Differential Diagnosis for Localized Tenderness on the Lateral Aspect of the Knee

Single Most Likely Diagnosis

• Iliotibial (IT) Band Syndrome: This condition is characterized by inflammation of the iliotibial band, a ligament that runs down the outside of the thigh from the hip to the knee. The symptoms described, such as localized tenderness on the lateral aspect of the knee just above the joint line and pain reproduction during passive flexion of the knee, are classic for IT band syndrome. The pain typically occurs at a specific angle of knee flexion, often around 30-40 degrees, which matches the scenario provided.

Other Likely Diagnoses

• Lateral Meniscal Tear: Tears in the lateral meniscus can cause pain on the lateral aspect of the knee. The pain can be exacerbated by movements that put stress on the meniscus, such as flexion and rotation of the knee.
• Lateral Collateral Ligament (LCL) Sprain: An injury to the LCL, which connects the femur to the fibula on the outside of the knee, can cause lateral knee pain and tenderness.
• Popliteus Tendinitis: Inflammation of the popliteus tendon, which is located at the back of the knee and helps to rotate the knee inward, can cause pain on the lateral aspect of the knee.

Do Not Miss Diagnoses

• Osteonecrosis of the Femoral Condyle: Although less common, osteonecrosis (death of bone tissue due to lack of blood supply) in the lateral femoral condyle can present with lateral knee pain and must be considered to avoid missing a potentially serious condition.
• Infection or Septic Arthritis: Infections within the knee joint or surrounding tissues can cause severe pain, swelling, and tenderness. These conditions are medical emergencies and must be promptly diagnosed and treated.
• Tumor: Bone or soft tissue tumors around the knee can cause localized pain and tenderness. While rare, these conditions are critical not to miss due to their potential impact on prognosis and treatment.

Rare Diagnoses

• Nerve Entrapment (Peroneal Nerve): Entrapment or compression of the peroneal nerve, which runs down the outside of the knee, can cause pain, numbness, and weakness in the area it supplies.
• Ganglion Cyst: A ganglion cyst is a lump under the skin that occurs when fluid leaks from a joint or tendon, forming a cyst. Although more common on the wrist, they can occur near the knee and cause localized pain and tenderness.