BASIC PROBLEMS WITH THE USE OF RESILIENT DENTURE LINING MATERIALS: LITERATURE REVIEW

Complicated cases in the treatment of totally edentulous patients, as well as in the fabrication of obturators and epitheses, require the use of resilient lining materials (RLMs). These materials have several disadvantages, which is why many dental practitioners do not recommend them. The tendency to breaking the bond between the hard acrylic resin and the resilient liner is a fact with unpleasant consequences, mainly found with the use of cold-curing siliconebased resilient liners. Methods to improve the bond between the hard denture base and the RLM have been the subject of numerous publications. The efforts of the authors are focused in two directions: mechanical or chemical treatment of the denture base or a combination of both. Other commonly discussed problems with the use of RLMs are the difficult and even impossible repair and the retention of oral fluids, fungi and other microorganisms due to the porosity of RLMs. Despite the wide variety of tools offered by different manufacturers, the issue of optimal and high-quality mechanical treatment of the polymerized resilient material has not yet been fully resolved.

. Lower denture, lined with a silicone-based resilient material (authors' clinical case) According to Grant et al. [1], the thickness of the resilient liner should be at least 3.0 mm, and, according to Basker, Davenport and Thomason [2], a thickness greater than 2.0 mm is not needed. In 2013, Lima et al. [5] also found that the stress in the underlying mucosa is the lowest when the thickness of the lining material is not more than 2.0 mm. The greater thickness of the liner reduces the thickness of the base of hard acrylic resin, which may lead to its fracturing, Fig. 3.

Fig. 3. Fractures of dentures lined with a siliconebased resilient material (authors' clinical cases)
Recent studies have indicated that the retention of microorganisms is the result of insufficient cleaning. According to Basker, Davenport and Thomason [2], a diet with fewer carbohydrates would reduce the supply of nutrients to the microorganisms, Fig. 2. According to the authors, cleaning with sodium hypochlorite is most effective, although it may lead to discoloration and corrosion of the metal elements (if any).

Fig. 2.
Colonization of Candida albicans on lower dentures, lined with a silicone-based resilient material after 6 months of use (authors' clinical cases) Methods for strengthening the base are provided in the literature. Basker, Davenport and Thomason [2] and others [6] propose the inclusion of a metal plate. A number of other authors [7 -10] recommend the use of fiberglass. Vergani et al. [11] and other authors [12] have reported the effect of post-polymerization microwave radiation on the flexural strength of Lucitone 550 hard heat-curing acrylic resin, which may increase the durability of the dentures.
Another major problem is the bond between the hard acrylic resin and the RLM.

Bond characteristics
The bond between the hard acrylic resin and the RLM can be of two types: mechanical and chemical, depending on the chemical composition of the materials. Acrylic-based RLMs chemically bond with the acrylic resin of the denture. In the case of silicone-based lining materials, the bond is predominantly micromechanical, and additional means -adhesives or polymeric solvents (primers), are used to improve it, Fig. 4a, b. When bonded by a polyurethane adhesive containing an A-silicone reactive component, a bond resistant to the tensile strength of 200 N/cm 2 is formed [13]. Adhesives contain a polymeric substance in a solvent. The polymeric substance may be a reactive molecule (organosilane) or a molecule such as PMMA in a solvent. The use of adhesive is appropriate for the indirect technique since the drying time is prolonged. In the direct technique, primers are more commonly used. The solvent contained in the primer dissolves the plastic surface and introduces the adhesive bonding polymer. After evaporation of the solvent, the hydrocarbon chains of the in the primer bind to the polymer of the matrix. The formed network in the superficial dissolved layer is a mechanical-chemical bond. On the other hand, the polymer contains reactive Si-H and vinyl groups that are ready for chemical bonding with the A-silicone. The primer technology is affordable, easy to implement, held for a short time, and provides tensile strength and shear strength of 190 -220 N/cm 2 [13]. The most commonly used primers are: ethyl acetate, dichloromethane, a mixture of methoxy and ethoxy silanes, methylene chloride, etc. The durability of the bond depends on the mechanical stresses, the coefficient of thermal expansion, the modulus of elasticity of the various materials, the solubility and the imbibition [14].
Bond breaking can occur at the border between the different materials, and then it is defined as adhesive. If the cause of the bond breaking is a tear of the resilient material, it is defined as cohesive, Fig. 5 and 6.

