Fracture Force Evaluation, Deflection, And Toughness On Three Repaired Injection Molded Pmma Denture Base Resins
Arun K Ch Sivakala1, Brintha J Jeyaraj1*, Murugesan Krishnan1, Muthu K Balasubramanian1
1 Department of prosthodontics, SRM Dental College, Chennai- 600089, Tamilnadu , India. Brinthajei @ yahoo.co.in
To date, the management of fractured dentures made from high-impact acrylic polymers is challenging. Hence this study is aimed to evaluate fracture force, toughness, and deflection on three types of repaired injection-molded polymethyl methacrylate thermoplastic denture base resins.
In this in-vitro study, the sample size was estimated to 20 samples per group (group A, B, and C) using G*power 3.0.10 software with power 80% and alpha error of 5%. So, a total of 60 high-impact injection-molded acrylic resin samples (39mm x 4mm x 8mm) were prepared and a pre-crack was made with a diamond disc to a depth of (3.0 ± 0.2) mm along the marked centreline. Repair of the fractured segments was done with Probase cold, Triplex SR cold, and Lukafix light cure resin. A three-point bending test was carried out to test the repaired site, and the obtained values were statically analyzed with one-way ANOVA and Tukey’s post hoc test (α ≤ 0.05).
The statistical analysis has shown significant differences in flexural force, deflection, and fracture toughness between groups. The increase in fracture toughness was observed in group A, which was 245.06N, the deflection was 0.14cm and flexural strength was 24.0 Mpa
The fracture force, deflection, and toughness were found to be significant in samples repaired with auto polymerizing PMMA resin. Hence the study concludes that auto polymerizing resin can be better used in repair auto polymerizing PMMA resin when compared to other resins.
Key words: Injection molded acrylic resin, Denture fracture, Denture repair, Deflection, Toughness.
The construction of dentures can be done by compression molding, injection molding the acrylic resin, or microwave processing [1-3]. PMMA is extensively processed by compression molding method for fabrication of denture for its desirable properties . However, a dimensional change attributed to this technique can be inevitable. According to ISO standard (1567:19881), the denture base acrylic resin should possess Kmax (maximum factor of the loading intensity), fracture work Wf of 1.90 MPam1/2 and 900 J/m2 respectively to resist fracture. However, fractures and the formation of cracks are common complications to both compression molded and injection molded removable prostheses. The most common of which is midline fractures and cracks at the posterior cantilever area, as a result of fatigue failure, extreme thin area, thin flange near frenum [5, 6]. So, Pryor in 1942 developed an injection molding system of plastics for dentures . The continuous injection process under constant hydraulic pressure in the closed mold compensates for excessive shrinkage and produces a dense strong plastic-free from porosity. Studies had shown that this injection-molded PMMA system had better dimensional stability, wear strength, better deflection, and water sorption [8-10]. However, fractures and the formation of cracks are common complications to both compression molded and injection molded removable prostheses. The denture base repair was previously done with auto polymerizing resin glass fiber reinforcement, woven metal, visible light polymerized reline material, salinized glass fibers, wires reinforced with Co-Cr or San-cobalt palatal bars which showed significant results on conventional compression-molded PMMA dentures [11-14]. The main aim of denture repair is to reimpose the denture to its initial strength. Final strength after repair depends on certain factors like the width of the fracture gap, fracture surface bevelling, and properties of repair resin. So the resistance to fracture of the repaired denture base material is affected by fracture strength and fracture toughness . Dentures made with injection-molded PMMA thermoplastic resin can also fracture but it is not feasible to repair with the same material. So it is mandatory to determine the flexural properties like toughness, deflection, and strength after the fractured denture base was repaired with other auto polymerizing and light cure resins. However, no studies have been previously done to analyze the fracture toughness of repaired Injection-molded polymethyl methacrylate denture base resins. Hence this study is aimed to evaluate fracture force, toughness, and deflection of injection-molded polymethyl methacrylate thermoplastic denture base resins repaired with two different auto polymerizing resins and one light-curing resin. The null hypothesis of this study stated that there is no difference in fracture force, deflection, and toughness between the three repair materials tested.
Materials and Methods
This in-vitro study was conducted in SRM Dental College from March 2018 to December 2019 and was approved by the institutional review board with the IRB number SRMDC/IRB/2017/MDS/NO.202 The sample size was estimated to 20 samples per group, using G*power 3.0.10 software with power 80% and alpha error 5% the sample size was calculated.
A master brass die was prepared according to ISO 20795.1.2013 with dimensions of 65mm x 40mm x 5mm in brass. The master die was duplicated with an additional silicone impression material (Aquasil soft putty, Dentsply, Germany) to prepare the mold. The wax blocks were prepared from the mold (Figure 1) and processing was done using injection-molded PMMA resin (SR Ivocap High Impact, Ivoclar Vivadent, Liechtenstein) based on the manufacturer’s instruction. The acrylic specimens were retrieved after the curing cycle is completed and checked for any irregularities. Then the specimens were trimmed and finished using acrylic stone trimmers and 600 grit sandpaper. Each specimen was cut into six equal samples measuring 39mm length, 4mm width, and 8mm height using a milling machine .
A total of 60 samples were made and their dimensions were verified using a digital micrometer (Digimatic Micrometer, Japan). The test samples were stored at 37 °C in water for 24 hrs before testing. The samples thus obtained were fixed lengthwise in the holding device and a mark was set exactly on the centreline of the sample. A pre-crack was cut with a diamond disc according to ISO 20795.1.2013 to a depth of (3.0 ± 0.2) mm along the marked centreline. Then the pre-crack was wet with a drop of glycerol and a sharp notch was made with a Double-sided 0.25*22mm NTI Flex disc (Val Lab diamond disk, US) (Figure 2). The notched samples were stored in a container with water at (37 ± 1) °C for 7 days before testing.
Figure 1. Wax pattern