Methodological Analysis of Finite Helical Axis Behavior in Joint Kinematics

Cescon, Corrado and Barbero, Marco and Cattrysse, Erik (2013) Methodological Analysis of Finite Helical Axis Behavior in Joint Kinematics. In: 3° Congresso Nazionale S.I.F, 24-25 Maggio 2013, Napoli.

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Poster SIF2013 FHA Methodology.pdf - Accepted Version

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BACKGROUND AND AIM Although the approach of the Finite Helical Axis (FHA) is very common in spacecraft dynamics and graphic imaging, it struggles with interpretational and representational difficulties compared to a six degrees of freedom analysis especially in clinical context and among medical professionals. The dispersion of the 3D-motion axis has been used to express the stability of the motion in knee kinematics and cervical spine analyses. However, those graphical representations do not allow mathematical and statistical comparison of data in a larger dataset. The aim of the present study is to investigate the effect of noise and angle intervals on the estimation of FHA parameters and to introduce a novel approach for the quantification of the FHA behavior. MATERIALS AND METHODS The algorithm used in this study is based on the method first presented by Mozzi in 1763. The helical axis parameters n, Θ, c and t can be derived with simple mathematical calculations. A simulation of body movement and FHA extraction has been performed introducing random noise on position and orientation of a virtual sensor (100 repetitions). RESULTS The study of the effect of input noise on the accuracy of the FHA-parameters estimation shows a linear relation between the simulated noise and the error in the corresponding parameter (see Figure). FHA axis stability can be determined by calculating the intersection points of the IHA (Instantaneous Helical Axis) for every plane perpendicular to the FHA using the Convex Hull (CH) technique. The minimum CH is defined as convex polygon with minimum area, which includes all points in a plane. The angle between the FHA and every IHA was also computed and its distribution was analyzed. CONCLUSIONS Input noise has a linear relationship and an inversely proportional relationship with the angle steps has been demonstrated with FHA-estimation accuracy. The FHA dispersion can be represented by the minimum convex hull and the distribution of angles of the IHA relative to the FHA.

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