I considered Speedplay pedals and was talked out of it by my LBS. I was told there is a thing as too much float and the weight of the cleats and lack of support of pedals was a question. I wound up with Look carbon keos, I love them. The great thing is that there are two different cleats with different levels of float. You definitely need at least 4.5 degrees with your knee problem. Go to a good bike shop and talk to them. I think it will be less stress on your knees than the cages. Good luck.