title: Semi-rigid screws provide an auxiliary option to plate working length to control interfragmentary movement in locking plate fixation at the distal femur.
abstract: BACKGROUND
  Extent and orientation of interfragmentary movement (IFM) are crucially affecting course and quality of fracture healing. The effect of different configurations for implant fixation on successful fracture healing remain unclear. We hypothesize that screw type and configuration of locking plate fixation profoundly influences stiffness and IFM for a given load in a distal femur fracture model.
  
  
  METHODS
  Simple analytical models are presented to elucidate the influence of fixation configuration on construct stiffness. Models were refined with a consistent single-patient-data-set to create finite-element femur models. Locking plate fixation of a distal femoral 10mm-osteotomy (comminution model) was fitted with rigid locking screws (rLS) or semi-rigid locking screws (sLS). Systematic variations of screw placements in the proximal fragment were tested. IFM was quantitatively assessed and compared for different screw placements and screw types.
  
  
  RESULTS
  Different screw allocations significantly affect IFM in a locking plate construct. LS placement of the first screw proximal to the fracture (plate working length, PWL) has a significant effect on axial IFM (p < 0.001). Replacing rLS with sLS caused an increase (p < 0.001) of IFM under the plate (cis-cortex) between +8.4% and +28.1% for the tested configurations but remained constant medially (<1.1%, trans-cortex). Resultant shear movements markedly increased at fracture level (p < 0.001) to the extent that plate working length increased. The ratio of shear/axial IFM was found to enhance for longer PWL. sLS versus rLS lead to significantly smaller ratios of shear/axial IFM at the cis-cortex for PWL of ≥ 62 mm (p ≤ 0.003).
  
  
  CONCLUSIONS
  Mechanical frame conditions can be significantly influenced by type and placement of the screws in locking plate osteosynthesis of the distal femur. By varying plate working length stiffness and IFM are modulated. Moderate axial and concomitantly low shear IFM could not be achieved through changes in screw placement alone. In the present transverse osteotomy model, ratio of shear/axial IFM with simultaneous moderate axial IFM is optimized by the use of appropriate plate working length of about 42-62 mm. Fixation with sLS demonstrated significantly more axial IFM underneath the plate and may further contribute to compensation of asymmetric straining.
authors.affiliation: Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration, Charité - University Medicine Berlin, Augustenburger Platz 1, Berlin, Germany. Electronic address: mark.heyland@charite.de.
authors.forename: Mark
authors.initials: M
authors.lastname: Heyland
authors.affiliation: Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration, Charité - University Medicine Berlin, Augustenburger Platz 1, Berlin, Germany.
authors.forename: Georg N
authors.initials: GN
authors.lastname: Duda
authors.affiliation: Centre for Musculoskeletal Surgery, Charité - University Medicine Berlin, Augustenburger Platz 1, Berlin, Germany.
authors.forename: Norbert P
authors.initials: NP
authors.lastname: Haas
authors.affiliation: Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration, Charité - University Medicine Berlin, Augustenburger Platz 1, Berlin, Germany.
authors.forename: Adam
authors.initials: A
authors.lastname: Trepczynski
authors.affiliation: BG Trauma Center, Eberhard Karls University Tübingen, Schnarrenbergstraße 95, Tübingen, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Döbele
authors.affiliation: BG Trauma Center, Eberhard Karls University Tübingen, Schnarrenbergstraße 95, Tübingen, Germany.
authors.forename: Dankward
authors.initials: D
authors.lastname: Höntzsch
authors.affiliation: Centre for Musculoskeletal Surgery, Charité - University Medicine Berlin, Augustenburger Platz 1, Berlin, Germany.
authors.forename: Klaus-Dieter
authors.initials: KD
authors.lastname: Schaser
authors.affiliation: Centre for Musculoskeletal Surgery, Charité - University Medicine Berlin, Augustenburger Platz 1, Berlin, Germany.
authors.forename: Sven
authors.initials: S
authors.lastname: Märdian
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates; D001863:Bone Screws; D004867:Equipment Design; D005264:Femoral Fractures / Q000473:pathology / Q000601:surgery*; D020342:Finite Element Analysis; D005593:Fracture Fixation, Internal / Q000295:instrumentation / Q000379:methods*; D017102:Fracture Healing; D018460:Fractures, Comminuted / Q000473:pathology / Q000601:surgery*; D017408:Guidelines as Topic; D006801:Humans; D013312:Stress, Physiological
keywords: angular stable; distal femur; finite element analysis; flexible fixation; locking plate; rigid fixation; semi-rigid screw