RETENTION AND FATIGUE RESISTANCE OF PEEK AND ACETAL THERMOPLASTIC RESIN CLASPS

Aim; The objective of this study was to evaluate the retentive force of clasps made from two thermoplastic resins by the insertion/removal test simulating two years. Methods: Over standardized premolar and molar metal crowns 36 clasps were fabricated, including 18 clasps (1.5 mm thick) from each of the following thermoplastic resins: polyetheretherketon (PEEK) and polyoxymethylene (POM). Each clasp assembly was subjected to an insertion/removal test on its abutment crown. Retention of each clasp at pre-test (Baseline) and after insertion/removal cycling intervals (360, 730, 1080, 1440, 2116 and 2880 cycles by chewing simulator) were measured by applying withdrawal force to it using universal testing machine. To analyze the retention over the course of insertion/removal test, retention was measured every 360 cycles. Data were statistically analyzed using 3-way ANOVA (α= 0.05). Results. PEEK Resin material group recorded statistically significant higher mean value than Acetal material group mean value (P=0.0013<0.05). Molar abutment group recorded statistically non-significant higher mean value than premolar abutment group mean value (P= 0.0620>0.05). Retention decreased significantly as indicated by three-way ANOVA (P= 0.007<0.05). Conclusion: Retention forces of PEEK clasps were clinically acceptable, and superior to retention forces of Acetal clasps. However, both the retention of adequately designed resin clasps might be enough for clinical use. Clasps made of PEEK showed the more promising retentive force with a non-fracture tendency at tooth undercut. KEYWORDS; PEEK, Acetyl, Clasps, Insertion Force, Removable Partial Denture (4020) Azza A. ElSegai and Muhammad Abbas E.D. J. Vol. 64, No. 4 clasps should not unduly stress abutment teeth or be permanently distorted during service (2). Various metallic materials have been used to fabricate the clasps of PRDPs and the physical properties of these materials have been examined. The most common alloys used for clasps are cobalt–chromium (CoCr) alloys (3). For the clasps to be retentive they must be flexible and retain the PRDPs satisfactorily (4). They donot have to transmit excess stress to the abutment tooth. Previous studies have shown that clasps made of more elastic materials present better resistance to loss of retention(5). On the other hand, if the clasps are excessively flexible, they may not provide adequate retention when positioned on the abutment tooth (6). Polyoxymethylene (POM) also known as acetal resin, an injection-molded resin has been introduced as an alternative to conventional Polymethylemethacrylate (PMMA). Polyoxymethylene is formed by the polymerization of formaldehyde. The homopolymer, polyoxymethylene is a chain of alternating methyl groups linked by an oxygen molecule. It has a relatively high proportional limit with little viscous flow enabling it to behave elastically over a great enough range to be used as a material for clasp construction (7). PEEK (polyetheretherketon) is part of a relatively new family of high temperature thermoplastic polymers. These polymers have great physical resistance, hydrological stability and rigidity, with a modulus of elasticity similar to bone. It is a main aromatic chain, connected with ketone and ether groups (8). In medicine, PEEK has been shown to be an excellent substitute for titanium in different therapeutic applications, and in dentistry as a temporary implant abutment (5). The low modulus of elasticity of the thermoplastic resins presents superior flexibility compared to the conventional Cr-Co alloys (9). The progressive loss of mechanical properties after repeated loads is an important consideration for the choice of materials. Although, clasp fatigue is based on recurrent deflection by repeatedly inserting and removing the prostheses(10). There are few studies that examined mechanical properties of thermoplastic resins to determine the appropriate design for PRDP clasp(11). However, to our knowledge there are little studies evaluating the use of PEEK as clasp materials. Therefore, this in-vitro study explored the retentive force of two thermoplastic resin clasps after repeated insertion and removal on two different abutment teeth. The null hypothesis was that there would be no difference in the retentive force between resin clasps or abutment teeth. MATERIALS AND METHODS Two thermoplastic resins (PEEK and Acetal) were evaluated in this study. Materials used are presented in Table 1. TABLE (1) Materials used for clasp fabrication. Brand name Composition Manufacturer Batch number PEEK Polyetheretherketon Bredent, Senden, Germany 540XS016901 Acetal Polyoxymethylene (POM) Thermoflex Acetal Resin Densply UK 01065 Abutment fabrication An artificial maxillary first premolar and molar (KaVo, Germany) were embedded in autopolymerizing acrylic resin (Acrostone, Cairo, Egypt) using custom-made copper holders with a diameter of 15 mm and 20mm high. The teeth were prepared for a surveyed complete metal crown. The prepared teeth were dublicated twice using a rubber impression material (Impregum Penta H RETENTION AND FATIGUE RESISTANCE OF PEEK AND ACETAL THERMOPLASTIC (4021) and L; 3M Espe, Germany). The impressions were poured in Type IV stone (GC, Belgium), and a complete crown was waxed on each preparation (Crowax; GmbH, Germany). The waxed crowns were surveyed to provide an undercut of 0.25 mm Occlusal rests, 2.5 mm long, 2.5 mm wide, and 2 mm deep, were placed dismally for premolar and mesially for molar. Mesial and lingual guide planes, two thirds the length of the crown, were prepared with a surveyor blade to standardize the path of insertion. The waxed 2 crowns were duplicated with silicon material (Speedy, Augsburg, Germany), and then 18 premolar crowns and 18 molar crowns were made by inserting heated liquid wax into the silicone mold. Then the crowns were cast in Co-Cr alloy (Wironit 99; Bego, Bremen, Germany). After fitting and finishing, the crowns were cemented inplace on the abutments with zinc phosphate cement (Hoffmann, Berlin, Germany). The guide planes were evaluated for parallelism.