Evaluating the child’s knee is different from evaluating the knee of the adult because disorders are more likely to be due to some underlying generalized dysplasia or to focal congenital or developmental deformity.
Screen for some underlying abnormality, such as nail–patella syndrome. Asyptomatic dislocation of the patella is common in Down syndrome. Dimpling over the knee is common in arthrogryposis. Recurvatum occurs in spina bifida and in arthrogryposis. Genu varum and valgum are common in ricketic disorders, and genu valgum is common in Morquio and Ellis–Van Creveld syndromes.
The physical examination usually provides the diagnosis or at least the basis for ordering further studies.
General inspection Look for obvious deformity, check the knee angle, and perform a rotational profile.
Knee Observe the child standing and note symmetry, knee angle, position of patella, masses, joint effusion, muscle definition and atrophy, and signs of inflammation. Is there full extension or hyper-extension?
Patellar tracking Ask the child to sit and slowly flex and extend the knee. Observe the tracking of the patella. Does it move in a linear fashion or displace laterally as the knee extends? Repeat the examination with the hand on the patella as the knee is flexed both actively and passively. Does the patella move smoothly and track in the midline? Does the knee fully flex and extend?
Q angle is the angle formed by a line connecting the anterior superior iliac spine with the midportion of the patella and a second line from the patellar midpoint to the tibial tubercle. Normally, the enclosed angle is less than about 15°. Be aware that the Q angle has no direct relationship with knee pain or patellar instability.
Point of maximum tenderness Locate the PMT by systematically examining the entire knee and tibia. The PMT often establishes a working diagnosis.
Palpate to assess temperature, swelling, and tenderness. Is the affected knee warmer than the other knee? Is a joint effusion present? Parapatellar fullness suggests a joint effusion. Evaluate any fullness by extending the knee, compressing the suprapatellar region, and checking for a fluid wave in the knee. A posttraumatic effusion is a sign of a significant intraarticular injury such as a torn peripheral meniscus, anterior cruciate ligament injury, or osteochondral fracture. Do not confuse prepatellar swelling with an intraarticular effusion.
Manipulate to determine if the patella is displaceable. In loose-jointed children, the patella is very mobile and more likely to dislocate.
Patellar apprehension is elicited by extending the knee and attempting to displace the patella laterally. Patients with recurrent dislocations who sense that this may cause the patella to dislocate may become apprehensive and may reach out to stop the examination.
Knee motion Is the arch of motion free and unguarded? Is crepitation or snapping present?
Lachman test Test anteroposterior laxity with this test. Flex the knee about 15°–20° and attempt to displace the tibia anterior in its relationship to the femur. Normally, a firm endpoint will be felt. Check for instability with varus and valgus stress. With the knee flexed to a right angle, evaluate for anterior or posterior drawer signs.
Rotational instability test Test the pivot-shift for ACL injury and capsular laxity by extending the knee fully and apply valgus and internal rotation stress to demonstrate anterolateral tibial subluxation.
Perform the reverse pivot shift test by first flexing and externally rotating the knee. Next, extend the knee to demonstrate postero-lateral capsular laxity associated with PCL injury.
Special radiographic projects such as sunrise and notch views may be useful. If conventional radiographs are not adequate, order special imaging studies.
Bone scans may be helpful in determining the location or activity of lesions. The study is sensitive but not specific.
MRI studies are overused and not appropriate for screening and are frequently overread, even in normal knees. They may be useful for ligamentous and meniscal injuries when correlated with clinical findings.
Ultrasound is useful for cysts and pre-patellar swelling evaluation.
Arthroscopy is essential for assessing meniscal injuries and for other ligamentous and osteochondral problems in children. It is less valuable for assessing pain.