Volume 101 Issue 2
Shirakura et al
2. Verzijden CW, Feilzer AJ, Creugers NH, Da-vidson CL. The influence of polymerization shrinkage of resin cements on bonding to metal. J Dent Res 1992;71:410-3. 3. Caughman WF, O’Connor RP, Williams
HA, Rueggeberg FA. Retention strengths of three cements using full crown prepara-tions restored with amalgam. Am J Dent 1992;5:61-3.
4. Rubo JH, Pegoraro LF. Tensile bond strength of a composite resin cement for bonded prosthesis to various dental alloys. J Prosthet Dent 1995;74:230-4.
5. Sen D, Nayir E, Pamuk S. Comparison of the tensile bond strength of high-noble, noble, and base metal alloys bonded to enamel. J Prosthet Dent 2000;84:561-6. 6. Gurbuz A, Inan O, Kaplan R, Ozturk AN.
Effect of airborne-particle abrasion on retentive strength in overtapered fixed prosthodontic restorations. Quintessence Int 2008;39:e134-8.
7. Matsumura H, Taira Y, Atsuta M. Adhesive bonding of noble metal alloys with triazine dithiol derivative primer and an adhesive resin. J Oral Rehabil 1999;26:877-82. 8. Parsa RZ, Goldstein GR, Barrack GM,
LeGeros RZ. An in vitro comparison of tensile bond strengths of noble and base metal alloys to enamel. J Prosthet Dent 2003;90:175-83.
9. Laufer BZ, Nicholls JI, Towsend JD. SiOx-C coating: a composite-to-metal bonding mechanism. J Prosthet Dent 1988;60:320-7.
10.Tanaka T, Atsuta M, Nakabayashi N, Masu-hara E. Surface treatment of gold alloys for adhesion. J Prosthet Dent 1988;60:271-9. 11.Tanaka T, Hirano M, Kawahara M,
Mat-sumura H, Atsuta M. A new ion-coating surface treatment of alloys for dental adhe-sive resins. J Dent Res 1988;67:1376-80.
12.Watanabe F, Powers JM, Lorey RE. In vitro bonding of prosthodontic adhesives to dental alloys. J Dent Res 1988;67:479-83. 13.Antoniadou M, Kern M, Strub JR. Effect of
a new metal primer on the bond strength between a resin cement and two high-noble alloys. J Prosthet Dent 2000;84:554-60. 14.Yoshida K, Taira Y, Matsumura H, Atsuta
M. Effect of adhesive metal primers on bonding a prosthetic composite resin to metals. J Prosthet Dent 1993;69:357-62. 15.Yoshida K, Kamada K, Atsuta M. Adhesive
primers for bonding cobalt-chromium alloy to resin. J Oral Rehabil 1999;26:475-8. 16.Imbery TA, Burgess JO, Naylor WP. Tensile
strength of three resin cements following two alloy surface treatments. Int J Prost-hodont 1992;5:59-67.
17.Zidan O, Ferguson GC. The retention of complete crowns prepared with three dif-ferent tapers and luted with four difdif-ferent cements. J Prosthet Dent 2003;89:565-71. 18.Wiskott HW, Nicholls JI, Belser UC. The
relationship between abutment taper and resistance of cemented crowns to dynamic loading. Int J Prosthodont 1996;9:117-39. 19.Browning WD, Nelson SK, Cibirka R,
My-ers ML. Comparison of luting cements for minimally retentive crown preparations. Quintessence Int 2002;33:95-100. 20.Ergin S, Gemalmaz D. Retentive
proper-ties of five different luting cements on base and noble metal copings. J Prosthet Dent 2002;88:491-7.
21.Wilson AD. Resin-modified glass-ionomer cements. Int J Prosthodont 1990;3:425-9. 22.Piwowarczyk A, Lauer HC, Sorensen JA.
The shear bond strength between luting cements and zirconia ceramics after two pre-treatments. Oper Dent 2005;30:382-8. 23.Piwowarczyk A, Lauer HC, Sorensen JA.
In vitro shear bond strength of cementing agents to fixed prosthodontic restorative materials. J Prosthet Dent 2004;92:265-73.
24.Abreu A, Loza MA, Elias A, Mukhopadhyay S, Rueggeberg FA. Effect of metal type and surface treatment on in vitro tensile strength of copings cemented to minimally retentive preparations. J Prosthet Dent 2007;98:199-207.
25.International Organization for Standardiza-tion. ISO 4049:2000. Dentistry -- polymer-based filling, restorative and luting materi-als. Available at: http://www.iso.org/iso/ store.htm
26.Conover WJ, Iman RL. Rank transforma-tions as a bridge between parametric and nonparametric statistics. Am Stat 1981;35:124-9.
27.Piegorsch WW, Richwine KA. Large-sample pairwise comparisons among multinomial proportions with an application to analysis of mutant spectra. JABES 2001;6:305-5. Corresponding author:
Dr Amara Abreu
Medical College of Georgia School of Dentistry
Department of Oral Rehabilitation AD-3239A 1120 15th St
Augusta, GA 30912 Fax: 706-721-8349 E-mail: aabreu@mcg.edu Acknowledgements
The authors thank 3M ESPE Puerto Rico, Henry Schein, and Brasseler USA for materials supplied for this research. Also, thanks to Mr Don Mettenburg and Dr Cesar Arrais for their talent and dedication in the research testing phase of this study, and The Medical College of Georgia for providing testing facilities and research support.
Copyright © 2009 by the Editorial Council for The Journal of Prosthetic Dentistry.
Statement of problem. In some clinical situations, the length of either a prepared tooth or an implant abutment is shorter than ideal, and the thickness of a porcelain crown must be increased. Thickness of the coping and the veneer-ing porcelain should be considered to prevent mechanical failure of the crown.
Purpose. The purpose of this study was to investigate the influence of veneering porcelain thickness for all-ceramic and metal ceramic crowns on failure resistance after cyclic loading.
Material and methods. All-ceramic and metal ceramic crowns (n=20) were fabricated on an implant abutment (RN Solid Abutment) for the study. Two different framework designs with 2 different incisal thicknesses of veneering por-celain (2 mm and 4 mm) were used for each all-ceramic and metal ceramic crown system, resulting in 4 experimental groups (n=10) with identically shaped crowns. The all-ceramic crown consisted of alumina (Procera AllCeram) works and veneering porcelain (Cerabien), while metal ceramic crowns were made of high noble metal (Leo) frame-works and veneering porcelain (IPS Classic). All crowns were cemented on the corresponding abutments using a resin cement (Panavia 21). They were subjected to 1000 cycles of thermal cycling (5°C and 55°C; 5-second dwell time). The crowns were tested with a custom-designed cyclic loading apparatus which delivered simultaneous unidirectional cyclic loading at 135 degrees, vertically, at an rpm of 250, with a load of 49 N. Each specimen was loaded for 1.2 x 106 cycles or until it failed. The specimens were thoroughly evaluated for cracks and/or bulk fracture with an opti-cal stereomicroscope (x10) and assigned a score of success, survival, or failure. The specimens without bulk fracture after cyclic loading were loaded along the long axis of the tooth, on the incisal edge, in a universal testing machine at a crosshead speed of 1.5 mm/min, until fracture. Fisher’s exact test was used to compare the success and survival rate between the 2 different materials (α=.05). Two-way ANOVA was used to analyze data in terms of material, porcelain thickness, and interaction effect. Also, a 2-sample t test was performed to compare between 2 thicknesses within the same material (α=.05).
