Q U I N T E S S E N C E I N T E R N AT I O N A L

PROSTHODONTICS

Dominik L. Büchi

Marginal and internal fit of curved anterior CAD/CAMmilled zirconia fixed dental prostheses: An in-vitro study Dominik L. Büchi, Dr med dent1/Sabine Ebler, DMD2/Christoph H.F. Hämmerle, DMD, Prof Dr med dent3/ Irena Sailer, Prof Dr med dent 4 Objective: To test whether or not different types of CAD/CAM systems, processing zirconia in the densely and in the pre-sintered stage, lead to differences in the accuracy of 4-unit anterior fixed dental prosthesis (FDP) frameworks, and to evaluate the efficiency. Method and Materials: 40 curved anterior 4-unit FDP frameworks were manufactured with four different CAD/CAM systems: DCS Precident (DCS) (control group), Cercon (DeguDent) (test group 1), Cerec InLab (Sirona) (test group 2), Kavo Everest (Kavo) (test group 3). The DCS System was chosen as the control group because the zirconia frameworks are processed in its densely sintered stage and there is no shrinkage of the zirconia during the manufacturing process. The initial fit of the frameworks was checked and adjusted to a subjectively similar level of accuracy by one dental technician, and the time taken for this was recorded. After cementation, the frameworks were embedded into resin and the abutment teeth were cut in mesiodistal and orobuccal directions in four specimens. The thickness of the cement gap was measured at 50× (internal adaptation) and 200× (marginal adaptation) magnification. The measurement of the accuracy was performed at four sites. Site 1: marginal adaptation, the marginal opening at the point of closest perpendicular approximation between the die and framework margin. Site 2: Internal adaptation at the chamfer. Site 3: Internal adaptation at the axial wall. Site 4: Internal adaptation in the occlusal area. The data

were analyzed descriptively using the ANOVA and Bonferroni/ Dunn tests. Results: The mean marginal adaptation (site 1) of the control group was 107 ± 26 μm; test group 1, 140 ± 26 μm; test group 2, 104 ± 40 μm; and test group 3, 95 ± 31 μm. Test group 1 showed a tendency to exhibit larger marginal gaps than the other groups, however, this difference was only significant when test groups 1 and 3 were compared (P = .0022; Bonferroni/Dunn test). Significantly more time was needed for the adjustment of the frameworks of test group 1 compared to the other test groups and the control group (21.1 min vs 3.8 min) (P < .0001; Bonferroni/Dunn test). For the adjustment of the frameworks of test groups 2 and 3, the same time was needed as for the frameworks of the control group. Conclusions: No differences of the framework accuracy resulting from the different CAM and CAD/CAM procedures were found; however, only after adjustment of the fit by an experienced dental technician. Hence, the influence of a manual correction of the fit was crucial, and the efforts differed for the tested systems. The CAM system led to lower initial accuracy of the frameworks than the CAD/CAM systems, which may be crucial for the dental laboratory. The stage of the zirconia materials used for the different CAD/CAM procedures, ie presintered or densely sintered, exhibited no influence. (Quintessence Int 2014;10:837–846; doi: 10.3290/j.qi.a32565)

Key words: fixed dental prosthesis, internal fit, marginal fit, zirconia 1

Postgraduate Student, Clinic of Fixed and Removable Prosthodontics and Dental Material Science, University of Zurich, Zurich, Switzerland.

2

Clinician, Private Practice, Zürich, Switzerland.

3

Head and Chairman, Clinic of Fixed and Removable Prosthodontics and Dental Material Science, University of Zürich, Zürich, Switzerland.

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4

Chair, Division of Fixed Prosthodontics and Biomaterials, Clinic of Dental Medicine, University of Geneva, Geneva, Switzerland.

