Wear behavior of human enamel against lithium disilicate glass ceramic and type III gold Ahreum Lee, BDS, DClinDent,a Michael Swain, BSc, PhD,b Lihong He, BDS, MDS, PhD,c and Karl Lyons, BDS, MDS, PhDd Department of Oral Rehabilitation, Faculty of Dentistry, University of Otago, Dunedin, New Zealand Statement of problem. The wear behavior of human enamel that opposes different prosthetic materials is still not clear. Purpose. The purpose of this in vitro study was to investigate and compare the friction and wear behavior of human tooth enamel that opposes 2 indirect restorative materials: lithium disilicate glass ceramic and Type III gold. Material and methods. Friction-wear tests on human enamel (n¼5) that opposes lithium disilicate glass ceramic (n¼5) and Type III gold (n¼5) were conducted in a ball-on-flat configuration with a reciprocating wear testing apparatus. The wear pairs were subjected to a normal load of 9.8 N, a reciprocating amplitude of approximately 200 mm, and a reciprocating frequency of approximately 1.6 Hz for up to 1100 cycles per test under distilled water lubrication. The frictional force of each cycle was recorded, and the corresponding friction coefficient for different wear pairs was calculated. After wear testing, the wear scars on the enamel specimens were examined under a scanning electron microscope. Results. Type III gold had a significantly lower steady-state friction coefficient (P¼.009) and caused less wear damage on enamel than lithium disilicate glass ceramic. Enamel that opposed lithium disilicate glass ceramic exhibited cracks, plow furrows, and surface loss, which indicated abrasive wear as the prominent wear mechanism. In comparison, the enamel wear scar that opposed Type III gold had small patches of gold smear adhered to the surface, which indicated a predominantly adhesive wear mechanism. Conclusions. A lower friction coefficient and better wear resistance were observed when human enamel was opposed by Type III gold than by lithium disilicate glass ceramic in vitro. (J Prosthet Dent 2014;-:---)
Clinical Implications Greater wear may occur when enamel is opposed by ceramic instead of gold restorations. An understanding of the mechanisms where this occurs and the controlling factors involved may help reduce this wear process.
The wear of teeth and restorative materials is a complex and multifactorial phenomenon that involves the interplay of biologic, mechanical, and chemical factors.1 Wear is a net result of a number of fundamental processes known as abrasion, adhesion, fatigue,
and corrosive effects, which act in a synchronous or additive manner.2-4 The rate of wear in the oral environment may be affected by various factors, such as neuromuscular forces, lubricants, patient habits, and the type of the restorative material used.5,6 This
underlines the complex nature of the wear process and the consequent difficulties encountered in conducting in vivo and in vitro tooth-wear studies. The annual wear rate for sound enamel under friction from mastication has been reported to range between 20mm
This study was supported by Kirkpatrick Bequest Fund Scholarship (Grant PL108227) from the Sir John Walsh Research Institute. a
Professional practice fellow, Department of Oral Rehabilitation. Professor and Head of Biomaterials, Department of Oral Rehabilitation. c Senior Lecturer, Department of Oral Rehabilitation. d Professor and Head of Prosthodontics, Department of Oral Rehabilitation. b
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Volume and 40 mm.7 Ideally, the wear rate of a dental restorative material should be compatible with that of natural tooth enamel.8,9 The rate of wear, however, may be altered by introducing a restoration with wear properties different from those of the natural teeth. The consequences of introducing abrasive restorative materials can be detrimental, which results in unacceptable damage to the occluding surfaces, the alteration of the functional path of masticatory movement, the loss of anterior guidance, and esthetics.5,10 Results of several studies have indicated that ceramic materials cause more wear on opposing enamel tooth structure than on cast gold alloy.9,11,12 The wear potential of ceramic materials depends on various factors, such as microstructure, surface roughness, and fracture toughness.8,13,14 In vitro studies reported that polished ceramics produce less wear on opposing enamel than on glazed ceramics and thus recommend polishing instead of glazing the adjusted area.14-16 Although the in vitro model cannot simulate the oral environment with all its biologic variables, with a well-controlled experimental design, the factors that lead to a certain wear mechanism of the material can be identified.17,18 The wear propensity of a dental restorative material reported in vitro will help researchers and clinicians understand and predict the response of a particular material in a clinical setting.19 Results of recent studies in the engineering field found that the wear of materials is closely related to the frictional energy dissipated during the sliding movement20,21 and that the energy dissipated by friction can be used to characterize the wear resistance of a material.22 This energy-wear approach has been introduced in dental tribology to evaluate the wear resistance of various dental biomaterials.23,24 The in vitro wear simulation devices developed for dental research incorporate a sensor that records frictional force as a function of time to obtain kinetic characteristics.25 These data help researchers interpret
wear mechanisms and use energy criteria to evaluate the wear resistance of the test materials. Results of studies have confirmed a positive correlation between volumetric wear and the friction energy dissipated during sliding contact of different dental restorative materials.26,27 The purpose of this study was to evaluate and compare the friction and wear behavior of human tooth enamel that opposes 2 different indirect restorative materials with a reciprocating wear testing apparatus: heat pressable lithium disilicate glass ceramic (IPS e.max Press; Ivoclar Vivadent) and Type III gold (Maxigold; Ivoclar Vivadent).
