Posterior dislocation of tibia on femur is prevented by:
**Core Concept**
The posterior dislocation of the tibia on the femur is prevented by the structures that form the posterior aspect of the knee joint, particularly the cruciate ligaments and the menisci. The primary mechanism of prevention involves the bony and ligamentous constraints that limit excessive movement of the tibia in relation to the femur.
**Why the Correct Answer is Right**
The posterior cruciate ligament (PCL) is the primary stabilizer of the posterior aspect of the knee joint. It prevents posterior subluxation or dislocation of the tibia on the femur. The PCL is composed of two bundles: the anteroinferior bundle and the posterousuperior bundle. The anteroinferior bundle is taut in extension and prevents excessive posterior movement of the tibia, while the posterousuperior bundle is taut in flexion and prevents excessive anterior movement. The menisci also play a role in stabilizing the knee joint by providing a cushioning effect and preventing excessive movement of the tibia.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify which ligament or structure prevents posterior dislocation of the tibia. While the anterior cruciate ligament (ACL) is crucial for preventing anterior subluxation of the tibia, it does not prevent posterior dislocation.
**Option B:** This option is incorrect because it is not clear what "medial ligament" refers to. While the medial collateral ligament (MCL) provides stability to the medial aspect of the knee joint, it is not primarily responsible for preventing posterior dislocation of the tibia.
**Option C:** This option is incorrect because it does not specify a particular structure or ligament that prevents posterior dislocation of the tibia.
**Option D:** This option is incorrect because it is not a specific ligament or structure that prevents posterior dislocation of the tibia.
**Clinical Pearl / High-Yield Fact**
The posterior cruciate ligament is often injured in conjunction with the anterior cruciate ligament in high-energy trauma, such as dashboard injuries. It is essential to assess the PCL in patients with knee injuries to prevent long-term consequences such as knee arthritis.
**Correct Answer: C. Posterior Cruciate Ligament (PCL).**