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The purpose of this study was to determine the accuracy of a radiographic model-based tracking technique that measures the three-dimensional in vivo motion of the tibio-femoral joint during running. Tantalum beads were implanted into the femur and tibia of three subjects and computed tomography (CT) scans were acquired after bead implantation. The subjects ran 2.5m/s on a treadmill positioned within a biplane radiographic system while images were acquired at 250 frames per second. Three-dimensional implanted bead locations were determined and used as a "gold standard" to measure the accuracy of the model-based tracking. The model-based tracking technique optimized the correlation between the radiographs acquired via the biplane X-ray system and digitally reconstructed radiographs created from the volume-rendered CT model. Accuracy was defined in terms of measurement system bias, precision and root-mean-squared (rms) error. Results were reported in terms of individual bone tracking and in terms of clinically relevant tibio-femoral joint translations and rotations (joint kinematics). Accuracy for joint kinematics was as follows: model-based tracking measured static joint orientation with a precision of 0.2 degrees or better, and static joint position with a precision of 0.2mm or better. Model-based tracking precision for dynamic joint rotation was 0.9+/-0.3 degrees , 0.6+/-0.3 degrees , and 0.3+/-0.1 degrees for flexion-extension, external-internal rotation, and ab-adduction, respectively. Model-based tracking precision when measuring dynamic joint translation was 0.3+/-0.1mm, 0.4+/-0.2mm, and 0.7+/-0.2mm in the medial-lateral, proximal-distal, and anterior-posterior direction, respectively. The combination of high-speed biplane radiography and volumetric model-based tracking achieves excellent accuracy during in vivo, dynamic knee motion without the necessity for invasive bead implantation.

No abstract

The mechanical properties of cancellous bone and the biological response of the tissue to mechanical loading are related to deformation and strain in the trabeculae during function. Due to the small size of trabeculae, their motion is difficult to measure. To avoid the need to measure trabecular motions during loading the finite element method has been used to estimate trabecular level mechanical deformation. This analytical approach has been empirically successful in that the analytical models are solvable and their results correlate with the macroscopically measured stiffness and strength of bones. The present work is a direct comparison of finite element predictions to measurements of the deformation and strain at near trabecular level. Using the method of digital volume correlation, we measured the deformation and calculated the strain at a resolution approaching the trabecular level for cancellous bone specimens loaded in uniaxial compression. Smoothed results from linearly elastic finite element models of the same mechanical tests were correlated to the empirical three-dimensional (3D) deformation in the direction of loading with a coefficient of determination as high as 97% and a slope of the prediction near one. However, real deformations in the directions perpendicular to the loading direction were not as well predicted by the analytical models. Our results show, that the finite element modeling of the internal deformation and strain in cancellous bone can be accurate in one direction but that this does not ensure accuracy for all deformations and strains.

Background: Accurately measuring in-vivo motion of the knee's patellofemoral (PF) joint is challenging. Conventional measurement techniques have largely been unable to accurately measure three-dimensional, in-vivo motion of the patella during dynamic activities. The purpose of this study was to assess the accuracy of a new model-based technique for measuring PF joint motion.