Medical knowledge | Dentistry » Josephine-Kenneth-Megan - Fracture resistance of all ceramic and metal ceramic inlays

Josephine F. Esquivel-Upshaw, DMD, MSa Kenneth J. Anusavice, PhD, DMDb Megan Reidc Mark C. K Yangd Robert B. Leee Fracture Resistance of All-Ceramic and Metal-Ceramic Inlays Purpose: Metal-ceramic inlay designs were developed to determine if the esthetic qualities of all-ceramic inlays could be duplicated and at the same time improve their strength and stability. The objectives of this study were to: (1) compare the fracture resistance of metal-ceramic inlays with that of all-ceramic inlays; (2) determine the correlation between the degree of preparation taper and fracture resistance; and (3) determine the correlation between marginal gap width and fracture resistance. Materials and Methods: Inlay preparations were made on 60 Dentoform teeth, with 30 teeth allocated for metal-ceramic inlays and 30 teeth for all-ceramic inlays. Each group was further subdivided into 5-, 10-, and 20-degree taper preparations. Metal-ceramic inlays were fabricated using Goldtech Bio 2000 metal and

Ceramco porcelain extending to the margin, while all-ceramic inlays were made from Empress II ceramic. Marginal gap widths were measured at six critical areas after fabrication. The load at failure was measured using an Instron Universal Testing Machine. Results: The mean fracture load for all-ceramic inlays and metal-ceramic inlays at 5, 10, and 20 degrees was 70 ± 40 N, 48 ± 37 N, 33 ± 7 N, and 40 ± 23 N, 29 ± 22 N, and 14 ± 4 N, respectively. The mean gap width was 105 µm and 126 µm for all-ceramic and metal-ceramic inlays, respectively. Conclusion: The mean fracture load for Empress inlays was significantly higher than that for metal-ceramic inlays. Inlays with a 5-degree taper were significantly more fracture resistant than those with a 20-degree taper. There was no relation between marginal gap width and fracture resistance. Int J Prosthodont 2001;14:109–114 T he appeal of tooth-colored restorations has led to the use and popularity of inlays made from these

materials. As more and more patients spurn the placement of dental amalgam in their mouths, there is an increasing interest in the development of these toothcolored restorations. The porcelain inlay, which originated as early as 1862,1 made its way into the twentieth century with technologic advancements such as the high-temperature firing oven and improved particle distribution. During the past 20 years, further developments in dental adhesives and resin cements have increased the popularity of ceramic inlays because of improved fracture resistance. However, fractures of ceramic inlays still occur Most of the ceramic systems available today are mainly designed for complete-crown coverage of anterior teeth. The use of porcelain inlays is associated with the problems of ceramic fracture even prior to cementation.2–4 Inlays are also associated with an increased risk of tooth fracture Fractographic analysis of ceramic inlays and crowns has proven that crack initiation in the weaker

ceramic products often begins aAssistant Professor, Department of Prosthodontics, University of Florida College of Dentistry, Gainesville. b Professor and Chairman, Department of Dental Biomaterials, University of Florida College of Dentistry, Gainesville. c Dental Student, University of Florida College of Dentistry, Gainesville. dProfessor, Department of Statistics and Division of Biostatistics, University of Florida College of Dentistry, Gainesville. eLab Manager, Department of Biostatistics, University of Florida College of Dentistry, Gainesville. Reprint requests: Dr Josephine F. Esquivel, Department of Prosthodontics, University of Florida College of Dentistry, PO Box 100435, Gainesville, Florida 32610. Fax: + 352-846-0248 e-mail: jesquivel@dental.ufledu Volume 14, Number 2, 2001 109 The International Journal of Prosthodontics All-Ceramic and Metal-Ceramic Inlays Esquivel-Upshaw et al Materials and Methods within the bonded surface, and initial cracks become apparent

after 55% to 60% of the applied load for fracture has been reached.5 Ceramic restorations also exhibit low tensile stress and static fatigue.6 Another deficiency of ceramic restorations is their marginal fit, which is inferior to that observed with gold inlays.7–9 Although this drawback is partially offset by using the acid-etch technique for cementation, poor marginal adaptation appears to increase tooth sensitivity.2 Additionally, luting agents have been found to clinically degrade after only 2 years of clinical use.10 Later studies have confirmed that most ceramic inlay systems have acceptable marginal fit, although “acceptable” is deemed to be any marginal gap less than 100 µm.11,12 A third disadvantage of ceramic inlays is the lack of long-term data to support their longevity as a durable restorative material. A Danish study revealed that over a 40-month period, almost 50% of the inlays failed because of fracture, secondary caries, and the presence of a large marginal gap.3

