IMPLANT DENTISTRY / VOLUME 22, NUMBER 6 2013
623
CAD/CAM Glass Ceramics for Single-Tooth Implant Crowns: A Finite Element Analysis Kivanç Akça, DDS, PhD,* Yeliz Cavusoglu, DDS, PhD,† Elcin Sagirkaya, DDS, PhD,‡ Buket Aybar, DDS, PhD,§ and Murat Cavit Cehreli, DDS, PhDk
oncrystalline ceramics are structurally in glassy nature and have been the gold standard in material selection for dental restorations because of their biological and mechanical properties.1,2 Despite the high optical properties of these ceramics, their clinical use is somewhat burdened with inadequete fracture strength.3 At present, advances in adhesive dentistry associated with CAD/CAM technology allows 1-visit in-office produced crowns from monoceramic blocks.4,5 Proof of concept has been established based on the longterm successful clinical outcomes.6 Essentially, adhesive cementation of machinable glass matrix ceramic crowns to natural tooth surface is crucial to achieve high survival rates.6 In-laboratory digital solutions to fabricate surgical guides, customized abutments and superstructures are well documented in implant dentistry.7,8 On the contrary, the scientific evidence level
N
*Professor, Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, Ankara, Turkey. †Post-Doc researcher, Section of Prosthodontics, Acıbadem Atas¸ehir Medical Surgery Center, Atas¸ehir, Istanbul, Turkey. ‡Assistant Professor and Chairperson, Department of Prosthodontics, Faculty of Dentistry, Ordu University, Ordu, Turkey. §Professor, Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, Istanbul University, Istanbul, Turkey. kProfessor, Department of Prosthodontics, Faculty of Dentistry, Ordu University; Director-Assignee, Institute of Health Sciences, Ordu University, Ordu, Turkey.
Reprint requests and correspondence to: Kivanç Akça, DDS, PhD, Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, Faculty of Dentistry, Ankara 06100, Turkey, Phone: +90-312-3054074, Fax: +90-312-3113741, E-mail: [email protected] ISSN 1056-6163/13/02206-623 Implant Dentistry Volume 22 Number 6 Copyright © 2013 by Lippincott Williams & Wilkins DOI: 10.1097/01.id.0000433589.33926.79
Purpose: To evaluate the load distribution of CAD/CAM monoceramic crowns supported with single-tooth implants in functional area. Materials and Methods: A 3dimensional numerical model of a soft tissue–level implant was constructed with cement-retained abutment to support glass ceramic machinable crown. Implant-abutment complex and the retained crown were embedded in a Ø 1.5 3 1.5 cm geometric matrix for evaluation of mechanical behavior of monoceramic CAD/CAM aluminosilicate and leucite glass crown materials. Laterally positioned axial load of 300 N was applied on the crowns. Resulting principal stresses in the mono-ceramic crowns were evaluated in relation to different glass ceramic materials.
Results: The highest compressive stresses were observed at the cervical region of the buccal aspect of the crowns and were 89.98 and 89.99 MPa, for aluminosilicate and leucite glass ceramics, respectively. The highest tensile stresses were observed at the collar of the lingual part of the crowns and were 24.54 and 25.39 MPa, respectively. Conclusion: Stresses induced upon 300 N static loading of CAD/ CAM aluminosalicate and leucite glass ceramics are below the compressive strength of the materials. Impact loads may actuate the progress to end failure of mono-ceramic crowns supported by metallic implant abutments. (Implant Dent 2013;22:623–626) Key Words: CAD/CAM, singletooth implant, glass ceramics, finite element analyses
for in-office CAD/CAM single-tooth implant applications to lessen the number of visits is only limited to case reports in comparison with natural tooth restorations.9,10 Additionally, the preclinical information with regard to the mechanical behavior of glass matrix ceramic crown on implant abutments is insufficient.11 Wolf et al12 reported increased fracture strength of aluminasilicate glass-ceramic crowns adhesively cemented either to zirconium or titanium abutments under static loading. Lately,
Çavus¸oglu et al13 presented early fatigue failure both for aluminosilicate and leucite glass ceramic single-tooth implant crowns on titanium abutments. Although the mechanical properties under dynamic loading conditions for both monoblock ceramic materials were found to be insufficient for clinical use, additional information regarding the characteristics of load distribution within the glass matrix ceramic materials used with metallic implant abutments is needed. The purpose of this study was, therefore, to
624
GLASS CERAMICS
FOR
SINGLE-TOOTH IMPLANT CROWNS
Fig. 1. CAD model of the ceramic crown on the implant.
