title: The effect of repetitive pilot-hole use on the insertion torque and pullout strength of vertebral system screws.
abstract: METHODS
  In vitro biomechanical investigation of the screw-holding capacity.
  
  
  OBJECTIVE
  To evaluate the effect of repetitive screw-hole use on the insertional torque and retentive strength of vertebral system screws.
  
  
  BACKGROUND
  Placement and removal of vertebral system screws is sometimes necessary during the surgical procedures in order to assess the walls of the pilot hole. This procedure may compromise the holding capacity of the implant.
  
  
  METHODS
  Screws with outer diameter measuring 5, 6, and 7 mm were inserted into wood, polyurethane, polyethylene, and cancellous bone cylindrical blocks. The pilot holes were made with drills of a smaller, equal, or wider diameter than the inner screw diameter. Three experimental groups were established based on the number of insertions and reinsertions of the screws and subgroups were created according to the outer diameter of the screw and the diameter of the pilot hole used.
  
  
  RESULTS
  A reduction of screw-holding capacity was observed between the first and the following insertions regardless the anchorage material. The pattern of reduction of retentive strength was not similar to the pattern of torque reduction. The pullout strength was more pronounced between the first and the last insertions, while the torque decreased more proportionally from the first to the last insertions.
  
  
  CONCLUSIONS
  Insertion and reinsertion of the screws of the vertebral fixation system used in the present study reduced the insertion torque and screw purchase.
authors.affiliation: Department of Biomechanics, Medicine and Rehabilitation of the Locomotor Apparatus, University of São Paulo, São Paulo, Brazil. hladefin@fmrp.usp.br
authors.forename: Helton L A
authors.initials: HL
authors.lastname: Defino
authors.forename: Rodrigo César
authors.initials: RC
authors.lastname: Rosa
authors.forename: Patrícia
authors.initials: P
authors.lastname: Silva
authors.forename: Antonio Carlos
authors.initials: AC
authors.lastname: Shimano
authors.forename: José Batista
authors.initials: JB
authors.lastname: Volpon
authors.forename: Francisco José Albuquerque
authors.initials: FJ
authors.lastname: de Paula
authors.forename: Philipp
authors.initials: P
authors.lastname: Schleicher
authors.forename: Klaus
authors.initials: K
authors.lastname: Schnake
authors.forename: Frank
authors.initials: F
authors.lastname: Kandziora
mesh_terms: D000818:Animals; D001863:Bone Screws; D001842:Bone and Bones / Q000601:surgery; D002417:Cattle; D006801:Humans; D008422:Materials Testing / Q000379:methods*; D028223:Pinus; D020959:Polyethylene; D011140:Polyurethanes; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013131:Spine / Q000601:surgery*; D019415:Torque; D014934:Wood
chemical_list: D011140:Polyurethanes; D020959:Polyethylene