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  • 1.
    André, Maria
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Kou, Wen
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Sundh, Anders
    Research and Development, Cad.esthetics AB, Skellefteå, Sweden.
    Effects of pretreatments and hydrothermal aging on biaxial flexural strength of lithium di-silicate and Mg-PSZ ceramics2016Ingår i: Journal of Dentistry, ISSN 0300-5712, E-ISSN 1879-176X, Vol. 55, s. 25-32Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Objectives: To evaluate the effect of specimen thickness, pretreatment and hydrothermal aging on the biaxial flexural strength (BFS) of lithium di-silicate glass (e.max Cad) and magnesia-stabilized zirconia (ZirMagnum) ceramic discs. Methods: The e.max Cad discs was studied: i) crystallized, ii) crystallized and glazed and iii) crystallized, glazed and unglazed side etched with hydrofluoric acid. The ZirMagnum discs were studied: i) as delivered, ii) after sandblasting and iii) after heat treatment similar to veneering. Hydrothermal aging was simulated by autoclave treatment. Results: The BFS of all the ZirMagnum specimens was superior (p < 0.001) to all the e.max Cad specimens. Glazing the 0.4 mm e.max Cad discs reduced (p < 0.05) their BFS compared with the unglazed 0.8 mm specimens, whereas glazing of 0.8 mm discs had no influence (p > 0.05) on the strength. Etching and autoclaving of e.max Cad did not affect (p > 0.05) the BFS. For ZirMagnum sandblasting with 0.2 MPa or 0.6 MPa did not influence the biaxial flexural strength (p > 0.05), whereas heat treatment reduced (p < 0.01) the BFS of 0.6 MPa sandblasted ZirMagnum. Autoclaving reduced the strength (p < 0.05) compared with ZirMagnum as delivered, whereas autoclaving of the 0.6 MPa sandblasted and heat treated specimens did not influence (p > 0.05) the BFS. Glazing, etching and sandblasting increased (p < 0.05) surface roughness. Conclusions: The effects of glazing, heat treatment, aging and mechanical treatment of the materials evaluated should be considered since their strength could be affected. Clinical significance: Mechanical properties of restorations made from prefabricated ceramic blocks could be affected of various treatments and could change over time.

  • 2.
    Kou, Wen
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    On dental ceramics and their fracture: a laboratory and numerical study2010Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Background Surface treatments and irregularities in the surfaces may affect the fracture of ceramics. The effects of various treatments on the surface texture of different types of ceramic cores/substructures was therefore qualitatively, quantitatively and numerically evaluated. Since fractures in ceramics are not fully understood, the fracture behavior in dental ceramic core/substructures was also studied using both established laboratory methods and newly developed numerical methods.

    Methods The surfaces of dental ceramic cores/substructures were studied qualitatively by means of a fluorescence penetrant method and scanning electron microscopy, quantitatively evaluated using a profilometer and also numerical simulation. In order to study fracture in zirconia-based fixed partial denture (FPD) frameworks, fractographic analysis in combination with fracture tests and newly developed two-dimensional (2D) and three-dimensional (3D) numerical modeling methods were used. In the numerical modeling methods, the heterogeneity within the materials was described by means of the Weibull distribution law. The Mohr–Coulomb failure criterion with tensile strength cut-off was used to judge whether the material was in an elastic or failed state.

    Results Manual grinding/polishing could smooth the surfaces on some of the types of dental ceramic cores/substructures studied. Using the fluorescence penetrant method, no cracks/flaws apart from milling grooves could be seen on the surfaces of machined zirconia-based frameworks. Numerical simulations demonstrated that surface grooves affect the fracture of the ceramic bars and the deeper the groove, the sooner the bar fractured. In the laboratory tests the fracture mechanism in the FPD frameworks was identified as tensile failure and irregularities on the ceramic surfaces could act as fracture initiation sites. The numerical modeling codes allowed a better understanding of the fracture mechanism than the laboratory tests; the stress distribution and the fracture process could be reproduced using the mathematical methods of mechanics. Furthermore, a strong correlation was found between the numerical and the laboratory results.

