If you are truly concerned about how fragile your knees are, and you want to continue riding well into the future on the same bike, you really should consider a triple

.And maybe even look at an MTB or what Shimano calls a trekking crankset -- that is, 26-36-48.

You might lose some top-end speed, but realistically, how often do you use the big ring anyway? And the difference between a 48t on an MTB and a 50T on a compact double is not that significant.

The wider Q might be an issue, but the option of going even lower on the granny with a 22T might be something you will be grateful for into the future... or at least your knees will be.