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An ACL injury is the tearing of the anterior cruciate (KROO-she-ate) ligament, or ACL, inside your knee joint. An ACL injury most commonly occurs during sports that involve sudden stops and changes in direction — such as basketball, soccer, tennis and volleyball.
Immediately after an ACL injury, your knee may swell, feel unstable and become too painful to bear weight. Many people hear or feel a "pop" in their knee when an ACL injury occurs.
Depending on the severity of your ACL injury, treatment may include surgery to replace the torn ligament followed by rehabilitation exercises to help you regain strength and stability. If your favorite sport involves pivoting or jumping, a proper training program may help reduce your chances of an ACL injury.