Results. According to the Fisher’s exact test, the all-ceramic group showed significantly higher success (P=.003) and survival rates (P=.001) than the metal ceramic group. For the failure load, the 2-way ANOVA showed significant ef-fects for material (P<.001) and porcelain thickness (P=.004), but not a significant interaction effect (P=.198). For the metal ceramic groups, crowns with a 2-mm porcelain thickness showed a significantly greater failure load than crowns with a 4-mm porcelain thickness (P=.004). However, all-ceramic groups did not show a significant difference between the 2 different thicknesses of veneering porcelain (P=.198).
The influence of veneering porcelain
thickness of all-ceramic and metal
ceramic crowns on failure resistance
after cyclic loading
Akihiko Shirakura, RDT, DDS,
aHeeje Lee, DDS,
bAlessandro
Geminiani, DDS,
cCarlo Ercoli, DDS,
dand Changyong Feng, PhD
eUniversity of Rochester Eastman Dental Center, Rochester, NY;
Louisiana State University School of Dentistry, New Orleans, La;
School of Medicine and Dentistry, University of Rochester,
Rochester, NY
This study was partially supported by a Greater New York Academy of Prosthodontics Student Grant.
aPrivate practice, Armonk, NY; former postgraduate student, Division of Prosthodontics, University of Rochester Eastman Dental
Center.
bAssistant Professor, Department of Prosthodontics, Louisiana State University School of Dentistry; former postgraduate student,
Division of Prosthodontics, University of Rochester Eastman Dental Center.
cPostgraduate student, Advanced Education in General Dentistry Program, University of Rochester Eastman Dental Center. dAssociate Professor, Chair and Program Director, Division of Prosthodontics, University of Rochester Eastman Dental Center. eAssistant Professor, Department of Biostatistics and Computational Biology, School of Medicine and Dentistry, University of
Rochester.
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Volume 101 Issue 2
February 2009
Shirakura et al
Conclusions. The all-ceramic crowns showed significantly higher success and survival rates after cyclic loading, butlower failure loads than metal ceramic crowns. The thickness of the veneering porcelain affected the failure load of the metal ceramic crowns, but not that of the all-ceramic crowns. (J Prosthet Dent 2009;101:119-127)
Clinical Implications
Procera AllCeram crowns may allow up to approximately 4
mm of feldspathic porcelain on the incisal area without
increasing the failure rate or decreasing the failure load.
There are various restorative ma-terials commercially available for the replacement of single and multiple teeth. While metal ceramic systems have had a longer track record,1,2
various types of all-ceramic crown systems are also available.3,4 Several
methods and products, including con-ventional powder-slurry techniques, and castable, machinable, pressable, and infiltrated ceramics, are used to fabricate all-ceramic crowns.5 These
systems can be broadly divided into the following categories with respect to the presence of a ceramic core: (1) core systems which use a ceramic core, characterized by high fracture toughness,6,7 veneered with
feldspath-ic porcelain to simulate the esthetfeldspath-ics of a natural tooth, and (2) coreless systems which are fabricated com-pletely of a specific ceramic material. These systems achieve a toothlike pearance with the selection of an ap-propriately colored ceramic and the application of surface-coloring tech-niques. The ceramic core is produced by either slip casting or machine milling, commonly combined with a computer-aided design/computer-aided manufacturing (CAD/CAM) method.8
The simplest shape of any core or framework, both for all-ceramic and metal ceramic systems, covers all of the surfaces of the prepared abutment teeth, including the margins, with an even thickness of the material, which is then veneered with feldspathic por-celain to achieve the desired tooth shape. For metal ceramic systems, the longevity of the restoration, spe-cifically, the integrity of the veneering porcelain layer, is thought to be
de-pendent on framework design.9-12 Two
principles have been suggested to in-crease the long-term prognosis of the veneering porcelain of a metal ceramic system: (1) the porcelain is veneered with the minimum thickness compat-ible with good esthetics, and (2) the porcelain is supported by the coping so that tensile or shear fractures can be minimized.11 The assumption is
that an excessively thick layer of ve-neering porcelain may be more prone to shear and tensile force-induced fractures under occlusal loading.
A survey of crown and fixed par-tial denture failures indicated that the incidence of porcelain fractures is the second most common cause for met-al ceramic prosthesis replacement,13
while other studies showed porcelain fracture to occur only in 2.5% to 4.5% of single metal ceramic crowns14-16 and
2% of fixed partial dentures.16 Kelly17
reported that the structural problems of metal ceramic prostheses can be as low as 3% to 4% at 10 years of service. Libby et al18 showed a prevalence of
8% of porcelain failures with 5-unit fixed partial dentures (FPDs) over 14.4 years of service.
For the all-ceramic prosthesis which uses a ceramic framework, it has been reported that, similarly to metal ceramic restorations, veneer-ing porcelain fracture remains one of the primary complications affecting longevity.3,19 While a certain number
of fractures are expected as a con-sequence of fatigue after long-term service, it is assumed that an improp-erly designed core/framework which requires the application of an exces-sively thick layer of veneering porce-lain may result in a higher incidence
of failure, not only for metal ceramic systems but also for all-ceramic pros-theses.
The restoration of anterior teeth with crowns and FPDs that have a framework for porcelain support is further complicated by the require-ment, generally placed on the veneer-ing porcelain, to simulate a lifelike tooth appearance. This can be partic-ularly challenging for anterior restora-tions for which a high level of trans-lucency, generally in the incisal and middle third of the tooth, is required. While the presence of the framework in these areas is thought necessary to provide mechanical resistance to fracture, it may be detrimental to es-thetics, and specifically, to achieving translucency. In these situations, to satisfy esthetic demands, a clinician may seek a framework that does not properly extend into the incisal third to support the veneering porcelain. What remains unclear is the effect that such a framework design would have on the longevity of metal ceramic and all-ceramic anterior crowns.
Two studies, published by the same group, investigated the mean load at fracture of alumina all-ceram-ic crowns (Procera AllCeram; Nobel Biocare AB, Göteborg, Sweden) with different thicknesses of veneering por-celain, and found different results with 2 similar experimental designs.20,21 The
authors used a brass die simulating a posterior tooth, with the veneering porcelain thicknesses ranging from 0 (alumina coping only) to 1.4 mm, and the specimens were tested with a single load-to-failure application. The authors reported that increasing the thickness of the veneering
porce-Table I.
Materials used for coping and veneering porcelain1 Long coping design. 2 Short coping design. lain increased the compressive load at
fracture in one study,20 and reported
no relation between the thickness and the compressive load in the second.21
The purpose of the present study was to investigate the influence of in-cisal veneering porcelain thickness of all-ceramic and metal ceramic crowns on failure resistance after thermal cy-cling, cyclic mechanical loading, and load-to-failure testing. The null hy-pothesis was that there would be no significant differences in the failure resistance between 2 different thick-nesses of veneering porcelain for the individual crown systems. Also, it was hypothesized that there would be no significant difference in the failure re-sistance between the tested metal ce-ramic and all-cece-ramic crown systems. Moreover, this study sought to inves-tigate the effects of thermal cycling and cyclic loading on the occurrence of cracks in the veneering porcelain of the tested systems. The null
hypothe-ses were that there would be no differ-ence in the crack occurrdiffer-ence between the metal ceramic and all-ceramic sys-tems, and no difference between the 2 different thicknesses of veneering porcelain in each system.
