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Show Timothy Plaizier Electrical Engineering SIMULTANEOUS MEASUREMENT OF THREE-DIMENSIONAL JOINT KINEMATICS AND TISSUE STRAINS WITH OPTICAL METHOD Timothy Plaizier 1,2; (Jeff Weiss, Jl,3) 1 Musculoskeletal Research Laboratories, Department of Bioengineering University of Utah Faculty Sponsor Jeffrey Weiss Introduction The objective of this study was to assess the precision and accuracy of a three-dimen-sional (3D) motion analysis system for the simultaneous measurement of soft tissue strains and joint kinematics. The system consisted of software for calculating the 3D coordinates of contrast markers and two high-resolution digital cameras. System precision was assessed by examining the variation in the coordinates of static mark-ers over time. The measurement of the 3D strain accuracy was accomplished by mov-ing contrasting markers a fixed distance in the field of view and then calculating the error in predicted strain. The 3D kinematic measurements were obtained by simulat-ing the measurements that are required for recording joint kinematics. The aim of this study was to develop a meth-odology for simultaneous measurement of 3D soft tissue strain and joint kinematics using digital cameras, and to quantify the errors associated with these measurements in a test setup that resembled the study of knee ligament biomechanics. Methods The measurement system consisted of two high-resolution digital cameras (Pulnix TM-1040, 1024x1024x30 frames per second (fps)) equipped with 50 mm 1:1.8 lenses and extension tubes, two frame grabbers and Digital Motion Analysis Software (DMAS). The cameras required 2.1 MB of memory per frame when configured to 6 fps as we had them configured. The cameras were focused at a target with a 190 mm diago-nal field of view (FOV). The DMAS software tracked marker centroids in both camera views automatically and applied the modi-fied direct linear transformation (DLT) to calculate the 3D centroid coordinates [1]. Preliminary tests demonstrated that black markers against a white background pro-vided superior contrast and therefore sys-tem accuracy in comparison to markers covered with reflective tape, while two 100 W incandescent lights provided the best contrast. A 3D calibration frame was manufac-tured. Twenty-seven white Delrin spheri-cal markers (4.75 mm dia.) were arranged in three levels of nine marker grids, posi-tioned 60 mm apart. The exact coordi-nates of each marker centroid were de-termined with a coordinate measuring machine (Zeiss Eclipse 4040, ac-curacy ±0.0004 mm). These val-ues were used for DLT calibration. Due to ongoing improvements in the sen-sitivity and resolution of charge-coupled devices (CCDs), it is now possible to obtain images of very high quality and resolu-tion, allowing the computation of strain in three dimensions with accuracies ±0.1-0.5% error in percent strain [2]. Another advantage besides providing non-contact measurements, is that these systems have become especially attractive because sys-tem integrators have taken advantage of OEM digital cameras and framegrab-b ers, therefore eliminating the need for proprietary, vendor-specific hardware. These improvements suggest that it may be possible to use a field of view large enough to track markers for calculation of both soft tissue strain and joint kinematics. Results The field of view (190 mm) was chosen to al-low simultaneous recording of markers for soft tissue strain measurement and knee joint kinematics. From ourtesting procedure we can repot an average system precision was between ±0.004 mm and ±0.028 mm, depending on marker size and camera angle. Absolute error in strain measure-ment varied from a minimum of ±0.005% to a maximum of ±0.109%, depending on the angle between cameras. Kinematic accuracy for translations was between ±0.018 and ±0.034 mm, while rotational accuracy was ±0.082 to ±0.160 degrees. |
