A healthy 27-year-old Caucasian man is brought to the emergency department after being involved in a motor vehicle accident. He complains of right knee pain and swelling. He was sitting in the front seat without a seat belt. His knees hit the front dashboard. His vital signs are stable. On examination, there is mild swelling and stiffness. Which of the following was most likely injured in this patient's knee joint?

Question

A healthy 27-year-old Caucasian man is brought to the emergency department after being involved in a motor vehicle accident.  He complains of right knee pain and swelling.  He was sitting in the front seat without a seat belt.  His knees hit the front dashboard.  His vital signs are stable.  On examination, there is mild swelling and stiffness.  Which of the following was most likely injured in this patient's knee joint?

Quizplus · Accepted Answer

Explanation: This patient has a classic 'dashboard' injury, wherein a posteriorly directed force is placed on the anterior aspect of the proximal tibia, with the knee in a flexed position. This results in disruption of the posterior cruciate ligament (PCL). A similar injury can be seen in an athlete who falls on a flexed knee with the foot in plantar flexion. Unlike patients with ACL injury, these patients do not complain of a typical 'popping' sound, and may not have an unstable knee. (Choice A) ACL injuries are common in athletic activities. An isolated injury may occur after hyperextension of the knee. Up to 33% to 90% of patients with an ACL tear experience a 'popping' sensation. The rapid onset of a knee effusion and an unstable knee are common. Lachman's test is a very sensitive physical test for acute anterior cruciate ligament tear. It is done with the patient's knee flexed at 20 degrees; one hand pulls the proximal tibia, while the other hand stabilizes the femur. (Choices C) Meniscal injuries principally occur after a twisting injury to the knee with one foot fixed to the ground (e.g. with sudden turning while running). The medial meniscus is more commonly injured, as compared to the lateral meniscus. A bucket handle tear is the most common type of medial meniscus tear. Patients generally complain of pain and swelling of the knee, and a popping sensation at the time of injury. In medial meniscus injuries, tenderness is generally felt along the medial side of the knee. Effusion with meniscus injuries takes about 24 hours to form, and is often not significantly bloody, unlike ACL or osteochondral injuries. Locking of the knee joint on extension is generally seen. McMurray's sign is snapping felt with tibial torsion and the knee flexed at 90 degrees. (Choices D and E) Medial and lateral collateral ligaments are injured when the line of force strikes from the side of the joint. An abduction injury with a torsional component causes a medial collateral ligament tear in most of the instances. On examination, the knee joint is swollen due to the effusion, with tenderness over the medial aspect of the knee. As the medial collateral ligament resists valgus angulations at the knee, injury to this ligament leads to increased angulation of the affected knee on valgus stress. A valgus stress test is generally performed with the knee flexed at 20-30 degrees. Educational Objective: The classic dashboard injury results from a posteriorly directed force on the anterior aspect of the proximal tibia with the knee in a flexed position. This results in disruption of the posterior cruciate ligament (PCL).

A Healthy 27-Year-Old Caucasian Man Is Brought to the Emergency