MATERIAL AND METHODS
Two different coping designs based on the thickness of the incisal veneer-ing porcelain were used (Figs. 1 and 2; Table I). Each design was used for the 2 different systems (metal ceramic and all-ceramic), resulting in 4 experi-mental groups (n=10). The sample size was determined from a pilot study based on 2-way ANOVA (metal versus ceramic framework; long framework versus short framework), and a sam-ple size of 6 in each combination (24 total) was deemed sufficient to have 90% power to detect differences for the comparison of interest.
For the long ceramic coping group
(CL), an implant abutment (RN Solid Abutment; Institut Straumann AG, Waldenburg, Switzerland), 5.5 mm in height, was connected to an im-plant analog (RN synOcta analog; Institut Straumann AG) with 35 Ncm torque using a torque control device (ratchet and torque control device; Institut Straumann AG). A plastic coping (048.245; Institut Straumann AG) was placed on the abutment, and the length of the plastic coping was adjusted to accommodate fabri-cation of a full contour waxing. The full contour waxing possessed the di-mensions shown in Figure 1. An index of the full contour waxing was made using polydimethylsiloxane putty im-pression material (Sil-Tech; Ivoclar Vi-vadent, Amherst, NY) (index A). The index consisted of buccal and lingual halves that could be separated and re-assembled for fabrication of the wax patterns. The waxing was cut back to provide the space for the veneering
All ceramic
Metal ceramic
*Composition (wt%): Au, 45.0; Pd, 41.0; Ag, 6.0; Sn, 2.2; In, 3.4; Ga, 1.8; Ru, <1.0; Re, <1.0; Li, <1.0 Nobel Biocare AB Ivoclar Vivadent Procera AllCeram Leo* Manufacturer Name Alumina High noble metal alloy Material Brand Noritake Ivoclar Vivadent Cerabien IPS Classic Manufacturer Name Brand
Coping Veneering Porcelain
Volume 101 Issue 2
February 2009
Shirakura et al
Conclusions. The all-ceramic crowns showed significantly higher success and survival rates after cyclic loading, butlower failure loads than metal ceramic crowns. The thickness of the veneering porcelain affected the failure load of the metal ceramic crowns, but not that of the all-ceramic crowns. (J Prosthet Dent 2009;101:119-127)
Clinical Implications
Procera AllCeram crowns may allow up to approximately 4
mm of feldspathic porcelain on the incisal area without
increasing the failure rate or decreasing the failure load.
There are various restorative ma-terials commercially available for the replacement of single and multiple teeth. While metal ceramic systems have had a longer track record,1,2
various types of all-ceramic crown systems are also available.3,4 Several
methods and products, including con-ventional powder-slurry techniques, and castable, machinable, pressable, and infiltrated ceramics, are used to fabricate all-ceramic crowns.5 These
systems can be broadly divided into the following categories with respect to the presence of a ceramic core: (1) core systems which use a ceramic core, characterized by high fracture toughness,6,7 veneered with
feldspath-ic porcelain to simulate the esthetfeldspath-ics of a natural tooth, and (2) coreless systems which are fabricated com-pletely of a specific ceramic material. These systems achieve a toothlike pearance with the selection of an ap-propriately colored ceramic and the application of surface-coloring tech-niques. The ceramic core is produced by either slip casting or machine milling, commonly combined with a computer-aided design/computer-aided manufacturing (CAD/CAM) method.8
The simplest shape of any core or framework, both for all-ceramic and metal ceramic systems, covers all of the surfaces of the prepared abutment teeth, including the margins, with an even thickness of the material, which is then veneered with feldspathic por-celain to achieve the desired tooth shape. For metal ceramic systems, the longevity of the restoration, spe-cifically, the integrity of the veneering porcelain layer, is thought to be
de-pendent on framework design.9-12 Two
principles have been suggested to in-crease the long-term prognosis of the veneering porcelain of a metal ceramic system: (1) the porcelain is veneered with the minimum thickness compat-ible with good esthetics, and (2) the porcelain is supported by the coping so that tensile or shear fractures can be minimized.11 The assumption is
that an excessively thick layer of ve-neering porcelain may be more prone to shear and tensile force-induced fractures under occlusal loading.
A survey of crown and fixed par-tial denture failures indicated that the incidence of porcelain fractures is the second most common cause for met-al ceramic prosthesis replacement,13
while other studies showed porcelain fracture to occur only in 2.5% to 4.5% of single metal ceramic crowns14-16 and
2% of fixed partial dentures.16 Kelly17
reported that the structural problems of metal ceramic prostheses can be as low as 3% to 4% at 10 years of service. Libby et al18 showed a prevalence of
8% of porcelain failures with 5-unit fixed partial dentures (FPDs) over 14.4 years of service.
For the all-ceramic prosthesis which uses a ceramic framework, it has been reported that, similarly to metal ceramic restorations, veneer-ing porcelain fracture remains one of the primary complications affecting longevity.3,19 While a certain number
of fractures are expected as a con-sequence of fatigue after long-term service, it is assumed that an improp-erly designed core/framework which requires the application of an exces-sively thick layer of veneering porce-lain may result in a higher incidence
of failure, not only for metal ceramic systems but also for all-ceramic pros-theses.
The restoration of anterior teeth with crowns and FPDs that have a framework for porcelain support is further complicated by the require-ment, generally placed on the veneer-ing porcelain, to simulate a lifelike tooth appearance. This can be partic-ularly challenging for anterior restora-tions for which a high level of trans-lucency, generally in the incisal and middle third of the tooth, is required. While the presence of the framework in these areas is thought necessary to provide mechanical resistance to fracture, it may be detrimental to es-thetics, and specifically, to achieving translucency. In these situations, to satisfy esthetic demands, a clinician may seek a framework that does not properly extend into the incisal third to support the veneering porcelain. What remains unclear is the effect that such a framework design would have on the longevity of metal ceramic and all-ceramic anterior crowns.
Two studies, published by the same group, investigated the mean load at fracture of alumina all-ceram-ic crowns (Procera AllCeram; Nobel Biocare AB, Göteborg, Sweden) with different thicknesses of veneering por-celain, and found different results with 2 similar experimental designs.20,21 The
authors used a brass die simulating a posterior tooth, with the veneering porcelain thicknesses ranging from 0 (alumina coping only) to 1.4 mm, and the specimens were tested with a single load-to-failure application. The authors reported that increasing the thickness of the veneering
porce-Table I.