    Conclusion Based on the findings in the current thesis, smooth surfaces in areas of concentrated tensile stress would be preferable regarding the survival of ceramic restorations, however, the surfaces of only some of the ceramic cores/substructures could be significantly affected by manual polishing. The newly developed 3D method clearly showed the stress distribution and the fracture process in ceramic FPD frameworks, step by step, and seems to be an appropriate tool for use in the prediction of the fracture process in ceramic FPD frameworks.

  • 3.
    Kou, Wen
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap. Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Kou, Shaoquan
    Department of Civil and Mining Engineering, Luleå University of Technology, Luleå, Sweden.
    Liu, Hongyuan
    Department of Civil and Mining Engineering, Luleå University of Technology, Luleå, Sweden.
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    Numerical modeling of the fracture process in a three-unit all-ceramic fixed partial denture2007Ingår i: Dental Materials, ISSN 0109-5641, E-ISSN 1879-0097, Vol. 23, nr 8, s. 1042-1049Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    OBJECTIVES: The main objectives were to examine the fracture mechanism and process of a ceramic fixed partial denture (FPD) framework under simulated mechanical loading using a recently developed numerical modeling code, the R-T(2D) code, and also to evaluate the suitability of R-T(2D) code as a tool for this purpose. METHODS: Using the recently developed R-T(2D) code the fracture mechanism and process of a 3U yttria-tetragonal zirconia polycrystal ceramic (Y-TZP) FPD framework was simulated under static loading. In addition, the fracture pattern obtained using the numerical simulation was compared with the fracture pattern obtained in a previous laboratory test. RESULTS: The result revealed that the framework fracture pattern obtained using the numerical simulation agreed with that observed in a previous laboratory test. Quasi-photoelastic stress fringe pattern and acoustic emission showed that the fracture mechanism was tensile failure and that the crack started at the lower boundary of the framework. The fracture process could be followed both in step-by-step and step-in-step. SIGNIFICANCE: Based on the findings in the current study, the R-T(2D) code seems suitable for use as a complement to other tests and clinical observations in studying stress distribution, fracture mechanism and fracture processes in ceramic FPD frameworks.

  • 4.
    Kou, Wen
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    Li, Decong
    Institute of Mechanics, Chinese Academy of Sciences, Beijing, China.
    Qiao, Jiyan
    Institute of Mechanics, Chinese Academy of Sciences, Beijing, China.
    Chen, Li
    Institute of Mechanics, Chinese Academy of Sciences, Beijing, China.
    Ding, Yansheng
    Institute of Mechanics, Chinese Academy of Sciences, Beijing, China.
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    A 3D numerical simulation of stress distribution and fracture process in a zirconia-based FPD framework2011Ingår i: Journal of Biomedical Materials Research. Part B - Applied biomaterials, ISSN 1552-4973, E-ISSN 1552-4981, Vol. 96B, nr 2, s. 376-385Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this study, a numerical approach to the fracture behavior in a three-unit zirconia-based fixed partial denture (FPD) framework was made under mechanical loading using a newly developed three-dimensional (3D) numerical modeling code. All the materials studied were treated heterogeneously and Weibull distribution law was applied to describe the heterogeneity. The Mohr-Coulomb failure criterion with tensile strength cut-off was utilized to judge whether the material was in an elastic or failed state. For validation, the fracture pattern obtained from the numerical modeling was compared with a laboratory test; they largely correlated with each other. Similar fracture initiation sites were detected both in the numerical simulation and in an earlier fractographic analysis. The numerical simulation applied in this study clearly described the stress distribution and fracture process of zirconia-based FPD frameworks, information that could not be gained from the laboratory tests alone. Thus, the newly developed 3D numerical modeling code seems to be an efficient tool for prediction of the fracture process in ceramic FPD frameworks.

