title: The instantaneous helical axis of the subtalar and talocrural joints: a non-invasive in vivo dynamic study.
abstract: BACKGROUND
  An understanding of rear-foot (talocrural and subtalar joints) kinematics is critical for diagnosing foot pathologies, designing total ankle implants, treating rear-foot injuries and quantifying gait abnormalities. The majority of kinematic data available have been acquired through static cadaver work or passive in vivo studies. The applicability of these data to dynamic in vivo situations remains unknown. Thus, the purpose of this study was to fully quantify subtalar, talocrural and calcaneal-tibial in vivo kinematics in terms of the instantaneous helical axis (IHA) in twenty-five healthy ankles during a volitional activity that simulated single-leg toe-raises with partial-weight support, requiring active muscle control.
  
  
  METHODS
  Subjects were each placed supine in a 1.5 T MRI and asked to repeat this simulated toe-raise while a full sagittal-cine-phase contrast (dynamic) MRI dataset was acquired. From the cine-phase contrast velocity a full kinematic description for each joint was derived.
  
  
  RESULTS
  Nearly all motion quantified at the calcaneal-tibial joint was attributable to the talocrural joint. The subtalar IHA orientation and position were highly variable; whereas, the talocrural IHA orientation and position were extremely consistent.
  
  
  CONCLUSIONS
  The talocrural was well described by the IHA and could be modeled as a fixed-hinge joint, whereas the subtalar could not be.
authors.affiliation: Functional and Applied Biomechanics Section, Rehabilitation Medicine Department, National Institutes of Health, Bethesda, MD, USA. fsheehan@cc.nih.gov.
authors.forename: Frances T
authors.initials: FT
authors.lastname: Sheehan