Which ligament of knee is affected when a direct blow strike from the lateral side of knee in a medial direction?

Question

Accepted Answer

The correct option is (1):
A direct blow to the lateral side of the knee, which forces the knee inward in a motion known as valgus stress, is a common mechanism for severe knee trauma. While the initial force is directed to stress the medial side, a high-velocity or high-impact valgus blow often causes the joint to buckle and rotate, leading to a complex multi-ligament injury. In such a severe situation, the extreme forces cause the knee to subluxate (partially dislocate), subjecting the Anterior Cruciate Ligament (ACL) to a non-physiological shearing and rotational load that exceeds its tensile strength. The ACL's critical function as the primary stabilizer against both anterior displacement of the tibia and rotational forces makes it highly susceptible to rupture under this severe, multi-planar stress.

The incorrect option is (2):
The Posterior Cruciate Ligament (PCL) is the main stabilizer that prevents the tibia from moving too far backward relative to the femur. The typical mechanism for an isolated PCL injury is a direct force applied to the front of the shin bone (tibia) while the knee is bent, a common example being a fall directly onto the knee or a "dashboard injury." Since the question describes a blow from the *lateral* side, which primarily creates inward (valgus) and rotational forces, this type of trauma does not directly create the posterior displacement required to cause an isolated PCL tear.

The incorrect option is (3):
The Medial Collateral Ligament (MCL) is anatomically positioned to be the primary ligamentous restraint against valgus stress. Therefore, a direct blow from the lateral side of the knee, driving the knee medially, is the classic mechanism for an MCL sprain or tear. However, the question implies a significant level of trauma, and in cases of severe valgus stress, the MCL often tears first, but the extreme force continues to be transmitted across the joint, leading to a subsequent tear of the Anterior Cruciate Ligament (ACL). Thus, while the MCL is the initial restraint, the ACL tear signifies the ultimate failure of the joint's core stabilizers.

The incorrect option is (4):
The Lateral Collateral Ligament (LCL) is located on the outer side of the knee and serves to resist varus stress—a force applied from the inner (medial) side of the knee that pushes the lower leg outward. The injury described in the question—a direct blow from the *lateral* side—is a valgus force. This valgus force compresses the lateral side of the knee joint, which effectively causes the LCL to become slack and unstressed. Therefore, the mechanism of a lateral blow directly protects the LCL and is the opposite of the force required to cause an LCL injury.