Pressure switch for prosthesis?

[hamonrye]

I am a prosthetist and as such I design and fabricate artificial limbs for amputees. I have noticed a recurring problem with dysvascular diabetics in that they often have no protective sensation. When their prosthesis is fitting correctly the pressures within the socket (the part their "stump" goes in) are evenly distributed. When for whatever reason (usually swelling resulting in a volume increase) it is not fitting properly the pressures become very high at the bottom of the socket and can cause the skin to break down. This particular patient population has no protective sensation (pain) to tell them to take the prosthesis off and address the problem. I was wondering how difficult it would be to build a device to warn them of high pressures in the end of the socket by a warning light or a vibration or sound.

If there was a pressure switch made that I could put in the end of the socket and wire to a light and battery they could see the light come on and know that there was a problem. I was wondering if anyone here has any ideas on how to build such a device that they would like to share.

Thanks

 

[IRstuff]

Maybe you can use a thin-film strain gage?

TTFN

 

[sreid]

Not exactly what you are looking for but this is interesting.

http://www.oandp.org/jpo/library/1990_02_130.asp

 

[Mike2468]

Does higher pressure = more frictional heat = skin breakdown? Implanting a few small surface mount thermocouples or RTDs in the artificial limb would be very easy to do.

 

[TurboXS]

I vaguely remember years ago in a robotics course that they were experimenting with a pad that was conductive. As the pressure on the pad increased the resistance of the pad decreased.

Perhaps something like this;

http://www.rehabpub.com/features/112003/4.asp

or this;

http://www.iop.org/EJ/abstract/0960-1317/10/2/320

may help.

regards
Pete
(TurboXS)

 

[itsmoked]

hamonrye: Are you talking about the circumferential forces?

The arm growing in circumference?

This would be the easiest force to monitor.

 

[hamonrye]

IRstuff:Maybe you can use a thin-film strain gage?

Well I did find this and it looks like it may be ideal, if I knew how it worked. Looks like it may be pretty expensive too.

http://www.smdsensors.com/detail_pgs/s400_detail.htm

This brings up another problem which is cost. Insurance companies and Medicare won't pay for this device since it won't have its own procedure code and I'm sure the patients won't pay for it either. Since leaves either I or the patient will have to pay for it I would like for the design to utilize the mose inexpensive components available. I have no plans to profit from or pursue a patent rights on this device, I just want something to help my patients.

Mike2468oes higher pressure = more frictional heat = skin breakdown? Implanting a few small surface mount thermocouples or RTDs in the artificial limb would be very easy to do.

To be honest I really don't know if this would work.

TurboXS:I vaguely remember years ago in a robotics course that they were experimenting with a pad that was conductive. As the pressure on the pad increased the resistance of the pad decreased.

Yes I have seen that resistance film before and it is pretty cool. It would be wonderful to get that kind of detail regarding pressure distribution within the prosthetic socket but I'm not sure it is practical from a cost standpoint. It would more than likely require some very costly custom work to develop a film that could map the inside of a socket. I was looking for something much more primitive.

itsmoked: Are you talking about the circumferential forces?
The arm growing in circumference?
This would be the easiest force to monitor.

After reading my post I noticed I didn't do a very good job describing what I want and what it should monitor. I mentioned that I wanted something to alarm an insensate prosthesis wearer of high distal socket pressures. What I didn't mention is how the edema and resulting limb volume change leads to the high pressures.

I'll give an example here. Bob is an elderly dysvascular diabetic with neuropathy which is the cause of his below the knee amputation. He gets up one the morning and grabs his prosthetic leg and begins to put it on. The fit of prosthetic sockets is adjusted by adding or removing combinations of thin socks which act as "shims". When applied they tighten the socket evenly over the entire surface within the socket. Bob knows that he usually wears 5 socks, but this morning he can't get his limb down into the prosthesis with all 5 socks(it is swollen)so he tries it without socks. It slips right in and feels comfortable, so Bob gets up and starts his day. As he walks around, his body weight shifting onto and off of the prostheses, generates enough pressure to begin to force the fluid that accumulated in his leg overnight up and out of the socket reducing his volume. Before long, Bob has slipped down into the socket as a result of the volume loss and as a result of this he is now bearing weight on the cut ends of the bones of his leg. The problem is that he doesn't feel any discomfort and may continue to walk on the ill fitting limb until he gets bruising of worse, skin breakdown.

