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Dr. Michael S. Sacks
09-28-1995, 12:12 AM
Dear all,

Some time back I posted an open question regarding
soft tissue strains measurements. I received some
interest on this, and would like to get some more
action on this topic before summarizing things.

My view on this issue stems from my experience in
the testing of soft tissues. Way back when I was'
an undergraduate, we used to test tendons directly
clamped to metal grips (i.e. not bone-tendon-bone,
as is correctly done today). We would measure
strain using markers on the surface of the tissue
near the middle of the tendon, and ignore all
other strain information. The assumptions of this
approach are many, and I think the following pretty
much sums them up:

1) surface strains represent the exact strains throughout
the tissue substance or at least the average strain (thanks
to J.K. Suh).

2) The strain field is homogeneous within the tissue area
demarkated by the video markers.

3) Local fiber-to-fiber shearing is ignored.

4) Gripping effects (this can include clamping, or stress
concentrations from the use of a suture in the case of biaxial
testing, etc.) are only local, and do not propagate through
the tissue (i.e. St. Venant's principle).

5) The stress field within the marked region is uniform.

For those tissues specimens in which the relative tissue thickness
is small in comparison to the other tissue dimensions, and where
the marked region is small in comparision to the overall tissue
specimen size, these assumptions appear to be reasonable. However,
there is evidence that this may not allways be the case. For
example, if tissue structure is very complex (e.g. multiple
layers of tissues with a wide range of orientations), the strain
field will certainly vary through the thickness of the tissue. Finally,
the degree of cross-linking between fiber layers will influence the
"cross-talk" between the surface layers (where the strain is measured)
with the interior substance.

The real question is: how bad is the problem. One example I can
think of is the ACL, which has a very complex architecture. David
Butler some time back showed a non-uniform strain field along the
ligament. Clearly, surface strains for this tissue may not be
always be representative of the complete strain field. This information
may be of importance to ligament grafts, etc.


I would really appreciate input from all who are interested (or
are aggravated) by this problem.

-Michael Sacks
U. Miami.