Evaluation of Thermoplastic Resin as a New Pattern Material for Posts: Dimensional Accuracy and Detail Reproduction

Traditionally, dark-cure (cold-cure) acrylic resin is used as pattern material for cast post and core. A new post system that incorporates a specially designed plastic support rod and a thermoplastic resin has been introduced to the market and is claimed to simplify the procedure for cast posts fabrication. The aim of the present work was to evaluate the dimensional accuracy and detail reproduction of thermoplastic resin when used as pattern or impression material for posts, and the amount of solid residue after ignition when used as pattern material. Materials used were thermoplastic resin (Trigger Feed, Glue Gun, China), polyether rubber base material (Permadyne, ESPE), and Duralay acrylic resin (Relience Dental Manufacturing Co). A machined cut circular stainless steel (st.st) mold similar to those described in ADA specification No. 19 and BS 4269 was constructed with two horizontal and three vertical sharp line grooves. The scribed horizontal lines were used to measure the dimensional changes. The scribed vertical lines provided a measure of surface detail reproduction. A ring of 5 mm thickness was machined cut that placed on the circular st.st. mold shoulder leaving 2 mm thickness for the impression material. Thirty impressions of st. st. die were made, ten from each material used in the study. Three readings were made for each impression disc to the nearest 0.001 mm using a travelling microscope (Unitron Bi5-3174) at ×10 magnification. The percentage of dimensional change was calculated. Surface detail reproduction was evaluated, by inspecting each vertical line under a travelling microscope (Unitron Bi5-3174) at X 10 magnification using low-angle illumination. The reproduction was considered acceptable if two of three of the vertical lines were reproduced continuously and well defined for the full length. Two grams of thermoplastic resin and two grams of cold cure resin were ignited separately in porcelain crucible at 700 °C. The nonvolatile residue left was weight using electronic balance. Results of the study showed that there was no statistically significant difference in surface detail reproduction as well as dimensional stability during hardening between thermoplastic resin, cold-cure resin pattern material or polyether elastomeric impression material. There was no statistical significant difference in the percentages of solid residue left after burning out of thermoplastic resin or cold cure resin. Clinical relevance: Thermoplastic resin is a cheap, low cost, easily manipulated, elastic material, and as accurate and dimensionally stable as the rigid cold cure resin pattern. Hence, it must replace the traditional post-core pattern materials or impression materials for post hole(s).