Response to Dr. Chip Southerland's Question  Thursday 5 November  1998

Dear Chip,

	Call me Harry. Thanks for your question and explanation 
of the background to your question which hopefully helps my response.  
Thanks also for carefully reading the handout which was part of my 
presentation on "Variables In the Construction of Functional Foot 
Orthoses" as part of the PFOLA seminar in California.  As you know, 
I replaced another speaker at late notice but stand on my statement 
that it is important to "pour the cast vertical".  Three important 
factors discussed in the lecture were that phasic control of the foot 
throughout the kinetic weight bearing stance phase of gait, from 
contact through midstance and into liftoff phases are essential for 
functional control.  Prior to casting for the orthotic device, 
measurements are made non-weight bearing on a part to part
basis to determine the neutral position of the subtalar joint axis, 
confirmed in stance with measurement of the vertical bisection of the 
calcaneus compared to the supporting surface and lower one third of 
the leg, as well as the influence of tibial varum or genu valgum on 
the resting calcaneal stance position.  

	The second factor is that the standard casting method is 
with the foot held in the neutral non-weight bearing position 
which captures non-weight bearing shape of the heel.  Therefore, 
in response to your concern "if the cast is poured perpendicular 
to the ground based on the rearfoot bisector and no additional 
inversion is placed into the pour, aren't we cupping
the rearfoot in the maximally pronated position?"  My response to that
is no.  The foot compensates in stance by pronating to the vertical 
calcaneal position. However, if the cast captures the non-weight 
bearing shape of the heel, then the heel cup of the orthosis can be 
canted, inverted or everted to stabilize the position of the heel 
bone by the prescribed amount.  Your calculations with regard to 
NCSP rearfoot position combining subtalar and tibial influence
on the heel bone position are 11 degrees inverted.  However, I 
believe it is not the heel cup that holds the heel in position 
but the posting under the device.  It is common to post two thirds of 
the N.P.  For example, a Kirby medial skive is ineffective during 
gait without a varus post underneath it.  In static stance the Kirby 
medial skive or a varus heelpost tilts the heel into an inverted 
position but during function, the Kirby skive or rearfoot post prevent 
eversion of the heel during the contact phase.  

	The third point to be made is that for phasic control 
throughout the gait cycle a functional orthotic device first 
controls motion of the heel, then controls motion and position 
of the forefoot, and finally influences position of the forefoot 
in liftoff by locking the midtarsal joint through the midstance 
phase.   This raises the question "What does a heel cup do?"  
Frankly, I do not think it controls motion of the heel 
(the outer skin is round in appearance and the weight bearing 
portions under the medial and lateral tuberosity create a platform).  
Therefore, it is not the heel cup that controls motion but the shape 
of the platform underneath the heel bone, specifically the medial and 
lateral calcaneal tuberosities, so a Kirby skive is more effective in 
controlling heel motion than a heel post.  My experience is that a 
Kirby skive improves function of an orthotic device during the 
contact phase of the dynamic foot but the Kirby skive in stance 
activities, such as those patients who work standing, such as grocery 
checkers,assembly line workers, barbers etc., the Kirby skive can be 
irritant and in stance, you are correct, that the medial skive would 
raise the medial side of the forefoot off the plate if the midtarsal
joint is locked against the supporting surface.  

	I see very little if any reason to ever pour a cast inverted 
and I am not comfortable with the explanation of the function of the 
Blake inverted orthotic device, and have not found it necessary to 
prescribe an inverted cast device which totally distorts the 
relationship between the orthosis and the foot morphology.  
The only reason to pour a cast inverted is that the lab would make 
a higher heel cup laterally before posting. However, if the 
laboratory looks at the doctor's prescription for specific heel 
posting (4, 6, 8 degrees, never more then 8 degrees varus posting) 
then they should press the orthosis over the cast deep 
enough to accommodate a varus post and a flat heel cup parallel 
to the supporting surface.  Chip, I enjoyed your questions and hope 
this explains the rationale for my statements.  If the heel cup is 
formed over the non-weight bearing shape of the foot and posted or 
skived in order to control, more specifically prevent compensation 
during the contact phase of gait, then the post will be effective, 
and it is appropriate to demand that the laboratory pour the cast 
with the bisection of the calcaneus in the vertical position.  

Sincerely, Harry 


-- 
Harry F. Hlavac, DPM
Hlavac Podiatry
100 Shoreline Highway, Suite 150
Building B
Mill Valley, CA  94941
Voice: (415) 331-4500 FAX: (415) 331-9035
Email: [log in to unmask]


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