Naofumi Shigeto

Corrosion properties of soldered joints. Part I: Electrochemical action of dental solder and dental nickel-chromium alloy

The corrosion properties of contact between six Ni-Cr alloys and two dental solders were evaluated. Ni-Cr alloys with low Cr content coupled with silver solder induced continuous galvanic current, whereas Ni-Cr alloys with high Cr and some Mo content coupled with silver or gold solder induced galvanic current for a short time. Six Ni-Cr alloys were divided into two groups according to the dissolution of Ni, one with low resistance and the other with high resistance. The initial equilibrium was induced between high resistance Ni-Cr alloys and both solders by less current density, which implies a smaller degree of corrosive current than that in low-resistance alloys. Corrosion was observed in the silver solder coupled with high-resistance Ni-Cr alloy, but was not found in the gold solder coupled with both alloys. In soldering Ni-Cr alloy, it is better to use a gold solder and a high-resistance Ni-Cr alloy for corrosion resistance.

A decade of progress for the adhesive fixed partial denture

The history of the development of adhesive fixed partial dentures was reviewed and present clinical problems discussed. Longitudinal studies are not available because adhesive prostheses with 4-META adhesive resin were introduced only a few years ago. However, scientific advances will make placement of resin-bonded metal prostheses a more stable, standard technique in fixed prosthodontics.

Corrosion properties of soldered joints. Part II: Corrosion pattern of dental solder and dental nickel-chromium alloy

Six nickel-chromium (Ni-Cr) alloys joined with two solders were evaluated for degree of corrosion after they were immersed in corrosive solution for 100 days. The corroded surface of the Ni-Cr alloy and the cross section of the soldered joint were observed and the solution of nickel and copper (Cu) ions in each combination was measured. No corrosion was observed in the high-resistant Ni-Cr alloys with high Cr content. Pitting corrosion was seen on low-resistant Ni-Cr alloys, which had less Cr content. In the cross section of the joint, the joint of silver solder became porous by corrosion and the corrosion advanced especially along the interface. Solution of Ni ions was detected in all of the combinations of Ni-Cr alloys and solders with exception of the combination of high-resistant Ni-Cr alloys and gold solder. On the other hand, solution of Cu ions was detected in all of the combinations with silver solder.

Adhesion between copper-plated acrylic tray resin and a polysulfide rubber impression material

The method of copper plating on acrylic resin was developed, and the bond strength to polysulfide rubber was measured in comparison with that of the adhesive coating. The bond strength with copper plating was greater than with adhesive coating. The specimens with adhesive coating failed due to a rupture occurring at the elastomer-adhesive interface, while those with copper plating had cohesive-adhesive failure. The method of copper plating on acrylic resin improved the adhesion between the polysulfide rubber and the custom-built acrylic resin tray.

Evaluation of the methods for dislodging the impression tray affecting the dimensional accuracy of the abutments in a complete dental arch cast

The dimensional accuracy of dies was evaluated in complete dental arch casts made by three different methods of dislodging the impression tray. By one method, the impression tray was dislodged parallel to the long axis of the abutment and by the other two the trays were inclined with the fulcrum at either the posterior or anterior region of the complete dental arch. The dimensional changes of the molar-die were significantly affected by the dislodging method in the inclined way but not those of the incisor-die. The anteriorly inclined method showed fewer dimensional changes than the posteriorly inclined method. On the other hand, the dimensional changes of the incisor-die were not significant by any dislodging method, but were observed when some impression material was used. If the impression tray is removed by the inclined way, the fulcrum should be chosen at a region remote from the abutment instead of at the proximal region.

Use of plastic bar molded casting in the modification of variably expanding investment to compensate for casting shrinkage of nickel-chromium alloy

Trial plastic bar molded castings were compared for accuracy with cast crowns by means of a nickel-chromium alloy, which was used to establish the liquid density of variably expanding investments. The plastic bars were invested to evaluate the change of expansion rate in a wide range of six liquid densities from 0% to 100%, and the distances between the sections were measured before and after casting. Wax crowns were cast to obtain a more detailed relation around 0% expansion at seven liquid densities, and the gaps between the base of the die and the margin of the crown were measured before and after casting. The expansion rates of both castings were calculated and thus were proportional to the liquid density; both regression curves indicated high correlation coefficients. As a result of the statistics of the Students t-test, the difference between the two methods was not significant. The trial plastic bar was useful in establishing the variably expanding investment for precise casting of artificial crowns.

[Galvanic corrosion of dental Ni-Cr alloy by contact with dental solders. Part 3. Observation of soldered joints].

In this study, contact corrosion of Ni-Cr alloy soldered by gold and silver solder was investigated. After being soldered, Ni-Cr alloys were immersed in physiological saline solution for 100 days. A detailed observation was made of the surface of the soldered alloy and the interface between Ni-Cr alloy and solder, using an optical microscope. In addition, distribution of alloy elements in Ni-Cr alloy-solder interface and strength of the soldered joint were examined.The following results were obtained.1. In case of soldering by gold solder, no corrosion occured at the soldered joint.2. In case of soldering by silver solder, corrosion occured at Ni-Cr alloy-solder interface, and the solder became porous.3. Pitting occured only at the surface of Ni-Cr alloys containing less than 19% Cr.4. At Ni-Cr alloy-gold solder interface, a thin layer which might be caused by alloying with each other was observed using an optical microscope and EPMA. Tensile testing confirmed that the soldered joint between Ni-Cr alloy and gold solder was stronger than a layer of gold solder.

Galvanic corrosion of dental Ni-Cr alloy by contact with dental solders. Part 2. Effect of Cr concentration of the alloys.

In case of soldered crowns and bridges made of Ni-Cr dental casting alloy, contact corrosion possibly occurs in oral environment. Generally, corrosion resistance of Ni-Cr alloy is related to content of chromium, but there are few reports about contact corrosion between Ni-Cr alloy and solder. Thus authers investigated about relation between contact corrosion and content of chromium in Ni-Cr alloys.In this study, six kinds of Ni-Cr dental casting alloys were selected, these contents of chromium were different. Galvanic cells were constituted with combination of Ni-Cr alloys and gold solder or silver solder in saline solution, and galvanic currents were recorded continuously for a week. Then amounts of Ni and Cu dissolved in saline solution were measured after four months.Results1. Ni-Cr dental casting alloys with less than 19% content of chromium became anode either in combination with gold solder or silver solder. And these Ni-Cr alloys were promoted to corrode by contact with solders.2. Pitting was observed on surfaces of Ni-Cr alloys corroded.3. Ni-Cr alloys with more than 22% content of chromium did not corrode in this experimental environment.