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The effect of endodontic access preparation on the failure load of lithium disilicate glass-ceramic restorations
The effect of endodontic access preparation on the failure load of lithium disilicate glass-ceramic restorations Dana Qeblawi, DDS, MS,a Thomas Hill, PhD,b and Kelly Chlostac State University of New York at Buffalo, School of Dental Medicine, Buffalo, NY; Ivoclar Vivadent Inc, Amherst, NY Statement of problem. Endodontic access preparation through lithium disilicate ceramic restorations may damage the restoration and compromise its load-bearing capability. Purpose. The purpose of this in vitro research was to investigate the effect of simulated endodontic access preparation through lithium disilicate glass-ceramic restorations on their load to failure. Material and methods. Sixty lithium disilicate glass-ceramic (IPS e.max CAD) complete-coverage restorations were milled and crystallized. Five coats of die relief were applied internally in the crown to provide a cement space approximately 60 µm in thickness. Composite resin dies were fabricated by backfilling each crown. The specimens were then stored at 37°C and 100% humidity for 30 days. The crowns with their respective dies were divided into 6 groups: Groups M-C, M-ZR, M-SC, and M-CRF were adhesively bonded with a resin cement (Multilink Implant), and Groups F-C and F-ZR were conventionally cemented with zinc phosphate cement (Fleck’s). After storing all groups for 1 week, Groups M-C and F-C served as the intact controls for the 2 cementation techniques, while Groups M-ZR and F-ZR had an access prepared with a 126 µm grit-size diamond rotary instrument. For Groups M-SC and M-CRF, the endodontic access was prepared with 150 µm and 180 µm grit-size diamond rotary instruments, respectively. Access preparations were restored with composite resin. All specimens were stored at 37°C and 100% humidity for 1 week before they were loaded to failure with a universal loading apparatus (crosshead speed=1mm/min). The results were analyzed with a 1-way ANOVA followed by Tukey’s HSD test (α=.05). Results. The highest failure loads were achieved with Groups M-C (3316 N ±483) and M-ZR (3464 N ±645) Larger grit rotary instruments resulted in lower failure-loads in Groups M-SC (2915 N ±569) and M-CRF (2354 N ±476). Groups F-C (2242 N ±369) and F-ZR(1998 N ±448) had significantly lower failure loads than their adhesively bonded counterparts (P<.05). The use of 126 µm grit size did not significantly alter the failure loads of the restorations in either cementation technique. Conclusions. Adhesively bonded restorations sustained significantly higher loads to failure than those conventionally cemented. The use of a high efficiency, smaller-grit diamond rotary instrument for endodontic access preparation did not alter the load to failure of lithium disilicate restorations, regardless of the cement used. The use of a larger-grit rotary instrument did not improve the cutting efficiency and reduced the failure load of bonded restorations. (J Prosthet Dent 2011;106:328-336)
Clinical Implications
Endodontic treatment through ceramic restorations is not uncommon. When the restoration will not be replaced, proper rotary instrument selection, such as a high-efficiency, 126-μm grit diamond rotary cutting instrument, may impact the longevity of the restored restoration.
Presented as a poster at the American Association of Dental Research annual meeting, San Diego, Calif, March 2010. Clinical Assistant Professor, Department of Restorative Dentistry, State University of New York at Buffalo. Manager of Scientific Services, Research and Development, Ivoclar Vivadent Inc. c Research Assistant, Research and Development, Ivoclar Vivadent Inc. a
b
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November 2011 The use of ceramic restorations has expanded in the past decade because of increased esthetic demands by patients. Lithium disilicate is a contemporary glass-ceramic system that is gaining popularity among dentists because of its high strength and enhanced optical properties. The need for an efficient grinding instrument becomes essential during crown removal or endodontic access preparation through a restoration. For crown removal purposes, the efficiency of the grinding instrument is the only concern. However, for endodontic access preparation, the potential damage caused by the grinding instrument on the restoration must be evaluated. It has been reported that 4% of teeth receiving complete coverage restorations develop pulpal necrosis.1 In ceramic crowns, the need for endodontic treatment has been reported as 1% with a range of 0% to 5%.2 Optimally, a restoration with an endodontic access preparation should be replaced when endodontic therapy is completed. However, if the restoration is not to be replaced, the effect of the access preparation on the longevity of the restoration is crucial. Earlier studies have reported varying degrees of damage to ceramic restorations after endodontic access preparation.3-6 Damage ranged from minor chipping to microcrack formation and catastrophic fractures. A novel technique was described for creating an endodontic access preparation through leucite-reinforced ceramic restorations by using airborneparticle abrasion.7 However, while this technique caused no catastrophic fractures, chipping, or microcracks, preparation takes longer to complete. It has been determined that the strength of dental ceramics is flaw dependent.8 The aqueous oral environment is detrimental to existing cracks, thus facilitating their propagation.9 According to the Griffith theory of brittle materials, microcracks are the origins of ceramic failures because they act as stress concentrators.10 It can be surmised, therefore, that pre-
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paring endodontic access through ceramic restorations may compromise their longevity. Few studies have investigated the effect of endodontic access preparation on the fracture strength of ceramic restorations.11,12 Strokes et al11 reported lower fracture strength for endodontically prepared and repaired ceramic restorations than for intact controls. Wood et al12 reported that endodontic access preparation followed by repair reduced the reliability of both zirconia and alumina restorations but resulted in a significant loss of strength only in zirconia-based restorations. The effect of endodontic access preparation on the failure load of lithium disilicate glass-ceramic restorations has not been evaluated. Adhesive resin bonding has been shown to increase the load-bearing capability of ceramic restorations.13 Bindl et al14 demonstrated that adhesive cementation increased the fracture load of lithium disilicate crowns compared to conventional cementation. This advantage of the adhesive interphase may be important for the survival of the restoration after endodontic therapy and repair. Michanowicz et al15 were the first to describe a method for endodontic access preparation through ceramic crowns with a #2 diamond rotary instrument with water coolant. Another study also recommended using a diamond rotary instrument in a high-speed handpiece with light brush strokes and copious water spray.16 Because it has a higher crystalline content than other glass ceramics, lithium disilicate is not easily penetrated by commonly-used grinding instruments. Several diamond cutting instruments have recently been introduced to prepare high-strength ceramics such as zirconia. The manufacturers of these instruments incorporated various technologies such as proprietary grit, optimal density, multilayered designs, and durable bonding of the diamond crystals to the shank of the instrument to increase their efficiency and longevity when
used on strong ceramic materials. Regardless of the instrument, it has been established that copious water irrigation is paramount when grinding through ceramics.3,16 Water acts as a lubricant, improves cutting efficiency, and prevents heat buildup.17-19 The purpose of this in vitro study was to evaluate the effect of endodontic access preparation on the failure load of both adhesively bonded and conventionally cemented lithium disilicate restorations. It was hypothesized that adhesively bonded ceramic restorations would sustain higher loads to failure than conventionally cemented restorations. It was also hypothesized that the use of a high efficiency, smaller grit, diamond rotary instrument for the access preparation would result in less damage and strength reduction than a larger-grit diamond rotary instrument.
