Comparison of the accuracy of impressions made of 2 implants with interfering axial convergence with CAD-CAM impression copings and the altered cast technique: An in vitro study

Studies that evaluated the accuracy of different methods for making an impression of 2 implants in close proximity or with adverse axial convergence are lacking.The purpose of this in vitro study was to compare the accuracy of impressions made with metal and resin computer-aided design and computer-aided manufacturing (CAD-CAM) impression copings and an altered cast technique.Two implant analogs with an interimplant angulation of 50 degrees were placed in an acrylic resin master model. Thirty impressions were made with 3 techniques (n=10). Two angled abutments were scanned to print 40 impression copings from a castable resin, half of which were cast with nickel-chromium alloy (M-CAD) and half of these were used in the R-CAD group. Impressions were made and poured. For the altered cast technique (Alt-cast), casts with only the mesial analogs were made and hollowed at the site of the distal analogs. The position of the distal analogs was transferred by resin indices (n=10) fabricated with light-polymerized resin on the master model. The linear displacements of mesial (ΔR1) and distal (ΔR2) analogs and the interanalog distance (ΔRt) were measured with a coordinate measuring machine. The data were analyzed by using the Kruskal-Wallis and Dunn post hoc tests (α=.05).No significant difference was found between the M-CAD and R-CAD groups regarding ΔR1 (P=.722), ΔR2 (P=.576), or ΔRt (P=.939). However, the Alt-cast group differed from the M-CAD group in ΔR1 (P.001), ΔR2 (P.001), and ΔRt (P=.002) and also from the R-CAD group in ΔR1 (P=.001), ΔR2 (P=.002), and ΔRt (P=.003).The impressions made with metal and resin CAD-CAM impression copings were more accurate than those made with the Alt-cast technique although all techniques had acceptable accuracy.