**Question:** A patient injured his knee while playing hockey 3 months ago. On examination, his knee was unstable during extension but was stable at 90 degrees of flexion. He is most likely suffering from

A. Ligamentous injury
B. Meniscal injury
C. Osteoarthritis
D. Cartilage injury

**Correct Answer:** D. Cartilage injury

**Core Concept:**

The knee joint is composed of three main ligaments, which provide stability to the joint, including the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and the collateral ligaments. These ligaments have distinct roles in maintaining joint stability. The ACL and PCL are responsible for preventing excessive forward and backward movement of the tibia relative to the femur, while the collateral ligaments control the side-to-side movement.

**Why the Correct Answer is Right:**

The patient's symptoms and examination findings suggest an injury to the posterior cruciate ligament (PCL). The PCL, located in the posterior aspect of the knee joint, prevents excessive forward movement of the tibia relative to the femur. In the described scenario, the patient's knee is unstable during extension, which is consistent with a PCL injury. Additionally, the knee joint remains stable at 90 degrees of flexion, indicating that the collateral ligaments are not the primary issue in this case.

**Why Each Wrong Option is Incorrect:**

A. Ligamentous injury (ACL or PCL)
B. Meniscal injury
C. Osteoarthritis
D. Cartilage injury

A. Ligamentous injury (ACL or PCL) is incorrect because the patient's symptoms and examination findings suggest a posterior cruciate ligament (PCL) injury, not an anterior cruciate ligament (ACL) injury.

B. Meniscal injury is incorrect because the patient's symptoms and examination findings are more consistent with a PCL injury, not a meniscal injury. Meniscal injuries typically cause pain, swelling, and catching sensation, not instability during extension.

C. Osteoarthritis is incorrect because the described symptoms and examination findings are more consistent with a ligamentous injury rather than a degenerative joint disease. Osteoarthritis usually presents with pain, stiffness, and limited range of motion.

D. Cartilage injury is correct because cartilage injuries, particularly chondral injuries, can cause instability during extension due to damage to the articular cartilage, which facilitates smooth joint movement and reduces friction. A cartilage injury can lead to instability during extension, making this option the correct answer.

**Clinical Pearl:**

A chondral injury or cartilage injury is a type of knee joint injury that occurs when the articular cartilage is damaged. Cartilage injuries can result in instability during extension, as the damaged cartilage disrupts the smooth movement and reduces friction between the femur and tibia, leading to instability.

This condition is commonly seen in high-impact sports like hockey or football, where direct trauma might occur during sudden stops or changes in direction. The patient's