The meniscus is between the bones. It can swell without showing on the outside. My wife had hers repaired last May. It didn't swell on the outside until the surgery. One thing's for sure - it won't heal on it's own. An xray won't show anything. Only an MRI will show what's happened and it can be tricky to read. My wife's MRI showed a tear across it on one side and a lengthwise split on the other. When the surgeon got in there, he repaired the tear but the split turned out to be that it was just folded over, not torn. He simply unfolded it and somehow tacked it down to hold it in place. It's slower to heal than a replacement. How long depends on what's in there. My wife's been since May and it'll probably another 4 or 5 months before she can kneel on it without pain.
The surgeon said that when he operates on one, he often has an artificial joint ready to go in case he finds something serious that the MRI doesn't show. He can install it while he's in there so a 2d surgery isn't needed. The MRI is a fantastic tool but it's not foolproof.
