Abstract
Objectives
The purpose of the study was to evaluate the influence of preparation design, and ceramic material, masticatory fatigue and fracture resistance of non-retentive all-ceramic full-coverage restorations luted on human mandibular molars.
Methods
Full-coverage occlusal restorations were laboratory fabricated from leucite reinforced glass-ceramic (IPS Empress Esthetic) or lithium disilicate glass-ceramic (IPS e.max Press). For each ceramic material four groups with eight specimens each were randomly assigned. Groups had either a non-retentive, occlusal preparation with chamfer finishing line or straight-beveled finishing line and the preparation was either completely within enamel or within dentin with a finishing line in enamel. Restorations were adhesively luted to the teeth using composite resin. After storage in water for 1 week specimens were cyclic loaded 600,000 times with a weight of 10 kg and additionally, thermocycled 3500 times (5/55 °C) in a masticatory simulator. Surviving specimens were loaded until, fracture in a universal testing machine. Statistical analysis was done using three-way ANOVA.
Results
All specimens survived the masticatory fatigue. Mean fracture resistance ranged from 2895 to 4173 N. Influence of ceramic material on fracture resistance was significant ( p = 0.0001). Lithium disilicate glass-ceramic restorations had higher fracture resistances than leucite reinforced glass-ceramic restorations. Different preparation designs showed no significant influence on fracture resistance ( p = 0.0969). The design of the finishing line did not influence the fracture resistance ( p = 0.9461).
Significance
The fracture resistance of adhesively luted non-retentive full-coverage molar restorations, made of lithium disilicate or leucite reinforced glass-ceramic is promising and seems to permit clinical application.
1
Introduction
An ideal restorative material should satisfy functional as well as esthetic requirements. It should also provide long-term reliability in conjunction with tooth structure preserving methods . With improvements in the biocompatibility, esthetics and in the mechanical properties of ceramic materials, the application of all-ceramic restorations next to conventional metal restorations is justified . Fabrication techniques such as heat pressing or CAD/CAM (computer-assisted design/computer-assisted machining) methods are used to produce even small-sized all-ceramic restorations, such as inlays or onlays, with high fracture resistance . In case of a large carious tooth defect, several studies have well documented the good performance of ceramic onlays and crowns .
In case of occlusal abrasion or malpositioned teeth without caries occlusal restorations might be required. Conventional treatment concepts are based on retentive inlay or partial crown preparations . However, a less invasive non-retentive preparation design might be possible when adhesive luting techniques are used.
Studies evaluated the influence of adhesive surface on fracture resistance of ceramic restorations. Ceramics bonded to enamel with resin showed much higher fracture resistance and bond strength than those bonded to dentin .
No data for non-retentive occlusal full-coverage all-ceramic restorations are available evaluating the influence of the bonding tooth substrate, the design of the finishing line or different classes of glass-ceramics.
The hypothesis of this study was that neither the preparation design nor the bonding tooth substrate nor the type of glass-ceramic will influence the fracture resistance of all-ceramic full-coverage molar restorations. Therefore in this study the masticatory fatigue and fracture resistance of all-ceramic full-coverage molar restorations luted to different bonding surfaces (enamel or dentin with a finishing line within enamel) with different non-retentive preparation designs were evaluated. A leucite reinforced glass-ceramic (IPS Empress Esthetic, Ivoclar-Vivadent, Schaan, Liechtenstein) and a lithium disilicate glass-ceramic (IPS e.max Press, Ivoclar-Vivadent) were used to fabricate the all-ceramic restorations.
2
Materials and methods
2.1
Tooth preparation
Sixty-four extracted caries free and crack free human mandibular molars were cleaned of both calculus deposits and soft tissues and then stored in a 0.1% thymol solution at room temperature. All teeth were fixed in metal rings. First the root portion, 2 mm away from the cemento-enamel junction was coated with an artificial periodontal membrane made of gum resin (Anti-Rutsch-Lack, Wenko-Wenselaar GmbH & Co. KG, Hilden, Germany). Then the teeth were fixed in 15 mm-diameter metal rings, using fast-setting polyester resin (Technovit 4000, Heraeus Kulzer, Wehrheim, Germany), which simulated the human alveolar bone. To avoid the teeth rotating or moving in the metal rings each root was first provided with a 0.9 mm steel-wire before fixing with the resin. During polymerization of the resin specimens were stored in cold water to avoid a thermal impact of the exothermic reaction on the specimens.