Correspondence: Prof Dr Irena Sailer, Division of Fixed Prosthodontics and Biomaterials, University of Geneva, 19 Rue Barthélemy-Menn, 1205 Geneva, Switzerland. Email: [email protected]

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The high-strength ceramic zirconia is well established as a ceramic framework material for fixed dental prostheses (FDPs) in anterior and posterior regions of the jaws.1,2 Numerous studies report very good clinical stability of FDPs with zirconia frameworks.3-5 However, besides these promising findings some studies report problems with the fit of the ceramic frameworks.3,6 These problems were associated with the computeraided fabrication procedures needed to process zirconia.3,7-9 Zirconia frameworks are milled out of prefabricated blanks by means of computer-aided-design/computeraided-manufacturing (CAD/CAM) systems.10 A large variety of CAD/CAM systems for the processing of zirconia is available today.11,12 The originally developed CAD/ CAM systems processed zirconia in its densely sintered stage.13,14 Most of today’s CAD/CAM procedures focus on the processing of zirconia in a soft, pre-sintered stage.15 It may be assumed that the differences in the CAD/ CAM systems lead to differences in the outcomes, more specifically of the clinical accuracy of the frameworks. The findings reported in the literature about the accuracy depending on the milling process, however, are contradictory.11 Some studies show no differences in the accuracy of frameworks made out of densely sintered and pre-sintered zirconia blanks.16 Other investigations show that frameworks made out of the densely sintered zirconia exhibited significantly better accuracy.17,18 The accuracy not only depends on the type of the zirconia blanks used but also on the type of the manufacturing process and the CAD/CAM system itself. The manufacturing processes are different with respect to scanning (laser or structure light scanning or mechanical touch-probe scanning), design software, milling, and the final shrinkage during the sintering process. Besides the fabrication technique, clinical factors such as preparation design, span length, geometry, and localization of the reconstruction may influence the accuracy of the frameworks.11 As previously mentioned, problems with the clinical fit of zirconia-ceramic reconstructions have been

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reported in studies using different CAD/CAM systems.3,6 Hence, the choice of the CAD/CAM procedure appears to be crucial for the clinical success. Furthermore, besides technical and clinical parameters, the time and effort of the dental technician to adjust the frameworks to a subjectively acceptable accuracy may differ between different manufacturing procedures. If the accuracy has to be significantly improved after the milling and/or sintering process, the efficiency, ie, the cost effectiveness of the respective CAD/CAM system, may be low for the laboratory. As yet, no scientific information on the efficiency or cost effectiveness of CAD/CAM systems is available. The aim of this study was to test whether or not different types of CAD/CAM systems, processing zirconia in the densely and in the pre-sintered stages, lead to differences in the accuracy of four-unit anterior FDP frameworks. Furthermore, the efficiency, ie, the time and effort needed by an experienced dental technician to adjust the fit of the different frameworks to a subjectively similar accuracy, was evaluated. The null-hypotheses of the study were: • The type of CAD/CAM system does not exhibit influence on the accuracy of the frameworks. • Processing zirconia in a densely-sintered or presintered stage does not exhibit influence on the accuracy of the frameworks. • The time and effort for the dental technician to adjust the accuracy do not differ between the different CAD/CAM procedures.

METHOD AND MATERIALS Curved anterior four-unit FDP frameworks were manufactured with four different CAD/CAM systems: • DCS Precident (DCS) (control group) • Cercon (DeguDent) (test group 1) • Cerec InLab (Sirona) (test group 2) • Kavo Everest (Kavo) (test group 3). The frameworks made with the DCS Precident system were used as control group because this CAD/CAM system processed zirconia in its densely sintered stage

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and no sintering shrinkage had to be taken into account. All other frameworks (test groups 1 to 3) were milled out of pre-sintered zirconia and then sintered to full density. A study master cast with standardized abutment teeth in the region of the maxillary right central incisor and first premolar (11 and 14 according to FDI notation), and pontics in the region of the maxillary right lateral incisor and canine (12 and 13) was manufactured as follows.

