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Surface or internal nucleation and crystallization of glass-ceramics
Optical Materials 35 (2013) 1756–1758
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Surface or internal nucleation and crystallization of glass-ceramics W. Höland ⇑, V.M. Rheinberger, C. Ritzberger, E. Apel Ivoclar Vivadent AG, Research and Development, Bendererstr. 2, LI-9494 Schaan, Liechtenstein
a r t i c l e
i n f o
Article history: Received 18 November 2010 Accepted 4 April 2013 Available online 18 May 2013 Keywords: Crystallization Mechanisms Glass-ceramic Dental materials Translucence
a b s t r a c t Fluoroapatite (Ca5(PO4)3F) was precipitated in glass-ceramics via internal crystallization of base glasses. The crystals grew with a needle-like morphology in the direction of the crystallographic c-axis. Two different reaction mechanisms were analyzed: precipitation via a disordered primary apatite crystals and a solid state parallel reaction to rhenanite (NaCaPO4) precipitation. In contrast to the internal nucleation used in the formation of fluoroapatite, surface crystallization was induced to precipitate a phosphate-free oxyapatite of NaY9(SiO4)6O2-type. Internal nucleation and crystallization have been shown to be a very useful tool for developing high-strength lithium disilicate (Li2Si2O5) glass-ceramics. A very controlled process was conducted to transform the lithium metasilicate glass-ceramic precursor material into the final product of the lithium disilicate glass-ceramic without the major phase of the precursor material. The combination of all these methods allowed the driving forces of the internal nucleation and crystallization mechanisms to be explained. An amorphous phosphate primary phase was discovered in the process. Nucleation started at the interface between the amorphous phosphate phase and the glass matrix. The final products of all these glass-ceramics are biomaterials for dental restoration showing special optical properties, e.g. translucence and color close to dental teeth. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Nucleation and crystallization mechanisms used in the development of glass-ceramics are controlled by taking advantage of various solid state reactions. Surface nucleation and surface crystallization are the two most important mechanisms. Their fundamentals are described in numerous textbooks [1,2]. In order to obtain a deeper understanding of special partial reactions within the solid state reactions, extensive studies have been carried out on base glasses with a stoichiometric composition. This means that the base glass features exactly the same chemical composition as the crystal phase that should be precipitated by controlled nucleation and crystallization in the glass-ceramic. The cordierite system (2MgO2Al2O35SiO2) [3], the diopside system (MgOCaO 2SiO2) [4], and the devitrite system (Na2O3CaO6SiO2) [5,6], for example, were selected for the fundamental research on the surface crystallization processes. For a long time, surface crystallization processes were considered difficult to control. With the establishment of the elastic theory [7,8], however, it became quite clear that these processes could be controlled. Internal nucleation (also known as volume nucleation) and crystallization processes in the lithium disilicate system (Li2O2SiO2) [9,10] in particular were studied. The role of possible pre⇑ Corresponding author. E-mail address: [email protected] (W. Höland). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.04.023
liminary phases and metastable transition phases were examined in depth. Nevertheless, non-stationary processes were also discovered in the process [11]. The research carried out on non-stoichiometric materials systems led to the discovery of new ways of developing glassceramics tailor-made for specific applications. For example, Beall and co-workers [12,13] produced high-strength glass-ceramics containing chain and layer silicates as the main crystal phase. These materials include glass-ceramics of the enstatite, canasite, richterite and lithium disilicate type, which were produced by internal nucleation with different nucleation agents. In one special glass system, Headley and Loehman [14] showed that following the formation of Li3PO4 at high temperatures, the epitaxial growth of lithium disilicate on this orthophosphate is possible. Furthermore, as more knowledge was gained about liquid–liquid phase separation processes (even on the nanometer scale), possibilities of internal nucleation and crystallization were discovered, which would allow these processes to be specifically developed in complex composites. These include a phosphoalumino-silicate system, which allows glass-ceramics containing fluoroapatite to be formed [15,16]. Controlled surface crystallization of non-stoichiometric materials systems was used in the development of powder compacts. For example, a glass-ceramic containing leucite (KAlSi2O6) was developed, which exhibited favorable strength (up to 180 MPa flexural strength) and translucent properties.
