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unknown user
10-18-1992, 01:37 AM
Subject: Radiographic landmarks
.................................................. .......
Carolyn Small, Associate Professor Phone (613) 545-2581
Mechanical Engineering, Queen's University FAX 545-6489
Kingston, Ontario, Canada K7L 3N6 smallc@qucdn.queensu.ca
*** Forwarding note from SMALLC --QUCDN 10/18/92 11:16 ***
Subject: Radiographic landmarks

Sorry for the delay; term time y'know (us academics always have an excuse)

Re: your question about whether referencing surface markers to bone landmarks
is necessary - it all depends on what you're trying to do. In a strictly
clinical monitoring situation, say for gait analysis for screening or
evaluation of treatment progress, probably not, as you would be looking only
to discriminate between "normal" and "abnormal"patterns. On the other hand,
to tease out the motions of the skeletal structure from surface data, in a
3D analysis, we find the ability to express the positions of markers in a
frame of reference embedded in the underlying bone extremely useful, if not
absolutely necessary. And if one wishes to try to estimate musculotendinous
moment arms in various positions, it is required to know where the bones are,
not where the surface markers are.

The bad news is that bone positions are often difficult to locate accurately
in living subjects who naturally might object if we were to start implanting
lead balls in their bones. My application is the hand and wrist; these bones
are physically small and error propagation when you start taking the differ-
ences between adjacent landmarks is tragic. Over the past two years we have
done some work on estimating the precision and accuracy of locating bone
landmarks, imbedded markers (cadavers, natch) and surface markers on stereo-
pair X-rays, and comparing the results of kinematic analyses done on surface
data to those done from bone landmark data. We have three papers in the queue
on this subject, to appear "next available issues" in the Journal of Biomedi-
cal Engineering. And a couple more on the way. Our conclusions are that we
can identify surface markers much more accurately than many bone landmarks,
and that trying to identify bone landmarks in anything other than a neutral
position (for 3D reconstructions) can make you go blind. So for diagnostic
info we use surface marker data directly; for the "basic research" aspects
we reference to underlying bony structures.

I'm going to post this on BIOMCH-L as well as trying to send it to you on
the net; your given address doesn't match what my mailer thinks your address
is. Please acknowledge receipt of this message if you get it directly...
I'm curious about connections to other nets. Do all CompuServ types get
hooked into Internet automatically?

Hope this is of some help.
.................................................. .......
Carolyn Small, Associate Professor Phone (613) 545-2581
Mechanical Engineering, Queen's University FAX 545-6489
Kingston, Ontario, Canada K7L 3N6 smallc@qucdn.queensu.ca