The influence of finish line curvature on the marginal gap width of ceramic copings

The influence of finish line curvature on the marginal gap width of ceramic copings Chutima Asavapanumas, DDSa and Chalermpol Leevailoj, DDS, MSDb School of Dentistry, Chulalongkorn University, Bangkok, Thailand Statement of problem. As a result of natural tooth anatomy or gingival recession, anterior teeth are more likely to present increased abutment finish line curvature. Purpose. The purpose of this study was to investigate the influence of the curvature of the finish line on the marginal gap widths of ceramic copings. Material and methods. An ivorine maxillary central incisor was prepared for 3 different abutment finish line curvatures (1, 3, and 5 mm). Thirty-six copings were fabricated for each of these curvatures by using Cercon, IPS e.max, and Lava systems. The marginal gap width was measured by using a stereomicroscope, and the data were subsequently analyzed by means of a 2-way ANOVA and a 1-way ANOVA (α=.05). Results. A significantly higher mean marginal gap width was found for the 5-mm curvature group (Cercon, 76.59 ±23.01 µm; IPS e.max, 106.44 ±18.48 µm; Lava, 128.34 ±20.79 µm) than for both the 3-mm curvature group (Cercon, 60.18 ±9.74 µm; IPS e.max, 81.79 ±16.20 µm; Lava, 99.19 ±15.32 µm) and the 1-mm curvature group (Cercon, 38.3 ±6.85 µm; IPS e.max, 52.22 ±10.66 µm; Lava, 69.99 ±6.77 µm). Conclusions. The greater the finish line curvature, the wider the marginal gap widths for the 3 ceramic systems. (J Prosthet Dent 2013;109:226-233)

Clinical Implications

For ceramic restorations, preparations with higher degrees of abutment finish line curvature should be avoided. Supragingival margin design may be considered in an effort to reduce the degree of finish line curvature of abutment teeth. The natural appearance of ceramic restorations has made them the treatment of choice for anterior teeth. However, this advantage must be considered against the possible lack of good marginal adaptation, which is essential for the clinical success and quality of a ceramic restoration.1,2 Insufficient adaptation of restorations may result in an increase in plaque accumulation, ultimately leading to periodontal disease3,4 and secondary caries, which can result in pulpal inflammation.2 Furthermore, exposure of the dental

luting agent at the marginal gap to the oral environment also leads to a rapid increase in cement dissolution,5 a situation which is widely recognized as a major cause of restoration failure. Christensen6 found that in the visually accessible surfaces of a cast restoration, subgingival marginal openings in the range of 39 to 119 µm and supragingival margins of 2 to 51 µm were judged to be clinically acceptable. McLean and von Fraunhofer7 subsequently undertook a 5-year clinical study of 1000 restora-

tions and concluded that 120 µm was the maximum acceptable marginal opening (ranging from 100 to 120 µm); however, Byrne et al8 reported that discrepancies of less than 10 µm were routinely possible. There are several studies reporting the marginal gap widths in different ceramic systems. Balkaya et al9 reported a value of 57 µm as a mean marginal gap widths of In-Ceram Alumina coping. Cho et al10 demonstrated a mean marginal opening of 27.5 µm for IPS e.max Press (Ivoclar

Supported by the Graduate School Thesis Grant, Chulalongkorn University. Graduate student, Esthetic Restorative and Implant Dentistry, Faculty of Dentistry. Associate Professor, Program Director of Esthetic Restorative and Implant Dentistry, Faculty of Dentistry.