Materials used for coping and veneering porcelain1 Long coping design. 2 Short coping design. lain increased the compressive load at
fracture in one study,20 and reported
no relation between the thickness and the compressive load in the second.21
The purpose of the present study was to investigate the influence of in-cisal veneering porcelain thickness of all-ceramic and metal ceramic crowns on failure resistance after thermal cy-cling, cyclic mechanical loading, and load-to-failure testing. The null hy-pothesis was that there would be no significant differences in the failure resistance between 2 different thick-nesses of veneering porcelain for the individual crown systems. Also, it was hypothesized that there would be no significant difference in the failure re-sistance between the tested metal ce-ramic and all-cece-ramic crown systems. Moreover, this study sought to inves-tigate the effects of thermal cycling and cyclic loading on the occurrence of cracks in the veneering porcelain of the tested systems. The null
hypothe-ses were that there would be no differ-ence in the crack occurrdiffer-ence between the metal ceramic and all-ceramic sys-tems, and no difference between the 2 different thicknesses of veneering porcelain in each system.
MATERIAL AND METHODS
Two different coping designs based on the thickness of the incisal veneer-ing porcelain were used (Figs. 1 and 2; Table I). Each design was used for the 2 different systems (metal ceramic and all-ceramic), resulting in 4 experi-mental groups (n=10). The sample size was determined from a pilot study based on 2-way ANOVA (metal versus ceramic framework; long framework versus short framework), and a sam-ple size of 6 in each combination (24 total) was deemed sufficient to have 90% power to detect differences for the comparison of interest.
For the long ceramic coping group
(CL), an implant abutment (RN Solid Abutment; Institut Straumann AG, Waldenburg, Switzerland), 5.5 mm in height, was connected to an im-plant analog (RN synOcta analog; Institut Straumann AG) with 35 Ncm torque using a torque control device (ratchet and torque control device; Institut Straumann AG). A plastic coping (048.245; Institut Straumann AG) was placed on the abutment, and the length of the plastic coping was adjusted to accommodate fabri-cation of a full contour waxing. The full contour waxing possessed the di-mensions shown in Figure 1. An index of the full contour waxing was made using polydimethylsiloxane putty im-pression material (Sil-Tech; Ivoclar Vi-vadent, Amherst, NY) (index A). The index consisted of buccal and lingual halves that could be separated and re-assembled for fabrication of the wax patterns. The waxing was cut back to provide the space for the veneering
All ceramic
Metal ceramic
*Composition (wt%): Au, 45.0; Pd, 41.0; Ag, 6.0; Sn, 2.2; In, 3.4; Ga, 1.8; Ru, <1.0; Re, <1.0; Li, <1.0 Nobel Biocare AB Ivoclar Vivadent Procera AllCeram Leo* Manufacturer Name Alumina High noble metal alloy Material Brand Noritake Ivoclar Vivadent Cerabien IPS Classic Manufacturer Name Brand
Coping Veneering Porcelain
Volume 101 Issue 2
February 2009
Shirakura et al
porcelain following the coping design in Figure 1. Another index (index B) was made of the cut-back wax pattern using the same material and the same method as for index A. The cut-back wax pattern and the abutment were both scanned (in-and-out scanning) by a touch-probe scanner (Procera Scanner Mod 40; Nobel Biocare AB), and the electronic file was sent to the manufacturer’s processing center (Procera manufacturing facility; No-bel Biocare, Mahwah, NJ) to obtain 10 identical alumina copings (Procera AllCeram; Nobel Biocare AB) with the dimensions illustrated in Figure 1.
Veneering porcelain was applied with the aid of index A as follows: 2 layers of shade base porcelain (Cera-bien; Noritake Dental Supply Co Ltd, Aichi, Japan) were applied and fired in a porcelain furnace (Programat P100; Ivoclar Vivadent), followed by 2 layers of dentin and enamel porcelain. All of the specimens were glazed according to the manufacturer’s instructions. For the short ceramic coping group (CS), only the abutment was scanned, using the Procera system described above, to obtain 10 alumina copings with an even thickness of 0.4 mm (Fig. 2). The veneering porcelain was applied using index A, as in the CL group.
For the long metal coping group (ML), the abutment was connected to the implant analog and the plastic coping was placed. Index B was used to duplicate a wax pattern having a shape and dimensions identical to the coping, as for the CL group. Index B was replaced on the implant analog assembly, and the molten wax was poured into the space between index B and the plastic coping. Ten wax pat-terns were fabricated, invested (Cera-Fina; Whip Mix Corp, Louisville, Ky), and cast with high noble metal alloy (Leo; Ivoclar Vivadent). The veneer-ing porcelain (IPS Classic; Ivoclar Vi-vadent) was applied with the aid of index A with the same methods as for CL. For the short metal coping group (MS), 10 plastic copings, 0.5 mm thick, were adjusted to have a 0.4-mm-thick occlusal surface, and then
were invested and cast as in the ML group. Porcelain was applied by us-ing the same methods used for the CS group.
All of the crowns were cemented onto the corresponding abutments using resin cement (Panavia 21; Ku-raray Medical, Inc, Okayama, Japan) according to the manufacturer’s in-structions. No internal grinding, air-borne-particle abrading, nor etching of the ceramic coping group was per-formed before the cementation. Mix-ing and cementMix-ing procedures were performed at room temperature by a single investigator. After cementation, the crown-abutment-analog assem-blies were stored in saline solution at 37°C for 1 week. They were then subjected to 1000 cycles of thermal cycling. Each 70-second-long cycle consisted of 5 seconds of dwell time in 2 baths of 5°C and 55°C, with 2 transport times (30 seconds each) be-tween the 2 baths.
Each specimen was mechanically tested with a custom-designed cyclic loading apparatus (Fig. 3). This appa-ratus delivered simultaneous unidirec-tional cyclic loading at 135 degrees to the long axis of the tooth to simulate the force application to a maxillary in-cisor, at an average rpm of 250, with a load of 49 N.22 The load was applied
to the lingual aspect of the specimens at 2.5 mm below the incisal edge, us-ing a round stainless steel indenter with a diameter of 6 mm.23 The
fre-quency was monitored at least once each day during each testing with a
contact tachometer (Model 461891, rpm range of 0.5-19,999, accuracy 0.05%; Extech Instruments Corp, Waltham, Mass). Each specimen was kept continuously wet by applying sa-line solution with a custom-made de-livery system and was loaded for 1.2 x 106 cycles, simulating 5 years of
clini-cal service, or until it failed.24,25
The specimens were thoroughly evaluated for the presence of cracks with an optical stereomicroscope at x10 magnification (BM-1; Meiji Tech-no America, Santa Clara, Calif ). The specimen was considered as “success” if there was neither bulk fracture nor cracks. The specimen was considered as “failure” if there was bulk fracture or if the crack occurred on the facial aspect of the crown. If these compli-cations occur in a clinical situation, the crown will likely be replaced. The specimen that was not categorized as “failure” was categorized as “survival.” It included the “success” specimens, and the specimens that had cracks not involving the facial surface.
The specimens that did not show bulk fracture were further tested. They were loaded on the incisal edge along the long axis of the tooth with an 8-mm-diameter flat stainless steel piston until fracture, using a universal testing machine (MTS Alliance; MTS, Eden Prairie, Minn) at a crosshead speed of 1.5 mm/min. To decrease the possibility that a localized stress application would cause fracture of the porcelain, a 1-mm layer of tin was interposed between the crown and 3 Custom-designed cyclic loading apparatus.
the loading apparatus.
