The effect of different ceramic materials on the fracture resistance of dentin-bonded crowns

Using stereomicroscopy, light microscopy, and scanning electron microscopy, we investigated the development of vasa vasorum in the proliferated neointima of the autovein graft and its anastomoses implanted in the canine femoral artery against a background of poor distal runoff. In the stereomicroscopic examination, a microfil silicone rubber compound (MF) was injected transluminally or via perivascular vasa, and the vascular specimen was prepared for clearing by immersion in a methyl-salicylate solution. Vessel interstices filled with MF were found adjacent to the suture materials within 5 days of grafting. Fourteen days after implantation, luminally originating vasa vasorum were often visible in the neointima along the suture line and distributed into the media and adventitia connecting to the original vasa vasorum. At 6 months or more after grafting, many orifices of luminally originating vasa vasorum were seen along the suture line of both proximal end-to-end and distal end-to-side anastomoses and distributed into the thickened neointima forming a vasa network when the neointima had proliferated to over 250 microm in depth. On the other hand, some clefts filled with MF were found in mural thrombi deposited on the vascular sinus of the graft within 5 days, and these appeared to be one of the sources of luminally originating vasa vasorum on the graft distant from the suture line. Moreover, the development of numerous vasa vasorum was constantly demonstrated in the neointima when it had proliferated to over 250 microm in depth.     

不同弹性模量的基台--种植体组合对周围成骨性能的影响

利用有限元分析软件计算了不同静力作用下的多种基台-种植体周围骨组织的应力分布.模拟结果显示,基台-种植体组合中Ti6Al4V钛合金-聚醚醚酮(TC4-PEEK)相对于其他实验组其应力集中程度现象可以有效降低,周围骨组织的应力分布较为均匀,最大应力值为40～60 MPa.在轴向加载条件下,不同基台-种植体系统中PEEK种植体的应力水平较小,而周围骨组织应力水平较大;在斜向45°加载条件下,相对于其他两种基台-种植体系统,TC4-PEEK的应力水平更低,其周围骨组织中的皮质骨承受的最大应力值为55 MPa,松质骨承受的最大应力值为5 MPa,综合来看的应力水平最小,有助于骨沉积和成骨量增加,从而有效提高种植体的界面稳定性.     

不同弹性模量的基台-种植体组合对周围成骨性能的影响

利用有限元分析软件计算了不同静力作用下的多种基台-种植体周围骨组织的应力分布.模拟结果显示, 基台-种植体组合中Ti6Al4V钛合金-聚醚醚酮(TC4-PEEK)相对于其他实验组其应力集中程度现象可以有效降低, 周围骨组织的应力分布较为均匀, 最大应力值为40~60 MPa.在轴向加载条件下, 不同基台-种植体系统中PEEK种植体的应力水平较小, 而周围骨组织应力水平较大; 在斜向45°加载条件下, 相对于其他两种基台-种植体系统, TC4-PEEK的应力水平更低, 其周围骨组织中的皮质骨承受的最大应力值为55 MPa, 松质骨承受的最大应力值为5 MPa, 综合来看的应力水平最小, 有助于骨沉积和成骨量增加, 从而有效提高种植体的界面稳定性.      

Marginal discrepancy of all-ceramic crowns cemented on implant abutments

PURPOSE: The purpose of this investigation was to determine the mean marginal discrepancy of all-ceramic crowns cemented on implant abutments. MATERIALS AND METHODS: Five Br?nemark CeraOne abutments were connected to implant fixtures embedded in acrylic resin blocks. The marginal discrepancy was measured using a video camera connected to a digitizing board at four locations on each abutment for five samples in each of the following groups: all-ceramic caps (caps), all-ceramic crowns (crowns), and all-ceramic crowns cemented with zinc phosphate cement (cemented crowns). RESULTS: The mean marginal discrepancies were as follows: caps, 99.0 micrometers (SD, 16); crowns, 117.8 micrometers (SD, 20); and cemented crowns, 168.8 micrometers (SD, 23). Statistically significant differences were found between all three groups at the 99% level of confidence. In addition there were two significant differences between abutments. CONCLUSIONS: Subgingival marginal discrepancies of the magnitude measured in this study have been shown to cause periodontal problems. It is probable that the marginal discrepancy of CeraOne restorations, as tested, can be improved. Efforts should be made to enhance the marginal accuracy of these restorations.     

Color change and fracture resistance of two preveneered stainless-steel crowns after sterilization

A negative image of community care prevails. This method of care is perceived to be a relatively novel phenomenon and has received mixed media coverage. The negative image of community care has led to the growing belief that this care method has failed. This failure has largely been ascribed to the lack of powers available to control patients in the community and to the method's relative novelty. However, this paper contends that there are two flaws to the above assertion: first, community care is far from new, and second, the inherent problem is not the lack of powers available to control patients in the community, but, essentially, the absence of a secure and stable environment within the community.     

The effect of adhesive luting agent-dentinal surface interactions on compressive strength of two types of all-ceramic crowns

This research compared the compressive strength of two types of all-ceramic crown (Hi-Ceram and Duceram) as affected by selected luting cements (Zn phosphate, glass ionomer and composite resin cement). Thirty crowns of similar size and shape were constructed (15 crowns of each tested material) to fit a standard posterior tooth preparation. Five crowns from each material were cemented by one of the tested cements. The cemented crowns were loaded until catastrophic failure. A two-way analysis of variance was performed and showed that the type of utilized cement had a significant effect on the compressive strength being that Panevia Ex. resin cement the most effective one followed by glass ionomer and then finally zn phosphate cement. Statistical analysis also showed that Hi-Ceram crowns were more resistant to occlusal load than Duceram.     