a

b

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228

Volume 109 Issue 4 Vivadent, Schaan, Liechtenstein). Several authors2,11-13 reported the mean marginal gap range of 36.6 to 61.94 µm for IPS Empress II (Ivoclar Vivadent) and range of 43.02 to 77.0 µm for Cercon (Ceramco; Dentsply Intl, York, Pa). Two studies from Reigh et al14,15 reported a mean marginal gap width of 65 and 80 µm for fixed dental prostheses made with the Lava system (3M ESPE, St Paul, Minn). Grenade et al16 demonstrated the mean marginal gap width of a single tooth zirconia coping of 51 µm for the Procera computer-aided design and computer-aided manufacturing (CAD/CAM) system. The natural gingival architecture and tooth anatomy of the anterior region leads to the greater likelihood of an abutment preparation with a higher degree of finish line curvature in that region than in the posterior region.17 Furthermore, the labial finish lines of both incisor and canine teeth are often found to be located more apically, a phenomenon attributable to gingival recession. These factors contribute to the need for greater degrees of curvature for abutment teeth in the anterior region. Tao and Han18 and Tao et al19 concluded that the increased finish line curvature of metal ceramic crowns was related to wider marginal gaps, although in both studies, the abutment finish line curvature was found to have no significant influence on the marginal fit of Cercon ceramic CAD/ CAM crowns.18 However, few studies are currently available on the effects of the curvature of the abutment finish line for other ceramic restoration systems. The purpose of the present study was to investigate the influence of the curvature of the finish line on the marginal gap widths of 3 ceramic coping systems at different crown margin locations. The null hypothesis was that the marginal gap width of ceramic copings at different locations of crown margin was not influenced by the abutment finish line curvature or ceramic system.

1 mm

3 mm

5 mm

A

B

C

1 Distal views of 3 types of finish line curvature abutments. A, Distal view of 1-mm finish line curvature abutment. B, Distal view of 3-mm finish line curvature abutment. C, Distal view of 5-mm finish line curvature abutment.

2 mm

1.2 mm 5 mm

2 mm

1.2 mm 5 mm 1.2 mm

5 mm 1.2 mm

2 Labial (left) and distal views (right) of 5- mm curvature metal abutment dimensions.

MATERIAL AND METHODS An ivorine maxillary right central incisor tooth (A5A-500; Acteon Group, Bordeaux, France) was prepared by a single operator for ceramic copings with 3 types of abutment finish line curvatures (1, 3, and 5 mm), as shown in Figure 1. By using a high speed handpiece (SUPERtorque 660B; KaVo Dental Products, Lake Zurich, Ill), a 3-degree diamond rotary cutting instrument (NTI Diamond Instrument Z847KR 016; NTI-Kahla GmbH, Thuringia, Germany) and a tooth preparation guide, the tooth was prepared with a 1.2 mm shoulder margin, 2-mm incisal reduction, 1.5mm labial and axial reduction, and a total occlusal convergence of 6 degrees as illustrated in Figure 2.20 For the 1-mm curvature finish line

The Journal of Prosthetic Dentistry

abutment, the tooth was initially prepared with a buccolingual margin level which was 1 mm apical to the proximal margin level, as shown in Figure 1A; the tooth was then further prepared apically to create 3- and 5-mm curvature abutments, as shown in Figure 1B and 1C, with the proximal margin level remaining unchanged. At every stage, impressions were made of each new preparation design by using a polyether impression material (Impregum Penta Soft; 3M ESPE). A die was subsequently cast in cobalt chromium molybdenum casting alloy (Vitallium Alloy; Dentsply Intl) by using the lost wax technique. For each metal abutment, 36 polyether impressions were made for all 3 types of abutment finish line curvatures; these were poured into molds with a Class IV resin-reinforced die

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April 2013 material (Galaxy; Ultima, Seiches-surle-Loir, France). Twelve of the 36 dies for each type of finish line curvature were used to fabricate copings by using the 3 different types of ceramic systems (Cercon, IPS e.max, and Lava). The sample size of 12 was calculated based on the findings of Tao and Han18 at 95% confidence interval and 90% power. A die was scanned with a CAD scanner unit for the Cercon (Ceramco; Dentsply Intl) and Lava (3M ESPE) system copings, which were fabricated according to the recommendations of the respective manufacturers. The data subsequently were transferred to the CAM procedure for the design of an yttrium tetragonal zirconia polycrystal (Y-TZP) framework. The constant coping margin thickness of 0.4 mm on a 30 µm die spacer was modeled by a single technician from each of the centers (Cercon Center; Bangkok, Thailand and Lava Milling Center; Chon Buri, Thailand). The presintered blank was then milled to fabricate the ceramic coping (Table I). The fabrication of the IPS e.max Press system copings was undertaken in accordance with the manufacturer’s recommendations (IPS e.max Press; Ivoclar Vivadent). A coping with a constant thickness of 0.6 mm was waxed on a die for each type of finish line curvature with a 30 µm die space made with 3 coats of approximately 10-μm thickness of die spacer (Yeti die spacer; Yeti Dental Products GmbH, Engen, Germany), after which each of the wax patterns was evaluated by a single investigator. All of the wax patterns were invested and pressed by a single dental laboratory technician (Dental Art Laboratory, Bangkok, Thailand). After cooling the investment ring to room temperature, the investment was removed by using a separating disk and glass polishing beads. Finally the reaction layer formed during the press procedure was removed by using IPS e.max Press Invex Liquid, and the ceramic copings were cleaned in an ultrasonic cleaner and airborne-particle abraded.

