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H W Wiskott

Publications and source records attributed to H W Wiskott.

At least 19 recordsLinked to original sources
PubMed2001

Effect of water exposure on the fracture toughness and flexure strength of a dental glass.

OBJECTIVES: The low fusing dental glass (Duceram LFC) has been advertised as presenting a superior chemical resistance and augmented strength after 16h exposure to water or 4% acetic acid. The purpose of this study was to evaluate the effect of prolonged exposure to water on two mechanical properties (fracture toughness and flexure strength) of LFC. METHODS: Disks and bars were mirror polished and annealed prior to aging in: (1) air (control), (2) water for 24h at 80 degrees C and (3) water for 8 weeks at 80 degrees C. Fracture toughness (K(Ic)) was determined by indentation fracture (IF) and indentation strength (IS) using a 19.6N Vickers indentation load. Flexure strength values were obtained from three-point bending at 0.1mm/min. Statistical analysis was performed using the Weibull distribution, Tukey and Bartlett tests (P<0.05). RESULTS: Both techniques (IS and IF) showed a significant improvement in the K of Duceram LFC after 8 weeks in water (0.88 and 1.14MPa m(0.5)) as opposed to the 24-h values both in water and air (0.77-0.78MPa m(0.5)). However, for flexure strength the Weibull characteristic (S(0)) and the m parameter did not change significantly with water storage (S(0)=90-100MPa, Weibull m =7-8). SIGNIFICANCE: The increase in toughness of Duceram LFC after aging in water is an interesting and favorable observation for a restorative material exposed to the oral environment. Nevertheless, in comparison with other contemporary ceramics, the toughness of this LFC remains in the range of soda-lime-glass or classic feldspar porcelains.

Acetic Acid↗
PubMed2001

Incidence of fractures and lifetime predictions of all-ceramic crown systems using censored data.

PURPOSE: To determine Kaplan-Meier survival estimates and Weibull lifetime predictions for four all-ceramic crown systems from long-term data (> 5 yrs). MATERIALS AND METHODS: Single unit crowns of Cerestore (n = 30), Dicor (n = 30), Hi-Ceram (n = 22) and In-Ceram (n = 68) were placed in 95 patients treated in a university clinic. They were cemented using glass-ionomer (GI) for Cerestore, zinc phosphate (ZP) for Dicor, and 75% ZP (n = 51), 20% GI (n = 13) and 5% resin-based cement (n = 4) for In-Ceram crowns. The follow-up times were 8 yrs for Cerestore, 7 yrs for Dicor, 6 yrs for Hi-Ceram and 5 yrs for In-Ceram. The statistical analyses were based on censored data sets. A progressively censored Weibull distribution allowing for lifetime predictions beyond the actual observation time was used as well as the Kaplan-Meier Survival Product Limit Estimate (PLE), which provides survival estimates up to the maximum time of follow-up. RESULTS: Cerestore, Dicor and Hi-Ceram demonstrated molar fractures in the first 2 yrs, whereas for In-Ceram these occurred during the third and fourth year of the study. The Kaplan-Meier Survival PLE was 69% for Cerestore at 8 yrs, 86% for Dicor at 7 yrs, 81% for Hi-Ceram at 6 yrs, 92% for In-Ceram at 5 yrs. The predicted Weibull characteristic time T0 (time at which 63% of the restorations would have failed) was 23 yrs for Cerestore, 34 yrs for Dicor, 31 yrs for Hi-Ceram, and 16 yrs for In-Ceram. However, when using data sets arbitrarily limited to the three first years of follow-up, T0 decreased significantly for Cerestore, Dicor and Hi-Ceram due to the number of early fractures. Such Weibull lifetime data illustrate the risk of predicting long-term (> 5 yrs) survival using short-term (< or = 3 yrs) data on ceramic restorations, which exhibit fracturing in the initial years.

Aluminum Oxide↗
PubMed2000

Fracture toughness of aged dental composites in combined mode I and mode II loading.

