**1. Introduction**

The dynamic development of new multidisciplinary areas has a direct impact over the possible treatments and the rehabilitation of the dental function. Teeth rehabilitation with removable denture prosthesis is an established form of treating both partial and complete dentition in edentulous patients [1]. The developments in recent decades with dental implants dominate the current dental research, not only medical contraindications but also a negative attitude toward implants [2] and economic limitation [3] are the major disadvantages for their universal applicability, so the rehabilitation with dental prostheses still makes up a significant portion of everyday clinical practice [4].

The PMMA material revolutionized the preparation techniques used so far since Walter Wright introduced the acrylic resin as the denture base material in 1937 [5]. The acrylic resin became the preferred material for making denture bases, due to its ability to overcome many of the deficiencies of the materials used at that time [6].

Conversely, removable dentures are used in critical conditions of the oral cavity. There are about 500 microorganisms in the mouth, which produce a biofilm in an acidic environment causing several diseases [7], such as denture stomatitis [8], deterioration of the periodontal status of the remaining teeth [9], or carious lesions in abutment teeth [10]. Therefore, it is very important to choose a suitable material for dental prosthesis.

Poly(methyl methacrylate) (PMMA) is an acrylic resin usually used with a long tradition for prosthetic purposes [11]. It can be classified as chemically or thermally polymerized material depending on the factors that initiate the reaction. For dental prosthesis, thermally polymerized materials are used and the heat can be generated by hot water bath or microwave energy [12]. It was suggested that residual monomer concentration is the most important parameter in the determination of the final properties of the PMMA for dental prosthesis [12, 13]. It was found that in the chemical structure of PMMA, the alpha methyl groups tend to remain in the outer layer surface, whereas the methylene groups are in the inner layer of the PMMA surface, which gives an idea of the arrangement of the polymer [13]. In other words, PMMA has exhibited moderate cytotoxicity in bulk material and polymerized form [14, 15].

The aim of this chapter is to present the trends for the processing of PMMA, including the chemical synthesis, conventional processing (thermal polymerization), the new technique of thermal polymerization assisted with ultrasound, the antibacterial effect on PMMA with nanoparticles, and biocompatibility (cytotoxicity, genotoxicity, and mutagenesis).
