RESILIENT LINING MATERIALS FOR REMOVABLE DENTURES: TYPES, COMPOSITION AND TECHNOLOGY

For patients with removable dentures, requiring uniform distribution of masticatory forces, the use of elastic relining materials is indicated. They are classified with respect to various characteristics: chemical composition, technology, polymerisation process, final hardness – Shore A, etc. Two basic relining methods have been described in the scientific literature – direct (clinical), and indirect (laboratory). The combination of a laboratory method and a clinical procedure is named direct-indirect method. Direct and direct-indirect techniques are used for cold curing relining materials, while indirect techniques are used for heat curing elastic relining materials. The good knowledge of the types, techniques, processing and basic characteristics of elastic relining materials is an important factor for predictable results in the prosthetic treatment with removable dentures.


INTRODUCTION
Excessive bone resorption, thin and non-pliable mucosa, exostosis or painful neurogenic points in edentulous patients require more specific methods of treatment with complete dentures [1, 2], which provide a more even distribution of masticatory pressure [3] and reduce mucosal trauma. Such clinical cases can be resolved by using resilient lining materials for complete dentures.

I. Classification and composition of resilient lining materials (RLMs)
Resilient lining materials can be classified according to different characteristics: chemical composition, technology, type of polymerization, final hardness (by Shore A), etc. Based on their chemical composition, resilient resins were classified into acrylic, silicone and vinyl by Todorov [4] in Representatives of this group are: Orthoplast (Russia), Palasiv 62 (Germany), the heat-curing Coe Super soft (GC America Inc., USA) and others. [4] Today, the most common plasticizers are dibutyl phthalate or dibutyl glycolate [5]. The bond with the solid PMMA base is chemical. Some resilient resins contain the hydrophilic acrylic monomer hydroxyethyl methacrylate (HEMA) [5].
Healing liners, also called tissue conditioners, are acrylic-based resilient materials (PEMA) that remain soft for a limited time (days to weeks). Their chemical composition is shown in Silicone resilient resins [ Table 3] have a composition similar to silicone impression materials. By polycondensation or polymerization, the linear polymer chain of the principal component -polydimethyl siloxane -is extended and cross-linked and the material acquires elastic consistency. The bond with the denture base is mechanical [5]. They can be divided into heat-curing and cold-curing, as well as temporary and long-term resilient materials. Polysiloxanes are on the basis of silicone resilient resins. The known in the past in Bulgaria resilient resin Ortosil (Russia) is a polydimethyl siloxane rubber with a cross-linking agent -methyl triacetoxy silane -added in the liquid.
One of the most popular lining materials, known for more than 20 years, is Molloplast B (Detax, GmbH & Co., Germany). It is a heat-curing single component of silicone resin. The polymerization is activated by heat (boiling water for 2 hours), and the bond with the denture base is accomplished by an adhesive, which is a silicone polymer in a solvent. Heat-curing silicones consist of polydimethylsiloxane and benzoyl peroxide. Cross-linking is initiated by free radicals generated by the decomposition of organic peroxides at high temperatures.
Flexibase (Flexico Developments Ltd., London, UK) is available as powder and liquid. The liquid comprises a catalyst, a mixture of dibutyltin dilaurate and ethyl polysilicate. Here again, the bond with the denture base is accomplished with an adhesive that is a silicone polymer in a solvent.
Vinyl resilient resins can be polyvinyl chloride or polyvinyl acetate polymers with added plasticizers, crosslinking agents and color stabilizers. They are mechanically bound to PMMA. Plasticizers for polyvinyl chloride polymers are dibutyl acrylate and dioctyl phthalate.
Famous brands from this group are Elladent 100 (Russia) for lining and Elastoplast (Russia) for boxing bracelets.
Today, on the market of dental products, there are silicone-based RLMs that have the same chemical composition as the polymerization (addition) silicones known as A-silicones. The main chemical element is polydimethylsiloxane with terminal vinyl groups (-CH = CH2) or vinyl-polydimethylsiloxane. They are two-component pastes containing polyvinyl siloxanes and a platinum catalyst.
Some authors [24,25,26] suggest a simplified classification of RLMs, based on their chemical composition, the first group comprising acrylic-based lining materials and the second -silicone-based lining materials, and depending on the time of stay in the mouth -temporary and long-term RLMs. Abe [27] notes that resilient resins can be acrylic-, silicone-and polyolefin-based. He describes the material Mucopren soft (Kettenbach GmbH & Co. KG, Germany), which falls into the new subgroup of silicone resilient resins (the group of vinyl polysiloxanes -A silicones).
In 2014, a group of authors from Poland [7] made an extremely thorough analysis of RLMs. They describe in detail the chemical composition of resilient resins. The analyzed resilient acrylic-based materials are available as two-component powder and liquid systems. The powder is polyethyl methacrylate (PEMA) with an organic peroxide initiator. The liquid contains a high molecular weight monomer, such as: ethyl methacrylate, n-propyl methacrylate, n-butyl methacrylate.
Plasticizers are aromatic esters, such as dibutyl phthalate and ethyl acetate. The composition of these materials includes also cross-linking agents of ethylene glycol dimethacrylate type.
According to the technology, RLMs are divided into: 1

II. Lining techniques
The scientific literature describes two main types of lining techniques -direct (clinical) and indirect (laboratory). When the laboratory technique is associated with an immediate clinical procedure, it is called direct-indirect [31]. Direct and direct-indirect techniques are applied to coldcuring RLMs, while indirect techniques -to heat-curing RLMs.