MATERIAL AND METHODS Ethical approval was granted from the University of Otago Ethics Committee for the collection and testing of extracted human teeth for this study (reference no. 10/113). Extracted teeth were examined under a light microscope (Alphaphot YS2; Nikon) at 10 magnification, and teeth with caries or enamel defects were discarded. The collected teeth were stored in Hanks balanced salt solution (Gibco Life Technologies) to avoid dehydration until they were prepared for testing. The enamel specimens (n¼5) were prepared from 5 different extracted permanent premolars from individuals between 18 and 30 years old. The roots were sectioned along the cementumenamel junction with a diamond disk attached to a high-speed cutting machine (Accutom-50; Struers). The dental pulp was removed with a spoon excavator, and the crowns then were sectioned in half in a mesio-distal direction by using a high-speed cutting machine. The cutting was performed at a speed of 3000 rpm under copious water irrigation to minimize the influence of generated heat, which could result in dehydration and changes in the microstructure and chemistry of human teeth. The buccal halves were embedded in autopolymerizing epoxy resin (Epofix; Struers) with the enamel surface exposed. Autopolymerizing
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acrylic resin (Pattern Resin; GC Corp) was used to stabilize the crown and to expose a 33-mm enamel surface on a clear glass slide. The enamel specimens were ground with abrasive papers (TegraPol-21; Struers) from 1000, 1500, to 2500 grit and then were polished with 6-mm diamond suspensions (DiaPro; Struers) followed by 1mm diamond suspensions (DiaPro). The polished enamel specimens were cleaned ultrasonically to remove any surface debris and were stored in Hanks balanced salt solution until use. Each enamel specimen (n¼5) was opposed by 2 different restorative specimens; IPS e.max Press (n¼5) and Type III gold (n¼5). Wear scars on the same enamel specimen were 2 mm apart to prevent overlapping of the wear zones. The hardness (H) and the elastic modulus (E) of each enamel specimen were measured with a nanobased indentation system (UMIS-200; CSIRO). Nine indentations under a load of 25 mN were produced with a calibrated Berkovich indenter. The maximum load was held for 10 seconds to minimize the effect of creep on the unloading curve. The force (F) and displacement (D) data during the loading-unloading indentation cycle were used to calculate the H and E values of each enamel specimen with the Oliver-Pharr analysis method.28 Heat pressable lithium disilicate glass ceramics (IPS e.max Press) specimens (n¼5) were fabricated with the conventional lost-wax technique. Prefabricated wax patterns in the shape of a small cylinder with a spherical end (4 mm in diameter) were prepared and invested. The ceramic ingot was pressed at 925 C in a combination furnace (Programat EP 5000; Ivoclar Vivadent). The surfaces of the ceramic specimens were autoglazed in the furnace at 800 C for 1 minute. The finished ceramic specimens were examined under a light microscope (Alphaphot YS2; Nikon) at 20 magnification to screen for surface cracks and flaws. Specimens that displayed visible defects or cracks were discarded from the study. For the gold specimens (n¼5), prefabricated
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wax patterns were invested and cast according to manufacturers’ instructions. The specimens were then polished with brown and green abrasive wheels (Shofu Dental Corp). Friction-wear tests on human enamel were conducted in a ball-on-flat configuration by using a reciprocating horizontal wear testing apparatus, as seen in Figure 1. The device consisted of 2 main components, the fixed maxillary and the movable mandibular compartments. The restorative specimens were secured in the maxillary specimen holder. The spherical end of the
restorative specimens made contact with the enamel specimen and established a ball-on-flat surface contact geometry. The enamel specimens were glued onto aluminum plates and attached to the mandibular compartment. The latter moved back and forth on a friction-free platform, driven through the crank-and-rocker mechanism by the gear motor. The maxillary compartment was designed to support weights applied to the specimens. The wear pairs were subjected to a normal load of 9.8 N, a reciprocating amplitude of approximately 200 mm, and a
9.8N Force Load cell
Spherical shaped restorative specimen Flat enamel specimens immersed in distilled water
LVDT
Friction-free platform
1 Schematic view of wear testing apparatus.
reciprocating frequency of approximately 1.6 Hz for up to 1100 cycles under distilled water lubrication. A linear variable differential transformer was used to measure the displacement of the mandibular compartment on the friction-free platform (Fig. 2). The calibrated load cell attached to the mandibular compartment measured the frictional force. The baseline frictional force from sliding of the mandibular compartment without enamel specimens was deducted to obtain the true frictional force. Variations in frictional force and displacement were recorded with a computer-based data acquisition system (PowerLab; ADInstruments Pty Ltd). These data were used to calculate the friction coefficient (m) for different wear pairs as a function of cycles. The m values were presented as the mean (standard error of the mean). The data were analyzed with statistical software (SPSS 16.0 for Windows; SPSS Inc). The paired t test was performed to compare the average steady-state m values of enamel that opposed IPS e.max Press and Type III gold. After completion of the wear testing, the enamel wear scars were carbon coated and examined under a scanning electron microscope (SEM) (S360; Cambridge Instruments) at low (115) and high (500, 1500) magnification in the back-scattered mode.
RESULTS
Friction Coefficient (µ)
2 Photograph that shows wear-testing apparatus. 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
IPS e.max Press (n=5) Gold (n=5)
0
100 200 300 400 500 600 700 800 900 1000 1100
Number of Cycles 3 Evolution of friction coefficient (m) as function of number of cycles for 2 wear pairs investigated.
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The average H values for enamel ranged between 4.4 and 5.5 GPa, and the E values were between 100 and 130 GPa, which corresponded to the ranges reported in the literature.29 For the lithium disilicate, the typical values of H and E are 5.5 and 90 GPa,30 respectively, whereas those for Type gold are 1.5 and 85 GPa, respectively.31 Typical variations in the friction coefficient (m) as a function of the number of cycles for the 2 wear pairs are shown in Figure 3. The difference in the evolutionary pattern of the m differs widely between the 2 wear pairs. For the enamel and IPS e.max Press wear pairs,
4
Volume the m usually was low at the early wear stage, but it increased sharply to 0.5 within the first 100 cycles. The increase in the m continued steadily up to 400 cycles and, thereafter, attained a steady-state value. In comparison, m for the enamel and Type III gold wear pairs increased slowly but steadily up to 500 cycles and then reached a relatively steady-state condition. The average steady-state m value for the enamel and IPS e.max Press wear pair after 500 cycles was 0.64 0.039, whereas, for the enamel and Type III gold wear pair, the
Table I.
m value was 0.25 0.052 (Table I). The
paired t test revealed that the steadystate m value after 500 cycles was significantly higher for enamel opposed by IPS e.max Press than when opposed by Type III gold (P¼.009). The enamel wear scar that opposed IPS e.max Press exhibited distinct striations and plowing in the sliding direction (Fig. 4A). The higher magnification SEM image found numerous cracks and delaminated areas of the worn surface (Fig. 4B). The exposed underlying enamel that resulted from delamination
Average steady-state m values after 500 cycles for 2 wear pairs
Wear Pair
Average Steady-State m Values After 500 Cycles
Enamel/IPS e.max Press (n¼5)
0.64 0.039
Enamel/Type ___ gold (n¼5)
0.25 0.052
P
m friction coefficient.
