The anterior cruciate ligament prevents the tibia from sliding out in front of the femur and provides rotational stability.There are also two C-shaped structures made of cartilage called the medial meniscus and lateral meniscus that sit on top of the tibia in the knee joint and serve as cushion for the bones. Diagnosis
Manual tests
Most ACL injuries can be diagnosed by examining the knee and comparing it to the other, non-injured knee. When a doctor suspects ACL injury in a person who reports a popping sound in the knee followed by swelling, pain, and instability of the knee joint, they can perform several tests to evaluate the damage to the knee. These tests include the pivot-shift test, anterior drawer test, and Lachman test. The pivot-shift test involves flexing the knee while holding onto the ankle and slightly rotating the tibia inwards. In the anterior drawer test, the examiner flexes the knees to 90 degrees, sits on the persons feet, and gently pulls the tibia towards themself. The Lachman test is performed by placing one hand on the persons thigh and the other on the tibia and pulling the tibia forward. These tests are meant to test whether the ACL is intact and therefore able to limit the forward motion of the tibia. The Lachman test is recognized by most authorities as the most reliable and sensitive of the three.
Force applied to a less stiff knee is more likely to result in ACL tears.In addition, the quadriceps femoris muscle is an antagonist to the ACL. According to a study done on female athletes at the University of Michigan, 31% of female athletes recruited the quadriceps femoris muscle first as compared to 17% in males. Because of the elevated contraction of the quadriceps femoris muscle during physical activity, an increased strain is placed onto the ACL due to the "tibial translation anteriorly". Pathophysiology
The knee joint is formed by three bones: the femur (thighbone), the tibia (shinbone), and the patella (kneecap). These bones are held together by ligaments, which are strong bands of tissue that keep the joint stable while an individual is walking, running, jumping, etc. There are two types of ligaments in the knee: the collateral ligaments and the cruciate ligaments.The collateral ligaments include the medial collateral ligament (along the inside of the knee) and the lateral or fibular collateral ligament (along the outside of the knee). These two ligaments function to limit sideways movement of the knee.The cruciate ligaments form an "X" inside the knee joint with the anterior cruciate ligament running from the front of the tibia to the back of the femur, and the posterior cruciate ligament running from the back of the tibia to the front of the femur.