A method to study 3D knee pseudo-kinematics using low-dose stereoradiography during static squat

To investigate clinical questions as kinematic or postural differences between healthy and pathological populations, the current trend is to couple imagery to movement or to posture. To do so, researchers generally register a threedimensional (3D) model acquired from either CT scan or magnetic resonance imaging on 2D fluoroscopic images (registration on dynamic activities) or several radiographs (registration on posture or pseudo-kinematics) (Moro-oka et al. 2007). The literature suggests the use of so-called ‘extrinsic 2D/3D registration’. This implies the need of geometrical objects in the radiographic scene. Embedded tantalum beads or prostheses (Scarvell et al. 2010; Sharma et al. 2012),well detectable on the radiographs, are generally used. They allow attaining the expected measurement accuracy ,18 and ,1mm. When no tantalum beads or prostheses are present, the registration process lacks reliability and accuracy because of the difficulty to extract precise information from the radiographic images (blurry contours, superimposition of structures and cylindrical nature of long bones) (Belvedere et al. 2013). The goal of this work was to present an intrinsic 2D/3D registration method and to validate its reliability on patients’ images aswell as its accuracy on simulated radiographs in a pseudo-kinematic context.