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Biomedical subjects

H W Wiskott

Publications and source records attributed to H W Wiskott.

11 recordsLinked to original sources
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

Stress fatigue: basic principles and prosthodontic implications.

Clinical evidence indicates that the majority of fractures that occur in prosthodontic structures do so after a period of many years. Such failures generally are not related to an episode of acute overload but result from fatigue failure. This paper reviews the current knowledge of fatigue failure and test methods. An overview of published studies is given, and the authors suggest guidelines for future prosthodontic studies of this nature.

Chemical Phenomena↗
PubMedDate not supplied

The relationship between abutment taper and resistance of cemented crowns to dynamic loading.

This study investigated the relationship between abutment total occlusal convergence angle (taper) and the resistance of cemented crowns subjected to dynamic loading. Crown and abutment analogs were placed using zinc-oxide-eugenol, zinc-phosphate, glass-ionomer, or resin composite cement. Total occlusal convergence angles of 2.5, 5, 10, 15, 20, 30, and 40 degrees were used. Dynamic stresses were applied to the luted components until the bond failed or the components reached 10(6) load cycles. The data were analyzed using the staircase technique. The relationship between convergence and resistance was approximately linear for all the cements tested. Crowns luted with resin composite cement were more resistant to dynamic lateral loading than those placed using glass-ionomer or zinc-phosphate cements. Crowns luted with zinc-oxide-eugenol cement presented the least resistance to cyclic lateral stresses.

Cementation↗
PubMedDate not supplied

The effect of tooth preparation height and diameter on the resistance of complete crowns to fatigue loading.

This study investigated the relationship between tooth preparation height and diameter and the resistance of cemented crowns to dynamic loading. Crown and abutment analogs were cemented using zinc oxide-eugenol, zinc phosphate, glass-ionomer, and resin composite luting agents. Abutment height varied from 1 to 7 mm in increments of 1 mm. Diameters of 3, 4, 5, 6, 7, and 8 mm were tested. A testing apparatus designed to apply dynamic stresses was used. The cemented crowns were loaded until the cement bond failed or the components reached 1 million stress cycles. The staircase technique was applied for data analysis. The results showed that: (1) the relationship between abutment height (or diameter) and resistance to dynamic lateral loading is approximately linear; and (2) crowns luted using resin composite cement were more resistant than those placed using glass-ionomer cement, which in turn were more resistant than crowns cemented with zinc phosphate or zinc oxide-eugenol cements.

Cementation↗