Marginal and internal fits of fixed dental prostheses zirconia retainers

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Marginal and internal fits of fixed dental prostheses zirconia retainers Florian Beuer a,∗ , Hans Aggstaller a , Daniel Edelhoff a , Wolfgang Gernet a , John Sorensen b a b

Department of Prosthodontics, Ludwig-Maximilians-University, Goethestr. 70, 80336 Munich, Germany Pacific Dental Institute, Portland, OR, USA

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. CAM (computer-aided manufacturing) and CAD (computer-aided design)/CAM

Received 30 August 2007

systems facilitate the use of zirconia substructure materials for all-ceramic fixed partial

Received in revised form

dentures. This in vitro study compared the precision of fit of frameworks milled from semi-

11 April 2008

sintered zirconia blocks that were designed and machined with two CAD/CAM and one CAM

Accepted 15 April 2008

system. Methods. Three-unit posterior fixed dental prostheses (FDP) (n = 10) were fabricated for standardized dies by: a milling center CAD/CAM system (Etkon), a laboratory CAD/CAM system

Keywords:

(Cerec InLab), and a laboratory CAM system (Cercon). After adaptation by a dental techni-

Zirconia

cian, the FDP were cemented on definitive dies, embedded and sectioned. The marginal and

Fixed dental prosthesis

internal fits were measured under an optical microscope at 50× magnification. A one-way

Marginal fit

analysis of variance (ANOVA) was used to compare data (˛ = 0.05).

Internal fit

Results. The mean (S.D.) for the marginal fit and internal fit adaptation were: 29.1 ␮m (14.0)

CAD/CAM

and 62.7 ␮m (18.9) for the milling center system, 56.6 ␮m (19.6) and 73.5 ␮m (20.6) for the

Accuracy

laboratory CAD/CAM system, and 81.4 ␮m (20.3) and 119.2 ␮m (37.5) for the laboratory CAM system. One-way ANOVA showed significant differences between systems for marginal fit (P < 0.001) and internal fit (P < 0.001). Significance. All systems showed marginal gaps below 120 ␮m and were therefore considered clinically acceptable. The CAD/CAM systems were more precise than the CAM system. © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

All-ceramic restorations offer excellent esthetics and were successfully used for restoration of single anterior and posterior teeth [1–6]. Similar to metal–ceramics, all-ceramic fixed dental prostheses (FDP) rely on a high-strength ceramic substructure material to provide resistance to cyclic fatigue loading. Apart from the mechanical properties and aesthetics, the long-term clinical success of all-ceramic prosthodontics can be influenced by marginal discrepancies [7–12]. Poor

∗

marginal adaptation of fixed prostheses increases plaque retention and changes the distribution of the microflora, which can induce the onset of periodontal disease [1,13,14]. Poor marginal fit can also cause secondary caries and lead to clinical failure of fixed prosthodontics [15]. Microleakage from the oral cavity may cause endodontic inflammation [7]. Several clinical studies have shown that zirconia ceramic frameworks have sufficient strength to function as FDP [12,16,17].

Corresponding author. Tel.: +49 89 51609573; fax: +49 89 51609502. E-mail address: fl[email protected] (F. Beuer). 0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2008.04.018