Material and Methods Selection of rotary grinding instruments Five rotary instrument brands were evaluated for their efficiency in grinding lithium disilicate restorations. The rotary instruments, their manufacturer information, and their intended use are listed in Table I. Two rotary instrument shapes per brand were evaluated: round for starting the endodontic access and cylindrical for finishing the preparation with a high speed handpiece and water irrigation. A subjective evaluation was performed on lithium disilicate milled crowns (IPS e.max CAD; Ivoclar Vivadent AG, Schaan, Liechtenstein) which had been cemented with a resin cement (Multilink Automix; Ivoclar Vivadent AG) and stored in an incubator for 24 hrs at 37°C and 100% humidity. The rotary instruments were evaluated by the same operator and were subjectively ranked as poor, fair, good, or excellent according to their grinding efficiency. A coarse-grit diamond rotary instrument designed for cutting zirconia (ZR diamond; round-
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Table I. Rotary instruments, manufacturer, grit size, indication for use and subjective efficiency ranking Rotary Instrument
Manufacturer
Grit
Indication
Rotary Instrument Efficiency
DuraCut
Brasseler USA
Coarse 151µm
Cutting hard materials
Good
including zirconia and lithium disilicate ZIR.CUT
Axis Dental
Special Z grit
Cutting ceramics including
Good
zirconia and aluminous oxide. TSZtech
ZR
Premier
Komet
Diamond Crosstech
Dentalree
Fine
Cutting zirconia
Good
(45µm)
Indicated for multiple uses
Coarse
Cutting zirconia
(126µm)
Indicated for multiple uses
Excellent
Coarse
Multipurpose
(150µm)
diamond rotary instrument
Fair
Indicated for multiple uses Neodiamond
Microcopy
end taper diamond, 126-µm grit size; Komet, Besigheim, Germany) was ranked the most efficient and did not demonstrate dulling or charring when used to cut through lithium disilicate (Table I). Therefore, this rotary instrument was selected and was compared with 2 standard larger-grit diamond rotary instruments (Super-coarse; 150-µm grit size and CRF, 180-µm grit size round-end taper diamond rotary instruments; Brasseler USA, Savannah, Ga) to evaluate the effect of grit size on the failure load of the restorations after endodontic access preparation. Specimen Preparation Minimum sample size was calculated by using a power analysis (power=0.8) with an effect size of 1.72 and a significance level of .05. Sixty identical maxillary first molar completecoverage IPS e.max CAD restorations were milled with a CAD/CAM milling
Medium
Single-use diamond
(100-110µm)
rotary instrument
unit (Cerec MCXL; Sirona Dental Systems GmbH, Bensheim, Germany). The crowns were glazed (IPS e. max Crystall; Ivoclar Vivadent AG) and crystallized (speed glaze/crystallization cycle at 840°C, approximately 23 minutes). Five coats of die relief (P.D.Q. spacer; Whip Mix, Louisville, Ky) in alternating blue and gray colors were applied internally in each crown 1 mm short of the margin. Each layer was allowed 20 seconds to dry according to the manufacturer’s instructions before the subsequent layer was applied. With each layer measuring 12 µm in film thickness, a cement spacer measuring approximately 60 µm was introduced. Dies were fabricated by filling each crown with a composite resin material (Tetric Evo Ceram; Ivoclar Vivadent AG). A vinyl polysiloxane mold (Virtual Heavy Body; Ivoclar Vivadent AG) was used for die fabrication beyond the crown margin. The composite resin material was applied in 2 mm incre-
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Good
ments and light polymerized with an LED light (Bluephase G2; Ivoclar Vivadent AG) at 1,200 mW/cm2 for 20 seconds. The light output was verified periodically during specimen fabrication with an LED radiometer (Bluephase meter; Ivoclar Vivadent AG). The die spacer was removed from the internal surface of the crowns. The composite resin dies and their respective crowns were divided into 6 groups (n=10) and labeled. All dies were stored in water at 37°C for 30 days to allow for complete hydration, thereby eliminating any chance of dimensional expansion or distortion as a result of water uptake after crown cementation.20 After 30 days of moist storage for the resin dies, the crowns were cemented on their respective dies by using the following protocol. For Groups M-C, M-ZR, M-SC. and M-CRF, the crowns were etched for 20 seconds with a 4.5% hydrofluoric acid etching gel (IPS ceramic etching gel; Ivoclar Vivadent AG) and
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1 Specimen in loading apparatus after cementation.
2 Three points of indenter contact marked.
3 Access preparation.
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silanated for 60 seconds (Monobond Plus; Ivoclar Vivadent AG). A bonding agent (Heliobond; Ivoclar Vivadent AG) was applied on the composite resin die as a wetting agent. A dual-polymerizing resin cement (Multilink Implant (M); Ivoclar Vivadent AG) was placed into the crowns. The crowns were seated on the composite resin dies and excess cement eliminated. Each restoration was then loaded by using a 49 N static loading apparatus (Fig. 1), which was adapted to the specimen with vinyl polysiloxane impression material (Virtual; Ivoclar Vivadent AG). The load was maintained for 1 minute before any additional excess cement was removed. The cement was polymerized (Bluephase G2; Ivoclar Vivadent AG) at 1,200 mW/cm2 for 20 seconds on both buccal and lingual surfaces. The load was removed, and the cement was then polymerized for an additional 20 seconds at 1,200 mW/cm2 (Bluephase G2) through the occlusal surfaces. For Groups F-C and F-ZR, the crowns were etched (IPS ceramic etching gel) and then luted with zinc phosphate cement (Flex’s Zinc Phosphate (F); Cherry Hill, NJ). Excess cement was removed. The crowns were subjected to a 49 N load for 5 minutes before any additional excess cement was removed. After cementation, all specimens were stored for 1 week at 37°C and 100% humidity. Before preparing the endodontic access, the hemispherical steel indenter to be used for loading the specimens was adapted vertically on each specimen with a dental surveyor. Articulating paper was used to mark the 3 points of contact between the indenter and the restoration (Fig. 2). The endodontic access was prepared with a high-speed handpiece and copious water irrigation, avoiding marked contacts. The preparation was started with a round diamond rotary instrument until complete perforation of the restoration was achieved. A round-end tapered diamond rotary instrument was then used to complete
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Table II. Summary of control and test groups indicating cement type and rotary instrument used for access preparation Endodontic Access
Rotary Instrument (Grit Size)
Manufacturer
Group
Cement
Crown/Die Pretreatment
M-C
M
Yes
No
N/A
N/A
M-ZR
M
Yes
Yes
ZR Diamond (126µm)
Komet, Germany
Round: ZR6801.314.014 Cylinder: ZR6856.314.025 M-SC
M
Yes
Yes
Super coarse (150 µm)
Brasseler, USA
Round: 5801.31.018 Cylinder: 5856.31.018 M-CRF
M
Yes
Yes
CRF(180 µm)
Brasseler, USA
Round: 2801.31.023 Cylinder: 2856.31.018 F-C
F
Crown etched
No
N/A
N/A
ZR Diamond (126µm)
Komet, Germany
only F-ZR
F
Crown etched only
Yes
Round: ZR6801.314.014 Cylinder: ZR6856.314.025
M: Multilink Implant F: Fleck’s Zinc Phosphate
4 Access preparation restored with composite resin. the access preparation (Fig. 3). The diamond rotary instruments used to prepare the endodontic access for the different groups are listed in Table II. The ceramic margins of the endodontic access were etched (IPS ceramic etching gel) for 20 seconds
5 Specimen under loading apparatus.
and silanated (Monobond Plus) for 1 minute. The access preparation, measuring 4 mm in depth, was restored in two 2-mm increments with composite resin (Tetric EvoCeram) and each increment was light polymerized for 20 seconds (Bluephase G2) (Fig.
