The physician will take a thorough history addressing how the injury occurred and ascertaining when the pain may have first appeared. Questions regarding any earlier knee injuries are important as often ligaments and cartilage structures may have been previously strained. Any previous episodes of knee instability or the knee giving way is important information.
The physician can determine the integrity of the ACL by conducting special ligament stability tests. One simple but important test is called the Lachmans test. With the knee bent to 30 degrees, the physician gently pulls on the tibia to check the forward motion of the lower leg in relation to the upper leg. A normal knee will have less than two to four mm of forward movement, with a firm stopping felt when no further movement is observed. In contrast, a knee with an ACL tear will have increased forward motion and a soft end feel at the end of the movement. This is because of the loss of restraint of the forward movement of the tibia due to the torn ACL.
A similar test, with the knee bent to 90 degrees, is called the anterior drawer test. A more complex test is called the pivot shift test, in which greater stresses are put on the knee as it is straightened by the physician from a bent and inwardly rotated position. If the knee "gives," this is an indication that other stabilizing structures inside the knee must be torn besides the ACL. This test can sometimes only be done when the knee is completely relaxed. Because of this it may best be observed under anesthesia during the surgical procedure.
Acute knee injuries generally warrant x-ray films. They are carefully evaluated for any possible tearing away of bone where the ligaments attach. Also, the x-ray will show any loose bone fragments or other fractures if they are present.
Magnetic resonance imaging is a noninvasive test that produces an excellent image of all parts of the knee. In this test, the individual lies in a hollow cylinder while powerful magnets create signals from inside the knee. These signals are then converted into a computer image that clearly shows any damage to the structures inside the joint. The images are valuable not only to determine the presence of an ACL tear, but also the degree of the tear along with any damage to related structures, such as the meniscus and other ligaments.
An instrument called the KT 1000 is a device that can give information about the integrity of the ACL. It allows the doctor or therapist to measure the amount of forward motion of the lower leg in relation to the upper leg. It is used frequently. Both knees are usually tested and then compared. This device also accurately measures the amount of backward movement of the knee joint.