Search Results - (Author, Cooperation:Wiskott)

article

1998

Bibliothekar ; Fachhochschule ; Studienrichtung ; Bibliothekswissenschaft ; Dokumentation ; Genf ; Kanton ; Schweiz

Arbido (Verein Schweizerischer Archivarinnen und Archivare), (1998) H. 9, S. 10-13, 0258-0764

1420-102X

French

1532-849X

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

The objectives of the study were: 1) to map the stresses acting on the cement interface of crown and abutment analogs during loading; and 2) to provide a theoretical basis for the hypothesis that resistance to lateral dislodgment is a function of the distribution of compressive force vectors acting on the cement lute.〈section xml:id="abs1-2"〉〈title type="main"〉Materials and MethodsThree-dimensional finite element (FE) meshes of crown and abutment analogs were constructed and loaded in a direction perpendicular to the axes of symmetry of the abutments. Three parameters were investigated: taper (10° and 20° of convergence), abutment substrate (Ni-Cr alloy and dentin), and type of cement (zinc oxide eugenol, zinc phosphate, glass ionomer, and composite resin). The tensile and compressive components of the resulting force systems were plotted along two axes (z: parallel to the axis of symmetry of the crown/abutment complex; and y: perpendicular to z, ie, parallel to the direction of loading). Von Mises stresses were also generated.〈section xml:id="abs1-3"〉〈title type="main"〉ResultsFirst, it was shown that the restoration's axis of rotation was located inside the abutment cone and was perpendicular to and intersected the axis of symmetry of the crown/abutment complex. Second, stress distribution was dependent on the three parameters investigated. Varying taper led to shifts due mainly to alterations in specimen geometry, whereas the abutment substrate and the cement type had a bearing on the level of the axis of rotation. The smaller the modulus of elasticity of the abutment substrate or the cement lute, the farther apical the location of the axis of rotation.〈section xml:id="abs1-4"〉〈title type="main"〉ConclusionsConventional schemes for explaining crown dislodgment in which the restoration rotates around an axis located at the preparation margin should be reassessed. The results of the FE analysis are compatible with the hypothesis that resistance to lateral dislodgment is a function of the distribution of compressive force vectors acting on the cement interface.

Electronic Resource

1600-0765

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

This study is part of a research program that aims to develop a constitutive three-dimensional model of the periodontal ligament (PDL) through the identification of pertinent material parameters. As part of this program, bovine PDL was utilized to establish stress-strain responses under tensile and compressive loading conditions. Fresh bovine molars were secured, frozen and prepared to appropriate dimensional specifications. Bar-shaped specimens that comprised portions of dentine, PDL and bone were produced. Push-pull tests were conducted using a specifically constructed loading machine. Full range monotonic stress-strain diagrams were generated. The effect of a rate increase on cyclic S–E diagrams was also determined. The influence of specimen thickness was expressed in terms of modulus of elasticity, strength, uniaxial maximizer strain, and strain energy density. The overall load-response was heavily hysteretic in compression. On the tensile side, after a steep rise, the curve tended to flatten out asymptotically. Variations in rate that spanned four orders of magnitude had no effect on reciprocal load responses. The E-modulus was in the 4–8 MPa range, the strength of the PDL was 1–2 MPa, the maximizer strain was at 45–60% and the strain energy density ranged between 0.3 and 0.4 MPa.

Electronic Resource

0040-4020

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Electronic Resource

0040-4039

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Electronic Resource

1600-0501

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Abstract:  A number of algorithms have been proposed to model the adaptive behavior of bone under load. However, the predictions of several models have neither been compared nor have they been systematically related to in vivo data. To this end, the stress states of loaded implant–bone interfaces were analyzed before and after osseointegration using finite element (FE) techniques.In a preliminary step, an FE mesh of a cylindrical implant encased in a cancellous core surrounded by a cortical layer was constructed, and the stresses and strains that developed at the interface were determined. The implant was loaded with 100 N vertical and 30 N lateral loads. Using this structure, the peak compressive and tensile stresses were determined. Then bone remodeling predictions were assessed using three different models: von Mises equivalent strain, strain energy density and effective stress. Finally, a systematic search of the literature was conducted to relate the numerical predictions to existing in vivo data.The FE simulations led to the following conclusions: (1) calculated compressive stresses were lower than the ultimate compressive stresses of cortical and cancellous bone. (2) Calculated tensile stresses were generally superior to experimental data on the tensile strength of the bone–implant interface. (3) With one exception, the predictions of all models were homogeneously grouped on the stimulus scales. (4) The predictions of the models as to bone gain or loss were not consistent and at times contradictory. It is hypothesized that this effect is linked to a lazy zone that is too narrow. With respect to the application of the numerical models to in vivo data, peak strains and strain energy densities were consistent with in vivo data. No in vivo data were found that supported effective stress as a stimulus.