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4). The specimens were stored in an incubator for 1 more week at 100% humidity and 37°C. After 1 week of storage, the apical portion of each specimen was embedded in an autopolymerizing acrylic resin custom tray material (Bosworth Fastray; Bo-
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November 2011 sworth Company, Skokie, Ill) to form a base. The crowns were subjected to an axial compressive load on the occlusal surface (Fig. 5) in a universal testing machine (Instron Loading Frame, model 33R4204; Instron Corp, Norwood, Mass) with a 5-kN load cell at 1.00 mm/min speed until
Results
failure of the specimen occurred. The results were analyzed by using a 1-way ANOVA (α=.05), followed by a multiple comparison test (Tukey’s HSD). Each specimen was visually inspected to determine whether the failure involved the restoration only or the restoration and the die combined.
Descriptive statistics for the failure load (N) of the 6 groups are illustrated in Figure 6. One-way ANOVA revealed a significant difference among the groups (F5,54=14.28, P<.001). The highest mean load to failure was
5000
Failure Load (N)
4000
3000
2000
1000
0
M-C
M-ZR
M-SC M-CRF Groups
F-C
F-C
6 Box plot of failure load (N) for all groups.
Table III. Mean differences and confidence intervals of differences between groups 95% Confidence Interval Lower Bound Upper Bound
(X) Groups
(Y) Groups
Mean Difference (X-Y)
M-C
M-ZR
-147.8
.986
-816.5
520.9
M-ZR
M-SC
M-CRF F-C
P
M-SC
400.3
.494
-268.4
1069.0
M-CRF
962.0*
.001
293.3
1630.7
F-C
1073.8*
<.001
405.1
1742.5
F-ZR
1317.0*
<.001
648.3
1985.7
M-SC
548.1
.167
-120.6
1216.8
M-CRF
1109.8*
<.001
441.1
1778.5
F-C
1221.6*
<.001
552.9
1890.3
F-ZR
1464.8*
<.001
796.1
2133.5
M-CRF
561.7
.148
-107.0
1230.4
F-C
673.5*
.047
4.8
1342.2
F-ZR
916.7*
.002
248.0
1585.4
F-C
111.8
.996
-556.9
780.5
F-ZR
355.0
.622
-313.7
1023.7
F-ZR
243.2
.889
-425.5
911.9
Values are significantly different for P<.05. M: Multilink Implant
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F: Fleck’s Zinc Phosphate
ZR: ZR Diamond-coarse
SC: Super-coarse
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7 Catastrophic crown failure only.
8 Combined failure of crown and die.
Table IV. Percentage distribution of failure modes among groups Failure Mode Crown Only Crown and Die
Groups
Mean Failure Load (N)
M-C
3316
0%
100%
M-ZR
3464
0%
100%
M-SC
2915
0%
100%
M-CRF
2354
40%
60%
F-C
2242
50%
50%
F- ZR
1999
70%
30%
M: Multilink Implant
achieved in Groups M-ZR (3464 N), M-C (3316 N), and M-SC (2915 N). Group M-CRF was significantly lower (2354 N) than Groups M-C and M-ZR (P<.05) but not statistically different from Group M-SC (P=.14). The lowest failure loads were measured in groups F-ZR (1999 N) and F-C (2242 N) and were statistically lower than Groups M-C and M-ZR (P<.001) and Group M-SC (P<.05). Confidence intervals for the differences among the groups are illustrated in Table III. Two types of failures were observed: failure of the crown only (Fig. 7) and combined failure of the crown and composite resin die (Fig. 8). Specimens in Groups M-C, M-ZR, and M-SC demonstrated combined failures (crown and resin die). Groups M-CRF, F-C, and F-ZR demonstrated different ratios of the 2 failure modes. Higher failure loads were associated with failure of the crown and resin die
F: Fleck’s Zinc Phosphate
ZR: ZR Diamond-coarse
combined. Table IV lists the percentile distribution of failure modes among the groups.
Discussion Both research hypotheses were accepted. With conventional cementation, the repaired restorations sustained only 57% of the failure load measured with adhesive bonding, thus emphasizing the superiority of an adhesive interphase. The 126-µm diamond rotary instrument demonstrated efficient grinding with no evidence of sparking or charring in the preliminary evaluation phase. This rotary instrument was compared with 2 commonly used diamond rotary instruments with a larger-grit size (150 and 180µm). The endodontic access was completed in less time with the 126-µm grit-size rotary instrument than with the larger-grit rotary in-
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SC: Super-coarse
struments, and the failure load of the repaired restoration was lower when larger-grit rotary instruments were used for access preparation. The findings of this study are in agreement with previous reports indicating that adhesive cementation improves the failure resistance of ceramic restorations.13,14 When comparing Groups M-C and F-C (control groups), the adhesively bonded group (M-C) sustained significantly higher loads to failure than the conventionally cemented group (F-Z). The failure mode in all specimens of Group M-C was a combined failure (crown and resin die), while in Group F-C, both failure modes were equally observed. The combined failure mode (crown and resin die), observed in all specimens of Groups M-C, M-ZR, and M-SC, demonstrated how an adhesive interphase can increase the loadbearing capability of ceramic restora-
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November 2011 tions by transmitting the loads to the supporting structure. Four specimens from Group M-CRF demonstrated failures in the crown only, resembling the conventionally cemented groups. It is suggested that the larger-grit rotary instrument used in Group M-CRF disrupted the adhesive interphase in some specimens, resulting in the same failure mode observed with conventional cementation. An efficient grinding instrument with a smallergrit size will not only cause less damage to the restoration but may also maintain the integrity of the adhesive interphase. Two studies have compared the strength of ceramic restorations with and without an endodontic access.11,12 The first study evaluated feldspathic ceramic restorations, while the second investigated the fracture resistance of alumina-veneered and zirconia-veneered restorations. Both studies reported significantly lower strength values with access preparation than with the intact control. The present study was the first identified by the authors to compare the failure load of lithium disilicate monolithic restorations before and after simulated endodontic access preparation. Unlike the results reported for other ceramics,11,12 simulated endodontic access did not result in reduced failure load in any of the test groups. The difference in findings may have been due to the use of a high efficiency diamond rotary instrument, the high strength of lithium disilicate, or its monolithic design. Despite the fact that conventional in vitro failure tests of fixed restorations do not faithfully reproduce intraoral loading conditions,21 they do offer a controlled environment for comparative evaluation of the variables under investigation. Failure loads measured in this study were in the range of 1900 N to 3500 N, which exceeds the maximum measured occlusal forces previously reported to be in the range of 585 N22 to 740 N23. Static loading under dry conditions is one of the limitations of this
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study and may explain the high failure loads recorded. While wet storage alone reduces failure loads significantly,24 a combination of cyclic loading in the presence of water was reported to produce failure loads that fall within a clinically meaningful range.25 Another limitation of this study was the use of composite resin dies rather than natural teeth. Resin or steel dies have been used in in vitro studies26,27 to standardize the preparation and mechanical properties of the substructure. It has been suggested that the elastic modulus of the supporting structure may influence the fracture susceptibility of ceramic restorations.28,29 The composite resin material used for die fabrication in this study had a modulus of elasticity of 10 GPa as described by the manufacturer—close to the range reported for human dentin.28,30 The slight difference in the elastic modulus of the composite resin dies and human dentin may explain the combined failure observed in most adhesively bonded specimens, a fact which has not been reported clinically. The dry static loading conditions used may further explain the combined failure phenomenon in that they could have initiated ceramic failure at the external surface. This differs from failures observed intraorally.21 That the bond strength between the composite resin cement and the composite resin dies is superior to human dentin is another possible factor resulting in the combined failure mode. Intraoral cyclic loading conditions may degrade such bonds, explaining the lack of this observation in clinical reports. Future research in this area should address dynamic and fatigue loading in wet conditions and possibly use artificial aging to better simulate clinical circumstances.
