I recently had an accident aboard my dirtbike that resulted in a 40mph bash to my knee. It was not direct in respect to the kneecap, more towards the inside of the patella. However, an MRI(although expen$ive) is the best way to remove all doubt, well almost. MRI's are not 100% and in some cases the OS and Radiologist do NOT see eye to eye. One sees something the other doesn't, so forth and so on. Ask for a copy of the films before they stick you in the tube (they should be free) and get a second or maybe even a third opinion on the results.