Transverse elastic moduli of unidirectional fiber composites with interfacial debonding

A mathematical model is developed to predict the transverse elastic moduli of unidirectional fiber composites. Two cases of fiber/matrix interfacial bondings are investigated: perfect bonding and complete debonding,i.e., no transferring of tensile stress on surfaces where the fiber and matrix materials are separated. Fibers are assumed to exhibit transverse isotropy, and the elastic interactions among fibers are also considered. In the analysis of the completely debonded case, the cavity formation model is adopted and the original fiber and surrounding cavities are replaced by an imaginary anisotropic inclusion. Closed form solutions of effective elastic moduli are obtained for transverse tensile and compressive moduli, and transverse shear moduli. The limiting case of fiber-like voids is also studied. Numerical results are presented for two metal matrix composites, where the optimal wetting between the fiber and matrix is difficult to be obtained and hence the interfacial bonding and transverse property have been major concerns. Formerly Visiting Associate Professor at The University of Delaware     

Resistance to fracture of three all-ceramic systems

This paper explores the notion of emotional competence for professional practitioners and the impact of different levels of emotional competence on communication in mental health nursing practice. It comments on available approaches to assessing competence in this area and on factors that obstruct and impair such assessment. Practitioner awareness of both the concept of emotional competence and its assessment is examined at a professional and personal level. Strategies for the achievement and maintenance of effective communication and emotionally competent professional practice are proposed.     

A systematic investigation of elastic moduli of Wc-Co alloys

Measurements of Young’s modulus, shear modulus, compression modulus, and Poisson’s ratio on WC-(1 ~ 30) wt pct Co alloys were carried out by dynamic resonance method. The effects of volume fraction of the WC phase, the carbon content, and the WC particle-size on the elastic moduli were investigated in some detail. The result shows that the various elastic moduli of the alloys depend solely on volume fractions of the constituent phases and they invariably fall within Hashin and Shtrikman’s bounds. Moreover, the moduli are found to be approximated by Paul’s “strength of materials” type formulas. It is concluded that elastic behavior of the alloys can be essentially predicted on the basis of a simplified geometrical model of WC particles embedded in continuous cobalt matrix.     

Effective elastic moduli of fiber-matrix interphases in high-temperature composites

This article describes a theoretical model and an experimental method for determination of interphasial elastic moduli in high-temperature composites. The interphasial moduli are calculated from the ultrasonically measured composite modulivia inversion of multiphase micromechanical models. Explicit equations are obtained for determination of interphasial stiffnesses for an interphase model with spring boundary conditions and multiphase fiber. The results are compared with the exact multiphase representation. The method was applied to ceramic and intermetallic matrix composites reinforced with SiC SCS-6 fibers. In both composites, the fiber-matrix interphases include approximately 3-μm-thick carbon-rich coatings on the outer surface of the SiC shell. Although the same fiber is used in both composite systems, experimental results indicate that the effective interphasial moduli in these two composite systems are very different. The interphasial moduli in intermetallic matrix composites are much greater than those in ceramic matrix composites. After taking the interphase microstructure into account, we found that the interphasial moduli measured for the intermetallic matrix composites are very close to the estimated bulk moduli of the pyrolytic carbon with SiC particle inclusions. Our analysis shows that the lower effective interphasial moduli in the reaction-bonded Si₃N₄ (RBSN) ceramic matrix composites are due to imperfect contact between the interphasial carbon and the porous matrix and to thermal tension forces which slightly unclamp the interphase. Thus, measured interphase effective moduli give information on the quality of mechanical contact between fiber and matrix. Possible errors in the interphasial moduli determined are analyzed and the results show that these errors are below 10 pct. In addition, the use of the measured interphasial moduli for assessment of interphasial damage and interphase reactions is discussed.     

Microleakage of dentin-bonded crowns placed with different luting materials

This review defines the term "compound allergy" in the context of new findings, and discusses evidence that allergenic reaction products have been identified. Material was gathered by searching Index Medicus and the Science Citation Index, and reviewing several standard texts. Issues regarding the validity of patch test results are addressed and we introduce the term "pseudocompound allergy" to cover cases of false-negative patch tests. We present new theories regarding the mechanisms by which new allergens are formed and a means of classification.     

The fracture resistance of dental materials

A novel test procedure was developed that provides the basis for a standard plain strain fracture toughness test to evaluate fracture resistance of resin-based materials. This procedure utilizes a notched disc specimen. Using this technique, a new concept of Torque to initiate fracture (T)' was suggested. Also, fracture resistance can be assessed by means of critical stress intensity factor (K[IC]). Using this method, the fracture resistance of direct restorative materials, resin-based inlay/onlays and luting cements can be evaluated.     

The fracture resistance of metal-ceramic interfaces

The effects of interface structure and microstructure on the fracture energy, Γ_i, of metal-ceramic interfaces are reviewed. Some systems exhibit a ductile fracture mechanism and others fail by brittle mechanisms. In the absence of either interphases or reaction products, Γ_i is dominated by plastic dissipation (for both fracture mechanisms), leading to important effects of metal thickness, h, and yield strength, σ₀. Additionally, Γ_i is larger when fracture occurs by ductile void growth (for the same h and σ₀). A fundamental understanding now exists for the ductile fracture mechanism. However, some basic issues remain to be understood when fracture occurs by brittle bond rupture, particularly with regard to the role of the work of adhesion, W_ad. Interphases and reaction products have been shown to have an important effect on Γ_i. A general trend found by experiment is that Γ_i scales with the fracture energy of the interphase itself, wherein Γ_i tends to increase for the interphase sequence: amorphous oxides > crystalline oxides > intermetallics. However, there also appear to be important effects of the residual stresses in the interface (which influence the fracture mechanism and the layer thickness.