Differential Diagnosis for 85 year old male with swollen red knee post knee replacement

• Single most likely diagnosis:
  • Chronic prosthetic joint infection: This is a likely diagnosis given the patient's history of knee replacement and current presentation of a swollen, red knee with pain and resistance to bending. The recent treatment for cellulitis in the foot could be a source of hematogenous spread of infection to the prosthetic joint.
• Other Likely diagnoses:
  • Aseptic loosening of the prosthetic joint: This condition, where the prosthetic joint becomes loose due to wear and tear or other non-infectious causes, could explain the pain and resistance to bending. It's a common complication of knee replacements, especially in older patients.
  • Osteoarthritis in the surrounding native joints: Although the patient has had a knee replacement, osteoarthritis could still be affecting the surrounding joints, such as the hip or ankle, causing referred pain to the knee.
  • Recent cellulitis complicating a pre-existing condition: The recent cellulitis treatment could be masking or complicating a pre-existing condition of the knee, such as a chronic infection or inflammatory arthritis.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Septic arthritis: Although less likely than chronic prosthetic joint infection, septic arthritis is a medical emergency that requires prompt diagnosis and treatment to prevent serious complications, including joint destruction and sepsis.
  • Deep vein thrombosis (DVT): Given the patient's recent history of cellulitis and immobility, there is a risk of developing DVT, which could lead to pulmonary embolism if not promptly diagnosed and treated.
  • Necrotizing fasciitis: This rare but life-threatening condition could be a complication of the recent cellulitis and requires immediate medical attention.
• Rare diagnoses:
  • Rheumatoid arthritis or other autoimmune arthropathies: Although less common in this age group, these conditions could still be a cause of the patient's symptoms, especially if they have a history of autoimmune disorders.
  • Hematogenous metastasis to the knee: In rare cases, cancer could metastasize to the knee, causing pain and swelling. This would be more likely in patients with a known history of cancer.
  • Complex regional pain syndrome (CRPS): This condition, characterized by chronic pain and inflammation, could be a rare cause of the patient's symptoms, especially if they have a history of trauma or nerve damage.