Conclusions Within the limitation of this study, the following conclusions were drawn: 1. The preparation of an endodontic access with a 126-µm dia-
mond rotary instrument did not alter the failure load values of lithium disilicate restorations, regardless of the cement used. 2. Larger-grit diamond rotary instruments resulted in lower failure loads for the repaired restorations than did smaller-grit high efficiency rotary instruments. 3. Adhesively bonded restorations sustained significantly higher failure loads than those that were conventionally cemented with and without the access preparation.
REFERENCES 1. Cheung GSP. A preliminary investigation into the longevity and causes of failure of single unit extracoronal restorations. J Dent 1991;19:160-3. 2. Goodacre CJ, Bernal G, Rungcharassaeng K, Kan JY. Clinical complications in fixed prosthodontics. J Prosthet Dent 2003;90:31-41. 3. Teplitsky PE, Sutherland JK. Endodontic access of Cerestore crowns. J Endod 1985; 11:555-8. 4. Sutherland JK, Teplitsky PE, Moulding MB. Endodontic access of all-ceramic crowns. J Prosthet Dent 1989;61:146-9. 5. Cohen BD, Wallace JA. Castable glass ceramic crowns and their reaction to endodontic therapy. Oral Surg Oral Med Oral Pathol 1991;72:108-10. 6. Haselton DR, Lloyd PM, Johnson WT. A comparison of the effects of two burs on endodontic access in all-ceramic high lucite crowns. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:486-92 7. Sabourin CR, Flinn BD, Pitts DL, Gatten TL, Johnson JD. A novel method for creating endodontic access preparations through all-ceramic restorations: air abrasion and its effect relative to diamond and carbide bur use. J Endod 2005;31:616-9. 8. Thompson JY, Anusavice KJ, Naman A, Morris HF. Fracture surface characterization of clinically failed all-ceramic crowns. J Dent Res 1994;73:1824-32. 9. Kelly JR. Dental ceramics: current thinking and trends. Dent Clin N Am 2004;48:513-30. 10.Griffith AA. The phenomena of rupture and flow in solids. Phil Trans Roy Sot A 1921;224:163-198. 11.Stokes AN, Hood JA, Casley PB, Cawley RM, Cho GJ. Endodontic access cavities in porcelain jacket crowns--two methods of repair compared. Restorative Dent 1988;4:56-8. 12.Wood KC, Berzins DW, Luo Q, Thompson GA, Toth JM, Nagy WW.. Resistance to fracture of two all-ceramic crown materials following endodontic access. J Prosthet Dent. 2006;95:33-41. 13.Chen J-H, Matsumura H, Atsuta M. Effect of different etching periods on the bond strength of a composite resin to a machinable porcelain. J Dent 1998;26:53-8.
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Volume 106 Issue 5 14.Bindl A, Lüthy H and Mörmann W H. Strength and fracture pattern of monolithic CAD/CAM-generated posterior crowns. Dental Mater 2006;22:29-36. 15.Michanowicz AE, Michanowicz JP. Endodontic access to the pulp chamber via porcelain jacket crowns. Oral Surg Oral Med Oral Pathol 1962;15:1483-8. 16.Davis MW. Providing endodontic care for teeth with ceramic crowns. J Am Dent Assoc 1998;129:1746-7. 17.Galindo DF, Ercoli C, Funkenbusch PD, Greene TD, Moss ME, Lee HJ, et al. Toothpreparation: a study on effect of different variables and a comparison between conventional and channeled burs. J Prosthodont 2004;13:3-16. 18.von Fraunhofer JA, Siegel SC, Feldman S. Handpiece coolant flow rates and dental cutting. Oper Dent 2000;25:544-8. 19.Lloyd BA, Rich JA, Brown WS. Effect of cooling techniques on temperature control and cutting rate for high-speed dental drills. J Dent Res 1978;57:675-84. 20.Huang M, Thompson VP, Rekow ED, Soboyejo WO. Modeling of water absorption induced cracks in resin-based composite supported ceramic layer structures. J Biomed Mater Res 2007;84:124.
21.Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 1999;81:652-661. 22.Kikuchi M, Korioth TW, Hannam AG. The association among occlusal contacts, clenching effort, and bite force distribution in man. J Dent Res 1997;76:1316-25. 23.Gibbs CH, Mahan PE, Lundeen HC, Brehnan K, Walsh EK, Holbrook WB. Occlusal forces during chewing and swallowing as measured by sound transmission. J Prosthet Dent 1981;46:443-9. 24.Kelly JR, Rungruanganunt P, Hunter B, Vailati F. Development of a clinically validated bulk failure test for ceramic crowns. J Prosthet Dent 2010;104:228-238 25.Rekow D, Thompson VP. Engineering long term clinical success of advanced ceramic prostheses. J Mater Sci Mater Med 2007;18:47-56. 26.Castellani D, Baccetti T, Giovannoni A, Bernardini UD. Resistance to fracture of metal ceramic and all-ceramic crowns, Int J Prosthodont 1994;7:149-54. 27.Yoshinari M, Derand T. Fracture strength of all-ceramic crowns. Int J Prosthodont 1994;7:329-338.