Fig. 2. Conversion of the CAD model to numeric model for FE analysis. Arrow indicates 1.5 mm laterally positioned axial loading with respect to implant long axis to simulate similar loading conditions at lu et al.13 Çavus¸og
analyze the mechanical stresses in machinable mono-ceramic single-tooth implant crowns using finite element (FE) analysis.
MATERIALS
AND
METHODS
The CAD model of a tissue-level (neck Ø 4.8 mm) cylindrical screw dental implant (Ø 4.1 3 10 mm) (043.052S; Institut Straumann AG, Basel, Switzerland) with a cementable abutment of 5.5 mm in height (048.541; Institut Straumann AG) was constructed in 1 piece using I-DEAS Artisan Series 3.0 (Structural Dynamics Research Corporation,
AKÇA
ET AL
Fig. 3. Similar load distribution of compressive stresses around (A) aluminosilicate glass and (B) leucite glass ceramic crowns from lateral view. Higher stresses concentrated at the applied load side at cervical region of crown for both the glass ceramic materials.
Fig. 4. Compressive stress values viewed from the buccal aspect of (A) aluminosilicate glass and (B) leucite glass ceramic crowns. Highest values were recorded at the crown margin in contact with implant shoulder.
Milford, OH). To simplify the modeling, the threads of the implant and the abutment were not represented in their spiral characteristics but as symmetrical rings.14 The implant-abutment complex was transferred to a preprocessor (MSC. MarcMetat 2000; MSC. Software Corporation, Los Angeles, CA) for FE model conversion and was embedded vertically in the center of a Ø 1.5 3 1.5-cm cylinder representing 1-mm cortical bone in the upper surface and the rest trabecular bone15 (Fig. 1). A mandibular premolar all-ceramic crown in anatomical dimensions was placed over the abutment with 1, 2, and 3 mm material thickness at cervical, middle, and occlusal thirds of abutment, respectively. The buccal contour of the crown in occlusal third was enlarged,
and the buccal cusp was flattened for laterally positioned axial loading (Fig. 2). The 3-dimensional FE model was constructed by using 32,137 elements resulting in 8423 elements for the implant-abutment complex, 10,096 elements for the crown, and 13,798 elements for the cortical-trabecular bone. Boundary conditions were established by constraining the bone cylinder from its bottom and lateral surfaces.15 A static load of 300 N was applied vertically on the top of the buccal cusp surface to simulate laterally positioned axial loading.8 The loading case was processed separately for 2 different glass ceramic materials (aluminosilicate and leucite glass) using an FE analysis solver (Abaqus 6.5.1; Simulia, Providence, RI). All the materials were assumed to be isotropic,
IMPLANT DENTISTRY / VOLUME 22, NUMBER 6 2013
Fig. 5. Load distribution behavior of tensile stresses from lateral view were similar at the opposite applied load side in comparison to compressive stresses around (A) aluminosilicate glass and (B) leucite glass ceramic crowns.
Fig. 6. Highest tensile stress values but lower in comparison to compressive stresses were recorded at margin both for (A) aluminosilicate glass and (B) leucite glass ceramic crowns.
homogeneous, and linearly elastic, and the Young module and Poisson ratio of aluminosilicate and leucite glass ceramics were 68 GPa and 0.316 and 70 GPa and 0.3 (manufacturer’s data), respectively. Principal stresses were recorded and visualized comparatively for both the glass ceramic materials using a postprocessor (Abaqus/Viewer; Simulia).