Since the sample size was rela-tively small, and for some cells, the count was less than 5, the Pearson’s chi-square test was inappropriate. Therefore, Fisher’s exact test was used to compare the success and survival rate between the 2 different systems (α=.05). A 2-way ANOVA was used to assess the significances of mate-rial, porcelain thickness, and interac-tion effect. Also a 2-sample t test was performed to compare between the 2 porcelain thicknesses within the same material.
RESULTS
Success, survival, and failure of the specimens under cyclic loading are summarized in Table II. Five spec-imens from the CL and CS groups were considered success, whereas only 1 from ML and none from MS were considered a success. Accord-ing to the Fisher’s exact test, the all-ceramic group showed significantly higher success (P=.003) and survival rates (P=.001) than those of the met-al ceramic group after cyclic loading. The CL and CS groups had 3 and 1 failures, respectively, due to the pres-ence of cracks on the facial surfaces of the specimens. None of the speci-mens from either CL or CS showed a bulk fracture after the cyclic loading. The ML group had 7 failures, which consisted of 1 oblique fracture in-cluding the incisal edge and 6 crack occurrences on the facial surfaces of the specimens. The MS group had 1 fracture of the solid abutment and 7 facial cracks, which resulted in 8 fail-ures in total, although only 7 were considered as crown failures, for the purpose of comparison. The metal ce-ramic group had 18 crack occurrenc-es in total, and all of them involved the lingual loading site, whereas the all-ceramic group demonstrated 1 crack at the loading site and 9 cracks on the cervical area (Figs. 4 and 5).
The results of the load-to-failure test are summarized in Table III. All of the specimens from the
all-ce-Table II.
Number of specimens in success, survival, and failure (n=10) categories after cyclic loading4 Cracks of metal ceramic crowns occurring at cyclic loading site.
5 Crack observed on cervical area for all-ceramic crown. CL
CS ML MS
CL - ceramic core, long; CS - ceramic core, short; ML - metal core, long; MS - metal core, short *One failure was not counted since it was caused by fracture of abutment. 5 5 1 0 7 9 3 2 Success Survival 3 1 7 7* Failure Group
ramic group demonstrated core and veneering porcelain fracture under the failure load (Fig. 6). The 2-way ANOVA was significant for the fac-tors, material (P<.001) and porcelain thickness effect (P=.004), but not for interaction effect (P=.198) (Table
IV). Within the same material group, ML showed significantly greater fail-ure loads than MS (P=.004) while, for the all-ceramic system, CL and CS were not significantly different (P=.198).
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porcelain following the coping design in Figure 1. Another index (index B) was made of the cut-back wax pattern using the same material and the same method as for index A. The cut-back wax pattern and the abutment were both scanned (in-and-out scanning) by a touch-probe scanner (Procera Scanner Mod 40; Nobel Biocare AB), and the electronic file was sent to the manufacturer’s processing center (Procera manufacturing facility; No-bel Biocare, Mahwah, NJ) to obtain 10 identical alumina copings (Procera AllCeram; Nobel Biocare AB) with the dimensions illustrated in Figure 1.
Veneering porcelain was applied with the aid of index A as follows: 2 layers of shade base porcelain (Cera-bien; Noritake Dental Supply Co Ltd, Aichi, Japan) were applied and fired in a porcelain furnace (Programat P100; Ivoclar Vivadent), followed by 2 layers of dentin and enamel porcelain. All of the specimens were glazed according to the manufacturer’s instructions. For the short ceramic coping group (CS), only the abutment was scanned, using the Procera system described above, to obtain 10 alumina copings with an even thickness of 0.4 mm (Fig. 2). The veneering porcelain was applied using index A, as in the CL group.
For the long metal coping group (ML), the abutment was connected to the implant analog and the plastic coping was placed. Index B was used to duplicate a wax pattern having a shape and dimensions identical to the coping, as for the CL group. Index B was replaced on the implant analog assembly, and the molten wax was poured into the space between index B and the plastic coping. Ten wax pat-terns were fabricated, invested (Cera-Fina; Whip Mix Corp, Louisville, Ky), and cast with high noble metal alloy (Leo; Ivoclar Vivadent). The veneer-ing porcelain (IPS Classic; Ivoclar Vi-vadent) was applied with the aid of index A with the same methods as for CL. For the short metal coping group (MS), 10 plastic copings, 0.5 mm thick, were adjusted to have a 0.4-mm-thick occlusal surface, and then
were invested and cast as in the ML group. Porcelain was applied by us-ing the same methods used for the CS group.
All of the crowns were cemented onto the corresponding abutments using resin cement (Panavia 21; Ku-raray Medical, Inc, Okayama, Japan) according to the manufacturer’s in-structions. No internal grinding, air-borne-particle abrading, nor etching of the ceramic coping group was per-formed before the cementation. Mix-ing and cementMix-ing procedures were performed at room temperature by a single investigator. After cementation, the crown-abutment-analog assem-blies were stored in saline solution at 37°C for 1 week. They were then subjected to 1000 cycles of thermal cycling. Each 70-second-long cycle consisted of 5 seconds of dwell time in 2 baths of 5°C and 55°C, with 2 transport times (30 seconds each) be-tween the 2 baths.
Each specimen was mechanically tested with a custom-designed cyclic loading apparatus (Fig. 3). This appa-ratus delivered simultaneous unidirec-tional cyclic loading at 135 degrees to the long axis of the tooth to simulate the force application to a maxillary in-cisor, at an average rpm of 250, with a load of 49 N.22 The load was applied
to the lingual aspect of the specimens at 2.5 mm below the incisal edge, us-ing a round stainless steel indenter with a diameter of 6 mm.23 The
fre-quency was monitored at least once each day during each testing with a
contact tachometer (Model 461891, rpm range of 0.5-19,999, accuracy 0.05%; Extech Instruments Corp, Waltham, Mass). Each specimen was kept continuously wet by applying sa-line solution with a custom-made de-livery system and was loaded for 1.2 x 106 cycles, simulating 5 years of
clini-cal service, or until it failed.24,25
The specimens were thoroughly evaluated for the presence of cracks with an optical stereomicroscope at x10 magnification (BM-1; Meiji Tech-no America, Santa Clara, Calif ). The specimen was considered as “success” if there was neither bulk fracture nor cracks. The specimen was considered as “failure” if there was bulk fracture or if the crack occurred on the facial aspect of the crown. If these compli-cations occur in a clinical situation, the crown will likely be replaced. The specimen that was not categorized as “failure” was categorized as “survival.” It included the “success” specimens, and the specimens that had cracks not involving the facial surface.
The specimens that did not show bulk fracture were further tested. They were loaded on the incisal edge along the long axis of the tooth with an 8-mm-diameter flat stainless steel piston until fracture, using a universal testing machine (MTS Alliance; MTS, Eden Prairie, Minn) at a crosshead speed of 1.5 mm/min. To decrease the possibility that a localized stress application would cause fracture of the porcelain, a 1-mm layer of tin was interposed between the crown and 3 Custom-designed cyclic loading apparatus.
the loading apparatus.
Since the sample size was rela-tively small, and for some cells, the count was less than 5, the Pearson’s chi-square test was inappropriate. Therefore, Fisher’s exact test was used to compare the success and survival rate between the 2 different systems (α=.05). A 2-way ANOVA was used to assess the significances of mate-rial, porcelain thickness, and interac-tion effect. Also a 2-sample t test was performed to compare between the 2 porcelain thicknesses within the same material.
