title: Abutment screw loosening and bending resistance of external hexagon implant system after lateral cyclic loading.
abstract: BACKGROUND
  Rigorous efforts to reduce the recurrence of abutment screw loosening in single-tooth implant restorations have recently been made. However, the behavior of the implant/abutment joint components with respect to critical bending force is still unclear.
  
  
  OBJECTIVE
  This study investigated the effect of different cyclic loading periods on abutment screw loosening and bending resistance of a single-tooth external hexagon implant system.
  
  
  METHODS
  Fifteen Brånemark implant assemblies were divided equally into groups A, B, and C. Each assembly consisted of a Brånemark System Mk IV 4 x 10 mm implant (Nobel Biocare AB, Gothenburg, Sweden) mounted in a brass block, a CeraOne 3 mm abutment (Nobel Biocare AB), and an experimental cement-retained superstructure. For groups A and B a cyclic load of 50 N was applied centrally and perpendicular to the long axis of the implant. Targets of 1.0 x 10(6) cycles (40 months of simulated function) and 0.5 x 10(6) cycles (20 months of simulated function) were defined for groups A and B, respectively. Group C (control) was left unloaded for the same loading time period as was group B. Reverse torque was recorded before and after loading, and the difference was calculated. After cyclic loading, specimens were mounted in a testing machine, and the yielding and bending strengths were measured. The data were analyzed with one-way analysis of variance and were compared by means of the Tukey test (p < .05).
  
  
  RESULTS
  There were statistically significant differences (p < .001) in the reverse torque difference values of group A ([-5.6 to -3.4] +/- 0.86 Ncm) as compared to those of group B ([-2.4 to -1.6] +/- 0.32 Ncm) and group C ([-0.7 to 0.0] +/- 0.26 Ncm). Likewise, group B showed a significant difference compared to group C (p = .002). On the other hand there was no statistically significant difference in the mean values among the test groups in regard to the yielding and bending strengths (p >.050).
  
  
  CONCLUSIONS
  Within the limitations of this study, long-term fatigue significantly affected the reverse torque values under centric lateral load (p <.001) whereas it had no significant effect on the resistance of the implant/abutment joint to static bending.
authors.affiliation: Department of Conservative Dentistry and Prosthodontics, Faculty of Dentistry, University of Jordan, Amman, Jordan. khraisat@lycos.com
authors.forename: Ameen
authors.initials: A
authors.lastname: Khraisat
authors.forename: Osama
authors.initials: O
authors.lastname: Abu-Hammad
authors.forename: Najla
authors.initials: N
authors.lastname: Dar-Odeh
authors.forename: Awni M
authors.initials: AM
authors.lastname: Al-Kayed
mesh_terms: D000704:Analysis of Variance; D000044:Dental Abutments; D019250:Dental Implants, Single-Tooth; D017268:Dental Prosthesis Retention / Q000295:instrumentation*; D003799:Dental Stress Analysis; D018583:Pliability; D018709:Statistics, Nonparametric; D019415:Torque; D016474:Weight-Bearing