| BackgroundAll ceramic materials mainly included glass ceramics, the glass infiltration oxide ceramic, dense alumina ceramic and zirconia ceramic. And they present esthetic potential and excellent biocompatibility in common. However, clinical reports showed that the inherent brittleness and low flexural strength of conventional glass and alumina ceramics had been limited to their extensive use. Zirconia-based ceramics have been promoted among dentists and patients as suitable for dental use, due to its excellent strength and superior fracture resistance, enhanced biocompatibility, low radioactivity, and interesting optical properties. The introduction of computer-aided design/computer-aided manufacturing(CAD/CAM) techniques has increased the general acceptance of zirconia in dentistry. However, there is a bad bond between zirconia ceramic and veneering porcelain, veneering porcelain fracture is the most common failure mode of zirconia-based ceramic restorations. Recently researches focus on how to take advantage of the good mechanical property of zirconia, avoid veneering porcelain failure, enhance the bond strength between zriconia ceramic and veneering porcelain. Full-contour zirconia ceramics, with the use of CAD/CAM techniques, are directly made from zirconium oxide materials with the full dental anatomy. Compared to the conventional zirconia based ceramic restorations, there is a monolithic structure of full-contour zirconia restorations, avoiding the veneering porcelain fracture problem. Also, through the stains materials it can achieve aesthetic results. Consequently all-contour zirconia ceramics could be a promising alternative restoration in the future. However, most of studies were about zirconia ceramic material, mechanical properties of the full-contour zirconia restorations is rarely reported.ObjectivesTo build a standard3D preparation model with the use of CAD/CAM techniques. To design and manufacture the mandibular first molar single crown restorations with different thickness on the base of the designed preparation. To test fracture load of all the crowns through a single-load-to-failure test. To study the influence of materials and thickness of the crown on the fracture load. To preliminarily study the suitable thickness of the full-contour zirconia crowns in clinical use.Materials and methodsPart ⅠThe crown size of the mandibular first molar model resin tooth was measured. On the base of the model tooth, the preparation was designed in accordance with the basic requirements of Prosthodontics textbook about tooth preparations. Parameters of the preparation were5°convergence angle,1mm width shoulder,1.5to2.0mm occlusal surface reduction. The designed model file was imported to the CAM milling machine and PMMA preparation dies were fabricated. The dimensions of all final dies were measured to make sure the exact duplication of the standard preparation model as designed.Part Ⅱ1. Full crown and coping model design:On the base of the first part of the designed preparation, the mandibular first molar crown and coping model were generated by the Dental Designer software of the CAD/CAM systems. 2. Fabrication of heat-pressed ceramic crowns:The designed model files were imported to the CAM milling machine and wax crowns were made. According to the IPS e.max press technicians manual, all the10porcelain crown were manufactured.3. Fabrication of full-contour zirconia crown:The same crown model file was imported to the CAM milling machine and the enlarged zirconia crowns were made. After the fully sintered and glazed,10full-contour zirconia crowns were produced.4. Fabrication of zirconia based ceramic crown:The copings were fabricated by the CAD/CAM systems. After fully sintered10zirconia coping were made. And three silicone impressions of the full crown were made. They were cut from mesial side to distal side, labial side to palatal side and occlusal side to gingival side respectively, serving as a guide for porcelain veneering. Then the traditional layering technique was applied to veneer the copings.8points of occlusal surface were checked to ensure the correct and consistent dimensions. Finally the zirconia based ceramic crown were fabricated.5. Fabrication of PFM:The same coping model file were imported to the CAM milling machine, and10wax copings were generated. After invested, melted and casted, the cobalt chromium coping were manufactured. The veneered procedure was identical to the zirconia based crowns. After glazed the production of PFM crowns were completed.6. Cementment and embedding of crowns:The inner surfaces of the all heat-pressed ceramic crowns were etched with5%hydrofluoric acid for20s and silanized for60s. Metal/Zirconia Primer were applied to the all full-contour zirconia crowns and PFM crowns. Then all the crowns were cemented to the preparation dies by Variolink Ⅱ.40crowns were stored in distilled water at37℃for24h after cured completely. The cemented crowns were vertically embedded in self-curing resin, leaving buccal, lingual, and interproximal cervical margins evenly exposed2mm above the potting surface. 