Differential Diagnosis for a Cystic Area Posterior to the Posterior Horn of Medial Meniscus in a 15-Year-Old with Knee Pain

• Single Most Likely Diagnosis
  • Meniscal cyst: This is the most likely diagnosis given the location and description of the cystic area. Meniscal cysts are known to occur in association with meniscal tears, and the posterior horn of the medial meniscus is a common location. The patient's age and presentation with knee pain also support this diagnosis, as meniscal injuries can occur in adolescents, especially those involved in sports.
• Other Likely Diagnoses
  • Ganglion cyst: While less common than meniscal cysts in this specific location, ganglion cysts can occur around the knee and could present as a cystic area posterior to the posterior horn of the medial meniscus. These cysts are benign and filled with mucinous fluid.
  • Bursitis (e.g., semimembranosus bursitis): Inflammation of the bursae around the knee can present as fluid-filled structures on imaging. The location posterior to the medial meniscus could be consistent with involvement of the semimembranosus bursa.
• Do Not Miss Diagnoses
  • Soft tissue sarcoma (e.g., synovial sarcoma): Although rare, soft tissue sarcomas can present as cystic or solid masses around the knee. It is crucial to consider these malignancies to ensure timely diagnosis and treatment.
  • Infection (e.g., abscess): An infectious process could present as a fluid collection in the soft tissues around the knee. This would be a medical emergency requiring prompt diagnosis and treatment.
• Rare Diagnoses
  • Synovial chondromatosis: This condition involves the growth of cartilage in the synovium, which can form cystic structures or loose bodies within the joint. It is less common and might present with a cystic area in the context of joint disease.
  • Pigmented villonodular synovitis (PVNS): A rare condition characterized by proliferation of the synovium, which can form cystic structures and lead to joint degeneration. It is an uncommon cause of knee pain and cystic lesions around the menisci.