title: The effect of clockwise and counterclockwise twisting moments on abutment screw loosening.
abstract: OBJECTIVE
  Abutment screw loosening is one of the most common complications of dental implants. When occlusal loading is applied, bending and twisting moments are counteracted by the implant-abutment connection held with the abutment screw. Restorations in different quarters of the oral cavity might bear clockwise or counterclockwise twisting moments that result from a regular chewing pattern. The aim of this study was to investigate the effect of different directional twisting moments on abutment screws.
  
  
  METHODS
  Twenty 3i implants were divided into four groups of C, R, L, and O. Each assembly consisted of an implant, abutment, and superstructure. For group C, a cyclic load of 10-100 N for 10(6)  cycles was applied centrally and perpendicular to the long axis of the implant, whereas for groups R and L, the same load conditions were applied eccentrically in clockwise and counterclockwise directions, respectively. Group O was left unloaded. The difference between the initial torque and post-load reverse torque was defined as the total torque loss. The data were analyzed by Kruskal-Wallis test. The surface of the abutment hexagon corners were examined with SEM after loading.
  
  
  RESULTS
  No abutment screw loosening was found after loading. Total torque loss of groups C, R, L, and O were 10.50 ± 0.58, 9.56 ± 1.01, 9.98 ± 1.81, and 9.58 ± 0.94 Ncm, respectively. There were no statistical differences among the four groups. SEM observations showed marked burnishing at the hexagonal corners on the compression sides of the R and L groups.
  
  
  CONCLUSIONS
  Within the limitations of this study, there was little effect of twisting moment direction on the total torque loss of an internal hexagon connection implant system. This could be attributed to the anti-twisting mechanism of the internal hexagon connection.
authors.affiliation: Department of Dentistry, National Yang-Ming University, Taipei, Taiwan.
authors.forename: Kuang-Ta
authors.initials: KT
authors.lastname: Yao
authors.forename: Hung-Chan
authors.initials: HC
authors.lastname: Kao
authors.forename: Cheng-Kung
authors.initials: CK
authors.lastname: Cheng
authors.forename: Hsu-Wei
authors.initials: HW
authors.lastname: Fang
authors.forename: Shing-Wai
authors.initials: SW
authors.lastname: Yip
authors.forename: Ming-Lun
authors.initials: ML
authors.lastname: Hsu
mesh_terms: D000044:Dental Abutments; D019250:Dental Implants, Single-Tooth; D017267:Dental Prosthesis Design; D017268:Dental Prosthesis Retention / Q000295:instrumentation*; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D018709:Statistics, Nonparametric; D013499:Surface Properties; D019415:Torque