A 17-year-old boy from Calcutta was playing football when he was tackled by another player. The opponent hit the lateral aspect of his knee. He presented to a hospital 1 week after the injury, complaining of swelling and pain in his right knee. On physical examination of his right knee, there is a large effusion. There is increased laxity (as compared to his uninjured knee) of his knee when his knee is passively placed in a valgus (abducted) position. In addition, there is significant anterior translation of his tibia with respect to his femur when his knee is tested at ninety degrees of flexion. Injury to which of the following structures most likely accounts for the increased laxity of his knee when his knee is passively placed in a valgus position?
A 17-year-old boy from Calcutta was playing football when he was tackled by another player. The opponent hit the lateral aspect of his knee. He presented to a hospital 1 week after the injury, complaining of swelling and pain in his right knee. On physical examination of his right knee, there is a large effusion. There is increased laxity (as compared to his uninjured knee) of his knee when his knee is passively placed in a valgus (abducted) position. In addition, there is significant anterior translation of his tibia with respect to his femur when his knee is tested at ninety degrees of flexion. Injury to which of the following structures most likely accounts for the increased laxity of his knee when his knee is passively placed in a valgus position?
💡 Explanation
**Core Concept**
The patient's symptoms suggest a knee injury involving ligamentous instability, specifically valgus instability. The valgus stress test is used to evaluate the integrity of the medial collateral ligament (MCL) and the posteromedial corner of the knee.
**Why the Correct Answer is Right**
The valgus stress test is positive when there is increased laxity of the knee in a valgus position, indicating a possible injury to the MCL or its associated structures. The MCL is a strong ligament that provides medial stability to the knee, preventing excessive valgus stress. When the MCL is injured, the knee becomes more susceptible to valgus stress, leading to increased laxity.
**Why Each Wrong Option is Incorrect**
**Option A:** The anterior cruciate ligament (ACL) is primarily responsible for preventing excessive anterior translation of the tibia with respect to the femur, not valgus stress. ACL injuries often result in a positive Lachman test or a positive anterior drawer test.
**Option B:** The posterior cruciate ligament (PCL) is responsible for preventing excessive posterior translation of the tibia with respect to the femur. PCL injuries do not typically result in valgus instability.
**Option C:** The medial meniscus is a cartilaginous structure that provides cushioning and stability to the knee joint. While meniscal injuries can occur in conjunction with ligamentous injuries, they do not directly contribute to valgus instability.
**Clinical Pearl / High-Yield Fact**
Remember the "5 Ps" of MCL injuries: Pain, Palpable tenderness, Positive valgus stress test, Painful range of motion, and Periarticular swelling. These symptoms can help differentiate MCL injuries from other knee injuries.
**Correct Answer:** C. The medial collateral ligament (MCL).
✓ Correct Answer: C. Medial collateral ligament
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