An in vitro evaluation of impression techniques for multiple internal- and external-connection implant prostheses.

Abstract

This in vitro study evaluated the accuracy of three different impression techniques, using polyether impression material, to obtain a precise definitive cast for a multiple-unit implant restoration with internal- or external-connection implants. Two reference resin models, one with four internal-connection implants (RRMI) and one with four external-connection implants (RRME), were fabricated. The longitudinal axes of all implants diverged by 15 to 20 degrees from the midline toward the labial/buccal. Six groups of 15 specimens each (45 RRMI, 45 RRME) were made with medium-consistency polyether: nonmodified square impression copings (NMIC and NMEC groups), square impression copings joined together with autopolymerizing acrylic resin before the impression procedure (RIC and REC groups), and square impression copings that were airborne particle-abraded and coated with impression adhesive (MIC and MEC groups). A single calibrated examiner evaluated the positional accuracy of the implant replica heads with a profile projector. These measurements were compared to the measurements calculated on the RRMI and RRME, which served as controls. All differences between the distances measured on the RRMI and on the RRME and those measured on the definitive casts of the six groups were transformed into the differences in microns each 1,000 μm of length (per mil difference) and statistically analyzed. Considering anterior and posterior per mil median differences together, the highest precision of the definitive casts was in the groups MEC, REC, and RIC. Within the limitations of this study, splinted square impression copings (RIC and REC) and airborne particle-abraded and coated (MIC and MEC) impression techniques showed greater accuracy in replicating the control models (RRMI and RRME) than the nonmodified technique (NMIC and NMEC). The definitive casts of the model with internal-connection implants achieved the highest precision with splinted square impression copings (RIC).