Prosthetic Knee Oil Change

Intro: Prosthetic Knee Oil Change

This Instructable is about how I did an oil change on a Total Knee 2100.

STEP 1: What Needs to Be Done, and How the Knee Works.

New Prosthetic knees are very expensive, but used ones can be purchased cheaply. This is because the prosthetic device manufactures won't maintain, service, or repair their products unless you are the original purchaser of that product, and can prove it.

I am an above the knee amputee, but can not afford a new prosthetic knee, so I got a used "Total Knee 2100" from Ebay. It made squishy noises, like there was air in the hydraulic system.

I was unable to get information from the manufacturer regarding oil type, quantity, or the proper procedure. I was forced then, to change the oil myself, with information I learned myself.

All the information in this Instructable is from my experience, and from my thinking. Nothing is from the manufacturer, and nothing is guaranteed to be the way it should be done.

Inside the top of the knee is a piston. As the knee changes angle, the piston pushes fluid through different variable orifices to add resistance to the motion. The Total knee 2100 has 3 adjustments.

I downloaded the Ossur patent applications, to see how the insides looked and worked.

STEP 2: Tools Needed

Small "spanner" head screw driver. I didn't have one, so I used a cutting disc on a dremel tool , and cut a notch in a jeweler screwdriver.

Medical syringe 20 ml is ideal, but any size will work. I used a 60ml and a 5ml.

Aquarium piping.

Something small and soft that will fit tightly into the filler hole and seal. I used a fitting from a garden irrigation system.

STEP 3: Choosing the Proper Oil.

Viscosities of oils are measured in cSt (centi-stokes). I got about 200ml of inert Vacuum Pump oil that had been mixed to a viscosity of 21cSt, and about 200ml that was 10cSt. Oil is manufactured in two types. thick, and thin. The factory then mixes the oils to make the viscosity you wish.

I just got some of this oil from a hobby shop. It is 100% silicone oil used in the shock absorbers in Radio Controlled cars. (Thank you very much, jwalker32)
It comes in many different viscosities from 200cSt to 850cSt. I bought some 200cSt. and some 450cSt

The 450cSt oil turned out to almost perfect.

STEP 4: Front View

On the front is a spanner head screw that removes the filter. The big black plug screw allows access to the piston top.

STEP 5: Left Side

On the left side is an adjustment screw labeled "C". This is the extension resistance.

STEP 6: Right Side

On the right side are two adjustments. The forward most one is for flexion resistance. The one behind it is for limiting heel rise.

The small spanner head screw at the rear on the right side, is the oil filler hole.

STEP 7: Remove Filler Screw.

Remove the oil filler screw on the right side rear.

STEP 8: Remove the Oil Filter

STEP 9: Remove Black Plug.

Remove the big black plug, and you can look down inside the cylinder and see the top of the piston. I used a coin as a screwdriver.

STEP 10: Remove All the Old Oil.

Now move the knee through it's full range of motion. As you do this, you will have oil squirt out of all three holes. This was old dirty oil. Very messy.

I was able to get about 6ml of oil out. It was very black.

I guessed the viscosity to be about 21cSt.

An oil expert looked at my old oil and guessed that it was closer to 15cSt. I mixed 4ml of 10cSt and 4ml of 21cSt to make some 16cSt (-ish).

STEP 11: Adding New Oil

Next was to screw back in the oil filter.

I then filled the cylinder with as much 16cSt oil as it would hold, the put the black cap on tight.

I used a garden irrigation fitting, to screw into the oil fill hole, then put on some aquarium piping on the other end.

I put oil into a syringe, and plugged the syringe into the piping. Then started moving the knee through its full range of motion.

The 16cSt oil was way to thin, and offered no resistance. I tried the 21cSt oil with the same result.

I then put in 200cSt oil. This gave a good working knee, but only at the maximum adjustments possible.

Next was the 450cSt oil. That was close to perfect.

STEP 12: Removing the Air.

After about 7 ml of oil was inside the knee, I would let the knee set for an hour, then move it again through it's full range. At this time many small bubbles of air would escape into the syringe.

In total I put in 9.8ml of oil.

I did this for about 6 hours until there were no more bubbles, and the knee no longer produced any squishy sound when moved.

I screwed in the filler screw.

The knee was now all sealed and not leaking.

STEP 13: Removing the Last of the Air With Vacuum.

I was sure I had removed all the air from the oil, but I had not yet removed air from the system. AIr can get trapped in the many nooks and drilled passages.

Once I had a hydraulic knee that had air in the system, and made squishy noises. I flew overseas with the knee in checked baggage, and on landing the air was gone. Maybe the reduced pressure from the altitude sucked the air out.