W. Höland et al. / Optical Materials 35 (2013) 1756–1758
1757
The present publication also describes non-stoichiometric base glass compositions, which enable the production of biocompatible glass-ceramics for restorative dental applications. First, however, in accordance with the title of this publication, the mechanisms of internal crystallization and surface crystallization of glass will be discussed separately. The ways in which the two mechanisms can be effectively combined will be shown at the end of this discussion.
2. Mechanisms of crystallization 2.1. Surface or internal crystallization 2.1.1. Apatite formation: one type of crystal – two different mechanisms The formation of fluoroapatite (Ca5(PO4)3F) was achieved in the SiO2–Al2O3–Na2O–K2O–CaO–P2O5–F materials system. The mechanism of internal nucleation and crystallization was clearly responsible for the result. Surface effects could be discounted. Amorphous, spherical nano-phases of P2O5-rich components can play a special part in the process. The controlled nucleation and crystallization of this apatite enabled the growth of the phase along the crystallographic c-axis in needle-like form. The crystals grow separately in relatively isolated form. The resulting flexural strength was measured at approx. 100 MPa. The Ktip value (the critical stress intensity factor at the crack tip) measured 0.7 MPa m0.5 [17]. Another surprising effect of apatite crystallization was discovered in the SiO2–Al2O3–La2O3–Y2O3–Li2O–Na2O–K2O system. It was clearly shown that the mechanisms of internal nucleation and crystallization could not be used to form oxyapatite (NaY9(SiO4)6O2) [18]. Instead, the mechanism of surface nucleation/crystallization must be used to precipitate these crystals in the base glass. When comparing the two glass-ceramic systems and their main crystal phases with each other, it is important to note that either internal or surface nucleation must be applied.
2.1.2. Lithium disilicate crystallization: internal crystallization In the multicomponent system SiO2–Al2O3–Li2O–K2O–ZnO– P2O5 and in the SiO2–Al2O3–Li2O–K2O–P2O5–ZrO2 system, the mechanism of internal nucleation was used to control the crystallization of lithium disilicate [19,20]. This process consists of several stages and various sequential and parallel reactions. In this process, the formation of a lithium metasilicate phase and its transformation into a lithium disilicate phase plays an important role. High-temperature X-ray diffraction (HT-XRD) investigations and Rietveld measurements have shown that the complete transformation to lithium disilicate is possible [21]. The microstructure of this type of glass-ceramic is shown in Fig. 1. As shown in this figure, the material has an interlocking microstructure. The crystalline fraction of approx. 60 vol.% is very high. The resulting flexural strength was established at 741 ± 80 MPa [22] in laboratory test specimens and at 537–617 MPa [23] in the commercial product. If a crack is induced in the glass-ceramic, for example, with Vickers indentation, the crack always grows along the glass matrix and does not pass through the crystals. The crack, therefore, has to make considerable detours as it propagates. Consequently, the KIC value of 2.3–2.9 MPa m0.5, measured according to the single-edge V-notch beam (SEVNB) method, is very high. The Ktip value of 1.3 MPa m0.5 at the crack tip is significantly higher than that of the fluoroapatite glass-ceramic. However, both glass-ceramics can be joined because of their similar coefficients of thermal expansion (CTE).
Fig. 1. Microstructure of a translucent lithium disilicate glass-ceramic derived from the SiO2–Al2O3–Li2O–K2O–P2O5 system.
2.2. The combination: twofold crystallization mechanisms Thus far, the solid state mechanisms of controlled crystallization discussed here proceed either according to the mechanism of surface crystallization or the mechanism of internal crystallization. Therefore, two clearly separate reactions have been presented. In the discussion, it has become evident that either a number of solid state reactions can occur at the same time or that sequential reactions take place. Despite all the described complexities, these two very different mechanisms can be combined. For example, the crystallization of leucite (KAlSi2O6) can be achieved using the mechanism of surface crystallization and the simultaneous formation (a classical parallel reaction of solid state phase formation) of fluoroapatite. The finished product is a glass-ceramic that contains both apatite and leucite crystals. In order to produce these materials, powder compacts made with glasses from the SiO2–Al2O3–Na2O–K2O–CaO–P2O5–F materials system had to be fabricated. This powder crystallized at the grain surface and the leucite crystals grew into the interior of the glass grains. At the same time, fluoroapatite crystals also grew in the volume of the glass grains through the mechanism of internal crystallization. If this complex twofold crystallization mechanism is combined
Fig. 2. Microstructure of a translucent leucite–apatite glass-ceramic processed as powder compact. SEM, HF-etched sample.
