Practical inertial sensing-based method for estimating 3d joint kinematics in the Vicon Clinical Manager anatomical frame definition

Typically, body segment and segment joint 3D kinematics reference systems are defined using bony landmarks (marker based acquisition) or using a (neutral) pose or helical axes calibration (inertial sensing with MIMUs). This prevents inertial sensing multi-moment and natural-environment application in patients for which pose/helical axes calibration methods are physically not possible or inaccurate/unreliable (e.g. some neurological disorders). Methods were proposed for translating inertial sensor data to the ISB standard definition using a sensorized double pointer device (1), which need two people to operate and are not optimized for clinical use in terms of user-friendliness, speed or user error robustness. Also, no translation is available to the much-applied anatomical reference frame definition of Vicon Clinical Manager (VCM), which prevents direct integration with the huge body of previous research and knowledge acquired using the VCM reference frame definition. This paper examines the clinical feasibility, accuracy and repeatability of an improved method to facilitate the estimation of body segment and joint 3d kinematics through inertial sensing in the VCM anatomical reference frame. MIMUs were positioned on lower body segments plus one on the double-pointer device A protocol-driven clinical application instructs/shows clinicians to place the pointers on 2 bony landmarks for the first calibration recording. Pointer distance adjustment is achieved by sliding the pointers over their ruler using a thumb-operated clamping release mechanism. A button is pushed to start a three-second calibration recording (wirelessly). After assisted inspection of the recording, the clinician accepts/rejects the recording and follows instructions for the rest of the calibration protocol recordings, and then for the actual (walking) trials. The software corrects accidental up/down and left/right placement errors. From the first walking trial, real-time full 3d body segment and joint kinematics are available by applying the pointer-based calibration (animated 3d avatar, graphs). For lower body validation trials synchronized inertial sensing and Vicon data were acquired for 12 subjects (10 ACL rehabilitation and 2 adult CP patients). In addition to the markers for the VCM protocol, 3 additional markers were placed on a rig under each inertial sensor. Inertial sensing-based joint angles were compared against VCM joint angles.Download : Download high-res image (217KB)Download : Download full-size image Fig. 1: The new double pointer device applied to l upper leg bony landmarks The first results indicate an average relative difference between the new inertial sensing-based method and VCM gold standard (defined as rotation component in the difference quaternion in e.g. knee joint) of around 5 degrees. After some practice, the new method with added double pointer device with one extra inertial sensor could be easily applied by one person and appeared fast and practical. The first results indicate that joint kinematics are comparable to VCM results.