TO COMPARE AND EVALUATE THE FRACTURE TOUGHNESS AND FLEXURAL STRENGTH OF PROVISIONAL RESTORATIVE MATERIALS WITH AND WITHOUT REINFORCEMENT OF KEVLAR FIBRES - AN INVITRO STUDY

1. Post Graduate, Co.Org. Institute of Dental Science, Karnataka, India. 2. Prof & Hod, Co.Org. Institute of Dental Science, Karnataka, India. 3. Prof & Chair, Co.Org. Institute of Dental Science, Karnataka, India. 4. Professor, Co.Org. Institute of Dental Science, Karnataka, India. 5. Reader, Co.Org. Institute of Dental Science, Karnataka, India. 6. Senior Lecturer, Co.Org. Institute of Dental Science, Karnataka, India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 20 December 2019 Final Accepted: 22 January 2020 Published: February 2020

1137 methacrylate has shown poor wear resistance & poor esthetics. Thus PMMA and bisacryl resin composite materials possess a larger market value. 4 These materials must be strong enough to withstand the masticatory forces, particularly for long span FPD's for long term use or for patients with parafunctional habits. Therefore the flexural strength of these material is an inevitable property. 5,6 The resistance of provisional resins to crack propogation is helpful when assessing the strength and serviceability of treatment restorations. 7 A fracture toughness test measures the resistance of a material to crack extension. 8 Fibre reinforcement has been used to overcome the mechanical limitations of provisional resins. The fibres that have been incorporated in this manner are carbon, aramid, polyethylene and glass fibers. 3,9 Kevlar fibres (para aramid) are popular as they exhibit superior mechanical properties than nylon & E glass fibres and have superior wettability compared to carbon fibres and do not require treatment with a coupling agent. They were found to improve tensile strength, elastic modulus, impact strength and fracture resistance and found to be biocompatible with no evidence of toxicity. 10 Since very few evidence has been found where Kevlar fibres have been incorporated in provisional restorative materials such as polymethyl methacrylate and bis-acryl composite, this study aims to investigate the effect of addition of Kevlar fibres to provisional restorative materials and the changes in the properties like fracture toughness and flexural strength.
Methodology:-Fracture Toughness: Specimens of polymethyl methacrylate and bisacryl composite provisional restorative material were prepared according to ASTM (American society for testing and materials) no. E 399 83 ( Figure:1). The specimens were in the form of a double cantilever beam, with a slot that originates from the center of one edge, extending along the specimen's center line to a 60 degree terminal apex located slightly beyond the midpoint of the specimen. Two loading holes pierces the specimen. The design of the assembled mold provides 3 triangular ports, which allows the escape of excess resin during mold assembly and exposure to pressure during polymerization. PMMA specimens were fabricated at room temperature by mixing the polymer and monomer in a clean glass slab with a stainless steel spatula at the 2:1 ratio (Figure: 3). When the mix reached the dough stage, it is packed into the mold cavity slowly to avoid entrapping of air, the cover and the 2 circular rods of the mold were placed in position, and the entire assembly were placed in a hand press and compressed to allow the material to completely flow into the mold. The bisacryl specimens were prepared in the same manner, except that the material is supplied in an automixing cartridge. The mix is packed directly into the mold cavity using application tips supplied with the kit. (Figure: 4) The fibre reinforced specimens were made by precutting the fibres into 12 mm lengths and wetted using the polymer monomer mix for the PMMA specimens and a bonding agent for the bis acryl resin. (Figure:5). The mold cavity were filled with the resin, and then the fibres were placed perpendicular to the end of the slot and 1 mm away from it, aligning the fibres perpendicularly to the direction of the crack. After the resin is completely polymerized, the specimens were separated from the mold, and the flash was removed using a razor blade. The specimens were examined for any voids, and all defective specimens were discarded. Specimens were stored in water at 37˚C for 24 hours before testing.
A precrack was placed in the compact test specimens by placing a sharp scalpel at the end of the slot and applying hand pressure.
Polymethyl methacrylate Pmma ft a -control group -no reinforcement (n=10) Pmma ft b -reinforced with kevlar fibre (n=10) Bis acryl composite Bac ft a -control group-no reinforcement (n=10) Bac ft b -reinforced with kevlar fibre(n=10) The specimens were tested in tension in a universal testing machine with the direction of the force perpendicular to the plane of the preformed crack. The peak force (F) in newtons, which caused fracture of the specimens, was recorded and used to calculate the fracture toughness (K1C) measured in MPa.m 1/2 from the following equation; K1c= pc/bw 1/2 . F(a/w) Where pc is the maximum load before crack advance; b is the average specimen thickness (cm); w is the width of the specimen (cm), (a) = crack length (cm).