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with sintering principles, powder compacts can be produced (Fig. 2). This type of glass-ceramics is used either in the form of glassceramic granules, or as bulk performs. In granular form, the product can be sintered to produce powder compacts. In the shape of a bulk perform, the material can be molded by taking advantage of viscous flow principles. 3. Conclusions The authors conclude that both controlled internal nucleation and controlled surface nucleation can be used to produce glassceramics from multi-component systems. The fabrication of biomaterials for restorative dentistry represents a fine example of these materials. High-strength and high-toughness glass-ceramics of the lithium disilicate type can be veneered with fluoroapatite glass-ceramics. Thus, these two types of glass-ceramics can be combined to produce a very attractive biomaterial. The glassceramics show translucency similar to dental tooth structures. References [1] P.W. McMillan, Glass-Ceramics, Academic Press, New York, 1979. 2. [2] W. Höland, G.H. Beall, Glass-Ceramic Technology, The American Ceramic Society, Westerville, 2002.
[3] R. Müller, D. Tamm, W. Pannhorst, Bol. Soc. Esp. Ceram. VID. 31-C (1992) 105– 110. [4] E.D. Zanotto, J. Non-Cryst. Sol. 130 (1991) 217–219. [5] E.D. Zanotto, J. Non-Cryst. Sol. 129 (1997) 183–190. [6] K.L. Narayan, K.F. Kelton, J. Non-Cryst. Sol. 220 (1997) 222–230. [7] R. Müller, E.D. Zanotto, V.M. Fokin, J. Non-Cryst. Sol. 274 (2000) 208–231. [8] J. Schmelzer, R. Pascova, J. Möller, I. Gutzow, J. Non-Cryst. Sol. 162 (1993) 26– 39. [9] G.F. Neilson, M.C. Weinberg, J. Non-Cryst. Sol. 34 (1979) 137–147. [10] P.F. James, J. Non-Cryst. Sol. 73 (1985) 517–540. [11] J. Deubener, R. Brückner, M. Sternitzke, J. Non-Cryst. Sol. 163 (1993) 1–12. [12] G.H. Beall, Solid State Sci. 3 (1989) 333–354. [13] L.M. Echeveria, Bol. Soc. Esp. Ceram. VID. 5 (1992) 183–188. [14] T.J. Headley, R.E. Loehmann, J. Am. Ceram. Soc. 67 (1984) 620–625. [15] W. Höland, M. Frank, M. Schweiger, V. Rheinberger, Glastech. Ber. Glass Sci. Technol. 67C (1994) 117–122. [16] W. Höland, V. Rheinberger, S. Wegner, M. Frank, J. Mater. Sci.: Mater. Med. 11 (2000) 1–7. [17] E. Apel, J. Deubener, A. Bernard, M. Höland, R. Müller, H. Kappert, V. Rheinberger, Höland, J. Mech. Behav. Biomed. Mat. 1 (2008) 313–325. [18] C. van’t Hoen, V.M. Rheinberger, W. Höland, E. Apel, J. Europ. Ceram. Soc. 27 (2007) 1579–1584. [19] E. Apel, C. van’t Hoen, V.M. Rheinberger, W. Höland, J. Europ. Ceram. Soc. 27 (2007) 1571–1577. [20] W. Höland, V. Rheinberger, M. Schweiger, Phil. Trans. R. Soc. Lond. A 361 (2003) 575–589. [21] W. Höland, G.H. Beall, Glass-Ceramic Technology, second ed., Wiley, 2012. [22] W. Höland, V.M. Rheinberger, E. Apel, C. van’t Hoen, J. Europ. Ceram. Soc. 27 (2007) 1521–1526. [23] M. Schweiger, D. Tauch, H. Bürke, H.F. Kappert, V.M. Rheinberger, J. Dent. Res. 89 (Special Issue A) (2010). Abstract#420.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Surface or internal nucleation and crystallization of glass-ceramics W. Höland ⇑, V.M. Rheinberger, C. Ritzberger, E. Apel Ivoclar Vivadent AG, Research and Development, Bendererstr. 2, LI-9494 Schaan, Liechtenstein
a r t i c l e
i n f o
Article history: Received 18 November 2010 Accepted 4 April 2013 Available online 18 May 2013 Keywords: Crystallization Mechanisms Glass-ceramic Dental materials Translucence