Asavapanumas and Leevailoj

Table I. Fabrication materials of CAD/CAM systems Materials Scanner

CAD/CAM System Cercon Lava Cercon Eye

Lava Scan ST Scanner I

Software design

Cercon Art v2.2

Lava Software Design 5

Milling machine

Cercon Brain

CNC240 Milling Machine

Lot number of zirconia blank

18 000 749

458428

The metal abutments were labeled with measurement points; on the labial and lingual sides, the labels were marked at the most apical finish line point, while on the mesial and distal sides, the labels were marked at the most coronal finish line point. This was then followed by full seating of the specimens on their respective metal abutments by using a digital caliper micrometer (Mitutoyo Corp, Kanagawa, Japan) to stabilize a specimen until the distance between the coping and metal abutment could not be changed. Measurements of the marginal gap were performed by using a ×45 stereomicroscope (ML 9300; MeijiCorp, Nagaya, Japan) and a camera (EOS 100; Canon, Tokyo, Japan) with computer software (Image-Pro Plus, v4.5.11.22; Media Cybernetics, Inc, Bethesda, Md) used to measure the magnified images at the 4 (mesial, distal, labial, and lingual) sites. Each measurement was repeated 3 times, with all measurements being performed by a single investigator, and the mean of the data was calculated. The analysis of the data was undertaken by using statistical software (SPSS program v17.0; SPSS Inc, Chicago, Ill), with the mean marginal gaps of all measurement sites (labial, lingual, mesial, and distal) being analyzed with a 2-way ANOVA test, followed by a 1-way ANOVA and Tamhane T2 post hoc test at α=.05 to determine the statistical significance of the abutment finish line curvatures and ceramic systems. For each ceramic system (Cercon, IPS e.max, and Lava), a 2-way repeated measures ANOVA test was performed, followed

by the Tukey multiple comparison test at α=.05 with statistical software (SigmaStat v3.5; Systat Software Inc, Point Richmond, Calif ) to indicate the statistical significance of the abutment finish line curvatures and the measurement sites.

RESULTS The marginal gaps were measured on 108 copings, with 36 copings for each of the ceramic systems (Cercon, IPS e.max, and Lava). These were further divided into the 3 types of abutment finish line curvature (1, 3, and 5 mm). The 1-sample KolmogorovSmirnov test showed the results to be normally distributed. The 2-way ANOVA results in Table II indicated significant effects for both the finishing line curvature (P<.001) and ceramic system (P<.001); however, no interactions were found between those factors (P=.214). For each of the 3 ceramic systems, significant differences were observed in the mean marginal gaps among the 3 degrees of finish line curvature, as shown in Table III. For each ceramic system, the greater the finish line curvature, the larger the mean marginal gaps observed (the 5-mm group was greater than the 3-mm group which was greater than the 1-mm group). However, no significant differences were observable in the mean marginal gaps between the 3-mm and 5-mm finish line curvatures of the Cercon and IPS e.max copings. Statistically significant differences were found among the mean marginal gaps of the Cercon, IPS e.max, and Lava systems, with the Lava system exhibiting greater mean marginal gap

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Volume 109 Issue 4

Table II. Two-way ANOVA results of significant effects between abutment finishing line curvature and ceramic system. Dependent Variable: Mean marginal gaps

