title: Dimensional change in complete dentures fabricated by injection molding and microwave processing.
abstract: BACKGROUND
  Acrylic resin complete dentures undergo dimensional changes during polymerization. Techniques with injection molding and polymerization and microwave polymerization are reported to reduce these changes and thereby improve clinical fit. These dimensional changes need to be quantified.
  
  
  OBJECTIVE
  The purpose of this study was to compare differences in dimensional changes of simulated maxillary complete dentures during polymerization and storage in water after injection molding and conventional polymerization, or microwave polymerization against a control of conventionally packed and polymerized simulated maxillary complete dentures.
  
  
  METHODS
  Forty identical maxillary denture bases were prepared in dental wax with anatomic teeth. They were invested and the wax eliminated from the molds. Ten specimens each were randomly assigned to 1 of 4 groups. Group 1 was compression molded and conventionally polymerized; group 2 was injection molded and conventionally polymerized (Success); group 3 was injection molded and microwave polymerized (Acron MC); and group 4 was injection molded and microwave polymerized (Microbase). Intermolar width and changes in vertical dimension of occlusion, were determined after polymerization and after storage in water for 28 days. Measurements in triplicate were made between points scribed on the second molar teeth with a traveling microscope (accurate to 0.005 mm). Vertical dimension of occlusion was measured between points scribed on the upper and lower members of an articulator by use of an internal micrometer (accurate to 0.05 mm). Data were analyzed by use of a 1-way analysis of variance with Tukey post-hoc contrasts (P <.05).
  
  
  RESULTS
  Polymerization contractions (intermolar widths) for each group were: group 1, -0.24%; group 2, -0.27%; group 3, -0.35%; and group 4, -0.37%. The Microbase specimens had greater shrinkage than conventionally polymerized specimens, but there were no significant differences between the groups. All injection methods had less postpolymerization increase in vertical dimension of occlusion (0.63 to 0.41 mm) than the conventional Trevalon control (0.74 mm), but only group 4 was significantly different (P<.004). After storage in water for 28 days, all specimens increased in vertical dimension of occlusion (0.10% to 0.16%) from polymerization techniques, but there were no significant differences between groups.
  
  
  CONCLUSIONS
  Within the limitations of this study, injection molding resulted in a slightly less increase of vertical dimension of occlusion than conventional polymerization techniques, the difference being significant for Microbase compared with the conventional Trevalon control.
authors.affiliation: Guy's, King's and St. Thomas' Dental Institute, King's College, London, United Kingdom.
authors.forename: Phillip L J
authors.initials: PL
authors.lastname: Keenan
authors.forename: David R
authors.initials: DR
authors.lastname: Radford
authors.forename: Robert K F
authors.initials: RK
authors.lastname: Clark
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry* / Q000528:radiation effects; D000704:Analysis of Variance; D003735:Dental Casting Technique; D003764:Dental Materials / Q000737:chemistry* / Q000528:radiation effects; D003819:Denture Bases; D003779:Denture Design / Q000379:methods*; D003827:Denture, Complete, Upper; D006801:Humans; D007267:Injections; D008768:Methylmethacrylates / Q000737:chemistry; D008872:Microwaves; D011108:Polymers / Q000737:chemistry / Q000528:radiation effects; D011140:Polyurethanes / Q000737:chemistry; D011312:Pressure; D013223:Statistics as Topic; D013499:Surface Properties; D013997:Time Factors; D014716:Vertical Dimension; D014867:Water / Q000737:chemistry
chemical_list: D000180:Acrylic Resins; D003764:Dental Materials; D008768:Methylmethacrylates; C441199:Microbase; D011108:Polymers; D011140:Polyurethanes; D014867:Water; C075051:Acron MC; C029883:Trevalon universal