Differential Diagnosis for Left Knee Back Pain with Bruising

• Single most likely diagnosis
  • Hemarthrosis: This condition, characterized by blood in the joint space, is a common cause of knee pain and bruising, especially if there's a history of trauma or anticoagulant use. The presence of bruising visible on the back of the knee suggests bleeding into the soft tissues or joint.
• Other Likely diagnoses
  • Ligament sprain or strain: Injuries to the ligaments surrounding the knee can cause pain and bruising, especially if the mechanism of injury involves a twisting motion or direct blow to the knee.
  • Meniscal tear: Tears in the cartilage of the knee joint can lead to pain, swelling, and sometimes bruising, particularly if the tear is large or the injury is severe.
  • Bursitis: Inflammation of the bursae around the knee can cause pain and swelling, and if the bursitis is due to a traumatic injury, bruising may also be present.
• Do Not Miss diagnoses
  • Fracture: While less common than soft tissue injuries, fractures around the knee can cause significant pain and bruising. Missing a fracture could lead to improper healing and long-term disability.
  • Vascular injury: Damage to the blood vessels around the knee, such as the popliteal artery, is a medical emergency. Symptoms can include severe pain, swelling, bruising, and pallor or coolness of the affected limb.
  • Septic arthritis: This is an infection within the joint space, which can cause severe pain, swelling, and sometimes bruising. It's a medical emergency requiring prompt antibiotic treatment.
• Rare diagnoses
  • Osteonecrosis: Also known as avascular necrosis, this condition involves the death of bone tissue due to a lack of blood supply. It can cause knee pain and, in advanced stages, might lead to visible bruising if there's a fracture.
  • Tumor: Both benign and malignant tumors can cause knee pain and, if they bleed or cause a fracture, might lead to bruising. However, this would be a rare cause of the symptoms described.