title: A biomechanical testing system to determine micromotion between hip implant and femur accounting for deformation of the hip implant: Assessment of the influence of rigid body assumptions on micromotions measurements.
abstract: BACKGROUND
  Accurate pre-clinical evaluation of the initial stability of new cementless hip stems using in vitro micromotion measurements is an important step in the design process to assess the new stem's potential. Several measuring systems, linear variable displacement transducer-based and other, require assuming bone or implant to be rigid to obtain micromotion values or to calculate derived quantities such as relative implant tilting.
  
  
  METHODS
  An alternative linear variable displacement transducer-based measuring system not requiring a rigid body assumption was developed in this study. The system combined advantages of local unidirectional and frame-and-bracket micromotion measuring concepts. The influence and possible errors that would be made by adopting a rigid body assumption were quantified. Furthermore, as the system allowed emulating local unidirectional and frame-and-bracket systems, the influence of adopting rigid body assumptions were also analyzed for both concepts. Synthetic and embalmed bone models were tested in combination with primary and revision implants. Single-legged stance phase loading was applied to the implant - bone constructs.
  
  
  RESULTS
  Adopting a rigid body assumption resulted in an overestimation of mediolateral micromotion of up to 49.7μm at more distal measuring locations. Maximal average relative rotational motion was overestimated by 0.12° around the anteroposterior axis. Frontal and sagittal tilting calculations based on a unidirectional measuring concept underestimated the true tilting by an order of magnitude.
  
  
  CONCLUSIONS
  Non-rigid behavior is a factor that should not be dismissed in micromotion stability evaluations of primary and revision femoral implants.
authors.affiliation: Department of Mechanical Engineering, Biomechanics Division. KU Leuven, Celestijnenlaan 300c, Box 2419, 3001 Leuven, Belgium. Electronic address: steven.leuridan@kuleuven.be.
authors.forename: Steven
authors.initials: S
authors.lastname: Leuridan
authors.affiliation: Department of Mechanical Engineering, Smart Instrumentation, KU Leuven, Andreas Vesaliusstraat 13, 3000 Leuven, Belgium.
authors.forename: Quentin
authors.initials: Q
authors.lastname: Goossens
authors.affiliation: Department of Orthopedic Surgery, Leuven University Hospitals, Weligerveld 1, 3212 Pellenberg, Belgium.
authors.forename: Jorg
authors.initials: J
authors.lastname: Roosen
authors.affiliation: Department of Mechanical Engineering, Smart Instrumentation, KU Leuven, Andreas Vesaliusstraat 13, 3000 Leuven, Belgium.
authors.forename: Leonard
authors.initials: L
authors.lastname: Pastrav
authors.affiliation: Department of Mechanical Engineering, Smart Instrumentation, KU Leuven, Andreas Vesaliusstraat 13, 3000 Leuven, Belgium.
authors.forename: Kathleen
authors.initials: K
authors.lastname: Denis
authors.affiliation: Department of Orthopedic Surgery, Leuven University Hospitals, Weligerveld 1, 3212 Pellenberg, Belgium.
authors.forename: Michiel
authors.initials: M
authors.lastname: Mulier
authors.affiliation: Department of Mechanical Engineering, PMA, KU Leuven, Celestijnenlaan 300c, 3001 Leuven, Belgium.
authors.forename: Wim
authors.initials: W
authors.lastname: Desmet
authors.affiliation: Department of Mechanical Engineering, Biomechanics Division. KU Leuven, Celestijnenlaan 300c, Box 2419, 3001 Leuven, Belgium.
authors.forename: Jos
authors.initials: J
authors.lastname: Vander Sloten
mesh_terms: D019644:Arthroplasty, Replacement, Hip / Q000379:methods*; D001696:Biomechanical Phenomena; D005269:Femur / Q000502:physiology*; D006622:Hip Prosthesis; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology; D009038:Motion; D011474:Prosthesis Design
keywords: Experimental micromotion measurement; Hip implants; Rigid body assumption