RESULTS In both ceramic materials, the magnitude and distribution of minimum principal stresses (compressive stresses) were comparable (Fig. 3). The highest stresses were observed predominantly at the cervical region of the buccal aspect of the crowns, where mating between the crown and the abutment shoulder is
achieved. The highest compressive stresses obtained for aluminosilicate and leucite glass ceramics were 89.98 and 89.99 MPa, respectively (Fig. 4). Likewise, the magnitude and distribution of maximum principal stresses (tensile stresses) were comparable in both ceramic materials (Fig. 5). The highest tensile stresses were observed at the cervical region of the lingual aspect of the crowns. The highest tensile stresses obtained for aluminosilicate and leucite glass ceramics were 24.54 and 25.39 MPa, respectively (Fig. 6).
DISCUSSION The rationale behind this study was to study the mechanical effects of static loading on all-ceramic CAD/CAM
625
aluminosilicate and leucite glass ceramic crowns on implant abutments by numeric analysis, which had unfavorable fate on dynamic loading in a previous in vitro study.13 The FE model and the crowns were the same as in the in vitro model, and the only difference was that 300 N force was applied in a static manner in this study. The in vitro study revealed visible cracks and fractures located in the buccal aspect of the crowns arising from the margin, and this was confirmed by high minimum principal stresses located at the collar of the crowns. Indeed, high compressive stresses at the crown-implant junction was the main cause for the failure. In addition, the amplitude of compressive stresses in both types of ceramic crowns were very close (;90 MPa), suggesting that the mechanical fate of both crowns would be comparable, as observed in the in vitro study.13 However, this is not the case. The flexural strength of a material is approximately 12% to 20% of its compressive strength. The flexural strengths of aluminosilicate and leucite glass ceramics are 103 and 127 MPa, respectively,17 and therefore, the compressive strengths of the materials are between 515 to 858 MPa and 635 to 1058 MPa, respectively. As the outcome of static loading (;90 MPa) is lower than the compressive strength of the materials, one could presume that static loading would not lead to the failure of these crowns. Considering the percentage decrease between start and end of failure values of monolithic aluminosilicate and leucite glass ceramics,18 the crack initiation of these glass ceramics are between 335 to 531 MPa and 431 to 719 MPa. The outcome of 300-N static loading in FE analysis is still below for possible start of failure. The present results suggest that implant-supported monoblock CAD/ CAM aluminosilicate and leucite glass ceramic crowns may withstand impact forces of 300 N. However, this interpretation does not necessarily imply that the structural integrity of the glass ceramic crowns remains intact. On the contrary, stresses at the collar region of the crowns may be responsible to crack propagation that would result in fracture of these crowns in time as indicated
626
GLASS CERAMICS
FOR
SINGLE-TOOTH IMPLANT CROWNS
(Figs. 3 and 4).19–21 Additionally, lack in adhesive integration of cement layer to metal abutment that impairs adhesive bonding needs additional considerations in mechanical support of glass ceramic implant crowns.22
CONCLUSION The following could be drawn for clinical application of machinable glass ceramics for single-tooth implant crowns: Functional occlusal forces can be tolerated at static contact. Impact forces may initiate or propagate crack formation. Glass ceramics modified in structure should be considered to optimize mechanical failures.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ACKNOWLEDGMENT This study was partly supported by _ TÜBITAK (The Scientific and Technological Research Council of Turkey; Project #109S295).
REFERENCES 1. Magne P, Perroud R, Hodges JS, et al. Clinical performance of novel-design porcelain veneers for the recovery of coronal volume and length. Int J Periodontics Restorative Dent. 2000;20:440–457.
