I am not writing the following to whine, but as a reminder that you should listen to your body.
About 10 years ago I was diagnosed with a patella dysplasia, which is a deformation of the kneecap. You don't see this with the naked eye, but it has caused some considerable pain now and then.
Anyway, doc told me that if things get worse, knee surgery might be necessary.
Up to now I had been using a meditation bench (Pi form) and it was excellent. Pain in the knees was there, but still ok. However, already during our zazenkais here I felt my knees reached a certain limit.
I ignored it (I am stubborn)...
However, about two weeks ago the pain was really, really intense, just after 30 minutes of sitting (sometimes tears of pain welling up). I should have listened to my inner alarm bells by then (did not though).
Last Saturday when I came home from shopping my right knee was stiff and swollen. I cooled it and from then on I've been doing zazen on a chair.
This has been quite a change for my sitting practice, I must admit, but I think I will carry on sitting like that. I am still (relatively) young and I'd like to have well functioning knees...
So, if anyone of you feels pain during sitting that goes way beyond the normal pain on a regular basis (I know that some pain is normal during zazen), listen to your body.
Oh, and before I forget:
If any of you have to use a chair for zazen as well, I'd be happy for any tips/recommendations!