Then, teeth were prepared for full-coverage occlusal restorations from a leucite reinforced glass-ceramic (IPS Empress Esthetic) or from a lithium disilicate glass-ceramic (IPS e.max Press). For each ceramic material four groups with eight specimens each were randomly assigned. Groups had either a non-retentive occlusal preparation within enamel (occlusal reduction 0.5 mm) or within dentin with a finishing line in enamel ( Table 1 ). Two different finishing line designs were prepared, a 0.8 mm chamfer finishing line or a straight-beveled finishing line with a 170-degree taper ( Fig. 1 a and b) . To reduce the risk of ceramic fractures all cusps were totally covered and surfaces were smoothened.
| Full-coverage molar restorations from leucite reinforced glass-ceramic (IPS Empress Esthetic, n = 32) | Full-coverage molar restorations from lithium disilicate glass-ceramic (IPS e.max Press, n = 32) | ||||||
|---|---|---|---|---|---|---|---|
| Straight-beveled finishing line ( n = 16) | Chamfer finishing line ( n = 16) | Straight-beveled finishing line ( n = 16) | Chamfer finishing line ( n = 16) | ||||
| Enamel | Dentin | Enamel | Dentin | Enamel | Dentin | Enamel | Dentin |
| 64 specimens | |||||||
| S1-EE | D1-EE | S2-EE | D2-EE | S1-EM | D1-EM | S2-EM | D2-EM |
2.2
Fabrication of the ceramic restorations
A dual-mixed impression technique with light body and heavy body polyether (Permadyne Penta H und L, 3M-Espe, Seefeld, Germany) was used for every prepared molar. After building plaster-samples (GC Fuji Rock EP, Leuven, Belgium) a silicone mould fabricated according to the dimensions of the lower left first molar was used to standardize the wax pattern dimensions, which were 1.5 mm thick at the fissure and 2 mm thick at the cusps. The wax patterns were made of Speedy Injektionswachs 70 and a wax injector (Wachsinjektor 1500) to get 64 identical full anatomic occlusal surfaces. Castable ceramic ingots of the leucite reinforced glass-ceramic and the lithium disilicate glass-ceramic were heated and pressed into an investment mould after the burn out of the wax analogue. Pressing of the ceramic was performed according to the manufacturer’s instructions for each ceramic system. The intaglio and outer surface of the pressed restoration were cleaned with airborne particle abrasion using 100 μm glass polishing beads at a pressure of 2 bar. The lithium disilicate glass-ceramic restorations were then cleaned in a special etching liquid (IPS e.max Press Invex Liquid, Ivoclar-Vivadent) using an ultrasonic cleaning device. A porcelain finishing stone was used for removing excess ceramic at the required areas after checkup with high viscosity silicone (GC Fit Checker, GC, Leuven, Belgium). After cleaning the surfaces again with airborne particle abrasion (100 μm glass polishing beads at pressure of 1 bar) a glazing firing at 725 °C (IPS e.max Press Ceram Glasurpaste, Ivoclar-Vivadent) was performed for each ceramic occlusal surface.
2.3
Adhesive luting
The intaglio surface of the restoration was first cleaned using 36% phosphoric acid etching gel and then etched for 60 s for the leucite reinforced glass-ceramic or 20 s for the lithium disilicate glass-ceramic using 5% hydrofluoric acid etching gel (IPS Keramik Ätzgel, Ivoclar-Vivadent). The etched surface was thoroughly cleaned using water spray for 60 s. Oil free compressed air was used for drying the intaglio surface. Then a silane coupling agent (Monobond S, Ivoclar-Vivadent) was applied immediately to the intaglio surface of each restoration. Liquid two of Optibond FL (adhesive, KerrHawe, Bioggio, Switzerland) was then applied to the surface, dried with oil free compressed air and light cured (Optilux 500, Demetron, USA) for 20 s. The total etching technique was performed according to the instructions of the dentin–adhesive manufacturer. The exposed dentin of the tooth was etched for 15 s, while exposed enamel was etched for 30 s with 36% phosphoric acid etching gel and thoroughly rinsed with water spray for at least 30 s. The preparation surface was dried for 5 s with gentle air. Then liquid one of Optibond FL (primer) was applied with a disposable brush performing a scrubbing motion for 30 s. The surface was then gently air dried for 5 s before liquid two of Optibond FL (adhesive) was applied and light cured for 20 s. Variolink II basis paste and catalyst paste (Ivoclar-Vivadent) were mixed for 10 s then applied to the intaglio surface of the restoration. The ceramic occlusal surface was seated with 10 N to its perspective prepared tooth with a special loading apparatus. The excess luting cement at the margin was removed and an air-inhibiting gel (Liquid Strip, Ivoclar-Vivadent) was applied along the margin to prevent formation of an oxygen inhibited unpolymerized resin layer. Two sides of the tooth were light cured for 60 s. One hour after cementation all specimens were stored in water at 37 °C for 1 week.
