We have been having some discussions lately regarding the
anatomical ankle joint axis (talo-crural) versus the mechanical
axis that we incorporate into our ankle foot orthoses.
Traditionally, the mechanical ankle joint axis has been
determined by using the distal tip of the medial malleolus as a
reference, and place the lateral joint axis at the same height.
The anterior/posterior location of the joints are determined by
using the distal tip of the medial maleolus again as a starting
point, and then place the lateral joint in a position which sets
the foot in about 5-7 degrees of external rotation.
Our concerns have been in the area of mechanical alignment
versus skeletal alignment of the joints. We have been
experimenting with placing the joint axes in a more anatomical
position by moving the lateral joint posterior to be more
congruent with the lateral malleolus. Also, we have been tilting
the joint axis a few degrees to match the angle of the medial
versus the lateral malleolus. The end result is an orthosis
which looks and acts quite differently from the traditionally
aligned orthosis, but mimics the actual motion of the foot more

Discussion points:

How critical is mechanical joint alignment on an orthosis?

Since tibial progression over the foot is made up of more than
just talo-crural motion, do you think that skeletal alignment of
the mechanical ankle joints is significant.

Any info that could be used in the determination of more
appropriate mechanical joint axes, using skeletal landmarks as
reference points, would be helpful.

Thank you very much.

Paul E. Prusakowski, C.O.
Shands Hospital at the University of Florida
Orthotics and Prosthetics Dept.