AKÇA
ET AL
2. Cehreli MC, Kokat AM, Ozpay C, et al. A randomized controlled clinical trial of feldspathic versus glass-infiltrated alumina all-ceramic crowns: A 3-year followup. Int J Prosthodont. 2011;24:77–84. 3. Johnston WM, O’Brien WJ. The shear strength of dental porcelain. J Dent Res. 1980;59:1409–1411. 4. Qin F, Zheng S, Luo Z, et al. Evaluation of machinability and flexural strength of a novel dental machinable glass-ceramic. J Dent. 2009;37:776–780. 5. Behr M, Rosentritt M, Mangelkramer M, et al. The influence of different cements on the fracture resistance and marginal adaptation of all-ceramic and fiber-reinforced crowns. Int J Prosthodont. 2003;16:538–542. 6. van Dijken JW, Hasselrot L. A prospective 15-year evaluation of extensive dentin-enamel-bonded pressed ceramic coverages. Dent Mater. 2010;26:929–939. 7. Drago C, Saldarriaga RL, Domagala D, et al. Volumetric determination of the amount of misfit in CAD/CAM and cast implant frameworks: A multicenter laboratory study. Int J Oral Maxillofac Implants. 2010;25:920–929. 8. Pozzi A, Sannino G, Barlattani A. Minimally invasive treatment of the atrophic posterior maxilla: A proof-of-concept prospective study with a follow-up of between 36 and 54 months. J Prosthet Dent. 2012; 108:286–297. 9. Fritzsche G. Treatment of a singletooth gap with a Cerec 3D crown on an implant: A case report. Int J Comput Dent. 2004;7:199–206. 10. Gougaloff R, Stalley FC. Immediate placement and provisionalization of a dental implant utilizing the CEREC 3 CAD/CAM Protocol: A clinical case report. J Calif Dent Assoc. 2010;38:170–173, 176–177. 11. Hosseini M, Kleven E, Gotfredsen K. Fracture mode during cyclic loading of implant-supported single-tooth restorations. J Prosthet Dent. 2012;108:74–83. 12. Wolf D, Bindl A, Schmidlin PR, et al. Strength of CAD/CAM-generated
esthetic ceramic molar implant crowns. Int J Oral Maxillofac Implants. 2008;23: 609–617. lu Y, Sahin E, Gürbüz R, 13. Çavus¸og et al. Fatigue resistance of 2 different CAD/CAM glass-ceramic materials used for single-tooth implant crowns. Implant Dent. 2011;20:374–378. 14. Merz BR, Hunenbart S, Belser UC. Mechanics of the implant-abutment connection: An 8-degree taper compared to a butt joint connection. Int J Oral Maxillofac Implants. 2000;15:519–526. 15. Akca K, Cehreli MC. Biomechanical consequences of progressive marginal bone loss around oral implants: A finite element stress analysis. Med Biol Eng Comput. 2006;44:527–535. 16. Lawn BR, Deng Y, Lloyd IK, et al. Materials design of ceramic-based layer structures for crowns. J Dent Res. 2002; 81:433–438. 17. Bindl A, Lüthy H, Mörmann WH. Fracture load of CAD/CAM-generated slot-inlay FPDs. Int J Prosthodont. 2003; 16:653–660. 18. Bindl A, Lüthy H, Mörmann WH. Strength and fracture pattern of monolithic CAD/CAM-generated posterior crowns. Dent Mater. 2006;22:29–36. 19. Rekow D, Thompson VP. Engineering long term clinical success of advanced ceramic prostheses. J Mater Sci Mater Med. 2007;18:47–56. 20. Rekow D, Thompson VP Nearsurface damagedA persistent problem in crowns obtained by computer-aided design and manufacturing. Proc Inst Mech Eng H. 2005;219:233–243. 21. Jung YG, Peterson IM, Kim DK, et al. Lifetime-limiting strength degradation from contact fatigue in dental ceramics. J Dent Res. 2000;79:722–731. 22. Malament KA, Socransky SS. Survival of Dicor glass-ceramic dental restorations over 16 years. Part III: Effect of luting agent and tooth or tooth-substitute core structure. J Prosthet Dent. 2001;86: 511–519.
