title: Effect of abutment screw length and cyclic loading on removal torque in external and internal hex implants.
abstract: OBJECTIVE
  The purpose of this study was to evaluate the effects of abutment screw length and cyclic loading on the removal torque (RTV) in external hex (EH) and internal hex (IH) implants.
  
  
  METHODS
  Forty screw-retained single crowns were connected to external and internal hex implants. The prepared titanium abutment screws were classified into 8 groups based on the number of threads (n = 5 per group): EH 12.5, 6.5, 3.5, 2.5 and IH 6.5, 5, 3.5, 2.5 threads. The abutment screws were tightened with 20 Ncm torque twice with 10-minute intervals. After 5 minutes, the initial RTVs of the abutment screws were measured with a digital torque gauge (MGT12). A customized jig was constructed to apply a load along the implant long axis at the central fossa of the maxillary first molar. The post-loading RTVs were measured after 16,000 cycles of mechanical loading with 50 N at a 1-Hz frequency. Statistical analysis included one-way analysis of variance and paired t-tests.
  
  
  RESULTS
  The post-loading RTVs were significantly lower than the initial RTVs in the EH 2.5 thread and IH 2.5 thread groups (P<.05). The initial RTVs exhibited no significant differences among the 8 groups, whereas the post-loading RTVs of the EH 6.5 and EH 3.5 thread groups were higher than those of the IH 3.5 thread group (P<.05).
  
  
  CONCLUSIONS
  Within the limitations of this study, the external hex implants with short screw lengths were more advantageous than internal hex implants with short screw lengths in torque maintenance after cyclic loading.
authors.affiliation: Department of Prosthodontics, College of Dentistry, Wonkwang University, Iksan, Republic of Korea.
authors.forename: Hnd Hadi
authors.initials: HH
authors.lastname: Mohammed
authors.affiliation: Department of Prosthodontics, College of Dentistry, Wonkwang University, Iksan, Republic of Korea.
authors.forename: Jin-Han
authors.initials: JH
authors.lastname: Lee
authors.affiliation: Department of Dental Biomaterials, College of Dentistry, Wonkwang University, Iksan, Republic of Korea.
authors.forename: Ji-Myung
authors.initials: JM
authors.lastname: Bae
authors.affiliation: Department of Prosthodontics, College of Dentistry, Wonkwang University, Iksan, Republic of Korea.
authors.forename: Hye-Won
authors.identifier: 0000-0003-0623-5647
authors.initials: HW
authors.lastname: Cho
keywords: Abutment screw length; Cyclic loading; Dental implant-abutment connection; Removal torque value