Source

Type III Mean df Sum of Squares Square

Curvature

2

45589

Ceramic system

2

F

P

22794

97.6

<.001

30044

15022

64.3

<.001

1.5

.214

Curvature × Ceramic

4

1382

345

Error

99

23120

234

R2=0.769

Table III. Mean marginal gap widths of coping margins Finishing Line Curvature 1 mm (µm) 3 mm (µm) 5 mm (µm) Ceramic System Mean SD Mean SD Mean SD Cercon

38.30

6.85A

60.18

9.74B,C

76.59 23.01B,C,E,F

IPS e.max

52.22

10.66B

81.79

16.20C,D

106.44 18.48D,F,G

Lava

69.99

6.77C

99.19

15.32D,E

128.34

20.79G

Statistically significant differences in mean marginal gaps were indicated by different upper case letters by using 1-way ANOVA tests, followed by Tamhane T2 post hoc tests.

Table IV. Two-way Repeated Measures ANOVA between finishing line curvature and site of measurement of different ceramic systems. Dependent Variable: Marginal gap widths

Source df

Panel A: Cercon SS MS F

P

Panel B: IPS e.max df SS MS F

P

df

Panel C: Lava SS MS F

P

C

2

35428 17714

18.2

<.001

2

71954 35977

36.1

<.001

2

81710 40855

47.8

<.001

S

3

59043 19681

35.2

<.001

3

5658

1886

2.6

.066

3

28617

9539

12.2

<.001

C×S

6

28984

4831

9.8

<.001

6

10364

1727

2.1

.069

6

18567

3094

6.7

<.001

Error

66 32517

493

66 55076

834

66 30669

465

C=Curvature. S=Site of measurement. SS = Sum of Squares. MS=Mean Square.

widths than both the IPS e.max and Cercon systems (Table III). The marginal gap widths of the 4 measurement locations were analyzed by using the 2-way Repeated Measures ANOVA and the Tukey multiple comparison test. As shown in Table IV, the results in Panels A (Cercon) and C (Lava) revealed significant differences in the marginal gap widths for different finish line curvatures (P<.001) and measurement sites (P<.001) while also showing interaction between fac-

tors (P<.001); however, the results in Panel B (IPS e.max) indicated no significant effects for different measurement sites (P=.066) or interactions between factors (P=.069). For the Cercon (Panel A) and Lava (Panel C) systems, the marginal gap widths at the labial and lingual sites were found to be greater than those at the mesial and distal sites; however, no significant differences were observable among any of the measurement sites in the IPS e.max sys-

The Journal of Prosthetic Dentistry

tem (Panel B), as shown in Table V. There was no significant difference among the mean marginal gap of the 1-, 3-, and 5-mm groups at the mesial and distal margin site of Cercon copings and no difference between the mean marginal gap width of the 3and 5-mm groups at the distal margin site of Lava copings. Statistically significant differences were found among the mean marginal gap widths of the 1-, 3-, and 5-mm groups of IPS e.max copings (Table V).

Asavapanumas and Leevailoj

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April 2013

Table V. Tukey test on marginal gap widths of crown margins for each ceramic system

Finishing Line Curvature 1 mm (µm) 3 mm (µm) 5 mm (µm) Ceramic System Mean SD Mean SD Mean SD Panel A: Cercon Labial