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Computer-aided manufacturing (CAM) of zirconia substructures currently utilizes two different strategies for the type of milling blocks used. The hardness of the zirconia blocks and hence the difficulty in milling the substructure is determined by the degree of sintering of the blocks. Originally, blocks were fully sintered by a process known as hot isostatic pressing (HIP) of the zirconia material. Milling the HIP zirconia has some disadvantages such as high wear rates of the diamond milling burs in the CAM machines and long milling times due to slower feed rates of the densely sintered zirconia. Since no further sintering is necessary and therefore no sintering shrinkage, the marginal fit of these substructures is excellent. The reported values of marginal fit for this technique are 60.4 and 74.0 ␮m [11]. Another study showed that high precision can be achieved using milling devices for densely sintered zirconia [9]. A second method of milling block fabrication utilizes a semi-sintered zirconia material. The semi-sintered material has a chalk-like consistency making it much more rapidly and easily machineable in a CAM unit. After milling, the substructure is sintered to full density [18–20]. The postmilling sintering results in a linear shrinkage in the range of 15–30% [15,21]. The increased milling efficiency of the softer semi-sintered block has the trade-off of a potentially poorer fit from a 20% sintering shrinkage, the scanning process, compensatory software design and milling. There is consensus between various authors that marginal openings below 120 ␮m are clinically acceptable [22–25]. A clinical study showed a mean marginal gap size of 80 ␮m for FDPs fabricated with one computer-aided design (CAD)/CAM system (Lava, 3M ESPE, Seefeld, Germany) that mills from semi-sintered block [21]. Another clinical study on a CAM only system (DCM prototype of Cercon, DeguDent, Hanau, Germany) reported poor marginal fit and a 22% rate of secondary caries after 5 years [12]. Numerous studies have examined the marginal fit of ceramic crowns, but little data is available on the marginal and internal fits of CAD/CAM systems machining semi-sintered zirconia [7,10,11,21,25–30]. The working hypothesis was that there would be significant differences in marginal and internal fits between the three systems evaluated. The purpose of this study was to compare the marginal and internal fits of three milling systems used for fabrication of zirconia substructures, as clinical studies have reported different marginal fits [15,21].

2.

Material and methods

A typodont model with a missing mandibular right first molar was used (Frasaco, Tettnang, Germany). A 1.2-mm, 360◦ chamfer preparation was made on the second premolar and second molar. To control axial reduction, a silicone impression (Optosil, Heraeus Kulzer, Hanau, Germany) was made prior to tooth preparation. Additionally, the provisional crown (Protemp Garant, 3M ESPE) was used to verify the thickness, so the circumferential and occlusal reduction could be quantified (Dial Caliper, Kori Seiki, Tokyo, Japan). The preparation was completed with a surveyor (F1, DeguDent) using a carbide bur (Komet H 356 RGE 103.031, Brasseler GmbH, Lemgo, Germany) to ensure that the preparation had a total taper of

95

8◦ . Thirty impressions were made (Adisil blau, Siladent Dr. Boehme und Schoeps GmbH, Goslar, Germany) with custom impression trays (U3 # 141163 Orbilock, Orbis Dental, Munster, Germany) and poured in class IV resin-reinforced (ISO type IV) die stone (ResinRock, Whip Mix Corp, Louisville, KY). The same investigator made all impressions, and the same experienced technician fabricated all dies. The dies were divided into three groups. The precision of fit of the substructure was measured without veneering porcelain [7]. The amount of internal relief and resulting tightness of fit was controlled with the cement space thickness setting of the design software. The optimal seating for each system was determined in pre-study trial runs as follows: Five retainers were designed with each of the CAD/CAM systems having five different cement spaces. After milling and sintering the retainers were returned to their respective dies and evaluated visually by the investigators. The settings that showed the least space between the preparation margin and the margin of the retainer and did not allow rotation of the abutment on the sectioned die under finger pressure were chosen for the study. The Cerec InLab required a virtual cement space of −20 ␮m, Etkon needed a virtual cement space of 30 ␮m while Cercon had an actual cement space thickness of approximately 20 ␮m.

2.1. Milling center CAD/CAM system (Etkon, Etkon AG, Graefelfing, Germany) Ten definitive dies were digitized and the retainers were designed in the dental laboratory using a CAD-program (etkon visual 3.1, Etkon AG). A wall thickness of 0.5 mm and a cement space of 30 ␮m were chosen for the copings. The data were sent to the milling center, where the enlarged frames were machine milled (IMES Premium, Wieland Imes, Pforzheim, Germany) from presintered zirconia (Xawex G 100, Xawex Dental systems, Ebmatingen, Switzerland). After milling, the copings were sintered in a special furnace (Xawex Sinterofen, Xawex Dental systems) at a temperature of 1400 ◦ C for 10 h. The sintered frames were placed on their definitive dies.

