An anterior cruciate ligament injury is the over-stretching or tearing of the anterior cruciate ligament (ACL) in the knee. A tear may be partial or complete. The knee joint is located where the end of the thigh bone (femur) meets the top of the shin bone (tibia). Four main ligaments connect these two bones: Medial collateral ligament (MCL) runs along the inside of the knee. It prevents the knee from bending inward. Lateral collateral ligament (LCL) runs along the outside of the knee. It prevent