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seemed rough and irregular. The enamel surface that opposed Type III gold exhibited no remarkable surface characteristics apart from small patches of gold smear on the low magnification image (Fig. 4C). However, at high magnification, fine crack lines could be seen with gold smears in the direction perpendicular to the sliding direction. Minute areas of enamel surface loss were detected under the highest magnification shown in Figure 4D. During initial loading and under the sliding conditions, considerably higher contact stresses developed beneath and about the area of contact. An estimate of the contact stresses and the area of contact may be obtained by using the analysis presented by Ashby32 and by Lawn and Wilshaw.33 In this study, 4-mm-diameter, spherical-ended cylinders of Type III gold and lithium disilicate were in contact with enamel; the E values of these 3 materials were all
4 Wear scar on enamel surface. A, Opposing IPS e.max Press specimen 500 magnification. B, Opposing IPS e.max Press specimen 1500 magnification. C, Opposing Type III gold specimen 500 magnification. D, Opposing Type III gold specimen 1500 magnification. Arrow in A indicates sliding direction of IPS e.max Press, whereas highlighted features in D show cracks with some loss of enamel material.
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similar (86, 95, and 100 GPa). According to Ashby,32 when E values are similar, the radius of contact, a, for a ball on a flat surface is given by a¼0.7(FR/E)1/3, in which F is the maximum force, R is the ball radius, and E is the elastic modulus. By using an averaged value of 95 GPa for E, R¼2 mm, and F¼10 N, the radius of contact a¼45 mm. The average contact pressure, P, beneath the area of contact is given by P¼F/sy a2. By using this equation, the averaged contact pressure was 1.6 GPa. This value was well below the H of enamel and the lithium disilicate but exceeded the yield stress of the gold alloy. According to the manufacturer, the yield stress (sy) of the gold alloy is 310 MPa and, as such, this material will yield or plastically deform on contact. An estimate of the contact radius of the plastically deformed gold may be obtained from Ashby,32 namely, 3.sy¼F/(pa2), which results in a radius of contact of 58 mm. During sliding contact, the stress field is distorted and the tensile stresses behind the sliding area of contact increase well above those that develop during static loading. The maximum tensile stresses occur at the edge of the area of contact and are given by st¼F/ (6pa2). The resulting maximum tensile stress was 250 MPa for the lithium disilicate and 155 MPa for the gold in contact with enamel. These stresses were not sufficient to generate cracking in the glass ceramic, and gold is a very tough material. However, they exceeded the tensile strength of enamel, although previous studies indicated that the surface cracking of enamel under sliding is influenced by the size of the indenter, sliding speed, and direction.34 During sliding contact, the tensile stresses that develop are dependent on the friction coefficient (m). The maximum tensile stresses at the trailing edge of contact are given by stf ¼ st([1-2n]/2)(1þAm), in which A¼([3p/8] [4þn]/[1-2n]), n is the Poisson ratio and m is the coefficient of friction. In the present situation, the steady-state m was 0.65 for the glass ceramiceenamel and 0.25 for the
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5 Sliding Direction
IPS e.max Press
N Compression Area
Tensile Area
Tension plus normal loading area
Shear Stress/ Deformation
Compression plus normal loading area
5 Schematic illustration of stress field distribution during localized sliding contact between enamel and IPS e.max Press. gold-enamel. The resultant maximum tensile stresses when v¼0.3 were 665 MPa for the glass ceramic and 187 MPa for the gold. These sliding stresses, especially for the lithium disilicate contact, were very high and would generate partial Hertzian cone cracks at the trailing edge. With each transit of the ball across the surface, the extent of such cracking would become more severe. In the case of the gold, the larger contact area and the lower sliding friction resulted in much lower tensile stresses, and, even after 1100 sliding passages, only faint traces of shallow surface cracks were generated.
DISCUSSION For the enamel and IPS e.max Press wear pair, the m was low initially but increased sharply within 100 cycles. This continued to increase steadily up to approximately 500 cycles and then remained almost constant (Fig. 3). Similar evolutionary patterns of m were observed in in vitro studies that evaluated the wear behavior of enamel.23,24 However, the steady-state m value varied, depending on the antagonist material and the wear testing conditions, which indicated that the frictional behavior of dental enamel is both material- and system-dependent. The reason for a sharp increase in the m within the initial cycles is interpreted to reflect the apparent ductility of enamel against the lithium disilicate glass
ceramic under conditions of sliding.34 As the glass ceramic specimen transverses the enamel, surface asperities of harder glass ceramic plow and deform the softer enamel surface.2 This raises the friction and initiates the surface damage of enamel. In addition, the complex microstructure of lithium disilicate with glass and crystalline needles results in a rough abrasive surface that further enhances the friction and wear damage. The SEM observations of the enamel surface opposing IPS e.max Press found deep plowing and striations along the sliding direction, which suggests abrasive wear as one of the predominant wear mechanisms.4 Areas of irregular concavities and gaps on the enamel surface result from localized bulk delamination and surface fracture.23 This type of wear is caused by repetitive loading on brittle enamel surface. As the IPS e.max Press specimen slides over the enamel surface, sliding contact generates a frictional force, which results in tensile, compressive, and shear stresses on the enamel surface, as shown schematically in Figure 5. Microcracks develop within the subsurface, and these propagate and emerge to eventually form a particle that becomes displaced.18 The in vitro study by Arsecularatne and Hoffman,35 which used focused ion beam milling and field emission SEM, found that such behavior occurs. The displaced enamel particles may contribute to