Fabrication of the study master cast At a standardized acrylic model of a human maxilla (Typodont, Frasaco), the resin teeth 11 and 14 were removed and replaced by titanium abutment teeth with a standardized preparation design. These abutment teeth were virtually designed based on the published preparation recommendations for CAM reconstructions,13 and then fabricated out of titanium by means of rapid prototyping. The abutment teeth comprised a 1.2-mm circular chamfer marginal preparation and a total occlusal convergence (TOC) of 8 degrees.14 The titanium abutment teeth were fixed in the acrylic model in the positions 11 and 14 by means of resin (Duralay, Reliance Dental) using a parallelometer for the positioning and fixation during the polymerization of the resin. The teeth 12 and 13 were removed and the “alveolar” regions were filled with resin, representing the pontic areas. In order to duplicate this study master cast for the four tested groups, 10 impressions were made with an A-silicone impression material (Matrix duplicate silicon, Anaxdent). Each of these 10 impressions was poured four times with epoxy resin (Mirapont, Hager & Werken). These 40 identical epoxy resin copies of the original master cast represented the “patients” and were divided into the test and control groups (n = 10). Subsequently, impressions were made of the 40 epoxy “patients” with polyether impression material (Permadyne, 3M Espe), analogous to standard clinical procedures. The polyether impressions were then poured with stone, resulting in 10 stone master casts per group.

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In test group 1 (Cercon system), a class IV resinreinforced die stone (Fujirock, GC) was used for the fabrication of the master casts, according to the manufacturer’s recommendations at the time of the study. In the remaining three groups the master casts were fabricated with scan stone (Cerec stone BC, Sirona). Finally, for each master cast one framework was fabricated as described below.

Control group: DCS Precident The study cast was prepared for scanning according to the manufacturer’s recommendations. The area above the abutment tooth preparation margin was colored with a white color (Caran d’ache), whereas the area below the preparation margin was colored in black (Caran d’ache). With this procedure the contrast between the two areas was increased for scanning. The abutment teeth were subsequently placed in the scanner (DCS Preciscan) and scanned one by one. Then, the entire stone cast was scanned. The frameworks were designed using a CAD software (Precident version 3.43). The thickness of the framework was set at 0.6 mm, the cementation gap at 0.07 mm. Ten analogous frameworks were milled according to this design out of HIP zirconia blanks (TKT Metoxit) (DCS Precimill unit).

Test group 1: Cercon Ten wax models (Inlay wax soft, GC) of the frameworks were manually made on the 10 stone casts. Two layers of die spacer (REF 6590 0001, DeguDent) were applied, resulting in a spacer thickness of 76 μm.19 The thickness of the frameworks was 0.7 mm. Subsequently, each wax framework was fixed in the scanner (Cerconbrain unit) and covered with scan powder (Degussa Dental). A corresponding white-stage zirconia blank (Cerconbase 30, Degussa Dental) was placed in the respective holder of the milling unit (Cerconbrain). The wax framework was digitized with the laser scanner. The corresponding software of the system (Cercon software version 240405) enlarged the digitized data of the framework by approximately 20%.10 The enlarged framework was then milled out of the white-stage zirconia blank. The frameworks were sintered to full density in a furnace

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(CerconHeat) at 1,350°C for 6 hours, whereby the sintering shrinkage of approximately 20% to the desired dimensions occurred.

Test group 2: Cerec The scan stone models were scanned and the framework was designed virtually. The thickness of the framework was also 0.7 mm. The cement layer was set at 10 μm and the setting for the spacer was −70 μm. These values are according to similar studies.17 The frameworks were milled (Cerec inLab milling station) out of the pre-sintered zirconia (e.max ZirCad B 40 blanks, Ivoclar Vivadent) and subsequently sintered to full density.

Test group 3: Kavo Everest The scan stone models were digitized by first scanning the abutment teeth individually and then the entire master cast (Everest scanner). The digitally designed framework was milled in a five-axis milling machine (Everest Engine, Kavo) out of the pre-sintered blanks, and the restorations were sintered to their final dimensions (Everest therm, Kavo) for 6 hours at 1,450°C.

Adjustment of the framework fit The initial fit of the frameworks was checked on the corresponding stone casts by one dental technician highly experienced in CAD/CAM reconstructions. The fit was analyzed using Fit Checker (GC) and a light microscope. The fit was adjusted by removing interfering areas in the frameworks with a diamond bur (Intensiv). This procedure was performed until the subjectively best possible fit was achieved at all frameworks. Care was taken to adjust all frameworks to a subjectively similar level of accuracy. The time the dental technician needed for these procedures was recorded in minutes.

tal) was used according to the manufacturer’s recommendations. During the setting, a standardized force (50 N) was applied to the center of the pontic area with a custom-made device. The excess cement was removed and the marginal region of the FDPs was covered with a glycerine gel (Oxygard, Kuraray).

