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Show These results demonstrate that simul-taneous measurement of 3D soft tis-sue strain and 3D joint kinematics can be performed while achieving excellent ac-curacy for both sets of measurements. Discussion This study demonstrated that the 3D sys-tem could accurately measure strain and kinematics using a FOV that accommo-dates simultaneous tracking of markers for both measurements. Results showed that a reduced camera angle marginally im-proved uncertainty for frontal plane (z-axis) displacements, while an increased camera angle slightly improved uncertainties of sag- gital plane (x-axis) displacements. More over, in comparison to similar systems us-ing proprietary vendor-specific hardware, this system is a small fraction of the cost. The measurements of system precision were calculated using the length of the position vectors of the markers, and thus these mea-surements should be considered average er-rors that take into account the precision in all three spatial directions. In contrast, the mea-surements of strain were made along the z-axis direction. Since the z-axis and y-axis are closest to perpendicular to the camera line of sight, these directions will correspond to the best accuracy for the camera angles used in this study. Thus, measurements of strain accuracy should be considered a best case when considering general measure-ments in 3D using similar camera angles. Although a similar argument applies to the kinematic measurements, the "gold stan-dard" for these measurements was based on a combination of digitizer, actuator en-coder, or digital caliper measurements. Because of the differences in the accu-racy of these measurement techniques and the propagation of errors in the ki- nematic measurements, the results for translational and rotational kinematic ac- curacies likely represent worst cases. As with any research or testing system based on video or digital cameras, the most important determinants of precision and accuracy are the resolution of the CCD and the FOV used for measurements. In this study, the FOV was chosen to allow simulta-neous tracking of markers for strain and kine-matic measurements in the context of study-ing the human MCL [2,3]. Limitations on the rate of data transfer from the camera to the framegrabber cards and then to computer memory, primarily imposed by the bandwidth of the computer system's bus, result in a trad-eoff between the frame rate and spatial reso-lution of the CCD. Cameras with higher reso-lution CCDs typically have slower frame rates. This limitation will likely disappear with im-provements in computer architecture. Marker contrast is also very important, with improved contrast yielding better system precision and thus accuracy. Finally, the physical size of the CCD influences the sensitivity, with larger CCDs yielding better sensitivity and thus better image quality. The cameras used in this study had 1" CCDs, the largest size that is available. In summary, the 3D measurement sys-tem provided excellent accuracy for strain measurement and very good accuracy for kinematic measurements. The absolute and percent errors are considered to be more than acceptable for use in our stud-ies of ligament strains and joint kinematics. References [1]Hatze, H, 1988, "High-Precision Three-Di- mensional Photogrammetric Calibration and Object Space Reconstruction Using a Modified Dlt-Approach," J Biomech, 21, pp. 533-8. [2]Gardiner, J. C, Weiss, J. A., and Rosenberg, T. D., 2001, "Strain in the Human Medial Col-lateral Ligament During Valgus Loading of the Knee," Clin Orthop, pp. 266-74. [3]Gardiner, J. C. and Weiss, J. A., 2003, "Sub-ject-Specific Finite Element Analysis of the Human Medial Collateral Ligament During Valgus Knee Loading," J Orthop Res, 21, pp. 1098-106. |