Bond improving methods
The improvement of the bond between the hard denture base and the RLM has been the subject of numerous publications. The efforts of the authors are focused in two directions: mechanical or chemical treatment of the denture base or a combination of both. In 2012, Philip, Ganapathy and Ariga [27] examined the tensile strength (after preliminary mechanical and chemical treatment) between a polyvinylsiloxane soft liner and an acrylic denture base and concluded that pretreatment with a sandblasting apparatus and a monomer significantly increased its value. They obtained the following results for the different treatments: acetone treatment -0.043 MPa, monomer treatment -0.054 MPa, surface sandblasting -0.0615 MPa, jet-abrasive treatment -0.0727 MPa, sandpaper treatment and a monomer -0.082 MPa and jet-abrasive treatment plus a monomer -0,111 MPa. After being stored in water, all values increased.
In the same year, Kazanji and Abid Al-Kadder [28] investigating Molloplast B (heat-curing silicone), GS Reline (cold-curing silicone), and Bony plus (cold-curing acrylicbased material), offered a 032-bur treatment to increase bond strength. Tensile strength was increased more for Molloplast B than for GS Reline and Bony plus. Arafa [29] also has reported that treatment with a monomer increases bond strength. According to him and other authors [30], treatment with a laser weakens the bond, and treatment with Al2O3 sand strengthens it. On the other hand, a number of other publications do not recommend jet-abrasive treatment, but only one with a monomer [31 -33]. Atsü [34], after testing several types of treatment (adhesive, sandblasting, silica coating, and combination) with Ufi Gel P, found that the strongest bond was when using an adhesive alone (1.35 MPa). The values for the jet-abrasive treatment and the silica coating were 0.28 MPa and 0.34 Mpa, respectively. The low bond strength can be explained by the stresses occurring in the border band and the reduced ability of the material to penetrate the PMMA irregularities. This was confirmed by other authors [35 -37]. Korkmaz et al. [19] applied several conditioning methods: The minimum binding strength acceptable for clinical use is 0.44 MPa [15 -17]. Popular methods of evaluating the bond strength are tests for tensile strength, shear strength [14] and peel bond strength [17 -20]. The peel bond strength is a test to measure the strength of the bond between two materials. Although tensile strength tests do not simulate clinically common forces, the method is considered indicative.
Wiêckiewicz et al. [21] investigated the adhesion strength by comparing tensile strength and shear strength.  46 MPa), and it was not recommended, especially for polyamide resins (Deflex). Akin et al. [38] confirmed the conclusion of Korkmaz et al. [19]. They treated acrylic denture bases with sandblasting and various lasers (Er: YAG, Nd: YAG and KTP lasers) and reported that treatment with an Er: YAG laser increases the bonding strength and sandblasting and Nd: YAG and KTP lasers reduce it. Tugut et al. [39] found that long pulse length Er: YAG (300 mJ, 3W) causes significant changes in texture and improves adhesion between the denture base and the resilient liner.
Gupta [15], after treating the denture base surface with acetone for 30 seconds, the monomer for 180 seconds and methylene for 15 seconds, has found that the flexural strength of the acrylic resin decreases. The flexural strength values were determined after simulating accelerated aging by thermal cycling (500 cycles at 5°C -55°C) and 30 minutestorage in water. For the control untreated base surfaces, the flexural strength values were 781.19 kg/cm 2 , when treated with monomer for 180 seconds -725.09 kg/cm 2 , when treated with methylene chloride and acetone, 715.78 kg/cm 2 and 711.81 kg/cm 2, respectively. Demir et al. [40] studied the effect of different reactive surface agents on the tensile strength between the RLMs and the denture base. Maleic anhydride (MA) is a reactive monomer that contains an unsaturated double bond and acid anhydride groups that allow chemical bonding with the resin. Treatment with 2% MA in butanone, added to the primer prior to adhesive application, has shown the highest tensile strength values of 2.53 Mpa, and the lowest values were obtained with 20% MA. Hatamleh et al. [8] have reported increased bond strength of Molloplast B to a fiber-reinforced StickTech acrylic base. According to Lassila et al. [9], denture base reinforcement with fiberglass should be carefully considered. Increasing bond strength is not valid for all materials. According to the authors, the bond strength of the cold-curing silicones studied by them is determined by the binding agent. Only the Softreliner Tough with the ethyl acetate primer was registered to increase the bond strength. For a primer polymer in 2-butanone (Ufi Gel SC) and polyacryl in dichloromethane (Vertex SoftSil 25), the bond strength even decreases, as with the Eversoft acrylic-based resin material. In 2017, Hristov [41] suggested a method for improving the bonding by using retentive pearls in the hard resin/resilient material border zone.
Studies in recent years have confirmed that the bond strength between acrylic resilient lining materials and polymethyl methacrylate denture base is of the highest quality. Bonding to acrylic plastic for CAD-CAM -IvoBase CAD is the weakest and the strongest when the resilient material is applied to non-polymerized polymethyl methacrylic resin.
The latter is only possible with acrylic-based resilient materials and with Molloplast B -a resilient silicone-based heatcuring material. A number of authors have confirmed that pretreatment of the denture base with oxygen plasma, monomer, acetone or isobutyl methacrylate (iBMA) leads to improved bond strength with the resilient material and thermocycling and sandblasting -to its deterioration [42].

