Influence of cavity preparation design on fracture resistance of ...

More documents

Recommendations

Info

THE JOURNAL OF PROSTHETIC DENTISTRY SOARES ET AL advocated for posterior ceramic restorations have been based on traditional cast metal restoration <strong>design</strong>s, but with more occlusal tooth reduction and a slightly increased taper. 3 These <strong>preparation</strong>s can involve the removal <strong>of</strong> considerable tooth structure, 13 and as more structure is removed, a tooth will have less resistance to fracture. 14 However, in <strong>preparation</strong>s for posterior ceramic restorations, some authors have demonstrated that occlusal reduction results in a reduced chance <strong>of</strong> restoration failure, likely increasing longevity <strong>of</strong> the restoration. 4,6,15 Fracture resistance tests have been used to determine the forces that may induce fracture <strong>of</strong> such restorations, and thus enable a <strong>preparation</strong> <strong>design</strong> to be suggested for providing greatest resistance to fracture. 10,16-26 Dental ceramics are considered to be esthetic restorative materials with desirable characteristics, such as translucence, fluorescence, and chemical stability. 17,27 They are also biocompatible, have high compressive strength, and their thermal expansion coefficient is similar to that <strong>of</strong> the tooth structure. 27 In spite <strong>of</strong> their many advantages, ceramics are fragile under tensile strain, making them susceptible to fracture during the luting procedure and under occlusal force. 28-31 This dichotomy raises an important question as to which is the best <strong>cavity</strong> <strong>preparation</strong> <strong>design</strong> for posterior teeth restored with ceramic restorations. Therefore, the aim <strong>of</strong> this study was to assess the in vitro resistance to fracture <strong>of</strong> Leucite-reinforced ceramic-restored posterior teeth, and to analyze the modes <strong>of</strong> fracture with various <strong>preparation</strong> <strong>design</strong>s. The null hypothesis was that different <strong>preparation</strong> <strong>design</strong>s have no effect on the fracture resistance <strong>of</strong> teeth restored with Leucite-reinforced ceramics. MATERIAL AND METHODS Fig. 1. Cavity <strong>preparation</strong> <strong>design</strong>s <strong>of</strong> different experimental groups. Ninety freshly extracted, sound, caries-free human mandibular molars <strong>of</strong> similar size and shape were selected by measuring the buccolingual and mesiodistal widths in millimeters, allowing a maximum deviation <strong>of</strong> 10% from the determined mean. Teeth were stored in 0.2% thymol solution. Calculus and s<strong>of</strong>t-tissue deposits were removed with a hand scaler. The teeth were cleaned using a rubber cup and fine pumice water slurry and then stored in 0.9% saline solution at 4°C until completion <strong>of</strong> the experiment. The roots were covered with a 0.3-mm layer <strong>of</strong> a polyether impression material (Impregum; 3M ESPE, St Paul, Minn) to simulate the periodontal ligament, and embedded in a polystyrene resin (Cristal, Piracicaba, Sao Paulo, Brazil) up to 2 mm below the cementoenamel junction to simulate the alveolar bone. 17,24 The teeth were divided into 9 groups (n=10) as follows: IT, intact teeth (control group); CsI, conservative inlay; ExI, extensive inlay; CsO/mb, onlay with conservative isthmus covering the mesiobuccal cusp; ExO/mb, onlay with extensive isthmus covering the mesio-buccal cusp; CsO/b, onlay with conservative isthmus covering all buccal cusps; ExO/ b, onlay with extensive isthmus covering all buccal cusps; CsO/t, onlay with conservative isthmus covering all cusps; ExO/t, onlay with extensive isthmus covering all cusps (Fig. 1). Using a 6-degree taper diamond rotary cutting instrument (3131; KG Sorensen, Barueri, Sao Paulo, Brazil), 8 different <strong>preparation</strong>s, with internal rounded angles, were defined. A <strong>preparation</strong> machine (Federal University <strong>of</strong> Uberlandia, Uberlandia, Minas Gerais, Brazil) was used to standardize <strong>preparation</strong> dimensions (Fig. 2). 17 This device consists <strong>of</strong> a high-speed handpiece (KaVodo Brasil Ltd, Joinville, SC, Brazil) coupled to a mobile base. The mobile base moves vertically and horizontally with the aid <strong>of</strong> 3 micrometers (Mitutoyo, Tokyo, Japan) with a 0.1-mm level <strong>of</strong> accuracy. The isthmus floor <strong>of</strong> the mesio-occluso-distal (MOD) cavities was prepared following principles for ceramic and indirect composite resin <strong>preparation</strong>s. 32 The pulpal floor was prepared to a depth <strong>of</strong> 2.5 mm from the occlusal 422 VOLUME 95 NUMBER 6
