title: Interference screws should be shorter than the hamstring tendon graft in the bone tunnel for best fixation.
abstract: OBJECTIVE
  Interference screw fixation of hamstring tendon grafts in bone has to overcome the challenges that tendons have a slippery surface and viscoelastically adapt under pressure. As the typical failure mode of the graft is to slip past the interference screw, it was hypothesized that the position and configuration of the graft end may be of influence on the fixation strength.
  
  
  METHODS
  Different configurations of the graft ending and its effect to primary fixation with interference screws after viscoelastic adaptation were tested in six groups: I: graft and the screw inserted at the same depth, II/III: the graft overlaps the tip of the screw (interference screw of 28 and 19 mm in length, respectively), IV: strengthening of the graft ending with additional suture knots, V: Endopearl, respectively, and VI: effect of partial retraction of the screw after excessive insertion. In vitro tests were performed with fresh calf tendon grafts and interference screws in bone tunnels (fresh porcine distal femur) all of 8 mm in diameter.
  
  
  RESULTS
  The relative position of the graft ending to the tip of the interference screw thereby was recognized as a significant factor on pullout forces. Further strengthening at the graft endings with additional suture knots or an Endopearl device could improve primary hold as well.
  
  
  CONCLUSIONS
  Better fixation strength is achieved if the tip of interference screw does not extend past the end of a tendon graft. Enforcement of the tendon end with sutures or an implant can further improve fixation.
authors.affiliation: Department of Orthopedics, University of Zurich, Balgrist, Forchstrasse 340, 8008, Zurich, Switzerland.
authors.forename: Michael
authors.initials: M
authors.lastname: Stalder
authors.forename: Mazda
authors.initials: M
authors.lastname: Farshad
authors.forename: Jess G
authors.initials: JG
authors.lastname: Snedeker
authors.forename: Dominik C
authors.initials: DC
authors.lastname: Meyer
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002417:Cattle; D005269:Femur / Q000601:surgery*; D018482:Muscle, Skeletal / Q000637:transplantation; D013552:Swine; D013710:Tendons / Q000502:physiology / Q000637:transplantation*