623
CAD/CAM Glass Ceramics for Single-Tooth Implant Crowns: A Finite Element Analysis Kivanç Akça, DDS, PhD,* Yeliz Cavusoglu, DDS, PhD,† Elcin Sagirkaya, DDS, PhD,‡ Buket Aybar, DDS, PhD,§ and Murat Cavit Cehreli, DDS, PhDk
oncrystalline ceramics are structurally in glassy nature and have been the gold standard in material selection for dental restorations because of their biological and mechanical properties.1,2 Despite the high optical properties of these ceramics, their clinical use is somewhat burdened with inadequete fracture strength.3 At present, advances in adhesive dentistry associated with CAD/CAM technology allows 1-visit in-office produced crowns from monoceramic blocks.4,5 Proof of concept has been established based on the longterm successful clinical outcomes.6 Essentially, adhesive cementation of machinable glass matrix ceramic crowns to natural tooth surface is crucial to achieve high survival rates.6 In-laboratory digital solutions to fabricate surgical guides, customized abutments and superstructures are well documented in implant dentistry.7,8 On the contrary, the scientific evidence level
N
*Professor, Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, Ankara, Turkey. †Post-Doc researcher, Section of Prosthodontics, Acıbadem Atas¸ehir Medical Surgery Center, Atas¸ehir, Istanbul, Turkey. ‡Assistant Professor and Chairperson, Department of Prosthodontics, Faculty of Dentistry, Ordu University, Ordu, Turkey. §Professor, Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, Istanbul University, Istanbul, Turkey. kProfessor, Department of Prosthodontics, Faculty of Dentistry, Ordu University; Director-Assignee, Institute of Health Sciences, Ordu University, Ordu, Turkey.
Reprint requests and correspondence to: Kivanç Akça, DDS, PhD, Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, Faculty of Dentistry, Ankara 06100, Turkey, Phone: +90-312-3054074, Fax: +90-312-3113741, E-mail: [email protected] ISSN 1056-6163/13/02206-623 Implant Dentistry Volume 22 Number 6 Copyright © 2013 by Lippincott Williams & Wilkins DOI: 10.1097/01.id.0000433589.33926.79
Purpose: To evaluate the load distribution of CAD/CAM monoceramic crowns supported with single-tooth implants in functional area. Materials and Methods: A 3dimensional numerical model of a soft tissue–level implant was constructed with cement-retained abutment to support glass ceramic machinable crown. Implant-abutment complex and the retained crown were embedded in a Ø 1.5 3 1.5 cm geometric matrix for evaluation of mechanical behavior of monoceramic CAD/CAM aluminosilicate and leucite glass crown materials. Laterally positioned axial load of 300 N was applied on the crowns. Resulting principal stresses in the mono-ceramic crowns were evaluated in relation to different glass ceramic materials.
Results: The highest compressive stresses were observed at the cervical region of the buccal aspect of the crowns and were 89.98 and 89.99 MPa, for aluminosilicate and leucite glass ceramics, respectively. The highest tensile stresses were observed at the collar of the lingual part of the crowns and were 24.54 and 25.39 MPa, respectively. Conclusion: Stresses induced upon 300 N static loading of CAD/ CAM aluminosalicate and leucite glass ceramics are below the compressive strength of the materials. Impact loads may actuate the progress to end failure of mono-ceramic crowns supported by metallic implant abutments. (Implant Dent 2013;22:623–626) Key Words: CAD/CAM, singletooth implant, glass ceramics, finite element analyses
for in-office CAD/CAM single-tooth implant applications to lessen the number of visits is only limited to case reports in comparison with natural tooth restorations.9,10 Additionally, the preclinical information with regard to the mechanical behavior of glass matrix ceramic crown on implant abutments is insufficient.11 Wolf et al12 reported increased fracture strength of aluminasilicate glass-ceramic crowns adhesively cemented either to zirconium or titanium abutments under static loading. Lately,
Çavus¸oglu et al13 presented early fatigue failure both for aluminosilicate and leucite glass ceramic single-tooth implant crowns on titanium abutments. Although the mechanical properties under dynamic loading conditions for both monoblock ceramic materials were found to be insufficient for clinical use, additional information regarding the characteristics of load distribution within the glass matrix ceramic materials used with metallic implant abutments is needed. The purpose of this study was, therefore, to
624
GLASS CERAMICS
FOR
SINGLE-TOOTH IMPLANT CROWNS
Fig. 1. CAD model of the ceramic crown on the implant.
