The objective of this in vitro study was to visualize and to quantify the marginal and internal fit of heat-pressed ceramic restorations by a novel three-dimensional procedure. Accuracy and reproducibility of the employed measuring method were determined.
An acrylic model of a lower left first molar was prepared to receive a partial crown and duplicated by single step dual viscosity impressions. Corresponding working casts were formed from Type IV die stone and indirect restorations were fabricated from heat-pressable lithium disilicate ceramics (IPS e.max Press, Ivoclar Vivadent AG, Schaan, Liechtenstein). The acrylic tooth model and the ceramic partial crowns were digitized by a structure light scanner with a measurement-uncertainty of 4 μm and subjected to computer-aided quality inspection. Visual discrepancies in marginal and internal fit were displayed with colors. For quantitative analysis, mean quadratic deviations (RMS) were computed and analyzed by Student’s t -test ( n = 5, α = 0.05).
Mean RMS-values for accuracy (reproducibility) ranged from 34 (14) μm for internal areas to 78 (23) μm for marginal surfaces. Differences in accuracy ( p = 0.003) and reproducibility ( p < 0.001) were statistically significant. In general, areas with sharp internal line angles such as occlusal ridges and the preparation finish line exhibited oversized dimensions, whereas areas with rounded and soft internal line angles were undersized.
The viability of a computer-aided and three-dimensional approach for assessing marginal and internal fit of indirect restorations was demonstrated. Thereby, the obtained results track complex form changes as they occur during laboratory processing.
A plethora of ceramic materials suitable for different indications such as inlays, onlays, crowns or fixed partial dentures are commercially available . The clinical success of these restorations is closely connected to their mechanical properties, adequate cementation and bonding, accurate adaption and reasonable esthetics . Thereby, marginal and internal fit to the underlying tooth structure are essential criteria which predetermine the longevity of a ceramic restoration . Since poor adaption might lead to marginal discoloration, exposure of luting resin, dissolution of cement, microleakage, increased plaque retention and secondary decay , a variety of clinical trials and in vitro studies have been conducted to examine marginal and internal gap sizes ( Table 1 ). Acceptable fit-discrepancies have been reported to range from 50 to 150 μm .
|Author||Year||Measuring method/study type||Marginal gap (SD) in μm||Internal gap (SD) in μm|
|Addi et al.||2003||Optical microscopy/in vitro||147 (45)–167 (30)||206 (60)|
|Yeo et al.||2003||Optical microscopy/in vitro||46 (16)||–|
|Quintas et al.||2004||Optical microscopy/in vitro||68 (47)||–|
|Romao et al.||2004||Optical microscopy/in vitro||65 (15)–89 (14)||–|
|Bindl and Mörmann||2005||Scanning electron microscopy/in vitro||44 (23)||105 (53)|
|Stappert et al.||2005||Optical microscopy/in vitro||51 (4)||–|
|Frankenberger et al.||2008||Scanning electron microscopy/in vitro||166–246 (n/a)||–|
|Reich||2008||Scanning electron microscopy/in situ||56 (31)||–|
|Stappert et al.||2008||Optical microscopy/in vitro||54 (4)–61 (4)||–|
|Al-Rabab’ah et al.||2008||Optical microscopy/in vitro||85 (18)||118 (23)|
|Baig et al.||2010||Optical microscopy/in vitro||35 (36)||–|
|Keshvad et al.||2011||Optical microscopy/in vitro||56 (18)||17 (5)|
|Yuksel and Zaimoglu||2011||Optical microscopy/in vitro||93 (10)||–|
Heat-pressable ceramics were developed to decrease inhomogeneities and porosities that usually occurred during conventional sintering . With the introduction of the IPS Empress 2 material and its successor IPS e.max Press (both Ivoclar Vivadent AG, Schaan, Liechtenstein), lithium disilicate crystals embedded into a glassy matrix prevent the propagation of microcracks , thereby providing improved mechanical stability . Thus, contemporary lithium disilicate based ceramic cores may be used for replacing a single second premolar as a pontic in posterior fixed partial dentures . Also, frameworks can be veneered with fluoroapatite porcelain to provide natural semi-translucent esthetics .
Although indirect ceramic restorations undergo complex three-dimensional form changes during their laboratory fabrication process , marginal and internal discrepancies have generally been evaluated in a one- or two-dimensional context. In particular, specimens are sectioned to evaluate internal and marginal gap sizes by either optical or scanning electron microscopy. Common sample sizes range from 5 to 10 specimens per group, with 2–150 different measuring locations, selected in a systematic or random manner. Additionally, a light-bodied vinylpolysiloxane can be injected between the restoration and the underlying die to replicate the cement space. The resulting layer may be digitized by optical systems, evaluated photometrically, or analyzed regarding its density and weight .
As enlarged internal incongruities might lead to incomplete bonding interfaces, that can compromise the integrity of ceramic restorations , precise information on marginal and internal adaption are prerequisite towards long-term clinical success. The objective of this in vitro study was to visualize and to quantify the marginal and internal fit of pressed lithium disilicate partial crowns by a novel three-dimensional procedure. Dimensional differences were examined spatially for the entire surface of a prepared molar. Accuracy and reproducibility of the employed measuring method were determined. The primary null hypothesis was that differences between marginal and internal fit of heat-pressed ceramics were not statistically significant.
