I discussed the issues raised about shear stresses on the nipple while breastfeeding with my engineer
husband (degree in physics) and e-mailed the question to my daughter (degree in physics, works on a
linear accelerator).  Both of them were hampered by a limited knowledge of breast and oral anatomy,
but they did the best they could with the information I provided.

My daughter sent back a lengthy and very thoughtful e-mail.  It's attached here as a quote, complete
with pictures.

My husband says the problem is too complicated to model effectively because it involves communicating
systems (the nipple and the baby), both of which respond to the shape and activities of the other and
each of which is unique to that individual.  He thinks it would take a large team weeks to months to
design appropriate measurement instruments and then a second large team several more weeks to months
to study each mother-baby unit and create a working model of their interactions.  In short, it
probably wouldn't be terribly helpful in finding a practical solution to an individual breastfeeding
problem.

They both thought it was a very interesting exercise, however, and think that the original poster is
definitely onto something....


> I'm not exactly clear on what the breastfeeding difficulty is that is
> being discussed, but I can help to model the forces on a nipple given the
> described conditions.
>
> what I would have envisioned prior to hearing the theory about pressure
> causing nipple retraction would be one gelatinous object shaped like a
> cylindar attatched to a (slightly convex) sheet.  In that case, there is a
> force from the vacuum, and an equal and opposite force keeping the breast
> attatched to the body.  So that would give simple tension, lengthening the
> nipple, and (just as described for the cube under tension) making it
> thinner.
>
> Of course, if you add the idea that pressure on tissues behind the nipple
> can cause nipple retraction, then the shape of the nipple and presence of
> shear forces depends on the diameter of the piece of tissue connecting the
> tissues on which the pressure is exerted and the tip of the nipple.  I
> envision a piston within the breast:
>
>
>     _______ tip of nipple
>     |  |  |
>     |  |  |
> ____|  |  |_____skin
>        |
>   _____|_____ piston
>
> if the rod between the nipple tip and surface of the piston is narrow, the
> nipple will probably be distorted by shear forces and end up being shaped
> like this (in cross section):
>
>
>      __   __
>      | \_/ |
>      |  |  |
> _____|  |  |____skin
>         |narrow piston rod
>     ____|____
>
>
> with the vacuum forces pulling the perimiter fo the nipple up more than
> the center (which is being pulled down by the pressure force on the
> piston).  However, if the rod between the piston and the nipple tip is the
> entire width of the nipple, it would end up looking more like this:
>
>
>
>
>
>     ______
> __  ||  ||  ___skin
>   \ ||  || /
>    \||  ||/
>      |  |  broad piston rod
>    __|__|__
>
> in this case the shear stresses are experienced by the tissue surrounding
> the nipple, rather than the nipple itself.  There may be problems caused
> by breast tissue partially covering the nipple, but there isn't
> defformation of the nipple itself.
>
> I don't know enough about the actual physical structure of the breast and
> nipple to know which of these models is best for envisioning what happens
> to actual breasts, but it is true that if there is a pressure force that
> can push the nipple away from the baby, this force is in oppositoun to the
> vacuum force.  There may or may not be shear forces within the nipple
> depending on how the nipple and piece of tissue that is being pressed
> against are connected.  Given either of the pictures above, I can see
> where there may be obsruction of milk flow due to the defformation of the
> breast/nipple.  If this is an appropriate model, then the important
> parameter is the proportion of the vacuum force to the pressure on the
> piston.  If the baby sucks hard enough to counterbalance the
> piston mpressure and thereby  make the nipple the optimum shape for milk
> flow, then there is no problem.  The problem would arize if the baby
> either pushes too hard against the piston, or doesn't suck hard enough to
> counterbalance the pressure that it is exerting on the piston.
>
>

Cheri Van Hoover, CNM, CLE
San Francisco, CA

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