RESULTS
Success, survival, and failure of the specimens under cyclic loading are summarized in Table II. Five spec-imens from the CL and CS groups were considered success, whereas only 1 from ML and none from MS were considered a success. Accord-ing to the Fisher’s exact test, the all-ceramic group showed significantly higher success (P=.003) and survival rates (P=.001) than those of the met-al ceramic group after cyclic loading. The CL and CS groups had 3 and 1 failures, respectively, due to the pres-ence of cracks on the facial surfaces of the specimens. None of the speci-mens from either CL or CS showed a bulk fracture after the cyclic loading. The ML group had 7 failures, which consisted of 1 oblique fracture in-cluding the incisal edge and 6 crack occurrences on the facial surfaces of the specimens. The MS group had 1 fracture of the solid abutment and 7 facial cracks, which resulted in 8 fail-ures in total, although only 7 were considered as crown failures, for the purpose of comparison. The metal ce-ramic group had 18 crack occurrenc-es in total, and all of them involved the lingual loading site, whereas the all-ceramic group demonstrated 1 crack at the loading site and 9 cracks on the cervical area (Figs. 4 and 5).
The results of the load-to-failure test are summarized in Table III. All of the specimens from the
all-ce-Table II.
Number of specimens in success, survival, and failure (n=10) categories after cyclic loading4 Cracks of metal ceramic crowns occurring at cyclic loading site.
5 Crack observed on cervical area for all-ceramic crown. CL
CS ML MS
CL - ceramic core, long; CS - ceramic core, short; ML - metal core, long; MS - metal core, short *One failure was not counted since it was caused by fracture of abutment. 5 5 1 0 7 9 3 2 Success Survival 3 1 7 7* Failure Group
ramic group demonstrated core and veneering porcelain fracture under the failure load (Fig. 6). The 2-way ANOVA was significant for the fac-tors, material (P<.001) and porcelain thickness effect (P=.004), but not for interaction effect (P=.198) (Table
IV). Within the same material group, ML showed significantly greater fail-ure loads than MS (P=.004) while, for the all-ceramic system, CL and CS were not significantly different (P=.198).
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Shirakura et al
Table III.
Mean and SD of failure loadTable IV.
Results of 2-way ANOVA for failure load6 Core and veneering porcelain fracture of all-ceramic crown after load-to-failure test. Arrow indicates junction between solid abutment and prosthetic platform of implant analog (junction A). Arrowhead indicates cement layer along junction A.
CL CS ML MS
CL - ceramic core, long; CS - ceramic core, short; ML - metal core, long; MS - metal core, short
*One specimen was excluded due to considerable chipping during cyclic loading.
**One specimen was excluded due to abutment fracture during cyclic loading.
10 10 9* 9** 1619.82 1339.80 3116.42 2429.62 n Mean (N) 414.97 212.82 628.65 572.90 SD Group Material Porcelain thickness
Material x porcelain thickness Error Total 1 1 1 34 37 df 15843779 2122754 391912 7744753 26103198 15843779 2122754 391912 227787
Sum of SquaresType I SquareMean
69.56 9.32 1.72 F Source of Variation <.001 .004 .198 P
A
B
DISCUSSIONFor the present study, 2 null hy-potheses were addressed for the test-ing of the ultimate failure load: (1) there would be no significant differ-ences in the failure resistance between 2 different thicknesses of veneering porcelain in the individual crown systems, and (2) there would be no significant difference in the failure resistance between 2 different crown systems. The first null hypothesis was rejected for the metal ceramic system, but it was accepted for the all-ceramic system, which showed no significantly different failure load between 2 differ-ent veneering porcelain thicknesses. The results support rejection of the second hypothesis.
For the occurrence of cracks after thermal and mechanical cyclic load-ing, 2 null hypotheses were addressed: (1) there would be no significant dif-ferences in the occurrence of cracks between 2 different thicknesses of veneering porcelain in the individual crown systems, and (2) there would be no significant difference in the oc-currence of cracks between 2 different crown systems. The first null hypothe-sis was accepted, while the significant difference in the occurrence of cracks after cyclic loading between the 2 dif-ferent crown systems indicated that the second hypothesis should be re-jected.
It is important to recognize that any in vitro study design that aims to reproduce a complex biomechanical environment, such as that of masti-cation, has certain limitations, and the results must be interpreted with caution. For example, the present unidirectional cyclic loading design reproduced only 1 (vertical) vector of forces generally found in the mastica-tory cycle, and, therefore, does not entirely simulate the complexity of the oral biomechanical environment. In addition, although the loading of ceramic restorations by a round in-denter has been frequently used in other studies23,26,27 to simulate cyclic
occlusal contact, it has also been
ar-gued that this type of loading might cause the level of stress in the ceramic to exceed what is found intraorally.28
In this regard, however, it is in-teresting to note some fundamen-tal differences in the behavior of the tested metal ceramic specimens when compared to alumina framework/ce-ramic ones. The same cyclic loading regimen produced distinctly differ-ent crack locations for the 2 groups. Ten specimens from the Procera AllCeram groups resisted the cyclic loading without any crack develop-ment (“success”), as opposed to the metal ceramic groups that had only a single specimen with a score of “suc-cess.” For the all-ceramic groups, cracks occurred on 10 specimens (6 crowns scored as “survival” and 4 as “failure”), whereas for the metal ce-ramic groups, cracks occurred on 18 specimens (4 were assigned a score of “survival” and 14 were categorized as “failure”).
The “failure” scores primarily re-sulted from the occurrence of cracks that compromised the esthetics, es-sentially extending onto the facial aspect of the crowns. This classifica-tion was arbitrarily designated by the authors based on clinical criteria. A crown would certainly be replaced if the loading conditions resulted in a bulk fracture, and it could be ar-gued that a visible fracture affecting the buccal surface would also not be deemed acceptable by most patients, therefore requiring replacement. While 14 cracks of the metal ceramic crowns involved the buccal surface, it is noteworthy to mention that all of the cracks in this group also involved the loading site (Fig. 4), whereas 9 out of 10 cracks in the all-ceramic groups appeared on the cervical area of the crowns (Fig. 5), and only 1 at the loading site.
For the metal ceramic crowns, it could be argued that the loading of porcelain specimens with a spherical indenter resulted in an excessive level of stress concentration at the loading site, initiating the crack in this area with a subsequent extension on the
buccal surface. However, the same loading conditions in the Procera All-Ceram crowns did not result in the same pattern of cracks. Only 1 crack was observed at the loading site, and 90% of the cracks involved only the cervical areas of the crowns. The au-thors speculated that this different behavior was specific to the Procera AllCeram crowns and could be due to 2 factors: (1) different mechani-cal properties of the crowns, and (2) greater thickness of the cement layer. A specific pattern of adaptation of the Procera copings was noted along the junction between the solid abutment and the prosthetic platform of the im-plant analog (junction A). When the solid abutment was screwed into the implant analog, it created an obtuse angle at the junction A (Fig. 6). Since the tip of the touch probe of the Pro-cera scanner is round, this angular joint was not completely captured by the scanner, with a resulting rounded internal angle of the coping. There-fore, the alumina coping was fabri-cated with greater cement space at the junction A area than at any other internal aspect of the crown. Figure 6 also showed the catastrophic fracture of the Procera AllCeram crown under the load-to-failure test, and the thick cement layer was observed along junc-tion A. It is possible that the greater thickness of the cement layer might have caused tipping of the all-ceramic crown during cyclic loading, possibly producing excessive hoop stresses at the cervical margin of the crowns and resulting in the occurrence of cracks. A study investigating the thickness of the cement layer of the Procera AllCe-ram crowns29 found an increased
ce-ment layer on the round slope of the chamfer margin, and it was caused by the same reason explained above.