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Volume 3-body wear by acting as a rolling abrasive particle between the rubbing surfaces.4 In comparison, enamel is comparatively harder than Type III gold. As the gold specimen slides against the enamel surface, the frictional force results in plastic deformation of the gold specimen, and, at the same time, adhesive forces develop at the asperity junctions between the enamel and gold in contact. The adhesive forces at the asperity junctions may become stronger than the interatomic metallic bonds of the gold material and result in fragments of gold that adhere to the enamel surface during subsequent sliding movement.3 As the gold specimen deforms, the contact surface area increases and the contact pressure decreases. The rising coefficient of friction with sliding distance would increase the tensile stresses at the trailing edge (as outlined earlier), potentially generating additional small surface cracks, which seem to be the sites where gold is sheared from the antagonist. The SEM observation of the enamel surface that opposes the gold specimen revealed a minimally damaged wear scar characterized by small patches of gold smear adhered to the enamel surface (Fig. 4C). Highmagnification SEM images of the wear scar found fine crack lines perpendicular to the sliding direction and minute areas of surface loss (Fig. 4D). These crack features are typical of a partial cone crack formation at the trailing edge of the indenter.23 The transfer of material from the gold specimen to the antagonist has been reported by previous in vitro wear studies11,12 and indicates that adhesive wear is the predominant wear mechanism. The adhesive wear mechanism of this wear pair relates to the structure of the gold material, which possesses high ductility due to a large number of slip systems.11 In addition, a thin layer of gold particles may assist with the trapping of fluid between the specimens in contact and thereby assist with boundary lubrication. Kaidonis et al36 suggest that the displacement of the liquid lubricant from the interface may result
in a rapid increase in the wear rate because the enamel particles that act as a solid lubricant would be washed away. If the enamel-gold couple were subjected to longer cycles of loading, then the equilibrium steady-state friction condition would probably depend on the extent of metal transfer to the enamel and its tendency to remain locked onto the enamel surface. If the surface of enamel were completely covered with gold, then metal-to-metal contact would occur, which results in micro welding and an increase in friction that would be expected to fragment the surface of the enamel and of the rougher gold surface. However, this phenomenon is not evident in clinical evaluations because the transferred material seems susceptible to removal by food particles and mechanical brushing. The purpose of this study was to explore the onset of deformation and damage to enamel that opposed lithium disilicate glass ceramic and Type III gold. Longer tests could better evaluate the long-term effect of the tested restorative materials on the wear behavior of enamel. The limitations of this study include the absence of physiological factors such as artificial saliva and food simulation slurry, and the evaluation of the surface of the abrading restorative materials. Introducing food slurry or artificial saliva may affect the wear behavior of enamel and alter the results, and should be the basis of further study. In addition, knowledge of the surface roughness of the restorative specimens before and after wear testing would have been useful. The surfaces used were close to those typically supplied by laboratories: polished gold and glazed ceramic.
CONCLUSIONS Within the limits of this in vitro study, the following conclusions were drawn: 1. Lower friction coefficient (m) and better wear resistance were observed when human enamel was opposed by Type III gold than by IPS e.max Press. 2. Abrasive wear was the main mechanism for the
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IPS e.max Press and enamel wear pair, whereas adhesive wear occurred predominantly for the Type III gold and enamel wear pair. In vitro and clinical research is required to characterize and predict the wear behavior of human tooth enamel against a range of prosthetic dental materials.
REFERENCES 1. Kelleher M, Bishop K. Tooth surface loss: an overview. Br Dent J 1999;186:61-6. 2. Mair L, Stolarski T, Vowles R, Lloyd C. Wear: mechanisms, manifestations and measurement. Report of a workshop. J Dent 1996;24: 141-8. 3. Hutchings IM. Tribology: friction and wear of engineering materials. London: ButterworthHeinemann; 1992. p. 20-40. 4. Zum Gahr KH. Microstructure and wear of materials. Amsterdam: Elsevier Science; 1987. p. 80-131. 5. Mahalick J, Knap F, Weiter E. Occlusal wear in prosthodontics. J Am Dent Assoc 1971;82: 154-9. 6. Kim SK, Kim KN, Chang IT, Heo SJ. A study of the effects of chewing patterns on occlusal wear. J Oral Rehabil 2001;28:1048-55. 7. Lambrechts P, Braem M, VuylstekeWauters M, Vanherle G. Quantitative in vivo wear of human enamel. J Dent Res 1989;68: 1752-4. 8. Seghi RR, Rosenstiel SF, Bauer P. Abrasion of human enamel by different dental ceramics in vitro. J Dent Res 1991;70:221-5. 9. Hudson J, Goldstein G, Georgescu M. Enamel wear caused by three different restorative materials. J Prosthet Dent 1995;74:647-54. 10. Joshi N, Patil NP, Patil SB. The abrasive effect of a porcelain and a nickel-chromium alloy on the wear of human enamel and the influence of a carbonated beverage on the rate of wear. J Prosthodont 2010;19:212-7. 11. Ramp MH, Suzuki S, Cox CF, Lacefield WR, Koth DL. Evaluation of wear: enamel opposing three ceramic materials and a gold alloy. J Prosthet Dent 1997;77:523-30. 12. Hacker CH, Wagner WC, Razzoog ME. An in vitro investigation of the wear of enamel on porcelain and gold in saliva. J Prosthet Dent 1996;75:14-7. 13. Oh W, Delong R, Anusavice K. Factors affecting enamel and ceramic wear: a literature review. J Prosthet Dent 2002;87:451-9. 14. Park JH, Park S, Lee K, Yun KD, Lim HP. Antagonist wear of three CAD/CAM anatomic contour zirconia ceramics. J Prosthet Dent 2014;111:20-9. 15. Janyavula S, Lawson N, Cakir D, Beck P, Ramp LC, Burgess JO. The wear of polished and glazed zirconia against enamel. J Prosthet Dent 2013;109:22-9. 16. Jagger D, Harrison A. An in vitro investigation into the wear effects of unglazed, glazed, and polished porcelain on human enamel. J Prosthet Dent 1994;72:320-3.
