Alton M. Lacy

Time-dependent accuracy of elastomer impression materials. Part II: Polyether, polysulfides, and polyvinylsiloxane

T here are currently four basic types of elastomer impression materials in use in the dental profession: (1) silicone rubbers which polymerize by a condensation reaction, (2) polysulfide (mercaptan) rubbers, (3) polyethers, and (4) silicones which polymerize by an addition reaction. The latter have been introduced relatively recently and are also called polyvinylsiloxanes. The basic properties of these materials have been reviewed by Craig’ and Phillips.’ Two clinical advantages of these materials over reversible and irreversible hydrocolloids are that (1) multiple dies may be made from repetitive pours of a single impression and (2) the clinician can wait for an extended time period to pour the stone cast after making the impression. In addition, elastomers can be metal plated for the production of electrodeposited metal dies. There is disagreement in the literature regarding the superiority of dimensional stability between polysulfide rubbers and condensation silicones.:‘-” Polyether has been suggested as being more dimensionally stable when compared with its predecessors,‘. 7. ” and the polyvinylsiloxanes have recently claimed even superior accuracy and stability over polyethers.‘, ” The purpose of this study was to make a quantitative comparison of the accuracy and dimensional stability of representative products in each class by

Effect of bonded amalgam on the fracture resistance of teeth

The purpose of this study was to determine whether amalgam bonded to tooth structure with an adhesive resin cement can increase the fracture resistance of restored teeth. Extracted paired upper premolars were prepared for G.V. Black type mesioocclusodistal amalgam restorations. In one tooth of each pair (the experimental group), the enamel walls were etched with phosphoric acid and were painted with an adhesive resin (Panavia), and amalgam was condensed and carved. For the other tooth in each pair (the control group), amalgam was placed in the same manner but was not etched and lined with resin. The teeth were thermocycled and mounted for testing and then were loaded until fracture. A significant difference (p less than 0.05, the paired Students t-test) was found in the force needed to fracture the bonded amalgam group (70.5 +/- 21.6 kg) compared with that needed to fracture the conventional amalgam group (60.3 +/- 16.8 kg). SEM examination of fractures at the interface occurred predominantly within the resin.

Time-dependent accuracy of elastomer impression materials. Part I: Condensation silicones

1. Although condensation silicone elastomers can be used to produce multiple dies from a single impression, one can expect changes in dimensions of dies produced from successive pours. 2. Putty/wash systems exhibit variable behavior among different brands. Some products produce a series of dies of increasing diameter over a 4-day period, while other brands may produce a series of dies whose diameters decrease as they are successively poured. 3. Light-body wash silicone impressions made in adhesive-lined custom acrylic resin trays produce dies which increase in diameter with time. 4. Putty/wash system, in general, show a greater eccentricity of dies than is seen with custom-tray/adhesive/wash systems, but there were some significant exceptions among the brands tested. 5. There does not appear to be any difference in behavior between single-wash and double-mix techniques in the condensation-silicone systems. 6. For maximum accuracy, the condensation-silicone systems should be poured as soon as possible after making the impression.

In vitro microleakage at the gingival margin of porcelain and resin veneers

This in vitro study investigated the effect of dental surface treatments on the seal of porcelain and resin laminate veneers to tooth structure. Nine groups of 20 teeth were prepared for labial veneers--six groups with the apical margin of the veneer in dentin, two groups with the veneer wholly bordered by enamel, and one group with the apical margin coincident with the apical margin of a previously placed glass ionomer restoration. The teeth were treated with a variety of dentin-bonding agents prior to restoration of eight groups with porcelain veneers and one group with Visio-Gem microfilled resin veneers. After placement, the veneers were stored in water for 4 days, then subjected to thermocycling before silver staining and sectioning. Results showed no leakage around the margins of the veneers wholly bordered by enamel. Complete leakage was found around the glass ionomer restorations, and nearly total leakage was found along the resin-dentin interface of the resin veneers. The porcelain veneers that extended onto dentin showed variable but limited leakage for four of five of the dentin-bonding treatments.

Effectiveness of hardening films on die stone

1. Cyanoacrylate resins and resinous die hardeners effectively improved the hardness and abrasion resistance of stone dies. 2. Application of liquid resins followed by blotting and/or blowing with compressed air produced an improved surface without clinically significant dimensional change. The extent of film buildup was qualitatively related to the viscosity of the resin. 3. Application of multiple coats of resin or simply soaking the dies in resin without further blotting or other mode of removal of the excess produced a thick film which can obliterate surface detail.

Three factors affecting investment setting expansion and casting size

T he production of accurate dental castings by the lost wax process involves casting molten alloy into a refractory mold which is sufficiently and precisely oversized to accommodate the shrinkage of the alloy during cooling. The total expansion of the investment mold is determined by the combined effects of setting behavior and thermal expansion during burnout. Several investigators have studied the multitude of variables which affect setting expansion.1-1o The major variables include (1) number of liners in the ring, (2) position of liners, (3) position of wax pattern in the ring, and (4) water/powder ratios of investments. The combined roles of each of these variables in regulating expansion has been well documented and generally accepted by the profession. Clinical experience has suggested, however, that other factors may also be significant in determining the setting expansion of investment and in the subsequent regulation of casting size. The purpose of this study was to investigate the related effects of (1) mixing rate, (2) ring liner position, and (3) storage conditions on the setting expansion of both gypsum-bonded and phosphatebonded investment molds; and subsequently to correlate casting size with measured expansion data. Although there are many studies of investment expansion behavior in the literature, few have actually correlated these data with actual casting size.

Xeroradiography for imaging biomaterials--A comparison with conventional radiography.

Our visual comparison of dental xeroradiographs versus conventional film radiographs demonstrated the superiority of the dental xeroradiographic technique for the imaging of all radiolucent and mild-to-moderately radiopaque materials. Both systems were equally valuable for the imaging of very dense radiographic materials such as gold and amalgam alloys.

Incidence of bubbles on samples cast in a phosphate-bonded investment

P hosphate-bonded investments are widely used to produce castings of high-fusing alloys for both ceramometal and all-metal dental restorations. These investments are generally stronger, more refractory, and can be manipulated for a greater degree of mold expansion than gypsum-bonded investments. The investments are primarily composed of 75% to 90% silica refractory in the form of quartz or cristobalite, which is initially bound together by magnesium and ammonium magnesium phosphate formed by reaction of magnesium oxide with monoammonium phosphate in water. High temperature burnout results in loss of water, crystallization, recrystallization of magnesium phosphates and formation of silicophosphate glasses, which give the mold high strength at casting temperatures.“’ An unfortunate sideeffect of the initial setting reaction is the liberation of gas (presumably ammonia) bubbles, which often adhere to the surface of the wax pattern and result in positively cast nodules on the surface of the casting (Fig. 1). Such nodules interfere with the fit of the casting and must be removed before the casting is clinically serviceable. This is time-consuming and troublesome for the dental technician and may compromise the fit of the casting if excessive internal grinding is required. It was the purpose of this study to examine the effect of a number of investing variables on the incidence of bubbles cast to wax patterns invested in a phosphatebonded investment. The following investing variables were examined. 1. Liquid/powder ratios 2. Mixing time under vacuum 3. Low speed (hand) vs. high speed (machine) mixing 4. Special liquid/water ratios for a fixed total L/P ratio 5. Use of a debubblizer