Hi, It could be from a few things. When you have a sharp blow to a bony area, it can actually cause the bone to overgrow just a little. This is called an exostosis, or sometimes referred to an a bone spur. Another possibility is that, when the hematoma (bruise) resolved, some of the tissue became fibrotic. This would cause a hard cartilage like lump. Both are benign. But, both can also cause discomfort if pressed on or bumped. But, the only way to know for sure would be to have the knee x-rayed. There is a bursa, directly under the skin, on top of the patella (kneecap). It can become inflamed after a direct blow (or from repeatedly pressing on it, such as when scrubbing the floor - so it used to be called washer woman s knee). But, when it is swollen, it feels like a water balloon. It usually does not become hard like bone. So, again, the only way to know for sure would be to have it check by a physician. Good luck.