Differential Diagnosis for Tendon Pain on the Left Side of Knee

Single Most Likely Diagnosis

• Iliotibial (IT) Band Syndrome: This condition is a 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 tenderness on the outside of the knee.

Other Likely Diagnoses

• 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.
• Popliteus Tendinitis: Inflammation of the popliteus tendon, which connects the popliteus muscle to the lateral femoral condyle, can cause pain on the lateral side of the knee.
• Peroneal Tendinitis: Inflammation of the peroneal tendons, which run down the outside of the ankle and attach to the lateral side of the knee, can cause pain and swelling on the outside of the knee.

Do Not Miss Diagnoses

• Septic Arthritis: Although less common, septic arthritis is a serious condition that requires prompt medical attention. It is characterized by joint pain, swelling, and redness, and can be life-threatening if not treated promptly.
• Osteonecrosis: Also known as avascular necrosis, this condition occurs when the blood supply to the bone is disrupted, leading to bone death. It can cause severe pain and disability if not treated promptly.
• Tumor: Although rare, a tumor on the lateral side of the knee can cause pain and swelling. It is essential to consider this possibility, especially if the pain is persistent and unexplained.

Rare Diagnoses

• Gout: Although gout typically affects the base of the big toe, it can also occur in other joints, including the knee. It is characterized by sudden, severe pain and swelling, often accompanied by redness and warmth.
• Pseudogout: Also known as calcium pyrophosphate deposition disease (CPPD), this condition is characterized by sudden, severe pain and swelling in the knee, often accompanied by redness and warmth.
• Ligamentous Sprain: A sprain of the lateral collateral ligament (LCL) can cause pain and instability on the lateral side of the knee. Although less common, it is essential to consider this possibility, especially if there is a history of trauma or injury.