Materials and methods
An acrylic model of a lower left first molar (AG-3 ZE 36, Frasaco GmbH, Tettnang, Germany) was prepared to receive a partial crown restoration. A standard set of diamond burs suitable for ceramic preparations (Set 4562, Brasseler GmbH, Lemgo, Germany) was used to achieve controlled tooth-substance removal. The preparation featured a 1.5 mm occlusal height reduction, a 1 mm rounded shoulder finish line of the buccal wall and a 1 mm deep occlusal box. The 3 mm deep approximal grooves were finished with oscillating diamond tips (SONICflex prep ceram, KaVo Dental GmbH, Biberach, Germany) in order to achieve 90° margins as well as rounded and soft internal line angles.
Impression taking and cast fabrication
The prepared tooth was fastened to a typodont (AG-3, Frasaco GmbH) and duplicated by single step dual viscosity impressions, using an elastomeric impression material (Identium, Kettenbach GmbH, Eschenburg, Germany) based on a hybrid chemical formulation of silicone and polyether structures (Vinylsiloxanether). Further details are listed in Table 2 , along with ISO 4823:2000 viscosity designations. All impressions were made at room temperature by a single investigator.
|Material||Mixing ratio||Mixing technique||ISO 4823 type||Working time in s||Setting time in s||Hardness in shore A||Batch number|
The jaw model was mounted on a rectangular base containing three conical guidance pins to allow for standardized tray positioning. Light-bodied impression material was injected on and around the prepared tooth and dispersed with syringe air for approximately 3 s. An individualized tray, filled with heavy-bodied impression material, was gently lowered into the seating position without applying any additional force. To ensure adequate polymerization at room temperature, the impressions were allowed to set three times longer than recommended by the manufacturer . A minimum material thickness of 3 mm around the prepared tooth minimized distortion once the impression was separated . After removal, all impressions were inspected for defects by using 2.7× magnification (starVision SV1, starMed GbR, Munich, Germany). Thereafter, corresponding gypsum casts were formed with Type IV die stone (Tewerock, Kettenbach GmbH). The recommended ratio of 20 mL of distilled water to 100 g of powder was vacuum mixed (Wamix-Classic, Wassermann Dental-Maschinen GmbH, Hamburg, Germany), then slowly vibrated (KV-16, Wassermann Dental-Maschinen GmbH) into the impressions and allowed to set for 45 min before removal and final inspection. Prior to die formation, the impressions were treated with a surfactant (Debubblizer Surfactant, Almore International Inc., Portland, OR, USA) to reduce surface tension and improve the quality of the resulting cast . A consistent amount of surfactant (three spray bursts, approximately 0.5 mL) was dispersed into the impression negatives and remained there for 30 s. Syringe air was used to remove excess surfactant and gently dry the impression surfaces.
Partial crown fabrication
Five partial crowns were fabricated from lithium disilicate glass ceramics (IPS e.max Press, Ivoclar Vivadent AG) using a combination of the lost-wax and heat-press techniques. Glass ceramic ingots (HO 2, Ivoclar Vivadent AG, LOT P30472) were plasticized at 930 °C and vacuum pressed (EP 500 press furnace, Ivoclar Vivadent AG) into an investment mold (IPS PressVEST Speed, Ivoclar Vivadent AG). After a holding time of 25 min the pressed crowns were divested, separated and cleaned by applying 1% hydrofluoric acid (IPS e.max Press Invex Liquid, Ivoclar Vivadent AG) for 10 min. Internal surfaces were sandblasted with 100 μm aluminum oxide at 2 bar pressure. Finally, the restorations were fitted to their corresponding gypsum dies. Therefore, interferences on internal aspects were marked (Fit Checker II, GC Germany GmbH, Bad Homburg, Germany) and systematically removed by water-cooled diamond burs. All restorations were manufactured under supervision by the same dental technician at a commercial laboratory (Böhme Zahntechnik, Jena, Germany).
The prepared tooth and the fabricated ceramics were digitized with a structure light scanner originally developed by the Frauenhofer Institute for Applied Optics and Precision Engineering (Flex 3A, Otto Vision Technology GmbH, Jena, Germany), featuring a measurement-uncertainty of 4 μm and a homogenous measuring-point-distance of 5 μm (data according to manufacturer). Single light bands were projected onto the resin and ceramic surfaces and simultaneously recorded with three high-resolution camera lenses mounted fixed at predefined angles (triangulation angles). Triangulation routines allowed the calculation of data points from the captured images that could be displayed in a common coordinate system. Three data points were combined to form triangles, leading to highly accurate virtual representations of the digitalized objects. Datasets for each restoration and the reference tooth were computed (Qualify 12, Geomagic GmbH, Stuttgart, Germany) and saved in a STL format (Surface Tessellation Language, standard for CAD/CAM data exchange). To compare congruent areas , surface normals of the virtual preparation were inverted. Afterwards, STL records of the preparation and restorations were superimposed one on the other by computing all possible orientations and selecting the one with the best object-to-object penetration (so called Best-Fit-Method, Fig. 1 b ) .
Color-coded difference images were used to examine the congruency of preparation and restorations qualitatively ( Fig. 1 c). Dimensional differences between the ceramic partial crowns and the prepared reference tooth were computed for every data point captured during digitalization. The mean quadratic deviation (root mean square, RMS) was calculated by the following formula:
RMS = ∑ i = 1 n ( x 1 , i − x 2 , i ) 2 n