1

Introduction

Natural esthetic appearance and longevity are major aims in restoring teeth with single crowns and fixed partial dentures. With increasing popularity of computer-aided design/computer-aided manufacturing (CAD/CAM), a rising number of machinable esthetic materials have been introduced. Currently, the most popular CAD/CAM ceramics for single crowns are lithium disilicate and zirconia. Both materials may be applied either for monolithic restorations or as substructure with subsequent veneering with porcelain for esthetic improvement. Recently, new classes of polymer-infiltrated ceramic-network materials (e.g. Enamic, Vita), resin-nanoceramics (e.g. Lava Ultimate, 3M Espe), and zirconia-reinforced lithium silicate (ZLS) ceramics (e.g. Celtra Duo, DeguDent) have been developed as alternative materials for CAD/CAM single-tooth restorations. While high translucency and high strength at the same time are mutually exclusive for most types of monolithic ceramics, zirconia-reinforced lithium silicate may offer improved esthetics due to increased glass content and a microcrystalline structure, and flexural strength values comparable to lithium disilicate.

While common glass ceramics exhibit lower strength values and require adhesive luting to increase the mechanical strength of the restoration , zirconia and high-strength lithium disilicate ceramics offer the possibility of conventional cementation, e.g. with glass-ionomer cement. Furthermore, self-adhesive resin cements might be a time-saving and less technique sensitive alternative to multi-step adhesive systems. In vitro studies indicated no significant influence of cementation on fracture load of lithium disilicate crowns . Accordingly, clinical data have shown that lithium disilicate crowns were highly successful irrespective of an adhesive or conventional cementation . Cumulative survival rates in clinical studies were reported to be about 97% after 5 years , and about 87% up to 9 years . However, evidence for medium- or even long-term survival of lithium disilicate crowns is limited and neither clinical nor in vitro data are available about the performance of new ZLS ceramics.

Besides esthetics and the resistance to fatigue and fracture, the marginal adaptation contributes to the success of dental crowns. Insufficient marginal adaptation already after incorporation of the prosthetic restoration, or deterioration of the marginal quality during clinical service time might lead to the accumulation of bacterial plaque and can cause gingival inflammation, caries, pulp and periodontal lesions, finally resulting in failure of the restoration . Besides manifold influencing factors like the finishing line configuration, the margin location in enamel or dentin, the fabrication process of the crown and the value of the predefined cementing space, the type of cementation is supposed to influence marginal adaptation and quality .

Prior to routine clinical application, in vitro tests may help to evaluate new materials and restorations by combining reproducible laboratory conditions with basic requirements (occlusal loading, thermocycling) of the clinical situation. In vitro thermal cycling and mechanical loading is supposed to allow a first prediction of the mechanical performance of a new material. A long-term testing might stimulate fatigue failures and marginal degradation. But even in cases without any catastrophic failures, aging and deterioration effects might occur, thus reducing strength, fracture resistance or marginal adaptation.

The hypothesis of this in vitro study was that the type of cementation influences

• (i)

  the in vitro performance,

• (ii)

  the fracture resistance, and

• (iii)

  the marginal adaptation

of single molar ZLS crowns during a simulated five-year oral service.

2

Materials and methods

Human molars (tooth 46, n = 40) were prepared according to ceramic guidelines. A circular and occlusal anatomical reduction of 1.5 mm was carried out with a preparation angle of 4°. The finishing line resulted in a 1 mm deep circular shoulder with rounded inner angles at an isogingival height of the tooth cervix. The resilience of the human periodontium was simulated by coating the roots of the teeth with a 1 mm polyether layer (Impregum, 3 M Espe, G). For achieving a constant layer, the roots were dipped in a wax bath, which was replaced by polyether in a second fabrication process, as previously described . Then the teeth were positioned in resin blocks (Palapress Vario, Heraeus-Kulzer, G).

The preparations of the teeth were digitalized and 32 full-contour crowns of a zirconia-reinforced lithium silicate ceramic (ZLS; Celtra Duo, DeguDent, G) as well as 8 full-contour crowns of a lithium disilicate ceramic (LDS; IPS e.max CAD, Ivoclar-Vivadent, FL) were milled (Cerec MC XL, Sirona, G), crystallized (only LDS), and glazed with the respective glazing materials according to the manufacturer’s instructions. The circular and occlusal wall thickness was 1.5 mm and the cervical wall thickness was 1 mm. ZLS crowns were randomly divided into four groups ( n = 8/group) for cementation with different glass-ionomer cements (GIC), resin, and resin-modified self-adhesive luting materials ( Table 1 ). The LDS crowns served as reference group with adhesive bonding. The inner faces of all crowns were etched with 5% hydrofluoric acid for 20 s (LDS) or 30 s (ZLS) before cementation. For the adhesive luting protocol the crowns were silanized (silane coupling agent Monobond S, 60 s, Ivoclar-Vivadent). The prepared teeth were treated with the bonding system Syntac classic (Syntac Primer/Syntac Adhesive/Heliobond; Ivoclar-Vivadent) according to the instructions of the manufacturer.

