I've had a seriously messed up knee for years now. The biggest help you can give yours is found at the gym, not the ski store. The second biggest help will be good custom braces from your orthopedist. The third biggest help will be good boots properly fit and canted.
Only then, IMO, do we get to the ski shop. As a rule of thumb, a damper, smoother ski is going to keep knees happier than something light and full of feedback. Damp often means metal inside. K2's of course are the exemplar for older skeletons, but for those of us who think the Apache series goes more toward deadness than smoothness, I'd consider Head, Stockli, and Volkl. Plenty to choose from among those. My personal favs are the Head Superhape and iM series, and the Stockli wood cores like the XL. Nice combo of dampness, strength, and energy. I've heard good things about Nordicas, but haven't skied any.
If you're more of a finesse skier, I've found Blizzards to be lighter and quicker than many of the above, but they still handle crud and rough hard snow in a knee friendly way. The new Sollie Tornados and Furies manage to be surprisingly smooth on groomed and in chop even though they're very light, but they can get nervous on hard rough snow. Also can recommend Dynastar Legend series although you have to get used to the tip deflection. Skis I personally have trouble with in this regard: Fischers, Atomics, many but not all Rossis, most any ski that's foregrounded as "light and lively."
Finally, think hard about bindings. Pistons or springs help absorb or smooth out shocks. This is a good thing.