7. Single-load-to-failure test:The fracture load of four different materials crowns were tested by the universal testing machine. The results were analyzed by the statistical methods. Fracture load and failure modes of the four kinds of crowns were compared among groups, and the influence of materials on the fracture load were studied.Part IIIWith the use of CAD/CAM systems, on the base of the crown model of the second part, the crown models of four different thickness with the same shape were designed by the Dental Designer software. A total of40full-contour zirconia crowns were produced by the CAM milling machine. The PFM crowns of the second part was taken as the control group. The fracture load of crowns with the different thickness were tested, and the influence of thickness on the fracture load of crowns was studied.Results1. With the use of CAD/CAM techniques, a3D standard preparation model of the mandibular first molar was designed successfully by the Abutment Designer software. And the PMMA dies were fabricated finally. The shape and dimension of the dies were identical to the designs and consistent with the Prosthodontics textbook on the basic requirements of tooth preparations.2. The fracture load mean value for heat-pressed ceramic crowns was1863.16±116.81N, for full-contour zirconia crowns4109.93±610.18N, for zirconia-based ceramic crowns2308.01±510.94N, for PFM crowns2284.77±355.60N. Different groups of crowns with the same thickness shown statistically different fracture load. The full-contour zirconia group shown the significantly highest value than the other three groups. The heat-pressed ceramic groups exhibit statistically lower fracture load than PFM group while other groups were not significantly different.3. In the monolithic structure, the heat-pressed ceramic crowns exhibited the similar failure mode to the full-contour zirconia crown, which both suffered catastrophic failure. Ring crack was also observed in the central fossa under the indenter, and the fracture propagated along the groove and fossa. In the multilayer structure, all the PFM crowns shown veneer chippings, indicated the good bond between the porcelain veneering and metal coping. While total fractures were more frequent observed in the zirconia-base ceramic crowns. It was difficult to identify the porcelain fracture first or the coping, or occurrence as the same time.4. With the use of CAD/CAM systems and Dental Designer software, different thickness of the full-contour zirconia restorations were successfully designed and fabricated. it was proved a exact duplicated model with a good thickness control5. The fracture load mean value to the full-contour zirconia crown with different thickness was1308.38±111.38N for0.6mm group,1814.60±68.21N for0.8mm group,2429.88±315.03N for1.0mm group,3068.31±233.88N for1.2mm groups. Different groups of crowns with different thickness have the significantly different fracture load value. Thickness had an influence on fracture load of the full-contour zirconia crown. With thickness of crowns increased, the fracture load increased. There were no significant differences in the fracture load between the full-contour zirconia crown with1.0mm thickness and PFM crowns. It was considered that fracture load of the full-contour zirconia crowns with1.0mm was similar to the PFM crowns.6. The full-contour zirconia crowns with different thickness present the similar failure modes, the same as the heat-pressed ceramic crowns in part2. The thicker crown of the full-contour zirconia restorations shown more fracture pieces.Conclusions1. With the use of CAD/CAM systems, the standard preparation model was generated by the Abutment Designer software. It can provide a platform for the follow-up study of the mechanical properties of full crown in vitro.2. The full-contour zirconia crown exhibited the highest fracture load significantly, compared to heat-pressed ceramic crown, zirconia-based ceramic crown and PFM crown. It could be a reference for the clinical use of the full-contour zirconia crowns.3. With the use of CAD/CAM systems and the Dental Designer software, the full crown restorations with different thickness were manufactured. The dimensions of restorations were well controlled. It can provide a new method for the study of full crown restorations in vitro.4. Thickness has an influence on fracture load of the full-contour zirconia crown. The much thicker of crown means the stronger fracture load resistance.5. The full-contour zirconia with1.0mm thickness has the comparative fracture load to the PFM crown. It could be used as a reference of the full-contour zirconia crown in the clinical use. |
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