Flexural Strength:
A specially designed split stainless steel mold was constructed to form rectangular specimens of dimensions 2mm × 2mm ×25 mm ( Figure: 2). The specimen preparation were similar to the fracture toughness specimens. When the mix reached the dough stage, it was packed into the mold cavity slowly to avoid entrapping of air; the mold was then covered to remove the excess resin and kept at room temperature for 15 minutes to allow for complete polymerization of the resin. The fibre-reinforced specimens were made from precut 23-mm long fibres, wetted using the polymer-monomer mix (PMMA) and bonding agent (bis-acryl), and then placed in the lower part of the mold cavity and the resin applied on top. After complete polymerization of the resin, the specimens were separated from the mold; flash was removed with the razor blade and examined for voids Polymethyl methacrylate Pmma fs a -control groupno reinforcement (n=10) Pmma fs b -reinforced with kevlar fibre (n=10) Bis acryl composite Bac-fs a -control group-no reinforcement (n=10) Bac -fs b -reinforced with kevlar fibre(n=10) The flexural strength for all the specimens was determined by loading the specimens in the same universal testing machine. The load was applied with a crosshead speed of 1 mm/min. The peak force (F) in newtons, from stress strain curve of each specimen was recorded and used to calculate the flexural strength in MPa from the following equation: where δβ is the flexural strength in MPa; F is the maximum applied load in newtons; I is the supporting width in millimeters; B is the breadth of the test specimens in millimeters; and h is the height of the test specimen in millimetre.

Statistical Analysis:
The data was collected and fed in SPSS (IBM version 23) for the statistical analysis. The descriptive statistics included mean and standard deviation. The inferential statistics included One way ANOVA for comparison within and between the groups followed by Post Hoc Tukey's test for the multiple comparisons. The level of significance was set at 0.05 at 95% Confidence Interval.

Discussion:-
Fixed partial dentures have become a well-established treatment modality for many partially edentulous patients. Because these restorations are made indirectly in a dental laboratory, several days or weeks are usually required for their completion. 1 Therefore provisional restoration is an essential element of fixed prosthodontic treatment. They should shield pulpal tissue against physical, biochemical and thermal injuries, maintain positional stability and occlusal function, should provide strength, retention and aesthetics for the prepared teeth. In addition, they may be used for correcting irregular occlusal plane, altering vertical dimensions and changing the contour of the gingival tissue. 2 The commonly used provisional restorative resins are polymethyl methacrylate (PMMA), polyethyl methacrylate (PEMA), composite resin (bisacryl composite) and poly urethane dimethacrylate. 3 PMMA is the most commonly used material for indirectly made provisional fixed partial dentures. Its strength, color stability, ease of manipulation and polishing make it a desirable material. Bis-acryl composite resins are more expensive but show low exothermic reaction on setting, good marginal fit, and moderate color retention and strength. Thus PMMA and bisacryl resin composite materials possess a larger market value. 3 Kevlar fibres are popular as they exhibit superior mechanical properties than nylon and E-glass fibres. The commercial name for aramid fibre is Kevlar and chemically it is an organic compound such as poly paraphenyleneterephthalamide with chemical formula (-CO-C6H4-CO-NH-C6H4-NH-)n. Polyaramidfibres have superior wettability compared to carbon fibres and do not require treatment with a coupling agent. They were found to improve both tensile strength and modulus of elasticity of denture bases. Acrylic resin appliances reinforced with fibre content up to 2% and with unidirectional orientation showed significantly higher impact strength and fatigue resistance.
Since very few evidence has been found where Kevlar fibres have been incorporated in provisional restorative materials such as poly methyl methacrylate and bis-acryl composite, this study aimed at evaluating the effect of Kevlar fibre reinforcement on fracture toughness and flexural strength of PMMA and bisacryl composite provisional restorative material. It also intended to compare between the two materials on fracture toughness and flexural strength without any fibre reinforcement.
The stress at which a brittle material fractures is called the fracture strength. In the present study specimens were tested in tension in a universal testing machine with the direction of the force perpendicular to the plane of the preformed crack. Each specimen was held in a specially designed tension device in the machine, and tension force was applied with a crosshead speed of 5 mm/min.3 The present study shows higher fracture toughness of the control BAC resin (0.8496) over the control PMMA resin (0.7878). The reason for greater fracture toughness of BAC may be because of its molecular structure when compared to other provisional restorative materials. This observation was in correlation with the studies of Geertset al 6 15,21,22,26 This study had some limitations. In vitro static load tests differ from the dynamic intraoral conditions. Cyclic loading can be incorporated in the testing method to simulate the clinical environment. Microcracks and defects that grow inherently during thermal and mechanical processes can significantly reduce strength measurements. No cyclic loading in a moist environment was performed in the present study, and this is a study limitation.
After filling the mold, hand pressure was applied for 30 seconds until contact was established between the mold. The pressure was not standardized and this is a study limitation. 1144