Fiber reinforcement of a resin modified glass ionomer cement a b b a Carina B. Tanaka , Frances Ershad , Ayman Ellakwa , Jamie J. Kruzic * a - School of Mechanical and Manufacturing Engineering, University of New South Wales (UNSW Sydney), Sydney NSW 2052, Australia b - School of Dentistry, The University of Sydney, Westmead NSW 2145, Australia *Corresponding author. Jamie J. Kruzic Address: School of Mechanical and Manufacturing Engineering, University of New South Wales (UNSW Sydney), Sydney NSW 2052, Australia. Tel.: +61 2 9385 4017 E-mail address: [email protected] Abstract Objectives. Understand how discontinuous short glass fibers and braided long fibers can be effectively used to reinforce a resin modified glass ionomer cement (RMGIC) for carious lesions restorations. Methods. Two control groups (powder/liquid kit and capsule) were prepared from a light cured RMGIC. Either discontinuous short glass fibers or braided polyethylene fiber ribbons were used as a reinforcement both with and without pre-impregnation with resin. For the former case, the matrix was the powder/liquid kit RMGIC, and for the latter case the matrix was the capsule form. Flexural strength was evaluated by three-point beam bending and fracture toughness was evaluated by the single-edge V-notch beam method. Compressive strength tests were performed on cylindrical samples. Results were compared by analysis of variances and Tukey’s post-hoc test. Flexural strength data were analyzed using Weibull statistical analysis. Results. The short fiber reinforced RMGIC both with and without pre-impregnation showed a significant increase of ~50% in the mean flexural strength and 160 – 220% higher fracture toughness compared with the powder/liquid RMGIC control. Reinforcement with continuous braded fibers gave more than a 250% increase in flexural strength, and pre- impregnation of the braided fibers with resin resulted in a significant improvement of nearly 400% relative to the capsule control. However, for the short fiber reinforced RMGIC there was no significant benefit of resin pre-impregnation of the fibers. The Weibull modulus for the flexural strength approximately doubled for the fiber reinforced groups compared to the control groups. Finally, compressive strength was similar for all the groups tested. Significance. By using a RMGIC as a matrix, higher flexural strength was achieved compared to reported values for short fiber reinforced GICs. Continuous braided polyethylene fibers can give much higher flexural strength than discontinuous glass fibers, and their effectiveness is enhanced by pre-impregnation of the fibers with resin. However, the short fibers were more effective to toughen the RMGIC matrix. Keywords: Resin modified glass ionomer cement, fiber reinforcement, resin Composite, mechanical properties, fracture toughness 1. Introduction The maintenance of oral health in geriatric patients is a significant issue in dental care. Age-related salivary changes such as xerostomia combined with other factors including poor oral hygiene or gingival recession exposing root surfaces tend to result in a higher prevalence of caries in elderly patients [1]. Currently, the minimal intervention approach known as Atraumatic Restorative Treatment (ART) has been the treatment of choice for these patients [2]. The ART technique has numerous advantages compared to conventional adhesive restorations such as it does not require local anesthesia, a rubber dam for isolation, or drilling and can be performed in a shorter treatment time, thus reducing patient discomfort. However, restorations placed using the ART technique only achieve high survival rates for relatively small cavities with sufficient support of tooth structure, such as in class I occlusal restorations [3]. In addition, clinical studies have shown that restorations placed in the root and cervical surfaces are one of the least durable types of restorations [4, 5]. Indeed, they commonly experience high loss of retention, failure at the restoration margins, and secondary caries [4, 6]. The high failure rates of complex cavities and Class V restorations are related to the poor mechanical properties of the materials used [7]. Currently, the preferred material for root and cervical lesion restorations are glass ionomer cements (GICs) and resin modified GICs (RMGICs) [8, 9] due to their excellent adhesion to the tooth and their natural capacity for fluoride release. Furthermore, compared to resin-based composites, GICs are easier to handle and require less steps during the placement procedure. Although RMGICs represent a significant improvement in mechanical properties compared with conventional GICs, the strength and fracture resistance remains much less than typical resin composites [10, 11], and