Resin-based laboratory dental composites for prosthetic restorations have been developed in the past years as a cost-effective alternative to conventional porcelain-fused-to-metal or full ceramic restorations. The fracture toughness at different stress states (K(Ic), K(IIc), and mixed-modes K(I), K(II) ) was assessed for three laboratory dental composite resins used for prosthetic restorations that were aged up to 12 months in a food simulating fluid (10% ethanol) at 37 degrees C. The materials were mainly di- methacrylate based resins reinforced with submicron glass filler particles. The Brazilian disk test was used on precracked chevron-notched specimens, and different stress states were obtained by angulating the precracked chevron notch relative to the diametral compressive loading direction. The stress intensity factors were calculated using Atkinson et al.'s relation. For all three materials, mode I fracture toughness values ranged between 0.48-0.64 MPa. m(0.5) and mode II values ranged between 0.93-1.2 MPa. m(0.5). Overall, aging time and storage media had little effect on toughness. Considering the inherently low toughness of these restorative materials, their use should be limited to low stress masticatory areas.

Aluminum Oxide↗
PubMed2000

A new method to quantify wear using implant supported restorations.

OBJECTIVES: To design a novel technique to assess the wear of prosthodontic veneering materials. Further to determine whether accurate transfer between the oral cavity and the measuring device is achievable and assess the reproducibility of the coordinates generated by the measuring system. METHODS: The system is based on the repositioning capacity of an octagonal connector of the ITI implant system. The same type of connector was screwed onto the clinical implants that supported the experimental restorations and secured to the x-y table of the measuring device. The measuring setup also comprised a z-axis LVDT displacement gauge that allowed the entire surface of the restorations to be profiled and digitized. The system was under the control of a PC equipped with custom-made software that set the position of the stepping motors, lifted and lowered the z-axis probe, and registered and wrote the x-, y- and z-axis coordinates. Final numerical adjustments and analyses were performed using a commercial array-oriented software package. Validation procedures were performed using a specially designed calibration surface. RESULTS: On repeated profile tracings, the measurement error was less than 2 microns. When the calibration surface was removed between measurements as during clinical trials, the measurement error increased to ca. 5 microns. SIGNIFICANCE: The measurement error of the testing procedure including transfer to and from the mouth is +/- 5 microns.

Calibration↗
PubMed2000

A continuing education programme for general practitioners. Status report after 5 years of function.

In 1992, following newly issued university regulations, the board of the School of Dental Medicine of the University of Geneva decided to establish a structured continuing education course programme for practising dentists in an effort to better meet the school's continuing education mandate. The programme started in January 1994 and was structured so that regular courses would be offered in each discipline of dentistry. The course contents were aimed at satisfying the demands of practising dentists, but it was also established that basic science issues and theoretical concepts should be included. Possible course formats were "conference", "hands-on", "clinical" and "seminar". The courses were meant as a form of knowledge transfer from the school to the practising community, but also as a means to generate revenue for research and teaching programmes. Operative aspects were supervised by a small staff which was assisted by computer software designed to handle all procedural steps of course administration, participant registration, accounting, communication. The dentists' responses were rewarding in that attendance was very satisfactory. Closer scrutiny of our data, however, indicates that our impact is still low since at best only 20% of the course-hours required by the Swiss dental association are actually taken. Both course and programme evaluations were satisfactory and are discussed using the Harden and Laidlaw CRISIS criteria.

Adult↗
PubMed1999

Lack of integration of smooth titanium surfaces: a working hypothesis based on strains generated in the surrounding bone.

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.

Alveolar Bone Loss↗
PubMed1999

Compressive and tensile zones in the cement interface of full crowns: a technical note on the concept of resistance.

PURPOSE: 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. MATERIALS AND METHODS: Three-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 degrees and 20 degrees 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, i.e., parallel to the direction of loading). Von Mises stresses were also generated. RESULTS: First, 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. CONCLUSIONS: Conventional 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.