Measurement procedures The cemented frameworks were embedded into resin (Paladur, clear 7, Heraus Kulzer) to avoid ruptures during the cross-sectioning process (Isomet low speed saw, Buehler). The embedded frameworks were cut in buccooral directions and the pontics were removed. Subsequently, each abutment tooth was cut in mesiodistal and orobuccal directions (Fig 1). The resulting eight specimens per framework were polished prior to the measurements (Silicon carbide paper FEPA, 1200 μm, Struers). The samples were positioned in a light microscope (Axioskop 2, Zeiss) and photographed (Nikon D100) at a 50× (internal adaptation) and 200× (marginal adaptation) magnification. The images were transferred to an imaging data program (Optimas 6.5, Media Cybernetics) for the analyses. The measurement of the marginal and internal adaptation was performed as follows: a series of points was placed on the die and on the internal surface of the frameworks with a distance of 50 μm between the points. These points at the two respective sides were then connected perpendicularly by means of the software of the imaging data program (Optimas 6.5, Media Cybernetics). The distance in between the points representing the gap between the framework and the abutment tooth was assessed automatically in μm (Fig 2). The measurement of the accuracy was performed at four sites (Fig 3).

Statistical analysis Cementation of the frameworks The adjusted frameworks were cemented to the corresponding “patient” models (epoxy models). Prior to the cementation the frameworks were cleaned with air abrasion (aluminum oxide sand for 10 seconds at 1 bar) and ethanol. A resin cement (Panavia 21, Kuraray Den-

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All the data were imported to a statistical program (SPSS 12.0, SPSS) and mean data for each type for each framework were calculated in mesiodistal and buccooral directions. These data were analyzed descriptively using the ANOVA and Bonferroni/Dunn test (StatView 5.0.1, SAS Institute). The level of significance was set at P < .01.

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Fig 1 The frameworks were cemented on the “patient casts” and the sections in bucco-oral and mesiodistal direction were drawn in the zirconia framework on the abutment teeth. In the presented figure a posterior four-unit bridge, investigated in a study with the same study design, is shown.

Fig 1b In this figure the position and the direction of the sections are shown schematically.

Site 4

Site 3

Site 2 Site 1

a

b

Figs 2a and 2b Specimen prepared for the measurement procedure of the cement gap.

Fig 3 The measurement of the accuracy was performed at 4 sites. Site 1: Marginal adaptation, the marginal opening at the point of closest perpendicular approximation between the die and framework margin. Site 2: Internal adaptation at the chamfer. Site 3: Internal adaptation at the axial wall. Site 4: Internal adaptation in the occlusal area.

RESULTS Detailed information on the respective measurements of accuracy is given in Tables 1 to 3. •

Mean adaptation of the frameworks •

Site 1: Marginal adaptation. No difference of the marginal adaptation was found for the test groups 2 and 3 as compared to the control group (Table 1 and Fig 4). Test group 1 showed a tendency to exhibit larger marginal gaps than the other groups; however, this difference was only significant when

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test groups 1 and 3 were compared (P = .0022; Bonferroni/Dunn test) (Table 1 and Fig 4). Site 2: Internal adaptation at the chamfer. Test group 3 exhibited significantly larger gaps at the chamfer compared with test group 2 (P = .0035; Bonferroni/ Dunn test) and the control group (P = .0001; Bonferroni/Dunn test). No further differences were found for the other groups (Table 1 and Fig 4). Site 3: Internal adaptation at the axial wall. Test group 2 exhibited significantly better adaptation at

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Table 1

Descriptive statistics of the marginal and internal gaps (mean ± SD, μm)

Site 1

P value

Site 2

P value

Site 3

P value

Site 4

P value

Cercon

140 ± 26

.0022

86 ± 16

ns

133 ± 27

.0006

115 ± 21

< .0001

Cerec

104 ± 40

ns

84 ± 13

.0035

103 ± 11

.0006

132 ± 14

< .0001

DCS

107 ± 26

ns

73 ± 12

< .0001

123 ± 13

ns

272 ± 29

< .0001

KaVo

95 ± 31

.0022

102 ± 11

< .0001, .0035

114 ± 18

ns

112 ± 14

< .0001

ns, not significant.

