Differential Diagnosis for Knee Pain

The patient is complaining of knee pain with no known injury, negative antinuclear antibodies (ANA), and negative for pediatric arthritis. Here's a differential diagnosis organized into categories:

• Single Most Likely Diagnosis
  • Osgood-Schlatter disease: This is a common cause of knee pain in adolescents, characterized by inflammation of the patellar tendon at its insertion on the tibial tubercle. The absence of a known injury and negative tests for autoimmune diseases make this a plausible diagnosis.
• Other Likely Diagnoses
  • Patellofemoral pain syndrome: This condition is characterized by pain around or behind the patella, often related to activities that put stress on the knee, such as climbing stairs, squatting, or jumping. It's common in adolescents and young adults.
  • Meniscal injuries or tears: Although the patient reports no known injury, minor traumas or overuse can lead to meniscal tears, which might not always have a clear history of trauma.
  • Overuse injuries (e.g., tendonitis): Repetitive stress on the knee from sports or physical activities can lead to overuse injuries, including tendonitis of the quadriceps, patellar, or hamstring tendons.
• Do Not Miss Diagnoses
  • Septic arthritis: Although less likely, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with sudden onset of pain, swelling, and decreased range of motion, often accompanied by systemic symptoms like fever.
  • Osteosarcoma or other bone tumors: These are rare but critical diagnoses to consider, especially in pediatric patients. Persistent, unexplained pain, especially at night or at rest, should raise suspicion for a bone tumor.
  • Lyme disease: In endemic areas, Lyme disease can cause knee pain and swelling, especially if the patient has been exposed to ticks.
• Rare Diagnoses
  • Rheumatoid arthritis (juvenile idiopathic arthritis): Although the patient tested negative for pediatric arthritis, it's essential to consider this diagnosis, especially if symptoms persist or worsen over time.
  • Sickle cell disease with bone infarct: In patients with sickle cell disease, bone infarcts can cause severe pain, including in the knee.
  • Neuropathic pain: Rarely, knee pain can be referred from a neuropathic source, such as a lumbar radiculopathy.

Each of these diagnoses has a different set of implications for treatment and management, emphasizing the importance of a thorough evaluation to determine the underlying cause of the patient's knee pain.