What’s wrong with the knee that rattles and hurts when you extend and flex it?

The first and most common cause of knee flexion and extension with rattling and pain is knee meniscal injury. The meniscus of the knee plays a cushioning role on the inside of the knee, but when the lower extremity is heavily weighted, or when the knee is frequently crushed or flexed and extended, it can cause damage to the meniscus of the knee. Once the meniscus is damaged, it can cause pain and ringing during joint movement, and in severe cases, joint interlocking can occur. The second cause of knee flexion and extension with pain is more common in older patients and is mainly due to osteoarthritis of the knee joint.