  • 5.
    Kou, Wen
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    Molin, Margareta
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Protetik.
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    Surface roughness of five different dental ceramic core materials after grinding and polishing2006Ingår i: Journal of Oral Rehabilitation, ISSN 0305-182X, E-ISSN 1365-2842, Vol. 33, nr 2, s. 117-124Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In clinical practice, core materials can be exposed after adjustments are made to previously-luted all-ceramic restorations. The purpose of this study was to evaluate the surface roughness of five different dental ceramic core materials after grinding and polishing. Five different ceramic core materials, Vita In-Ceram Alumina, Vita In-Ceram Zirconia, IPS Empress 2, Procera AllCeram, and Denzir were evaluated. Vita Mark II was used as a reference material. The surface roughness, Ra value (mum), was registered using a profilometer. The measurements were made before and after grinding with diamond rotary cutting instruments and after polishing with the Sof-Lex system. The surface of representative specimens was evaluated qualitatively using scanning electron microscopy (SEM). Results were statistically analysed using analysis of variance (anova) supplemented with Scheffe's and Bonferroni multiple-comparison tests. Before grinding, Procera AllCeram and Denzir had the smoothest surfaces, while IPS Empress 2 had the coarsest. After grinding, all materials except IPS Empress 2 became coarser. Polishing with Sof-Lex provided no significant (P > 0.05) differences between Denzir, Vita Mark II and IPS Empress 2 or between Procera AllCeram and In-Ceram Zirconia. There were no significant differences (P > 0.05) either between the ground and the polished Procera AllCeram or In-Ceram Alumina specimens. Polishing of Denzir, IPS Empress 2 and In-Ceram Zirconia made the surfaces smoother compared with the state after grinding, whereas the polishing effect on Procera AllCeram and In-Ceram Alumina was ineffective. The findings of the SEM evaluation were consistent with the profilometer readings.

  • 6.
    Kou, Wen
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Qiao, Jiyan
    Institute of Mechanics, Chinese Academy of Sciences, Beijing, China.
    Chen, Li
    Institute of Mechanics, Chinese Academy of Sciences, Beijing, China.
    Ding, Yansheng
    Institute of Mechanics, Chinese Academy of Sciences, Beijing, China.
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Numerical simulation of the fracture process in ceramic FPD frameworks caused by oblique loading2015Ingår i: Journal of The Mechanical Behavior of Biomedical Materials, ISSN 1751-6161, E-ISSN 1878-0180, Vol. 50, s. 206-214Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using a newly developed three-dimensional (3D) numerical modeling code, an analysis was performed of the fracture behavior in a three-unit ceramic-based fixed partial denture (FPD) framework subjected to oblique loading. All the materials in the study were treated heterogeneously; Weibull׳s distribution law was applied to the description of the heterogeneity. The Mohr-Coulomb failure criterion with tensile strength cut-off was utilized in judging whether the material was in an elastic or failed state. The simulated loading area was placed either on the buccal or the lingual cusp of a premolar-shaped pontic with the loading direction at 30°, 45°, 60°, 75° or 90° angles to the occlusal surface. The stress distribution, fracture initiation and propagation in the framework during the loading and fracture process were analyzed. This numerical simulation allowed the cause of the framework fracture to be identified as tensile stress failure. The decisive fracture was initiated in the gingival embrasure of the pontic, regardless of whether the buccal or lingual cusp of the pontic was loaded. The stress distribution and fracture propagation process of the framework could be followed step by step from beginning to end. The bearing capacity and the rigidity of the framework vary with the loading position and direction. The framework loaded with 90° towards the occlusal surface has the highest bearing capacity and the greatest rigidity. The framework loaded with 30° towards the occlusal surface has the least rigidity indicating that oblique loading has a major impact on the fracture of ceramic frameworks.