28.Wakabayashi N, Anusavice KJ. Crack initiation modes in bilayered alumina/porcelain disks as a function of core/veneer thickness ratio and supporting substrate stiffness. J Dent Res 2000;79:1398-404. 29.Scherrer SS, de Rijk WG. The effect of crown length on the fracture resistance of posterior porcelain and glass-ceramic crowns. Int J Prosth 1992;5:550-7. 30.van Meerbek B, Willems G, Celis JP, Roos JR, Braem M, Lambrechts P, et al. Assessment by nano-indentation of the hardness and elasticity of the resin dentin bonding area. J Dent Res 1993;72:1434-42. Corresponding author: Dr Dana M. Qeblawi 225 Squire Hall, 3435 Main St. Buffalo, NY 14214 Fax: 716-829-2440 E-mail: [email protected] Copyright © 2011 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Clinical performance of cervical restorations--a meta-analysis Heintze SD, Ruffieux C, Rousson V. Dent Mater. 2010;26:993-1000. Objectives. To carry out a meta-analysis in order to assess the influencing factors on retention loss and marginal discoloration of cervical restorations made of composites and glass ionomer (derivates). Methods. The literature was searched for prospective clinical studies on cervical restorations with an observation period of at least 18 months. Results. Fifty clinical studies involving 40 adhesive systems matched the inclusion criteria. On average, 10% of the cervical fillings were lost and 24% exhibited marginal discoloration after 3 years. The variability ranged from 0% to 50% for retention loss and from 0% to 74% for marginal discoloration. Hardly any secondary caries was detected. When linear mixed models with a study and experiment effect were used, the analysis revealed that the adhesive/restorative class had the most significant influence, with 2-step self-etching adhesive systems performing best and 1-step selfetching adhesive systems performing worst; 3-step etch-and-rinse systems, glass ionomers/resin-modified glass ionomers, 2-step etch-and-rinse systems and polyacid-modified resin composites were ranked in between. Restorations placed in teeth whose dentin/enamel had been prepared/roughened showed a statistically significant higher retention rate than those placed in teeth with unprepared dentin (P<0.05). Beveling of the enamel and the type of isolation used (rubberdam/cotton rolls) had no significant influence. Significance. The clinical performance of cervical restorations is significantly influenced by the type of adhesive system used and/or the adhesive class to which the system belonged and whether the dentin/enamel is prepared or not. 2-Step self-etching- and 3-step etch&rinse systems shall be chosen over 1-step self-etching systems and glass ionomer derivates. The dentin (and enamel) surface shall be roughened before placement of the restoration. Reprinted with permission of Quintessence Publishing.
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Clinical Implications
Endodontic treatment through ceramic restorations is not uncommon. When the restoration will not be replaced, proper rotary instrument selection, such as a high-efficiency, 126-μm grit diamond rotary cutting instrument, may impact the longevity of the restored restoration.
Presented as a poster at the American Association of Dental Research annual meeting, San Diego, Calif, March 2010. Clinical Assistant Professor, Department of Restorative Dentistry, State University of New York at Buffalo. Manager of Scientific Services, Research and Development, Ivoclar Vivadent Inc. c Research Assistant, Research and Development, Ivoclar Vivadent Inc. a
b
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November 2011 The use of ceramic restorations has expanded in the past decade because of increased esthetic demands by patients. Lithium disilicate is a contemporary glass-ceramic system that is gaining popularity among dentists because of its high strength and enhanced optical properties. The need for an efficient grinding instrument becomes essential during crown removal or endodontic access preparation through a restoration. For crown removal purposes, the efficiency of the grinding instrument is the only concern. However, for endodontic access preparation, the potential damage caused by the grinding instrument on the restoration must be evaluated. It has been reported that 4% of teeth receiving complete coverage restorations develop pulpal necrosis.1 In ceramic crowns, the need for endodontic treatment has been reported as 1% with a range of 0% to 5%.2 Optimally, a restoration with an endodontic access preparation should be replaced when endodontic therapy is completed. However, if the restoration is not to be replaced, the effect of the access preparation on the longevity of the restoration is crucial. Earlier studies have reported varying degrees of damage to ceramic restorations after endodontic access preparation.3-6 Damage ranged from minor chipping to microcrack formation and catastrophic fractures. A novel technique was described for creating an endodontic access preparation through leucite-reinforced ceramic restorations by using airborneparticle abrasion.7 However, while this technique caused no catastrophic fractures, chipping, or microcracks, preparation takes longer to complete. It has been determined that the strength of dental ceramics is flaw dependent.8 The aqueous oral environment is detrimental to existing cracks, thus facilitating their propagation.9 According to the Griffith theory of brittle materials, microcracks are the origins of ceramic failures because they act as stress concentrators.10 It can be surmised, therefore, that pre-
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paring endodontic access through ceramic restorations may compromise their longevity. Few studies have investigated the effect of endodontic access preparation on the fracture strength of ceramic restorations.11,12 Strokes et al11 reported lower fracture strength for endodontically prepared and repaired ceramic restorations than for intact controls. Wood et al12 reported that endodontic access preparation followed by repair reduced the reliability of both zirconia and alumina restorations but resulted in a significant loss of strength only in zirconia-based restorations. The effect of endodontic access preparation on the failure load of lithium disilicate glass-ceramic restorations has not been evaluated. Adhesive resin bonding has been shown to increase the load-bearing capability of ceramic restorations.13 Bindl et al14 demonstrated that adhesive cementation increased the fracture load of lithium disilicate crowns compared to conventional cementation. This advantage of the adhesive interphase may be important for the survival of the restoration after endodontic therapy and repair. Michanowicz et al15 were the first to describe a method for endodontic access preparation through ceramic crowns with a #2 diamond rotary instrument with water coolant. Another study also recommended using a diamond rotary instrument in a high-speed handpiece with light brush strokes and copious water spray.16 Because it has a higher crystalline content than other glass ceramics, lithium disilicate is not easily penetrated by commonly-used grinding instruments. Several diamond cutting instruments have recently been introduced to prepare high-strength ceramics such as zirconia. The manufacturers of these instruments incorporated various technologies such as proprietary grit, optimal density, multilayered designs, and durable bonding of the diamond crystals to the shank of the instrument to increase their efficiency and longevity when
used on strong ceramic materials. Regardless of the instrument, it has been established that copious water irrigation is paramount when grinding through ceramics.3,16 Water acts as a lubricant, improves cutting efficiency, and prevents heat buildup.17-19 The purpose of this in vitro study was to evaluate the effect of endodontic access preparation on the failure load of both adhesively bonded and conventionally cemented lithium disilicate restorations. It was hypothesized that adhesively bonded ceramic restorations would sustain higher loads to failure than conventionally cemented restorations. It was also hypothesized that the use of a high efficiency, smaller grit, diamond rotary instrument for the access preparation would result in less damage and strength reduction than a larger-grit diamond rotary instrument.
