The most common symptom of knee osteoarthritis is pain. The pain is often a dull ache, similar to a toothache. However, the pain can be sharp and stabbing. Another common symptom of knee osteoarthritis includes intermittent swelling, also known as a knee effusion or ‘water on the knee.’ This is caused by excessive joint fluid in the knee. Knee pain and swelling may be accompanied by knee stiffness.