Differential Diagnosis for 11 YOM with Knee Injury

Single Most Likely Diagnosis

• Ligamentous sprain (e.g., MCL or ACL sprain): Given the mechanism of injury (hit on the knee with a feeling of it "going backwards") and the patient's difficulty ambulating, a ligamentous sprain is a common and plausible diagnosis. The full range of motion and normal X-ray do not rule out ligamentous injuries.

Other Likely Diagnoses

• Meniscal injury: The mechanism of injury could also suggest a meniscal tear, especially if the knee was twisted or bent at the time of impact. Symptoms might include pain, swelling, and difficulty moving the knee, although full range of motion does not exclude this diagnosis.
• Contusion or bruising: Direct trauma to the knee could result in a contusion, which would cause pain and difficulty walking but might not be evident on an X-ray.
• Patellofemoral pain syndrome: This condition, often related to overuse or direct trauma, could be considered if the patient has anterior knee pain, especially with activities that load the patellofemoral joint.

Do Not Miss Diagnoses

• Fracture not visible on initial X-ray (e.g., occult fracture): Although the X-ray is normal, some fractures (especially stress fractures or certain types of fractures in children, like a slipped capital femoral epiphysis in younger patients or a fracture through a growth plate) might not be immediately visible. Repeat imaging or a bone scan might be necessary if suspicion remains high.
• Vascular injury: Direct trauma to the knee could potentially injure the popliteal artery or vein, leading to serious complications if not promptly addressed. Signs might include pallor, pulselessness, poikilothermia, paresthesias, pain, and paralysis.
• Neurological injury: Injury to the nerves around the knee (e.g., peroneal nerve) could occur from direct trauma, leading to symptoms like numbness, tingling, or weakness in the leg.

Rare Diagnoses

• Osteochondritis dissecans: A condition where a piece of cartilage and a thin layer of the bone beneath it becomes damaged due to inadequate blood supply, leading to pain and swelling. It might not be immediately apparent on a plain X-ray.
• Discoid meniscus: A rare condition where the meniscus is thicker and more circular than usual, which could lead to knee pain and snapping sensations, especially with certain movements.