An ACL tear is an injury to the anterior cruciate ligament (ACL) in the knee. The ACL is one of the key ligaments that help stabilize the knee joint, connecting the thighbone (femur) to the shinbone (tibia). ACL injuries most commonly occur during sports that involve sudden stops or changes in direction, jumping and landing, such as soccer, basketball, football, and downhill skiing. The injury can occur if the knee moves or twists more than it naturally can, or if there is a sudden, high-energy impact to the knee. Symptoms of an ACL tear include a popping sound at the time of injury, pain, swelling, and knee instability. Treatment for an ACL tear can vary depending on the severity of the injury and any other damage inside the knee, but surgery is often required to regain full function of the knee.