Fig. 2. Conversion of the CAD model to numeric model for FE analysis. Arrow indicates 1.5 mm laterally positioned axial loading with respect to implant long axis to simulate similar loading conditions at lu et al.13 Çavus¸og
analyze the mechanical stresses in machinable mono-ceramic single-tooth implant crowns using finite element (FE) analysis.
MATERIALS
AND
METHODS
The CAD model of a tissue-level (neck Ø 4.8 mm) cylindrical screw dental implant (Ø 4.1 3 10 mm) (043.052S; Institut Straumann AG, Basel, Switzerland) with a cementable abutment of 5.5 mm in height (048.541; Institut Straumann AG) was constructed in 1 piece using I-DEAS Artisan Series 3.0 (Structural Dynamics Research Corporation,
AKÇA
ET AL
Fig. 3. Similar load distribution of compressive stresses around (A) aluminosilicate glass and (B) leucite glass ceramic crowns from lateral view. Higher stresses concentrated at the applied load side at cervical region of crown for both the glass ceramic materials.
Fig. 4. Compressive stress values viewed from the buccal aspect of (A) aluminosilicate glass and (B) leucite glass ceramic crowns. Highest values were recorded at the crown margin in contact with implant shoulder.
Milford, OH). To simplify the modeling, the threads of the implant and the abutment were not represented in their spiral characteristics but as symmetrical rings.14 The implant-abutment complex was transferred to a preprocessor (MSC. MarcMetat 2000; MSC. Software Corporation, Los Angeles, CA) for FE model conversion and was embedded vertically in the center of a Ø 1.5 3 1.5-cm cylinder representing 1-mm cortical bone in the upper surface and the rest trabecular bone15 (Fig. 1). A mandibular premolar all-ceramic crown in anatomical dimensions was placed over the abutment with 1, 2, and 3 mm material thickness at cervical, middle, and occlusal thirds of abutment, respectively. The buccal contour of the crown in occlusal third was enlarged,
and the buccal cusp was flattened for laterally positioned axial loading (Fig. 2). The 3-dimensional FE model was constructed by using 32,137 elements resulting in 8423 elements for the implant-abutment complex, 10,096 elements for the crown, and 13,798 elements for the cortical-trabecular bone. Boundary conditions were established by constraining the bone cylinder from its bottom and lateral surfaces.15 A static load of 300 N was applied vertically on the top of the buccal cusp surface to simulate laterally positioned axial loading.8 The loading case was processed separately for 2 different glass ceramic materials (aluminosilicate and leucite glass) using an FE analysis solver (Abaqus 6.5.1; Simulia, Providence, RI). All the materials were assumed to be isotropic,
IMPLANT DENTISTRY / VOLUME 22, NUMBER 6 2013
Fig. 5. Load distribution behavior of tensile stresses from lateral view were similar at the opposite applied load side in comparison to compressive stresses around (A) aluminosilicate glass and (B) leucite glass ceramic crowns.
Fig. 6. Highest tensile stress values but lower in comparison to compressive stresses were recorded at margin both for (A) aluminosilicate glass and (B) leucite glass ceramic crowns.
homogeneous, and linearly elastic, and the Young module and Poisson ratio of aluminosilicate and leucite glass ceramics were 68 GPa and 0.316 and 70 GPa and 0.3 (manufacturer’s data), respectively. Principal stresses were recorded and visualized comparatively for both the glass ceramic materials using a postprocessor (Abaqus/Viewer; Simulia).
