title: Load distribution between cephalic screws in a dual lag screw trochanteric nail.
abstract: BACKGROUND
  It has been observed clinically that the Z-effect is a potential cause of failure of an intramedullary nail with two cephalic screws. It describes the migration behavior of the cephalic screws in the femoral head. The primary objective was to examine different cephalic screw configurations and test the load distribution between them as a function of their relative placement and their relative movement in the nail. It has been hypothesized that different cephalic screw positions may have an influence on the stress in the implant and bone and therefore on implant failures, such as the Z-effect.
  
  
  METHODS
  To quantify the load distribution of a dual cephalic screw intramedullary femoral nail (Citieffe, Calderara di Reno, BO, Italy), a finite element model of the femur, focusing on the loading of the cephalic screws, was prepared. Four different screw lengths (90-105 mm) were examined. The investigation considered the stresses and strains in the bone and implant as well as the relative movement of the screws.
  
  
  RESULTS
  If the inferior cephalic screw had a shorter length, then the superior one and the femoral nail had to bear higher loads. In that case, the "equivalent von Mises stress" increased up to 10 % at the superior cephalic screw and up to 5 % at the femoral nail. The analysis of the relative movement showed that sliding of the inferior cephalic screw occurred in the nail. The total movement ranged from 0.47 to 0.73 mm for the different screw configurations.
  
  
  CONCLUSIONS
  The stresses were distributed more equally between the two cephalic screws in the bone and the implant if a longer inferior screw was used. The stresses in the bone and implant were reduced with a longer inferior cephalic screw. Therefore, a configuration using a longer inferior cephalic screw is preferable for trochanteric fracture fixation with a dual cephalic screw intramedullary device.
authors.affiliation: Institute of Biomechanics, Trauma Center Murnau, Murnau, Germany. julia.henschel@bgu-murnau.de.|Paracelsus Medical University, Salzburg, Austria. julia.henschel@bgu-murnau.de.
authors.forename: Julia
authors.initials: J
authors.lastname: Henschel
authors.affiliation: Institute of Biomechanics, Trauma Center Murnau, Murnau, Germany.|Paracelsus Medical University, Salzburg, Austria.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Eberle
authors.affiliation: Institute of Biomechanics, Trauma Center Murnau, Murnau, Germany.|Paracelsus Medical University, Salzburg, Austria.
authors.forename: Peter
authors.initials: P
authors.lastname: Augat
mesh_terms: D001858:Bone Nails; D001863:Bone Screws; D005269:Femur / Q000601:surgery*; D020342:Finite Element Analysis; D005594:Fracture Fixation, Intramedullary / Q000295:instrumentation* / Q000379:methods; D006620:Hip Fractures / Q000601:surgery*; D006801:Humans; D008422:Materials Testing / Q000379:methods; D011475:Prosthesis Failure; D013314:Stress, Mechanical; D016474:Weight-Bearing
keywords: Cut-out; Dual cephalic screw implant; FEM; Z-effect