Poly Methyl Methacrylate (PMMA) Properties and Reactions

Introduction to PMMA

• PMMA stands for Poly(methyl methacrylate), a synthetic polymer derived from methyl methacrylate.
• It is also known as Acrylic, Acrylic glass, Plexiglas, and Acrylite.
• German company Röhm & Haas AG introduced PMMA to the market under the trademark Plexiglas in 1933.
• British chemists Rowland Hill and John Crawford discovered PMMA and registered it under the trademark Perspex.

Dental Applications

• Dental polymethyl-methacrylates (PMMAs) are used primarily for:
  • Dentures
  • Orthodontic devices
  • Individual impression trays
  • Temporary crowns
• The application of PMMA as a veneering material no longer plays a major role.

Non-dental Applications

• PMMAs are used in daily life for non-dental purposes as:
  • Bone cements
  • Acrylic glass
  • A base for various stains
  • Artificial fingernails
  • Nail varnish

Classification

• PMMAs are classified into three categories based on the setting reaction:
  • Light-cured
  • Heat-cured
  • Chemically (auto) cured
• Characteristics of each classification:
  • Light-cured: polymerizes through exposure to visible light, lower residual monomers, and reduced shrinkage.
  • Heat-cured: polymerizes when exposed to elevated temperatures, established technique, good mechanical properties, and suitable for chairside procedures.
  • Chemically (auto) cured: polymerizes through a chemical reaction between the powder and liquid components, simplicity, and no need for external heat or light sources.

Cytotoxicity

• Cytotoxicity of all products decreased after several extractions using aqueous cell culture medium.
• Auto polymerizing acrylics are significantly more toxic than heat-polymerizing products.
• Formaldehyde caused pronounced toxic effects, but MMA generated no noticeable toxic alterations at equivalent concentrations.
• The TC50 (median toxic concentration) of MMA in permanent L-929 cells was 5 μM after an incubation period of 2 days.

Microbial Effects

• The cross-linking agent EGDMA may increase the proliferation of Streptococcus sobrinus and Lactobacillus acidophilus.
• Candida albicans plays an important role in the etiology of “denture stomatitis”.
• Adherence or accumulation of Candida albicans was promoted by a rough surface structure of PMMA-based dentures.
• Permanent soft, porous relining materials promoted adhesion in some patients, but not in all.

Clinical Practice Advice

• Regular careful denture hygiene is important to prevent the transfer of pathogenic microbes from the oral cavity to the distal gastrointestinal tract or respiratory system.
• Patients at risk should disinfect their dentures regularly with, for instance, chlorhexidine.
• Temporary restorations should be removed before polymerization reaches the maximum setting temperature.
• Polymerization of PMMA temporaries should be completed outside the oral cavity.

Implantation Studies

• Local reactions are primarily dependent on the amount of substances leached and their toxicity.
• Heat-polymerized products should cause fewer effects than auto-polymerizing materials.
• Tissue irritation decreased between the 1st and 16th days of the implantation period.
• Rapid release of MMA from bone cements was verified under simulated in vivo conditions.

Pulp Reactions

• Auto-polymerizing PMMA resins caused pulp irritations when used for temporary restorations.
• This might be due to the release of residual monomers, particularly during the first days after polymerization.
• The setting of the material may cause high temperatures (80–120°C) that could be responsible for irritation of the dental pulp.