Table 2

Descriptive statistics of the mean marginal adaptation (site 1) of the walls facing the pontics and the walls away from the pontics (mean ± SD, μm) Site 1: mean marginal adaptation away from the pontics

P value

Site 1: mean marginal adaptation facing the pontics

P value

Cercon

115 ± 39

ns

132 ± 22

.0019

Cerec

92 ± 42

ns

113 ± 59

ns

DCS

98 ± 36

ns

69 ± 27

.0019

KaVo

88 ± 49

ns

111 ± 48

ns

ns, not significant.

Table 3

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Descriptive statistics of the marginal adaptation (site 1) of the buccal and oral sites of the frameworks (mean ± SD, μm) Site 1: mean marginal adaptation of the oral site of the framework

P value

Site 1: mean marginal adaptation of the buccal site of the framework

P value

Cercon

221 ± 69

.0018

93 ± 26

.0026

Cerec

162 ± 71

ns

51 ± 42

.0007

DCS

151 ± 65

ns

107 ± 32

.0026, .0007

KaVo

122 ± 57

.0018

58 ± 35

ns

Table 4

Time for adjustment of the framework adaptation by the dental technician, recorded in minutes

Framework

Cercon

Cerec

DCS

KaVo

1

20.0

5.0

7.5

4.2

2

20.0

3.0

2.0

2.2

3

18.0

2.5

3.5

12.0

4

25.0

3.0

3.5

8.0

5

30.0

4.0

3.7

6.5

6

12.0

4.0

2.8

4.0

7

22.0

6.0

4.5

13.0

8

25.0

3.5

4.5

6.8

9

24.0

3.0

4.0

12.8

10

15.0

4.0

3.5

2.5

Mean

21.1

3.8

3.95

7.2

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Mean marginal and internal gaps (μm)

Büchi et al

Cercon

Cercec

DCS

KaVo

350 300



250 200 150 100 50

Site 1

Site 2

Site 3

Site 4

Fig 4 Box plot with whiskers and outliners of the mean marginal and internal gaps.

Mean marginal adaptation (μm)

Cercon 220 200 180 160 140 120 100 80 60 40 20 0

Site 1: mean marginal adaptation away from the pontics

Cercec

DCS

KaVo

Site 1: mean marginal adaptation facing the pontics

Fig 5 Box plot with whiskers and outliners of the mean marginal adaptation (site 1) of the walls facing the pontics and the walls away from the pontics.

Mean marginal adaptation (μm)

Cercon

Cercec

DCS

KaVo

350 300 250 200 150 100 50 0

Site 1: mean marginal adaptation of the oral side of the framework

Site 1: mean marginal adaptation of the buccal side of the framework

Fig 6 Box plot with whiskers and outliners of the mean marginal adaptation (site 1) of the buccal and oral sites of the frameworks.

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the axial walls than test group 1 (P = .0006; Bonferroni/Dunn test). The other groups exhibited similar axial adaptation (Table 1 and Fig 4). Site 4: Internal adaptation at the occlusal area. The frameworks of the control group exhibited significantly larger gaps in the occlusal area than all other groups (P < .0001; Bonferroni/Dunn test). All other groups exhibited similar occlusal adaptation (Table 1 and Fig 4).

Comparisons of the mean marginal adaptation (site 1) of the walls facing the pontics and the walls away from the pontics The mean marginal adaptation of the sites facing the pontic was 69 ± 27 μm for the frameworks of the control group (DCS), followed by those of test group 3 (Kavo) (111 ± 48 μm) and test group 2 (Cerec) (113 ± 59 μm). The poorest marginal adaptation showed again in test group 1 (Cercon) (132 ± 22 μm) (Table 2 and Fig 5). The mean marginal adaptation of the sites away from the pontic was 88 ± 49 μm for the frameworks of test group 3 (Kavo), followed by those of test group 2 (Cerec) (92 ± 49 μm) and the control group (DCS) (98 ± 36 μm). The poorest adaptation was shown by test group 1 (Cercon) (115 ± 39 μm) (Table 2 and Fig 5).

