Terry E. Donovan

An evaluation of impression techniques for osseointegrated implants

A passive fit between osseointegrated implants and the prosthesis they will support has been advocated. An experimental model was developed to test the accuracy of three impression techniques and the components used to make the transfer records. Statistically, no significant difference was found between the three methods tested. From this initial study, it appears that further work is needed to isolate techniques that will predictably provide accurate registration of the position of endosseous implants.

Review and survey of medicaments used with gingival retraction cords

Data from a survey of 495 dentists indicate that most dentists used the mechanical-chemical method of gingival-deflection; 79.39% of those used cord containing epinephrine. It can be concluded that potentially significant amounts of epinephrine can be absorbed systemically from the local anesthetic solution, that secretion of endogenous epinephrine in response to stress occurs, often at levels sufficient to cause measurable changes in hemodynamic variables, and that absorption of epinephrine from impregnated strings occurs. The amount of absorption will vary with the exposure of the vascular bed, the length and concentration of the impregnated cord, and the length of time of application. It is possible that the actual total amount of circulating catecholamine would be cumulative, and the corresponding cardiovascular response would be related to the total amount of epinephrine in the bloodstream, regardless of the source. When the fact that we usually have inadequate data on the cardiovascular status of our patients is considered, as well as the tendency to make impressions of multiple prepared teeth, the continued use of epinephrine cord in dentistry must be viewed with alarm. Equally effective astringent gingival deflection agents such as alum, aluminum sulfate, and aluminum chloride exert no systemic effects. Therefore, there is little indication for use of epinephrine-containing retraction cords. Adequate medical evaluation, careful use of anesthetics that contain epinephrine, and sedative techniques when indicated will assure the safety of our patients.

Physical properties of acrylic resin polymerized by four different techniques.

A utopolymerizing acrylic resin is used frequently in fixed prosthodontics to make provisional restorations, many of which are expected to remain in service for extended periods of time (Fig. 1). Physical properties of strength, density, and hardness may predict the longevity of such restorations. Provisiona restorations may be made intraorally or by an indirect technique. The indirect technique has significant advantages, including improved marginal integrity and prevention of pulpal injury caused by exothermic polymerization of the acrylic resin.lm3 Several techniques for polymerizing acrylic resin have been advocated, including processing under pressure in water to improve the physical properties.4,5 Scant scientific documentation exists to justify a particular technique.6 The purpose of this study was to determine the transverse strength, porosity, and hardness of an autopolymerizing polymethyl methacrylate resin processed under the following conditions: (1) polymerization in air, (2) polymerization under water, (3) polymerization under air pressure, and (4) polymerization under water under air pressure. Fig. 1. Provisional fixed partial denture processed in pressure pot functioned successfully for several months.

The dimensionsal accuracy of improved dental stone, silverplated, and epoxy resin die materials

1 mproved dental stone is the predominant material in dentistry for making dies used in the lost-wax process. Although improved dental stones have served the profession well, the main disadvantages of their use as dies are (1) susceptibility to dimensional change due to abrasion, (2) limited reproduction of fine detail,’ and (3) lack of strength. The lack of strength can result in breakage of dies, especially those duplicating long, narrow preparations. In the fabrication of all-porcelain labial margins or some of the new generation cast porcelain restorations, dies that exhibit greater strength and abrasion resistance than gypsum dies are desirable. Alternate die materials that may yield dimensionally accurate dies, are more resistant to abrasion, and are stronger than the improved dental stone dies presently in use are available. The literature concerning dimensional accuracy of various die materials is somewhat equivocal. Gypsum die materials generally exhibit an average setting expansion of O.l%.* Such minimal expansion is thought to be beneficial in terms of aiding compensation for metal shrinkage, wax pattern dimensional change, and other unavoida.ble inaccuracies in the casting process. Electroplated dies have been found to be less accurate dimensionally than improved stone dies when formed with polysulfide rubber or condensation-reaction silicone impression m.aterials.3 One study demonstrated that silver-plated dies tend to be undersized by as much as 0.2%; whereas another found that they tend to be oversized.5 It has been shown that vinyl polysiloxane impression ma.terials are the most accurate for making silver-plated dies.+ Castings made on silver-plated dies have been shown to have smaller marginal openings than those made on improved stone dies.6 Epoxy resin dies have been found to be generally undersized, pri.marily because of polymerization shrinkage.’ The manufacturer of the new generation epoxy resin die systern claims to have controlled this shrinkage and that dimensional accuracy can be precisely con-

