The effect of selected variables on the retention of irreversible hydrocolloid impression material

The aim of this study was to investigate the effect selected variables have on the retention of irreversible hydrocolloid impression material in an impression tray.

To simulate custom impression trays, acrylic resin plates were fabricated from autopolymerizing tray resin and visible light-activated composite resin (VLC). A total of 240 plates of each material were fabricated. Half of the plates of each material were polymerized against baseplate wax and the other half against tinfoil. Specimens fabricated against wax were divided into four groups of 30 specimens each and were prepared as follows: Group 1 (control): plain, left untreated plates, Group 2: plates were perforated, Group 3: plates were coated with a tray adhesive, Group 4: plates were perforated and coated with adhesive. Specimens fabricated against tinfoil were also divided into four similar groups. Impression holders were prepared from autopolymerizing acrylic resin (AAR). Neocolloid (an irreversible hydrocolloid impression material) was mixed, applied to the tray plate, and seated over the impression holder. The test assembly was then placed in an incubator and allowed to set for 5 minutes. Tensile forces were applied to specimens using an Instron testing machine at a crosshead speed of 5 mm/min. The results were recorded in Kg/mm2. The mode of failure was recorded as either adhesive or cohesive. Data analysis was conducted using one-way analysis of variance (ANOVA) and the Scheffe post hoc tests (p<0.05).

The tensile bond strength of Neocolloid to AAR specimens fabricated against wax and specimens fabricated against tinfoil increased significantly when both methods of retention (perforation and adhesive) were applied. The same was observed with VLC specimens fabricated against wax and tinfoil. The best tensile bond strength of all specimens tested was obtained with the combination of VLC/tinfoil specimens having both methods of retention. Bond failure occurred at the Neocolloid-adhesive interface in all specimens.

Tray materials, type of spacers, and retention methods interacted to influence the tensile bond strength of irreversible hydrocolloid. The performance of the VLC resin fabricated against tinfoil spacer was consistently superior to AAR. Applying an adhesive to a perforated acrylic surface produced significantly greater tensile bond strength. All specimen failures occurred adhesively indicating a weak bond between the adhesive and the irreversible hydrocolloid impression material.

The results of this investigation suggested perforating custom trays, routinely applying irreversible hydrocolloid adhesive, and using a custom tray made from VLC resin that was fabricated against a tinfoil spacer is needed to achieve the maximum tensile bond strength during impression making.