ACL Tears

Diagnosing An ACL Tear

The diagnosis of an ACL tear is based on

Physical Examination

The doctor 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 damaged. Any previous episodes of knee instability or the knee giving way, or previous injury to the knee, is important information.

The doctor can determine whether the knee is stable on examination. One simple but important test is called the Lachmans test. With the knee bent to 30 degrees, the doctor 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 2 to 4 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 the normal 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 doctor 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.

Radiographic Evaluation

Acute knee injuries generally warrant x-ray films. An x-ray cannot show an ACL tear because it is a soft tissue injury (and x-rays show the bones). But it can show an ACL injury if it is an ‘avulsion’, when the tendon has been pulled away with a bone fragment. An x-ray is also useful to exclude a possible concurrent bone injury, or see whether a pre-existing problem of the knee is present (for example, arthritis, or a previous bone injury, or loose bone fragments inside the joint). 


Magnetic resonance imaging is a noninvasive test that produces an excellent image of all parts of the knee. It is the key investigation to determine whether an ACL tear is present. 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 a tear of the medial or lateral meniscus or of the collateral ligaments.

For further information about mri, go to MRI.


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