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As a briefing history of the denture lining materials, we can find that in 1942, Matthews introduced plasticized polyvinylchloride as one of the first soft lining materials used with acrylic resin dentures (Sultana, 1991) then Lammie & Storer published a summary of observations on different soft denture materials and classified the processed resilient materials as follows. (1) natural rubber, (2) polyvinylchloride, (3) polyvinylacetae, (4) methyl methacrylate copolymer, (5) silicone (Lammie & Storer, 1958). Furthermore, they stated that this material became hard due to loss of plasticizer after several months, and became stained and cracked in the mouth.
However, we tend to examine the composition and uses of both tissue conditioners and soft lining materials in prosthetic dentistry. Basically, Tissue conditioners composed of polyethylmethacrylate and mixture of aromatic ester and ethyl alcohol and they are found in three components system: polymer (powder), monomer (liquid) and dibutyl phthalate as plasticizer flow control (liquid) when mixed together a gel is formed (Braden M 1970). The major uses of these tissue conditioners materials are: tissue treatment, temporary obturator, baseplate stabilization, to diagnose the outcome of resilient liners, liners in surgical splints, trial denture base and as a functional impression material (Nallaswamy D. 2003).
Tissue treatment: the advantage of using a tissue conditioner is to prepare the selected oral structure to withstand all the stress from the prosthesis and to preserves the residual ridge. It is also used to heal irritated hyperemic tissues before denture fabrication.
Temporary obturator: tissue conditioners can be added as a temporary obturator over the existing complete or partial denture and this can be done directly in the month or indirectly after an impression of surgical area has been made.
Stabilization of baseplate and surgical splints or stents: when undercuts are present on an edentulous cast, an acrylic temporary denture base cannot be used as it may get locked into the undercut and break the cast during removal. In this case tissue conditioner may be used to stabilize the record base and prevent breakage of the cast.
Adjunct to an impression or as a final impression material: when it is difficult to determine the extent of the denture base due to movable oral structure, tissue conditioner can be used to record the extensions of the denture in a dynamic form that will later help in preparing an impression tray for the final impression.
To diagnose the outcome of resilient liners: patient with well-constructed denture sometimes develop chronic soreness and find it difficult to wear it comfortably. Tissue conditioners can be used to determine if this problem will be resolved with the use of a resilient liner. (Nallaswamy D, 2003).
Commercially, there are a wide range of tissue conditioner products, for example, GC tissue conditioner and Visco-gel and more but here we will compare between these two only:
Firstly, GC- tissue conditioner is composed of Polyethylmethacrylate (powder), methacrylate monomer (liquid) and dibutyl sebacate (plasticizer). Manufacturer claims that using of dibutyl sebacate as plasticizer instead of the conventional dibutyl phthalate has a merit of not being harmful to the human endocrine system. However, it is suitable for conditioning and relieving tissue (for example when there are areas of inflammation or pressure points on the inflamed alveolar ridge), but also for temporary soft relining of partial and complete dentures as well as functional impressions to fabricate new dentures or restore existing ones, for immediate dentures and as an interim solution for direct loading situations in implantology. It is manufactured by GC EUROPE, Belgium (www.gceurope.com).
On the other hand, Visco-gel tissue conditioner is composed of Polyethyl methacrylate (powder), Phthalyl butyl glycolate, Ethanol (liquid). It is can be used prior to denture replacement, extension or rebasing, tissue conditioner where the denture-bearing tissues have become disturbed by either denture trauma or infection, temporary lining immediately after extraction or oral surgery, temporary soft liner when normal dentures are not tolerated, particularly for patients with impairment of general health and as functional impression material in post-operative cases or when ill-fitting dentures require replacement or rebasing. Visco-gel is manufactured by DENTSPLY DeTrey GmbH, Germany (www.dentsply.com).
Regardless the trade, the following steps should be considered while applying a tissue conditioner, first, tissue part of the denture base, which crosses an undercut, should be reduced then the tissue surface of the denture, which covers the crest of the ridge, should be reduced by 1 mm to allow sufficient room for placement of the tissue conditioner (Nallaswamy D, 2003).
Soft lining material is the second part of this review and here it is wise to understand the difference between the tissue conditioner and the soft lining material. The International Organization for Standardization (ISO) has issued two international standards related to liner materials: (ISO 10139-1:1991): first, lining materials for removable dentures-Part 1: short-term materials and soft lining materials for removable dentures (ISO, 1991) and the second, lining materials for removable dentures-Part 2: materials for long-term use (ISO, 1999). A short-term liner is one that is used intraorally for up to 30 days. A long-term liner is one that maintains softness and elasticity for more than 30 days (Garcia & Jones, 2004). Practically, the ISO is differentiating between the temporary and the permanent soft ling material but theoretically, we can find that tissue conditioners can be used for more than 30 days depends on the loss of the plasticizer. Moreover, tissue conditioner and soft lining have been used for the same purpose clinically. So then, what is the difference? Viscosity is that difference so whenever the material is with low viscosity and flow easily you can use it as a tissue conditioner while material with high viscosity is indicated to be used as a soft liner. There are two groups of soft lining material the first is the acrylic resin-based liner which consists of powder and liquid components. The powder has poly (ethyl or methyl) methacrylate, and sometimes copolymers, while the liquid contains methyl methacrylate monomer and plasticizers that are added to lower the glass transition temperature. The distribution of the large plasticizer molecules minimizes entanglement of polymer chains, which allows individual chains to slide by one another, keeping the liner from fully hardening. The liquids used do not contain acrylic monomers (Craig & Gibbons, 1961; Anusavice, 2003). The second type of these soft lining materials is the silicone denture liners which are basically polymers of dimethylsiloxane. Poly(dimethylsiloxane) is a viscous liquid, which can be cross-linked to give rubber a good elastic property. The cross-linking agent is usually an alkyl-silane (silicate) and the reaction is catalysed by an organo-metal salt such as tinoctoate or benzoyl peroxide (McCabe, 1976). Silicones are not dependent on leachable plasticizers and remain elastic for longer periods of time (Anusavice, 2003; Munksgaard, 2004). These liners cannot bind to the acrylic denture base; however, the use of an adhesive can overcome this limitation, which was shown to be a method of failure (Dogan et al., 2006). These liner materials may be selected for the treatment of the following conditions: poor stability of denture, inadequate retention of denture, reduction of vertical dimension, degradation of the denture base, improper extension of borders into the muccobuccal fold regions, mucosal irritations, atrophic ridges, bony undercuts, denture opposing natural teeth, reduced thickness and viscoelasticity of the mucosa, pain from gingival irritation, maxillofacial defects, traumatic or pathologic tissue loss and for reline of an existing denture (Zarb et al., 1989; Christensen, 1995; Hayakawa et al., 2000; Hill & Rubel, 2011).
As a conclusion then, these viscoelastic materials either tissue conditioners or soft lining materials are contributing to solve a list of dental problems that might exist in the removable prosthesis or could be used in a range of dental procedures which are related to fully or partially edentulous patients. Furthermore, these materials need a continuous care and maintenance especially, the tissue conditioner which tends to harden and roughen due to the loss of the plasticizer. However, we should not forget the primary purpose of these materials which is they are used as temporary materials during transitional situations.
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