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17. Lambrechts P, Debels E, Van Landuyt K, Peumans M, Van Meerbeek B. How to simulate wear? Overview of existing methods. Dent Mater 2006;22:693-701. 18. Reid CN, Fisher J, Jacobsen PH. Fatigue and wear of dental materials. J Dent 1990;18: 209-15. 19. Wang W, DiBenedetto AT, Jon Goldberg A. Abrasive wear testing of dental restorative materials. Wear 1998;219:213-9. 20. Fouvry S, Liskiewicz T, Kapsa P, Hannel S, Sauger E. An energy description of wear mechanisms and its applications to oscillating sliding contacts. Wear 2003;255: 287-98. 21. Huq M, Celis J. Expressing wear rate in sliding contacts based on dissipated energy. Wear 2002;252:375-83. 22. Ramalho A, Miranda J. The relationship between wear and dissipated energy in sliding systems. Wear 2006;260:361-7. 23. Zheng J, Zhou ZR, Zhang J, Li H, Yu HY. On the friction and wear behaviour of human tooth enamel and dentin. Wear 2003;255:967-74. 24. Zheng J, Zhou ZR. Friction and wear behavior of human teeth under various wear conditions. Tribol Int 2007;40:278-84. 25. Sajewicz E, Kulesza Z. A new tribometer for friction and wear studies of dental materials and hard tooth tissues. Tribol Int 2007;40: 885-95.
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7 26. Sajewicz E. On evaluation of wear resistance of tooth enamel and dental materials. Wear 2006;260:1256-61. 27. Ramalho A, Antunes P. Reciprocating wear test of dental composites against human teeth and glass. Wear 2007;263:1095-104. 28. Oliver WC, Pharr GM. Improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7:1564-83. 29. He L, Swain M. Understanding the mechanical behaviour of human enamel from its structural and compositional characteristics. J Mech Behav Biomed Mater 2008;1:18-29. 30. Guazzato M, Albakry M, Ringer SP, Swain MV. Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part I. Pressable and alumina glass-infiltrated ceramics. Dent Mater 2004;20:441-8. 31. Wataha JC. Alloys for prosthodontic restorations. J Prosthet Dent 2002;87:351-63. 32. Ashby MF. Materials selection in mechanical design. 3rd ed. Boston: Elsevier ButterworthHeinemann; 2005. p. 488-93. 33. Lawn B, Wilshaw R. Indentation fracture: principles and applications. J Mater Sci 1975;10:1049-81. 34. Powers JM, Craig RG, Ludema KC. Frictional behavior and surface failure of human enamel. J Dent Res 1973;52:1327-31.
35. Arsecularatne J, Hoffman M. On the wear mechanism of human dental enamel. J Mech Behav Biomed Mater 2010;3:347-56. 36. Kaidonis JA, Richards L, Townsend G, Tansley G. Wear of human enamel: a quantitative in vitro assessment. J Dent Res 1998;77:1983-90. Corresponding author: Dr Ahreum Lee Department of Oral Rehabilitation Faculty of Dentistry University of Otago PO Box 647 Dunedin 9054, NEW ZEALAND E-mail: [email protected] Acknowledgments The authors dedicate this article to the late Dr Lihong He. His knowledge and enthusiasm for dental biomaterials research provided many insights and encouragement during the course of this study. Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
Clinical Implications Greater wear may occur when enamel is opposed by ceramic instead of gold restorations. An understanding of the mechanisms where this occurs and the controlling factors involved may help reduce this wear process.
The wear of teeth and restorative materials is a complex and multifactorial phenomenon that involves the interplay of biologic, mechanical, and chemical factors.1 Wear is a net result of a number of fundamental processes known as abrasion, adhesion, fatigue,
and corrosive effects, which act in a synchronous or additive manner.2-4 The rate of wear in the oral environment may be affected by various factors, such as neuromuscular forces, lubricants, patient habits, and the type of the restorative material used.5,6 This
underlines the complex nature of the wear process and the consequent difficulties encountered in conducting in vivo and in vitro tooth-wear studies. The annual wear rate for sound enamel under friction from mastication has been reported to range between 20mm
This study was supported by Kirkpatrick Bequest Fund Scholarship (Grant PL108227) from the Sir John Walsh Research Institute. a
Professional practice fellow, Department of Oral Rehabilitation. Professor and Head of Biomaterials, Department of Oral Rehabilitation. c Senior Lecturer, Department of Oral Rehabilitation. d Professor and Head of Prosthodontics, Department of Oral Rehabilitation. b
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Volume and 40 mm.7 Ideally, the wear rate of a dental restorative material should be compatible with that of natural tooth enamel.8,9 The rate of wear, however, may be altered by introducing a restoration with wear properties different from those of the natural teeth. The consequences of introducing abrasive restorative materials can be detrimental, which results in unacceptable damage to the occluding surfaces, the alteration of the functional path of masticatory movement, the loss of anterior guidance, and esthetics.5,10 Results of several studies have indicated that ceramic materials cause more wear on opposing enamel tooth structure than on cast gold alloy.9,11,12 The wear potential of ceramic materials depends on various factors, such as microstructure, surface roughness, and fracture toughness.8,13,14 In vitro studies reported that polished ceramics produce less wear on opposing enamel than on glazed ceramics and thus recommend polishing instead of glazing the adjusted area.14-16 Although the in vitro model cannot simulate the oral environment with all its biologic variables, with a well-controlled experimental design, the factors that lead to a certain wear mechanism of the material can be identified.17,18 The wear propensity of a dental restorative material reported in vitro will help researchers and clinicians understand and predict the response of a particular material in a clinical setting.19 Results of recent studies in the engineering field found that the wear of materials is closely related to the frictional energy dissipated during the sliding movement20,21 and that the energy dissipated by friction can be used to characterize the wear resistance of a material.22 This energy-wear approach has been introduced in dental tribology to evaluate the wear resistance of various dental biomaterials.23,24 The in vitro wear simulation devices developed for dental research incorporate a sensor that records frictional force as a function of time to obtain kinetic characteristics.25 These data help researchers interpret
wear mechanisms and use energy criteria to evaluate the wear resistance of the test materials. Results of studies have confirmed a positive correlation between volumetric wear and the friction energy dissipated during sliding contact of different dental restorative materials.26,27 The purpose of this study was to evaluate and compare the friction and wear behavior of human tooth enamel that opposes 2 different indirect restorative materials with a reciprocating wear testing apparatus: heat pressable lithium disilicate glass ceramic (IPS e.max Press; Ivoclar Vivadent) and Type III gold (Maxigold; Ivoclar Vivadent).
