PURPOSE: The aim of this study was to assess distortion inherent in casting, soldering, and simulated porcelain firings of screw-retained, implant-supported three-unit fixed partial dentures (FPDs). MATERIALS AND METHODS: Ten wax patterns were fabricated on a die-stone cast containing two implants, 20 mm apart from center to center. Five specimens were cast in a high-palladium alloy, exposed to simulated porcelain firings, sectioned, and then soldered with low-fusing solder. Five specimens were cast, sectioned, soldered with high-fusing solder, and then exposed to simulated porcelain firings. For each specimen, two horizontal and six vertical distances between appropriately scribed reference points were measured with a traveling microscope. Comparisons were made among the various measurements taken after wax-pattern fabrication, casting, high- and low-fusing soldering, and each porcelain firing. Data were analyzed using a repeated-measures factorial ANOVA (alpha = 0.05). RESULTS: Significant difference was detected in the amount of horizontal distortion during casting (53 +/- 24 microns) and high-fusing soldering (-49 +/- 50 microns), as well as in the amount of horizontal distortion during high-fusing soldering (-49 +/- 50 microns) and low-fusing soldering (17 +/- 26 microns). However, no clinically significant difference was found in the amount of horizontal distortion during casting, low-fusing, and high-fusing soldering. The greatest amount of distortion during the simulated porcelain firings took place during the oxidizing cycle. CONCLUSIONS: Soldering did not improve the casting misfit of a three-unit implant-retained FPD model. Metal-ceramic implant frameworks should be oxidized before intraoral fit evaluation.

Distortion of Three-Unit Implant Frameworks During Casting, Soldering, and Simulated Porcelain Firings

Strength properties of soldered joints from various ceramic-metal combinations

Tensile specimens of dental solder joints were tested to determine the effect of gap distance on the strength of solder joints. Two dental alloys were used; a Type III gold alloy and a gold-palladium (Au-Pd) alloy. The Type III alloy was joined with the recommended gold solder. The Au-Pd alloy was joined using preceramic and postceramic application soldering techniques. Gap distances of 0.13 mm, 0.5 mm, and 1 mm were used with each soldering technique. All solder specimens were machined to a uniform diameter and then tested for tensile strength. It was concluded that: 1. There was a significant increase in strength for the Type III gold as the gap distance was increased. 2. The trend toward stronger joints at narrow gap distances for Au-Pd alloy joined with presolder was not statistically significant. 3. The trend toward stronger joints at wide gap distances for the Au-Pd alloy joined with the postsolder was not statistically significant. 4. The postsolder joints were statistically stronger than the presolder joints. 5. The trends observed could be partially explained in terms of competing effects of yield strength (triaxially), wettability, and void or inclusion content at the various gap distances.

An investigation of tensile strength of dental solder joints

An investigation was conducted to evaluate the relative accuracy of one-piece castings of FPDs of three, four, and five units with those assembled by preceramic soldering and postceramic soldering. The preceramic soldered FPDs were subjected to six simulated porcelain firing cycles to assure a homogeneous sample. A total of 15 FPDs, five of each length, were evaluated for each test condition. A three-piece aluminum mold with stainless steel dies, which allowed for varying the length of the prosthesis, was used to fabricate the FPDs in 52% gold-palladium alloy. Marginal discrepancies of the castings on the dies were compared for the three test techniques. The mesiodistal length of the FPDs that were pre- and postsoldered were also measured at the occlusal and at the gingival margins and compared with the restorations before soldering.

Accuracy of one-piece castings, preceramic and postceramic soldering.

In identifying the criteria which influence soldering distortion, the results of this study indicate the following: 1. Linear distortion identified at the investment phase (a net increase in gap distance) is due to setting expansion of the plaster and soldering investment. 2. The linear distortion identified at the soldering phase (a net decrease in gap distance) occurs because of the contraction of the solder. 3. The rotational distortions measured were found to be minimal. 4. The use of a minimum gap distance, without contact, is desirable. 5. Adding solder results in a well-rounded, solid solder joint.

Distortion in dental soldering as affected by gap distance

This investigation evaluated preceramic and postceramic soldering procedures. Three gap spaces and two soldering methods were evaluated. Wider gaps produced stronger joints. The strongest joints were achieved by using wider gaps and the oven soldering technique. Very wide gaps (0.76 mm.) are not recommended because of possible distortion through excessive solder shrinkage. However, in practice, many solder joints exceed the 0.31 mm. recommended minimum; these joints will not show decreased strength.

J.D. Preston

Papers

Preceramic and postceramic solder joints.