**4.1 Polymethyl methacrylate (PMMA)**

Polymethyl methacrylate (PMMA) is a synthetic polymer that is used as an economical alternative to polycarbonate when extremely high strength is not necessary. Unlike polycarbonate, PMMA does not contain potentially harmful subunits like bisphenol-A. Moreover, it is easier to handle, process, and less expensive than polycarbonate, as illustrated in **Figure 1**.

In clinical practice, PMMA is mostly used as prosthesis for craniofacial tissue defects such as dentures. PMMA has great mechanical properties and low toxicity. PMMA is the most regular substance used to design complete and partial dentures. Despite its great features, it cannot accomplish all mechanical necessities of prosthesis. Flexural fatigue due to repeated masticatory and high-impact forces caused by dropping are the major causes of denture fractures. Features of PMMA denture are summarized in **Figure 2** [8].

**Figure 1.** *Sample of polymethyl methacrylate denture.*

*Adherence of* Candida albicans *on Polymethyl Methacrylate in Probiotics Solution DOI: http://dx.doi.org/10.5772/intechopen.112321*

**Figure 2.** *PMMA features.*

Polymethyl methacrylate (PMMA) is a popular material used in the fabrication of complete dentures, accounting for 95% of cases. This is due to its ease of processing, repair, and polishing, as well as favorable physicochemical properties and acceptable esthetics. For over 80 years, different processing techniques, such as pouring or mold filling (compression and injection molding), have been used to create dentures from PMMA. Each of these techniques presents its own benefits and drawbacks, making them more suitable for certain clinical procedures. Despite these differences, PMMA remains a versatile and popular option in complete denture fabrication because it is light weighted, easy to fabricate, and affordable. However, PMMA has some limitations, including low fracture resistance, poor physical properties in oral fluids, and potential allergic reactions. These limitations can impact clinical performance and denture longevity.

To overcome these drawbacks, several attempts have been made to improve the physical and mechanical properties of PMMA, such as material reinforcements, alternative material use with different compositions, and polymerization techniques, all of which aim to enhance the properties of PMMA to improve the clinical outcomes.

Digital denture fabrication has advanced with the use of computer-aided design and computer-aided manufacturing (CAD-CAM) technology. Two common methods used in this process include subtractive (milled) and additive (3D-printing) approaches.

In the milled method, a pre-polymerized PMMA disc is used to mill the denture base, resulting in high strength and adequate surface properties due to its fabrication under high temperatures and pressures. Compared to conventional fabrication methods, milled denture bases have no polymerization shrinkage and less residual monomer, providing significant advantages.

However, the performance of 3D-printed resins is currently lower than milled and conventional resins. The 3D-printing method builds the denture base layer-bylayer using photo-polymerized fluid resins, which leads to noticeable impacts on the strength and surface properties of the material after thermal cycling. Furthermore, 3D-printed resins exhibit higher levels of water sorption and solubility compared to traditional resins.

Although there are advantages and drawbacks to both the milled and 3D-printed denture base fabrication methods, further research is needed to improve the performance of 3D-printed resins and enhance their potential for use in dentistry [9, 10].
