Jack D. Preston

Measurement of paint-on die spacers used for casting relief.

Three paints used for die spacing were measured for film thickness. Two of the materials are marketed as die spacers, and the third is sold as a model airplane paint. The materials were applied to stone dies in multiple coats. They were embedded in resin, sectioned, and measured photographically through a microscope. The three die spacing materials were capable of achieving clinically acceptable relief in the range of 20 to 40 mu by using a variable number of coats. Different paints produced different thicknesses. The two paints sold specifically for die spacing gave 11 to 13 mu per coat. The hobby shop paint had the smallest film thickness per coat, yielding approximately 5 mu per layer. It also had the smallest standard deviation. This control plus the relative low cost and availabilty make it attractive for this purpose.

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-

An investigation of epoxy resin dies

1. Complete crown epoxy resin dies are undersized. 2. MOD onlay epoxy resin dies are accurate. 3. Detail duplication of epoxy resin dies is comparable to that of die stone. 4. Hardness values of epoxy resin are generally less than those of die stone. The hardness of Epoxydent closely approached that of die stone.

Post-ceramic soldering of various alloys

A base metal alloy was soldered to precious and semiprecious alloys using a post-ceramic soldering technique. The specimens were tested by the tensile mode and the results statistically analyzed. The solder joints involving the base metal had strengths comparable to the precious and semiprecious joints. However, a high percentage of the base metal joints were too weak to be tested. Further study using other solders is indicated.

Report of the Committee on scientific investigation of American Academy of restorative dentistry

Abstract Dental research continued to grow during 1988. Unfortunately the quality does not always parallel the quantity of publications. This report obviously does not encompass all of the literature but focuses on studies that are related to trends and to matters that are considered controversial. Likewise the particular interests of the members of the American Academy of Restorative Dentistry were taken into consideration. The subjects covered include periodontics, preventive dentistry, bulp biology, craniomandibular disorders, ceramics, color in restorative dentistry, and dental materials.

Therapy: Anecdote, experience, or evidence?

In dentistry, most changes in therapy come from new techniques and products that are introduced to the market. Clinicians (and patients) can be overwhelmed by advertisements and marketing, some obvious and some (e.g., paid clinical reports in non-peer-reviewed journals) not so obvious. Because most advances are made with small case studies, which are at a lower level of evidence, it is imperative that data clinicians read or see have the greatest validity possible. This validity is imperative to achieve evidence-based dentistry that uses relevant, high-quality, clinically oriented research that provides better information for the clinician and better treatment for the patient.