FIGURE 200-3 Diagram of the knee extensor mechanism. The major force exerted by the quadriceps muscle tends to pull the patella laterally out of the intercondylar sulcus. The vastus medialis muscle pulls medially to keep the patella centralized. (Modified from Smith JB: Knee problems in children, Pediatr Clin North Am 33:1439, 1986.)
The patellofemoral joint is the extensor mechanism of the knee and a common site of injury in the adolescent (Fig. 200-3).
Knee effusion or swelling is a common sign of injury. Whenthe fluid accumulates rapidly after an injury, it is usually ahemarthrosis (blood in the joint) and may indicate a fracture,ligamentous disruption (often of the anterior cruciate ligament[ACL]), or meniscus tear. Unexplained knee effusion mayoccur with arthritis (septic, Lyme disease, viral, postinfectious,juvenile idiopathic arthritis, systemic lupus erythematosus).It may also occur as a result of overactivity and hypermobilejoint syndrome (ligamentous laxity). An aspiration and laboratory evaluation of unexplained effusion can help expedite adiagnosis.
Each meniscus is normally semilunar in shape; rarely thelateral meniscus will be disk shaped. A normal meniscus isattached at the periphery and glides anteriorly and posteriorlywith knee motion. The discoid meniscus is less mobile and may tear more easily. When there is inadequate posterolateral attachment, the discoid meniscus can displace anteriorlywith knee flexion, causing an audible click. Most commonly,patients will present in late childhood or early adolescenceafter an injury with knee pain and swelling. The anteroposterior radiographs can show increased joint space and asquared-off appearance of the lateral femoral condyle; magnetic resonance imaging (MRI) can confirm the diagnosis.Treatment is usually arthroscopic excision of tears and reshaping of the meniscus.
A popliteal cyst (Baker cyst) is commonly seen in themiddle childhood years. The cause is the distension ofthe gastrocnemius and semimembranous bursa alongthe posteromedial aspect of the knee by synovial fluid. Inadults, Baker cysts are associated with meniscus tears. In childhood, the cysts are usually painless and benign. Theyoften spontaneously resolve, but it may take several years.Knee radiographs are normal. The diagnosis can be confirmed by ultrasound. Treatment is reassurance, becausesurgical excision is indicated only for progressive cysts or cysts that cause disability.
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Osteochondritis dissecans (OCD) most commonly involves theknee. It occurs when an area of bone adjacent to the articularcartilage suffers a vascular insult and separates from the adjacentbone. It most commonly affects the lateral aspect of the medialfemoral condyle. Patients may complain of knee pain or swelling.The lesions can be seen on anteroposterior, lateral, and tunnelview radiographs. MRI can be helpful in determining the extentof the injury. In young patients with intact articular cartilage,the lesion will often revascularize and heal with rest from activities. The healing process may take several months and requiresradiographic follow-up to document healing. With increasingage, the risk for articular cartilage damage and separation of thebony fragment increases. Older patients are more likely to needsurgical intervention. Any patient with a fracture of the articularcartilage will not improve without surgical intervention. Patientswith OCD should be referred to a specialist.
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