title: Does maximum torque mean optimal pullout strength of screws?
abstract: OBJECTIVE
  To determine the relationship between insertion torque and pullout strength of 3.5-mm-diameter cortical screws in cadaveric humeri with different bone mineral densities (BMDs).
  
  
  METHODS
  Five pairs of human humeri from each of 3 BMD groups (normal, osteopenic, and osteoporotic) were used. Holes were drilled in each humerus, and maximum insertion torque (T(max)) was measured by tightening a screw until stripping occurred. In the remaining holes, screws were tightened to 50%, 70%, or 90% of the T(max). A servohydraulic testing machine pulled each screw out at 1 mm/s while resulting force and axial displacement were recorded at 10 Hz. The authors checked for an effect of insertion torque (percent T(max)) on pullout strength using a general linearized and latent mixed model (Stata10), controlling for cortical thickness and BMD (T-score).
  
  
  RESULTS
  Pullout strength for normal and osteoporotic bone was greatest for screws inserted to 50% T(max) and was significantly greater than that at T(max) but not significantly different from that at 70% or 90% T(max). For osteopenic bone, pullout strength was greatest at 70% peak torque, but it was not significantly different from the pullout strength at the 50% or 90% T(max) levels.
  
  
  CONCLUSIONS
  Tightening screws beyond 50% T(max) does not increase pullout strength of the screw and may place bone at risk for damage that might result in loss of fixation. Even after adjusting for bone thickness and density, there is no clear relationship between pullout strength and screw torque.
authors.affiliation: International Center for Orthopaedic Advancement, Department of Orthopaedic Surgery, The Johns Hopkins University, Baltimore, MD 21224-2780, USA.
authors.forename: Sara E
authors.initials: SE
authors.lastname: Tankard
authors.forename: Simon C
authors.initials: SC
authors.lastname: Mears
authors.forename: Daniel
authors.initials: D
authors.lastname: Marsland
authors.forename: Evan R
authors.initials: ER
authors.lastname: Langdale
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Belkoff
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena; D015519:Bone Density; D001851:Bone Diseases, Metabolic / Q000503:physiopathology / Q000601:surgery*; D001863:Bone Screws; D005260:Female; D006801:Humans; D006811:Humerus / Q000502:physiology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011475:Prosthesis Failure; D019415:Torque