Introduction

As defined by the Glossary of Prosthodontic Terms, connectors are a component of a fixed partial dental prosthesis (FPD) “that unite retainers and pontics (1).Soldering is the process of joining two pieces of metal by fusing an intermediate alloy (Solder) with a melting point lower than the parent metal.  Joining or fusion of a metal or an alloy is required for assembling bridges in prosthodontics, uniting component parts of bridges for establishing esthetics, to develop harmony and also for uniting wrought clasps to a cast metal framework in removable prostheses (2)^. This procedure was conventionally done using methods like soldering, brazing and welding. Newer techniques such as laser fusion, infrared fusion and electron beam fusion have been recently introduced (2,3). Numerous investigations on accuracy and strength of various materials as connectors in dental brazing have been done with various methods.

Different soldering techniques have been used in dentistry especially in fixed prosthodontics. Accuracy of multiple-unit fixed partial dentures is a basic issue in prosthetic treatments. Generally, as the length of one piece casting increases, the potential for inaccuracy of fit also increases. To assure the fit of metal frameworks on abutments, a trial insertion is generally performed in a clinic. If any inadequacy is detected, sectioning and soldering of metal frameworks are required (4). Alloys that are used in crown and Bridge work include a wide range of precious, semi-precious and non-precious alloys, each of which has advantages and disadvantages. A steep rise in the cost of precious metals in 1973-1974 led to widespread use of more economical non-precious alloys, referred to as Base-metal alloys. These include Nickel-Chromium (Ni-Cr) and Chromium-Cobalt
(Cr-Co) and have suitable mechanical and physical properties such as increased strength and hardness, higher fusion temperature and less distortion during porcelain firing compared to precious metals. Disadvantages of base metal alloys are higher excess oxide formation during casting and soldering, difficulty in finishing and polishing and questionable biocompatibility (5). Consequently, a change in the technique of soldering is necessary for joining base metal alloys. Although many favorable properties of joints have been mentioned in literature, no definitive technique has been proven to produce the ideal properties (6). When base metal alloys are used, solders which match the parent alloy both in color and strength is required. Therefore, in such cases, nickel chromium alloy solders are used to fabricate joints, especially in metal ceramic bridges (2,7,8)^.

Other more recent techniques such as soldering by infra– red machine can be used; however, no difference in strength and porosity has been found between
infra-red and torch soldering (9) Several methods, including an infrared heating method and laser welding, have been investigated to join different metals such as high-palladium, cobalt-chromium and titanium alloys (10,12). Although these methods fulfill the need for mechanical strength, torch and oven soldering are still standard techiques for joining dental alloys. Although microwave technology is well known in industrial settings for it’s faster heating rate, shortened processing time, improved microstructure and energy efficiency, the application of microwave technology in dentistry has been limited to polymeric materials and dental ceramics and application of microwave technology to dental brazing alloys has not yet been  reported (13,15).Pre-veneer soldering is a more popular technique compared to post-veneer soldering because it is easier; however, some investigators found
post-veneer soldering to be stronger(16)

Multi-unit dental restorations are usually cast in small sections and then soldered by a gas-torch technique (17). Supercast is a non- precious alloy most commonly used in crown and bridge work, this alloy contains beryllium,  which has been identified as a potential carcinogen and also has to be bought from foreign countries (8). On the other hand, Minalux is also a non - precious alloy with no beryllium made in our   home-country, Iran. The goal of this study was to compare the strength of soldered joints of Supercast and Minalux alloy.