Chromium Alloys↗
PubMed1998

Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic.

OBJECTIVES: Various methods aimed at determining the fracture toughness of ceramics in mode I (KIc) have been described in the literature. The accuracy, scatter and the interexaminer reproducibility of KIc depend strongly on the procedural approach, the test parameters used and the conditioning of the specimens. The purpose of the present study was to compare fracture toughness values obtained using two indentation methods as well as a newly established fracture mechanics test. METHODS: The following methods for KIc determination were applied: (1) indentation fracture (IF), (2) indentation strength (IS) and (3) the single-edge-V-notched-beam test (SEVNB). The materials tested were a low-fusing dental glass (Duceram LFC) and a feldspar-based porcelain (IPS classic). Data were compared by ANOVA and Tukey's multiple comparison test (p < or = 0.05). RESULTS: For both materials, KIc coefficients of variation ranged between 10 and 14% for IF and 7 and 10% for IS. The IS technique demonstrated a load dependency for the IPS porcelain which was not observed when using the IF method. The SEVNB test provided consistent results with coefficients of variation between 1 and 3%. SEVNB toughness values for the IPS porcelain were in agreement with the IS technique. However, halfpenny shaped cracks were observed at the tip of the notch of all LFC specimens thus leading to underestimated KIc values. SIGNIFICANCE: The overall aim of this type of study is to select testing procedures that are as expedient and reliable as possible. This study has shown that all three methods agreed within 10%. However none of the procedures proved absolutely straightforward. Decision on which method to use should be based on a sound understanding of the conceptual limitations and technical difficulties inherent to each technique.

Aluminum Silicates↗
PubMed1997

Mechanical and elemental characterization of solder joints and welds using a gold-palladium alloy.

PURPOSE: This study was conducted to determine whether newer infrared or laser welding technologies created joints superior to traditional furnace or torch soldering methods of joining metals. It was designed to assess the mechanical resistance, the characteristics of the fractured surfaces, and the elemental diffusion of joints obtained by four different techniques: (1) preceramic soldering with a propane-oxygen torch, (2) postceramic soldering with a porcelain furnace, (3) preceramic and (4) postceramic soldering with an infrared heat source, and (5) laser welding. MATERIAL AND METHODS: Mechanical resistance was determined by measuring the ultimate tensile strength of the joint and by determining their resistance to fatigue loading. Elemental diffusion to and from the joint was assessed with microprobe tracings. Scanning electron microscopy micrographs of the fractured surface were also obtained and evaluated. RESULTS: Under monotonic tensile stress, three groups emerged: The laser welds were the strongest, the preceramic joints ranged second, and the postceramic joints were the weakest. Under fatigue stress, the order was as follows: first, the preceramic joints, and second, a group that comprised both postceramic joints and the laser welds. Inspection of the fractographs revealed several fracture modes but no consistent pattern emerged. Microprobe analyses demonstrated minor diffusion processes in the preceramic joints, whereas significant diffusion was observed in the postceramic joints. CLINICAL IMPLICATIONS: The mechanical resistance data conflicted as to the strength that could be expected of laser welded joints. On the basis of fatigue resistance of the joints, neither infrared solder joints nor laser welds were stronger than torch or furnace soldered joints.

Ceramics↗
PubMed1995

A rationale for a simplified occlusal design in restorative dentistry: historical review and clinical guidelines.

An occlusal contact pattern in which the number of occlusal contacts has been substantially reduced as compared with traditional schemes is described. Concepts that may have had a justification in balanced occlusions have been needlessly transferred to anterior disclusion mechanics. No natural dentition presents occlusal contacts as described in many texts and yet stability is established. The temporomandibular joint does present structural changes that should be accounted for when an occlusal anatomy is designed. The force vectors that are active on teeth are not directed along the longitudinal axes of the roots only, and thus occlusal contact locations will not determine the direction of functional forces. The stability of the teeth on the arch depends primarily on the forces of eruption from the periodontium and the balance between the resting pressures of the muscles of the cheeks and the tongue. The mechanics of the stomatognathic system are not as accurate as their counterpart on an articulator. The variability of the guiding surfaces inherent to the temporomandibular joints should be incorporated into an occlusal design. Occlusal contacts that do not fulfill a justifiable purpose may be eliminated, and the number of contacts may be reduced to one per tooth.

