Iliotibial (IT) Band Syndrome: This is the most common cause of lateral knee pain, especially in runners or 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.
Popliteus Tendinitis: Inflammation of the tendon that connects the popliteus muscle to the lateral femoral condyle can cause lateral knee pain, especially with activities that involve twisting or bending.

Osteonecrosis of the Lateral Femoral Condyle: A condition where the bone tissue dies due to lack of blood supply, which can cause severe pain and disability if left untreated.
Infection (e.g. Septic Arthritis): A bacterial infection in the knee joint can cause severe pain, swelling, and redness, and requires prompt medical attention to prevent serious complications.
Tumor (e.g. Osteosarcoma): A rare but potentially life-threatening condition that requires early diagnosis and treatment.

Peroneal Nerve Compression: Compression of the peroneal nerve, which runs down the outside of the knee, can cause pain, numbness, and weakness in the lateral knee and foot.
Fabella Syndrome: A rare condition where a small bone in the lateral knee, called the fabella, becomes inflamed or irritated, causing pain and stiffness.
Ganglion Cyst: A benign lump that can form on the outside of the knee, which can cause pain and stiffness if it becomes large enough to compress surrounding tissues.