2.2. Laboratory CAD/CAM system (Cerec InLab, Sirona, Bensheim, Germany) Ten sets of solid definitive dies were positioned in the scanning frame with modeling clay (Fullmaterial Kronen, Sirona) according to the manufacturer’s directions. The internal scanner component of the unit digitized the die and the CADdesign was performed (V 2.7 software, Sirona). A retainer wall thickness of 0.5 mm and a cement space of −20 ␮m were used. The enlarged frameworks were milled from semi-sintered zirconia (Vita InCeram YZ-cube, Vita-Zahnfabrik, Bad Säckingen, Germany) under water spray. Sintering was performed in the furnace (Thermo-Star, Thermo-Star GmbH, Aachen, Germany) for 6 h at 1520 ◦ C.

2.3.

Laboratory CAM system (Cercon, DeguDent)

Different from the previous systems this was a CAM system only, where a wax-up had to be made first, then scanned and converted to a digital design. Ten definitive die sets were prepared. Die spacer (InCeram Interspace varnish, Vita-

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Zahnfabrik) was applied to the die surface in two layers to compensate for the inability to use a virtual cement layer and spacer, as used in the CAD/CAM systems. Another spacer (InCeram Interspace Varnish, Vita-Zahnfabrik) was used instead of the system spacer (Cercon spacer, DeguDent), because of its control and better measurability. In unpublished pilot studies, the authors determined that fresh InCeram spacer makes even layers of approximately 10 ␮m thickness while Cercon spacer made layers up to 100 ␮m thick. A coat of varnish was applied on the axial surface of the die from the margin to the occlusal surface; after drying, a second layer was applied to the completed die, beginning 1.5 mm above the preparation margin. This die preparation methodology has proven its suitability in pre-study trials as described for the CAD/CAM systems. After preparation of the definitive die, the frameworks were manually fabricated out of wax (Nawax compact, Yeti Dental Products, Engen, Germany) and marginal fit was evaluated with a microscope (Stemi DV 4, Zeiss, Oberkochen, Germany) at 8× magnification. Two additional examiners evaluated the marginal fit. The examiners were experienced dental technicians who had more than 21 years experience in fabricating wax patterns and more than 8 years experience manufacturing zirconia restorations. Both technicians were calibrated before the present study by dividing crowns with different marginal openings in two groups (clinically acceptable and clinically unacceptable in their opinion). The inter-examiner agreement factor was 90%. If the dental technicians disagreed, a dentist with over 15 years clinical experience made the decision. Achieving uniformity of wall thickness was difficult, although every wax framework was evaluated for uniform thickness of 0.5 mm at 12 predefined points (Dial Caliper, Kori Seiki). To have comparable wax frameworks, a silicone guide was made over the first fabricated pattern using a putty material (Optosil, HeraeusKulzer). The silicone guide was used to evaluate and adapt the following nine wax patterns. The system’s internal laser scanner scanned the wax and the virtually enlarged framework was milled. Finally, the enlarged frameworks were sintered in the furnace (Cercon heat, DeguDent) for 4 h at 1350 ◦ C. All frameworks were examined for deformity and debris, and were steam cleaned (Triton SLA, Bego, Bremen, Germany). Each framework was seated on a definitive die. The frameworks were evaluated on the dies under 8× magnification (Stemi DV 4, Zeiss) for marginal discrepancy. Copings were rejected if they had a margin deemed visually unacceptable by two investigators. Under-contoured frameworks and frameworks that could be rotated on the sectioned dies were also rejected. New copings were fabricated on the same dies. Ten acceptable frameworks for each test group were achieved from a total of 41 frameworks. Five Cercon, three Cerec InLab and three Etkon specimens were rejected and replaced by new retainers. Next, the acceptable frameworks were adapted until the best possible fit was achieved. The adaptation was performed by an experienced technician under 8× magnification (Stemi DV 4, Zeiss) according to the literature [31]. The goal of adaptation was defined as the point when no more improvement of the marginal gap was visible by two or more investigators and retention would be lost if further adjustment was made. To identify areas that needed cor-

Fig. 1 – Embedded specimen during sectioning.