a b s t r a c t Fluoroapatite (Ca5(PO4)3F) was precipitated in glass-ceramics via internal crystallization of base glasses. The crystals grew with a needle-like morphology in the direction of the crystallographic c-axis. Two different reaction mechanisms were analyzed: precipitation via a disordered primary apatite crystals and a solid state parallel reaction to rhenanite (NaCaPO4) precipitation. In contrast to the internal nucleation used in the formation of fluoroapatite, surface crystallization was induced to precipitate a phosphate-free oxyapatite of NaY9(SiO4)6O2-type. Internal nucleation and crystallization have been shown to be a very useful tool for developing high-strength lithium disilicate (Li2Si2O5) glass-ceramics. A very controlled process was conducted to transform the lithium metasilicate glass-ceramic precursor material into the final product of the lithium disilicate glass-ceramic without the major phase of the precursor material. The combination of all these methods allowed the driving forces of the internal nucleation and crystallization mechanisms to be explained. An amorphous phosphate primary phase was discovered in the process. Nucleation started at the interface between the amorphous phosphate phase and the glass matrix. The final products of all these glass-ceramics are biomaterials for dental restoration showing special optical properties, e.g. translucence and color close to dental teeth. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Nucleation and crystallization mechanisms used in the development of glass-ceramics are controlled by taking advantage of various solid state reactions. Surface nucleation and surface crystallization are the two most important mechanisms. Their fundamentals are described in numerous textbooks [1,2]. In order to obtain a deeper understanding of special partial reactions within the solid state reactions, extensive studies have been carried out on base glasses with a stoichiometric composition. This means that the base glass features exactly the same chemical composition as the crystal phase that should be precipitated by controlled nucleation and crystallization in the glass-ceramic. The cordierite system (2MgO2Al2O35SiO2) [3], the diopside system (MgOCaO 2SiO2) [4], and the devitrite system (Na2O3CaO6SiO2) [5,6], for example, were selected for the fundamental research on the surface crystallization processes. For a long time, surface crystallization processes were considered difficult to control. With the establishment of the elastic theory [7,8], however, it became quite clear that these processes could be controlled. Internal nucleation (also known as volume nucleation) and crystallization processes in the lithium disilicate system (Li2O2SiO2) [9,10] in particular were studied. The role of possible pre⇑ Corresponding author. E-mail address: [email protected] (W. Höland). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.04.023
liminary phases and metastable transition phases were examined in depth. Nevertheless, non-stationary processes were also discovered in the process [11]. The research carried out on non-stoichiometric materials systems led to the discovery of new ways of developing glassceramics tailor-made for specific applications. For example, Beall and co-workers [12,13] produced high-strength glass-ceramics containing chain and layer silicates as the main crystal phase. These materials include glass-ceramics of the enstatite, canasite, richterite and lithium disilicate type, which were produced by internal nucleation with different nucleation agents. In one special glass system, Headley and Loehman [14] showed that following the formation of Li3PO4 at high temperatures, the epitaxial growth of lithium disilicate on this orthophosphate is possible. Furthermore, as more knowledge was gained about liquid–liquid phase separation processes (even on the nanometer scale), possibilities of internal nucleation and crystallization were discovered, which would allow these processes to be specifically developed in complex composites. These include a phosphoalumino-silicate system, which allows glass-ceramics containing fluoroapatite to be formed [15,16]. Controlled surface crystallization of non-stoichiometric materials systems was used in the development of powder compacts. For example, a glass-ceramic containing leucite (KAlSi2O6) was developed, which exhibited favorable strength (up to 180 MPa flexural strength) and translucent properties.