This doesn't make sense to me as the knee should be air tight. Maybe air can get past the o'rings in the bearings for the main shaft. If this is the case, then putting the whole knee in a vacuum would remove air from the system.

I cut up a 1 liter plastic coke bottle, and put the knee in the bottle, then put that into a vacuum bag used to flatten clothes for storage. I used my vacuum cleaner to suck out as much air as possible.

After 1 day of vacuum, there was no change in the movement of the knee.

Next I removed the filler plug, and screwed in my garden fitting and put on the syringe without the plunger in it. Then I put all of that into the vacuum bag. Now the vacuum should be applied to the entire fluid and knee. After 4 hours, there were no bubbles. I removed everything from the bag and bottle, and reinserted the filler plug.

If I ever do this again, I will not bother with the vacuum at all. No bubbles were released during any of the vacuum proceedures.

STEP 14: A Working Knee :-)

The knee moves smooth and quiet. I still think that there is a bit of air in the system. I also think that the knee must have an air chamber in it. The manual says to never lay the knee on its side. I think that laying it on it's side must allow air to be, where air shouldn't be.

I have the extension and flexion resistances set to the factory default values. maybe some more fine adjustments will be necessary, but the knee is very close to factory resistances. I put the knee on my leg, and it works. I walked on it all afternoon.

Thanks for all the comments and help.

Comments

[m2410s]

hello dear sir and thanks a lot for your generosity in sharing your useful and extinct experience with others.
I encountered with damaged one of T.K.2100 model that the oil has been leaked from its right bearing so that whole of the system has been worked dry. and just with its extension assist spring.

[atma.atma]

I wish to change the oil of my Ossur total knee 2100 which is about 5 years old. Some oil has leaked out of the knee over time. The knee is still functional thou. Advised to change the entire knee by the experts as they are not able to service it.
I am in the process of getting the tools and oil. I will try the to use the 450cSt Silicon oil as suggested/

What quantity of 450cSt Silicon oil will be enough for the replacement?

[kgwedi]

I wish you good luck on fixing your 2100.
Before opening it up, I would check very well, and be sure where the leak is coming from exactly. Maybe clean real good with water and paper towels, then baby powder all over? If leak is coming from one of the shafts, then it is beyond my knowledge. Most probable leak site will be from one of the adjustment screws.
I would use RC model car shock absorber silicon oil of 150 CST viscosity. I have rebuilt another 2100 after doing this instructable, and that worked perfect for me. In the real world, there is very little difference between 150 CST and 450 CST as far as the knee goes. I walked on a knee with 100 CST, and I walked on a knee with 450 CST. I will use 150 in the future.
FYI: I have also overhauled 2 College Park K2 hydraulic ankles. I ended up using 150 CST in those also.
The last 2100 I did, I filled a 10 ml syringe with 150 CST oil and almost used it all. Maybe 9.5 ml. It depends on how much old oil you get out, and how full the air chamber is on refilling.

[atma.atma]

Thank you for the pointers, much appreciated. I will find out where exactly the oil is leaking before proceeding. I think, using powder is a great idea to pinpoint the leak. I dont have knowledge about Silicon Oil CST. I will buy 30ml-60ml 150CST silicon oil.

The oil leak started after the knee got soaked with water about 15months ago. Then I sprayed silicon WD40 on the knee bearings. Not sure if that could have contributed to the problem.

[kgwedi]

I don't know why water should cause a leak. Maybe sea water with sand in it?
The bearings are garden variety roller bearings. Available at just about any bearing store. I used normal muliti-purpose grease from Home Depot. You will need a grease gun, and 2 special grease nipples (fittings) for the grease gun. I got the fittings from an industrial supply store. I showed them the knee, and they knew exactly what grease nipples were needed.
Just pump the grease in to get all that WD-40 out, and then clean up the mess. I think I way over greased mine. Mine continued to leak grease for weeks from the bearings. I got grease all over anything my knee touched.
The reason a thicker or thinner grease does not matter to the swing rates, is because you adjust those anyway. So a thicker grease can be countered by lowering the extension or flexion resistance.

[atma.atma]

Thank you on the details of greasing. I will get bearing greased at a garage with a grease gun.

My main issue remains of oil leaking from the knee.

The leak earlier was due to hardening of the o rings at the cap and filter screw. Not due to damage with water as I have thought.

About 8 weeks ago I topped up the oil but it leaked out again. The Teflon tape I used to fasten on the screw heads did not help to make a perfect seal. I just obtained the right o rings and topped up the oil again.

