The International Journal of Prosthodontics, 06/1991

The purpose of this study was to evaluate the marginal adaptation of all-porcelain labial margin metal ceramic crowns using a porcelain shoulder material containing a light-polymerizing resin and one that used a direct-lift technique. Conventional metal margin restorations served as controls. Scanning electron micrographs were made of a 1-mm mesiodistal width of the margin at midfacial and midlingual reference marks. Image processing and analysis techniques were accomplished using a computer. Statistical evaluation indicated that the mean labial marginal discrepancy of the control group was significantly less than that of either of the two porcelain shoulder methods.

There is currently no consensus of opinion regarding form of the finish line for porcelain inlays. This study compared beveled and nonbeveled finishing lines. Twenty-four Class II MOD cavities of a standardised design were prepared in extracted premolar teeth. Twelve were finished using a half enamel occlusal bevel and 12 were not beveled. Porcelain inlays were fabricated and luted with a dual-polymerizing resin material. The completed restorations were stored in water, thermocycled, and analysed using a scanning electron microscope. The quality of the enamel/composite resin interface was found to be considerably better than that of the inlay/composite resin interface. The adaptation of composite resin to enamel was of equal quality for both beveled and nonbeveled preparations.

This article describes a technique for fabricating an interim obturator for maxillectomy patients using a microwave-polymerized denture base resin. The use of microwave irradiation to polymerize this type of resin reduces the time required to fabricate interim obturators and results in prostheses comparable in physical properties and superior in dimensional accuracy to prostheses made with conventional heat-polymerized acrylic denture base resin. The technique is simple and time efficient, and it does not require any special or elaborate laboratory equipment.

The Michigan Computer Graphics Coordinate Measurement System was used to measure the wear of three composite resins and an amalgam alloy control. The validity of the Latin Square experimental design for analysis in a clinical study was demonstrated, and statistically significant differences in wear were established for main effect for the materials under investigation. None of the composite resins examined approximated the amalgam alloy control in wear properties. Rank ordering of the materials with respect to wear was possible using this experimental design. Ranking of the materials for wear resistance using this research approach opens up other future research opportunities to correlate such properties as filler particle size, composition, and distribution with the data derived.

The design of removable partial dentures has progressed both clinically and in laboratory research. Nonetheless, there still appears to be a conflict between the empirical practice and the scientific data available. On this basis, the authors question several aspects of the lingual connector. If lingual connectors are required to be rigid, how is this best achieved and can it be assured that designs impart sufficient rigidity? On the contrary, is it possible to make the connector too rigid? No conclusions are attempted in this theoretical discussion, but research to resolve the dilemma is encouraged.

A technique enabling the study of adherence occurring among implant materials and epithelial and connective cells cultured in vitro and freshly obtained from explants of human gingival tissue is described. The data show that cell adherence to titanium alloy is influenced by the modality of implant surface preparation.

Porcelains that are claimed to improve the appearance of metal ceramic restorations by increasing light refraction from a highly textured opaque surface have recently been introduced. This study compared the appearance and color of one of these opaque porcelains with a conventional smooth-surface opaque. The opaque porcelains were applied to alloy substrates, and samples were evaluated colorimetrically. After the application of body and incisal porcelains, the samples were evaluated subjectively by clinicians and again colorimetrically. Subjective determination of the most lifelike porcelain depended on the shade. Color differences existed between the opaque porcelains for all shades examined.

The wettability of eight nonaqueous elastomeric impression materials was studied by comparing their contact angles. The materials included three polyethers (one of which was light activated), three hydrophilic poly(vinyl siloxanes), one conventional poly(vinyl siloxane), and one poly(vinyl siloxane) putty. Extracted teeth were prepared to approximate the roughness of a tooth preparation. Contact angles were measured at different time intervals after the start of mixing but were not shown to be significant. The nonhydrophilic poly(vinyl siloxane) materials and the poly(vinyl siloxane) putty were found to be significantly less wettable.

Shear adhesive strength tests were conducted to examine the adhesion of a newly developed light-polymerized denture base resin (LPR) to resin teeth and denture base materials. When LPR was bonded to these materials with a new light-activated bonding agent, the adhesive strength improved greatly. Adhesive strengths of LPR to denture base metals were greater than, or similar to, the metal adhesive autopolymerized resin containing 4-META. Compared with the metal adhesive heat-polymerizing resin, the LPR adhesive values were less when bonded to pure titanium but greater for 18-8 stainless steel and the gold-silver-palladium alloy.

The effect of opacity on the colorimetric responses of large-area and small-area colorimeters was determined using an elastomer intended for maxillofacial prosthetics use and containing various pigments at different concentrations. Opacity was determined by calculating the contrast ratio of 2-mm-thick specimens against black and white backings, using Kubelka-Munk analyses to correct for thickness and backing color variations. The measure of comparison of the two colorimeters was the relative difference in tristimulus reflectance, with the tristimulus reflectance of the large-area colorimeter as the basis of the relative difference. A significant quadratic relationship was found between contrast ratio and the relative difference in tristimulus reflectance. This relationship may be used to describe opacity without the need to make optical observations or measurements of a thin layer of material on contrasting backings. The small-area colorimeter produced color parameters that are a measure of the combined effects of both color and opacity. The importance of beam size considerations of optical measuring devices for translucent natural and prosthetic materials was emphasized.

A holographic interferometer for investigating deformations of dental prostheses is described. Residual stress relaxation resulting from the polymerization of acrylic resin dentures is qualitatively studied during the first 48 hours following fabrication. The resulting fringe patterns show a symmetry that is correlated to the shape of the denture. A quantitative analysis of the interferograms was performed with the aid of a computer program. The computer-aided holographic interferometer was tested using disk-shaped acrylic resin materials. Dimensional deformations resulting from temperature changes were measured with an accuracy better than 0.2 microns. Because of the aging of the resin material, the reaction to temperature changes may differ as a function of time and may be studied using quantitative comparison of deformation plots. For convenience, the original object shape can be added to or subtracted from the graphical deformation data.