title: Direct assessment of 3D foot bone kinematics using biplanar X-ray fluoroscopy and an automatic model registration method.
abstract: BACKGROUND
  Quantifying detailed 3-dimensional (3D) kinematics of the foot in contact with the ground during locomotion is crucial for understanding the biomechanical functions of the complex musculoskeletal structure of the foot. Biplanar X-ray fluoroscopic systems and model-based registration techniques have recently been employed to capture and visualise 3D foot bone movements in vivo, but such techniques have generally been performed manually. In the present study, we developed an automatic model-registration method with biplanar fluoroscopy for accurate measurement of 3D movements of the skeletal foot.
  
  
  METHODS
  Three-dimensional surface models of foot bones were generated prior to motion measurement based on computed tomography. The bone models generated were then registered to biplanar fluoroscopic images in a frame-by-frame manner using an optimisation technique, to maximise similarity measures between occluding contours of the bone surface models with edge-enhanced fluoroscopic images, while avoiding mutual penetration of bones. A template-matching method was also introduced to estimate the amount of bone translation and rotation prior to automatic registration.
  
  
  RESULTS
  We analysed 3D skeletal movements of a cadaver foot mobilized by a robotic gait simulator. The 3D kinematics of the calcaneus, talus, navicular and cuboid in the stance phase of the gait were successfully reconstructed and quantified using the proposed model-registration method. The accuracy of bone registration was evaluated as 0.27 ± 0.19 mm and 0.24 ± 0.19° (mean ± standard deviation) in translation and rotation, respectively, under static conditions, and 0.36 ± 0.19 mm and 0.42 ± 0.30° in translation and rotation, respectively, under dynamic conditions.
  
  
  CONCLUSIONS
  The measurement was confirmed to be sufficiently accurate for actual analysis of foot kinematics. The proposed method may serve as an effective tool for understanding the biomechanical function of the human foot during locomotion.
authors.affiliation: Department of Mechanical Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku Yokohama, 223-8522 Japan.
authors.forename: Kohta
authors.initials: K
authors.lastname: Ito
authors.affiliation: Graduate School of Engineering Science, Osaka University, Toyonaka, 560-8531 Japan.
authors.forename: Koh
authors.initials: K
authors.lastname: Hosoda
authors.affiliation: Graduate School of Engineering Science, Osaka University, Toyonaka, 560-8531 Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Shimizu
authors.affiliation: Graduate School of Engineering Science, Osaka University, Toyonaka, 560-8531 Japan.
authors.forename: Shuhei
authors.initials: S
authors.lastname: Ikemoto
authors.affiliation: Graduate School of Engineering Science, Osaka University, Toyonaka, 560-8531 Japan.
authors.forename: Shinnosuke
authors.initials: S
authors.lastname: Kume
authors.affiliation: School of Medicine, Keio University, Tokyo, 160-8582 Japan.
authors.forename: Takeo
authors.initials: T
authors.lastname: Nagura
authors.affiliation: School of Medicine, Keio University, Tokyo, 160-8582 Japan.
authors.forename: Nobuaki
authors.initials: N
authors.lastname: Imanishi
authors.affiliation: School of Medicine, Keio University, Tokyo, 160-8582 Japan.
authors.forename: Sadakazu
authors.initials: S
authors.lastname: Aiso
authors.affiliation: School of Medicine, Keio University, Tokyo, 160-8582 Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Jinzaki
authors.affiliation: Department of Mechanical Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku Yokohama, 223-8522 Japan.
authors.forename: Naomichi
authors.initials: N
authors.lastname: Ogihara