A 27-year-old man is brought to the emergency department due to knee pain and swelling after a motor vehicle collision. He was unrestrained in the passenger seat, and his shins struck the dashboard on impact. Vital signs are stable. The right knee is swollen and diffusely tender, and the patient guards the knee on examination. X-ray of the right knee shows a large effusion but no fracture or dislocation. Which of the following was most likely injured in this patient's knee joint?
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This patient most likely injured the posterior cruciate ligament (PCL). The PCL originates from the lateral surface of the medial femoral condyle and inserts into the posterior intercondylar area of the tibia. Its function is to prevent posterior displacement of the tibia relative to the femur. During the classic dashboard injury, a strong posteriorly directed force strikes the anterior aspect of the proximal tibia while the knee is in a flexed position. This causes significant posterior translation of the tibia in relation to the femur and disrupts the PCL.
PCL injury due to high-energy trauma (eg, motor vehicle collision) usually causes an acute, large hemarthrosis and diffuse tenderness of the knee. Integrity of the PCL may be tested using the posterior drawer test; however, the test can be difficult to perform if significant pain and swelling are present or if the patient guards the knee. Plain x-rays may show an effusion. Definitive diagnosis is made with MRI.
(Choice A) The anterior cruciate ligament (ACL) functions to resist anterior displacement of the tibia relative to the femur. Therefore, a posteriorly directed force to the anterior tibia (eg, dashboard injury) is unlikely to cause an isolated ACL tear. Most (>70%) ACL injuries occur via noncontact mechanisms that cause the tibia to move anteriorly while under valgus stress (eg, sudden deceleration or pivot).
(Choices B and C) The lateral collateral ligament (LCL) and medial collateral ligament (MCL) are classically injured when a force strikes the side of the knee. LCL injury typically occurs with a blow to the medial knee (varus stress injury); MCL injury typically occurs with a direct blow to the lateral knee while the foot is planted (valgus stress injury). Isolated tenderness along the lateral (LCL) or medial (MCL) aspect of the joint is commonly present.
(Choice D) Meniscus injuries primarily occur due to a twisting injury to the knee (vs a posterior force) and typically cause joint line (vs diffuse) tenderness. Associated knee effusions, when present, are usually smaller and develop gradually (eg, over hours to days).
Educational objective:
The posterior cruciate ligament (PCL) functions to prevent posterior displacement of the tibia relative to the femur. It can be disrupted when a strong posteriorly directed force strikes the anterior aspect of the proximal tibia while the knee is in a flexed position (ie, classic dashboard injury).