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Steve Piazza
07-25-2007, 04:49 AM
Thanks to Richard Baker for making a good point about the potential
ill-effects of deviations from revolute behavior when fitting
revolutes. It's important to consider such secondary rotations and
also to consider whether those rotations accompany the "primary"
rotation in an obligatory fashion. If they do, as Richard suggests
is the case for screw-home at the knee, then you will see repeatable
helical axes that may fall far from the flexion axis.

Regarding Richard's comments about deviations from hinge-like
behavior in the subtalar joint during weightbearing, we'd agree that
such deviations occur, but there is also some pretty convincing
evidence that the subtalar joint is a fairly good hinge in many
circumstances and that it is the talocrural joint that deviates more
from hinge-like motion. van Langelaan [1] applied
supination-pronation motions to loaded cadaver specimens and found
helical axes throughout the movement that looked like a
tightly-clustered bundle of nearly-parallel sticks. In our own lab,
we've seen similar results [2] during varied and loaded movements
applied in cadavers, with the subtalar axes differing from the "mean"
axis (taken in a least-squares sense) less than did the talocrural
axes; Kevin Ball's recent post indicates that he has observed
comparable behavior in cadaver specimens. Arne Lundberg applied some
fairly extreme input motions in vivo (60 degrees of PF/DF ROM, 30
degrees of internal/external rotation ROM, 40 degrees of
supination/pronation ROM) under full weightbearing and still found
talocalcaneal axes that were more hingelike than the talocrural axes
[3,4]. Lundberg's talocrural axes were often close to the long axis
of the tibia when internal/external rotations were applied; Kevin
Ball has just commented on similar finding in Chris Nester's work.

Steve Piazza
Greg Lewis
Penn State University

References cited:
[1] van Langelaan, E.J., 1983, "A kinematical analysis of the tarsal
joints. An X-ray photogrammetric study," Acta Orthop Scand Suppl,
204, pp. 1-269.
[2] Lewis, G.S., Sommer, H.J., 3rd, and Piazza, S.J., 2006, "In vitro
assessment of a motion-based optimization method for locating the
talocrural and subtalar joint axes," J Biomech Eng, 128, pp. 596-603.
[3] Lundberg, A. and Svensson, O.K., 1993, "The axes of rotation of
the talocalcaneal and talonavicular joints," Foot, 3, pp. 65-70.
[4] Lundberg, A., et al., 1989, "The axis of rotation of the ankle
joint," J Bone Joint Surg Br, 71, pp. 94-9.


>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?


----
Stephen Piazza, PhD
Associate Professor
Departments of Kinesiology, Mechanical Engineering,
and Orthopaedics & Rehabilitation
29 Recreation Building
The Pennsylvania State University
University Park, PA 16802
steve-piazza@psu.edu