It also should be noted that, while a higher number of crowns in the met-al ceramic group showed the presence of cracks after the cyclic loading test-ing, their mean ultimate failure load was significantly greater than that of the all-ceramic crowns. While the di-rection of loading during this test was
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Shirakura et al
Table III.
Mean and SD of failure loadTable IV.
Results of 2-way ANOVA for failure load6 Core and veneering porcelain fracture of all-ceramic crown after load-to-failure test. Arrow indicates junction between solid abutment and prosthetic platform of implant analog (junction A). Arrowhead indicates cement layer along junction A.
CL CS ML MS
CL - ceramic core, long; CS - ceramic core, short; ML - metal core, long; MS - metal core, short
*One specimen was excluded due to considerable chipping during cyclic loading.
**One specimen was excluded due to abutment fracture during cyclic loading.
10 10 9* 9** 1619.82 1339.80 3116.42 2429.62 n Mean (N) 414.97 212.82 628.65 572.90 SD Group Material Porcelain thickness
Material x porcelain thickness Error Total 1 1 1 34 37 df 15843779 2122754 391912 7744753 26103198 15843779 2122754 391912 227787
Sum of SquaresType I SquareMean
69.56 9.32 1.72 F Source of Variation <.001 .004 .198 P
A
B
DISCUSSIONFor the present study, 2 null hy-potheses were addressed for the test-ing of the ultimate failure load: (1) there would be no significant differ-ences in the failure resistance between 2 different thicknesses of veneering porcelain in the individual crown systems, and (2) there would be no significant difference in the failure resistance between 2 different crown systems. The first null hypothesis was rejected for the metal ceramic system, but it was accepted for the all-ceramic system, which showed no significantly different failure load between 2 differ-ent veneering porcelain thicknesses. The results support rejection of the second hypothesis.
For the occurrence of cracks after thermal and mechanical cyclic load-ing, 2 null hypotheses were addressed: (1) there would be no significant dif-ferences in the occurrence of cracks between 2 different thicknesses of veneering porcelain in the individual crown systems, and (2) there would be no significant difference in the oc-currence of cracks between 2 different crown systems. The first null hypothe-sis was accepted, while the significant difference in the occurrence of cracks after cyclic loading between the 2 dif-ferent crown systems indicated that the second hypothesis should be re-jected.
It is important to recognize that any in vitro study design that aims to reproduce a complex biomechanical environment, such as that of masti-cation, has certain limitations, and the results must be interpreted with caution. For example, the present unidirectional cyclic loading design reproduced only 1 (vertical) vector of forces generally found in the mastica-tory cycle, and, therefore, does not entirely simulate the complexity of the oral biomechanical environment. In addition, although the loading of ceramic restorations by a round in-denter has been frequently used in other studies23,26,27 to simulate cyclic
occlusal contact, it has also been
ar-gued that this type of loading might cause the level of stress in the ceramic to exceed what is found intraorally.28
In this regard, however, it is in-teresting to note some fundamen-tal differences in the behavior of the tested metal ceramic specimens when compared to alumina framework/ce-ramic ones. The same cyclic loading regimen produced distinctly differ-ent crack locations for the 2 groups. Ten specimens from the Procera AllCeram groups resisted the cyclic loading without any crack develop-ment (“success”), as opposed to the metal ceramic groups that had only a single specimen with a score of “suc-cess.” For the all-ceramic groups, cracks occurred on 10 specimens (6 crowns scored as “survival” and 4 as “failure”), whereas for the metal ce-ramic groups, cracks occurred on 18 specimens (4 were assigned a score of “survival” and 14 were categorized as “failure”).
The “failure” scores primarily re-sulted from the occurrence of cracks that compromised the esthetics, es-sentially extending onto the facial aspect of the crowns. This classifica-tion was arbitrarily designated by the authors based on clinical criteria. A crown would certainly be replaced if the loading conditions resulted in a bulk fracture, and it could be ar-gued that a visible fracture affecting the buccal surface would also not be deemed acceptable by most patients, therefore requiring replacement. While 14 cracks of the metal ceramic crowns involved the buccal surface, it is noteworthy to mention that all of the cracks in this group also involved the loading site (Fig. 4), whereas 9 out of 10 cracks in the all-ceramic groups appeared on the cervical area of the crowns (Fig. 5), and only 1 at the loading site.
For the metal ceramic crowns, it could be argued that the loading of porcelain specimens with a spherical indenter resulted in an excessive level of stress concentration at the loading site, initiating the crack in this area with a subsequent extension on the
buccal surface. However, the same loading conditions in the Procera All-Ceram crowns did not result in the same pattern of cracks. Only 1 crack was observed at the loading site, and 90% of the cracks involved only the cervical areas of the crowns. The au-thors speculated that this different behavior was specific to the Procera AllCeram crowns and could be due to 2 factors: (1) different mechani-cal properties of the crowns, and (2) greater thickness of the cement layer. A specific pattern of adaptation of the Procera copings was noted along the junction between the solid abutment and the prosthetic platform of the im-plant analog (junction A). When the solid abutment was screwed into the implant analog, it created an obtuse angle at the junction A (Fig. 6). Since the tip of the touch probe of the Pro-cera scanner is round, this angular joint was not completely captured by the scanner, with a resulting rounded internal angle of the coping. There-fore, the alumina coping was fabri-cated with greater cement space at the junction A area than at any other internal aspect of the crown. Figure 6 also showed the catastrophic fracture of the Procera AllCeram crown under the load-to-failure test, and the thick cement layer was observed along junc-tion A. It is possible that the greater thickness of the cement layer might have caused tipping of the all-ceramic crown during cyclic loading, possibly producing excessive hoop stresses at the cervical margin of the crowns and resulting in the occurrence of cracks. A study investigating the thickness of the cement layer of the Procera AllCe-ram crowns29 found an increased
ce-ment layer on the round slope of the chamfer margin, and it was caused by the same reason explained above.
It also should be noted that, while a higher number of crowns in the met-al ceramic group showed the presence of cracks after the cyclic loading test-ing, their mean ultimate failure load was significantly greater than that of the all-ceramic crowns. While the di-rection of loading during this test was
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Shirakura et al
along the axis of the tooth, it could be speculated that the presence of cracks in metal ceramic crowns, at least as produced by the experimental testing conditions used in this study, is of sec-ondary importance in determining the ultimate resistance to fracture, and that the intrinsic characteristics of the materials, as compared to the all-ceramic crowns, have a dominant ef-fect on the occurrence of failure under single load-to-failure testing. It could be speculated that, since most of the metal-ceramic specimens that were loaded to failure had already shown the occurrence of cracks during cyclic testing and, therefore, stresses high enough to produce tensile failure had already occurred, the loading condi-tions in the load-to-failure test may have actually stressed the crowns that were deemed “success” or “survival” in different manners. Although this is experimentally possible, it is none-theless surprising to see that the raw values of the load-to-failure test did not show a separation or clear iden-tification between the specimens that were initially classified as success from those classified as survival.
