I assume that your swelling is nearly 10 cms below your knee on the front part (anterior).
Hard lump on leg can be of anything. You need to consult your surgeon.
There are many things to be noted
whether it moves when moved, moves when knee joint is moved, consistency of the swelling, whether it yeilds to pressure, temperature of the swelling etc,
Even with this you would be needing some biopsies to accurately come to a diagnosis.
Hard swelling may be of benign exostosis
, multiple exostosis (there may be similar swelling in other parts of body), ganglion cyst
etc. there is also a chance of tumours which need to be treated immediately.
SO consult your surgeon immediately.