Unfortunately, the oil leaked again. This time I observed the oil came out of a breather hole on top of the knee. The plastic cap and the filter screw seals are good. Could it be that I have topped up too much oil into the chamber?

The oil coming out of the tiny hole is it normal, when the knee is flex repeatedly? In your photo in step 11, I also see a hole at the top of knee. This hole is made by design. Not sure of its functionality.

Any pointers how to stop the oil leak from the top of the knee?

Looking forward for some advice.

Thank you.

[kgwedi]

I have one knee off e-bay that leaks out of that small hole in the top. I think "maybe" all hydraulic systems need a pressure relief valve in case the pressure gets to high. Maybe that is what happened.
I was walking to the shops on that knee when all of a sudden it went floppy with no resistance, and oil was everywhere. The culprit was that small hole.
I could not stop that leak.
My guess was that a one time relief part, relieved the pressure by allowing it to escape through that hole. Sort of like a fuse.
I am sorry that I can not share more information. Fortunately I have other 2100 knees, and so I don't need that one. If I "had" to fix it, I would put a small sheet metal screw into the hole with epoxy, to seal it shut. Of course that would defeat the purpose of a relief valve, but maybe it would make the knee usable again as long as the pressure never got to high.
I wish I knew what to tell you....

[shenld]

dear teacher.How to change the oil of mauch hydraulic cylinder

[LucasDomit]

another thing, to remove the air, bubbles air, whatever.... open the black screw at the top, fill with the oil and swing the knee like the movement of walking, but the base need be sit at yours socket , swing it , the oil will goes down flowing into the canals and the air will goes up, do it until do not have air going out, fill complete( the knee need to be in total flexion), make sure have enough oil on it! dont be afraid to put enough oil! when you spin the screw the excess of oil will go out! better, you get the point. now. you get an working total knee........

[LucasDomit]

hello! im a amputee and have a total knee 2000, he starts a leak oil... had contact the ossur they say 'we dont know' or go to 'your clinic', i bought the knee on the ebay.... so dont have the warranty.... them remove the screws, clean the filter, fill everything with sewing machine oil, sure the oil was into it is twice more viscosity of the sewing oil, i test the oil, i get an industrial seringy, remove the applicator and put 2ML of each oil, the total knee oil takes 1 minute and 10 seconds to flow down, and the sewing oil machine takes 30 seconds to flow out by gravity... at this moment im fuck out... had i knee doesnt work properly. i thinked convence myself to put the sewing oil machine to try to see if works! the flexibility and resistence is mutch better! im using day by day and works! i get fine ajustments on it, better than the original oil was into it. but i thing from the smell and that viscosity is from a hidraulic oil, or at least two times more viscosity of a sewing machine oil. i had another total knee is sit on my garage 'leaks a lot of oil' i will open and see who the o'rings are, from there will fix this what im using. [e-mail]

[keets]

I just read your sheelchair-mod. And I am really really impressed that you did this too!

And still the same: it really sucks that you not got a new knee from the government, healt-care or whatever.

However I can assume that this give more satisfaction!

[hugomaniac]

I would trust your friend that thought it was about 16. I have never replaced oil in a knee. Always used to send them out. Mauch units are no longer repairable. All screws have been replaced with press fittings. It also seems all prosthetic components will not be repaired if they are over 5 years old.

[jwalker32]

I'd try to get some silicon shock oil for RC cars. It comes it variable viscosities and is also made for this type of functionality so removing the air should be the same.

[kgwedi]

I will definitely look into silicone oil for RC cars. Great tip. Thanks.

[hugomaniac]

Ossur says that the knee needs to be filled in a vacuum chamber. I'm a O&P Tech of over 25 years and a practitioner had unscrewed one of the adjustment screws all the way and allowed air into the system and I had to send it back to Ossur for them to fix it. If you could build a vacuum chamber with a syringe filled with extra oil and the spanner driver with the plug stuck to it along with a push rod to move the knee from the oil to the plug you might be able to get all of the air out.

[kgwedi]

I bet you could teach me a lot about this knee. I was never sure if there was supposed to be any air in the system or not. Many hydraulic systems need an air buffer. Great that you say NO air. Thank you.

I can do the vacuum chamber thing. Do you know the correct viscosity of the oil? Will silicone oil be better than vacuum pump oil?

[FbO Vorcha]

Sucks that you can't afford a replacement, great that you're good with tools. Good luck in straightening that knee out.

[tytower]

Well that's pretty good now if I can just find out how to recharge the shock absorber mattress in my foot I'll have a new limb.

[Swansong]

Cool, I hadn't seen how this was done before. :)