MATERIAL AND METHODS Ethical approval was granted from the University of Otago Ethics Committee for the collection and testing of extracted human teeth for this study (reference no. 10/113). Extracted teeth were examined under a light microscope (Alphaphot YS2; Nikon) at 10 magnification, and teeth with caries or enamel defects were discarded. The collected teeth were stored in Hanks balanced salt solution (Gibco Life Technologies) to avoid dehydration until they were prepared for testing. The enamel specimens (n¼5) were prepared from 5 different extracted permanent premolars from individuals between 18 and 30 years old. The roots were sectioned along the cementumenamel junction with a diamond disk attached to a high-speed cutting machine (Accutom-50; Struers). The dental pulp was removed with a spoon excavator, and the crowns then were sectioned in half in a mesio-distal direction by using a high-speed cutting machine. The cutting was performed at a speed of 3000 rpm under copious water irrigation to minimize the influence of generated heat, which could result in dehydration and changes in the microstructure and chemistry of human teeth. The buccal halves were embedded in autopolymerizing epoxy resin (Epofix; Struers) with the enamel surface exposed. Autopolymerizing
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acrylic resin (Pattern Resin; GC Corp) was used to stabilize the crown and to expose a 33-mm enamel surface on a clear glass slide. The enamel specimens were ground with abrasive papers (TegraPol-21; Struers) from 1000, 1500, to 2500 grit and then were polished with 6-mm diamond suspensions (DiaPro; Struers) followed by 1mm diamond suspensions (DiaPro). The polished enamel specimens were cleaned ultrasonically to remove any surface debris and were stored in Hanks balanced salt solution until use. Each enamel specimen (n¼5) was opposed by 2 different restorative specimens; IPS e.max Press (n¼5) and Type III gold (n¼5). Wear scars on the same enamel specimen were 2 mm apart to prevent overlapping of the wear zones. The hardness (H) and the elastic modulus (E) of each enamel specimen were measured with a nanobased indentation system (UMIS-200; CSIRO). Nine indentations under a load of 25 mN were produced with a calibrated Berkovich indenter. The maximum load was held for 10 seconds to minimize the effect of creep on the unloading curve. The force (F) and displacement (D) data during the loading-unloading indentation cycle were used to calculate the H and E values of each enamel specimen with the Oliver-Pharr analysis method.28 Heat pressable lithium disilicate glass ceramics (IPS e.max Press) specimens (n¼5) were fabricated with the conventional lost-wax technique. Prefabricated wax patterns in the shape of a small cylinder with a spherical end (4 mm in diameter) were prepared and invested. The ceramic ingot was pressed at 925 C in a combination furnace (Programat EP 5000; Ivoclar Vivadent). The surfaces of the ceramic specimens were autoglazed in the furnace at 800 C for 1 minute. The finished ceramic specimens were examined under a light microscope (Alphaphot YS2; Nikon) at 20 magnification to screen for surface cracks and flaws. Specimens that displayed visible defects or cracks were discarded from the study. For the gold specimens (n¼5), prefabricated
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wax patterns were invested and cast according to manufacturers’ instructions. The specimens were then polished with brown and green abrasive wheels (Shofu Dental Corp). Friction-wear tests on human enamel were conducted in a ball-on-flat configuration by using a reciprocating horizontal wear testing apparatus, as seen in Figure 1. The device consisted of 2 main components, the fixed maxillary and the movable mandibular compartments. The restorative specimens were secured in the maxillary specimen holder. The spherical end of the
restorative specimens made contact with the enamel specimen and established a ball-on-flat surface contact geometry. The enamel specimens were glued onto aluminum plates and attached to the mandibular compartment. The latter moved back and forth on a friction-free platform, driven through the crank-and-rocker mechanism by the gear motor. The maxillary compartment was designed to support weights applied to the specimens. The wear pairs were subjected to a normal load of 9.8 N, a reciprocating amplitude of approximately 200 mm, and a
9.8N Force Load cell
Spherical shaped restorative specimen Flat enamel specimens immersed in distilled water
LVDT
Friction-free platform
1 Schematic view of wear testing apparatus.
reciprocating frequency of approximately 1.6 Hz for up to 1100 cycles under distilled water lubrication. A linear variable differential transformer was used to measure the displacement of the mandibular compartment on the friction-free platform (Fig. 2). The calibrated load cell attached to the mandibular compartment measured the frictional force. The baseline frictional force from sliding of the mandibular compartment without enamel specimens was deducted to obtain the true frictional force. Variations in frictional force and displacement were recorded with a computer-based data acquisition system (PowerLab; ADInstruments Pty Ltd). These data were used to calculate the friction coefficient (m) for different wear pairs as a function of cycles. The m values were presented as the mean (standard error of the mean). The data were analyzed with statistical software (SPSS 16.0 for Windows; SPSS Inc). The paired t test was performed to compare the average steady-state m values of enamel that opposed IPS e.max Press and Type III gold. After completion of the wear testing, the enamel wear scars were carbon coated and examined under a scanning electron microscope (SEM) (S360; Cambridge Instruments) at low (115) and high (500, 1500) magnification in the back-scattered mode.
RESULTS
Friction Coefficient (µ)
2 Photograph that shows wear-testing apparatus. 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
IPS e.max Press (n=5) Gold (n=5)
0
100 200 300 400 500 600 700 800 900 1000 1100
Number of Cycles 3 Evolution of friction coefficient (m) as function of number of cycles for 2 wear pairs investigated.
