Össur, a company that specializes in artificial limbs, has one of the most advanced artificial knees available (availability might be a stretch, since they are not readily available)
The next generation Power Knee is on of the most advanced above-knee prosthetics today. Adaptive technology allows the knee to adjust to the users actions and provides powered motion for walking and provides necessary power on inclines and stairs.
The technology built into the knee itself seems to be self contained. A bunch of sensors and processors calculate what the user is doing and lets the knee adapt. It doesn't seem to take instructions from nerve endings or synaptic activity, but is still impressive nonetheless.