Differential Diagnosis for Lateral Knee Pain

The following differential diagnosis is organized into categories to help guide the thought process:

• Single Most Likely Diagnosis
  • Iliotibial (IT) band syndrome: This 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 friction or inflammation of this band can cause pain.
• Other Likely Diagnoses
  • Lateral meniscal tear: A tear in the cartilage of the knee joint can cause pain on the lateral (outer) side of the knee.
  • Lateral collateral ligament (LCL) sprain: An injury to the LCL, which connects the femur (thigh bone) to the fibula (outer lower leg bone), can cause lateral knee pain.
  • Popliteus tendonitis: Inflammation of the popliteus tendon, which connects the popliteus muscle to the femur, can cause lateral knee pain.
• Do Not Miss Diagnoses
  • Osteonecrosis of the femoral condyle: A condition where the bone tissue dies due to lack of blood supply, which can cause severe pain and require prompt treatment.
  • Stress fracture: A small crack in the bone, often caused by overuse or repetitive stress, which can be difficult to diagnose and requires prompt treatment to prevent further injury.
  • Septic arthritis: A bacterial infection of the joint, which is a medical emergency requiring prompt treatment.
• Rare Diagnoses
  • Peroneal nerve compression: Compression of the peroneal nerve, which runs down the outside of the leg, can cause pain, numbness, and weakness in the lateral knee area.
  • Ganglion cyst: A non-cancerous lump that can develop on the lateral side of the knee, causing pain and stiffness.
  • Osteochondritis dissecans: A condition where a piece of cartilage and bone becomes detached from the surrounding bone, which can cause pain and require surgical treatment.

Indication for Ultrasound (US)

Ultrasound can be useful in diagnosing lateral knee pain, particularly for:

• Evaluating soft tissue injuries, such as IT band syndrome or LCL sprains
• Detecting fluid collections or effusions in the joint
• Guiding injections or aspirations of the joint or surrounding tissues
• Evaluating the integrity of the tendons and ligaments surrounding the knee joint

Note: While ultrasound can be a useful diagnostic tool, it may not be the first-line imaging modality for all cases of lateral knee pain. A thorough clinical evaluation and consideration of the patient's history and physical examination findings are essential in determining the appropriate diagnostic approach.