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The average H values for enamel ranged between 4.4 and 5.5 GPa, and the E values were between 100 and 130 GPa, which corresponded to the ranges reported in the literature.29 For the lithium disilicate, the typical values of H and E are 5.5 and 90 GPa,30 respectively, whereas those for Type gold are 1.5 and 85 GPa, respectively.31 Typical variations in the friction coefficient (m) as a function of the number of cycles for the 2 wear pairs are shown in Figure 3. The difference in the evolutionary pattern of the m differs widely between the 2 wear pairs. For the enamel and IPS e.max Press wear pairs,
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Volume the m usually was low at the early wear stage, but it increased sharply to 0.5 within the first 100 cycles. The increase in the m continued steadily up to 400 cycles and, thereafter, attained a steady-state value. In comparison, m for the enamel and Type III gold wear pairs increased slowly but steadily up to 500 cycles and then reached a relatively steady-state condition. The average steady-state m value for the enamel and IPS e.max Press wear pair after 500 cycles was 0.64 0.039, whereas, for the enamel and Type III gold wear pair, the
Table I.
m value was 0.25 0.052 (Table I). The
paired t test revealed that the steadystate m value after 500 cycles was significantly higher for enamel opposed by IPS e.max Press than when opposed by Type III gold (P¼.009). The enamel wear scar that opposed IPS e.max Press exhibited distinct striations and plowing in the sliding direction (Fig. 4A). The higher magnification SEM image found numerous cracks and delaminated areas of the worn surface (Fig. 4B). The exposed underlying enamel that resulted from delamination
Average steady-state m values after 500 cycles for 2 wear pairs
Wear Pair
Average Steady-State m Values After 500 Cycles
Enamel/IPS e.max Press (n¼5)
0.64 0.039
Enamel/Type ___ gold (n¼5)
0.25 0.052
P
m friction coefficient.
.009
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seemed rough and irregular. The enamel surface that opposed Type III gold exhibited no remarkable surface characteristics apart from small patches of gold smear on the low magnification image (Fig. 4C). However, at high magnification, fine crack lines could be seen with gold smears in the direction perpendicular to the sliding direction. Minute areas of enamel surface loss were detected under the highest magnification shown in Figure 4D. During initial loading and under the sliding conditions, considerably higher contact stresses developed beneath and about the area of contact. An estimate of the contact stresses and the area of contact may be obtained by using the analysis presented by Ashby32 and by Lawn and Wilshaw.33 In this study, 4-mm-diameter, spherical-ended cylinders of Type III gold and lithium disilicate were in contact with enamel; the E values of these 3 materials were all
4 Wear scar on enamel surface. A, Opposing IPS e.max Press specimen 500 magnification. B, Opposing IPS e.max Press specimen 1500 magnification. C, Opposing Type III gold specimen 500 magnification. D, Opposing Type III gold specimen 1500 magnification. Arrow in A indicates sliding direction of IPS e.max Press, whereas highlighted features in D show cracks with some loss of enamel material.
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similar (86, 95, and 100 GPa). According to Ashby,32 when E values are similar, the radius of contact, a, for a ball on a flat surface is given by a¼0.7(FR/E)1/3, in which F is the maximum force, R is the ball radius, and E is the elastic modulus. By using an averaged value of 95 GPa for E, R¼2 mm, and F¼10 N, the radius of contact a¼45 mm. The average contact pressure, P, beneath the area of contact is given by P¼F/sy a2. By using this equation, the averaged contact pressure was 1.6 GPa. This value was well below the H of enamel and the lithium disilicate but exceeded the yield stress of the gold alloy. According to the manufacturer, the yield stress (sy) of the gold alloy is 310 MPa and, as such, this material will yield or plastically deform on contact. An estimate of the contact radius of the plastically deformed gold may be obtained from Ashby,32 namely, 3.sy¼F/(pa2), which results in a radius of contact of 58 mm. During sliding contact, the stress field is distorted and the tensile stresses behind the sliding area of contact increase well above those that develop during static loading. The maximum tensile stresses occur at the edge of the area of contact and are given by st¼F/ (6pa2). The resulting maximum tensile stress was 250 MPa for the lithium disilicate and 155 MPa for the gold in contact with enamel. These stresses were not sufficient to generate cracking in the glass ceramic, and gold is a very tough material. However, they exceeded the tensile strength of enamel, although previous studies indicated that the surface cracking of enamel under sliding is influenced by the size of the indenter, sliding speed, and direction.34 During sliding contact, the tensile stresses that develop are dependent on the friction coefficient (m). The maximum tensile stresses at the trailing edge of contact are given by stf ¼ st([1-2n]/2)(1þAm), in which A¼([3p/8] [4þn]/[1-2n]), n is the Poisson ratio and m is the coefficient of friction. In the present situation, the steady-state m was 0.65 for the glass ceramiceenamel and 0.25 for the
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5 Sliding Direction
IPS e.max Press
N Compression Area
Tensile Area
Tension plus normal loading area
Shear Stress/ Deformation
Compression plus normal loading area
5 Schematic illustration of stress field distribution during localized sliding contact between enamel and IPS e.max Press. gold-enamel. The resultant maximum tensile stresses when v¼0.3 were 665 MPa for the glass ceramic and 187 MPa for the gold. These sliding stresses, especially for the lithium disilicate contact, were very high and would generate partial Hertzian cone cracks at the trailing edge. With each transit of the ball across the surface, the extent of such cracking would become more severe. In the case of the gold, the larger contact area and the lower sliding friction resulted in much lower tensile stresses, and, even after 1100 sliding passages, only faint traces of shallow surface cracks were generated.