Comparisons of the marginal adaptation (site 1) of the buccal and oral sites of the frameworks On the buccal side, the mean marginal adaptation was 51 ± 42 μm for the frameworks of test group 2 (Cerec). The mean marginal adaptation of the frameworks of test group 3 (Kavo) was 58 ± 35 μm, and the mean marginal adaptation of the frameworks of test group 1 (Cercon) was 93 ± 26 μm. The poorest mean marginal adaptation was shown by the frameworks of the control group (DCS) (107 ± 32 μm). On the buccal side the framework fitted better than on the oral side (Table 3 and Fig 6). On the oral side, the mean marginal adaptation was 122 ± 57 μm for the frameworks of test group 3 (Kavo). The mean marginal adaptation of the frameworks of the control group (DCS) was 151 ± 65 μm, and the

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mean marginal adaptation of the frameworks of test group 2 (Cerec) was 162 ± 71 μm. The poorest marginal adaptation was shown by the frameworks of test group 1 (Cercon) (221 ± 69 μm) (Table 3 and Fig 6).

Time for adjustment of the framework adaptation by the dental technician Interesting differences of the times needed for the adjustment of the four types of frameworks to a subjectively similar adaptation were found. Significantly more time was needed for the adjustment of the frameworks of test group 1 compared to the other test groups and the control group (21.1 minutes vs 3.8 minutes) (P < .0001; Bonferroni/Dunn test). For the adjustment of the frameworks of test groups 2 and 3, the same time was needed as for the frameworks of the control group (Table 4). Hence, the time and efforts differ depending on the type of the CAM procedure, which may influence the efficiency for a dental laboratory.

DISCUSSION The present study showed that, after the adjustment of the frameworks, no influence of the CAD/CAM systems on the accuracy was found. All frameworks exhibited similar fit. In this respect, the first hypothesis can only partially be accepted. However, the time the technician needed for the adjustment to this similar fit differed significantly between the CAD/CAM systems. This indicates that the type of CAD/CAM system did influence the initial accuracy of the frameworks. Therefore, the first and third hypotheses are rejected. No clear differences were found when the accuracy of the frameworks resulting out of white-stage processing was compared to that of densely sintered processing. Hence, the second hypothesis is accepted. The present findings indicate that the choice of the CAD/CAM manufacturing procedure for the fabrication of zirconia-based reconstructions has a significant impact on daily clinical practice. As mentioned before, good clinical accuracy is required for predictable longterm success of the reconstructions.20,21 In most cases, if

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a framework fit is poor after manufacturing, the dental technician can improve its accuracy to a clinically acceptable level. However, depending on the initial fit much time may be needed, which in theory increases the costs for the reconstruction. Hence, manufacturing procedures that deliver high initial accuracy are very desirable. The present study showed that in order to achieve similar accuracy of zirconia frameworks made with different CAD/CAM procedures, for some of the manufacturing procedures much effort by the experienced dental technician was needed. As a consequence, the efficiency of the respective systems may be questioned. However, since CAD/CAM systems are in constant development, the presented issues may already be overcome. Future studies are necessary to test this. A recent review of studies testing the accuracy of zirconia frameworks showed that the mean marginal gap of the zirconia frameworks made with different CAD/CAM procedures amounted to 73.8 μm.11 Compared to this general mean value, the frameworks investigated in the present study showed higher values for the marginal gaps. Differences in the fabrication procedures of the frameworks, whether or not the tested frameworks were cemented to the study models prior to the measurement of the fit, and the measurement technique itself have an influence on the results. Unfortunately, most studies of accuracy do not follow a standardized study design. Therefore, a comparison of different laboratory studies of the accuracy of zirconia frameworks is difficult. In the present study, the influence of different CAM and CAD/CAM procedures on the accuracy of the zirconia frameworks was tested and no differences were found after adjustment by the technician. In contrast, a previous study by Bindl and Mormann17 showed that the type of fabrication procedure influenced the accuracy of ceramic frameworks. In this study, CAD/CAM systems provided significantly better marginal and internal fit of the frameworks than CAM systems,17 and Beuer et al22 confirmed these observations. Another study showed no significant differences of the accuracy resulting from different fabrication procedures.12 Inter-