Effect of four pretreatment techniques on porcelain-to-metal bond strength

Porcelain bond strengths of three base metal alloys with different metal pretreatment techniques were studied. Statistically significant differences were found in bond strengths achieved between the different metals but not with the different pretreatments. Each metal displayed considerable variation in bond strength with each of the pretreatment techniques. Collectively, the base metal alloys evaluated displayed a disturbing tendency for adhesive failure through the porcelain-metal interface, although this was not universal. Manufacturers and ceramists should develop a specific regime for porcelain bonding for each alloy used. Standardized testing should demonstrate adequate bond strengths with cohesive failure through the body of the porcelains.

Inhibition of polymerization of polyvinyl siloxanes by medicaments used on gingival retraction cords.

Inhibition of polymerization of polyvinyl siloxane materials by latex products is well documented. It is thought to be caused by contamination of the chloroplatinic acid catalyst by sulfur compounds. Many medicaments, such as aluminum sulfate and ferric sulfate, used on gingival retraction cords have been accused of causing inhibition of set of polyvinyl siloxane materials. Several of these medicaments were tested with two polyvinyl siloxanes and found not to cause any inhibition of set.

An analysis of margin configurations for metal-ceramic crowns

Major types of margin configurations for ceramometal crowns are reviewed. This is one of the most commonly misunderstood and poorly practiced aspects of fixed prosthodontics. The resultant underprepared teeth produce discolored gingiva and chalky crowns.

The effect of vacuum-mixed autopolymerizing acrylic resins on porosity and transverse strength

Three brands of autopolymerizing acrylic resin were mixed and polymerized three different ways and tested for transverse strength and porosity. The procedures tested were bench-curing at ambient temperatures, vacuuming the mix before bench-curing, and curing in a pressure device. The data indicate that both vacuuming the mix and pressure-curing are successful in reducing porosity and increasing transverse strength. These improved properties would be valuable in the fabrication of provisional restorations and occlusal splints. The following conclusions can be drawn: 1. Use of a pressure device decreases porosity and increases the transverse strength in samples of autopolymerizing acrylic resin. 2. Use of a vacuum device on the mixed acrylic resin works as well as pressure polymerizing. 3. Use of either technique is indicated to improve the quality and longevity of autopolymerizing acrylic resin restorations, orthodontic appliances, and occlusal splints.

Effect of electrodeposition of gold on porcelain-to-metal bond strength

The effect of electrodeposition of two different thicknesses of gold on porcelain-metal bond strength with three different base metal alloys was determined and compared to a control high gold alloy. The following conclusions can be drawn. Mean porcelain-metal bond strengths of all four alloys were in the same range when conventional porcelain application techniques were used. However, the base metal porcelain-metal bond is clinically suspect because of adhesive failure through the porcelain-metal interface. A thin 180A gold coat increased the porcelain-metal bond strength and generally resulted in cohesive failure through the porcelain. A thicker gold coat (720A) decreased the bond strength and generally resulted in adhesive failure. The mechanism of action of the gold coat on bond strength is likely its effect on the oxide layer on the surface of the base metal alloy.

Use of paint-on die spacers in preparations with grooves

Film thickness of paint-on die spacer materials was measured on axial walls and in grooves. Two different materials and two different modes of application were tested. From the data presented, the following conclusions can be drawn: There are potentially significant differences in film thickness produced with different die spacing materials. The technique used to apply die spacer can affect the film thickness produced. Careful application of Pactra airplane dope will not result in significant accumulation of die spacer in the groove. Tru-fit die spacer produced generally erratic film thickness and demonstrated a potentially significant amount of pooling in the groove base. Further study is warranted to determine if use of die spacer in grooves will improve the seating of castings.