Electronic Resource

1600-0501

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

It has been observed that the polished neck of dental implants does not osseointegrate as do textured surfaces. Similar findings were reported in the orthopedic literature on artificial hip endoprostheses. In Dentistry, lack of osseointegration was attributed to increased pressure on the osseous bed during implant placement, establishment of a physiological “biologic width”, stress shielding and lack of adequate biomechanical coupling between the load-bearing implant surface and the surrounding bone. Among the many variables that may affect osseointegration, this analysis proposes to include stress transfer as a significant one. Therefore the present report discusses the relationship between the stresses applied and bone homeostasis. Any viable osseous structure (Including the tissue that surrounds the polished implant neck) is subjected to periodic phases of resorption and formation. Clinical and experimental data have shown the detrimental effects of lack of function in that bone mass decreases with time. Due to inadequate mechanical stimuli, bone that is resorbed during normal turnover is redeposited in lesser amounts than previously, a process observed clinically as resorption. The stress ranges which cause bone to resorb, maintain or increase its mass and the level that eventually causes bone to fracture have been delimited in the literature. Applying these values to the situation to dental implants, it follows that if it is to be stable, crestal bone must be subjected to suitable levels of mechanical stimulation. We suggest that smooth surfaces will not provide adequate biomechanical coupling with the bone surrounding the implant neck in that the stress range induced by a polished surface is limited. We propose that the surface texture of threaded, plasma-coated or sandblasted implants generates a heterogeneous stress field around an implant in function. By design, such a stress field includes force levels which are conducive to bone formation. Hence, during the formation phase of bone turnover, osteoblast lineages are much more likely to be stimulated by biomechanical signals of appropriate magnitude.

Electronic Resource

1600-0501

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Abstract: The Straumann Company has recently supplemented its standard ‘morse-taper’ configuration with an octagonal internal key. During the restorative phase of implant treatment, this additional feature was designed to ensure positional duplicability between the laboratory and the clinical environments. It was, however, unclear whether this keying mechanism would decrease the mechanical strength of the connection between the implant and the abutment. This applies to keyed male and female parts but also to combinations of the new and the standard designs. Specially constructed specimens analogs representing all three combinations were fitted with a T-shaped bar, preangled to 15° and subjected to vertical force applications provided by a servohydrolic fatigue tester. The loading frequency was 2 Hz and the maximum cycle number was 106. The data were evaluated using the staircase technique. The specimens were also modeled and analyzed numerically using finite element procedures. The samples' failure locations were recorded and the displacement vs. cycle number plots were patterned in four groups. The fatigue tests and staircase analysis showed no difference in mechanical resistance between the standard and the internally keyed connectors. The finite element models revealed a stress concentration located at the apical edges of the octagonal connector. However, it appeared that this phenomenon was based on computational rather than mechanical grounds. The locations of the failure sites were distributed randomly across the structures, thereby indicating the absence of a locus of minor resistance. The patterns of the displacement vs. cycle number could not be attributed to specific combinations between the standard and the internally keyed designs. It was concluded that both connectors are equal in their mechanical resistance to bending and torquing forces.

Electronic Resource

1600-0501

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

In order to improve the prosthetic versatility of the ITI-Bonefit implant system, a clinical technique was tested whereby a custom made post and core buildup was cemented into implants. Specially designed threaded and serrated posts were machined in precious metal. After the posts were seated into the threaded channel of the implants, cores were built using autopolymerizing resin. Those were then cast and cemented into the implants. For the remainder of the procedure, the buildups were treated like natural abutments using conventional prosthodontic techniques. Mechanical tests were performed to assess the clinical viability of these buildups. The ultimate tensile strength of annealed posts lies in the 700–800 N range. By comparison, the pull-out resistance of posts cemented into natural roots ranged between 108 and 177 N, and the maximum pull-out resistance of manufactured abutments is about 1040 N. When the resistance against lateral forces was tested, the cemented build-ups ranged between 981 and 1128 N. whereas natural teeth fractured between 206 and 903 N depending on the diameter of the root. Manufactured abutments failed at stress levels of about 1020 N. Considering these favorable results, we conclude that the technique described above can be applied clinically for further investigation. Additionally, we suggest some modifications in implant design that would enhance the versatility of the system.

