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Contemporary Materials for Removable Prosthodontics

Contemporary prosthodontic theories have evolved from the principles originally explored by such pioneers as Sears, Hanau, and Gysi in the early 1920s.1-3 Since these initial investigations, numerous developments have occurred in prosthodontic techniques and materials. The introduction of polymethylmethacrylate (PMMA) in 1938, for example, drastically altered this discipline. Removable prostheses are primarily fabricated utilizing heat-cured resins. Recent developments in microwave curing, injection molding, and silicoating, however, have continued to improve the manner in which prostheses are processed from PMMA and in which metal-to-resin gaps are eliminated in frameworks. The introduction of Bis-acrylate composite resin has extended its application to the production of custom trays and occlusion rims; it has subsequently provided clinicians with an expedient and cost-effective approach to their fabrication.

 

Selection of Tray Materials

Traditional means of obtaining border molded final impressions for complete dentures incorporated the use of PMMA custom trays, thermoplastic compounds, and central spacing of the trays with baseplate wax.4 This technique also fostered the selective pressure philosophy for preservation of underlying supportive tissues. In contemporary prosthodontics, custom trays with peripheral relief have been fabricated with polyvinylsiloxane border molding materials.5 Various clinicians question the use of border molding with even a viscous elastomeric material due to the material's inability to support itself. Practitioners have also criticized the utilization of thermoplastic compound as a result of the relatively high temperatures required to enable the material to flow under applied load, which can alter the existing tissues. Alternative border molding materials exhibit improved handling characteristics and flow without the elevation in temperature, yet are sufficiently rigid to be trimmed and to serve as effective tray borders when congealed. Although elastomeric border molding is technique sensitive, it can be expedient and cost effective when mastered due to its ability to allow proper denture flange extension and reasonable force distribution under functional loads.

An accurate and correctly extended preliminary impression of the alveolar ridge is necessary in order to fabricate a properly extended custom tray. Provided the patient has an existing prosthesis with an accurate and comfortable fit, the tissue extension of the existing denture can be duplicated utilizing a polyvinylsiloxane putty as a "cast" material adapted to its tissue surface. Once the putty has been polymerized, the prosthesis is removed to reveal a comfortably extended tissue base that facilitates border molding and impression making.

Although this technique appears to increase patient expense, the conservation of time, effort, sterilization, and materials surpasses the cost factor associated with polyvinylsiloxane materials. While the passage of time has not significantly altered traditional prosthodontic principles, the continuous evolution of material formulation and properties has allowed clinicians to modify contemporary techniques in order to deliver restorative treatment with increased success and predictability. 

References 

  1. Sears VH. Balanced occlusion. J Am Dent Assoc 1925;12:1448. 
  2. Hanau R. Articulation defined, analyzed, and formulated. J Am Dent Assoc 1926;13:1694-1709.
  3. Gysi A. Practical Conclusions from Scientific Research in Denture Construction. New York, NY: Dentist's Supply Company, 1929.
  4. Zarb GA, Bolender CL, Carlsson GE. Boucher's Prosthodontic Treatment for Edentulous Patients. 11th ed. St. Louis, MO: Mosby, 1997.
  5. Chaffee NR, Cooper LF, Felton DA. A technique for border molding edentulous impressions using vinyl polyvinylsiloxane material. J Prosthodont 1999;8(2):129-134.

 

* Department of Dental Specialties, The Mayo Clinic, MN; Associate Professor of Dentistry, Nebraska Medical Center, Omaha, NE; and private practice, Omaha, NE.

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