title: Stiffness characteristics and inter-fragmentary displacements with different hybrid external fixators.
abstract: OBJECTIVE
  To determine how the different approaches of constructing a hybrid external skeletal fixator affect the mechanical environment at the fracture site.
  
  
  METHODS
  A comparative biomechanical analysis of external fixators that represent a spectrum of designs from unilateral to circular fixators.
  
  
  BACKGROUND
  The most pertinent parameter that affects fracture healing is the inter-fragmentary displacement. Most studies on the mechanical performance of external fixators have concentrated on the overall stiffness of the fixators.
  
  
  METHODS
  Mechanical testing was performed on four types of hybrid fixators in which the overall stiffness of the fixator as well as the axial, transverse shear and angular displacements at the fracture site were measured.
  
  
  RESULTS
  The Ilizarov hybrid fixator with one wire and one screw on each ring behaved more like a unilateral fixator than a circular fixator. The bar-ring hybrid fixator that simply connected a unilateral fixator body to a wire/ring assembly was too flexible. Reinforcing the bar-ring hybrid system with diagonally placed struts eliminated the deformation associated with the unilateral body and the resulting construct performed considerably better. The two-ring hybrid fixator most closely resembled the Ilizarov fixator.
  
  
  CONCLUSIONS
  Depending on the construction characteristics hybrid fixators possess a spectrum of mechanical characteristics of both unilateral and circular fixators. The constructions using two rings in which at least one bone segment is supported entirely by wires appears to have the best mechanical characteristics.
  
  
  CONCLUSIONS
  Understanding how the different approaches of constructing a hybrid fixator affect the mechanical environment at the fracture site will enable surgeons to chose or build the most appropriate fixator for each clinical situation.
authors.affiliation: University of Sheffield, Orthopaedic and Traumatic Surgery Research Group, Division of Clinical Sciences (North), S5 7AU, Sheffield, UK. l.yang@sheffield.ac.uk
authors.forename: Lang
authors.initials: L
authors.lastname: Yang
authors.forename: S
authors.initials: S
authors.lastname: Nayagam
authors.forename: Michael
authors.initials: M
authors.lastname: Saleh
mesh_terms: D001863:Bone Screws; D001864:Bone Wires; D019245:Compressive Strength; D004548:Elasticity; D004867:Equipment Design; D019544:Equipment Failure Analysis / Q000295:instrumentation / Q000379:methods*; D016267:External Fixators / Q000145:classification*; D009038:Motion; D013314:Stress, Mechanical; D016474:Weight-Bearing