The effect of recasting nickel chromium and cobalt chromium alloys on the quality of fixed prosthodontic devices

Base metal dental alloys are among the oldest restorative dental materials used in dentistry. They are still widely applied and are expected to remain in demand in the coming years. They demonstrate adequate properties required to meet the needs of wide and variable applications. Base metal alloys can be used alone or in combination with other dental materials to construct a dental restoration. Due to environmental and financial factors, variable practices have emerged in the processing of these alloys. Among these practices is the reuse of surplus alloys. The aim of this study was to evaluate the effect of recycling nickel-chromium and cobalt-chromium alloys on the quality of dental restorations employing a suggested test protocol where the correlation between the different factors tested was investigated. Wax patterns of different shapes were prepared for the different tests conducted. For each alloy type, five different combinations of new and surplus alloys were prepared to cast the wax patterns using an induction casting machine. Castings were subjected to different quality assessment investigations; (i) the microstructure was assessed for the presence of pores and inclusions and the chemical composition was evaluated for the amount of elements present (mass%), castings produced from new alloy were used as a control; (ii) castings were subjected to a polishing procedure simulating the technique used in dental laboratories. Surface roughness (Ra) was analysed prior to and after the polishing procedure using light profilometer; (iii) a metal to ceramic bond strength test was conducted following the requirements of ISO 9693: 2000. ISO standardised test pieces were subjected to a three-point bending test using a universal testing machine and bond strength evaluated; and (iv) ions released from castings were measured, where castings were placed into artificial saliva at pH 4 and 6 for five weeks. The mean amounts released were measured using inductively coupled plasma atomic emission spectroscopy (ICP/OES). The findings of this study imply that although microstructure and chemical composition of castings containing surplus alloy varied from those produced entirely from new alloys, these variations were not always significant and depended on the alloy type and restoration type. Surface roughness evaluation suggests that consecutive laboratory procedures, such as finishing and polishing procedures can considerably enhance surface topography of casting produced partially and entirely from surplus alloy. For the bonding test investigation, all test groups demonstrated bond strength values higher than that recommended by ISO 9693: 2000. A similar observation was documented for the ion release test; despite the increase in the amount of ions released from castings containing surplus alloys, the detected amount of ions was however very small compared to those documented in the literature. Although some of the tested characteristics of castings containing 100% recycled alloys were marginally inferior to those produced from new alloy, the former castings gave acceptable results and would be expected to function and perform appropriately in the patient’s mouth in the equivalent manner to castings produced from new alloys. However the addition of a small amount of new alloy, as low as 25%, has been shown to enhance the performance of castings compared to those containing 100% recycled alloy in the different tests conducted.