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Richard Baker
07-19-2007, 02:03 PM
Dear all,



Mike Schwartz seems keen to know why I haven't commented so far on this
discussion and the answer is because I've little to add. In starting to look
in detail at kinematic fitting/functional calibration we conducted a review
of all current literature on hip, knee and ankle kinematics (which we still
haven't got around to publishing). The conclusion of this was that all
recent work supports a 3 degree of freedom ball joint at the healthy knee, a
two degree of freedom (Flexion about an axis fixed in the femur and internal
and external rotation about the long axis of the tibia) joint at the healthy
knee and a one degree of freedom joint for the healthy ankle joint
(talo-crural). The conclusion for the sub-talar joint however was that it
doesn't have a fixed axis of rotation (particularly in weightbearing). This
is kind of what you'd expected if you just look at an articulated foot
model. It is not a simple joint and I'm not sure that we should be expecting
a simple rotation to come out of it. If the movement doesn't actually occur
about this axis in weight bearing I'm not sure what the point of trying to
determine the location of the axis from non weight bearing calibration
exercises is. We've thus decided to ignore it in our kinematic fitting and
restrict our optimised model to pelvis, hip and tibia where we feel the
greatest benefits to using optimisation techniques are likely to be gained
at present.



In the CP work we do it is only really the joint angles that we need and I'm
assuming I will just hang a very simple hind-foot off the tibia by either a
ball joint or possibly a 6 DOF joint. Of course in the CP group we have the
addition complication that there is significant deformity and laxity of the
foot and quite often limitations in both active and passive ranges of
motion. I hope we can be excused for putting this one in the too hard box.



The one thing I would add in particular regard to the use of helical axes,
but which might also be relevant to more general approaches, is the
behaviour of joints which are modelled as having a restricted number of
degrees of freedom but in reality may have other components. I've done some
very simple work looking at the helical axes for synthetic knee data. If
there is a pure hinge joint then the helical axis clearly always lies along
it. However if a small degree of internal or external rotation about the
long axis of the tibia is added in (as in any screw home mechanism towards
full extension) then the helical axis moves a long way (between 30 and 90
degrees depending on relative magnitude of the two rotations and exactly how
you define helical axes) from the flexion axis (which may, coincidentally be
useful in locating a knee joint centre lying somewhere along the
flexion-extension axis using an approach such as Mike's). Has anyone done
any similar work at the hind-foot to see what happens if you add in movement
about axes other than the talo-crural and sub-talar axes but then assume
that movement only occurs about those two axes in performing the fitting
procedure? Or is this what Steve has done and I've been a bit slack in
picking up?



Richard



PS BJ Fregly is over in Melbourne at the moment and we were talking about
some of this correspondence together. He reminded me that there simply isn't
enough movement during gait to determine the sub-talar joint axis and if you
want to look at the sub-talar joint axis during weight bearing you are going
to need to plant the foot and do rather bizarre movement of the rest of the
body to get a good range of movement at the sub-talar joint. Is this what
people are doing when they try to determine a sub-talar joint during weight
bearing or are they trying to use that small range of movement that is
present during walking?







Richard Baker PhD CEng CSci

Director Gait CCRE/Gait Analysis Service manager

Murdoch Childrens Research Institute

Royal Childrens Hospital

Parkville 3052, Victoria, Australia



Tel (+613) 9345 5354, Fax (+613) 9345 5447