-six class II C A D / C A M manufactured ceramic inlays (Cerec) were placed in 27 patients. Each patient received at least 1 inlay cemented with a dual-cured composite and 1 inlay cemented with a 2-component chemically cured resin composite. Restorations were examined after 5 years postcementation with the California Dental Association (CDA) rating system. All inlays were made by 1 of 3 dentists in accordance with the manufacturer's instructions. Each inlays was evaluated by 2 calibrated examiners working in pairs but independently. Values from the CDA scores were analyzed statistically with Fisher's exact test for difference of proportions at a significance level of 0.05. Results. At recall 89% of the 66 inlays were rated "satisfactory." During the follow-up period, 3 inlays (4.5%) were replaced as a result of fracture and 1 (1.5%) inlay because of fracture of tooth substance. All of those inlays were luted with the dual-cured cement. O f the remaining 62 inlays, the CDA rating was "excellent" was given to 84% for color, 97% for surface, and 81% for anatomic form. "Excellent" marginal integrity was seen in 52% of the dual-cured luted inlays and in 61% of the chemically cured luted inlays. No statistically significant (P>.05) difference was observed between the 2 luting cements. 30 References. I R p Rennet