rection, lipstick (Shine Délicieux, L’Oréal, Paris, France) was applied to the definitive die, and the coping was placed without force. The red spots inside the framework were removed by a diamond rotary cutting instrument (Komet 8801014, Brasseler). This procedure was repeated until the marked indicator spots disappeared and a uniform and even contact of the retainer on the die was achieved. After each refinement the color was removed from the die using a steam cleaner (Triton SLA, Bego). The same calibrated dental technician adapted all retainers. Two technicians decided whether more correction would improve the fit; if they disagreed the supervising dentist decided. The time required for adaptation was measured for all specimens and the mean was calculated. To compare the data a one-factorial analysis of variance (ANOVA) and a post hoc test (Student–Newman–Keuls) were carried out. After the adaptation of the FDP to the best possible fit, each framework was cemented on its definitive die with glass ionomer (KetacCem Aplicap, 3M ESPE). The capsule of glass ionomer cement was activated for 2 s (Aplicap Activator, 3M ESPE) and mixed automatically (Rotomix, 3M ESPE) for 10 s. The abutments of the framework were filled (Aplicap Applier, 3M ESPE) with cement, and the excess cement was removed by a disposable brush until the complete surface was coated. The retainer was set back onto the definitive die with finger pressure, and the excess cement was removed. A special cementing device was used to ensure that the pontic was loaded centrally at a force of 50 N [32] for 10 min. The mid-point of the buccal and lingual surfaces of both abutment teeth was marked on the die in order to standardize sectioning. Twentyfour hours after cementation, every framework was embedded into gypsum (ResinRock, Whip Mix Corp). The embedded specimens were sectioned with a circular saw (Accutom 2, Struers, Willich, Germany) (Fig. 1), at the mesial and distal of the pontic, so that only the embedded abutment teeth remained. The pontic was discarded, and the abutment teeth were cut from

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Fig. 2 – Etkon specimen before adaptation. Red lines indicate the cross-sections.

buccal to lingual according to the pencil-lines at the middles of both abutment teeth, thus resulting in two specimens to be evaluated for each framework (Fig. 2). The frameworks were examined at original 50× magnification (Axioskop 2, Zeiss). Four digital images were made of each cross-sectional specimen. One image of a calibration slide was made at the same magnification and used as a reference for calibration at each imaging session. In addition, two images of the marginal area were made at original magnification 200× (Axioskop 2, Zeiss) along with a calibration slide at the same magnification. Photographs were made with a digital camera (S1 Pro, Fuji, Tokyo, Japan) and transferred to the imaging data program (Optimas 6.5, Media Cybernetics, Silver Spring, MD, USA). The measurements were divided into three different areas of interest for better comparisons (Fig. 3). Measurement location chamfer and vertical area (CVA) comprised the chamfer-area and the axial wall, starting near the marginal gap and continuing until the transition with the smallest diameter. Measurement location OA (occlusal area) included the occlusal portion of the coping between the two smallest diameters. A measurement was made every 50 ␮m for groups

Fig. 4 – Marginal gap measurement: example of a photomicrographic (magnification 200×). Closest distance between die (D) and zirconia retainer (R) represents the measured marginal opening.

CVA and OA, resulting in 350 measured points per slice. The measurement was performed using the following method. A series of points was placed manually with the assistance of the computer on the border between the cement and the zirconia framework to establish the first reference line (Fig. 4a and b). A second series of points was set on the border between the cement and the abutment to establish the second reference line (Fig. 4b). The computer program connected two points from one side, and a perpendicular was dropped from a point of the opposite border (Fig. 4c). The length of the perpendicular was the measured cement gap in microns (␮m) (Fig. 4c). The marginal gap (measurement location MG) was defined as the distance between the outermost point of the coping and the outermost point of the preparation. The marginal gap was quantified by connecting these two points. The mean value for each specimen was calculated to generate the database (Figs. 5 and 6). The data were imported into a statistical program (SPSS 14.0, SPSS, Munich, Germany). The mean marginal gap width (measurement location MG), the internal gap (cement gap) widths (Groups CVA and OA) and the standard deviations (S.D.) were calculated. A one-way analysis of variance (ANOVA) was used to detect significant differences between the abutment teeth, the systems and the measurement locations. In addition a post hoc test was carried out for further analysis of the systems used and the measurement locations. The level of significance was set at 5%.

3.

Fig. 3 – Diagram of retainer for three measurement locations: measurement location CV = chamfer and vertical area; measurement location OA = occlusal area; measurement location MG = marginal gap.

