title: Nonlinear stiffness profiles of external fixators constructed with composite rods.
abstract: OBJECTIVE
  To determine if composite connecting rods confer nonlinear stiffness characteristics on unilateral and bilateral external skeletal fixators (ESF) in cranial-caudal bending and axial loading.
  
  
  METHODS
  Mechanical testing performed on models.
  
  
  METHODS
  Six models of 6-pin ESF constructs composed of birch dowels, a commercial ESF system, and composite connecting rods.
  
  
  METHODS
  Unilateral and bilateral ESF configurations were assembled using either specially designed composite titanium and silicone (composite group) or solid titanium (solid group) connecting rods. Mechanical testing was performed in axial loading and 4-point cranial-caudal bending. Stiffness was determined at a low and high-load range, and was considered increasing and nonlinear if the stiffness at high loads was greater than at low loads.
  
  
  RESULTS
  The stiffness of the solid group was linear in all testing modes and configurations. Bilateral composite fixators had a nonlinear increasing stiffness in axial loading and cranial-caudal bending. Unilateral composite fixators had a nonlinear increasing stiffness in axial loading, but not cranial-caudal bending. Solid connecting rods conferred a higher stiffness in all testing modes and configurations.
  
  
  CONCLUSIONS
  Composite connecting rods resulted in nonlinear increasing axial and bending stiffness in bilateral fixators, and in axial load in unilateral fixators.
  
  
  CONCLUSIONS
  Conventional ESF can be constructed so that the stiffness increases as load increases. This provides the surgeon with additional options to control the local mechanical environment of a healing fracture, which may be used to enhance fracture healing.
authors.affiliation: Tufts University Cummings School of Veterinary Medicine, North Grafton, MA, USA.
authors.forename: Kimberly R
authors.initials: KR
authors.lastname: Carlson
authors.forename: Karl H
authors.initials: KH
authors.lastname: Kraus
authors.forename: Michael P
authors.initials: MP
authors.lastname: Kowaleski
mesh_terms: D000818:Animals; D004867:Equipment Design / Q000295:instrumentation / Q000379:methods / Q000662:veterinary*; D016267:External Fixators / Q000662:veterinary*; D005592:Fracture Fixation / Q000295:instrumentation / Q000379:methods / Q000662:veterinary*; D017102:Fracture Healing; D050723:Fractures, Bone / Q000601:surgery / Q000662:veterinary*; D008422:Materials Testing; D013314:Stress, Mechanical; D016896:Treatment Outcome