W. Höland et al. / Optical Materials 35 (2013) 1756–1758
1757
The present publication also describes non-stoichiometric base glass compositions, which enable the production of biocompatible glass-ceramics for restorative dental applications. First, however, in accordance with the title of this publication, the mechanisms of internal crystallization and surface crystallization of glass will be discussed separately. The ways in which the two mechanisms can be effectively combined will be shown at the end of this discussion.
2. Mechanisms of crystallization 2.1. Surface or internal crystallization 2.1.1. Apatite formation: one type of crystal – two different mechanisms The formation of fluoroapatite (Ca5(PO4)3F) was achieved in the SiO2–Al2O3–Na2O–K2O–CaO–P2O5–F materials system. The mechanism of internal nucleation and crystallization was clearly responsible for the result. Surface effects could be discounted. Amorphous, spherical nano-phases of P2O5-rich components can play a special part in the process. The controlled nucleation and crystallization of this apatite enabled the growth of the phase along the crystallographic c-axis in needle-like form. The crystals grow separately in relatively isolated form. The resulting flexural strength was measured at approx. 100 MPa. The Ktip value (the critical stress intensity factor at the crack tip) measured 0.7 MPa m0.5 [17]. Another surprising effect of apatite crystallization was discovered in the SiO2–Al2O3–La2O3–Y2O3–Li2O–Na2O–K2O system. It was clearly shown that the mechanisms of internal nucleation and crystallization could not be used to form oxyapatite (NaY9(SiO4)6O2) [18]. Instead, the mechanism of surface nucleation/crystallization must be used to precipitate these crystals in the base glass. When comparing the two glass-ceramic systems and their main crystal phases with each other, it is important to note that either internal or surface nucleation must be applied.
2.1.2. Lithium disilicate crystallization: internal crystallization In the multicomponent system SiO2–Al2O3–Li2O–K2O–ZnO– P2O5 and in the SiO2–Al2O3–Li2O–K2O–P2O5–ZrO2 system, the mechanism of internal nucleation was used to control the crystallization of lithium disilicate [19,20]. This process consists of several stages and various sequential and parallel reactions. In this process, the formation of a lithium metasilicate phase and its transformation into a lithium disilicate phase plays an important role. High-temperature X-ray diffraction (HT-XRD) investigations and Rietveld measurements have shown that the complete transformation to lithium disilicate is possible [21]. The microstructure of this type of glass-ceramic is shown in Fig. 1. As shown in this figure, the material has an interlocking microstructure. The crystalline fraction of approx. 60 vol.% is very high. The resulting flexural strength was established at 741 ± 80 MPa [22] in laboratory test specimens and at 537–617 MPa [23] in the commercial product. If a crack is induced in the glass-ceramic, for example, with Vickers indentation, the crack always grows along the glass matrix and does not pass through the crystals. The crack, therefore, has to make considerable detours as it propagates. Consequently, the KIC value of 2.3–2.9 MPa m0.5, measured according to the single-edge V-notch beam (SEVNB) method, is very high. The Ktip value of 1.3 MPa m0.5 at the crack tip is significantly higher than that of the fluoroapatite glass-ceramic. However, both glass-ceramics can be joined because of their similar coefficients of thermal expansion (CTE).
Fig. 1. Microstructure of a translucent lithium disilicate glass-ceramic derived from the SiO2–Al2O3–Li2O–K2O–P2O5 system.
2.2. The combination: twofold crystallization mechanisms Thus far, the solid state mechanisms of controlled crystallization discussed here proceed either according to the mechanism of surface crystallization or the mechanism of internal crystallization. Therefore, two clearly separate reactions have been presented. In the discussion, it has become evident that either a number of solid state reactions can occur at the same time or that sequential reactions take place. Despite all the described complexities, these two very different mechanisms can be combined. For example, the crystallization of leucite (KAlSi2O6) can be achieved using the mechanism of surface crystallization and the simultaneous formation (a classical parallel reaction of solid state phase formation) of fluoroapatite. The finished product is a glass-ceramic that contains both apatite and leucite crystals. In order to produce these materials, powder compacts made with glasses from the SiO2–Al2O3–Na2O–K2O–CaO–P2O5–F materials system had to be fabricated. This powder crystallized at the grain surface and the leucite crystals grew into the interior of the glass grains. At the same time, fluoroapatite crystals also grew in the volume of the glass grains through the mechanism of internal crystallization. If this complex twofold crystallization mechanism is combined
Fig. 2. Microstructure of a translucent leucite–apatite glass-ceramic processed as powder compact. SEM, HF-etched sample.
