SURFACE-MARKER CLUSTER DESIGN CRITERIA FOR 3-D BONE MOVEMENT RECONSTRUCTION

When three-dimensional (3-D) human or animal movement is recorded usin g a photogrammetric system, bone-embedded frame positions and orientat ions are estimated from reconstructed surface marker trajectories usin g either nonoptimal or optimal algorithms, The effectiveness of these mathematical procedures in accommodating far both photogrammetric erro rs and skin movement artifacts depends on the number of markers associ ated With a given bone as well as on the size and shape characteristic s of the relevant cluster, One objective of this paper deals with the identification of marker-cluster design criteria aimed at the minimiza tion of error propagation from marker coordinates to bone-embedded fra me position and orientation, Findings allow for the quantitative estim ation of these errors for any given cluster configuration and suggest the following main design criteria, A cluster made up of four markers represents a good practical compromise, Planar clusters are acceptable , provided in quasi-isotropic distribution, The root mean square dista nce of the markers from their centroid should be greater than ten time s the standard deviation of the marker position error, The second obje ctive of this paper deals with the identification of the optimal clust er position and orientation on the limb aimed at the minimization of e rror propagation to anatomical landmark laboratory coordinates, Cluste r position should be selected to minimize skin movement artifacts, The longest principal axis of the marker distribution should be oriented toward the relevant anatomical landmark position.