ReviewBiodegradation of acrylic based resins: A review
Introduction
Acrylic based resins consist of polymeric materials based on polymethylmethacrylate. These dental materials are the result of a free radical polymerization reaction. They can be classified as chemical, heat or light activated depending on the factor that initiates the reaction. Chemical or autopolymerized materials involve a chemical activator like N,N-dimethyl p-toluidine [1]. For heat-polymerizing materials, heat can be generated by hot water bath or microwave energy, while the light polymerizing uses visible light as energy source [2], [3], [4].
Acrylic based resins are frequently used in daily dental practice, as they are able to provide the essential properties and necessary characteristics to be used in diverse functions. Most common use of the materials includes denture bases and denture liners, orthodontic appliances and temporary crowns [4], [5], [6].
Denture bases are composed of pre-polymerized polymethylmethacrylate (PMMA) or polyethylmethacrylate (PEMA) powder particles along with a peroxide initiator and a pigment, which are mixed with methacrylate monomers (methylmethacrylate, hexamethyleneglycoldimethacrylate, hydroxylethylmethacrylate, n-butylmethacrylate, tetrahydrofurfurylmethacrylate) and cross-linking agents such as ethyleneglycoldimethacrylate, trimethylolpropane trimethacrylate or 1,6-hexanediol dimethacrylate [1], [7], [8], [9].
Denture liners are used to improve the fit of denture bases, thus re-establishing the retention, support and stability of removable prostheses [10]. Several types of these materials are available, they can be hard reline resins or soft lining materials. Soft lining materials can be divided into two groups [4]. The first comprises materials in which the liquid is made of monomer components, such as methyl, ethyl or butylmethacrylate and phthalates, citrates or sebacates as plasticizers. The second group is similar to tissue conditioners, in which the liquid contains a mixture of plasticizers and ethyl alcohol [4], [11], [12], [13], [14].
Orthodontic appliances are used for space maintenance, thumb deterrent, tipping teeth, overbite reduction, block movements and retention. PMMA is the material most commonly used for manufacturing the polymeric part of these orthodontic appliances [15].
Temporary crowns are used during the interval between tooth preparation and placement of the definitive crown. There are several types of acrylic resin materials available for provisional restorations as PMMA or PEMA resins [16].
An important issue regarding the clinical application of the acrylic based resins is their biodegradation. It can be defined as the changes on their chemical, physical and mechanical properties due to the oral environment conditions.
In the oral cavity the materials are exposed to a rather complex milieu that comprises different endogenous (proteins, enzymes, polysaccharides, bacteria) and exogenous substances (all different sorts of compounds coming from the diary intake diet). These components establish a complex and intricate interplay of interactions, which result along with an important mechanical action, in a general biodegradation phenomena towards the biomaterials present in the oral cavity. These processes may permanently alter the properties of the material and compromise its function.
In addition, biodegradation of a biomaterial can produce leachable products, which in turn may induce a series of biological responses on cells and tissues. Biodegradation impact on the biocompatibility of acrylic materials is controversial [17] but concern about its clinical significance is a fact as subjective and objective complaints about these materials are increasing [18].
Section snippets
Causes for biodegradation
The polymeric materials were classically recognized as large stable structures with a high degree of resistance to biodegradation. However, several studies conducted especially with composites materials, showed that polymers may be subject to a myriad of degradation processes in the oral cavity [19], [20].
Polymer degradation does not occur as a result of isolated processes, multiple factors as saliva, chewing, thermal and chemical dietary changes may be responsible for the biodegradation
Consequences of biodegradation
A major clinically significant consequence of acrylic based resins biodegradation is the release of potential toxic unbound/uncured monomers or/and additives from the polymer network. The released compounds may have a toxic effect on the oral cavity. With respect to materials stability, biodegradation may induce significant changes in materials physical and mechanical properties that may ultimately lead to the catastrophic failure of the material.
Unexplored topics and areas for future research
Biodegradation of acrylic based resins under the oral environment has been so far uncompleted studied. Some questions that need to be investigated include: which enzymes are involved in the in vivo process of acrylic resins degradation? What are their co-factors? Is the saliva of one individual more likely to degrade certain materials than that from another person? What is the level of the different products in vivo?
A gap in the published literature exists regarding in vitro studies that allow
Conclusions
The following conclusions are draw from this review:
- 1.
Acrylic based resins are intensively used in dentistry practice as restorative, liners or as denture base materials. These substances are made by polymerization of methacrylate related monomers. Materials can be classified as chemical, heat or light polymerizing depending on the factor that initiates the polymerization reaction.
- 2.
Increasing concern arises regarding the safe clinical application of these materials due to their biodegradation
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