Clinical Protocols↗
PubMed1994

Fatigue resistance of soldered joints: a methodological study.

OBJECTIVES: In this investigation, the fatigue resistance of solder joints under cyclic loading was evaluated. METHODS: Au-Pd alloy rods were machined, prepared for soldering and joined using 735 solder. After trueing and polishing the joints, the S-N diagram (cycles to failure vs. applied stress) was generated. A conventional endurance limit (SN) was determined for 10(6) load cycles. Testing was carried out in a machine specifically designed to apply flexural fatigue loading to cantilevered test specimens. These were rotated around their main axes, and the device applied a sinusoidal, reverse-bending stress to the solder joints. The applied stress ranged from 300 MPa to 75 MPa in decrements of 25 MPa. Twelve specimens were cycled for each stress level until fracture occurred or 10(6) cycles were sustained (run-outs). In this first series of tests, the cycling speed corresponded to an average chewing rate, i.e., 1 Hz (60 rpm). In order to reduce the time required for testing, the cycling speed was then increased to 5, 10 and 15 Hz (300, 600 and 900 rpm). RESULTS: At 1 Hz, SN was 133.0 MPa, while at the higher cycling speeds, SN increased to 139.3, 160.8 and 175.8 MPa. SIGNIFICANCE: It was concluded that rotational fatigue tests as applied in this study were a feasible fatigue testing procedure. However, SN's gathered at faster rates might need correction factors if relationships with data pertaining to clinically relevant chewing rates are to be established.

Dental Soldering↗
PubMed1992

Mechanical resistance of cemented post and core buildups for ITI-Bonefit implants.

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.

Analysis of Variance↗
PubMed1991

Fatigue strength of a Au-Pd alloy/585 solder combination.

The S-N diagram (applied stress vs. cycles to failure) for the gold-palladium alloy/585 solder combination was generated. Au-Pd alloy specimens were cast, prepared for soldering, and joined with 585 solder. After milling and polishing of the joint, testing was carried out in a specially designed machine. This device loaded the cantilevered test specimen in a unidirectional mode at a rate of 70 strokes per minute. The applied tensile stress ranged between 294.2 and 686.5 MPa. Three specimens were cycled for each stress level until fracture occurred. The number of fatigue cycles to failure ranged from one cycle (stress level, 686.5 MPa) to 4.83 x 10(6) cycles (stress level, 294.2 MPa). On a logarithmic scale, the relationship between the applied stress and the number of cycles to failure was linear. Under the present (ideal) experimental conditions, the Au-Pd/585 solder combination behaved favorably on the high-cycle end of the diagram. However, no definite endurance limit could be detected within the stress range examined.

Confidence Intervals↗
PubMed1991

[Preprosthetic orthodontics: technical aspects of mechanotherapy].

This paper describes the rationale as well as some of the therapeutic tools which may be used during simple preprosthetic orthodontic movements. First a typical bracket is examined: the overall design, the base, the slot and the tie-wings. All those components allow mechanical force transmission from the archwire to the tooth in all three orders of movement. In the classic "edgewise" form of treatment, final placement of the teeth is determined by the bends incorporated into the archwire. Conversely, in the more recent "straight-wire" approach, the angulation of the slot relative to the bracket base provides for an optimal positioning of each individual tooth on the dental arch. Mesio-distal angulation of the roots being referred to as "tip" and bucco-lingual angulation as "torque". The bracket also allows bodily bucco-lingual tooth placement relative to the dental arch as a whole (in-out). Prior to a comprehensive treatment, brackets are placed on teeth according to a standard set of criteria. They must be located midway mesio-distally on the buccal aspect in accordance with the true anatomic longitudinal axis of the root. Its occluso-gingival position however can be slightly adjusted according to the individual needs of the case. In more limited situations, where only a few teeth are moved, those rules have to be adapted since the anchorage teeth are kept stable. Examples are presented which illustrate the key features of limited orthodontic movements. Archwires present themselves in various sections and alloys. The most commonly used metals are stainless steels and the more recently introduced nickel-titanium superelastic alloys.(ABSTRACT TRUNCATED AT 250 WORDS)

