Validation of three-dimensional model-based tibio-femoral tracking during running

Abstract

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.

Figure 1

Overhead view of the biplane radiographic system. Subjects ran on treadmill toward image intensifiers with the study leg within the imaging volume shortly before and after footstrike. The laboratory coordinate system was defined by a control object placed within the imaging volume (x-axis directed between image intensifiers, y-axis directed perpendicular to x-axis in the horizontal plane, and z-axis vertical).

Figure 2

Digitally reconstructed radiograph (DRR) of the femur (yellow) superimposed on distortion corrected biplane radiographs (red). Note implanted beads are not present in DRR. External surface markers and electrode wires are also visible in biplane radiographs.

Figure 3

Flexion-extension plots for three trials of Subject C. Bead-based tracking curves are dashed lines, model-based tracking results are solid lines. Horizontal axis is time relative to foot touchdown, vertical axis is flexion angle.

Figure 4

Anterior-posterior translation plots for three trials of Subject C. Bead-based tracking curves are dashed lines, model-based tracking results are solid lines. Horizontal axis is time relative to foot touchdown, vertical axis is flexion angle.