Material and Methods Selection of rotary grinding instruments Five rotary instrument brands were evaluated for their efficiency in grinding lithium disilicate restorations. The rotary instruments, their manufacturer information, and their intended use are listed in Table I. Two rotary instrument shapes per brand were evaluated: round for starting the endodontic access and cylindrical for finishing the preparation with a high speed handpiece and water irrigation. A subjective evaluation was performed on lithium disilicate milled crowns (IPS e.max CAD; Ivoclar Vivadent AG, Schaan, Liechtenstein) which had been cemented with a resin cement (Multilink Automix; Ivoclar Vivadent AG) and stored in an incubator for 24 hrs at 37°C and 100% humidity. The rotary instruments were evaluated by the same operator and were subjectively ranked as poor, fair, good, or excellent according to their grinding efficiency. A coarse-grit diamond rotary instrument designed for cutting zirconia (ZR diamond; round-
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Table I. Rotary instruments, manufacturer, grit size, indication for use and subjective efficiency ranking Rotary Instrument
Manufacturer
Grit
Indication
Rotary Instrument Efficiency
DuraCut
Brasseler USA
Coarse 151µm
Cutting hard materials
Good
including zirconia and lithium disilicate ZIR.CUT
Axis Dental
Special Z grit
Cutting ceramics including
Good
zirconia and aluminous oxide. TSZtech
ZR
Premier
Komet
Diamond Crosstech
Dentalree
Fine
Cutting zirconia
Good
(45µm)
Indicated for multiple uses
Coarse
Cutting zirconia
(126µm)
Indicated for multiple uses
Excellent
Coarse
Multipurpose
(150µm)
diamond rotary instrument
Fair
Indicated for multiple uses Neodiamond
Microcopy
end taper diamond, 126-µm grit size; Komet, Besigheim, Germany) was ranked the most efficient and did not demonstrate dulling or charring when used to cut through lithium disilicate (Table I). Therefore, this rotary instrument was selected and was compared with 2 standard larger-grit diamond rotary instruments (Super-coarse; 150-µm grit size and CRF, 180-µm grit size round-end taper diamond rotary instruments; Brasseler USA, Savannah, Ga) to evaluate the effect of grit size on the failure load of the restorations after endodontic access preparation. Specimen Preparation Minimum sample size was calculated by using a power analysis (power=0.8) with an effect size of 1.72 and a significance level of .05. Sixty identical maxillary first molar completecoverage IPS e.max CAD restorations were milled with a CAD/CAM milling
Medium
Single-use diamond
(100-110µm)
rotary instrument
unit (Cerec MCXL; Sirona Dental Systems GmbH, Bensheim, Germany). The crowns were glazed (IPS e. max Crystall; Ivoclar Vivadent AG) and crystallized (speed glaze/crystallization cycle at 840°C, approximately 23 minutes). Five coats of die relief (P.D.Q. spacer; Whip Mix, Louisville, Ky) in alternating blue and gray colors were applied internally in each crown 1 mm short of the margin. Each layer was allowed 20 seconds to dry according to the manufacturer’s instructions before the subsequent layer was applied. With each layer measuring 12 µm in film thickness, a cement spacer measuring approximately 60 µm was introduced. Dies were fabricated by filling each crown with a composite resin material (Tetric Evo Ceram; Ivoclar Vivadent AG). A vinyl polysiloxane mold (Virtual Heavy Body; Ivoclar Vivadent AG) was used for die fabrication beyond the crown margin. The composite resin material was applied in 2 mm incre-
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Good
ments and light polymerized with an LED light (Bluephase G2; Ivoclar Vivadent AG) at 1,200 mW/cm2 for 20 seconds. The light output was verified periodically during specimen fabrication with an LED radiometer (Bluephase meter; Ivoclar Vivadent AG). The die spacer was removed from the internal surface of the crowns. The composite resin dies and their respective crowns were divided into 6 groups (n=10) and labeled. All dies were stored in water at 37°C for 30 days to allow for complete hydration, thereby eliminating any chance of dimensional expansion or distortion as a result of water uptake after crown cementation.20 After 30 days of moist storage for the resin dies, the crowns were cemented on their respective dies by using the following protocol. For Groups M-C, M-ZR, M-SC. and M-CRF, the crowns were etched for 20 seconds with a 4.5% hydrofluoric acid etching gel (IPS ceramic etching gel; Ivoclar Vivadent AG) and
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November 2011
1 Specimen in loading apparatus after cementation.
2 Three points of indenter contact marked.
3 Access preparation.
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silanated for 60 seconds (Monobond Plus; Ivoclar Vivadent AG). A bonding agent (Heliobond; Ivoclar Vivadent AG) was applied on the composite resin die as a wetting agent. A dual-polymerizing resin cement (Multilink Implant (M); Ivoclar Vivadent AG) was placed into the crowns. The crowns were seated on the composite resin dies and excess cement eliminated. Each restoration was then loaded by using a 49 N static loading apparatus (Fig. 1), which was adapted to the specimen with vinyl polysiloxane impression material (Virtual; Ivoclar Vivadent AG). The load was maintained for 1 minute before any additional excess cement was removed. The cement was polymerized (Bluephase G2; Ivoclar Vivadent AG) at 1,200 mW/cm2 for 20 seconds on both buccal and lingual surfaces. The load was removed, and the cement was then polymerized for an additional 20 seconds at 1,200 mW/cm2 (Bluephase G2) through the occlusal surfaces. For Groups F-C and F-ZR, the crowns were etched (IPS ceramic etching gel) and then luted with zinc phosphate cement (Flex’s Zinc Phosphate (F); Cherry Hill, NJ). Excess cement was removed. The crowns were subjected to a 49 N load for 5 minutes before any additional excess cement was removed. After cementation, all specimens were stored for 1 week at 37°C and 100% humidity. Before preparing the endodontic access, the hemispherical steel indenter to be used for loading the specimens was adapted vertically on each specimen with a dental surveyor. Articulating paper was used to mark the 3 points of contact between the indenter and the restoration (Fig. 2). The endodontic access was prepared with a high-speed handpiece and copious water irrigation, avoiding marked contacts. The preparation was started with a round diamond rotary instrument until complete perforation of the restoration was achieved. A round-end tapered diamond rotary instrument was then used to complete
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Table II. Summary of control and test groups indicating cement type and rotary instrument used for access preparation Endodontic Access
Rotary Instrument (Grit Size)
Manufacturer
Group
Cement
Crown/Die Pretreatment
M-C
M
Yes
No
N/A
N/A
M-ZR
M
Yes
Yes
ZR Diamond (126µm)
Komet, Germany
Round: ZR6801.314.014 Cylinder: ZR6856.314.025 M-SC
M
Yes
Yes
Super coarse (150 µm)
Brasseler, USA
Round: 5801.31.018 Cylinder: 5856.31.018 M-CRF
M
Yes
Yes
CRF(180 µm)
Brasseler, USA
Round: 2801.31.023 Cylinder: 2856.31.018 F-C
F
Crown etched
No
N/A
N/A
ZR Diamond (126µm)
Komet, Germany
only F-ZR
F
Crown etched only
Yes
Round: ZR6801.314.014 Cylinder: ZR6856.314.025
M: Multilink Implant F: Fleck’s Zinc Phosphate
4 Access preparation restored with composite resin. the access preparation (Fig. 3). The diamond rotary instruments used to prepare the endodontic access for the different groups are listed in Table II. The ceramic margins of the endodontic access were etched (IPS ceramic etching gel) for 20 seconds
5 Specimen under loading apparatus.
and silanated (Monobond Plus) for 1 minute. The access preparation, measuring 4 mm in depth, was restored in two 2-mm increments with composite resin (Tetric EvoCeram) and each increment was light polymerized for 20 seconds (Bluephase G2) (Fig.
The Journal of Prosthetic Dentistry
4). The specimens were stored in an incubator for 1 more week at 100% humidity and 37°C. After 1 week of storage, the apical portion of each specimen was embedded in an autopolymerizing acrylic resin custom tray material (Bosworth Fastray; Bo-
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November 2011 sworth Company, Skokie, Ill) to form a base. The crowns were subjected to an axial compressive load on the occlusal surface (Fig. 5) in a universal testing machine (Instron Loading Frame, model 33R4204; Instron Corp, Norwood, Mass) with a 5-kN load cell at 1.00 mm/min speed until
Results
failure of the specimen occurred. The results were analyzed by using a 1-way ANOVA (α=.05), followed by a multiple comparison test (Tukey’s HSD). Each specimen was visually inspected to determine whether the failure involved the restoration only or the restoration and the die combined.