A combined thermal cycling and mechanical loading (TCML: 3000 × 5 °C/3000 × 55 °C, 2 min each cycle, H ₂ O dist.; 1.2 × 10 ⁶ cycles à 50 N, 1.6 Hz) with human molar antagonists was performed in a chewing simulator (EGO, Regensburg, G). The crowns were loaded in three-point-contact. Parameters are based on literature data on zirconia and ceramic restorations expressing that chewing simulation using this parameters might simulate a maximum of five years of oral service . During TCML all crowns were controlled for failures. Failed crowns were excluded from further testing and investigated in detail (light-microscope) for failure analysis. After TCML, fracture force of surviving crowns was determined by mechanically loading the crowns to failure in a universal testing machine (Zwick 1446, G). The force was applied on the centre of the crowns using a steel ball ( Ø = 12 mm, v = 1 mm/min) with a 1 mm foil (Dentaurum, Ispringen, G) inserted between crown and ball. The failure determination was set to a 10% loss of the maximum loading force or acoustic signal (crack). Marginal adaptation was determined on resin replica (Rencast CW 2215/HY 5162, Huntsman, Switzerland) that were fabricated by making impressions (Impregum, 3 M Espe) before and after TCML. Both interfaces at cement/tooth and cement/crown were investigated with scanning electron microscopy (working distance: 20.4 mm; voltage: 5–10 keV; low vacuum; magnification: 200×; Quanta FEG 400, FEI Company, Hillsboro, USA). SEM pictures of the margins were evaluated and share of perfect margin was determined. Marginal quality was defined using the following criteria: (i) “intact margin” with smooth transition and no interruption of continuity, and (ii) “marginal gap” showing separation of the components due to cohesive or adhesive failure.

Power calculation (G*Power 3.1.3, Kiel, G) provided an estimated power of >90% using eight specimens per group. Distribution of the data was controlled with Kolmogorov–Smirnov test.

Calculations and statistical analysis were carried out using SPSS 22 (IBM, USA). Mean values and standard deviations (SD) 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. Weibull analysis was done and shape parameter b was calculated (Visual-XSel 12.1, München, G).

2

Materials and methods

Human molars (tooth 46, n = 40) were prepared according to ceramic guidelines. A circular and occlusal anatomical reduction of 1.5 mm was carried out with a preparation angle of 4°. The finishing line resulted in a 1 mm deep circular shoulder with rounded inner angles at an isogingival height of the tooth cervix. The resilience of the human periodontium was simulated by coating the roots of the teeth with a 1 mm polyether layer (Impregum, 3 M Espe, G). For achieving a constant layer, the roots were dipped in a wax bath, which was replaced by polyether in a second fabrication process, as previously described . Then the teeth were positioned in resin blocks (Palapress Vario, Heraeus-Kulzer, G).

The preparations of the teeth were digitalized and 32 full-contour crowns of a zirconia-reinforced lithium silicate ceramic (ZLS; Celtra Duo, DeguDent, G) as well as 8 full-contour crowns of a lithium disilicate ceramic (LDS; IPS e.max CAD, Ivoclar-Vivadent, FL) were milled (Cerec MC XL, Sirona, G), crystallized (only LDS), and glazed with the respective glazing materials according to the manufacturer’s instructions. The circular and occlusal wall thickness was 1.5 mm and the cervical wall thickness was 1 mm. ZLS crowns were randomly divided into four groups ( n = 8/group) for cementation with different glass-ionomer cements (GIC), resin, and resin-modified self-adhesive luting materials ( Table 1 ). The LDS crowns served as reference group with adhesive bonding. The inner faces of all crowns were etched with 5% hydrofluoric acid for 20 s (LDS) or 30 s (ZLS) before cementation. For the adhesive luting protocol the crowns were silanized (silane coupling agent Monobond S, 60 s, Ivoclar-Vivadent). The prepared teeth were treated with the bonding system Syntac classic (Syntac Primer/Syntac Adhesive/Heliobond; Ivoclar-Vivadent) according to the instructions of the manufacturer.

Table 1

Fracture force, Weibull shape parameter b and marginal quality (share of perfect margins) of the different groups of cementation (identical letters indicate no significant differences between groups; identical numbers indicate no significant differences before-after TCML for each material).

Cement	Type	Fracture force Mean ± SD [ N ] Weibull b	Marginal quality [%] before TCML after TCML
			Cement-tooth	Cement-crown
Syntac classic/Variolink II (Ivoclar-Vivadent, FL)	Adhesive	2612 ± 853 x 2.88	99.6 ± 0.8 a1 99.5 ± 1.4 c1	99.8 ± 0.3 b 97.5 ± 3.0 d
Smart Cem 2 (Dentsply, G)	Self-adhesive	1903 ± 438 x 4.43	99.0 ± 1.7 a 96.3 ± 3.4 c	97.2 ± 4.3 b4 95.2 ± 6.8 d4
Aqua Cem (Dentsply, G)	GIC	1848 ± 836 x 2.53	97.5 ± 3.4 a 93.8 ± 5.6 c	92.8 ± 9.0 b5 96.1 ± 4.0 d5
Ketac Cem (3M Espe, USA)	GIC	1891 ± 593 x 3.29	95.2 ± 6.8 a2 94.2 ± 5.7 c2	85.7 ± 10.7 b6 84.7 ± 6.6 d6
IPS e.max CAD; Syntac classic/Variolink II (Ivoclar-Vivadent, FL)	Adhesive (reference)	2528 ± 668 x 4.07	96.6 ± 6.0 a3 97.2 ± 3.9 c3	100.0 ± 0.0 b7 99.5 ± 1.6 d7

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