The socket fit must remain tight in order to keep the patient's body weight distributed evenly in a Total Surface Bearing prosthetic socket like Bob's. When the volume of the limb decreases the pressures at the bottom of the socket will skyrocket as the limb slips further and further into it. What I would like to build is a simple, low profile device that could make the patient aware that he/she needs to add a sock to accomodate for this volume loss and prevent injury. Sorry if this doesn't make much sense I'm horrible at expaining this sort of thing.

Thanks everyone for your responses.

 

[IRstuff]

Actually, I was referring to something more like this:

http://www.omega.com/toc_asp/frameset.html?book=Pressure&file=STRAIN_GAGES

The basic sensor is a foil-resistor that change resistance when strained. The cheapest one goes for about $5/ea.

The pressure mats work along a similar principle. But cost considerations, might be restricted to use during office visits.

You might need to get both. The mat would be used to characterize the pressure patterns over time on some subjects, which would then allow you to determine the best location for mounted the foil gauge as well as determining how to interpret the time history of the measurements.

TTFN

 

[itsmoked]

hamonrye That! Was a nice description.

The guys have mentioned some good ideas.

Can you give a number for the distance from the stump to the base surface in the prosthesis?

How thick could a sensor be? If the distance is enough a pressure pad SWITCH that essentially declares your're in too deep would be the easiest to deal with electrically. Pressure type sensors will require more circuitry and power consumption then a switch.

What about the very thin pressure sensitive matting used for intrusion detection on floors and under carpets. I believe they are on/off switches.

 

[nbucska]

Is the object is to prevent the end of the stump from
to touching the bottom of the prothesis [P] or to limit the pressure ? Would a flat cap at the bottom of the
P. with a stiff spring and microswitch be good enough?
( sw--battery -- buzzer or LED ?)


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[itsmoked]

Yup, that's where I was going. Switch, wire, battery, buzzer/LED.

 

[nbucska]

You could have a "cup" ( a convex inpression/mold of the end of the stump) mounted with e.g. rubber srips -- acting as springs -- magnify the motion with a lever, etc..

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[felixc]

The cup can be molded as per the limb, and the spring-switch could be placed under the cup.

 

[MikeHalloran]

The explanation was excellent. The problem is a extremely difficult, because the limb can change shape.

The proximal cause of damage is pressure on the end of the stump, so that's the thing to sense. I'm thinking of putting a bag of inert incompressible fluid into the socket. Think 'breast implant' or something similar.

The bag would be equipped with a pressure switch wired to a small alarm sounder, powered by a 9V battery. The bag also needs to have a septum so that a needle can be inserted to measure the pressure and to add or remove fluid.

When the limb is fitted, the bag is filled with enough fluid to give the ideal pressure, whatever that is, or is it's filled enough to trip the alarm and then partially drained. When the limb shrinks or the patient doesn't apply enough socks, the limb projects farther into the socket, and trips the alarm.

Upon reflection, it could probably be done with a permanently sealed bag, by inserting shims under the bag to adjust its relative position in the socket, and sensing pressure by using the bag as a diaphragm, i.e. a ball-ended microswitch plunger surrounded by a ring, both mechanically held against the bag's surface.

Okay, there are a few details to iron out..






Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[hamonrye]

Very good ideas guys! I think you are right on that a pressure switch is in fact what I need and not a sensor. A sensor would provide useful feedback during the initial fitting and testing process but in the end product a switch would be ideal.

