Chris and Colleagues:
Kevin wrote:
<<"Nester et al's comment in their paper, in my opinion, is not quite
complete. There are instances where motion does occur in joints,
but not in the motion pattern of the normal joint axis for that joint.
This can certainly occur in when pathological forces act across the
STJ and/or MTJ axes. Therefore, even though their comment "axes
of rotation do not determine the motion at a joint, the motion
determines the axis" would be accurate in non-pathological
circumstances, abnormal motion can occur about no joint axis, if
pathology exists (i.e. ligamentous laxity, ligamentous rupture, etc)">>
Chris replied:
<<It is of course true that motion occurs in a joint that is not in the
motion pattern of the normal axis. However, in this instance all that
can be said is that the axis of rotation, when calculated from the
motion at such a joint, would differ from that seen in a joint which
was undergoing (more) "normal" motion. When there are
"pathological" forces acting across the joint and the consequent
joint motion is different than if forces across the joint were "non
pathological" then the axis of rotation for that joint would simply be
different. (because the axis follows from the joint motion)>>
Kevin continued:
<<Therefore, even though their comment "axes of rotation do not
determine the motion at a joint, the motion determines the axis"
would be accurate in non-pathological circumstances, abnormal
motion can occur about no joint axis, if pathology exists (i.e.
ligamentous laxity, ligamentous rupture, etc)">>
Chris replied:
<<This cannot be the case. If there is motion then we can have an
axis of rotation. There is no circumstance where motion takes
place and there is "no" axis of rotation. Quite simply, the axis will
be positioned and orientated differently, abnormally if you prefer,
when motion is abnormal/pathological compared to normal. This is
where the axis of rotation can be most clearly seen as an indicator
of abnormal function, since it will differentiate between the normal
function and abnormal function.>>
Kevin replies:
I disagree here. Motion **can** occur between two bones of the foot or, if
you want to be more general, between two objects, and have no axis of
rotation. This would be the case of a pure translation motion of one object
(or bone) to another object (or bone). Therefore, my comment above that "
abnormal motion can occur about no joint axis", I believe, is a valid
statement for the simple reason that the motion could be involve 100%
translation and 0% rotation. Where is the axis of rotation when no
rotational motion takes place???
Chris continues:
<<There are some important underlying principles that people need to
grasp. The axis of rotation is simply a parameters we use to
describe joint motion. Nothing more. It is a reflection of the motion,
and indicator of the joint kinematics, it has no prescriptive "power"
in the complex algorithum of factors that determinine joint function.
They don't exist, they are simply something mathematicians
created to help model motion. As Kevin correctly states later " it is
the interaction of forces acting external to the joint along with the
forces acting internally within the joint which determines the
motions seen at any joint". It is important to remember that in the
JAPMA paper we were not really trying to discuss in detail the
factors that determine joint motion and consequently the axis of
rotation, merely clarify what the axis is and is not, and provide a
context for our subsequent points regarding the MTJ.
You can have a axis of rotation for anything. Spin on your office
chair, and we could have an axis of rotation describing the motion
of the chair relative to your office, or you relative to the chair (not
very useful granted!). It is perhaps the phrasing of how we describe
joint motion which I think leads to confusion, we say "the joint
rotates around the axis.....", and this has some implicit suggestion
that the axis prescribes the joint motion, when it actually describes
it.>>
Kevin replies:
You make good points here, Chris. As a matter of fact, the paper which
Chris and coworkers recently had published in JAPMA on the midtarsal joint
"axes" is an excellent piece of work. I have already read it twice and
plan on going back and read it a third time in the near future (Nester, C.,
Findlow, A., Bowker, P.: Scientific approach to the axis of rotation of the
midtarsal joint. JAPMA, 91(2):68-73, 2001.) In fact, I liked it so much
that we will be having Chris come over to Miami in November to lecture at
the PFOLA meeting on the findings of his research (along with Benno Nigg,
Irene McClay and Mark Cornwall).
All I can say is: Good job Chris!!!!
Chris continues:
<<I'm sorry if I have missed other discussions regarding our recent
paper. I look out for "MTJ" in the subject box but haven't seen it.
Hope this helps, looking forward to hammering this out in 10 days
or so Kevin!>>
Kevin replies:
Hoping that some California sunshine and beautiful spring weather will not
be too much for you!!
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]
********************************************
-----------------------------------------------------------------
This message was distributed by the Podiatry JISCmail list server
to leave the Podiatry email list send a message containing the text
leave podiatry
to [log in to unmask]
Please visit http://www.jiscmail.ac.uk for any further information
-----------------------------------------------------------------
|
