Prosthetic Knee Oil Change




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

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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.

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    11 Discussions


    7 months ago

    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........


    7 months ago

    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.


    2 years ago

    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!


    2 years ago

    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.


    2 years ago

    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.

    1 reply

    Reply 2 years ago

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


    2 years ago

    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.

    1 reply

    Reply 2 years ago

    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

    2 years ago

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


    2 years ago

    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.


    2 years ago

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