Earlier studies have reported a 90% success rate over a 2-year period, although the criteria for success were not clearly defined.13 A 5-year study reports a 92% satisfactory rating for three inlay systems, but the authors concede that their fracture rate is lower than those reported in most studies because of several factors.14 Resin composites represent another tooth-colored restorative material in use today for inlays and onlays. As with all composite materials, there are the problems of poor wear resistance, marginal leakage, bacterial adhesion, and poor hydrolytic stability.6 In light of these problems, it seems that the goldalloy inlay remains the most reliable posterior intracoronal restoration. However, many patients find its metallic color objectionable. An interesting compromise that has yet to be examined is the fabrication of metal-ceramic inlays. They meet the requirement of esthetics with the porcelain veneer and, at the same time, the alloy substructure provides the

strength needed to withstand occlusal loading. Because this concept is fairly new, there has been limited literature concerning its preparation and physical properties. Clinicians15,16 have proposed the fabrication of metalceramic inlays to conserve tooth structure and to optimize the esthetic qualities of the restoration. No further studies have been performed to date to examine other facets of this type of restoration. The objectives of this study were to: (1) test the fracture resistance of metal-ceramic inlays and compare it to that of all-ceramic inlays; (2) determine the correlation between the degree of preparation taper and fracture resistance among all-ceramic inlays and porcelainfused-to-metal inlays; and (3) determine the correlation between marginal gap width and fracture resistance. The International Journal of Prosthodontics Sixty mesioocclusal inlay preparations were made on first molar Dentoform teeth (Kilgore) using a highspeed handpiece (Kavo) and diamond burs

(IOP1018, IOP4-016, IOP5-012; Brasseler). Each inlay preparation was standardized to measure 3 mm pulpally, 4 mm gingivally at the proximal box, and 5 mm at its thickest point occlusally. Dentoform teeth were embedded in Type V stone (Die Keen, Heraeus Kulzer) encased in an aluminum block for easy manipulation. From these preparations, 30 metal-ceramic inlays and 30 all-ceramic inlays were fabricated Each group was further subdivided into three groups of ten with 5, 10, and 20 degrees of taper. The taper of each preparation was determined by making individual impressions of each tooth with polyvinyl siloxane (Extrude, Kerr) and trimming the dies to form a flat surface. The angles were then measured using a protractor (Staedtler). Each inlay was made from a 0.5-mm metal substructure cast with a noble alloy (Goldtech Bio 2000, Argen). The alloy substructure was placed 15 mm away from the margin (Fig 1) to mimic an all-ceramic inlay, thereby facilitating esthetics. A die was made from

refractory material (Polyvest, Whip Mix) and opaque, body, and enamel porcelains (Vita shade A3.5; Ceramco porcelain) were applied and sintered at a maximum temperature of 970°C. The all-ceramic inlays were made from a heat-pressed ceramic (Empress II, Ivoclar/Vivadent) after waxing the desired contour and anatomy. The manufacturer’s instructions were followed for pressing and firing the ceramic Marginal gap widths were measured at six areas (midbuccal, midlingual, dovetail, proximobuccal, proximolingual, and gingival) using a traveling microscope at 30 magnification (Unitron Universal Measuring Microscope). The largest gap width was located and measured for each site These were then averaged as the mean gap width for that area for the particular sample group. Each inlay was cemented using a resin-modified glass-ionomer cement, Protec CEM (Vivadent), using a vibration method. The vibratory action was achieved by placing an ultrasonic tip (Kavo Sonicflex LUX 2000L) covered with an