In addition, the thickness of the incisal porcelain was inversely related to the failure load in metal ceramic crowns, while it did not significant-ly affect that of Procera AllCeram crowns. This leads one to believe, sim-ilar to the ideas mentioned previously with respect to the cyclic loading test-ing, that this type of crown may have somewhat different behavior under loading. From a clinical perspective, the fact that a 4-mm incisal extension of the veneering feldspathic porcelain is not more prone to develop cracks under cyclic and single-load-to-failure testing in Procera AllCeram crowns may indicate that, if higher incisal translucency is required, it might be achieved by shortening the core thick-ness and adding more incisal feld-spathic porcelain.
The present study investigated only limited combinations of materi-als for the core and veneering porce-lain, and the results cannot be
gener-alized to other systems. Moreover, the results might only apply to situations in which the crowns were cemented on implant abutments with resin ce-ment, which provides a dry environ-ment at the crown-abutenviron-ment inter-face. The results might be different if the crowns were cemented on sound dentin, which might keep the inter-face wet with tubular fluid.
Future studies should concentrate on testing whether the current results are applicable to other metal ceramic and all-ceramic systems. If the results are confirmed with other metal and ceramic frameworks, it would then be interesting to assess, with appro-priately designed in vitro studies, the reasons for this observed difference in behavior.Moreover, it was speculated by the authors that the geometric characteristic and the resultant great-er cement thickness of the junction between the abutment and the pros-thetic platform of the implant analog could have caused the cervical cracks observed in these crowns. These fac-tors were not controlled for in the cur-rent study and should be included in future studies.
CONCLUSIONS
Within the limitations of this study, the following conclusions were drawn:
1. All-ceramic crowns tested showed significantly higher success and survival rates after the cyclic loading test than did metal ceramic crowns.
2. Metal ceramic crowns showed significantly greater failure loads than the all-ceramic crowns following cy-clic loading.
3. The thickness of the incisal ve-neering porcelain affected the failure load of the metal ceramic crowns, but not that of the all-ceramic crowns. The metal ceramic crowns with 2-mm porcelain demonstrated significantly higher failure loads than crowns with 4-mm porcelain.
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Corresponding author: Dr Heeje Lee
Department of Prosthodontics LSU School of Dentistry 1100 Florida Ave New Orleans, LA 70119 Fax: 504-941-8284 E-mail: hlee4@lsuhsc.edu
Copyright © 2009 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature
Bone metabolic activity around dental implants under loading observed using bone
scintigraphy
Sasaki H, Koyama S, Yokoyama M, Yamaguchi K, Itoh M, Sasaki K.
Int J Oral Maxillofac Implants 2008;23:827-34.
Purpose: The purpose of this study was to determine dynamic changes in bone metabolism around osseointegrated titanium implants under mechanical stress.
Materials and Methods: Two titanium implants were inserted parallel to each other in the tibiae of rats and perpen-dicular to the bone surface with the superior aspect of the implant exposed. Eight weeks after insertion, closed coil springs with 0.5, 1.0, 2.0, and 4.0 N were applied to the exposed superior portion of the implant for 7 weeks to apply a continuous mechanical stress. Bone scintigrams were performed using a gamma camera with a modified high-res-olution pinhole collimator. Images were made at 1, 4, 7, 10, 14, 21, 28, 49, and 56 days after insertion and at 3 days and at weekly intervals until 7 weeks after load application. The ratio of the metabolic activity around the implants to that around a reference site (uptake ratio) was established. The Friedman, Steel, and Tukey tests (P < .05) were used to assess statistical significance.
Results: In the process of osseointegration, the uptake ratio increased during the first week after implant insertion and then gradually decreased. During the initial 3 weeks the uptake ratio was significantly higher than at 1 day after insertion. In the process of load application, the uptake ratio increased with 2.0- and 4.0-N loads; it was significantly higher until 6 weeks than it had been before load application.
Conclusion: Bone metabolism around the implants increases with loading and depends on the magnitude and period of the loading.
Volume 101 Issue 2
February 2009
Shirakura et al
along the axis of the tooth, it could be speculated that the presence of cracks in metal ceramic crowns, at least as produced by the experimental testing conditions used in this study, is of sec-ondary importance in determining the ultimate resistance to fracture, and that the intrinsic characteristics of the materials, as compared to the all-ceramic crowns, have a dominant ef-fect on the occurrence of failure under single load-to-failure testing. It could be speculated that, since most of the metal-ceramic specimens that were loaded to failure had already shown the occurrence of cracks during cyclic testing and, therefore, stresses high enough to produce tensile failure had already occurred, the loading condi-tions in the load-to-failure test may have actually stressed the crowns that were deemed “success” or “survival” in different manners. Although this is experimentally possible, it is none-theless surprising to see that the raw values of the load-to-failure test did not show a separation or clear iden-tification between the specimens that were initially classified as success from those classified as survival.
In addition, the thickness of the incisal porcelain was inversely related to the failure load in metal ceramic crowns, while it did not significant-ly affect that of Procera AllCeram crowns. This leads one to believe, sim-ilar to the ideas mentioned previously with respect to the cyclic loading test-ing, that this type of crown may have somewhat different behavior under loading. From a clinical perspective, the fact that a 4-mm incisal extension of the veneering feldspathic porcelain is not more prone to develop cracks under cyclic and single-load-to-failure testing in Procera AllCeram crowns may indicate that, if higher incisal translucency is required, it might be achieved by shortening the core thick-ness and adding more incisal feld-spathic porcelain.
The present study investigated only limited combinations of materi-als for the core and veneering porce-lain, and the results cannot be
gener-alized to other systems. Moreover, the results might only apply to situations in which the crowns were cemented on implant abutments with resin ce-ment, which provides a dry environ-ment at the crown-abutenviron-ment inter-face. The results might be different if the crowns were cemented on sound dentin, which might keep the inter-face wet with tubular fluid.
Future studies should concentrate on testing whether the current results are applicable to other metal ceramic and all-ceramic systems. If the results are confirmed with other metal and ceramic frameworks, it would then be interesting to assess, with appro-priately designed in vitro studies, the reasons for this observed difference in behavior.Moreover, it was speculated by the authors that the geometric characteristic and the resultant great-er cement thickness of the junction between the abutment and the pros-thetic platform of the implant analog could have caused the cervical cracks observed in these crowns. These fac-tors were not controlled for in the cur-rent study and should be included in future studies.
CONCLUSIONS
Within the limitations of this study, the following conclusions were drawn:
1. All-ceramic crowns tested showed significantly higher success and survival rates after the cyclic loading test than did metal ceramic crowns.
2. Metal ceramic crowns showed significantly greater failure loads than the all-ceramic crowns following cy-clic loading.
3. The thickness of the incisal ve-neering porcelain affected the failure load of the metal ceramic crowns, but not that of the all-ceramic crowns. The metal ceramic crowns with 2-mm porcelain demonstrated significantly higher failure loads than crowns with 4-mm porcelain.
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