Deep Vein Thrombosis (DVT): This is a common complication following surgery, especially orthopedic procedures like knee surgery. The immobilization and surgical trauma increase the risk of blood clot formation. The acute swelling and pain in the ankle could be due to a DVT that has developed in the calf or thigh, which is a frequent site for clot formation.

Cellulitis: A bacterial skin infection that can cause redness, swelling, and pain in the affected area. Post-surgical patients are at increased risk due to potential breaches in skin integrity and compromised immune response.
Hematoma or Seroma: Fluid collections can occur at the surgical site or along the path of a vein or artery, leading to swelling and pain. These are relatively common after surgeries, especially if there was significant tissue disruption.
Nerve Injury or Neuropathy: Direct trauma to nerves during surgery or indirect injury from swelling or hematoma can cause pain and swelling in the distribution area of the affected nerve.

Septic Arthritis: Although less common than other complications, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can occur if the joint space becomes infected, either from direct contamination during surgery or hematogenously from another infected site. The presence of fever, significant pain out of proportion to expected post-surgical pain, and decreased range of motion should raise suspicion.
Compartment Syndrome: This is a condition where increased pressure within a muscle compartment leads to inadequate blood supply to the tissues inside the compartment. It is a surgical emergency that can result from swelling or hematoma within a closed fascial space, leading to nerve and muscle damage if not promptly treated.

Rhabdomyolysis: A serious syndrome due to direct or indirect muscle injury. It results from the death of muscle fibers and release of their contents into the bloodstream, which can lead to acute kidney injury. This could be considered if there was significant muscle trauma during surgery or in the postoperative period.
Osteonecrosis or Avascular Necrosis: Although more commonly associated with long-term steroid use or traumatic injuries, osteonecrosis could potentially occur post-surgically, especially if there was disruption to the blood supply of the bone during the procedure.

To eliminate septic arthritis, the following steps should be taken:

Clinical Evaluation: Assess for signs of infection such as fever, chills, and significant pain or swelling out of proportion to what is expected post-surgically.
Laboratory Tests: Obtain a complete blood count (CBC) to look for elevated white blood cell count, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) levels, which can be elevated in infection.
Imaging: Use X-rays to rule out other causes of pain and swelling, such as fractures. Ultrasound or MRI can help identify fluid collections or other soft tissue abnormalities.
Joint Aspiration: The definitive diagnostic test for septic arthritis is arthrocentesis (joint aspiration) to analyze the synovial fluid for cell count, culture, and crystals.

If septic arthritis is diagnosed, treatment involves:

Antibiotics: Immediate initiation of broad-spectrum intravenous antibiotics, which can be narrowed based on culture results.
Surgical Drainage: Often necessary to drain the infected joint to remove purulent material and debris.
Supportive Care: Pain management, immobilization of the affected joint, and monitoring for complications such as osteomyelitis or sepsis.