36.40

9.57A,a

77.24

24.17B,b

128.35 53.71D,c

Lingual

50.72

13.90A,d

87.70

34.87B,e

90.80

34.95E,e

Mesial

30.24

7.63

36.59

15.09

C,f

44.77

11.14F,f

Distal

35.82

13.04A,g

39.19

15.85C,g

42.42

17.27F,g

Labial

47.82

16.77G,h

87.31

40.98H,i

132.61

57.27I,j

Lingual

52.09

18.32G,h

88.66

13.67H,i

108.99

37.15I,j

Mesial

56.82

8.67

72.02

29.13

100.09

36.67I,j

Distal

52.16

14.44G,h

79.18

26.48H,i

84.07

15.16I,j

Labial

79.17

17.54J,k

91.16

18.41K,k

162.23 30.04M,l

Lingual

75.69

11.38J,m

130.0

6.88L,n

140.52 31.54M,n

Mesial

62.43

18.79J,o

86.46

17.11K,p

110.93 38.34N,q

Distal

62.68

14.79

89.05

36.39

99.70

A,f

Panel B: IPS e.max

G,h

H,i

Panel C: Lava

J,r

K,s

42.70N,s

Different superscripted upper case letters indicate statistically significant differences in marginal gaps for different measurement sites. Different superscripted lower case letters indicate statistically significant differences in marginal gaps for different abutment finish line curvature based on Tukey multiple comparison tests (P<.05).

DISCUSSION These results revealed statistically significant differences in the marginal gap widths of ceramic copings for different abutment finish line curvatures; thus, the null hypothesis of this study was rejected. This study demonstrated that ceramic crown preparations with greater finish line curvatures had wider marginal discrepancies, a finding similar to those of Tao and Han18 and Tao et al.19 However, statistically significant differences were observed among the marginal gap widths for the different finish line curvatures of the Cercon copings, which contradicted the findings of Tao and Han.18 Possible explanations for the difference between the findings of the Tao and Han18 study and those of the present study include the greater number of specimens used, the use of a ×45

Asavapanumas and Leevailoj

magnification stereomicroscope and the use of a 30-µm die spacer. A number of factors which could give rise to the discrepancies in the greater degree of abutment finish line curvature groups for ceramic copings are noted. The greater volume of ceramics in a greater degree finish line curvature could be related to the larger amount of shrinkage occurring during the firing (IPS e.max) or sintering (Cercon and Lava) fabrication process, thereby creating a greater marginal discrepancy for that group.21 Furthermore, the different margin levels relating to the greater degree abutment finish line curvature copings required much greater accuracy of the longer line of the crown margin than the equal margin level of crown margin copings. Thus, the abutment finish line curvature group, with the greatest degree should also exhibit the

highest level of marginal openings. In the present study, regardless of finish line curvature, the CAD/CAM groups were not only found to exhibit a lower mean marginal gap width (38.30 to 76.59 µm for Cercon), but also found to show a larger mean marginal gap width (69.99 to 128.34 µm for Lava) than the heat-pressed group (52.22 to 106.44 µm for IPS e.max). In a recent study undertaken by Korkut et al,11 the CAD/CAM group (43.02 µm) was found to exhibit a lower mean marginal gap width than the heat-pressed group (47.51 µm); however, the in vitro studies by Ural et al12 and Baig et al13 reported greater mean marginal gap widths for the CAD/CAM group (77.10 and 66.4 µm) than for the heat-pressed group (61.94 and 36.6 µm). The Lava and Cercon copings were fabricated by using the CAD/CAM technique, but there were differences between these 2 ceramic brands. A laser scanner (Cercon brain) was used for the vertically scanned Cercon copings, whereas an optical 3D (Scanner ST) scanner using a triangulation scanning approach with a fringe pattern was used for the Lava coping. The software design, milling machine, and composition of the zirconia blank were also different (Table I). Thus, different results were obtained for the 2 types of CAD/CAM ceramic copings in this study. A comparison between the mean marginal gap widths at the 4 measurement locations (labial, lingual, mesial, and distal) showed that the Cercon and Lava copings (CAD/CAM technique) exhibited greater marginal gap widths at both the labial and lingual sites than those at the mesial and distal sites. As noted previously, the greater volume and length of ceramics in the labial and lingual sites could be due to a significant amount of sintering shrinkage, leading to a greater marginal discrepancy than that found at the proximal sites.21,22 No significant differences were found in the marginal gap width at the different measurement sites for

