Dalkiz, Arslan, Tuncdemir, Bilgin, Aykul INtrODutcION The number of denture wearers is increas- ing as the number of elderly people is continually growing and polymethyl methacrylate (PMMA) is still the most frequently used material in denture base fabrication.1 Despite of its satisfactory aes- thetic properties, ease of processing and accu- rate fit, PMMA incurs dimensional changes during processing and use.2-5 Thickness of the denture, thermal expansion and contraction of the acrylic resin and the gypsum, structural variations be- tween polymer and monomer systems, polymer/ monomer ratios, packing pressure, polymeriza- tion shrinkage, investments, separators, internal stresses, water sorption and desorption and wa- ter content of the gypsum were reported as some variables influencing the dimensional stability of an acrylic resin denture.2-10 In recent years, various types of reinforcing fibers have been introduced and used to enhance the physical and mechani- cal properties of acrylic resin denture bases and to increase their clinical performance.11-22 Effects of the type11-16, the form17,18, the length19,20 and the concentration20 of reinforcing fibers, as well as the type of fiber-reinforced acrylic resin denture base material21on the dimensional stability were inves- tigated in previous studies. However, a study dem- onstrating the relation between the palatal vault shape and the dimensional stability of fiber rein- forced denture base materials was not conducted up to now. The purpose of this in vitro study was to determine the effect of different palatal vault shapes on the dimensional stability of a glass fiber reinforced heat polymerized acrylic resin denture base material. MAtErIALs AND MEtHODs Three edentulous maxilla with shallow, deep and normal shaped palatal vaults were selected and impressions were obtained with an elasto- meric impression material (Zetaplus; Zhermack Spa, Badia Polesine, Rovigo, Italy) (Figure 1). Each impression was cast and duplicated 30 times with Type IV dental stone (Begostone; Bego, Bremer GW Herbst GmbH & Co, Bremen, Germany) and totally 90 definitive casts were obtained. Heat polymerized acrylic resin denture bases (QC-20, Dentsply Int Inc, Waybridge, Surrey, UK) were fab- ricated onto each definitive cast according to the recommendations of the manufacturer and pre- January 2012 - Vol.6 pared specimens were assigned to 3 main experi- mental groups (Table 1) representing three differ- ent vault shapes: Group SHAL ROUNDED Group U (U-shaped) and Group V (V-shaped) (n=30). Each main group consisted of 3 subgroups (n=10) repre- senting the type of fiber used; subgroups includ- ing wowen type fibers (Groups ROUNDED/WOF, U/ WOF, V/WOF), subgroups including roving type fi- bers (Groups ROUNDED/ROF, U/ROF, and V/ROF), and finally subgroups which did not include any fi- ber (Groups ROUNDED/NOF, U/NOF, V/NOF). An epoxy resin maxillary cast (Polycast Indus- tries, Bay Shore, NY) with four reference points (A, B, C, and D) was prepared to serve as control for metal gauge measurements. Point (A) was marked in the anterior midline of the edentulous ridge in the incisive papillary region, points (B) and (C) were marked in the right and left posterior mid- lines of the edentulous ridge in the second molar regions, and point (D) was marked in the posterior palatal midline near the fovea palatina media (Fig- ure 2). To determine linear dimensional changes, distances between four reference points (A-B, A-C, A-D and B-C) were initially measured with a metal gauge accurate within 0.1 mm (Hu-Friedy, Chica- go, IL) under a binocular stereo light microscope (Baytronix, B000A7XBSQ, Silver Spring, MD) with 20x magnification and data (mm) were recorded. Preparation of specimens without fiber For specimen preparation of groups ROUND- ED/NOF, U/NOF and V/NOF, 2 sheets of modeling wax were spread onto the casts as to completely cover the surfaces. A small wax extension was adapted at the posterior border of each cast to provide shrinkage compensation during the po- lymerization process. Cast specimens covered with wax patterns were flasked with Type II den- tal stone (Moldano, Bayer, Germany) and were al- lowed to set for 45 minutes. The flasks were then placed in boiling water for five minutes to soften the waxes. The flasks were then opened and the wax elimination was performed. Cast surfaces were thoroughly cleansed with liquid soap and rinsed with boiling water (Boil Out Unit, EWL 5522, Kavo EWL, Germany). Two coats of separator (DVA Acrylic and Plaster Separator, DVA Inc, Corona, CA) were applied on cast surfaces and allowed to dry. Heat polymerized acrylic resin denture base material (QC-20, Dentsply Int Inc, Waybridge, Sur- 71 European Journal of Dentistry