title: Influence of thermal and mechanical stresses on the strength of intact and relined denture bases.
abstract: BACKGROUND
  Denture bases may become increasingly weaker as a result of thermal stress and flexural cyclic loading. Information regarding this potential problem and its relationship to the denture base reline is limited.
  
  
  OBJECTIVE
  This study evaluated the influence of thermal and mechanical stresses on the strength of intact and relined denture bases.
  
  
  METHODS
  Twenty-eight microwave-polymerized (Acron MC) intact denture bases were prepared in the shape of a 3-mm-thick maxillary denture. Additionally, fifty-six 2-mm-thick denture bases were relined with 1 mm of autopolymerizing resin (Tokuyama Rebase Fast II or New Truliner) (n=28). Intact and relined specimens were divided into 4 groups (n=7) as follows: without stress (control); a mechanical stress at 0.8 Hz for 10,000 cycles; 5000 thermal cycles between 5 degrees C and 55 degrees C; or a combination thermo-mechanical stress. The specimens were vertically loaded in compression with a rounded rod at 5 mm/min until failure, using a universal testing machine. Data on maximum fracture load (N), deflection at fracture (%), and fracture energy (N.mm) were analyzed by 2-way analysis of variance and Student-Newman-Keuls tests (alpha=.05).
  
  
  RESULTS
  The strength of the denture bases relined with New Truliner was not significantly affected by any of the experimental conditions, but comparing the control groups, New Truliner exhibited the lowest maximum fracture load values. The maximum fracture load of intact denture bases (P=.002) and those relined with Tokuyama Rebase Fast II (P=.01) showed a significant decrease after thermal stress. Additionally, cyclic loading significantly decreased the maximum fracture load (P<.001), deflection at fracture (P=.025), and fracture energy (P<.001) of intact denture bases and those relined with Tokuyama Rebase (P values of .002, .039, and .001, respectively).
  
  
  CONCLUSIONS
  Thermal and mechanical stresses exert deleterious effects on the strength of intact and/or relined denture bases, which vary according to the relining material used.
authors.affiliation: Department of Prosthetic Dentistry, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan. seo_br@yahoo.com.br
authors.forename: Rosangela Seiko
authors.initials: RS
authors.lastname: Seo
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Murata
authors.forename: Guang
authors.initials: G
authors.lastname: Hong
authors.forename: Carlos Eduardo
authors.initials: CE
authors.lastname: Vergani
authors.forename: Taizo
authors.initials: T
authors.lastname: Hamada
mesh_terms: D000180:Acrylic Resins; D000704:Analysis of Variance; D019245:Compressive Strength; D003799:Dental Stress Analysis; D003819:Denture Bases; D003822:Denture Liners; D003823:Denture Rebasing; D006358:Hot Temperature; D008689:Methacrylates; D018583:Pliability; D013314:Stress, Mechanical
chemical_list: D000180:Acrylic Resins; D008689:Methacrylates; C514244:tokuyama rebase fast II; C071514:truliner; C075051:Acron MC