this is a significant concern when using them for permanent restorations. A few studies have shown that adding fiber reinforcements represents one potential way to improve the strength and fracture resistance for this class of dental materials [12-16]. However, one challenge is achieving good bonding between the matrix and the fibers [12], and no studies have examined fiber reinforcement of RMGICs. For fiber composite reinforcement, it is well known that control of the fiber/matrix interfacial properties is essential for achieving good mechanical properties [17]. In the field of resin based dental restorative composites, surface treatments such as cold gas plasma, silanization, or etching have been used to improve the interfacial bonding properties [18, 19]. In addition, studies show that fibers pre-impregnated with resin can have improved interfacial bonding, resulting in higher flexural strength [20], and this methodology has been successfully applied to porous continuous fibers for periodontal and post traumatic splints [21] and orthodontic retainers [22]. The purpose of this study was to evaluate the mechanical properties of fiber reinforced resin modified glass ionomer cements when using discontinuous/short glass fiber and braided long fibers. Additionally, the second purpose was to examine the effect of resin pre- impregnation of the fibers on the mechanical properties. The hypotheses of this study were that 1) by using a resin modified GIC matrix, enhanced properties would be achieved relative to fiber reinforced GICs that have been reported in the literature, 2) the RMGIC reinforced with long braided fibers would perform better than with short discontinuous fibers and 3) that fibers pre-impregnated with resin would give improved mechanical properties compared to non- impregnated fibers. 2. Materials and method 2.1. Specimen preparation Two control groups (powder/liquid kit and capsule) were prepared from a light cured, resin modified glass ionomer cement (Riva Light Cure, SDI Limited, Australia) following the manufacturer dose and mixing recommendations. The hand mixed group was prepared with a powder-to-liquid mass ratio of 3.1:1. The capsule group was activated and mechanically mixed in an amalgamator for 10 s. Table 1 –Manufacturer and composition of the resin modified glass ionomer and composite materials used in this study. Material Manufacturer Composition Resin-modified Riva Light-Cure Liquid: polyacrylic acid, tartaric acid, glass-ionomer Powder/Liquid and HEMA; cement Capsule powder: fluoroaluminosilicate glass (SDI Limited, Australia) *Filler (wt%): 30-60; *Filler composition: glass, oxide, fumed Construct™ Flowable resin silica; (Kerr, USA) resin composition: bis-EMA, TEGDMA, light-cure initiators, and stabilizers Abbreviations: HEMA: 2-hydroxyethyl methacrylate; bis-EMA: ethoxylated bis-phenol-A- dimethacrylate; TEGDMA: triethylene glycol dimethacrylate. *Further proprietary details not provided by manufacturer Braided polyethylene woven fibers ribbons (Construct™, Kerr, USA) were used as a reinforcement with and without pre-impregnation with flowable resin (Construct™, Kerr, USA) in a laminate construction (Figure 1) that may be quickly applied in the clinical setting. For the impregnated group, the cut ribbons of fibers were placed on a glass slab and using a spatula the fiber was saturated with resin on both sides. The impregnated fiber was kept away from light until it was incorporated with the RMGIC and the whole composite structure was cured as described below. Additional details on the RMGIC and the flowable resin can be found in Table 1. For the flexural strength samples, the 1.0 mm wide fiber ribbons were cut into 25 mm lengths and placed at the base of a stainless steel split mold. The fiber ribbon was covered with the RMGIC capsule material to fill the mold. Compressive strength specimens were prepared with a 3 mm braided fiber ribbon. The fibers were stretched to 4 mm width and placed in the middle of the cylinder height between layers of RMGIC (Figure 1). Figure 1. Schematic of the braided fiber specimens for flexural strength (A) and compressive strength (B) tests. Discontinuous short glass fibers (S-2 glass® fibers, AGY, USA) with a diameter of 5 m were cut by the manufacturer into a uniform length of 0.5 mm to create fibers with an aspect ratio of ~100. Adding fibers to the capsules was trialed but did not allow for good mixing. So, instead the short fibers were hand mixed with the bulk powder/liquid kit formulation of the same RMGIC (Riva Light Cure, SDI Limited, Australia). The short fibers were treated in two different ways: (1) wetted with the mixing liquid contained from the RMGIC kit or (2) pre-impregnated with flowable resin (Construct™, Kerr, USA). For the former group,