232

Volume 109 Issue 4 the heat-press ceramic (IPS e.max) copings. The outcome of the distortion of restorations from the heatpress or lost-wax technique used in the fabrication of IPS e.max copings could be attributed to die spacer thickness,23 the 0.4% shrinkage of the wax pattern,21 or the 0.2% thermal shrinkage of the ceramic coping after casting.24 In the present study, it was expected that the respective setting and thermal expansion of the phosphate-bonded investment, at approximately 0.3% and 0.2%, would compensate for this thermal shrinkage.21 The contraction and expansion of the various materials used in the fabrication process occurred in all directions; thus, no significant differences were observable in the marginal misfits of the IPS e.max copings at the different measurement sites. McLean and von Fraunhofer7 concluded that 120 µm was the maximum acceptable marginal opening (ranging from 100 to 120 µm). According to the findings of the present study, the mean marginal gaps of the 1-mm and 3-mm finish line curvature for the Cercon and IPS e.max ceramics met the acceptable criterion (ranging from 36.4 to 88.7 µm), as did the 1-mm finish line curvature for the Lava ceramics; however, the mean marginal gaps of the 5-mm finish line curvatures of the Cercon and IPS e.max ceramics (marginal gap at labial site) and those of the 3-mm (marginal gap at lingual site) and 5-mm (marginal gap at labial and lingual site) finish line curvatures for the Lava ceramics failed to meet this criterion (ranging from 42.4 to 162.2 µm). In a prior related study,18 no significant differences were found between the marginal gap widths of the ceramic copings and the veneered ceramic crowns, and indeed, Sulaiman et al21 reported that the marginal gap widths of the ceramic copings were the same as the overall fit of a veneered crown. The present study was, therefore, performed without veneering material with the overall goal of reducing the potential errors attributable to the

greater number of steps involved in the crown fabrication process.25 The direct view method was used to perform the repeat seating of the specimens on their respective definitive dies without destroying them with sectioning.26 Since Ural et al12 reported increased marginal discrepancies as a result of the cementation process, which they attributed to hydrostatic pressure, the direct view method was considered more appropriate for use in the present study. Regarding clinical implications, this study revealed that greater increases in marginal discrepancies are to be expected for greater degrees of finish line curvature abutment; thus, any preparations, which conform to higher degrees of abutment finish line curvature in labio-lingual gingival recession, should be avoided. Supragingival margin design may be considered in an effort to reduce the degree of finish line curvature of abutment teeth. This study did have limitations. The use of a digital caliper micrometer could not show the exact seating force. However, the seating force was standardized by a constant number of click turns of the fine adjustment part of a micrometer on all the specimens. Another limitation of this study was because of a limited budget, not all brands could be evaluated. Therefore, only 3 commonly used ceramic systems were tested. Further study of the abutment finish line curvature could evaluate marginal gap width changes for other ceramic systems and would elucidate the influence of finish line curvature on the marginal gap width of complete veneered ceramic crowns.

CONCLUSIONS Within the limitations of this study, the following conclusions from the analysis were drawn: A 5-mm finish line curvature ceramic coping exhibited the greatest marginal gap width values, with lower values being found for the 3-mm and

The Journal of Prosthetic Dentistry

1-mm finish line curvatures. The Cercon copings exhibited the lowest marginal gap width for all levels of finish line curvature, with Lava demonstrating greater marginal gap widths than both the IPS e.max and Cercon copings. The marginal gap widths found in the Cercon and Lava copings at both the labial and lingual sites were greater than those found at the mesial and distal sites; however, no significant differences were observed for each of the measurement sites of the IPS e.max copings.

