Skin artefact compensation by double calibration in bone motion reconstruction

Bone motion assessment by means of photogrammetric, non-invasive methods can be severely corrupted by experimental errors. The largest fraction of such errors is represented by artefacts due to the soft tissues intervening between the bone and the external markers used for data acquisition. The errors affecting the estimates of anatomical landmarks trajectories in the laboratory frame can be considerably reduced by following the CAST protocol which entails: (i) a static calibration of the anatomical landmarks in a technical reference frame integral with the skin marker cluster and (ii) the use of a rigid model of the cluster. This paper illustrates how a modification of the above-mentioned protocol involving: (i) a double calibration of the anatomical landmarks in two different postures and (ii) the use of a deformable cluster model can effectively enhance bone kinematics estimation. The mean reconstruction error on an anatomical landmark trajectory drops from over 15 mm to less than 8 mm while the average error of about 5° relative to bone orientation assessment decreases to less than 3°