Sometimes difficult to straighten?

Knee pain: Wear and tear arthritis (osteoarthritis) is a common cause of knee and other joint pain and stiffness, but because of associated swelling and difficulty straightening your knee, a torn cartilage is a possibility. If you obtain no relief from applying heat for 20 minutes off and on as well as taking anti-inflammatory medications like ibuprofen or naproxen, an MRI of your knee would be informative.