acrylic resin ball on the inlay surface for 45 seconds. Excess cement was removed using a plastic filling instrument, and surfaces were wiped clean with a cotton tip. Inlays were then aged in distilled water at 37°C for 2 weeks. Fracture resistance was measured using an Instron Universal Testing Machine. A 238-mm-diameter ball bearing was placed on the central fossa of the 110 Volume 14, Number 2, 2001 Esquivel-Upshaw et al All-Ceramic and Metal-Ceramic Inlays 1.5 mm from cavosurface ➘ Mesiodistal view Fig 1 Metal-ceramic inlay design. Table 1 Fig 2 Fractured metal-ceramic inlay. Failure Load (N) Inlay All-ceramic Metal-ceramic 5-degree taper Mean SD 70 40 10-degree taper Mean SD 40 23 48 29 20-degree taper Mean SD 37 22 33 14 7 4 SD = standard deviation. inlay restoration, approximately 1.3 mm away from the buccal and lingual margins of the restoration. To ensure proper location, double-sided tape was attached to the piston of the Instron machine and used

to pick up the ball bearing positioned at the central fossa of the tooth. A rubber dam was placed between the ball bearing and the porcelain surface to prevent Hertzian fractures at area of contact.17 The force was measured upon initial fracture of the porcelain. An auditory aid (Lisle Mechanic’s Stethoscope, model 52500) was used to detect initial fracture. This was attached to the piston of the Instron machine using an elastic stabilizer so that minimal sounds of fracture could be easily heard. After initial fracture was ascertained, samples were viewed under a microscope to confirm the existence and area of fracture. Samples were stained with a fluorescent dye material (Zyglo penetrant kit, Magnaflux) to view each fracture pattern (Fig 2). A two-way analysis of variance (ANOVA) was used to determine any significant difference in the mean failure loads with samples of varying degrees of taper and material. An analysis of covariance was used to determine the correlation between the

marginal gap width and the fracture resistance between both materials. An ANOVA with multiple comparisons was used to analyze marginal gap width data, using log transformation to stabilize the variances. Volume 14, Number 2, 2001 Table 2 Mean Marginal Gap Width (mm) Area All-ceramic inlays Mean SD Midbuccal Midlingual Dovetail Proximolingual Proximobuccal Gingival 92 136 113 93 68 131 44 143 71 113 41 97 Metal-ceramic inlays Mean SD 166 196 191 239 139 190 103 196 186 318 216 85 SD = standard deviation. Results The mean failure load for Empress inlays was significantly greater than that of metal-ceramic inlays (Table 1; P ≤ 0.05) The mean difference between the failure load for the 5-degree taper group and that for the 20-degree taper group (P = 0.001) was statistically significant, but there was no significant difference between 5- versus 10-degree tapers or between 10-degree versus 20-degree taper groups (P > 0.05) Table 2 illustrates the areas and the mean gap

widths for both all-ceramic and metal-ceramic inlays. The mean marginal gap widths for metal-ceramic inlays were larger than for the all-ceramic inlays. The gap width values were not normally distributed. A few extremely large values dominated the means and the 111 The International Journal of Prosthodontics All-Ceramic and Metal-Ceramic Inlays Esquivel-Upshaw et al that was comparable to all-ceramic restorations in esthetic performance, a yellow-colored metal was chosen. The physical properties of this metal include a modulus of elasticity of 77.2 GPa, which is low compared with standard metals used for porcelain application19 Scherrer and de Rijk20 noted that “fracture resistance is dependent on the modulus of elasticity of the supporting material.” This was confirmed by Lee and Wilson,21 who recommended the use of high– elastic modulus cores for all-ceramic crowns after studying the effect of different elastic moduli of cores on fracture resistance. Because porcelain is

a brittle material, it cannot be expected to withstand moderately high tensile stresses. Thus, it should not be supported by a supporting substrate with a low modulus of elasticity.22 Another factor to be considered is the difficulty encountered in detecting the initial fracture. This can also explain the wide range of standard deviations present with all sample groups. We initially used the Instron machine graphic chart to detect slight load versus time pattern deviations, which would indicate initial fracture. This proved to be quite inaccurate, as evidenced by earlier samples that exhibited loss of the entire inlay and part of the Dentoform tooth before a pattern deviation became visible. As a result, the authors developed an auditory aid to assist in determining the initial fracture of the restoration The results were erratic in that sometimes they corresponded with a pattern deviation on the chart, and other times did not. All samples were examined under the microscope to