RESULTS In both ceramic materials, the magnitude and distribution of minimum principal stresses (compressive stresses) were comparable (Fig. 3). The highest stresses were observed predominantly at the cervical region of the buccal aspect of the crowns, where mating between the crown and the abutment shoulder is
achieved. The highest compressive stresses obtained for aluminosilicate and leucite glass ceramics were 89.98 and 89.99 MPa, respectively (Fig. 4). Likewise, the magnitude and distribution of maximum principal stresses (tensile stresses) were comparable in both ceramic materials (Fig. 5). The highest tensile stresses were observed at the cervical region of the lingual aspect of the crowns. The highest tensile stresses obtained for aluminosilicate and leucite glass ceramics were 24.54 and 25.39 MPa, respectively (Fig. 6).
DISCUSSION The rationale behind this study was to study the mechanical effects of static loading on all-ceramic CAD/CAM
625
aluminosilicate and leucite glass ceramic crowns on implant abutments by numeric analysis, which had unfavorable fate on dynamic loading in a previous in vitro study.13 The FE model and the crowns were the same as in the in vitro model, and the only difference was that 300 N force was applied in a static manner in this study. The in vitro study revealed visible cracks and fractures located in the buccal aspect of the crowns arising from the margin, and this was confirmed by high minimum principal stresses located at the collar of the crowns. Indeed, high compressive stresses at the crown-implant junction was the main cause for the failure. In addition, the amplitude of compressive stresses in both types of ceramic crowns were very close (;90 MPa), suggesting that the mechanical fate of both crowns would be comparable, as observed in the in vitro study.13 However, this is not the case. The flexural strength of a material is approximately 12% to 20% of its compressive strength. The flexural strengths of aluminosilicate and leucite glass ceramics are 103 and 127 MPa, respectively,17 and therefore, the compressive strengths of the materials are between 515 to 858 MPa and 635 to 1058 MPa, respectively. As the outcome of static loading (;90 MPa) is lower than the compressive strength of the materials, one could presume that static loading would not lead to the failure of these crowns. Considering the percentage decrease between start and end of failure values of monolithic aluminosilicate and leucite glass ceramics,18 the crack initiation of these glass ceramics are between 335 to 531 MPa and 431 to 719 MPa. The outcome of 300-N static loading in FE analysis is still below for possible start of failure. The present results suggest that implant-supported monoblock CAD/ CAM aluminosilicate and leucite glass ceramic crowns may withstand impact forces of 300 N. However, this interpretation does not necessarily imply that the structural integrity of the glass ceramic crowns remains intact. On the contrary, stresses at the collar region of the crowns may be responsible to crack propagation that would result in fracture of these crowns in time as indicated
626
GLASS CERAMICS
FOR
SINGLE-TOOTH IMPLANT CROWNS
(Figs. 3 and 4).19–21 Additionally, lack in adhesive integration of cement layer to metal abutment that impairs adhesive bonding needs additional considerations in mechanical support of glass ceramic implant crowns.22
CONCLUSION The following could be drawn for clinical application of machinable glass ceramics for single-tooth implant crowns: Functional occlusal forces can be tolerated at static contact. Impact forces may initiate or propagate crack formation. Glass ceramics modified in structure should be considered to optimize mechanical failures.
DISCLOSURE The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ACKNOWLEDGMENT This study was partly supported by _ TÜBITAK (The Scientific and Technological Research Council of Turkey; Project #109S295).
REFERENCES 1. Magne P, Perroud R, Hodges JS, et al. Clinical performance of novel-design porcelain veneers for the recovery of coronal volume and length. Int J Periodontics Restorative Dent. 2000;20:440–457.
