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kaball19
07-23-2007, 09:59 AM
At the ASB meeting in San Francisco at the end of August, a colleague
and I will be presenting a poster on simultaneous rigid-segment pose
analysis of the tibia, talus and calcaneus (P9-37 An Elusive Talus:
Re-Thinking the Ankle Complex, Kevin A Ball, Thomas M Greiner). Given
the current discussion on ankle complex modeling we thought our work
may be relevant.

In 18 cadaveric specimens we directly measured axes of rotation for
the tibiotalar and talocalcaneal (subtalar) joints, and compared
these findings with tibio-calcaneal estimates derived by solving the
two-axis decomposition problem. In evaluating the responses to three
different driving actions: PD - Plantar-Dorsiflexion; IE - Inversion-
Eversion; ML - Medial-Lateral Rotation, we found it was the latter
two driving actions that produced the most interesting results.

We found subtalar joint axis orientation varied according to task.
For the IE and ML driving actions we calculated mean axis
orientations of 38 and 50 deg in elevation, and 16 to 13 degrees in
deviation respectively. Thus, the subtalar axis was not fixed but
adjusted somewhat to align with the driving action. This tendency
occurred to an even greater extent in the "subtalar-like" estimates
we derived from the tibiocalcaneal joint (by bypassing the talus).
Orientation values were 30 and 73 deg in elevation, with deviation
angles of 7.8 and -8.8 deg for the IE and ML driving actions
respectively.

This reminded me of efforts I had made several years ago to try out
the two-axis decomposition problem in-vivo on weight-bearing gait
using skin surface markers. Calculated axes of rotation for the ankle
looked fine, but the "subtalar-like" axes consistently tended to be
too high in elevation.

In our cadaveric preparation we were now directly measuring the
talus. We were also able to avoid the skin marker problem. So how
could we explain this difference?

We looked more carefully at the motion we had attributed to the
tibiotalar joint. We found that a single- axis estimate was not
actually sufficient. Two axes of rotation were occurring: the obvious
one for plantar-dorsiflexion: and a second axis of rotation that
aligned relatively vertically through the tibia. This finding
experimentally matches previous contemplation by Nester et al.
(2003). Transverse plane motion at the ankle joint. Foot Ankle Int. 24
(2):164-168.

We acknowledge that our experimental preparation does not involve
significant loading. Perhaps our unloaded cadaveric joints are more
mobile than in-vivo. On the other hand, clinicians routinely "exert
over-pressure to examine joint laxity". The loads of walking,
cutting, etc. are very likely to be greater than anything possible
through manual motioning. We suggest that the tibiotalar (and by
extension the talocrural) joint may have a few surprises of its own.

Thanks Kevin


Kevin A. Ball PhD
Assistant Professor
University of Hartford
Department of Physical Therapy
200 Bloomfield Ave, West Hartford, CT 06117
Rm: ISET/Dana 410B
Phone: 860-768-5806
FAX: 860-768-4558
email: keball@hartford.edu