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rsahni25
07-23-2006, 08:02 PM
Dear Martin Twiste

I am glad to read your response to Chi Wei TAN of NUS
however when it comes to actual practical evaluation
it all depends on background experience.

How would you describe theoretical Prosthetic foot alignment,
when the the subject a unilateral bk amputee has
a severe varus stump.

Best Regards
Mr.Rabinder Sahni
Prosthetics R&D,Designer lower limbs,Self user
INDIA

Martin Twiste wrote:
Dear Chi

If you are looking for a relationship between socket angle and foot angle,
then you need to distinguish between the following two aspects:

1) Changes in socket angle that occur during gait due to motion of the
residuum (i.e. the changes from backward to forward leaning of the residuum
/ socket from heel strike to push off, respectively) - in this case the
angle between the socket and shank (often also referred to as pylon) remains
the same.

2) Changes in socket angle that occur during alignment changes (i.e. the
changes from backward to forward leaning of the residuum / socket from
extension to flexion of the socket, respectively) - in this case the angle
between the socket and pylon changes.


When you refer to any changes in socket angle (may that be due to aspect 1)
or aspect 2)), then it is import to distinguish between:

a) Changes in socket angle and the resultant foot angle (without sufficient
force applied through the pylon to passivley move the foot).

b) Changes in socket angle and the compensatory changes in foot angle that
are required in order to leave the foot flat on the ground.


In my explanations that follow, I will assume that we are dealing with
situation a) (from which you can obviously derive situation b)). Also, I
just want to reinforce what certain angle changes mean: backward leaning of
the socket / residuum / pylon implies that their proximal end is posterior
to their distal end, whereby forward leaning of the socket / residuum /
pylon implies that their proximal end is anterior to their distal end.


Regarding aspect 1):

If the residuum / socket is leaning backward at heel strike, then the foot
would appear to be dorsiflexed. The reason why the foot would only appear to
be dorsiflexed is because its angle relative to the pylon has not changed
(unless sufficient force is applied through the pylon to passivley
plantarflex the foot). The toe part of the foot is up in the air (and heel
on the ground) simply due to the backward leaning pylon.

Conversely, if the residuum / socket is leaning forward at push off, then
the foot would appear to be plantarflexed. The reason why the foot would
only appear to be plantarflexed is because its angle relative to the pylon
has not changed (unless sufficient force is applied through the pylon to
passivley dorsiflex the foot). The toe part of the foot is on the ground
(and heel and in the air) simply due to the forward leaning pylon.


Regarding aspect 2):

If the residuum / socket is leaning backward or forward due to extension or
flexion of the socket relative to the pylon, respectively, then the main
reason for making adjustments in this way is because of the natural angle of
the residuum relative to the thigh. Extending or flexing the socket to
maintain the natural angle of the residuum relative to the thigh is aimed at
keeping the pylon vertical during standing and during mid stance phase for
amputees with a hyperextended knee or flexion contractures, respetively. As
the pylon is supposed to stay vertical during standing and during mid stance
phase, the foot would therefore be in a plantigrade position (no
dorsiflexion or plantarflexion).

However, if the socket is, say, extended relative to the pylon and the
socket is held straight relative to (or in line with) the thigh, then the
pylon would be leaning forward, and the foot would appear to be
plantarflexed. Like in the explanation regarding aspect 1), the reason why
the foot would only appear to be plantarflexed is because its angle relative
to the pylon has not changed (unless sufficient force is applied through the
pylon to passivley dorsiflex the foot), but in order to achieve foot flat,
the foot would need to be dorsiflexed.

Conversely, if the socket is, say, flexed relative to the pylon and the
socket is held straight relative to (or in line with) the thigh, then the
pylon would be leaning backward, and the foot would appear to be
dorsiflexed. Like in the explanation regarding aspect 1), the reason why the
foot would only appear to be dorsiflexed is because its angle relative to
the pylon has not changed (unless sufficient force is applied through the
pylon to passivley plantarflex the foot), but in order to achieve foot flat,
the foot would need to be plantarflexed.


Finally, while the socket is held straight relative to (or in line with) the
thigh, as extension and flexion of the socket relative to the pylon makes
the pylon lean backward and forward, respectively, this therefore not only
changes the angle of the foot relative to the ground, but it also changes
its position on the ground, and therefore relative to the centre of mass
(COM) of the amputee. Positional changes of the foot consequently change the
location of the centre of pressure, which, in turn, affects the ground
reaction forces (GRFs) as these also change position relative to the COM. In
addition, positional changes of the GRFs can affect the stability of lower
limb joints, and in particular the knee joint.

Conclusively, angle changes of the socket should not be considered an
equivalent to angle changes of the foot.

I hope this helps.

Cheers.

Martin

Dr Martin Twiste BSc, PgCert, MSc, PhD
Lecturer
Directorate of Porsthetics & Orthotics
University of Salford
Manchester M5 4WT, England
http://www.healthcare.salford.ac.uk/prosthetic/
http://www.healthcare.salford.ac.uk/crhpr/



----- Original Message -----
From: "Tan Chi Wei"
To:
Sent: Thursday, July 20, 2006 4:04 PM
Subject: [BIOMCH-L] Prosthetic foot alignment


Hi All :



I am a student and am new here. I learnt about Biomch-L after one of my
professors sent me an email regarding a topic of interest.



I am writing in today to find out how many people would agree with me in
the following:



In the alignment of the prosthetic foot, dorsiflexion was the most
important alignment change (ref: Fridman, Ona and Isakov; P&O
International 2003, 27. 17-22)

Would you agree if I say extension of a BK socket is actually equivalent
to dorsiflexion of the prosthetic foot and that flexion of the BK socket
is equal to plantar flexion of the prosthetic foot? I say this because
during mid stance, the shank is to be perpendicular ( or close to) to
the floor. Therefore, if a BK socket is flexed, shank perpendicular to
the floor, then the foot will have to be plantar flexed. Vice versa.



Open for comments.



Thanks

Chi Wei TAN


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