Results

The Etkon system specimens showed a mean (S.D.) marginal gap (measurement location MG) of 21.0 (9.2) ␮m for premolars and 37.2 (13.5) ␮m for molars. Cerec InLab specimens had a mean (S.D.) marginal gap (MG) width 46.7 (17.9) for premolars versus 66.4 (16.6) ␮m for molars. The CAM system Cercon

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Fig. 5 – (a) Internal adaptation at measurement location CVA (D: die; G: cement gap; R: zirconia retainer). (b) Internal adaptation at measurement location CVA during the measurement procedure (D: die; R: zirconia retainer). Points for measurement placed at the border die-cement (green) and at the border cement-retainer (red). (c) Internal adaptation at measurement location CVA during the measurement procedure (D: die; R: zirconia retainer; P: perpendicular/measured cement gap in ␮m). Points connected and perpendiculars dropped from the opposite border. Length of the perpendiculars represents the measured cement gap in ␮m.

specimens had a mean marginal gap width of 82.4 (24.6) ␮m for premolars and 80.4 (16.3) for molars at measurement location MG. The results of the ANOVA are shown in Tables 1–3. The detailed information of the mean marginal and internal gap values of Groups MG, CVA and OA are shown in Fig. 2 and

Table 4. The post hoc test indicated significant differences between all systems tested at measurement locations MG (marginal fit) and CVA. Cerec InLab and Etkon specimens were not statistically different at measurement location OA (P = 0.082) but showed a significant difference to the Cercon system.

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Table 1 – Mean gap widths, standard deviations, minimum and maximum in dependency on system, abutment tooth and measurement location System

Abutment tooth

Etkon Etkon Etkon Etkon Etkon Etkon Cerec Inlab Cerec Inlab Cerec Inlab Cerec Inlab Cerec Inlab Cerec Inlab Cercon Cercon Cercon Cercon Cercon Cercon

Measurement location

Premolar Molar Premolar Molar Premolar Molar Premolar Molar Premolar Molar Premolar Molar Premolar Molar Premolar Molar Premolar Molar

Minimum

CVA CVA OA OA MG MG CVA CVA OA OA MG MG CVA CVA OA OA MG MG

27.9 40.9 51.4 55.1 8.4 18.1 43.3 34.2 57.4 64.1 25.8 39.1 58.5 54.8 131.2 84.3 43.1 59.5

Table 2 – One-way ANOVA of measurement locations (MG, CVA, OA) Source d.f. Sum of squares Mean squares F value P value a

Measurement location 2 78667.350 39222.675 38.320 0.000a

Significant at 0.05 level.

Maximum 88.0 93.7 92.8 102.4 34.2 58.2 102.3 105.6 109.8 125.5 75.1 93.8 194.2 153.1 176.0 233.3 107.2 110.1

Mean

standard deviation

52.3 60.7 68.8 81.5 21.0 37.2 64.7 67.8 82.9 93.2 46.7 66.4 106.3 96.2 155.6 154.7 82.4 80.4

19.0 15.1 12.9 15.8 9.2 13.5 14.8 20.7 17.0 18.2 17.9 16.6 27.9 21.6 14.1 44.3 24.6 16.3

The post hoc test showed significant differences between all measurement locations. Adaptation of specimens fabricated by the Etkon system required a mean of 6.9 min (±2.6) to accomplish the defined goal while 20.0 min (±7.1) was needed to adapt the Cerec inLab group. Specimens from the Cercon group needed a mean adaptation time of 34.0 min (±12.9). The ANOVA and the post hoc test showed that time needed for adaptation was statistically different between all three groups (P < 0.001).

Table 3 – One-way ANOVA of system-factor by measurement locations (MG, CVA, OA) Group

Source

d.f.

MG CVA OA

System System System

2 2 2

a

Sum of squares 27371.022 44340.630 73719.343

Mean squares 13685.511 22170.315 36859.671

F value

P value

41.328 53.058 69.855

0.000a 0.000a 0.000a

Significant at 0.05 level.

Table 4 – One-way ANOVA of between-retainer factor (premolar, molar) by measurement locations (MG, CVA, OA) System

Measurement location

Source

d.f.

