Diagnosis for ACL is done in three ways:

1. Lachman’s Test (20deg)
2. Anterior Drawer Test (Lachman’s 90deg)
3. Pivot Shift Test

The first diagnostic tool in assessing any potential injury, as with any medical or physical disorder, is through patients history. History with anterior cruciate ligament injuries is surprisingly typical. The patient usually describe hyperextension motion, hearing an actual “popping’ sound, a sensation of the knee giving way, and pain. All these symptoms are followed by swelling of the knee.

While the patient history provides important clues to the orthopaedist, an examination by an experienced clinician is the most valuable diagnostic tool available. A well-performed physical examination will confirm a suspected injury and assess the severity of the injury as well. The hallmark of an anterior cruciate ligament injury is increased or pathologic tibio-femoral translation (motion between the tibia and femur) as compared to the opposite knee. Following are the most common tests used to measure instability of the joint in various planes of motion, when an injury to the anterior cruciate ligament is suspected.

The most sensitive test is the Lachman test. The examiner grasps the leg at the level of the tibia tubercle with one hand, thumb on the joint line below the patella. The other hand grasps the leg just above the superior pole of the patella. An anterior displacing force is applied to the tibia while stabilising the femur. Translation of the tibia is palpated via the thumb on the joint line. In a normal knee, anterior translation will be minimal, and there is distinct end point to the movement a significant increase in anterior translation and a “soft” or non-existent end point indicate a positive response to the Lachman test.

The anterior drawer test is generally not used in acute cases, because the test can be very painful since it is performed with the knee in 900 of flexion. In the chronic cases, the examiner places both hands on the lower leg just below the knee, with the fingers extending posteriorly around the leg and the thumbs on the joint line. With the patient’s foot in neutral rotation, a strong anterior displacing force is exerted with the fingers against the back of the leg, and the amount of anterior movement of the tibia at the joint line is palpated with the thumbs. A positive response to the anterior drawer test is measured when there is a significant anterior translation compared to the opposite knee. The anterior drawer test is not as sensitive as the Lachman test because the 900 flexion bring the menisci into play, and they can have a stabilising effect on the knee by acting as break stops to the anterior translation.

In suspected cases of chronic anterior cruciate ligament injury, the pivot shift test, or lateral pivot shift test is not useful. This test is not used in acute injuries, since it almost always shows a negative response in fresh injuries, even when there is damage to the anterior cruciate ligament. The examiner grasps the foot with one hand and places the other against the lateral aspect of the leg, below the knee. Pressure with this hand produces a slight valgus stress. The leg is elevated with the knee in extension. The examiner begins to flex the knee, continuing to apply valgus stress. If the anterior cruciate ligament is ruptured, the lateral knee approaches 200 -300 of flexion. Extending the knee again subluxes the tibia anteriorly. Even though the pivot shift can be falsely negative, a positive response is definitive of an anterior cruciate ligament tear.

Although rarely needed, one of the most accurate diagnostic tools for determining the extent of injury to the anterior cruciate ligament is a diagnostic arthroscopy.

Radiographs are usually normal in anterior cruciate ligament injuries, but they will reveal small avulsion fractures at the ligament attachments sites if that is the mechanism of injury. MRIs are 90% accurate in anterior cruciate ligament injuries and so they can be useful when physical examinations are inconclusive.