ascertain the presence of a crack The accepted degree of taper for inlays is in the range of 5 to 10 degrees where resistance and retention form are met.23,24 We examined both metal-ceramic and all-ceramic inlays at 5, 10, and 20 degrees of taper to determine any significance in fracture resistance. It was determined that a correlation exists between the degree of taper and the fracture resistance The 5-degree taper inlays displayed the greatest fracture resistance, while the 20-degree taper inlays showed the lowest values. Farah et al25 predicted this relation with gold inlays through finite element analysis. However, they did not verify this result experimentally They concluded that “the lateral walls of an inlay preparation will result in less stress if both the taper of the wall and the inclination of the cavosurface bevel are minimal.” The results of the present study also revealed that there is no significant difference between the 5- versus 10-degree taper or the 10- versus

20-degree taper groups, but there is a significant difference between the 5- versus 20-degree groups. Burke et al26 reported similar fracture resistance for indirect composite inlays with a taper range of 2 to 6 degrees. A schematic diagram (Fig 3) illustrates how more Increased stresses Mesiodistal view Fig 3 Increased taper resulting in increased stresses. standard deviations (Table 2). A log transformation was used to make the distribution closer to normal. Subsequent statistical analysis (two-way ANOVA) showed that there was an interaction between material and position with respect to the gap size. Metalceramic inlays produced a significantly larger mean gap than all-ceramic inlays (P < 0.05), except at the proximobuccal position (P = 0.17) There was no statistically significant association between marginal gap width and mean failure stress (P > 0.05) Discussion The causes of ceramic inlay failure include formation of local stress raisers, cavity design, inadequate

inlay thickness, presence of pores or cracks, poor fit, or excessively deep occlusal fissures.2 Because the cavity preparations were standardized between both metalceramic and all-ceramic inlays, cavity design and inlay thickness cannot explain why one inlay system performed better than the other. The modulus of elasticity of the Dentoform tooth (10.2 GPa) and dentin (10.4 ± 29 GPa) are also closely matched18 The fracture resistance of metal-ceramic inlays was significantly lower than that for the all-ceramic inlays. Although the performance was less than expected, this outcome can be attributed to several factors The choice of the metal substructure is probably critical in the lowered fracture resistance of metal-ceramic inlays. We used Goldtech Bio 2000 because of its deep yellow color. The study was originally designed to use Olympia (Jelenko), a gold-palladium alloy, but the trial inlays showed a grayish hue along the borders of the inlays in spite of opaque application. Because

the objective was to find a stronger material The International Journal of Prosthodontics 112 Volume 14, Number 2, 2001 Esquivel-Upshaw et al All-Ceramic and Metal-Ceramic Inlays stresses can be placed along the axiogingival line angle when the taper of the preparation is increased. There is less substrate support for the porcelain, and thus there is an increased propensity for fracture. Most of the fractures occurred along the margin area of the restorations, particularly for the metal-ceramic inlays. This is consistent with the fact that porcelain is brittle and a lack of support will cause it to fracture. It can be argued that these inlays performed below the expectations of clinically reported results.3,4,7,8,27,28 It must be remembered that this in vitro study subjected the inlays to an accelerated degradation with the use of an Instron machine and a ball bearing to initiate fracture. Normal masticatory patterns do not concentrate load on one area of the tooth for an

extended period. Also, the study detected fracture resistance, which is the initial fracture of the restoration, with the use of a microscope and an auditory aid. Fractures may exist in the clinical setting, but because of difficulty in detection, most inlays may and can still be in function. Marginal gap widths were compared for metal-ceramic and all-ceramic inlays for the three degrees of taper. Although the mean gap widths for the metal-ceramic inlays were slightly larger than those for the allceramic inlays, the differences were not significant (P > 0.05) This could be the result of the confounding effect between the material and the gap width (Table 2). This makes the gap effect undetectable Thus, it may be inferred that unless the gap difference is larger than 100 µm, it has little influence on fracture resistance. As mentioned earlier, marginal integrity is one of the deficiencies associated with all-ceramic restorations. The main design problem with the metal-ceramic inlay

