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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.