A 20-year-old soccer player presents with posterior sag of the tibia after landing on a bent knee. Which ligament is most likely injured?

A 20-year-old soccer player presents with posterior sag of the tibia after landing on a bent knee. Which ligament is most likely injured?

• A. ACL
• B. PCL
• C. MCL
• D. LCL

Answer: (B)

The main idea is that the posterior cruciate ligament resists backward (posterior) movement of the tibia when the knee is flexed. When someone lands on a bent knee and the tibia sags posteriorly, it points to a torn PCL because that ligament normally prevents the tibia from sliding backward relative to the femur. In this scenario, the posterior sag is a classic sign of PCL injury, and tests such as a posterior drawer test would likely be positive. The other ligaments don’t fit this picture: the ACL mainly prevents the tibia from moving forward, so injuries there produce anterior instability rather than a posterior sag; the MCL and LCL resist valgus and varus stresses on the knee, causing medial or lateral instability instead of posterior translation.

The main idea is that the posterior cruciate ligament resists backward (posterior) movement of the tibia when the knee is flexed. When someone lands on a bent knee and the tibia sags posteriorly, it points to a torn PCL because that ligament normally prevents the tibia from sliding backward relative to the femur. In this scenario, the posterior sag is a classic sign of PCL injury, and tests such as a posterior drawer test would likely be positive.

The other ligaments don’t fit this picture: the ACL mainly prevents the tibia from moving forward, so injuries there produce anterior instability rather than a posterior sag; the MCL and LCL resist valgus and varus stresses on the knee, causing medial or lateral instability instead of posterior translation.