Materials and methods

Preparation of specimens for the tests

 The alloys used in this study were Supercast (Vera bond dental alloy Albadent, California, USA) and Minalux (mavadkaran co. Iran). A recommended Solder by the manufacturer, Vera – solder (Vera-solder Albadent. California. USA) was used for both alloys. Eighteen specimens were used and divided in two groups of nine (simple randomization). First, an aluminum model according to ADA/ISO 9693 specifications was made for tensile test, made of a
3-mm diameter bar which was cut and soldered in the middle (Fig. 1) (18). The processing flask was used, upper and lower half of the flask was ½ filled with type II dental plaster (Pars Dandan, Tehran, Iran). After stone was set, the other half of the lower half of the flask was filled with condensation silicone (Speedex, Coltene, Switzerland) before putty was set, the aluminum model was ½ sank into it. After putty was set, it was lubricated by Vaseline. The upper half of the flask was filled with Speedex putty, similar to the lower half. Before putty was set, upper and lower halves of the flask were placed together under pneumatic press machine (KAVO EWL, Germany).After tightening the screws on the machine, pressure was raised to 2000 N/cm² for 5minuts. After 5minutes, pressure was released and excess resin removed. By removing the aluminum model from the flask, the impression was left on the Speedex putty. Auto polymerizing acrylic resin (Duralay, Reliance Dental Mfg. Co., USA) was placed into an already made impression of the aluminum mold. Before acrylic resin was set, upper and lower halves of the flask were placed together under pressing machine at 2000 N/cm² for five minutes. Then, upper and lower halves of the flask were separated, in this way a duplicate acrylic pattern from an aluminum pattern was made. Eighteen acrylic patterns were made this way. These patterns were invested in a circular casting rings lined with one layer of ring liner with phosphate-bonded investment (Hi Temp II, whip mix Corp, Louisville, ky). The rings were then placed in burnout furnace. Temperature was raised to 30 and held for 30 min at this temperature, followed by an automatically raise to 860 keeping for 30 minutes. Rings were cast by use of gas/oxygen torch and a manually controlled centrifuge machine (Degutron, Degussa, Germany). The castings were then bench cooled for 1 hour, divested, sandblasted and cleaned ultrasonically.

Figure 1 ADA/ISO 9693 model for tensile strength test

Preparation for soldering procedures

A mounting jig was made by filling a cup with stone (plaster) and sinking one half of each specimen into it, each specimen was cut in half by an ultra-thin disc (Dedeco International, long eddy, N, Y) perpendicular to them.

The thickness of the disc was .008 inches (0.20mm), which approximated a thickness of a business card. The specimens were joined with chemically activated acrylic resin (Duralay, Illinois, U.S.A) .The specimens were removed from plaster and invested in a block of soldering investment (Hi Temp II, whip mix Corp, Louisville, ky) with one inch thickness. For the gas-oxygen torch soldering, the soldering block was pre-heated in a burnout furnace at 350ºc for 30 minutes. The soldering gaps were fluxed before the soldering procedure, Vera solder which was used in this study was also dipped into flux before soldering. After soldering, specimens were allowed to bench cool. Joint areas were finished and polished by disc and polishing stones so that the thickness of these areas was 3.1mm. Joint areas were evaluated radiographically to ensure a porosity free joint. All Specimens were positioned 12.5cm
(5 inches) from the focal point of the radiographic unit which was set at 0.35 seconds, 15 ma and 90 kVp.

Preparation of specimens for the tests

The specimens were secured in an optimum horizontal and vertical position for testing compression and tensile strengths respectively. A universal testing machine (Instron; Zwick 20, Germany) was used for testing tensile strength and Dartec machine (HC10, England) was used to test compressive and flexural strengths. Force was applied at a speed of 0.5 mm/min for all three tests and mega pascal was used as the unit of force. Statistical analysis was computed by SPSS software (IBM SPSS statistics 22) and Mann - Whitney test was used for comparison between groups (significant level =0.05).

Results

The mean compressive strength for soldered supercast alloy was 1014.018 MPa and for soldered Minalux was 938.17. The mean flexural strength for soldered supercast alloy was 44.01 MPa and for soldered Minalux alloy was 51.70. The mean tensile strength for soldered supercast alloy was 347.46 MPa and for soldered Minalux alloy was 350.03 (Table.1). It was concluded that there were no statistical differences between compressive, flexural and tensile strengths of supercast and Minalux alloys (p=0.27, 0.38 and 0.18).

Table 1 Comparison between strength of soldered joint of supercast and minalux alloy (MPa)