title: A mathematical stiffness matrix for characterising mechanical performance of the Orthofix DAF.
abstract: This study investigates a method for characterising the 3D support to a bone fracture site that is provided by a fracture stabilising device. The method is demonstrated with the Orthofix DAF unilateral external fixator, for which a mathematical stiffness matrix is defined using experimental measurements in six degrees of freedom. Single forces or bending moments are applied to a model fracture stabilised by the fixator, and 3D inter fragmentary displacements are measured by an instrumented spatial linkage. The 6x6 stiffness matrix for the fracture site is calculated from the product of the vector of forces and moments and the inverse of the vector of displacements. A transformation matrix is used to determine the stiffness matrix for a range of anatomical angles (between the plane of the fixator frame and the sagittal plane). Comparison between measured displacements (for an angle of 30 degrees) and the corresponding calculated displacements showed agreement to within 8%. The method enables fracture site stiffness for a fixation device to be characterised comprehensively, and its properties to be identified in comparison with other devices. It also provides the means of analysing inter fragmentary motion that may arise from physiological loading, regardless of complexity.
authors.affiliation: Oxford Orthopaedic Engineering Centre, University of Oxford, Nuffield Orthopaedic Centre, Headington, UK. trevor.gardner@ooec.ox.ac.uk
authors.forename: T N
authors.initials: TN
authors.lastname: Gardner
authors.forename: M
authors.initials: M
authors.lastname: Weemaes
mesh_terms: D000465:Algorithms; D004867:Equipment Design / Q000379:methods*; D005592:Fracture Fixation / Q000295:instrumentation* / Q000379:methods; D006801:Humans; D008962:Models, Theoretical; D009984:Orthopedic Fixation Devices; D013978:Tibial Fractures / Q000628:therapy