title: Effect of casting procedures on screw loosening in UCLA-type abutments.
abstract: BACKGROUND
  Screw loosening of implant restorations continues to be a complication in implant prosthodontics. Screw joints are subjected to a loss of initially applied torque because of friction and component misfit. It has been suggested that the loss of applied torque is less in machined metal abutments than in cast plastic abutments.
  
  
  OBJECTIVE
  This study compared the loss of applied torque (detorque) values in machined titanium and in cast UCLA-type abutments for external hex abutment/implant interface.
  
  
  METHODS
  Four groups of 12 samples each were evaluated: (1) machined titanium abutments, (2) premachined palladium abutments cast with palladium, (3) plastic abutments cast with nickel-chromium, and (4) plastic abutments cast with cobalt-chromium. Each abutment was torqued to 30 Ncm according to the manufacturer's instructions and detorqued three times. The mean loss of applied torque (detorque) was recorded as a percentage of the torque applied. Group means were calculated and compared using ANOVA and Tukey's LSD test.
  
  
  RESULTS
  Mean detorque values were (1) 92.3 +/- 2.9%, (2) 81.6 +/- 5.0%, (3) 86.4 +/- 4.6%, and (4) 84.0 +/- 7.0%. Machined abutments demonstrated significantly greater detorque values compared with all cast groups (p < 0.05). No significant differences were found among cast groups.
  
  
  CONCLUSIONS
  Machined abutments retained a significantly greater percentage of torque compared with cast abutments. Casting procedures decrease the percentage of applied torque, which may influence final screw joint stability.
authors.affiliation: Department of Preventive and Restorative Dental Sciences, University of California, San Francisco, School of Dentistry, USA. stekano@uol.com.br
authors.forename: Stefania C
authors.initials: SC
authors.lastname: Kano
authors.forename: Paul
authors.initials: P
authors.lastname: Binon
authors.forename: Gerson
authors.initials: G
authors.lastname: Bonfante
authors.forename: Donald A
authors.initials: DA
authors.lastname: Curtis
mesh_terms: D000704:Analysis of Variance; D001863:Bone Screws; D000044:Dental Abutments; D003735:Dental Casting Technique; D019415:Torque