Your symptom of grinding is characteristic of osteoarthritis
. If you are having tendon weakness, then it could be patellar sub-luxation or patellar dislocation.
When did your pain begin, what were you doing at the time, and what were the initial symptoms?
Do you experience any grinding, locking, catching, or giving way of the knee?
Are there any positions that make your knee more or less comfortable?
What is the quality of your pain (sharp, shooting, dull, etc.)?