is that for it to be comparable to the esthetics of the all-ceramic crown, there must be no appearance of metal. Therefore, the cavosurface margins would have to be subject to the same porcelain margins, which are deemed to be a deficiency of the all-ceramic restorations. The creation of the porcelain margin for the metal-ceramic inlay involves the creation of a refractory die that can withstand the firing temperatures of porcelain. This introduces an extra laboratory step in which the adaptation of the porcelain margin can be compromised. In a study by Molin and Karlsson,7 the marginal fit of gold inlays versus ceramic inlays was compared. Although their conclusion was that gold inlays offered the best marginal adaptation by far, they noted a marginal discrepancy between the fit of the restoration on the die and on the actual tooth. This was explained by the handling procedures and the different properties of all of the materials involved with the fabrication of the inlay. In the

present study, the same factors are at play, including errors from the refractory die and precautions taken to compensate for shrinkage of porcelain. Volume 14, Number 2, 2001 Clinical measurements of acceptable gap widths for metal restorations are in the range of 100 to 150 µm.29,30 Acceptable all-ceramic inlays often have larger gap widths because they are luted with adhesive cements, which compensate somewhat for poor marginal adaptation. However, several new studies indicate that acceptable marginal gap widths at baseline quickly deteriorate into unacceptable gaps within a matter of several years.27,28 The metal-ceramic inlays would probably be subject to the same problems because of their porcelain margins. This study was originally performed with the purpose of finding an alternative to porcelain inlays that exhibits a greater resistance to fracture. Fabrication of the metal-ceramic inlay clearly shows that it does not exhibit any significant increase in fracture resistance.

The laboratory work involved in its creation, starting from casting the metal to firing the porcelain, was tedious. It involved numerous steps, which introduced more variables and compromised the fit of the restoration. Perhaps the use of a metal with a higher modulus of elasticity would allow this restoration to perform better than its all-ceramic counterpart, although the increase in fracture resistance would have to be substantially higher to compensate for the additional effort involved in fabricating it. Another clinically relevant factor is that the marginal integrity of the metal-ceramic inlay is comparable, if not slightly worse, than that of the all-ceramic inlay. Although clinicians can argue that the strength and adhesion of resin-based luting cements can compensate for the poor marginal integrity of these restorations, studies have proven that the margin resin degrades over time and involves wear of the cement and chipping of the porcelain. It is therefore imperative that

marginal strength and integrity be established at baseline Conclusions This study examined the concept of metal ceramic as an alternative to all-ceramic restorations. Within the limitations of this study, we conclude that: • The fracture resistance of Empress II all-ceramic inlays is significantly greater than that of metal-ceramic inlays made of a high–gold content alloy. • The taper of the preparation affected the fracture resistance of the restorations; the greater the taper, the lower the fracture resistance. Evidence for the differences in fracture resistance of inlays with tapers of between 5 and 20 degrees is strong, but the fracture resistance differences between the 5- and 10-degree groups and the 10- and 20-degree groups were not statistically significant (P > 0.05) 113 The International Journal of Prosthodontics All-Ceramic and Metal-Ceramic Inlays Esquivel-Upshaw et al • Marginal gap width is not a good predictor of fracture resistance in that no

correlation exists between the two parameters. 14. Molin MK, Karlsson SL A randomized 5-year clinical evaluation of 3 ceramic inlay systems. Int J Prosthodont 2000;13:194–200 15. Shillingburg HT, Brooks TD, Clark TL A porcelain-fused-to-metal MOD onlay. Quintessence Dent Technol 1986;10:355–357 16. Sewitch T Resin bonded metal-ceramic inlays: A new approach J Prosthet Dent 1997;78:408–411. 17. Wakabayashi N, Anusavice KJ Crack initiation modes in bilayered alumina/porcelain disks as a function of core/veneer thickness ratio and supporting substrate stiffness J Dent Res 2000;79: 1398–1404. 18. Palamara JE, Wilson PR, Thomas CD, Messer HH A new imaging technique for measuring the surface strains applied to dentine J Dent 2000;28:141–146 19. Chew CL, Norman RD, Stewart GP Mechanical properties of metal-ceramic alloys at high temperature. Dent Mater 1990;6: 223–227. 20. Scherrer SS, de Rijk WG The fracture resistance of all-ceramic crowns on supporting structures with