Just to clarify one thing I saw mentioned above. There should not be any space between the distal end of the patient's limb and the bottom of the socket. The limb should be in total contact over the entire surface of the socket. A well fit limb will also exert uniform pressure (which varies depending on the wearer's weight and surface area of the limb)over the entire surface of the interior of the socket. This is what makes it so tricky a task. The ideal pressure exerted on the interior of the socket will be different with each person.

Whatever I put into the socket needs to be incorprated into the socket wall itself so as not to intrude into the space the limb is to occupy. Both the distal cup and fluid mass proposals would work well for this reason. It would be much harder to fabricate a custom shaped cup for every amputee than to just make a provision in the socket shape for something inserted in the bottom. A pocket in the bottom equal to the mass of the sensor object would work well. It is for this reason that at this time I think the "fluid mass" proposal has the most promise. Both proposals are excellent ideas and both would work but I think the fluid mass plan would be the easiest to implement. Primarly due to the fact that it would conform to the shape of the end of the limb.

What sort of switch would you use for such a thing? I'm going to try and make some drawings of how I see some of these ideas being implemented. When I get it done I will post a link to it here and let you guys critique it.

Thanks a bunch guys, I can't believe how many good ideas you have come up with. This place is home to some really smart folks!

 

[nbucska]

I would use the MICROSWITCH (tm) -- the same cheap room-thermostats use with a 1"x2" bimetal strip. It switches
with a very small motion.

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[itsmoked]

hamonrye.. Thanks for the feed back. Your further information does however, give me pause. If the limb always does contact the bottom of the socket I retract my "don't use a sensor" statement.

Why? Because I can well imagine the dynamics that this interface will be subjected too. Brief pressure spikes on every step, negotiating steps or curbs. With just a *dumb* switch you will get lots of false alarms beeps, etc., etc.
False alarms rapidly result in user avoidance of a product!

This means you will need logic with delays... This means a microprocessor. If you need a processor then you don't need liquid! You don't need a liquid/switch interface. You don't need leaks!! All you need is a very thin resistive strain gauge dropped into the socket. No liquids. No leaks. No strange manufacturing, by people who will be interested only if you agree to buy a zillion.

The micro makes a whole lot of things possible besides reducing the price of the *interface*. It allows you to set limits customer to customer. It allows you to set time-to-alarm periods to prevent false alarms. It allows you to do zeroing. It allows you to do logging! Check for user quality and attention. It also allows many different signalling possibilities; beeps; lights; vibrations; severity escalation; urgency, even wireless if required.

A processor also allows low battery warnings unavailable by a switch only system.

In my opinion, with your latest info, a *sensor* that is monitored with a micro is the correct path to take.

 

[IRstuff]

I'd go so far as to say that a pressure mat is still the preferred solution, since there's probably no way to predict where any individual's pressure points are going to be or whether the pressure points are even stationary.

A small PIC micro with a couple of 16:1 analog muxes could monitor a 16x16 sensor mat.

TTFN

 

[hamonrye]

That sounds great, but I lack the required knowledge of microprocessors and associated sensors to build such a device here in my shop. I was hoping for a simple easy to build solution, but I guess that is not the answer when it comes to addressing such a complex problem.

I found an old article that describes a sensor system used on a different part of the prosthesis. (on the bottom of the foot) It was something that never took off, but was an interesting idea regardless. It mentions it was done with grant money. Maybe one of you electronic gurus could get a grant to tackle the distal sensor project.

http://www.wheelchairnet.org/WCN_ProdServ/Docs/TeamRehab/RR_95/9511art5.PDF

 

[Beggar]

If you're looking to develop something custom but you don't have the requisite knowledge, go to your local university. Almost certainly you could find a grad student who'd do the work for free as part of their thesis or project.

I'd also contact some of the institutions that have both an engineering school and a medical school (in California, I'd go to Stanford, UCLA, UC Davis, or UC San Diego).

You could also talk to faculty at a place like MIT or CalTech - they've got lots of very bright students who like doing "cool" stuff like that.

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