DISCUSSION For the enamel and IPS e.max Press wear pair, the m was low initially but increased sharply within 100 cycles. This continued to increase steadily up to approximately 500 cycles and then remained almost constant (Fig. 3). Similar evolutionary patterns of m were observed in in vitro studies that evaluated the wear behavior of enamel.23,24 However, the steady-state m value varied, depending on the antagonist material and the wear testing conditions, which indicated that the frictional behavior of dental enamel is both material- and system-dependent. The reason for a sharp increase in the m within the initial cycles is interpreted to reflect the apparent ductility of enamel against the lithium disilicate glass
ceramic under conditions of sliding.34 As the glass ceramic specimen transverses the enamel, surface asperities of harder glass ceramic plow and deform the softer enamel surface.2 This raises the friction and initiates the surface damage of enamel. In addition, the complex microstructure of lithium disilicate with glass and crystalline needles results in a rough abrasive surface that further enhances the friction and wear damage. The SEM observations of the enamel surface opposing IPS e.max Press found deep plowing and striations along the sliding direction, which suggests abrasive wear as one of the predominant wear mechanisms.4 Areas of irregular concavities and gaps on the enamel surface result from localized bulk delamination and surface fracture.23 This type of wear is caused by repetitive loading on brittle enamel surface. As the IPS e.max Press specimen slides over the enamel surface, sliding contact generates a frictional force, which results in tensile, compressive, and shear stresses on the enamel surface, as shown schematically in Figure 5. Microcracks develop within the subsurface, and these propagate and emerge to eventually form a particle that becomes displaced.18 The in vitro study by Arsecularatne and Hoffman,35 which used focused ion beam milling and field emission SEM, found that such behavior occurs. The displaced enamel particles may contribute to
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Volume 3-body wear by acting as a rolling abrasive particle between the rubbing surfaces.4 In comparison, enamel is comparatively harder than Type III gold. As the gold specimen slides against the enamel surface, the frictional force results in plastic deformation of the gold specimen, and, at the same time, adhesive forces develop at the asperity junctions between the enamel and gold in contact. The adhesive forces at the asperity junctions may become stronger than the interatomic metallic bonds of the gold material and result in fragments of gold that adhere to the enamel surface during subsequent sliding movement.3 As the gold specimen deforms, the contact surface area increases and the contact pressure decreases. The rising coefficient of friction with sliding distance would increase the tensile stresses at the trailing edge (as outlined earlier), potentially generating additional small surface cracks, which seem to be the sites where gold is sheared from the antagonist. The SEM observation of the enamel surface that opposes the gold specimen revealed a minimally damaged wear scar characterized by small patches of gold smear adhered to the enamel surface (Fig. 4C). Highmagnification SEM images of the wear scar found fine crack lines perpendicular to the sliding direction and minute areas of surface loss (Fig. 4D). These crack features are typical of a partial cone crack formation at the trailing edge of the indenter.23 The transfer of material from the gold specimen to the antagonist has been reported by previous in vitro wear studies11,12 and indicates that adhesive wear is the predominant wear mechanism. The adhesive wear mechanism of this wear pair relates to the structure of the gold material, which possesses high ductility due to a large number of slip systems.11 In addition, a thin layer of gold particles may assist with the trapping of fluid between the specimens in contact and thereby assist with boundary lubrication. Kaidonis et al36 suggest that the displacement of the liquid lubricant from the interface may result
in a rapid increase in the wear rate because the enamel particles that act as a solid lubricant would be washed away. If the enamel-gold couple were subjected to longer cycles of loading, then the equilibrium steady-state friction condition would probably depend on the extent of metal transfer to the enamel and its tendency to remain locked onto the enamel surface. If the surface of enamel were completely covered with gold, then metal-to-metal contact would occur, which results in micro welding and an increase in friction that would be expected to fragment the surface of the enamel and of the rougher gold surface. However, this phenomenon is not evident in clinical evaluations because the transferred material seems susceptible to removal by food particles and mechanical brushing. The purpose of this study was to explore the onset of deformation and damage to enamel that opposed lithium disilicate glass ceramic and Type III gold. Longer tests could better evaluate the long-term effect of the tested restorative materials on the wear behavior of enamel. The limitations of this study include the absence of physiological factors such as artificial saliva and food simulation slurry, and the evaluation of the surface of the abrading restorative materials. Introducing food slurry or artificial saliva may affect the wear behavior of enamel and alter the results, and should be the basis of further study. In addition, knowledge of the surface roughness of the restorative specimens before and after wear testing would have been useful. The surfaces used were close to those typically supplied by laboratories: polished gold and glazed ceramic.
CONCLUSIONS Within the limits of this in vitro study, the following conclusions were drawn: 1. Lower friction coefficient (m) and better wear resistance were observed when human enamel was opposed by Type III gold than by IPS e.max Press. 2. Abrasive wear was the main mechanism for the
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IPS e.max Press and enamel wear pair, whereas adhesive wear occurred predominantly for the Type III gold and enamel wear pair. In vitro and clinical research is required to characterize and predict the wear behavior of human tooth enamel against a range of prosthetic dental materials.
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7 26. Sajewicz E. On evaluation of wear resistance of tooth enamel and dental materials. Wear 2006;260:1256-61. 27. Ramalho A, Antunes P. Reciprocating wear test of dental composites against human teeth and glass. Wear 2007;263:1095-104. 28. Oliver WC, Pharr GM. Improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 1992;7:1564-83. 29. He L, Swain M. Understanding the mechanical behaviour of human enamel from its structural and compositional characteristics. J Mech Behav Biomed Mater 2008;1:18-29. 30. Guazzato M, Albakry M, Ringer SP, Swain MV. Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part I. Pressable and alumina glass-infiltrated ceramics. Dent Mater 2004;20:441-8. 31. Wataha JC. Alloys for prosthodontic restorations. J Prosthet Dent 2002;87:351-63. 32. Ashby MF. Materials selection in mechanical design. 3rd ed. Boston: Elsevier ButterworthHeinemann; 2005. p. 488-93. 33. Lawn B, Wilshaw R. Indentation fracture: principles and applications. J Mater Sci 1975;10:1049-81. 34. Powers JM, Craig RG, Ludema KC. Frictional behavior and surface failure of human enamel. J Dent Res 1973;52:1327-31.
35. Arsecularatne J, Hoffman M. On the wear mechanism of human dental enamel. J Mech Behav Biomed Mater 2010;3:347-56. 36. Kaidonis JA, Richards L, Townsend G, Tansley G. Wear of human enamel: a quantitative in vitro assessment. J Dent Res 1998;77:1983-90. Corresponding author: Dr Ahreum Lee Department of Oral Rehabilitation Faculty of Dentistry University of Otago PO Box 647 Dunedin 9054, NEW ZEALAND E-mail: [email protected] Acknowledgments The authors dedicate this article to the late Dr Lihong He. His knowledge and enthusiasm for dental biomaterials research provided many insights and encouragement during the course of this study. Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
