Differential Diagnosis for Left Knee Pain

The patient presents with pain in the left knee, occasional swelling, soreness behind the knee cap, and a feeling of instability, particularly when descending stairs. Based on these symptoms, the following differential diagnoses are considered:

• Single Most Likely Diagnosis

  • Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or behind the patella (knee cap), often associated with activities that put stress on the knee, like climbing or descending stairs. The symptoms described, such as soreness behind the knee cap and a feeling of the knee wanting to buckle, are classic for PFPS.

• Other Likely Diagnoses

  • Meniscal Tear: A tear in the meniscus, which is the cartilage that cushions the knee joint, can cause pain, swelling, and a sense of instability in the knee. The location of the pain and the specific activities that exacerbate it can help differentiate this from PFPS.
  • Ligament Sprain: Sprains to the ligaments around the knee can cause pain, swelling, and instability. The mechanism of injury and the specific location of the pain can help identify which ligament might be involved.
  • Osteoarthritis: Although more common in older adults, osteoarthritis can cause knee pain, swelling, and instability, especially with weight-bearing activities like descending stairs.

• Do Not Miss Diagnoses

  • Deep Vein Thrombosis (DVT): While less likely, DVT is a potentially life-threatening condition that can cause swelling and pain in the leg. It's crucial to consider DVT, especially if there's a history of recent immobilization, cancer, or other risk factors.
  • Septic Arthritis: An infection within the knee joint can cause severe pain, swelling, and warmth. This condition requires urgent medical attention to prevent joint destruction.
  • Tumor: Although rare, a tumor in or around the knee can cause pain, swelling, and instability. A thorough history and appropriate imaging are necessary to rule out this possibility.

• Rare Diagnoses

  • Osgood-Schlatter Disease: More common in adolescents, this condition causes pain, swelling, and tenderness below the knee cap due to inflammation of the patellar tendon.
  • Bursitis: Inflammation of the bursae around the knee can cause localized pain and swelling. The specific location of the pain can help identify which bursa is involved.
  • Plica Syndrome: This condition involves irritation of the synovial folds within the knee joint, leading to pain and swelling, often mimicking a meniscal tear.

Each of these diagnoses should be considered in the context of the patient's full medical history, physical examination findings, and potentially, diagnostic imaging or other tests to determine the underlying cause of the knee pain.