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Orthopaedics
12-16-1998, 01:58 AM
Hello all!

I am a graduate student at The University of Texas Southwestern Medical
Center at Dallas working on a telemetry 6 DOF spatial linkage device for
knee joint assessment, and I am using the screw axis theory to determine a
joint coordinate system. The procedure involves placing the device on a
subject's limb and having them flex and extend a few degrees to estimate the
flexion/extension screw axis and then internally/externally rotate a few
degrees with the limb at 90 degrees of flexion to estimate the
internal/external rotaion screw axis. Then by taking the cross product of
the F/E and I/E axes, I should be able to compute the varus/valgus axis.

The problem is that the F/E axis and the I/E axis are described in terms of
direction cosines and a point on each axis. When I take the cross product
of the F/E and I/E axes, I get the V/V axis, which in theory is
perpendicular to the F/E and I/E axes, but I cannot find a point on the V/V
axis in order to fully describe a vector which describes that axis.

I could really use the help of those researchers who have extensive
experience in the mathematics of spatial linkage devices.

If anyone has any information about this, any insight on my problem, or a
very good reference article, I would really appreciate it.

____________________________________________
Ericka F. Calton
Graduate Student in Biomedical Engineering
Motion and Performance Laboratory
Baylor University Medical Center at Dallas
214.820.6300 (fax:214.820.1988)
e-mail: orthop@baylordallas.edu

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