AKÇA
ET AL
2. Cehreli MC, Kokat AM, Ozpay C, et al. A randomized controlled clinical trial of feldspathic versus glass-infiltrated alumina all-ceramic crowns: A 3-year followup. Int J Prosthodont. 2011;24:77–84. 3. Johnston WM, O’Brien WJ. The shear strength of dental porcelain. J Dent Res. 1980;59:1409–1411. 4. Qin F, Zheng S, Luo Z, et al. Evaluation of machinability and flexural strength of a novel dental machinable glass-ceramic. J Dent. 2009;37:776–780. 5. Behr M, Rosentritt M, Mangelkramer M, et al. The influence of different cements on the fracture resistance and marginal adaptation of all-ceramic and fiber-reinforced crowns. Int J Prosthodont. 2003;16:538–542. 6. van Dijken JW, Hasselrot L. A prospective 15-year evaluation of extensive dentin-enamel-bonded pressed ceramic coverages. Dent Mater. 2010;26:929–939. 7. Drago C, Saldarriaga RL, Domagala D, et al. Volumetric determination of the amount of misfit in CAD/CAM and cast implant frameworks: A multicenter laboratory study. Int J Oral Maxillofac Implants. 2010;25:920–929. 8. Pozzi A, Sannino G, Barlattani A. Minimally invasive treatment of the atrophic posterior maxilla: A proof-of-concept prospective study with a follow-up of between 36 and 54 months. J Prosthet Dent. 2012; 108:286–297. 9. Fritzsche G. Treatment of a singletooth gap with a Cerec 3D crown on an implant: A case report. Int J Comput Dent. 2004;7:199–206. 10. Gougaloff R, Stalley FC. Immediate placement and provisionalization of a dental implant utilizing the CEREC 3 CAD/CAM Protocol: A clinical case report. J Calif Dent Assoc. 2010;38:170–173, 176–177. 11. Hosseini M, Kleven E, Gotfredsen K. Fracture mode during cyclic loading of implant-supported single-tooth restorations. J Prosthet Dent. 2012;108:74–83. 12. Wolf D, Bindl A, Schmidlin PR, et al. Strength of CAD/CAM-generated
esthetic ceramic molar implant crowns. Int J Oral Maxillofac Implants. 2008;23: 609–617. lu Y, Sahin E, Gürbüz R, 13. Çavus¸og et al. Fatigue resistance of 2 different CAD/CAM glass-ceramic materials used for single-tooth implant crowns. Implant Dent. 2011;20:374–378. 14. Merz BR, Hunenbart S, Belser UC. Mechanics of the implant-abutment connection: An 8-degree taper compared to a butt joint connection. Int J Oral Maxillofac Implants. 2000;15:519–526. 15. Akca K, Cehreli MC. Biomechanical consequences of progressive marginal bone loss around oral implants: A finite element stress analysis. Med Biol Eng Comput. 2006;44:527–535. 16. Lawn BR, Deng Y, Lloyd IK, et al. Materials design of ceramic-based layer structures for crowns. J Dent Res. 2002; 81:433–438. 17. Bindl A, Lüthy H, Mörmann WH. Fracture load of CAD/CAM-generated slot-inlay FPDs. Int J Prosthodont. 2003; 16:653–660. 18. Bindl A, Lüthy H, Mörmann WH. Strength and fracture pattern of monolithic CAD/CAM-generated posterior crowns. Dent Mater. 2006;22:29–36. 19. Rekow D, Thompson VP. Engineering long term clinical success of advanced ceramic prostheses. J Mater Sci Mater Med. 2007;18:47–56. 20. Rekow D, Thompson VP Nearsurface damagedA persistent problem in crowns obtained by computer-aided design and manufacturing. Proc Inst Mech Eng H. 2005;219:233–243. 21. Jung YG, Peterson IM, Kim DK, et al. Lifetime-limiting strength degradation from contact fatigue in dental ceramics. J Dent Res. 2000;79:722–731. 22. Malament KA, Socransky SS. Survival of Dicor glass-ceramic dental restorations over 16 years. Part III: Effect of luting agent and tooth or tooth-substitute core structure. J Prosthet Dent. 2001;86: 511–519.