Sum of squares

Mean squares

F value

P value

Etkon

MG CVA OA

Retainer Retainer Retainer

1 1 1

1323.174 692.740 808.496

1323.174 692.740 808.496

9.891 2.356 3.904

0.006a 0.133 0.064

Cerec InLab

MG CVA OA

Retainer Retainer Retainer

1 1 1

1930.259 95.481 526.461

1930.259 95.481 526.461

6.840 0.295 1.697

0.020a 0.590 0.209

Cercon

MG CVA OA

Retainer Retainer Retainer

1 1 1

20.4662 1005.638 3.611

20.5662 1005.638 3.611

0.047 1.619 0.003

0.830 0.211 0.955

a

Significant at 0.05 level.

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Fig. 6 – Means and standard deviations of marginal gap (measurement area MG) widths and internal gap widths (measurement location CVA and OA).

4.

Discussion

Significant differences were found between the three systems investigated in terms of both marginal and internal fits. Marginal gaps of 1–161 ␮m were reported in the literature for conventionally fabricated all-ceramic crowns [4,25]. In contrast, marginal gaps of 23–74 ␮m were reported for CAD/CAM-fabricated all-ceramic crowns [7,9,11]. The fit of CAD/CAM fabricated FDP frameworks in this study were within the range reported in the literature [7,9,11]. When the two CAD/CAM systems were compared to the CAM system, there were significant differences in mean marginal fits for the CAD/CAM FDP. The long fabrication process and variability in hand fabrication required in the CAM system might cause the differences in the precision: (1) definitive die preparation with spacer, (2) waxing and (3) wax pattern removal from the die. Removal of the wax pattern from the die can cause distortion, negatively affecting the accuracy. Additionally, the scanner must scan the internal aspects of the wax pattern, which is much more difficult to scan than the die. Therefore, it appears there are two main factors to consider in the fit of restorations produced by this CAM system: the skill of the technician and the accuracy of the scanning process. Another reason for the differences in results might be the adaptation of the retainers by the dental laboratory technician. All frameworks were adapted by the same technician and verified by at least two calibrated examiners as being the best possible fit in their opinion. This influence can therefore be considered the minimal unavoidable degree of error inherent to the system. This procedure also reflects the manufacturing process in the dental laboratory and was reported in the literature [31]. The Cercon specimens required significantly more time for adaptation and improvement of fit plus the system had inferior marginal accuracy compared to the CAD/CAM systems. Cerec 3 CAD/CAM all-ceramic full crowns are fabricated using an optical impression technique. In the literature, marginal gaps in the range from 53 to 108 ␮m were reported,

depending on the thickness of the spacer setting [27]. Using the Cerec InLab and Vita InCeram zirconia blanks, singlecrown copings showed a mean marginal gap of 43 ± 23 ␮m, which is comparable to the results of the three-unit FDPs fabricated with Cerec InLab in the present study (57 ± 20 ␮m) [7]. The differences in marginal fit between the three systems were statistically significant. Whether these differences are relevant in the clinical setting is questionable, because the mean marginal gap values and even the maximum values of all three systems were below the recommended clinical limit of 120 ␮m [22–25,33]. However, the marginal gap of the Cercon system was 2.5 greater than the Etkon system. Considering that the tooth preparation was ideal it may be that in the clinical situation the poorer marginal fit of the Cercon system may be compounded. Reflecting this poor fit of the Cercon system a clinical study on posterior FDP recorded 21% secondary caries after 5 years [15]. The internal fit of the Etkon, Cerec InLab, and Cercon specimens were within the range of the values reported in the literature of 49–136 ␮m [7,11,27]. Most authors do not differentiate the internal fit into different measurement locations as in the present study [9,10,24–27]. Measurement locations CVA and OA were reported to influence the mechanical stability of zirconia restorations [34,35]. Thin cement layers (80 ␮m) at measurement location OA have been reported to be more favorable for the mechanical stability of zirconia based restorations [34]. One in vitro study showed that a lack of precision in internal fit may promote higher risks for veneering fracture [36]. Glass–ceramic crowns were reported to show greater compressive strength when the mean CVA was at a gap dimension of 73.0 ␮m while a significantly lower failure strength was observed if the mean gap dimension was increased to 122.0 ␮m [35]. Thicker cement layers as found in the Cercon-group might have negative influences on the clinical performance of the restorations [34–36]. The range of standard deviation was lower (9.6–32.0) than in comparable studies (49.0–68.0) [26]. However, it has to be considered that this study used the cross-sectional technique to obtain the data. This technique might lead to a lack of information concerning the precision of fit. It might be questioned if the measured areas represented the precision of fit of the whole specimen. However, several studies used the crosssection technique to evaluate the precision of fit [7,21,25,37]. The FDP were fabricated under optimal laboratory conditions. Clinically, the fit of all-ceramic restorations is influenced by other factors such as tooth preparation, impression technique and cementation methodology, which have not been evaluated in the present study. The following limitations apply to this study: (1) artificial teeth were prepared ideally; (2) the measurements were performed on the definitive dies, so the error of the transfer from patient to the dental laboratory was not included; (3) the frameworks were adapted by the dental laboratory technician; (4) only one die spacer thickness was evaluated for each system; (5) only one cementation technique was performed. Further studies are needed to evaluate the influence of the scanning process and the milling process on the accuracy of a CAD/CAM restoration as well as the influence of cement

