Evaluation of recasting base metal alloy on marginal fitness of cast crown with two type of finishing lines

Base metals are being increasingly used in casting procedures in dentistry. Since recasting these alloys in laboratory might have negative effects on marginal fitness, it seems necessary to study the effect of recasting procedures more carefully. The goal of this study was to study the effect of recasting base metals on vertical marginal fitness of crowns with different marginal configurations with different percentages of alloys. In this study, two dies made up of brass with dimensions of prepared premolars and marginal configurations of shoulder bevel 450 and shoulder 1350 were prepared by CNC machine. Total of 30 copings were waxed and cast, 15 copings for each die. Copings were further subdivided in to three groups A, B and C. Group A consisted of 100% new alloy, group B consisted of combination of 50% new alloy and 50% old alloy, group C consisted of 100% recast alloy. After copings were seated, vertical distance from margin of the restoration to margin of the die was measured by metric microscope with Moticam camera at four points of buccal, lingual, mesial and distal. Data were analyzed by two- and one-way ANOVA and t- test. Group A1 (135º shoulder finishing line cast with 100% new alloy): 156.11 micron. Group A2 (45º shoulder bevel finishing line cast with 100% new alloy): 99.74 micron. Group B1 (135º shoulder finishing line cast with 50% new alloy and 50% recast alloy): 273.38 micron. Group B2 (45º shoulder bevel finishing line cast with 50% new alloy and 50% recast alloy): 123.07 micron. Group C1 (135º shoulder finishing line cast with 100% recast alloy): 369.40 micron. Group C2 (45º shoulder bevel finishing line cast with 100% recast alloy): 106.97 micron. There were statistical differences between groups A, B and C in vertical gap when three different percentages of alloys (100% new with average gap of 127.93 micron, 50% new+50% recast with average gap of 198.22 micron, 100% recast with average gap of 238.18 micron) were used (p value = 0.036). There were statistical differences in mean vertical gap distance between different preparations (shoulder bevel 45º, shoulder 135º) within each group (100% new, 50% new + 50% recast, 100% recast). When three different percentages of alloys were used (100% new alloy, 50% new + 50% old, 100% old), 100% new alloy showed the least mean vertical gap compared to other two groups. In shoulder 135º there were statically differences between group A and C only and in shoulder bevel preparation there were no statically differences between groups A, B and C. In group C (100% recast) there were significant differences between two preparations (shoulder bevel 45º and shoulder 135º).