  • 7.
    Kou, Wen
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    Qiao, Jiyan
    Chinese Academy of Sciences, Beijing, China.
    Li, Decong
    Chinese Academy of Sciences, Beijing, China.
    Chen, Li
    Chinese Academy of Sciences, Beijing, China.
    Ding, Yansheng
    Chinese Academy of Sciences, Beijing, China.
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    A 3-D numerical simulation of the fracture process in three-unit FPD frameworks2009Konferensbidrag (Övrigt vetenskapligt)
  • 8.
    Kou, Wen
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap. Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi, Odontologisk materialvetenskap.
    Fracture behaviour of zirconia FPDs substructures: fractographic analysis of zirconia2010Ingår i: Journal of Oral Rehabilitation, ISSN 0305-182X, E-ISSN 1365-2842, Vol. 37, nr 4, s. 292-299Artikel i tidskrift (Refereegranskat)
    Abstract [en]

     The purpose of this study was to evaluate the occurrence of superficial flaws after machining and to identify fracture initiation and propagation in three-unit heat-treated machined fixed partial dentures (FPDs) substructures made of hot isostatic pressed (HIPed) yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) after loaded to fracture. Four three-unit HIPed Y-TZP-based FPDs substructures were examined. To evaluate the occurrence of superficial flaws after machining, the surfaces were studied utilizing a fluorescent penetrant method. After static loading to fracture, characteristic fracture features on both mating halves of the fractured specimens were studied using a stereomicroscope and a scanning electron microscope. Grinding grooves were clearly visible on the surfaces of the machined FPDs substructures, but no other flaws could be seen with the fluorescent penetrant method. After loading to fracture, the characteristic fracture features of arrest lines, compression curl, fracture mirror, fracture origin, hackle and twist hackle were detected. These findings indicated that the decisive fracture was initiated at the gingival embrasure of the pontic in association with a grinding groove. Thus, in three-unit heat-treated machined HIPed Y-TZP FPDs substructures, with the shape studied in this study, the gingival embrasure of the pontic seems to be a weak area providing a location for tensile stresses when they are occlusally loaded. In this area, fracture initiation may be located to a grinding groove.

  • 9.
    Kou, Wen
    et al.
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Tsukasa, Akasaka
    Department of Biomedical Materials and Engineering, Graduate School of Dental Medicine, Hokkaido University, Sapporo, Japan..
    Watari, Fumio
    Department of Biomedical Materials and Engineering, Graduate School of Dental Medicine, Hokkaido University, Sapporo, Japan..
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    An in vitro evaluation of the biological effects of carbon nanotube-coated dental zirconia2013Ingår i: ISRN Dentistry, ISSN 2090-4371, E-ISSN 2090-438X, Vol. 2013, nr Article ID 296727Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The purpose of this study is to evaluate functionalized multiwalled carbon nanotubes (fMWCNTs) as a potential coating material for dental zirconia from a biological perspective: its effect on cell proliferation, viability, morphology, and the attachment of an osteoblast-like cell. Osteoblast-like (Saos-2) cells were seeded on uncoated and fMWCNT-coated zirconia discs and in culture dishes that served as controls. The seeding density was 104 cells/cm2, and the cells were cultured for 6 days. Cell viability, proliferation and attachment of the Saos-2 cells were studied. The results showed that Saos-2 cells were well attached to both the uncoated and the fMWCNT-coated zirconia discs. Cell viability and proliferation on the fMWCNT-coated zirconia discs were almost the same as for the control discs. Better cell attachment was seen on the fMWCNT-coated than on the uncoated zirconia discs. In conclusion, fMWCNTs seem to be a promising coating material for zirconia-based ceramic surfaces to increase the roughness and thereby enhance the osseointegration of zirconia implants.

