Hello Ferry and everyone. I'm so sorry to hear of your battle with pain Ferry. That's so debilitating, and you have my sympathy 
I've just watched the tai chi link Polnan has put on for you, and it is excellent for what you need, which is something slow and gentle to keep you supple. However don't try to follow and do all that at first go. To start with, put a chair in front of you and stand behind it. Place your hands on the back, shoulder width apart, for support. Now place your feet so that they line up with the back legs of the chair, i.e. they will be shoulder width apart like your arms. Now, when you bend your knees do so by sitting back into your hip joints, keeping your shins straight. It's the tiniest move. Ensuring you keep your shins straight as you sit back means that you're not putting unnecessary pressure into your knees. Perfecting that move is your first step. If you can, place your chair side on to a full length mirror to make certain you're not moving your knees forward. As well as that, you're aiming to move slowly, back into the hips, and slowly up to standing again. You have the chair to hold on to so at this stage it's this tiny move you are concentrating on. The next stage, once you're confident with that little move, is to let go of the chair back (but keep your hands hovering above it so you can hold it again if you need to, and do the sit back move again (and again and again!). That's the very first part of tai chi and what you will have learnt is the correct tai chi stance - feet shoulder width apart, sitting slightly back into your hip joints and with knees slightly bent and directly above your feet (no knock knees). Tai chi (and good balance) is all about posture. You'll see from the video clip that the moves are very slow and smooth. Please feel free to pm me anytime if you want more or have questions. But don't go on to follow the video until you have mastered the stance. Enjoy, and I hope it helps. It's even more pleasant to practice outside in the garden!