Descriptive statistics for the failure load (N) of the 6 groups are illustrated in Figure 6. One-way ANOVA revealed a significant difference among the groups (F5,54=14.28, P<.001). The highest mean load to failure was
5000
Failure Load (N)
4000
3000
2000
1000
0
M-C
M-ZR
M-SC M-CRF Groups
F-C
F-C
6 Box plot of failure load (N) for all groups.
Table III. Mean differences and confidence intervals of differences between groups 95% Confidence Interval Lower Bound Upper Bound
(X) Groups
(Y) Groups
Mean Difference (X-Y)
M-C
M-ZR
-147.8
.986
-816.5
520.9
M-ZR
M-SC
M-CRF F-C
P
M-SC
400.3
.494
-268.4
1069.0
M-CRF
962.0*
.001
293.3
1630.7
F-C
1073.8*
<.001
405.1
1742.5
F-ZR
1317.0*
<.001
648.3
1985.7
M-SC
548.1
.167
-120.6
1216.8
M-CRF
1109.8*
<.001
441.1
1778.5
F-C
1221.6*
<.001
552.9
1890.3
F-ZR
1464.8*
<.001
796.1
2133.5
M-CRF
561.7
.148
-107.0
1230.4
F-C
673.5*
.047
4.8
1342.2
F-ZR
916.7*
.002
248.0
1585.4
F-C
111.8
.996
-556.9
780.5
F-ZR
355.0
.622
-313.7
1023.7
F-ZR
243.2
.889
-425.5
911.9
Values are significantly different for P<.05. M: Multilink Implant
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F: Fleck’s Zinc Phosphate
ZR: ZR Diamond-coarse
SC: Super-coarse
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7 Catastrophic crown failure only.
8 Combined failure of crown and die.
Table IV. Percentage distribution of failure modes among groups Failure Mode Crown Only Crown and Die
Groups
Mean Failure Load (N)
M-C
3316
0%
100%
M-ZR
3464
0%
100%
M-SC
2915
0%
100%
M-CRF
2354
40%
60%
F-C
2242
50%
50%
F- ZR
1999
70%
30%
M: Multilink Implant
achieved in Groups M-ZR (3464 N), M-C (3316 N), and M-SC (2915 N). Group M-CRF was significantly lower (2354 N) than Groups M-C and M-ZR (P<.05) but not statistically different from Group M-SC (P=.14). The lowest failure loads were measured in groups F-ZR (1999 N) and F-C (2242 N) and were statistically lower than Groups M-C and M-ZR (P<.001) and Group M-SC (P<.05). Confidence intervals for the differences among the groups are illustrated in Table III. Two types of failures were observed: failure of the crown only (Fig. 7) and combined failure of the crown and composite resin die (Fig. 8). Specimens in Groups M-C, M-ZR, and M-SC demonstrated combined failures (crown and resin die). Groups M-CRF, F-C, and F-ZR demonstrated different ratios of the 2 failure modes. Higher failure loads were associated with failure of the crown and resin die
F: Fleck’s Zinc Phosphate
ZR: ZR Diamond-coarse
combined. Table IV lists the percentile distribution of failure modes among the groups.
Discussion Both research hypotheses were accepted. With conventional cementation, the repaired restorations sustained only 57% of the failure load measured with adhesive bonding, thus emphasizing the superiority of an adhesive interphase. The 126-µm diamond rotary instrument demonstrated efficient grinding with no evidence of sparking or charring in the preliminary evaluation phase. This rotary instrument was compared with 2 commonly used diamond rotary instruments with a larger-grit size (150 and 180µm). The endodontic access was completed in less time with the 126-µm grit-size rotary instrument than with the larger-grit rotary in-
The Journal of Prosthetic Dentistry
SC: Super-coarse
struments, and the failure load of the repaired restoration was lower when larger-grit rotary instruments were used for access preparation. The findings of this study are in agreement with previous reports indicating that adhesive cementation improves the failure resistance of ceramic restorations.13,14 When comparing Groups M-C and F-C (control groups), the adhesively bonded group (M-C) sustained significantly higher loads to failure than the conventionally cemented group (F-Z). The failure mode in all specimens of Group M-C was a combined failure (crown and resin die), while in Group F-C, both failure modes were equally observed. The combined failure mode (crown and resin die), observed in all specimens of Groups M-C, M-ZR, and M-SC, demonstrated how an adhesive interphase can increase the loadbearing capability of ceramic restora-
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November 2011 tions by transmitting the loads to the supporting structure. Four specimens from Group M-CRF demonstrated failures in the crown only, resembling the conventionally cemented groups. It is suggested that the larger-grit rotary instrument used in Group M-CRF disrupted the adhesive interphase in some specimens, resulting in the same failure mode observed with conventional cementation. An efficient grinding instrument with a smallergrit size will not only cause less damage to the restoration but may also maintain the integrity of the adhesive interphase. Two studies have compared the strength of ceramic restorations with and without an endodontic access.11,12 The first study evaluated feldspathic ceramic restorations, while the second investigated the fracture resistance of alumina-veneered and zirconia-veneered restorations. Both studies reported significantly lower strength values with access preparation than with the intact control. The present study was the first identified by the authors to compare the failure load of lithium disilicate monolithic restorations before and after simulated endodontic access preparation. Unlike the results reported for other ceramics,11,12 simulated endodontic access did not result in reduced failure load in any of the test groups. The difference in findings may have been due to the use of a high efficiency diamond rotary instrument, the high strength of lithium disilicate, or its monolithic design. Despite the fact that conventional in vitro failure tests of fixed restorations do not faithfully reproduce intraoral loading conditions,21 they do offer a controlled environment for comparative evaluation of the variables under investigation. Failure loads measured in this study were in the range of 1900 N to 3500 N, which exceeds the maximum measured occlusal forces previously reported to be in the range of 585 N22 to 740 N23. Static loading under dry conditions is one of the limitations of this
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study and may explain the high failure loads recorded. While wet storage alone reduces failure loads significantly,24 a combination of cyclic loading in the presence of water was reported to produce failure loads that fall within a clinically meaningful range.25 Another limitation of this study was the use of composite resin dies rather than natural teeth. Resin or steel dies have been used in in vitro studies26,27 to standardize the preparation and mechanical properties of the substructure. It has been suggested that the elastic modulus of the supporting structure may influence the fracture susceptibility of ceramic restorations.28,29 The composite resin material used for die fabrication in this study had a modulus of elasticity of 10 GPa as described by the manufacturer—close to the range reported for human dentin.28,30 The slight difference in the elastic modulus of the composite resin dies and human dentin may explain the combined failure observed in most adhesively bonded specimens, a fact which has not been reported clinically. The dry static loading conditions used may further explain the combined failure phenomenon in that they could have initiated ceramic failure at the external surface. This differs from failures observed intraorally.21 That the bond strength between the composite resin cement and the composite resin dies is superior to human dentin is another possible factor resulting in the combined failure mode. Intraoral cyclic loading conditions may degrade such bonds, explaining the lack of this observation in clinical reports. Future research in this area should address dynamic and fatigue loading in wet conditions and possibly use artificial aging to better simulate clinical circumstances.
Conclusions Within the limitation of this study, the following conclusions were drawn: 1. The preparation of an endodontic access with a 126-µm dia-
mond rotary instrument did not alter the failure load values of lithium disilicate restorations, regardless of the cement used. 2. Larger-grit diamond rotary instruments resulted in lower failure loads for the repaired restorations than did smaller-grit high efficiency rotary instruments. 3. Adhesively bonded restorations sustained significantly higher failure loads than those that were conventionally cemented with and without the access preparation.
