1

Introduction

CAD/CAM technologies combined with high strength ceramics allow the fabrication of all-ceramic restorations, even the application of multi-span reconstructions in posterior areas. All-ceramic restorations usually consist entirely of porcelain. Alternatively, a high strength ceramic substructure can be used, which requires veneering with porcelain and glazing. The new CAD/CAM milling methods and the introduction of novel zirconia ceramics allow the manufacture of full-zirconia restorations with occlusal design but without subsequent veneering (e.g. Prettau, Zirkonzahn, I; Zeno Zr Bridge, Wieland, G). These zirconia-based restorations achieve good esthetic results, even without veneering. Partially stabilized zirconia substructures provide high hardness, fracture strength, and structural reliability and show a smaller range of strength variations than porcelain . Because the properties of substructure ceramics differ decisively from those of veneering porcelain , different wear behavior should be expected. Particularly mechanical properties, such as hardness, frictional resistance, or fracture toughness, are supposed to strongly influence wear performance .

Oral wear is a complex process, which is influenced by the thickness and hardness of enamel, the chewing behavior in combination with parafunctional habits and neuromuscular forces, as well as the abrasive influence of food and antagonists . Occlusal antagonistic contact is an important reason for wear and the gradual removal of dental material . Wear is caused by plowing of hard asperities into softer surfaces . Different aspects of wear are abrasion, attrition, fatigue, and corrosion: abrasion occurs during the mastication of food serving as a third body , whereas attrition is the result of antagonist contact during mastication, swallowing, and occlusal movements . Fatigue wear is caused by subsurface cracks that proceed due to repeated load cycles, and corrosive wear is a result of chemical reactions . Chewing, clenching, and moisture may cause wear of the ceramic surface, which is an assumed reason for the cracking or chipping of dental surfaces, particularly of the veneering porcelain . Since dental materials ideally achieve wear behavior similar to that of enamel, the wear of dental materials is usually characterized in relation to that of tooth tissues. These considerations imply that natural antagonistic teeth should not be damaged by restorative materials, such as ceramic; however, antagonistic enamel wear has been proven to be higher than that of ceramic under clinical conditions .

Enamel antagonists are required for achieving clinical conditions. However, morphological and structural differences of enamel complicate standardized wear testing. Earlier approaches to standardize enamel cusps by grinding did not reduce the range of wear results compared to those of non-standardized enamel antagonists . For this reason, only the application of identically shaped and structured antagonists, such as spheres, allows a standardization of antagonistic conditions and thus a valid quantification of wear results . Although steatite spheres may not be considered an ideal substituent for human enamel because of their mechanical and tribological properties , their suitability as an antagonist material for in vitro studies on wear resistance has been documented .

For characterizing the complex oral wear situation, clinical tests are essential. However, these in vivo evaluations are expensive and time-consuming, and some variables, such as individual chewing forces or ambient conditions, cannot be sufficiently controlled . In contrast, laboratory tests allow the investigation of the single parameters of wear processes, but the considerable variability even of in vitro wear simulations should be considered . Wear tests only show little correlation with clinical data but present a comparative evaluation of different materials under standardized conditions.

The aim of this in vitro study was to investigate the two-body wear resistance of different ceramics versus steatite and human enamel antagonists. The hypothesis of this study was that zirconia shows higher wear resistance than porcelains but also higher antagonistic wear.

2

Materials and methods

The specimens ( n = 16 per group; diameter 5 mm, thickness 2 mm) were manufactured from different ceramics. For fixation during the two-body wear test, the specimens were embedded in the middle of round alumina stubs (ALU Stubs, Balzers, Walluf, G) using a light-curing dental composite (Tetric Ceram, Ivoclar Vivadent, Schaan, FL). The zirconia materials represented either pre-sintered or hot isostatic pressed (hip) systems ( Table 1 ). The Zeno Zr Bridge zirconia system was used for fabrication without veneering. One zirconia system investigated was glazed, and the glaze was applied directly after polishing (9749M/F, Meisinger, Neuss, D) or sandblasting (120 μm, 2.5 bar). Veneering porcelains were selected in terms of their application on zirconia substructures. Human enamel and the porcelain Vita Omega 900, which is used for porcelain-fused-to-metal restorations, were used as references. Veneering porcelains were glazed with the corresponding glazing material. Details on the ceramic materials used and their processing are listed in Table 1 . Specimens were smoothed with silicon carbide grinding paper under permanent water cooling (grain 500; Buehler, Lake Bluff, USA). Surface roughness was determined before wear testing by means of a profilometer (Perthometer SP6, Perthen-Feinprüf, G; Traversing length (LT): 1.7 mm; standard critical wavelength: 0.25 mm, velocity: 0.1 mm/s, 2 μm diamond indenter).

For simulation of standardized wear, steatite balls (magnesium silicate, n = 8; d = 3 mm, CeramTec, Plochingen, G) were used as antagonists. A mean cusp radius of 1.5 mm was selected for these tests because individual human cusp radii between 0.6 mm and 2–4 mm . Human enamel ( n = 8) served as a reference antagonist for simulating a typical clinical situation. For the preparation of enamel antagonists, human molars (stored in 0.5% chloramine solution for no longer than 4 weeks) were separated into individual cusps. Randomly selected enamel cusps or steatite spheres were embedded in the middle of round alumina stubs (ALU Stubs, Balzers, Walluf, G) using a light-curing dental composite (Tetric Ceram, Ivoclar Vivadent, Schaan, FL). Non-treated antagonists were mounted into the chewing simulator (EGO, Regensburg, G). Specimens were pneumatically loaded in a pin-on-block design with a vertical load of 50 N for 1.2 × 10 ⁵ cycles at a frequency of 1.6 Hz (lateral movement: 1 mm, mouth opening: 2 mm) simulating a human chewing cycle. During wear simulation, specimens were subjected to 600 thermal cycles in distilled water at temperatures of 5 °C and 55 °C for 2 min for each cycle. Permanent thermal cycling with water removed wear debris from the specimens’ surface.

