Richard, Craig, Jeff and Colleagues:

Richard wrote:

<<I have recently been told that when casting a patient with ankle equinus
you
should cast them slightly pronated or the resulting device could have
negative
effects rather than be benificial.

Is this the case and if so why?>>

I would like to make a few comments regarding Richard's question and Jeff's
and Craig's answers.

As Craig mentioned, the theory used to explain why an orthosis would be made
"more pronated" in a patient with an equinus deformity of the ankle is that
this would allow the midfoot to flatten more so that the foot talus could
plantarflex on the forefoot during late midstance with either the foot not
being hurt by a relatively non-deformable orthosis or compress a more
deformable orthosis.

The problem with these ideas is that in many feet, taking a cast in the
pronated position using the standard neutral position suspension casting
technique will actually produce no decrease in the medial longitudinal arch
(MLA) height of the resultant negative cast.   I noted this first about 18
years ago when I took about 100 negative casts of  students of mine in
different subtalar joint (STJ) positions for a educational display on
negative casting errors (which is still on display at CCPM).  Therefore,
unless some other modification to the neutral suspension casting technique
is being used, taking a cast of the same foot in the STJ neutral position
and then in the STJ pronated position will not necessarily result in a
decrease in MLA height of the cast in the pronated cast compared to the
neutral cast in all feet.

In addition, I will not necessarily take the negative cast in a more
pronated position or try to lower the MLA height in the resultant orthosis
in a patient that has a significant equinus deformity.  Many times I will
simply have the lab leave the heel contact point thickness of the device a
little thicker than normal to reduce the tension on the Achilles tendon or I
may have the patient wear the orthosis in a shoe which has a higher heel
height differential than normal.  I wrote a more detailed discussion of this
mechanical concept in my chapter in Valmassy's text (Kirby, Kevin A.:
"Troubleshooting Functional Foot Orthoses", pp. 327-348, in Valmassy,
R.L.(editor), Clinical Biomechanics of the Lower Extremities, Mosby-Year
Book, St. Louis, 1996.)

One other interesting thing to ponder is that the spatial location of the
STJ axis has a very significant effect on how much the plane of the plantar
foot dorsiflexes in relation to the tibia when moving the loaded foot from
the STJ neutral position to the STJ maximally pronated position.  As the STJ
becomes more medially deviated, rotation of the loaded foot from STJ neutral
position to the STJ maximally pronated position will produce a larger change
in dorsiflexion of the plantar foot to the tibia than the same STJ motion
being done in a foot with a laterally deviated STJ axis.  This kinematic
effect of STJ spatial location on the three dimensional spatial movements of
the foot to the tibia seen in non-weightbearing examination on the kinetics
of the foot and lower extremity is **very significant**.   I touched on this
concept briefly in my paper which describes two methods to determine STJ
axis location (Kirby, Kevin A.:  "Methods for Determination of Positional
Variations in the Subtalar Joint Axis", Journal of the American Podiatric
Medical Association, 77: 228-234, May 1987.)

Finally, contrary to the technique which Jeff described, I measure ankle
joint dorsiflexion with the patient not assisting with their ankle joint
dorsiflexors.  The reason I perform the ankle joint dorsiflexion measurement
in this fashion is that it seems to be a much more consistent way of
applying ankle joint dorsiflexion moment than having the patient assist
which produces a variable magnitude of ankle joint dorsiflexion moment from
patient to patient.  It is a very common finding to have a heavily-muscled
man only have 1-2 degrees of ankle joint dorsiflexing with me pushing on the
plantar foot and then when they assist with their ankle joint dorsiflexors
they will have 10+ degrees of ankle joint dorsiflexion.  I acknowledge that
there are problems with both methods of measuring ankle joint dorsiflexion.
However, I feel I can compare differences from individual to individual much
more easily by not having the patient assist with their ankle joint
dorsiflexors during the test.

Cheers,

Kevin

********************************************
Kevin A. Kirby, DPM
Assistant Clinical Professor of Biomechanics
California College of Podiatric Medicine

Private Practice:
2626 N Street
Sacramento, CA  95816  USA

Voice: (916) 456-4768   Fax: (916) 451-6014
E-mail: [log in to unmask]
********************************************

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