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estingly, in the present study, the technician needed significantly more time to adjust the CAM frameworks (Cercon) to a similar fit to that of the other systems. One of the reasons for the differences between the fabrication systems might be the varying framework designing procedures. With CAD/CAM systems, the design of the frameworks is performed virtually by means of specific computer software, whereas for a CAM system the dental technician manually fabricates a wax-up of the framework on a study model. For this manual fabrication procedure, factors like the amount of die spacer applied on the study cast before the waxup may have a significant impact on the outcome.22 After modeling, the wax framework is scanned and virtualized, which again may have an impact on the resulting framework accuracy. The present study showed no influence of the type of zirconia, densely sintered or pre-sintered white-stage zirconia, on the marginal accuracy of the resulting frameworks. For the systems using pre-sintered zirconia, the frameworks were milled approximately 20% increased in size and subsequently sintered, whereby they shrank to the desired dimensions.10 The resulting accuracy was similar to the frameworks milled in the desired dimensions out of densely sintered zirconia. The recent data indicated that the accuracy of the systems using pre-sintered zirconia was promising and, therefore, the advantages of the simplified milling procedures may be applied in the clinical routine. The findings of studies testing pre-sintered and densely sintered zirconia are controversial. Two recent studies supported the present findings that no significant differences between systems using pre-sintered and densely sintered zirconia blanks occurred with respect to the marginal accuracy.23,24 Bindl and Mormann23 evaluated the marginal gap of single crowns and Reich and coworkers analyzed the marginal discrepancy of three-unit bridges.24 However, two further studies17,18 showed smaller marginal gaps of the frameworks milled out of densely sintered zirconia than out of presintered zirconia. These two studies investigated threeand four-unit frameworks. It might be assumed that the size of the framework is crucial for the outcomes of the

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different manufacturing procedures. This assumption, however, needs to be tested in further studies. Other reasons for differences in accuracy of CAD/ CAM zirconia reconstructions were the preparation design of the abutment tooth, the configuration and span length of the reconstructions, and whether or not porcelain veneering ceramic was applied in the marginal region, as has been shown in different studies.11,25,26 In the present study the abutment teeth were made according to the requirements of the CAD/CAM procedures by means of a rapid prototyping technique. With this, the preparations were standardized. Three studies tested at the effect of increasing the span length on the accuracy of the frameworks.27-29 All showed a tendency for a reduction of accuracy of the frameworks with an increase in span length. A study by Komine et al12 investigated the influence of the configuration of four-unit frameworks (curved anterior vs straight posterior) on the marginal fit and concluded that a straight configuration, where pontics are in the same line to the abutment teeth, showed a better marginal fit compared to curved FDPs. The authors attributed their findings to the shrinkage of pre-sintered zirconia during the sinter process, which influenced the accuracy. As mentioned before, the present study indicated that the adjustment of the accuracy of zirconia frameworks to a “clinically acceptable level” by a dental technician played an important role in the outcome. Significant differences in the times needed for the adjustment were found. To date, there is no evidence in the literature and there are no scientific guidelines on what constitutes a “clinically acceptable” marginal and internal accuracy. Several authors have considered that marginal discrepancies between 100 μm to 120 μm are clinically acceptable.20,21 These studies were retrospective studies and did not test zirconia-based reconstructions. In order to define a clinically acceptable accuracy of zirconia frameworks, randomized controlled clinical trials including the different manufacturing procedures are needed.

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Q U I N T E S S E N C E I N T E R N AT I O N A L Büchi et al

CONCLUSION In the present study no differences in the framework accuracy resulting from the different CAM and CAD/ CAM procedures were found; however, only after adjustment of the fit by an experienced dental technician. Hence, the influence of a manual correction of the fit was crucial, and the efforts differed for the tested systems. The CAM system led to lower initial accuracy of the frameworks than the CAD/CAM systems, which may be crucial for the dental laboratory and daily clinical practice. The stage of the zirconia materials used for the different CAD/CAM procedures, ie, pre-sintered or densely sintered, exhibited no influence.

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VOLUME 45 • NUMBER 10 • NOVEMBER/DECEMBER 2014

CAM-milled zirconia fixed dental prostheses: an in-vitro study.

To test whether or not different types of CAD/CAM systems, processing zirconia in the densely and in the pre-sintered stage, lead to differences in th...
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