I, too, have R.A., and inflammatory osteoarthritis as well. I haven't been kind to my body as I assumed it would heal, so I spend years crashing MX bikes, was in a couple of horrific car wrecks (not my choice, nor my fault). I tumbled around dojos for well over 15 years, and made a run at the "40 40's" in Shito Ryu. That's punching a makiwara board, which has only a little "give", 40 times a day for 40 days with each fist, making contact only with the first two knuckles. I made it to about day 32 or so and had to quit because I used my hands at work (I was a mine surveyor at the time, and spent a lot of time chiseling holes in the mine roof to make benchmarks) and I simply couldn't continue. At that point I'd bridged the first and second "punching knuckles" of my right hand, and nearly so the left. A few years later I got involved with the Society of Creative Anachronism and was what they called a "stick jock". Yeah, I'll wear the odd clothing, but I came to fight. Later I found that unsatisfying, and helped form a "Company of Foot" which is like the SCA with the training wheels off. The only rule was "please try not to break your toys", meaning the other fighters. Oops, I forgot a ~10 foot fall onto a slab of sandstone, and a 30 foot "skipper" while free-climbing a rock face. I should have known all of that would come home to bite me, but I was taught that the more you stress yourself the stronger you get. That may be true if you don't have a rheumatoid disease. Mine was diagnosed when I was in the hospital getting 2/3rds of my right lung removed because of cancer, long after I'd done most of the damage. Who knows how long I'd had it, and was breaking my body instead of building it.
I haven't tried all the possible knives, but the one I love and carry now is a Spyderco Paramilitary 2 in Cruwear steel with canvas Micarta scales. Frankly, I didn't like the scales at first, but they grew on me. The Spydie-hole on this particular knife is large so it's easy to snag with the meat of my thumb, and it's far enough away from the pivot that it takes little force to open (the latter's a problem I have with almost all thumb-stud openers). I don't "pinch" the compression lock to open it, I choke up on the handle and twist the knife a little in my hand and push the lock with my thumb, letting the choil fall on my index finger. I then adjust my grip and close the blade with my index finger.
If you want a less expensive knife, I've had good luck with the Ken Onion Leek, and the Dozier assisted folders. The problem with the Leek is that it's small, maybe too small for arthritic hands, and I have 2-3 Doziers's in drawers scattered around here because the clip failed.
Finally, you need a good sharpener as it will be difficult to keep the same "hand" while sharpening on a stone as you did before the R.A. got bad. I bought a Spyderco Tri-Angle Sharpener, and while it's not state of the art I can use it to get a knife with good steel sharp enough that it's dangerous to handle. If you have some older knives that need the edges re-shaped, the diamond sticks that fit that sharpener will do so in short order. I'm debating whether to buy the ultra-fine stones - I don't think they would get the knife any sharper, but they would put a pretty mirror finish on it. I'm not a collector, just a knife user for 65 years and care more about the steak than the sizzle.
If you're looking at a fixed blade, I recently bought a Bark River that's very nice. It has a horizontal carry sheath that prints badly for me, I'm still trying to figure out how I can carry it.