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spacer thickness and cementation technique on the marginal and internal fits of zirconia restorations.

5.

Conclusions

Under the conditions of this study, the following conclusions were drawn: 1. The laboratory fitting and adjustment procedure required a mean time (min) of 6.9 ± 2.6 for Etkon, 20.0 ± 7.1 for Cerec InLab and 34.0 ± 12.9 for Cercon. 2. All systems tested showed marginal gaps within the standard of clinical acceptability. 3. The Etkon system produced the best marginal fit. 4. The Cercon system produced the worst marginal gap, which was 2.5 times larger than the Etkon system.

references

[1] Glauser R, Sailer I, Wohlwend A, Studer S, Schibli M, Scharer P. Experimental zirconia abutments for implant-supported single-tooth restorations in esthetically demanding regions: 4-year results of a prospective clinical study. Int J Prosthodont 2004;17:285–90. [2] Luthardt RG, Holzhuter M, Sandkuhl O, Herold V, Schnapp JD, Kuhlisch E, et al. Reliability and properties of ground Y-TZP-zirconia ceramics. J Dent Res 2002;81:487–91. [3] Nothdurft FP, Pospiech PR. Clinical evaluation of pulpless teeth restored with conventionally cemented zirconia posts: a pilot study. J Prosthet Dent 2006;95:311–4. [4] Schaerer P, Sato T, Wohlwend A. A comparison of the marginal fit of three cast ceramic crown systems. J Prosthet Dent 1988;59:534–42. [5] Sorensen JA, Choi C, Fanuscu MI, Mito WT. IPS Empress crown system: three-year clinical trial results. J Calif Dent Assoc 1998;26:130–6. [6] Sorensen JA, Kang SK, Torres TJ, Knode H. In-Ceram fixed partial dentures: three-year clinical trial results. J Calif Dent Assoc 1998;26:207–14. [7] Bindl A, Mormann WH. Marginal and internal fit of all-ceramic CAD/CAM crown-copings on chamfer preparations. J Oral Rehabil 2005;32:441–7. [8] Cho L, Song H, Koak J, Heo S. Marginal accuracy and fracture strength of ceromer/fiber-reinforced composite crowns: effect of variations in preparation design. J Prosthet Dent 2002;88:388–95. [9] Coli P, Karlsson S. Precision of a CAD/CAM technique for the production of zirconium dioxide copings. Int J Prosthodont 2004;17:577–80. [10] Rinke S, Huls A, Jahn L. Marginal accuracy and fracture strength of conventional and copy-milled all-ceramic crowns. Int J Prosthodont 1995;8:303–10. [11] Tinschert J, Natt G, Mautsch W, Spiekermann H, Anusavice KJ. Marginal fit of alumina- and zirconia-based fixed partial dentures produced by a CAD/CAM system. Oper Dent 2001;26:367–74. [12] Sailer I, Holderegger C, Jung RE, Suter A, Thievent B, Pietrobon N, et al. Clinical study of the color stability of veneering ceramics for zirconia frameworks. Int J Prosthodont 2007;20:263–9. [13] 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.

