Iliotibial (IT) Band Syndrome: This condition is the most common cause of lateral knee pain, especially in runners and cyclists. The IT band is a ligament that runs down the outside of the thigh from the hip to the knee, and when it becomes inflamed, it can cause pain and swelling on the outside of the knee.

Lateral Meniscus Tear: A tear in the cartilage of the knee can cause pain on the lateral (outer) side of the knee, especially with twisting or bending movements.
Lateral Collateral Ligament (LCL) Sprain: An injury to the LCL, which connects the femur to the fibula, can cause pain and instability on the outside of the knee.
Peroneal Tendinitis: Inflammation of the tendons that connect the peroneal muscles to the bones in the lateral knee can cause pain and swelling in the area.

Osteonecrosis of the Lateral Femoral Condyle: A condition where the bone tissue in the lateral part of the knee dies due to lack of blood supply, which can cause severe pain and disability if not treated promptly.
Infection (Septic Arthritis or Osteomyelitis): An infection in the knee joint or bone can cause severe pain, swelling, and redness, and requires urgent medical attention.
Tumor (e.g., Osteosarcoma): Although rare, a tumor in the lateral knee can cause persistent pain and swelling, and should not be missed.

Nerve Entrapment (e.g., Peroneal Nerve Entrapment): Compression of the peroneal nerve can cause pain, numbness, and weakness in the lateral knee and foot.
Popliteal Cyst (Baker's Cyst): A fluid-filled cyst in the back of the knee can cause pain and swelling in the lateral knee, although it is more commonly associated with medial knee pain.
Stress Fracture of the Lateral Femoral Condyle: A small crack in the bone of the lateral knee can cause pain and swelling, especially in athletes who participate in high-impact sports.