Two days after signing a three-year contract extension worth $50 million, Arizona Cardinals quarterback Carson Palmer sustained a season-ending ACL injury. Injuring the Anterior Cruciate Ligament (ACL) is a fear all athletes share and rightfully so. The ACL is responsible for 90 percent of stability in the knee joint. This video courtesy of Elara Systems shows exactly what happens inside of a knee when an ACL injury occurs.

According to the American Academy of Orthopedic Surgeons, sprains or tears of the ACL are among the most common types of knee injury. ACL injuries are more prevalent in physically demanding sports such as soccer, basketball, and football. Depending on severity and the patient’s activity level, most ACL injuries require surgery and physical rehabilitation before the patient can regain full function of their knee.

The ACL is one of four major ligaments connecting bone to bone inside the knee joint while coordinating function and promoting stability. It also prevents forward movement of the tibia in the adult knee. Around 200,000 ACL injuries occur each year, half of which require reconstructive surgery. Due to a more physically active lifestyle and a higher participation in sports, one in 1,750 ACL injuries occur among patients between the age of 15 and 45.