Differential Diagnosis for Right Knee Pain

The patient's presentation of right knee pain after a fall, with some relief from ibuprofen and no obvious deformities or positive tests, suggests several potential diagnoses. These can be categorized as follows:

• Single Most Likely Diagnosis
  • Chronic Knee Sprain or Ligamentous Injury: Given the history of a fall four months prior and the ongoing pain, a chronic knee sprain or ligamentous injury is plausible. The fact that ibuprofen helps suggests an inflammatory component, which is consistent with this diagnosis.
• Other Likely Diagnoses
  • Osteoarthritis (OA): Although OA can be primary, a traumatic event like a fall can exacerbate or precipitate symptoms in a previously unaffected joint. The patient's age and the chronic nature of the pain make OA a consideration.
  • Meniscal Tear: Meniscal tears can occur from acute trauma, like a fall, and may not always present with a clear mechanism or positive tests initially. Chronic pain and stiffness could be indicative of a meniscal tear, especially if the tear is not large enough to cause significant mechanical symptoms.
  • Bursitis: Prepatellar or pes anserine bursitis could be considered, especially if the fall caused direct trauma to the knee. These conditions can cause chronic pain and might respond to NSAIDs like ibuprofen.
• Do Not Miss Diagnoses
  • Infection (Septic Arthritis): Although less likely given the chronic nature and response to ibuprofen, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. A history of trauma could potentially introduce bacteria into the joint.
  • Tumor: Bone or soft tissue tumors around the knee could cause chronic pain. While rare, missing a diagnosis of a tumor could have significant consequences.
  • Osteonecrosis: Trauma can lead to osteonecrosis, particularly of the femoral condyles. This condition might not present immediately after the injury and could be a cause of chronic pain.
• Rare Diagnoses
  • Complex Regional Pain Syndrome (CRPS): This condition can follow trauma and is characterized by chronic pain, inflammation, and hypersensitivity. It's less common but should be considered if other diagnoses are ruled out and the pain seems disproportionate to the injury.
  • Neuropathic Pain: Direct nerve injury from the fall could lead to chronic neuropathic pain. This would be less common but could be a consideration if other causes are excluded.

Each of these diagnoses should be considered in the context of the patient's full history, physical examination, and potentially, further diagnostic testing such as imaging studies.