normal if the injury is a nondisplaced Salter–Harris type I fracture Although not necessary as part of the ED evaluation, magnetic resonance imaging (MRI) may be needed when the physis is tender or a large effusion is present Acute traumatic avulsion of the tibial tuberosity is caused by acute stress on the knee’s extensor mechanism The quadriceps muscle group extends the knee by way of the patellar ligament The patellar ligament inserts on the tibial tuberosity and may avulse it during sudden acceleration (e.g., beginning a jump) or deceleration (e.g., landing after a jump) The patient will have tenderness and swelling over the tibial tubercle and be unable to extend the knee fully (or perform a straight leg raise) A lateral radiograph is diagnostic Fractures of the patella are rare in younger children because the patella does not ossify until to years of age, leaving it with a thick cartilage layer that protects it from direct trauma In addition, the soft tissue anchors of the patella are flexible which diffuses blunt forces However, a direct impact on the patella into the distal femur can cause transverse or comminuted fractures Much more common in children are avulsion fractures of the patella resulting from forceful contraction of the quadriceps With patellar fractures, the patient’s knee will be swollen, the patella tender, and knee extension painful A radiograph is usually diagnostic although care must be taken not to miss small avulsion fragments including sleeve fractures which are unique to pediatrics (egg-shell–like bony fragment that dislocates with avulsed soft tissue) Bipartite patellae are a normal variant and may be confused with an acute fracture Osteochondral fractures are fractures of articular cartilage and underlying bone not associated with ligamentous attachments These fractures often involve the femoral condyles or the patella The injury may follow a direct blow to the knee, a twisting injury, or patellar dislocation The patient will have severe pain, immediate swelling, and will hold the knee partially flexed Hemarthrosis may be present Knee radiographs should include an intercondylar view because the fracture fragment may be in the intercondylar notch Osteochondral fractures can be missed because only the small ossified portion of the osteochondral fragment is radiopaque MRI may be necessary for diagnosis Analogous to an adolescent who ruptures the anterior cruciate ligament (ACL), patients to 16 years old may sustain avulsion fractures of the tibial spine at the point where the ACL inserts The tibial spine is incompletely ossified and may avulse before the ligament ruptures The patient may have a hemarthrosis and will be unable to bear weight If the patient tolerates an examination, the Lachman test (see Fig 42.3 ) may be positive because the injury is similar mechanically to an ACL tear AP, lateral, and intercondylar or tunnel-view radiographs will show the