SOARES ET AL Fig. 2. Cavity <strong>preparation</strong> instrument. (A) Micrometer controls quantity <strong>of</strong> vertical movement, which is moved by connecting rod, B; (C) metal device allows 180-degree movement around its longitudinal axis and 360-degree movement around its transverse axis; (D) high-speed handpiece with diamond rotary cutting instrument; (E) specimen being prepared; (F) micrometer. cavosurface margin <strong>of</strong> the <strong>preparation</strong>; the occlusal isthmus was 5 mm wide for extensive isthmus groups and 2.5 mm wide for conservative isthmus groups. The buccolingual widths on mesial and distal boxes were similar to the occlusal isthmus width. Each box had a gingival floor depth <strong>of</strong> 1.5 mm mesiodistally and an axial wall height <strong>of</strong> 2 mm. Margins were prepared with 90-degree cavosurface angles. A single-stage impression was made <strong>of</strong> each prepared tooth using a double-viscosity vinyl polysiloxane (Panasil; Kettenbach GmbH, Eschenburg, Germany) in a stock plastic tray (Tigre, Sao Paulo, Brazil). After 2 hours, the impressions were poured with Type IV stone (Velmix; Kerr Italia SpA, Scafati, Italy). One technician fabricated all restorations using a standardized technique and following the manufacturer’s instructions. 27 Restorations were made with a leucite-reinforced ceramic (Cergogold; Degussa Dental, Hanau, Germany). A spacer (Isolit; Degussa Dental) was applied over the high-density stone dies, and a 0.7-mm-thick wax coping (Plastodent; Degussa Dental) was fabricated. The wax coping was invested (Cerg<strong>of</strong>it Investment; Degussa Dental) and placed in a burnout furnace (F1800 1P; EDG, Sao Paulo, Brazil) to eliminate the wax. The burnout furnace was preheated to 270°C, and the temperature was gradually increased to approximately 850°C for 40 minutes with the alumina plunger kept inside the furnace. Ingots (Cergogold, shade A3; Degussa Dental) were pressed in an automatic press furnace (Cerampress Qex; Dentsply Ceramco, York, Pa). After cooling, the specimens were divested using 50-mm glass THE JOURNAL OF PROSTHETIC DENTISTRY Fig. 3. Compressive loading using 6-mm-diameter steel sphere placed in center <strong>of</strong> occlusal leucite-reinforced ceramic molar restoration. beads at 4-bar pressure, followed by airborne-particle abrasion with 100-mm aluminum oxide at 2-bar pressure to remove the refractory material. Finally, the specimens were treated with 100-mm aluminum oxide airborne-particle abrasion at 1-bar pressure. The ceramic restorations were then luted (RelyX ARC; 3M ESPE), following the manufacturer’s instructions. Ceramic inlays were etched with 10% hydr<strong>of</strong>luoric acid (Condicionador de Porcelanas; Dentsply, Sao Paulo, Brazil) for 60 seconds, and then a silane agent (Rely-X ceramic primer; 3M ESPE) was applied for 60 seconds and dried. 27 The <strong>cavity</strong> <strong>preparation</strong>s (enamel and dentin) were etched using 37% phosphoric acid for 15 seconds, rinsed, and blotted dry with absorbent paper. With a fully saturated brush tip, 2 consecutive coats <strong>of</strong> an adhesive system (Adper Single Bond; 3M ESPE) were applied to the tooth, gently dried for 5 seconds with compressed air, and polymerized with a halogen light-polymerization unit (XL 3000; 3M ESPE) for 20 seconds at an intensity <strong>of</strong> 800 mW/cm 2 and a sourceto-specimen distance <strong>of</strong> 1 cm. The resin luting agent was dispensed onto a mixing pad and mixed for 10 seconds. A thin layer <strong>of</strong> the material was applied to the ceramic restoration, which was seated in place. Excess luting agent was removed with a brush. The luting agent was polymerized (800 mw/cm 2 , XL 3000; 3M ESPE) from the facial, lingual, and occlusal directions for 40 seconds in each direction. Finishing rotary cutting instruments (#2135 F and #2135 FF; KG Sorensen) were used to remove excess luting agent. The teeth were subjected to axial compressive loading with a metal sphere 6 mm in diameter (Fig. 3) at a crosshead speed <strong>of</strong> 0.5 mm/min in a universal testing machine (Model DL2000; EMIC, Sao Jose dos Pinhais, Brazil). The force required (N) to cause fracture was JUNE 2006 423
Page 1: Influence
Page 5 and 6: SOARES ET AL Fig. 5. Mean fracture
Page 7 and 8: SOARES ET AL et al, 1 St-Georges et
Page 9: SOARES ET AL Reprint requests to: D

Influence of cavity preparation design on fracture resistance of ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?