REFERENCES 1. Cheung GSP. A preliminary investigation into the longevity and causes of failure of single unit extracoronal restorations. J Dent 1991;19:160-3. 2. Goodacre CJ, Bernal G, Rungcharassaeng K, Kan JY. Clinical complications in fixed prosthodontics. J Prosthet Dent 2003;90:31-41. 3. Teplitsky PE, Sutherland JK. Endodontic access of Cerestore crowns. J Endod 1985; 11:555-8. 4. Sutherland JK, Teplitsky PE, Moulding MB. Endodontic access of all-ceramic crowns. J Prosthet Dent 1989;61:146-9. 5. Cohen BD, Wallace JA. Castable glass ceramic crowns and their reaction to endodontic therapy. Oral Surg Oral Med Oral Pathol 1991;72:108-10. 6. Haselton DR, Lloyd PM, Johnson WT. A comparison of the effects of two burs on endodontic access in all-ceramic high lucite crowns. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:486-92 7. Sabourin CR, Flinn BD, Pitts DL, Gatten TL, Johnson JD. A novel method for creating endodontic access preparations through all-ceramic restorations: air abrasion and its effect relative to diamond and carbide bur use. J Endod 2005;31:616-9. 8. Thompson JY, Anusavice KJ, Naman A, Morris HF. Fracture surface characterization of clinically failed all-ceramic crowns. J Dent Res 1994;73:1824-32. 9. Kelly JR. Dental ceramics: current thinking and trends. Dent Clin N Am 2004;48:513-30. 10.Griffith AA. The phenomena of rupture and flow in solids. Phil Trans Roy Sot A 1921;224:163-198. 11.Stokes AN, Hood JA, Casley PB, Cawley RM, Cho GJ. Endodontic access cavities in porcelain jacket crowns--two methods of repair compared. Restorative Dent 1988;4:56-8. 12.Wood KC, Berzins DW, Luo Q, Thompson GA, Toth JM, Nagy WW.. Resistance to fracture of two all-ceramic crown materials following endodontic access. J Prosthet Dent. 2006;95:33-41. 13.Chen J-H, Matsumura H, Atsuta M. Effect of different etching periods on the bond strength of a composite resin to a machinable porcelain. J Dent 1998;26:53-8.
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Volume 106 Issue 5 14.Bindl A, Lüthy H and Mörmann W H. Strength and fracture pattern of monolithic CAD/CAM-generated posterior crowns. Dental Mater 2006;22:29-36. 15.Michanowicz AE, Michanowicz JP. Endodontic access to the pulp chamber via porcelain jacket crowns. Oral Surg Oral Med Oral Pathol 1962;15:1483-8. 16.Davis MW. Providing endodontic care for teeth with ceramic crowns. J Am Dent Assoc 1998;129:1746-7. 17.Galindo DF, Ercoli C, Funkenbusch PD, Greene TD, Moss ME, Lee HJ, et al. Toothpreparation: a study on effect of different variables and a comparison between conventional and channeled burs. J Prosthodont 2004;13:3-16. 18.von Fraunhofer JA, Siegel SC, Feldman S. Handpiece coolant flow rates and dental cutting. Oper Dent 2000;25:544-8. 19.Lloyd BA, Rich JA, Brown WS. Effect of cooling techniques on temperature control and cutting rate for high-speed dental drills. J Dent Res 1978;57:675-84. 20.Huang M, Thompson VP, Rekow ED, Soboyejo WO. Modeling of water absorption induced cracks in resin-based composite supported ceramic layer structures. J Biomed Mater Res 2007;84:124.
21.Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 1999;81:652-661. 22.Kikuchi M, Korioth TW, Hannam AG. The association among occlusal contacts, clenching effort, and bite force distribution in man. J Dent Res 1997;76:1316-25. 23.Gibbs CH, Mahan PE, Lundeen HC, Brehnan K, Walsh EK, Holbrook WB. Occlusal forces during chewing and swallowing as measured by sound transmission. J Prosthet Dent 1981;46:443-9. 24.Kelly JR, Rungruanganunt P, Hunter B, Vailati F. Development of a clinically validated bulk failure test for ceramic crowns. J Prosthet Dent 2010;104:228-238 25.Rekow D, Thompson VP. Engineering long term clinical success of advanced ceramic prostheses. J Mater Sci Mater Med 2007;18:47-56. 26.Castellani D, Baccetti T, Giovannoni A, Bernardini UD. Resistance to fracture of metal ceramic and all-ceramic crowns, Int J Prosthodont 1994;7:149-54. 27.Yoshinari M, Derand T. Fracture strength of all-ceramic crowns. Int J Prosthodont 1994;7:329-338.
28.Wakabayashi N, Anusavice KJ. Crack initiation modes in bilayered alumina/porcelain disks as a function of core/veneer thickness ratio and supporting substrate stiffness. J Dent Res 2000;79:1398-404. 29.Scherrer SS, de Rijk WG. The effect of crown length on the fracture resistance of posterior porcelain and glass-ceramic crowns. Int J Prosth 1992;5:550-7. 30.van Meerbek B, Willems G, Celis JP, Roos JR, Braem M, Lambrechts P, et al. Assessment by nano-indentation of the hardness and elasticity of the resin dentin bonding area. J Dent Res 1993;72:1434-42. Corresponding author: Dr Dana M. Qeblawi 225 Squire Hall, 3435 Main St. Buffalo, NY 14214 Fax: 716-829-2440 E-mail: [email protected] Copyright © 2011 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Clinical performance of cervical restorations--a meta-analysis Heintze SD, Ruffieux C, Rousson V. Dent Mater. 2010;26:993-1000. Objectives. To carry out a meta-analysis in order to assess the influencing factors on retention loss and marginal discoloration of cervical restorations made of composites and glass ionomer (derivates). Methods. The literature was searched for prospective clinical studies on cervical restorations with an observation period of at least 18 months. Results. Fifty clinical studies involving 40 adhesive systems matched the inclusion criteria. On average, 10% of the cervical fillings were lost and 24% exhibited marginal discoloration after 3 years. The variability ranged from 0% to 50% for retention loss and from 0% to 74% for marginal discoloration. Hardly any secondary caries was detected. When linear mixed models with a study and experiment effect were used, the analysis revealed that the adhesive/restorative class had the most significant influence, with 2-step self-etching adhesive systems performing best and 1-step selfetching adhesive systems performing worst; 3-step etch-and-rinse systems, glass ionomers/resin-modified glass ionomers, 2-step etch-and-rinse systems and polyacid-modified resin composites were ranked in between. Restorations placed in teeth whose dentin/enamel had been prepared/roughened showed a statistically significant higher retention rate than those placed in teeth with unprepared dentin (P<0.05). Beveling of the enamel and the type of isolation used (rubberdam/cotton rolls) had no significant influence. Significance. The clinical performance of cervical restorations is significantly influenced by the type of adhesive system used and/or the adhesive class to which the system belonged and whether the dentin/enamel is prepared or not. 2-Step self-etching- and 3-step etch&rinse systems shall be chosen over 1-step self-etching systems and glass ionomer derivates. The dentin (and enamel) surface shall be roughened before placement of the restoration. Reprinted with permission of Quintessence Publishing.
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