The cause of knee pain varies and can be accompanied by a wide variety of symptoms. Sprains occur when ligaments are overstretched or torn, and symptoms include pain, swelling, bruising and decreased flexibility within the knee joint, making it difficult to straighten the leg. There may also be a popping or clicking sound within the knee. During a sprain more than one ligament maybe affected, tears may be partial or complete and will affect the severity of symptoms. Sprains can be caused by twisting of the knee during activity or by landing incorrectly on the feet. Sprains are often graded in stages. Mild stages describe symptoms which are minimal, and in which the individual is still able to move the knee and put weight on the joint. Moderate sprains lead to moderate pain, some loss of mobility within the joint, and may be caused by a partial tear. A severe sprain leads to intense pain, and may require surgery. Typically there is a complete tear of the ligament.

Strains occur when muscles or tendons are overstretched or torn and may also involve partial or complete tears. They are often caused by injuries, such as those acquired during certain activities and contact sports, or from overuse and repetitive use of the knee joint. There may be muscle weakness, cramping, spasms, swelling, and immobility of the knee joint. Strains are also graded due to their severity.

Dislocation of the knee or patella dislocation is a condition where the knee cap twists or slips out of position. It can lead to knee pain, rapid swelling, and instability within the knee joint. It may also include bruising of the knee and numbness. Inflammation can occur in many different areas of the knee. For example, inflammation of the bursae in the knee joint is known as bursitis. Inflammation of the tendons is known as tendinitis, and inflammation of the synovial fluid which provides lubrication to the knee joint is known as synovitis. Inflammation can lead to stiffness, reduced mobility, swelling, and pain. Inflammation may occur due to direct damage to the knee, or it may be caused by other conditions, such as arthritis. Damage or conditions which affect other areas of the leg such as the hamstrings, may cause referred pain to the knee. Infections, cysts, tumors and the natural aging process which leads to the degeneration of the joint, can also lead to pain.

As treatment may vary depending on the cause, it is important to have any signs of knee pain, diagnosed by your health care provides, particularly pain which is chronic, severe or accompanied by other symptoms. In addition to a physical examination, other diagnostic procedures may be carried out. MRIs, X rays or CT scans can evaluate the structures within the knee and rule out any fractures or breaks within the bones. A joint fluid analyses in which a sample of the fluid is taken, can help to determine the presence of infection, bacteria and help to diagnose inflammatory conditions such as gout or arthritis.