REFERENCES 1. Naert I, Van der Donck A, Beckers L. Precision of fit and clinical evaluation of all-ceramic full restorations followed between 0.5 and 5 years. J Oral Rehabil 2005;32:51-7. 2. Bindl A, Mormann WH. Marginal and internal fit of all-ceramic CAD/CAM crowncopings on chamfer preparations. J Oral Rehabil 2005;32:441-7. 3. Lang NP, Kiel RA, Anderhalden K. Clinical and microbiological effects of subgingival restorations with overhanging or clinically perfect margins. J Clin Periodontol 1983;10:563-78. 4. Valderhaug J, Heloe LA. Oral hygiene in a group of supervised patients with fixed prostheses. J Periodontol 1977;48:221-4. 5. Jacobs MS, Windeler AS. An investigation of dental luting cement solubility as a function of the marginal gap. J Prosthet Dent 1991;65:436-42. 6. Christensen GJ. Marginal fit of gold inlay castings. J Prosthet Dent 1966;16:297-305. 7. McLean JW, von Fraunhofer JA: The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107-11. 8. Byrne G, Goodacre CJ, Dykema RW, Moore BK. Casting accuracy of high-palladium alloys. J Prosthet Dent 1986;55:297-301. 9. Balkaya MC, Cinar A, Pamuk S.Influence of firing cycles on the margin distortion of 3 all-ceramic crown systems.J Prosthet Dent 2005;93:346-55. 10.Cho SH, Nagy WW, Goodman JT, Solomon E, Koike M. The effect of multiple firings on the marginal integrity of pressable ceramic single crowns. J Prosthet Dent 2012;107:17-23. 11.Korkut L, Cotert HS, Kurtulmus H. Marginal, internal fit and microleakage of zirconia infrastructures: an in-vitro study. Oper Dent 2011;36:72-9. 12.Ural C, Burgaz Y, Saraç D. In vitro evaluation of marginal adaptation in five ceramic restoration fabricating techniques. Quintessence Int 2010;41:585-90. 13.Baig MR, Tan KB, Nicholls JI. Evaluation of the marginal fit of a zirconia ceramic computer-aided machined (CAM) crown system. J Prosthet Dent 2010;104:216-27.

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April 2013 14.Reich S, Wichmann M, Nkenke E, Proeschel P. Clinical fit of all-ceramic three-unit fixed partial dentures, generated with three different CAD/CAM systems. Eur J Oral Sci;113:174-9. 15.Reich SM, Peltz ID, Wichmann M, Estafan DJ. A comparative study of two CEREC software systems in evaluating manufacturing time and accuracy of restorations. Gen Dent 2005;53:195-8. 16.Grenade C, Mainjot A, Vanheusden A. Fit of single tooth zirconia copings: comparison between various manufacturing processes. J Prosthet Dent 2011;105:249-55. 17.Lindhe J, Karring T, Lang NP. Clinical periodontology and implant dentistry. 4th ed. Oxford, UK: Blackwell Munksgaard; 2003. p. 5-8. 18.Tao J, Han D. The effect of finish line curvature on marginal fit of all-ceramic CAD/ CAM crowns and metal-ceramic crowns. Quintessence Int 2009;40:745-52.

19.Tao J, Yoda M, Kimura K, Okuno O. Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature. Dent Mater 2006;22:397-404. 20.Leevailoj C, Platt JA, Cochran MA, Moore BK. In vitro study of fracture incidence and compressive fracture load of all-ceramic crowns cemented with resin-modified glass ionomer and other luting agents. J Prosthet Dent 1998;80:699-707. 21.Sulaiman F, Chai J, Jameson LM, Wozniak WT. A comparison of the marginal fit of InCeram, IPS Empress, and Procera crowns. Int J Prosthodont 1997;10:478-84. 22.Hobo S. Distortion of occlusal porcelain during glazing. J Prosthet Dent 1982;47:154-6. 23.Yeo IS, Yang JH, Lee JB. In vitro marginal fit of three all-ceramic crown systems. J Prosthet Dent 2003;90:459-64. 24.Dong JK, Luthy H, Wohlwend A, Schärer P. Heat-pressed ceramics: technology and strength. Int J Prosthodont 1992;5:9-16.

25.Balkaya MC, Cinar A, Pamuk S. Influence of firing cycles on the marginal distortion of 3 all-ceramic crown systems. J Prosthet Dent 2005;93:346-55. 26.Sorensen JA. A standardized method for determination of crown margin fidelity. J Prosthet Dent 1990;64:18-24. Corresponding author: Dr Chutima Asavapanumas Esthetic Restorative and Implant Dentistry Faculty of Dentistry, Chulalongkorn University 34 Henri-Dunant Rd Patumwan, Bangkok 10330 THAILAND Fax: +66-2255-3058 E-mail: [email protected] Acknowledgments The authors thank The Cercon Center, The Lava Milling Center, and Ivoclar Vivadent, Thailand for support of the ceramic materials. Copyright © 2013 by the Editorial Council for The Journal of Prosthetic Dentistry.

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