Welcome, Joey! Be sure and take that test ride...or even a couple. The Spyder can be either good or bad on the knees. My wife had a badly broken leg, and found the Spyder didn't bother her bad knee at all, except for the need to stretch her legs occasionally due to the arthritis and staying in one position too long. She hasn't tried it yet to see the effect on her new total knee replacement.
I have had work on both knees. The Spyder is very comfortable for one of them, far better than my bikes. The other one aches when I ride the Spyder, due to how far my legs are spread on this wide machine. I have found if I change the position in which I hold my legs, this pain goes away, but riding in that posture is still a bit unnatural for me, so I often forget. It is not a deal breaker, but is certainly noticeable on a long run (multiple tanks of gas). You really need to judge this one for yourself, in person, before you decide for certain. All our injuries are slightly different.
-Scotty