Direct technique
Direct technique is a completely clinical procedure comprising a complex of intraoral and extraoral steps. A basic requirement is to use cold-curing RLMs, the most popular of which are silicone-based. In the direct technique, a new base of resilient material is added to a denture of hard acrylic resin, pre-fabricated according to conventional technology. In order to achieve successful relining, it is necessary to treat the existing denture in a way to provide enough space for the new material and a good bond with the hard acrylic base.
Providing the necessary distance can be done in two main ways: 1) by using a spacer or 2) by abrading (a step, described in almost all instructions of resilient material manufacturers).
By using a spacer Abe [27] describes a technique, in which 10 mm long, 5 mm wide and 2 mm thick hard resin stoppers are placed in the frontal and the two molar areas after adjustment of the fabricated denture. Relining is done in two stages. First, the resilient material is uniformly applied to the mucosal surface of the denture and then, the denture is inserted into the patient's mouth and pressed against the prosthetic field. The stoppers limit the excessive sinking and provide the required uniform thickness (2 mm) of the resilient material. After 2-3 minutes, the denture is removed from the patient's mouth and the stoppers are removed. Additional material is placed on their sites and the denture is returned to the mouth for final polymerization.
Another way to provide the necessary space for the new material by using a spacer is described by Tata and Nandeeshwar [32]. The method is applied, when relining is pre-planned as the last stage of denture fabrication. In this method, the denture is fabricated on a duplicate working cast in the following manner: A 2 mm thick wax plate is adapted to the first working cast. Window frames are cut along the alveolar ridge in the area of canines and molars. The cast is duplicated by using an alginate impression material. The denture, fabricated on the duplicated cast will have resin feet (stoppers) corresponding to the window frames, thus providing the required space for the resilient material. It is not clear from the publication whether the stoppers are later replaced with a resilient material or not.
By abrading Providing sufficient space for the resilient material by abrading is performed with a metal milling cutter, after preliminary marking of the required depth of the relining layer with a round bur [ Figure 1].  The described method of mechanical treatment does not apply to resilient materials, which have a weak bond with the hard acrylic resin. In such cases, the relining layer should reach the prosthetic edge [9].
The next clinical steps include application of an ad-hesive (primer) for the time defined by the manufacturer's instructions; application of the resilient material; inserting the denture into the patient's mouth and pressing against the prosthetic field [ Figure 3]. Generally, polymerization is completed in about 5 minutes.

Direct-indirect technique
The first step of the direct-indirect technique is to make a functional impression with the pre-fabricated hard acrylic denture. The denture is used as a customized impression tray. It is recommended to obtain the functional impression with the mouth closed (by Popov) and by using soft tests [31]. The next technological steps are laboratory.
A gypsum working model is cast, which, together with the denture, is mounted on an articulator or a special cuvette-key, to preserve the intermaxillary relation [33] [ Figure 4].

Fig. 4. Mounting on an articulator
The impression material is removed and the prosthetic surface is treated by abrading as in the direct technique. When the provision of a uniform space for the resilient material is done by abrading, it should be borne in mind that the impression material also provides a certain space, so the abrasion has to be reduced.
Providing a uniform space for the resilient material can also be done by the method, described by Abe [32], by placing hard resin spacers before taking the impression. The application of resilient material is planned at the start of treatment, to prevent errors in determining the level of the masticatory plane and the intermaxillary relation. The next steps include application of a primer on the mucosal surface of the denture, application of the resilient material and remounting on an articulator (cuvette-key) [ Figure 5]. According to other sources [8], already in 1992, a modified technique for extending the life of tissue conditioners included additional heat treatment.

Indirect Technique
All heat-curing resilient materials require indirect, all-laboratory technique. The hard acrylic denture has no contact with the patient before application of the resilient lining. The so-called double-layer dentures are fabricated, with pre-planned lining with resilient material.
Some silicone-based heat-curing resilient materials, such as Molloplast B (Detax, GmbH & Co. KG, Germany), Luci-Sof (Dentsply Int., USA) and others, require heat treatment for 2.0-2.5 h. The bond with the hard acrylic resin is accomplished by means of a primer, when the prosthesis is polymerized in advance. A better bond is obtained, when the resilient material is applied against unpolymerized acrylic resin and the polymerization is performed simultaneously for both materials [35,36,37]. This technology is described in detail by MacEntee [38] and Hayakawa [9]. Using a spacer is a must.
The laboratory steps to the application of acrylic resin are not different from conventional technology. After evaporation of the wax, a spacer, which can be made of wax [9,38], silicone [39], self-curing or light-curing resin [40] or vacuum-pressed thermofoil plate [41], is placed on the model. The use of a spacer provides a uniform thickness of the resilient material [42]. An acrylic resin is placed on the other half of the cuvette, followed by a test-closure and pressing, using a sheet of polyethylene foil. The cuvette is opened, the spacer is removed and the resilient material is placed in place. Again, a test closure and pressing is performed, the cuvette is opened and the excess resilient material is removed. The cuvette is closed, tightened in a mechanical or hydraulic press for 10-15 minutes and placed in a polymerization apparatus.
The polymerization of Molloplast B (Detax, GmbH