2.4
Cyclic loading, mastication simulator and fracture load
All specimens were cyclic loaded 600,000 times with a weight of 10 kg and additionally thermal cycled between 5 and 55 °C in tap water with a 60-s dwell time at each temperature and 3500 times in a computerized masticatory simulator (Willytec Kausimulator, Willytec, Munich, Germany). A loading cycle frequency of 1.2 Hz with a maximum load of 10 kg and a lateral component of 0.3 mm ( Fig. 2 ) was selected to simulate high physiological masticatory forces . The weight was lifted completely from the specimens in each loading cycle and resulted in a maximum peak loading of about 150 N, because of its descending velocity at 1.2 Hz . Steatite ceramic balls (5 mm in diameter, Hoechst Ceram Tec, Wunsiedel, Germany) were used as antagonistic surfaces to simulate the opposite teeth. The position of each specimen was adjusted to ensure that the opposite ceramic ball contacted the triangular ridge of the medio-buccal cusp. All specimens survived the chewing simulator.
For determination of the fracture resistance, a stainless steel bar with a 6 mm in diameter ball end-mounted in a screw driven universal testing machine ( Fig. 3 ) with a stepping motor (Zwick Z010/TN2A, Ulm, Germany) was used to apply compressive load along the long axis of restored teeth at a cross-head speed of 1 mm/min until fracture. The compressive load was centered on the midline fissure of each specimen, so that the load was applied to the triangular ridges of both lingual and facial cusps. The compressive load required to cause fail fracture was recorded in Newton [N] for each specimen. The Shapiro–Wilk-Normality-Test was used to determine a normal distribution. Because of a skewed distribution ( p = 0.0017), all values were transformed to logarithms ( p = 0.14). These logarithmic values were evaluated with three-way ANOVA.
2
Materials and methods
2.1
Tooth preparation
Sixty-four extracted caries free and crack free human mandibular molars were cleaned of both calculus deposits and soft tissues and then stored in a 0.1% thymol solution at room temperature. All teeth were fixed in metal rings. First the root portion, 2 mm away from the cemento-enamel junction was coated with an artificial periodontal membrane made of gum resin (Anti-Rutsch-Lack, Wenko-Wenselaar GmbH & Co. KG, Hilden, Germany). Then the teeth were fixed in 15 mm-diameter metal rings, using fast-setting polyester resin (Technovit 4000, Heraeus Kulzer, Wehrheim, Germany), which simulated the human alveolar bone. To avoid the teeth rotating or moving in the metal rings each root was first provided with a 0.9 mm steel-wire before fixing with the resin. During polymerization of the resin specimens were stored in cold water to avoid a thermal impact of the exothermic reaction on the specimens.
Then, teeth were prepared for full-coverage occlusal restorations from a leucite reinforced glass-ceramic (IPS Empress Esthetic) or from a lithium disilicate glass-ceramic (IPS e.max Press). For each ceramic material four groups with eight specimens each were randomly assigned. Groups had either a non-retentive occlusal preparation within enamel (occlusal reduction 0.5 mm) or within dentin with a finishing line in enamel ( Table 1 ). Two different finishing line designs were prepared, a 0.8 mm chamfer finishing line or a straight-beveled finishing line with a 170-degree taper ( Fig. 1 a and b) . To reduce the risk of ceramic fractures all cusps were totally covered and surfaces were smoothened.