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W. Höland et al. / Optical Materials 35 (2013) 1756–1758
with sintering principles, powder compacts can be produced (Fig. 2). This type of glass-ceramics is used either in the form of glassceramic granules, or as bulk performs. In granular form, the product can be sintered to produce powder compacts. In the shape of a bulk perform, the material can be molded by taking advantage of viscous flow principles. 3. Conclusions The authors conclude that both controlled internal nucleation and controlled surface nucleation can be used to produce glassceramics from multi-component systems. The fabrication of biomaterials for restorative dentistry represents a fine example of these materials. High-strength and high-toughness glass-ceramics of the lithium disilicate type can be veneered with fluoroapatite glass-ceramics. Thus, these two types of glass-ceramics can be combined to produce a very attractive biomaterial. The glassceramics show translucency similar to dental tooth structures. References [1] P.W. McMillan, Glass-Ceramics, Academic Press, New York, 1979. 2. [2] W. Höland, G.H. Beall, Glass-Ceramic Technology, The American Ceramic Society, Westerville, 2002.
[3] R. Müller, D. Tamm, W. Pannhorst, Bol. Soc. Esp. Ceram. VID. 31-C (1992) 105– 110. [4] E.D. Zanotto, J. Non-Cryst. Sol. 130 (1991) 217–219. [5] E.D. Zanotto, J. Non-Cryst. Sol. 129 (1997) 183–190. [6] K.L. Narayan, K.F. Kelton, J. Non-Cryst. Sol. 220 (1997) 222–230. [7] R. Müller, E.D. Zanotto, V.M. Fokin, J. Non-Cryst. Sol. 274 (2000) 208–231. [8] J. Schmelzer, R. Pascova, J. Möller, I. Gutzow, J. Non-Cryst. Sol. 162 (1993) 26– 39. [9] G.F. Neilson, M.C. Weinberg, J. Non-Cryst. Sol. 34 (1979) 137–147. [10] P.F. James, J. Non-Cryst. Sol. 73 (1985) 517–540. [11] J. Deubener, R. Brückner, M. Sternitzke, J. Non-Cryst. Sol. 163 (1993) 1–12. [12] G.H. Beall, Solid State Sci. 3 (1989) 333–354. [13] L.M. Echeveria, Bol. Soc. Esp. Ceram. VID. 5 (1992) 183–188. [14] T.J. Headley, R.E. Loehmann, J. Am. Ceram. Soc. 67 (1984) 620–625. [15] W. Höland, M. Frank, M. Schweiger, V. Rheinberger, Glastech. Ber. Glass Sci. Technol. 67C (1994) 117–122. [16] W. Höland, V. Rheinberger, S. Wegner, M. Frank, J. Mater. Sci.: Mater. Med. 11 (2000) 1–7. [17] E. Apel, J. Deubener, A. Bernard, M. Höland, R. Müller, H. Kappert, V. Rheinberger, Höland, J. Mech. Behav. Biomed. Mat. 1 (2008) 313–325. [18] C. van’t Hoen, V.M. Rheinberger, W. Höland, E. Apel, J. Europ. Ceram. Soc. 27 (2007) 1579–1584. [19] E. Apel, C. van’t Hoen, V.M. Rheinberger, W. Höland, J. Europ. Ceram. Soc. 27 (2007) 1571–1577. [20] W. Höland, V. Rheinberger, M. Schweiger, Phil. Trans. R. Soc. Lond. A 361 (2003) 575–589. [21] W. Höland, G.H. Beall, Glass-Ceramic Technology, second ed., Wiley, 2012. [22] W. Höland, V.M. Rheinberger, E. Apel, C. van’t Hoen, J. Europ. Ceram. Soc. 27 (2007) 1521–1526. [23] M. Schweiger, D. Tauch, H. Bürke, H.F. Kappert, V.M. Rheinberger, J. Dent. Res. 89 (Special Issue A) (2010). Abstract#420.