Anterior Cruciate Ligament

Case 3: A 16-year-old lacrosse player injured her right knee after she planted her right leg to take a shot on goal during a game. As she twisted her knee she felt a pop and had immediate pain. She had significant swelling within a few hours and has a hard time putting full weight on her knee. She has a large knee effusion and a positive Lachman test.

The ACL is the stabilizing ligament of the knee that prevents the tibia from moving forward off of the femur. The ACL is injured 2-8 x more frequently in adolescent females than males. Improper landing mechanisms are thought to be one of the reasons for this female predominance. ACL prevention programs are popular in sports such as soccer, basketball, and volleyball. The goal of these programs is to teach proper landing techniques to help prevent ACL injury.

The mechanism of injury is usually noncontact, with the foot planted when a twisting injury occurs at the knee. Athletes usually hear or feel a pop and have immediate pain and swelling within 24 hours. Contact injuries may also damage the ACL. On examination they can usually weight bear but have a large effusion. They have a positive Lachman test suggesting that the ACL is torn (Fig. 7). Radiographs are usually normal although a Segund Fracture is pathognomonic for an ACL tear (Fig. 8). An MRI may be useful in equivocal cases or in those with a soft endpoint on Lachman testing. Athletes with a torn ACL need surgical repair if they wish to return to cutting and pivoting sports as without an ACL, cutting and pivoting will damage the meniscus and may lead to early arthritis. Adolescents may also sustain a tibial spine fracture with the same mechanism that tears the ACL. Mechanisms that injure the ACL can also cause damage to the MCL or meniscus.

Figure 7 Lachman test for anterior cruciate ligament (ACL) integrity. With the patient's hip relaxed and slightly externally rotated, place your hands on the femur and tibia. You should be able to move the leg up and down with the hand supporting the distal femur. Place the tibia hand with the thumb just medial to the tibial tuberosity and the rest of the hand supporting the calf musculature. Hold the relaxed knee at 20-30° of flexion. While stabilizing the femur quickly pull upwards toward your shoulder (without rotating the leg) with the hand holding the tibia. If the ACL is intact, an endpoint (taught rubberband ending) will be appreciated. If the ACL is torn, no endpoint or a soft stoppage will be felt.

Figure 7 Lachman test for anterior cruciate ligament (ACL) integrity. With the patient's hip relaxed and slightly externally rotated, place your hands on the femur and tibia. You should be able to move the leg up and down with the hand supporting the distal femur. Place the tibia hand with the thumb just medial to the tibial tuberosity and the rest of the hand supporting the calf musculature. Hold the relaxed knee at 20-30° of flexion. While stabilizing the femur quickly pull upwards toward your shoulder (without rotating the leg) with the hand holding the tibia. If the ACL is intact, an endpoint (taught rubberband ending) will be appreciated. If the ACL is torn, no endpoint or a soft stoppage will be felt.

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