extension and in 30 degrees of flexion, as shown in Figure 42.4 Nonemergent orthopedic referral may be indicated if the examination reveals lateral or medial laxity ACL injuries occur in many scenarios, but usually involve rotational forces on a fixed foot The patient often reports the sensation of a “pop.” The joint usually swells rapidly as a result of hemarthrosis and has a marked decrease in ROM The Lachman test ( Fig 42.3 ) is sensitive (0.7 to 0.9) in detecting ACL injuries but may be falsely negative soon after the injury, when the knee is swollen and painful Examining the uninjured knee can be helpful for comparison MRI and occasionally arthroscopy are often needed for definitive diagnosis ACL injuries are rare before adolescence because in a child, the ACL’s insertion point, the tibial spine, is incompletely ossified and more likely to be injured than the ligament Radiographs may detect an associated epiphyseal fracture, tibial spine fracture, or an avulsed bone fragment due to concurrent MCL or LCL injury