Differential Diagnosis for Left Knee Pain
Single most likely diagnosis
- Anterior Cruciate Ligament (ACL) injury: The mechanism of injury (hyperextension during soccer) and the presence of an effusion in the suprapatellar pouch and at the lateral anterior joint line on ultrasound are highly suggestive of an ACL injury. ACL injuries are common in sports that involve sudden stops, changes of direction, and jumping, such as soccer.
Other Likely diagnoses
- Meniscal tear: The presence of an effusion and the mechanism of injury could also suggest a meniscal tear. Meniscal tears can occur due to twisting or hyperextension injuries, and they often present with joint line tenderness and effusion.
- Ligament sprain (MCL or LCL): Although the mechanism of injury is more typical of an ACL injury, a ligament sprain of the medial collateral ligament (MCL) or lateral collateral ligament (LCL) is also possible, especially if there was a valgus or varus stress component to the injury.
- Patellofemoral pain syndrome: This condition could be considered if the patient had a history of anterior knee pain or if the examination revealed patellar tenderness or maltracking. However, the acute onset and presence of an effusion make this diagnosis less likely.
Do Not Miss diagnoses
- Fracture (e.g., occult fracture or stress fracture): Although the X-ray is negative, it is essential to consider the possibility of an occult fracture, especially if the patient has a high-impact mechanism of injury or if there are other risk factors for osteoporosis or stress fractures.
- Septic arthritis: This is a medical emergency that requires prompt diagnosis and treatment. Although the patient's presentation does not strongly suggest septic arthritis (e.g., no fever, no recent infection), it is crucial to consider this diagnosis, especially if the patient is immunocompromised or has a history of recent joint injection or surgery.
- Osteonecrosis: This condition can occur due to trauma or other factors, such as steroid use or sickle cell disease. Although it is less likely in a young patient with a recent injury, it is essential to consider osteonecrosis in the differential diagnosis.
Rare diagnoses
- Osteochondritis dissecans: This condition involves a fragment of cartilage and bone that becomes detached from the surrounding bone. It can occur due to trauma or other factors, such as genetics or metabolic disorders.
- Plica syndrome: This condition involves inflammation of the synovial folds (plicae) within the knee joint. It can cause knee pain and effusion, but it is a rare condition and usually requires arthroscopic evaluation for diagnosis.
- Neoplastic conditions (e.g., osteosarcoma, synovial sarcoma): These conditions are rare and usually present with more systemic symptoms, such as weight loss or night pain. However, it is essential to consider them in the differential diagnosis, especially if the patient has a persistent or unexplained effusion.