Dioracy Fonterrada Vieira

Influence of some factors on the fit of cemented crowns

In the study, cast gold crowns were cemented similar to methods used for patients. Conclusions were that: 1. Fresh cement painted with a camel brush in the part of the crown to be cemented promotes a better fit than when the crown is completely filled with cement. 2. Mechanical vibration of the crown at cementation promotes a better fit. 3. Venting the crown, an internal relief by acid etching, or a combination of both these procedures improve the seating of the crown during cementation. 4. The association of one or numerous variables used in this study considerably improves the fit of the cemented crown.

Influence of certain variables on the abrasion of acrylic resin veneering materials

T wo OF THE RECOGNIZED PROBLEMS associated with acrylic resins have been its low hardness and lack of resistance to abrasion1 The resin may be abraded not only by attrition between opposing teeth during mastication but also by continuous use of toothbrush and dentifrice. While certain studies have been concerned with the influence of toothbrushing on abrasion of acrylic resins,2-5 those investigations generally have not employed resins specifically designed for the fabrication of plastic veneers in crown and bridge prosthesis. Likewise, variables such as curing procedures and the type of dentifrice and toothbrush have not been correlated with abrasion resistance. Thus, the purpose of this research was to determine some of the factors which might influence the amount of wear caused by toothbrushing on acrylic resin veneering materials.

Fluoride release and physical properties of a fluoride-containing amalgam

1. The amount and percentage of fluoride released from amalgam were less for the 1:1.6 alloy/mercury ratio than for the 1:1 ratio. 2. Higher fluoride concentrations resulted in a greater quantity but lower percentage of fluoride released from the amalgam. 3. Longer immersion times resulted in a greater amounts and percentages of released fluoride. However, the fluoride released per hour decreased with immersion time. 4. Immersion of the amalgam in lactic acid caused a greater release of fluoride than immersion in distilled water. 5. Disintegration of the amalgam was greater in lactic acid than in distilled water. The contribution of calcium fluoride to weight loss was greater in water than in lactic acid, indicating a corrosive effect for lactic acid. 6. The higher calcium fluoride content (2 per cent) decreased the compressive strength of the amalgam but did not alter the dimensional change.

Fracture strength of Class II amalgam restorations condensed over protective bases

A protective base, serving as a substitute for dentin, guards the pulp against thermal, electrical, chemical, and mechanical injury. In addition, a medicated base promotes the formation of reparative dentin. All bases should possess sufficient strength to resist the forces of condensation of dental amalgam or gold foil’; in addition, they must resist the stresses of mastication. The most important physical requirement of a base is its ability to resist condensation forces.” A cement base needs to resist more than 170 p.s.i. to preclude fracture and displacement resulting from condensation3 Zinc oxide and eugenol cement (ZOE) lacks the necessary strength when compared to zinc phosphate cement in Class I and II amalgam restorations.‘+ The recommendation that two or more layers of varnish be applied to cavity walls has added another factor to base techniques.” When tested, the varnish failed to alter the strength of cylindrical specimens of amalgam.7 A previous report discussed pertinent factors which relate to the physical problems of protective bases5 The class of preparation, the size of the cavity, and whether or not the line angles are created in dentin relate to the physical needs of a base. The views in that report agreed with earlier discussions of this problem.P. :’ Calcium hydroxide, protected with zinc oxide and eugenol cement (ZOE), may suffice for small bases, but larger areas require zinc phosphate cement, These concepts and other clinical practices have, in part, been investigated. and the results are described in this report.

Physical properties of dental amalgam containing metal pins

No type of pin inclusion investigated in this study produced consistent increases or decreases in the strength of amalgam specimens under a variety of loading conditions and specimen ages. However, comparison of relative values for specimens tested under the same conditions showed that the highest fracture loads were found more often for specimens containing threaded gold pins than for any other type of specimen.

Fracture strength of Class II composite resin restorations

T he superior physical properties of composite resins have aroused the possibility of their use beyond the accepted indications for acrylic resin and silicate cement. The literature has suggested the use of composite resin for Class I,‘m4 Class II,” and Class IV39 * restorations. Other reports advocated the use of composite resin or tested the influence of pins on the physical properties of composite resin.“, i Clinical tests which compared composite resin and amalgam for Class II restorations showed that, while amalgam retained its anatomic form, the composite resin had superior marginal integrity for as much as three years of service.5, 8, !’ Standards for strength have not yet been established for posterior restorations,5 despite the many tests conducted under standardized conditions in laboratories. This procedure avoids the complications found in clinical testing of restorations of various forms, sizes, and volumes. This report presents the results of a laboratory test of Class II composite restorations retained with and without cemented pins.

Crystallization of zinc phosphate cement

Abstract A microscopic study indicated that ZnHPO 4 ·3H 2 O was formed during the crystallization of zinc phosphate cement powder and liquid. Long crystals of ZnHPO 4 ·3H 2 O radiated from a nucleus of crystallization. Apparently, the surface tension force plays a role in partially inhibiting the growth of these crystals. This same surface tension may be one of the causes of the shrinkage that occurs during setting of zinc phosphate cement.