FIGURE 42.2 Anatomy of the knee—sagittal section FIGURE 42.3 Testing for anterior cruciate ligament injury with the Lachman test Flex the knee 20 to 30 degrees, support the thigh with one hand, and grasp the calf with the other hand Move the tibia forward on the femur Observe the tibial tubercle for movement and feel for excessive forward movement of the tibia in relation to the femur If the history is consistent with dislocation but the patient is no longer in pain and has a normal examination, the patella may have subluxated or dislocated and self-reduced On examination, the patella may be high riding or laterally displaced The patellar apprehension test can be performed by gently attempting to move the patella laterally If the patient has guarding or apprehension, this suggests a previously subluxated (or fully dislocated) patella Radiographs should be obtained to look for an associated osteochondral fracture Soft Tissue Injuries Significant medial collateral ligament (MCL) or lateral collateral ligament (LCL) injuries are rare when the physis is open because the involved ligaments are stronger than the growth plate The LCL inserts on the fibular head proximal to the physis, and the MCL inserts on the tibia distal to the physis In older patients, the MCL or LCL may be damaged by a blow to the lateral or medial side of the knee, respectively This can occur during contact sports, or when stressed during noncontact injuries such as with skiing or activities that involve pivoting Severe collateral ligament injury may be associated with ACL or meniscal damage On examination, the knee may be swollen or tender over the involved ligament The knee should be tested for lateral laxity (i.e., valgus and varus stress tests) in full 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 FIGURE 42.4 Testing for collateral ligament injury Test the knee in full extension and in 30 degrees of flexion To test for medial collateral ligament injury (A ), hold and apply force to the medial side of the ankle with one hand and apply pressure over the fibular head with the other hand To test for lateral collateral ligament injury (B ), hold and apply force to the lateral side of the ankle with one hand and apply pressure just below the medial side of the knee with the other