Differential Diagnosis for Right Posterior Knee Pain Radiating to Right Buttocks

Single Most Likely Diagnosis

• Piriformis Syndrome: This condition is characterized by pain in the posterior knee and buttocks due to compression of the sciatic nerve by the piriformis muscle. The symptoms described, including the radiation of pain from the posterior knee up to the buttocks, align closely with piriformis syndrome, especially given the duration of the symptoms since March.

Other Likely Diagnoses

• Hamstring Strain: A strain in the hamstring muscles could cause pain in the posterior knee that radiates up to the buttocks, especially if the strain is severe or involves the proximal hamstring tendons.
• Sciatica: Sciatica refers to pain that radiates along the path of the sciatic nerve, which runs from the lower back through the hips and buttocks and down each leg. The symptoms could be consistent with sciatica, particularly if there is an underlying lumbar spine issue.
• Baker's Cyst: A Baker's cyst, which is a fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee, could cause posterior knee pain. If large enough, it might refer pain to the buttocks area.

Do Not Miss Diagnoses

• Lumbar Disc Herniation: Although less likely, a herniated disc in the lumbar spine could cause sciatic pain radiating down to the knee and potentially up to the buttocks. Missing this diagnosis could lead to prolonged pain and potential neurological deficits.
• Deep Vein Thrombosis (DVT): DVT in the lower extremities can cause pain and swelling, typically in the calf but potentially radiating up the leg. It's a critical diagnosis not to miss due to the risk of pulmonary embolism.
• Osteonecrosis of the Femoral Head: This condition, also known as avascular necrosis, could cause hip and buttock pain that might radiate down to the knee. It's a serious condition that requires prompt treatment to prevent collapse of the femoral head.

Rare Diagnoses

• Tumors (Bone or Soft Tissue): Although rare, tumors in the bone or soft tissues of the knee, thigh, or buttocks could cause localized pain that radiates. These would be unusual causes of the symptoms described but are important to consider in the differential diagnosis to ensure no serious condition is overlooked.
• Infection (Osteomyelitis or Septic Bursitis): Infections such as osteomyelitis (bone infection) or septic bursitis (infection of a bursa) could cause pain and swelling in the affected area. These conditions are less common but require urgent medical attention.