title: Screw Stripping After Repeated Cortical Screw Insertion-Can We Trust the Cancellous "Bailout" Screw?
abstract: BACKGROUND
  During osteosynthesis standard nonlocking cortical screws often require reinsertion, raising concern over possible decrease in their effectiveness. This study aims to quantify that potential loss of fixation with reinsertions as well as examine the ability of a cancellous "bailout screw" to regain insertion torque in a previously stripped screw hole.
  
  
  METHODS
  Four different types of bone surrogates were chosen to represent normal cortical bone, osteoporotic cortical bone, high-density (normal) cancellous bone, and low-density (osteoporotic) cancellous bone; nonlocked 3.5-mm cortical screws were inserted into the predrilled holes 1, 2, 3, 4, or 5 times before being torqued maximally to the point of stripping. A 4.0-mm cancellous "bailout" screw was then placed into the same hole and torqued until stripping. Torque was measured continuously using a torque-measuring screwdriver and maximal insertion torque (MIT) of 3.5 and 4.0 screws before stripping was recorded.
  
  
  RESULTS
  MIT decreased with reinsertion of nonlocked cortical screws. By the third reinsertion in all but the normal bone surrogates, the screws lost approximately one third to one half of their original MIT (50%-71% of original torque). The bailout screw succeeded in restoring the original MIT in the osteoporotic cancellous bone surrogate and the normal cortical bone surrogate. In the normal cancellous and osteoporotic cortical bone surrogates, the bailout screw was only able to restore an average of 50% (range 31%-63%) of the original MIT.
  
  
  CONCLUSIONS
  Screw reinsertion may significantly reduce the MIT of 3.5-mm nonlocked cortical screws. Use of the bailout cancellous screw for a stripped cortical screw should be expected to restore MIT only in normal cortical bone and osteoporotic cancellous bone. In other scenarios, the bailout screw should not be expected to uniformly restore full insertion torque.
authors.affiliation: *UCSF/SFGH Orthopaedic Trauma Institute, UCSF Department of Orthopaedic Surgery, San Francisco, CA; and †Department of Orthopaedic Surgery, Hennepin County Medical Center, Minneapolis, MN.
authors.forename: Meir
authors.initials: M
authors.lastname: Marmor
authors.forename: Gudrun
authors.initials: G
authors.lastname: Mirick
authors.forename: Amir
authors.initials: A
authors.lastname: Matityahu
mesh_terms: D001863:Bone Screws; D001842:Bone and Bones / Q000502:physiology* / Q000601:surgery*; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D006801:Humans; D011475:Prosthesis Failure; D019919:Prosthesis Implantation; D012086:Reoperation / Q000295:instrumentation* / Q000379:methods; D013314:Stress, Mechanical; D013718:Tensile Strength; D019415:Torque