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[14] Valderhaug J, Heloe LA. Oral hygiene in a group of supervised patients with fixed prostheses. J Periodontol 1977;48:221– 4. [15] Sailer I, Feher A, Filser F, Gauckler LJ, Luthy H, Hammerle CH. Five-year clinical results of zirconia frameworks for posterior fixed partial dentures. Int J Prosthodont 2007;20:383– 8. [16] Raigrodski AJ, Chiche GJ, Potiket N, Hochstedler JL, Mohamed SE, Billiot S, et al. The efficacy of posterior three-unit zirconium-oxide-based ceramic fixed partial dental prostheses: a prospective clinical pilot study. J Prosthet Dent 2006;96:237–44. [17] Vult von Steyern P, Carlson P, Nilner K. All-ceramic fixed partial dentures designed according to the DC-Zirkon technique. A 2-year clinical study. J Oral Rehabil 2005;32:180–7. [18] Curtis AR, Wright AJ, Fleming GJ. The influence of simulated masticatory loading regimes on the bi-axial flexure strength and reliability of a Y-TZP dental ceramic. J Dent 2006;34:317–25. [19] Curtis AR, Wright AJ, Fleming GJ. The influence of surface modification techniques on the performance of a Y-TZP dental ceramic. J Dent 2006;34:195–206. [20] Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L. The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater 1999;15:426–33. [21] 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 2005;113:174–9. [22] Belser UC, MacEntee MI, Richter WA. Fit of three porcelain-fused-to-metal marginal designs in vivo: a scanning electron microscope study. J Prosthet Dent 1985;53:24–9. [23] Karlsson S. The fit of Procera titanium crowns. An in vitro and clinical study. Acta Odontol Scand 1993;51:129–34. [24] McLean JW, von Fraunhofer JA. The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107–11. [25] Sulaiman F, Chai J, Jameson LM, Wozniak WT. A comparison of the marginal fit of In-Ceram, IPS Empress, and Procera crowns. Int J Prosthodont 1997;10:478–84. [26] Behr M, Rosentritt M, Mangelkramer M, Handel G. The influence of different cements on the fracture resistance and marginal adaptation of all-ceramic and fiber-reinforced crowns. Int J Prosthodont 2003;16:538–42. [27] May KB, Russell MM, Razzoog ME, Lang BR. Precision of fit: the Procera AllCeram crown. J Prosthet Dent 1998;80:394–404. [28] Nakamura T, Dei N, Kojima T, Wakabayashi K. Marginal and internal fit of Cerec 3 CAD/CAM all-ceramic crowns. Int J Prosthodont 2003;16:244–8. [29] Pera P, Gilodi S, Bassi F, Carossa S. In vitro marginal adaptation of alumina porcelain ceramic crowns. J Prosthet Dent 1994;72:585–90. [30] Yeo IS, Yang JH, Lee JB. In vitro marginal fit of three all-ceramic crown systems. J Prosthet Dent 2003;90:459– 64. [31] Witkowski S, Komine F, Gerds T. Marginal accuracy of titanium copings fabricated by casting and CAD/CAM techniques. J Prosthet Dent 2006;96:47– 52. [32] Proussaefs P. Crowns cemented on crown preparations lacking geometric resistance form. Part II. Effect of cement. J Prosthodont 2004;13:36–41. [33] McLean JW. Polycarboxylate cements. Five years’ experience in general practice. Br Dent J 1972;132:9–15.

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[34] Rekow ED, Harsono M, Janal M, Thompson VP, Zhang G. Factorial analysis of variables influencing stress in all-ceramic crowns. Dent Mater 2006;22:125–32. [35] Tuntiprawon M, Wilson PR. The effect of cement thickness on the fracture strength of all-ceramic crowns. Aust Dent J 1995;40:17–21.

[36] Rekow D, Thompson VP. Near-surface damage—a persistent problem in crowns obtained by computer-aided design and manufacturing. Proc Inst Mech Eng [H] 2005;219:233–43. [37] Bindl A, Mormann WH. Fit of all-ceramic posterior fixed partial denture frameworks in vitro. Int J Periodontics Restorative Dent 2007;27:567–75.