As a proud owner of both rheumatoid and osteo arthritic conditions, I sympathize with your situation. As far as medications go, I've had (and continue to have) good results from Naprocyn (Naproxyn) and Darvocet D (Propoxyphene) pills on a daily basis and Enbrel (Etanercept) injections once a week.

I can't give you any suggestions about positions per se, however I have a few alternative suggestions that you might be able to incorporate.

1) Don't continue to attempt all the "traditional" contortionist positions. Substitute partial mummification (plastic wrap from shoulders to hips) in place of ropes, tethers, etc. Substitute bending over a larger piece of furniture at the hips (desk, sewing machine stand, etc.) instead of ottomans, stools, and the like.

2) Have your dominant partner task you with different types of punishment/training such as nude driving during the daytime or something similar.

I've found that I've had to be more creative/imaginative with my BDSM activities. I hope that you are able to adapt to your situation satisfactorily.

Good luck.