Treatment of RLMs
After compression in the laboratory or functional molding in the patient's mouth, there is always a surplus of resilient material that has to be removed. Initially, this is done with a sharp scalpel. This is followed by machining with various cutting tools [13]. Despite the wide variety of tools offered by different manufacturers, the issue of optimal and high-quality machining of polymerized resilient material is not fully resolved. It is necessary to distinguish between shape correction and smoothing. In the resilient material/hard resin border zone, materials with clearly different properties come in contact. The roughness of the materials is undesirable because it creates conditions for bacterial retention and the development of fungal infections. All these features greatly increase the requirements for the treating tools. The various RLM manufacturers offer their own cutting tools in the commercial kits, Fig. 7 [13].
The special milling cutters of Komet, Germany are GSQ or FSQ. When working with them, low pressure and speed of 15,000-30,000 r/min are used. For Mucopren soft® products of Kettenbach, Germany and Elite® soft relining of Zhermack, Italy, double-cut hard alloy cutters are available.  The elastic material UfiGel C® of Voco, Germany is treated with non-woven abrasive discs with various sizes of the abrasive particles under the name Lisko (Erkodent, Germany). The discs are designed for pre-polishing and are of four types -rough, medium, fine and super fine. In the first three varieties, the abrasive particles are coated with synthetic resin and in the finest -with latex. For its products, Molloplast B®, Mollosil® and Mollosil plus®, Detax, Germany offers a toolkit containing cutters, caps with different abrasive properties (abrasive particle size of 150 µm and 180 µm) and pre-polishing discs, Fig. 8.
The instrument Soft Wizard ® of NTI-Êahla, Germany, is a soft, elastic wheel with an optimum size of the abrasive particles, Fig. 9. Pesun, Hodges and Lai [43] examined the surfaces of RLMs after treatment and have reported that all tools adequately reduce the resilient liner volume. According to the authors, machining with diamond files leaves many scratches and should be followed by machining with carbide finishing burs or carbide milling cutters. The authors compared the surfaces of polished and unpolished material and noted that the final polishing with pumice and tin oxide paste gives the best results. The pumice erases scratches on the surface and provides a smooth surface for tin oxide polishing. It is confirmed that Molloplast B (Detax, GmbH & Co, KG, Germany) should be treated with stones of different abrasive properties prior to polishing. Immersion of soft liner dentures in ice water facilitates their treatment. Silicone-based RLMs are difficult to polish. To optimize their surface treatment, different varnishes for surface sealing are available on the market for dental products. These are incompletely filled, low-viscosity Asilicones that cannot chemically bind to the base material due to the lack of free binding sites in the polymerized silicone. The lacquer coating remains as a film on the silicone, which is peeled off after a few months and needs new application.

CONCLUSION
Despite the efforts of many authors to find a solution to improve the compromised stability of the full dentures [44][45][46][47] and the bond between the denture base and the resilient lining material in the case of cold-curing silicone-based resilient liners, this problem is still unsolved and remains a matter of scientific interest.