Trond Hegdahl

Contraction stresses of composite resin filling materials

The polymerization shrinkage of composite resin filling materials and the tensile stresses developed when the shrinkage is restrained were measured in an in vitro experiment. This allows an estimation to be made of the forces exerted upon the enamel walls of cavities filled with the resin in the acid etch technique. The results indicate that the stresses acting on the enamel are low compared to the tensile strength of the enamel.

On the measuring of roughness

For the present study some roughness parameters often used in dental research were defined and some of their properties discussed. The technique of filtration or cut-off as a means of distinguishing between roughness and curvature was explained and demonstrated in some specimens of dental amalgam.

The Structure and Mineralization of the Carapace of the Crab (Cancer pagurus L.)

Hegdahl, T., Silness, J. & Gustavsen, F. 1977. The structure and mineralization of the carapace of the crab (Cancer pagurus L.). 1. The endocuticle. Zool. Scr. 6: 89-99. The endocuticle of the dorsal carapace of Cancer pagurus in the intermoult condition was studied by means of light microscopic, electron microscopic, microradiographic, X-ray diffraction, and electron probe techniques. Within the lamellae the fibrils and the long axes of the calcite crystals were parallel and ran horizontally. The cuticle appeared lamellar for two reasons: (1) an alternation in the direction of the fibrils as well as the long axes of the calcite crystals in various vertical levels gave rise to the lamellar appearance; (2) in a zone subjacent to the exocuticle differences in the degree of mineralization often brought about a marked layering of the endocuticle. The fibrils were contained in tubes made up by the inorganic crystals. In undecalcified preparations the pore canals were elliptic in cross section. They contained filamentous material in which very small crystals were indicated. The walls of the pore canals did not possess any structurally differentiated lining. At diametrically opposite ends of the pore canal cross sections very large crystals occurred. Results of measurements of dimensions of fibrils, inorganic tubes, pore canals, and crystals in various sites are presented. Trond Hegdahl, John Silness & Finn Gustavsen, School of Dentistry, University of Bergen, Arstadveien 17, N-5000 Bergen, Norway.

The Structure and Mineralization of the Carapace of the Crab (Cancer pagurus L.) 2. The exocuticle

Decalcified and undecalcified preparations of the crab Cancer pagurus in the intermoult condition were studied to examine the mineralization and structure of the epicuticle, using light microscopic, electron microscopic, and microradiographic methods. The epicuticle was found to be composed of two layers, one superficial membrane, and one thicker layer, measuring 1‐2 μm. From the base layer spines or microtrichia projected. These were approximately 18 μm long and built like the remainder of the epicuticle. The spines and the base layer of the epicuticle contained vertical canals which in undecalcified sections accomodated columns of crystals. These canals were the only location in which minerals occurred in the epicuticle. In decalcified preparations filamentous strands were observed in the canals. Elsewhere in the epicuticular tissue no fibrillar structure was observed. The canals and their contents seemed to extend across the junctional zone between the epicuticle and the exocuticle.

Porosity of resin filling materials

Four paste-paste types of resin filling materials and three capsulated products were tested with respect to porosity content and size distribution of the pores. The data were obtained by using an automatic image analyzer. The paste-paste products were inserted into the test cavities by a bulk-packing technique and an injection technique. The results showed that the use of a syringe reduced the porosity content, mainly by the elimination of big pores. The capsulated products were not less porous than the paste-paste materials.

Physical Properties of Some Zinc Phosphate and Polycarboxylate Cements

Several physical properties were measured for two zinc phosphate and three polycarboxylate cements. The specimens were tested in compression 24 h after they had been made. Two series of specimens were examined. In one series the cement powders were given a correct treatment, protected from atmospheric humidity. In the other series the powders were exposed for 1 week to ambient conditions with the temperature varying between 20 degrees and 24 degrees C and the relative humidity between 40% and 59%. The zinc phosphate cements were characterized by high values of modulus of elasticity and by plastic deformations less than 0.2%. The polycarboxylate cements were more flexible and also exhibited large plastic deformations. Compared with the zinc phosphate cements, therefore, the polycarboxylate cements had high values of resilience and toughness. The storing of cement powders exposed to atmospheric humidity for 1 week did not change the measured properties of the polycarboxylate cements. However, both strength and resilience were significantly reduced for one phosphate cement.

HARDNESS, STRENGTH, AND DUCTILITY OF PREFABRICATED TITANIUM RODS USED IN THE MANUFACTURE OF SPARK EROSION CROWNS

Depending on the size of the prepared tooth spark eroded and milled, Procera crowns (Nobelpharma AB, Göteborg, Sweden) are manufactured from one of five diameters of pure titanium rods. In this study microindentation hardness tests were performed on the outer 400 microns and center of 10 samples for each type of rod. Five tensile samples were also machined for each of the diameters and tested in tension in a universal testing machine. Cast titanium samples were similarly prepared and tested. Some significant differences in hardness and considerable differences in strength and ductility were identified between the prefabricated rods. In comparison, cast titanium was significantly harder and stronger but less ductile.

Area of the organic-inorganic interface of dental enamel

The area of the organic-inorganic interface of several kinds of dental enamel was determined by examining on a computer images based on electron micrographs of decalcified sections. The mean values varied between 0.1363 m2/mm3 and 0.2229 m2/mm3. Assuming a specific gravity for dental enamel of 3.15 the corresponding values were 43 m2/g and 71 m2/g, respectively. The possibility that fluoride may influence the area of the organic-inorganic interface was discussed.

Flexural strength of a high-temperature soldered cobalt-chromium alloy

The flexural strength of a high-temperature soldered cobalt-chromium alloy was compared with that of the intact alloy. In one test group the specimens were soldered by Vitallium welding rods and in the other group Bego cobalt-chromium solder was used. The cylindrical portion of all specimens was mounted equally in an Instron testing machine. No statistically significant differences were found in flexural strength between the groups.

Porosity of resin veneer materials

The present investigation has studied the effects of different types of resins and different processing and curing methods on the number and size of pores occurring in resin facing materials. The mean number of pores per square millimeter varied between 0.5 and 4. The highest numbers occurred in unfilled heat-polymerized acrylic resin and in light-activated resin, whereas the lowest numbers were found in heat-polymerized micro-filled resin. Specimens processed in accordance with the free technique and cured under pressure consistently obtained lower numbers of pores than the corresponding specimens processed in a flask. Most of the pores were less than 50 micron diameter in all test groups. From a clinical point of view, a dense resin with a limited content of pores may give the most favorable results in terms of mechanical, esthetic, and hygienic properties of the veneer.