title: Mechanical factors responsible for the obstruction of the gliding mechanism of a dynamic hip screw for stabilizing pertrochanteric femoral fractures.
abstract: BACKGROUND
  In treatment of pertrochanteric femoral fractures with dynamic hip screws (DHSs) (135-degree, Synthes, Bettlach, Switzerland), damage was observed in removed lag screws, leading to the conclusion that the gliding mechanism must have been obstructed as a result of either inappropriate position of the implant or insufficient medial support in the fracture zone.
  
  
  METHODS
  The forces and moments transmitted in the screw socket are calculated using a mathematical model to find the optimal position of the implant.
  
  
  RESULTS
  The forces and moments depend on the position and orientation of the lag screw as well as on the position of the contact point between the two main fragments. By changing the point of contact, a better decrease of the load to the DHS can be achieved than by changing the position and orientation of the screw. For a low contact point, the model shows the lowest values for the forces in the socket.
  
  
  CONCLUSIONS
  Complete agreement was found between the results of the presented calculations and our own clinical experience in removed DHSs.
authors.affiliation: Unfallkrankenhaus Meidling, Vienna, Austria.
authors.forename: E
authors.initials: E
authors.lastname: Sim
authors.forename: H B
authors.initials: HB
authors.lastname: Schmiedmayer
authors.forename: P
authors.initials: P
authors.lastname: Lugner
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena; D001863:Bone Screws; D004867:Equipment Design; D004868:Equipment Failure; D005260:Female; D006620:Hip Fractures / Q000601:surgery*; D006801:Humans; D008297:Male; D008875:Middle Aged