Differential Diagnosis for Patellar Pain after Fall on Knee

• Single most likely diagnosis
  • Patellar contusion or bone bruise: This is the most likely diagnosis given the patient's history of a fall on the knee and the presence of a large joint effusion without an acute fracture. The mechanism of injury and the symptoms are consistent with a contusion or bone bruise.
• Other Likely diagnoses
  • Osteochondral injury: The fall could have caused an osteochondral injury, which is a damage to the cartilage and the bone underneath. This would explain the joint effusion and the patient's symptoms.
  • Internal derangement of the knee: This refers to an injury to the soft tissues within the knee joint, such as ligaments or menisci. The large joint effusion and the patient's symptoms could be consistent with an internal derangement.
  • Patellofemoral pain syndrome: This is a condition characterized by pain in the front of the knee, usually around or behind the patella. The patient's symptoms and the presence of a large joint effusion could be consistent with this diagnosis.
• Do Not Miss diagnoses
  • Fracture (e.g., occult fracture): Although the initial X-rays did not show an acute fracture, it is possible that a fracture could be present but not visible on the initial images. An occult fracture could be deadly if missed, as it could lead to further complications if not properly treated.
  • Ligamentous injury (e.g., ACL or MCL tear): A ligamentous injury could be present, even if the initial X-rays did not show any evidence of a fracture. This type of injury could be serious and require prompt treatment to avoid further complications.
  • Septic arthritis: Although less likely, septic arthritis is a serious condition that requires prompt treatment. The presence of a large joint effusion and the patient's symptoms could be consistent with septic arthritis.
• Rare diagnoses
  • Osteonecrosis: This is a rare condition characterized by the death of bone tissue due to a lack of blood supply. Although unlikely, osteonecrosis could be a possible diagnosis, especially if the patient has a history of steroid use or other risk factors.
  • Neuropathic arthropathy: This is a rare condition characterized by joint degeneration and destruction due to a nerve disorder. Although unlikely, neuropathic arthropathy could be a possible diagnosis, especially if the patient has a history of nerve damage or other underlying conditions.