The International Journal of Prosthodontics, 05/1991

A two-dimensional finite element method was used to analyze the changes in mechanical behavior of the supporting structures when a fixed prosthesis replaced a mandibular first molar. In the unrestored situation, as the degree of bone resorption increased there was a corresponding increase in stress in the periodontium. Tilting of the molar abutment induced additional stress on the mesial side of the root. The presence of a fixed prosthesis markedly reduced the magnitude and distribution of stress in the periodontium. The mechanical advantage obtained by a fixed prosthesis was greater in the situation of a tilted second molar with reduced bone support than with higher bone levels.

Infrared soldering was compared to gas-oxygen torch soldering by testing specimens made from each of the following four classes of metal ceramic alloys: gold-platinum-palladium, gold-palladium, palladium-silver, and nickel-chromium-beryllium. There was no significant difference between infrared and torch soldering for the gold-palladium, palladium-silver, and base metal alloy specimens. However, infrared soldering of the gold-platinum-palladium alloy samples produced significantly weaker joints than those produced by torch soldering.

The stability of the bioactive surface is a major concern in the chemical and physical design of apatite ceramic coatings. In clinical studies, no sintered-hydroxyapatite bulk coating material was present in the surrounding bone tissue after loading periods up to 11 years. In in vitro studies, hydroxyapatite plasma-sprayed coatings dissolved considerably, possibly because of loss of crystallinity during the plasma-spraying procedure. A heat treatment at 600 degrees C reconstituted the crystallinity and promoted the transformation of alpha-tricalcium phosphate into the more stable beta-tricalcium phosphate. Fluorapatite coatings did not lose their crystallinity and had the added advantage of stability as a result of the fluoride ion.

Three bonding agents were evaluated to determine their effect on the strength of the bond when Silskin II and Cosmesil SM4 silicone facial elastomers were polymerized to a light-activated denture resin. A silane bonding agent proved to be superior, whereas a light-activated bonding agent had inferior bonding characteristics. Intensive heat aging had no significant effect on bond strength.

One premixed and three powder-liquid light-polymerized, hard denture reline materials were evaluated for Knoop hardness, transverse strength, and tensile strength when bonded to a heat-activated denture resin. The chemical compositions of the components of the reline materials were analyzed. The premixed light-activated liner was harder, had a higher tensile bond strength to the heat-activated denture resin, and was somewhat stronger than the power-liquid reline materials. Properties of the resins were related to their compositions.

The influence of four denture adhesives (Fittydent, Super Poligrip, Super Wernet's, and tragacanth) on patients' opinions of 10 aspects of denture use was studied. The adhesives were applied in turn to the existing complete dentures of 32 patients in a sequence determined by eight randomly chosen 4 x 4 Latin squares of different treatments. Subjective views of the patients indicated that one product significantly improved denture retention and had other favorable features.

Postceramic soldering of collarless veneered retainers in which the porcelain margins were formed with a platinum foil matrix technique was investigated. The purpose of the study was to determine if removing the platinum foil matrix before or after indexing and soldering procedures would affect the porcelain margin integrity in fixed partial dentures. Prostheses were fabricated on a nickel-chromium laboratory model using both sequences. Six test cycles were performed. Each cycle included one soldering with matrix support and one without the matrix, for a total of 12 solderings. The fixed partial dentures were compared for degree of marginal seating with a measuring microscope and for configuration changes at the porcelain margins with scanning electron micrographs. In the microscopic analysis of marginal closure, soldering without foil matrix was statistically equal to soldering with foil in place. Removal of the platinum foil matrices prior to indexing provided for no further seating of the metal ceramic retainers. Evaluation by scanning electron microscopy demonstrated distinct configuration changes in the porcelain margins for the specimens soldered without matrix support.

A combined orthodontic/prosthodontic technique for the treatment of tipped abutment molars is described. The technique uses a cast metal removable partial denture with an uprighting wire and coil spring engaging an orthodontic tube bonded onto the tipped abutment. When the molar has been uprighted to the desired position, a clasp assembly is added to the prosthesis.

The maxillary and mandibular casts of 45 healthy and dentulous subjects, with conspicuous wear facets on canines and molars, were mounted on a semiadjustable articulator. The sagittal condylar guidance was determined by two methods: (1) using a protrusion wax interocclusal record and (2) by matching the wear facets of opposing canines and contralateral molars. There was a substantial bilateral symmetry of the right and left sagittal condylar guidance angles using both methods. There were, however, significant differences between the mean angles of the two methods, about 31 degrees (protrusion record) versus about 24 degrees (wear facets). This, in association with rather low coefficients of linear determination, suggested that the two methods of recording the sagittal condylar guidance angle were based on totally different phenomena.

A new type of highly resilient retainer for maxillary obturator prostheses is described. The obturator component is composed of an acrylic resin connector and a round resilient wing, with the combined shape resembling a mushroom. The retainer is designed to absorb mechanical stresses, which are transmitted from the denture portion to nasal soft tissues through the obturator extension, and to use tissue undercuts more effectively than do conventional hollow extension prostheses. Renewability of the retainer part is another advantage of this retainer. From these properties, the retainer seems to have many indications for the prosthodontic treatment of maxillary defects.

It has been traditional either to cast invested wax patterns immediately after the investment has set or to store the invested patterns in a humidor for casting later. This study compared the fit of MOD inlays to parent dies after casting immediately following set of the investment, after casting 1 hour following the setting of the investment in air, storage in a humidor for 7 days, and after storage in air for 7 days. The only significant differences in fit were in the group stored in a humidor, and then only at the axiogingival junctions.

Clinical researchers seek to eliminate specific subject attributes from becoming confounding variables to distinguish a treatment effect from an underlying patient effect. In prospective studies, the random assignment of subjects into experimental groups results in the equal distribution of subject attributes, thereby eliminating them as confounding variables. Retrospective study designs, often used in prosthodontics research, preclude the random assignment of subjects. This article describes the application of a standard research design that combines the use of intersubject and intrasubject controls to compensate for the potential imbalance in subject attributes in retrospective clinical studies. This approach should help prevent clinical investigators from declaring differences between experimental groups when, in fact, no differences exist.