  • 10.
    Sundh, Anders
    et al.
    Praktikertjänst AB.
    Kou, Wen
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Sjögren, Göran
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Effects of Pretreatment, Specimen Thickness, and Artificial Aging on Biaxial Flexural Strength of Two Types of Y-TZP Ceramics2019Ingår i: Operative dentistry, ISSN 0361-7734, E-ISSN 1559-2863Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Objectives: To evaluate the effects of surface treatment, specimen thickness, and aging on the biaxial flexural strength (BFS) of two types of yttria-stabilized, tetragonal zirconia polycrystal (Y-TZP) ceramics.

    Methods and Materials: Disc-shaped specimens, 0.4 and 1.3 mm thick, made from hot isostatic pressed (Denzir) and non–hot isostatic pressed (ZirPlus) Y-TZP, were sandblasted, heat treated, and autoclaved. The surface topography was assessed in accordance with European Standard 623-624:2004 and the BFS tests in accordance with International Organization for Standardization Standard 6872:2008. For statistical analyses, one-way Shapiro-Wilk test, analysis of variance (post hoc: least significant differences), Mann-Whitney U-test, and Pearson correlation tests (p<0.05) were used.

    Results: As delivered, the BFS of the 0.4-mm ZirPlus was >1.3-mm ZirPlus (p<0.01), and the BFS of the 0.4-mm Denzir was >1.3-mm Denzir (p<0.001). Sandblasting with 0.2 MPa reduced the BFS of the ZirPlus and Denzir discs (p<0.01), whereas sandblasting with 0.6 MPa increased the BFS of the 0.4-mm Denzir (p<0.001) and reduced the BFS of the 0.4-mm ZirPlus (p<0.05). Heat treatment significantly reduced the BFS of all the groups except for the 0.6 MPa sandblasted 0.4-mm ZirPlus. Autoclaving reduced the BFS of the as-delivered ZirPlus and Denzir specimens (p<0.001), whereas autoclaving the 0.6 MPa sandblasted and heat-treated specimens had no effect (p>0.05) on the BFS. The 0.6 MPa sandblasted, heat-treated, and autoclaved 0.4-mm Denzir exhibited higher BFS than the 0.6 MPa sandblasted, heat-treated, and autoclaved 0.4-mm ZirPlus (p<0.05).

    Conclusions: Thickness and surface treatment of Y-TZP–based ceramics should be considered since those factors could influence the BFS of the material.

  • 11. Ågren, Martin
    et al.
    Kou, Wen
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Molin Thorén, Margareta
    Umeå universitet, Medicinska fakulteten, Institutionen för odontologi.
    Bond strength of surface-treated novel high translucent zirconia to enamel2019Ingår i: Biomaterial Investigations in Dentistry, Vol. 6, nr 1, s. 35-42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aim: The aim of the present study was to evaluate the shear bond strength of zirconia, stabilised with 5% yttria, luted to enamel and to evaluate the fracture pattern at loss of retention.

    Methods: A total of 53 test specimen were manufactured from two partially stabilised zirconia materials, Zirkonzahn Prettau Anterior (ZPA) (n = 16) and Whitepeaks CopraSmile Symphony 5 layer (WCS) (n = 18), and a lithium disilicate (Ivoclar e.Max Press) (n = 19) acting as control. All test specimens were cemented to human enamel with Variolink Esthetic DC and then subjected to a shear bond strength test. Fracture and surface analysis were performed using light and scanning electron microscope.

    Results: No significant differences in shear bond strength were detected when analysing the three groups. Dividing them according to the fracture pattern significant difference in shear bond strength between the two zirconia groups could be seen analysing test bodies with failure of adhesion to the test body, but not to enamel. The ZPA had higher shear bond strength (23.68 MPa) than WCS (13.00 MPa). No significant differences were seen compared to the control group (19.02 MPa).

    Conclusion: Partially stabilised zirconia shows potential as a material to be used where macro mechanical bonding is not possible, although this study does not reveal how or if the bonding deteriorates over time.

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