Dental Bonding↗
PubMedDate not supplied

The effect of film thickness and surface texture on the resistance of cemented extracoronal restorations to lateral fatigue loading.

PURPOSE: The aim of the present study was to assess the effect of cement-film thickness and surface texture (roughness) on the resistance of cemented crowns to dynamic lateral loading. MATERIALS AND METHODS: Crown and abutment analogues were cemented using zinc-oxide-eugenol, zinc-phosphate, glass-ionomer, and composite cements. The space left for the cement lute was 0.02, 0.05, 0.1, 0.2, and 0.5 mm. The 3 degrees of surface texture subjected to investigation were (1) polished with up to 4,000-grit paper, (2) sanded using a 1,000-grit paper, and (3) sandblasted with 50-micron aluminum oxide. Testing was conducted according to the staircase procedure. The specimens were subjected to rotational fatigue loading until the cement bond failed or the components reached 1,000,000 stress cycles. RESULTS: The results showed that the relation between cement thickness and resistance to dynamic lateral loading is hyperbolic. For the zinc-oxide-eugenol, the zinc-phosphate, and the glass-ionomer cements increasing surface texture had a moderate effect. For composite cement, sandblasting doubled the resistance to dynamic lateral loading. For both parameters tested (cement thickness and surface texture), the ascending order of resistance was: zinc-oxide-eugenol, zinc-phosphate, and glass-ionomer cements. Crowns cemented with composite cement presented the highest resistance to dynamic lateral loading. CONCLUSION: Within the confines of the present experimental design, it is concluded that (1) decreasing the width of the cement layer increases the resistance to dynamic lateral loading, and (2) texturing the surface of the abutment and the restorations as after sandblasting increases the resistance to dynamic lateral loading.

Cementation↗
PubMedDate not supplied

Microstructures of brazings and welds using grade 2 commercially pure titanium.

PURPOSE: Microstructural analyses of commercially pure titanium (CpTi) are scarce. The present report presents the micrographs, fractographs, elemental characteristics, and hardness profiles of brazed joints and weldments using machined rods of CpTi. MATERIALS AND METHODS: CpTi rods were joined using four techniques: laser welding, electric-arc welding, electron-beam welding, and gold- and Ti-filler brazing. The specimens were then subjected to tensile and fatigue loading. After sectioning and patterning, optical micrographs of intact joints were obtained. Fractured surfaces were investigated using scanning electron microscopy (SEM). The joint's composition was determined by SEM-energy dispersive x-ray analysis. Hardness was determined at specific locations using a microindenter. RESULTS: While laser welding left the parent metal's equiaxed structure fairly intact, electric-arc welding, electron-beam welding, and brazing created a heat-affected zone in the vicinity of the joint. The extent and characteristics of the heat-affected zone depended on the amount of heat transferred to the specimens. In this respect, brazing essentially increased grain size and altered their shape. Electron-beam welding augmented this phenomenon, yielding grains that encompassed the full diameter of the joint. Electric-arc welding disrupted the granular pattern and generated highly lamellar/acicular structures. CONCLUSION: Hardness was not a good indicator of mechanical resistance, nor was the joint's structural continuity with the parent substrate. Still, acicular microstructures were characterized by a peculiar behavior in that such joints were highly resistant to tensile stresses while their fatigue strength ranged among the lowest of the joints tested.

Copper↗