Differential Diagnosis for Anterior Knee Pain Following Long Run

• Single Most Likely Diagnosis
  • Patellofemoral Pain Syndrome (PFPS): This is the most likely diagnosis due to the location of the pain (anterior knee) and the activity that triggered it (long run). PFPS is a common condition characterized by pain around or behind the patella, often associated with activities that put stress on the knee, such as running.
• Other Likely Diagnoses
  • Iliotibial (IT) Band Syndrome: Although IT band syndrome typically presents with lateral knee pain, it can sometimes radiate to the anterior aspect of the knee. It's common in runners due to the repetitive knee flexion and extension.
  • Quadriceps Tendinopathy: Inflammation or degeneration of the quadriceps tendon can cause anterior knee pain, especially after activities that involve running or jumping.
  • Patellar Tendinopathy: Also known as jumper's knee, this condition involves inflammation of the patellar tendon, leading to pain below the kneecap. It's common in athletes who frequently jump or run.
• Do Not Miss Diagnoses
  • Fracture (e.g., Stress Fracture of the Patella or Femur): Although less common, a fracture could present with acute onset of pain after a run. Missing this diagnosis could lead to further injury or complications.
  • Osteonecrosis of the Knee: A condition where the bone tissue dies due to lack of blood supply. It can cause severe knee pain and is more common in older adults or those with certain risk factors, but could be considered in the differential to avoid missing a serious condition.
  • Septic Arthritis: An infection within the knee joint, which is a medical emergency. While rare in this context, it's crucial to consider, especially if there are systemic symptoms like fever or if the patient is immunocompromised.
• Rare Diagnoses
  • Sinding-Larsen-Johansson Syndrome: A rare condition that affects the patellar tendon at its insertion on the tibia, more commonly seen in adolescents. It could be considered in younger runners.
  • Osgood-Schlatter Disease: Another condition more common in adolescents, characterized by inflammation of the growth plate at the top of the tibia, where the patellar tendon attaches. It could cause anterior knee pain in young runners.
  • Bursitis (Prepatellar or Infrapatellar): Inflammation of the bursae in front of or below the kneecap can cause anterior knee pain. While not as common in runners, it could be considered, especially if there's a history of direct trauma or repetitive kneeling.