Electronic Resource

1434-601X

Springer Online Journal Archives 1860-2000

Physics

Zusammenfassung Die Arbeit ergänzt eine kürzlich gegebene Theorie der anomalen Streuung von α-Teilchen, in welcher die Streuanomalie ℜ(ϑ) als Funktion des Streuwinkels ϑ bei fest vorgegebener Resonanzenergie berechnet wurde. Die zur vollständigen Beschreibung der Anomalie gehörige Abhängigkeit ℜ(E) von der α-Energie soll hier unter Verwendung eines einfachen graphischen Verfahrens nachgetragen werden. Bei Variation des Streuwinkels ϑ ergeben sich starke Veränderungen der Linienform ein und derselben Resonanzlinie. Die graphische Methode gestattet in einfachster Weise einen Überblick über die bei verschiedenen Streuwinkeln zu erwartenden Kurvenformen.

Electronic Resource

1434-601X

Springer Online Journal Archives 1860-2000

Physics

Zusammenfassung Es wird ein Verfahren angegeben, das insbesondere über den Verlauf reeller Lösungen derSchrödinger-Gleichung eine gute Orientierung in relativ kurzer Zeit ermöglicht. Die Anregung hierzu wurde durch eine Arbeit vonKleinwächter undWojtech gegeben, in der Kreiskonstruktionen zur Behandlung von Einschaltproblemen schwingungsfähiger Systeme verwandt werden.

Electronic Resource

1437-1596

Springer Online Journal Archives 1860-2000

Medicine

Law

Electronic Resource

1432-1440

Springer Online Journal Archives 1860-2000

Medicine

Electronic Resource

1432-1440

Springer Online Journal Archives 1860-2000

Medicine

Electronic Resource

1432-1440

Springer Online Journal Archives 1860-2000

Medicine

Electronic Resource

1432-1440

Cyclosporin A ; Renal transplantation ; Azathioprine ; Steroids ; Immunosuppression ; Nierentransplantation ; Azathioprin ; Steroide ; Immunsuppression

Springer Online Journal Archives 1860-2000

Medicine

Zusammenfassung In einer multizentrischen Studie mit acht beteiligten Zentren wurden 232 Empfänger von Leichennieren-Transplantaten nach dem Zufallsprinzip einer Behandlung mit Cyclosporin A (CyA-Gruppe: 117 Patienten) bzw. mit Azathioprin und Steroiden (Kontrollgruppe: 115 Patienten) zugewiesen. Nach einer Beobachtungsperiode zwischen 2 Wochen und 11 Monaten betrug die geschätzte Überlebenswahrscheinlichkeit der Transplantate 73% in der CyA- und 53% in der Kontrollgruppe. Während der Therapie gab es zwei Todesfälle in der CyA- gegenüber sieben in der Kontrollgruppe. 82% der CyA-Patienten mit funktionsfähigen Transplantaten erhielten CyA allein, 17% wurden auf die herkömmliche Therapie umgestellt, und ein Patient erhielt Prednisolon zusätzlich zu CyA. Steroide haben nie 27% erhalten. Die Nierenfunktion 6 Monate nach der Transplantation war in der CyA-Gruppe ähnlich wie in der Azathioprin- und Steroid-Gruppe. Nach diesen vorläufigen Ergebnissen scheint CyA wirkungsvoller als die konventionelle Immunsuppression zu sein, wobei Langzeit-Steroid-Therapie vermieden werden kann.

Summary In a multicentre trial conducted in eight European centres, 232 recipients of cadaveric renal allografts were randomly allocated to receive either cyclosporin A (CyA, 117 patients) or azathioprine and steroids (control, 115 patients) for immunosuppression. After a follow-up period of up to eleven months, graft survival probability estimates are 73% in the CyA group and 53% in the control group. Two deaths have occurred in the CyA group and seven in the control group. 82% of the CyA group with functioning grafts are receiving CyA alone, 17% have been changed to azathioprine and steroids and one patient is receiving prednisolone in addition to CyA; 27% have never received steroids. At six months post-transplant renal function is similar in patients receiving CyA and in those receiving azathioprine and steroids. On the basis of these preliminary results CyA appears to be more effective than conventional immunosuppression and avoids the necessity of longterm steroid therapy.

Electronic Resource

0040-4039

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Electronic Resource

0029-554X

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Energy, Environment Protection, Nuclear Power Engineering

Physics

Electronic Resource

1432-1076

Springer Online Journal Archives 1860-2000

Medicine

Electronic Resource

1432-1076

Springer Online Journal Archives 1860-2000

Medicine

Electronic Resource