different elastic moduli. Int J Prosthodont 1993;6:462–467. 21. Lee SK, Wilson PR Fracture strength of all-ceramic crowns with varying core elastic moduli. Aust Dent J 2000;45:103–107 22. Anusavice KJ Phillips’ Science of Dental Materials, ed 10 Philadelphia: WB Saunders, 1996:594–595. 23. Charbeneau GT Principles and Practice of Operative Dentistry Philadelphia: Lea & Febiger, 1988:352. 24. Smith GE, Grainger DA Biomechanical design of extensive cavity preparations for cast gold. J Am Dent Assoc 1974;89: 1152–1157. 25. Farah JW, Dennison JB, Powers JM Effects of design on stress distribution of intracoronal gold restorations J Am Dent Assoc 1977; 97:1151–1155. 26. Burke FJ, Wilson NH, Watts DC The effect of cavity wall taper on fracture resistance of teeth restored with resin composite inlays. Oper Dent 1993;18:230–236 27. Kramer N, Frankenberger R, Pelka M, Petschelt A IPS Empress inlays and onlays after four yearsA clinical study J Dent 1999;27: 325–331. 28.

Friedl KH, Schmalz G, Hiller KA, Saller A In vivo evaluation of a feldspathic ceramic system: 2-year results. J Dent 1996;24:25–31 29. Fransson B, Oilo G, Gjeitanger R The fit of metal-ceramic crowns, A clinical study. Dent Mater 1985;1:197–199 30. Karlsson S The fit of Procera titanium crowns An in vivo and clinical study. Acta Odontol Scand 1993;51:129–134 Acknowledgments This study was supported by NIDR grant DE06672-15, a UFCD Student Summer Research Fellowship, and A-dec. References 1. Qualtrough AJE, Wilson N, Smith G The porcelain inlay: A historical view Oper Dent 1990;15:61–70 2. Milleding P, Ortengren U, Karlsson S Ceramic inlay systems: Some clinical aspects. J Oral Rehabil 1995;22:571–580 3. Isidor F, Brondum K A clinical evaluation of porcelain inlays J Prosthet Dent 1995;74:140–144. 4. Bergman MA The clinical performance of ceramic inlays: A review Aust Dent J 1999;44:157–168 5. Peters MC, de Vree JH, Brekelmans WA Distributed crack analysis of ceramic

inlays. J Dent Res 1993;72:1537–1542 6. McLean JW The failed restoration: Causes of failure and how to prevent them. Int Dent J 1990;40:354–358 7. Molin M, Karlsson S A clinical evaluation of the Optec inlay system Acta Odontol Scand 1992;50:227–231 8. Molin M, Karlsson S The fit of gold inlays and three ceramic inlay systems: A clinical and in vitro study. Acta Odontol Scand 1993; 51:201–205. 9. Peutzfeldt A, Asmussen E A comparison of accuracy in seating and gap formation for three inlay/onlay techniques. Oper Dent 1990;15:129–135. 10. Thonemann B, Federlin M, Schmalz G, Schams A Clinical evaluation of heat-pressed glass-ceramic inlays in vivo: 2-year results Clin Oral Invest 1997;1:27–34. 11. Kawai K, Hayashi M, Torii M, Tsuchitani Y Marginal adaptability and fit of ceramic milled inlays. J Am Dent Assoc 1995;126: 1414–1419. 12. Reid JS, Saunders WP, Baidas KM Marginal fit and microleakage of indirect inlay systems Am J Dent 1993;6:81–84 13. Oram DA, Pearson GJ A

survey of current practice into the use of aesthetic inlays. Br Dent J 1994;176:457–462 The International Journal of Prosthodontics 114 Volume 14, Number 2, 2001