After the wear test, vertical substance loss [μm] of the different ceramics (with steatite and enamel antagonists) was determined with an optical 3D profilometer (Laserscan 3D, Willytec, Munich, G). A standardized wear area of the steatite antagonists [mm ² ] was quantified for evaluating antagonistic wear. Individual morphological and structural differences of human enamel complicate standardized wear testing and may cause high variations of the wear data. Therefore, we refrained from determining the wear areas of the enamel antagonists. Instead, for the qualitative characterization of wear patterns, all specimens and antagonists were subjected to scanning electron microscopy (Quanta FEG 400, FEI Company, Hillsboro, USA) after wear simulation. The surfaces were examined at a magnification of 30–4000 at 10 keV. Damage on enamel antagonists, which was caused by wear tests, was characterized.

Calculations and statistical analysis were carried out with SPSS 17.0 for Windows (SPSS Inc., IL, USA). Means and standard deviations were calculated and analyzed by means of one-way analysis of variance (ANOVA) and the Bonferroni multiple comparison test for post-hoc analysis. The level of significance was set to α = 0.05.

2

Materials and methods

The specimens ( n = 16 per group; diameter 5 mm, thickness 2 mm) were manufactured from different ceramics. For fixation during the two-body wear test, the specimens were embedded in the middle of round alumina stubs (ALU Stubs, Balzers, Walluf, G) using a light-curing dental composite (Tetric Ceram, Ivoclar Vivadent, Schaan, FL). The zirconia materials represented either pre-sintered or hot isostatic pressed (hip) systems ( Table 1 ). The Zeno Zr Bridge zirconia system was used for fabrication without veneering. One zirconia system investigated was glazed, and the glaze was applied directly after polishing (9749M/F, Meisinger, Neuss, D) or sandblasting (120 μm, 2.5 bar). Veneering porcelains were selected in terms of their application on zirconia substructures. Human enamel and the porcelain Vita Omega 900, which is used for porcelain-fused-to-metal restorations, were used as references. Veneering porcelains were glazed with the corresponding glazing material. Details on the ceramic materials used and their processing are listed in Table 1 . Specimens were smoothed with silicon carbide grinding paper under permanent water cooling (grain 500; Buehler, Lake Bluff, USA). Surface roughness was determined before wear testing by means of a profilometer (Perthometer SP6, Perthen-Feinprüf, G; Traversing length (LT): 1.7 mm; standard critical wavelength: 0.25 mm, velocity: 0.1 mm/s, 2 μm diamond indenter).

Table 1

Materials and manufactures (gray: veneering porcelain; white: zirconia; n.i.: no information available).

Ceramic	Glaze	Manufacturer	Type	Composition [wt.%]	Flexural strength [MPa]	Fracture toughness [MPa √m]	E -module [GPa]	Hardness [HV]	Particle size [μm]
Cercon Ceram Kiss	Cercon Ceram Kiss Glaze	DeguDent, Hanau, D	Feldspar	n.i.	n.i.	n.i.	n.i.	n.i.	n.i.
Creation Zi-F	Creation ZI Glaze	Creation Willi Geller, Meiningen, A	Feldspar	SiO 2 65–72; Al 2 O 3 8–10; K 2 O 5–6; Na 2 O 10–12; Li 2 O < 1; CaO 4–6; BaO 0.5–1.5; CeO 2 , CeF 3 < 1; pigments 0.1–3	90	n.i.	n.i.	n.i.	60
Lava Ceram	Lava Ceram Glaze	3M Espe, Seefeld, D	Feldspar	n.i.	100	1.1	80	530	25
Omega 900	Akzent Glaze	Vita Zahnfabrik, Bad Säckingen, D	Feldspar; fine structure	SiO 2 58–62; Al 2 O 3 14–16; K 2 O 8–11; Na 2 O 5–7; CaO 1–2; MgO 0.3–0.8; B 2 O 3 3.5; SnO 2 1–1.5	101	n.i.	n.i.	420	17.6
Ceramill ZI		Amann Girrbach, Pforzheim, D	Y-TZP	ZrO 2 + Y 2 O 3 + HfO 2 ≥ 99; Y 2 O 3 4.5–5.4; HfO 2 < 5; Al 2 O 3 < 0.5; oxides < 0.5	1200 ± 200	n.i.	>200	1300 ± 200	≤0.6
Digizon-A HIP		Amann Girrbach, Pforzheim, D	Y-TZP	ZrO 2 + Y 2 O 3 + HfO 2 = 99; Y 2 O 3 4.5–5.4; HfO 2 < 5; Al 2 O 3 < 0.5; oxides < 0.5	1300	>6	200	1200	0.6
Lava Zirkonoxid		3M Espe, Seefeld, D	Y-TZP	n.i.	>1100	5–10	>205	1250	0.5
Zeno Zr Bridge		Wieland, Pforzheim, D	Y-TZP	ZrO 2 + HfO 2 + Y 2 O 3 ≥ 99; Y 2 O 3 4.5–6; HfO 2 ≤ 5; Al 2 O 3 ≤ 0.5; oxides ≤ 0.5	1100	7	210	1300	n.i.
Cercon Base		DeguDent, Hanau, D	Y-TZP	ZrO 2 , Y 2 O 3 5; HfO 2 < 2; Al 2 O 3 , SiO 2 < 1	1200	n.i.	210	n.i.	n.i.
Cercon Base polished	Cercon Ceram Kiss Glaze
Cercon Base 2.5 bar 100 μm

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