Prosthetic Hydraulic Ankle Repair

I am a left, above knee amputee. I use a hydraulic Ossur 2100 knee and a College Park K2 ankle on my prosthesis.I bought the College Park Odyssey K2 foot on E-bay. It now leaks hydraulic fluid from where the piston comes out of the seal plate.

This instructable is about the K2, and how I fixed the leaking seal. This instructable is NOT, about how YOU, should fix or repair a leaky K2.

A prosthetic device must be very reliable. A 99.99% reliable prosthetic, means you will probably still fall at least once a day. Do anything in this instructable at your own risk. This is not a tutorial, it is simply showing what I did.

Tools Needed:

Every fitting on the K2 is American, except for the main shaft. There are no metric tools used.

A complete set of Allen wrenches down to a 1/16.

3/8 inch thin walled socket.

Step 1: About the K2 Foot.

The K2 is the a hydraulic ankle/foot made by College Park.

All K2s are both right and left. Only the foot shell determines which foot it is for. The main shaft of an ankle must be in the same plane as the main shaft of the knee. This means the K2 is always pidgin toed, and my weight passes over the middle toe, not the big toe.

The hydraulics are used to control the toe up, and toe down motion. There are variable orifices on both sides of the piston, and these create resistance to the movement.

Step 2: Remove Hydraulic Unit

Remove the foot shell, and the carbon fiber foot plates.

Now you have only the hydraulic unit.

Step 3: Parts of the Hydraulic Unit

Here you can see the hydraulic unit. Note the big bolt, the adjustment screw for the variable Dorsiflexion orifice, and the four small Allen screws that hold the piston plate on.

You can also see the curved piston. The leak is where the curved piston goes through the plate. The blue plastic is just a dust cover that protects the main shaft retaining screw.

Step 4: Removing the Bolt on the Pyramid Lever.

Use a 3/8 inch thin walled socket to remove the big bolt on the end of the curved piston. I had to grind down a normal 3/8 socket to make it thin walled.

Remove the blue plastic dust cover, and remove the main shaft retaining screw that is deep inside the pyramid.

You can now rotate the pyramid far back to free the pyramid from the curved piston. There will be two small aligning pins that go into the piston.

Next remove the two end screws on the main shaft.

Step 5: Remove the Main Shaft.

The main shaft is held in place by the two end screws, and the screw inside the top of the pyramid. The main shaft has a grove in it where the screw fits to lock the shaft.

The inside thread for the shaft is 12 x 24. I put a 12x24 bolt into the end of the shaft with some large washers, and as I screwed in the 12x24 bolt, it pushed against the washers, and pulled the shaft out.

Once the shaft is out, the top pyramid part can be removed, or just tilted back out of the way.

Step 6: Removing Seal Plate.

This allows me access to the 4 small screws holding the seal plate down. They are supposed to be 1/16 inch Allen head screws. Mine were so mangled, that I drove a #7 Torque driver into the holes with a hammer, and that worked fine.

Once the seal plate was removed, the top of the piston and the fluid can be seen.

A side view of the seal plate shows it is slanted to allow for the bent piston shaft.

Step 7: Changing the Fluid

Now that the fluid is visible, I used a syringe and removed the fluid, working the piston as I did. I removed about 4CC of old black fluid.

I guessed the viscosity of the fluid to be 350 CTS. I replaced the fluid with silicone oil from a radio controlled race car shock absorber. I could only buy 400 CTS and 250 CTS, so I mixed them until I had 350 CTS.

2CC 400 + 2CC 250 = 4 CC of 325. I figured that 325 was close enough. Using a syringe, I refiled the unit with 4CC of clean fluid.

Step 8: Replacing the Seal and O'ring.

I removed the seal from inside the seal plate. This is where my leak was coming from.

I decided to change the o'ring that goes around the outside of the seal plate. It is not a circle, because it has to go around the hydraulic port of the top of the piston. But it turns out that it is a normal o'ring that just follows the grove cut for the o'ring.

Both seal and o'ring were available at the first seal shop I went to.

Now to put it all back together.

Step 9: Reassembling the Hydraulic Unit.

After putting both seal and o'ring in place, I put the seal plate back on. The seal plate must be set at an angle to allow the curved piston to move freely.

Tighten the four screws.

Put the pyramid lever on.

Check the shaft, and make sure it is clean and smooth, then push the main shaft in. I lightly greased the shaft.

Put the two screws into the end of the shaft, and the shaft retaining screw that is deep inside the top of the pyramid.

Push the pyramid lever down on to the piston shaft, making sure the two small pins align properly.

Put the big bolt into the top of the piston, and tighten it.

Put the blue plastic dust cap onto the pyramid top.

The hydraulic unit is now completed.

Step 10: Finishing the Foot

Now bolt the carbon fiber foot plates back on, and fit the foot shell.

To test the foot/ankle, I walked on it for a few hours, and noticed that even with the Dorsiflexion and Plantarfexion resistance set to minimum, the ankle was slow and stiff. I think the fluid was to viscus.

Step 11: One Week Later

I replaced the fluid in the K2. It now has 150 CTS silicone fluid in it, and it is much better.

I can adjust it to loose, and I can adjust it to stiff.

I now have an ankle that allows walking on slopes, and uneven terrain. Total cost was about 5% of a new ankle.

I hope this helps someone to have a foot/ankle, that may not have been able to afford one new.