Zazen with spondylitis / Arthritis

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I try to sit zazen meditation regularly. I also have Ankylosing spondylitis, which is an inflammatory disease that attacks joints primarily in the spine. I'm worried that prolonged sitting with a totally erect spine may be causing further damage. It sure hurts a lot but my impression of zen is that it's not always supposed to be comfortable. Or is sitting with correct zazen posture actually helping to reduce damage? Any comments about meditation from those with similar conditions will be much appreciated.