The diagnosis of an ACL tear is commonly made shortly after the accident as it is often readily diagnosed on the physical examination. Imaging studies can also be carried out to confirm the diagnosis and to assess other parts of the knee, such as the menisci. An arthroscopy is not generally necessary to make a diagnosis of an ACL tear.

The specific nature of the accident:

The tear of an ACL is usually the result of a sudden violent twisting of the knee as seen during certain sports, such as football or skiing. The typical ski accident occurs at slow speed (or even with the skier standing still). The binding does not release and the skier notes a painful cracking in the knee and may even feel an immediate instability as he or she gets up to start skiing again. Commonly, the swelling in the knee is large and immediate. In such a setting, the skier should seek immediate medical attention.

when cleats maintain the foot in a fixed position on the ground, violent twisting of the knee can lead to a tear of the ACL.

 

Physical Examination:

The classic test for an ACL is the anterior drawer sign called the Lachman - carried out on a knee which is just slightly flexed. A full examination must of course include testing of other structures around the knee some of which are commonly injured simultaneously (e.g. the menisci).

the Lachman (anterior drawer on a slight flexed knee) can establish the diagnosis

Although the Lachman commonly establishes the diagnosis, it may be difficult or impossible to perform this test on a patient whose knee is particularly swollen or painful. This calls for imaging studies.

 

Imaging of the torn ACL:

Plain x-rays. This form of routine imaging is useful for ruling out major fractures. Specialized office imaging, e.g. side views with application of an anterior drawer force. This is an objective way of demonstrating the anterior drawer (Lachman) sign. This type of radiograph can be used quantitatively to assess the specific amount of knee instability.

the upper part of the shin is pressed upwards against two fixed counters in a 3 point bending more.

30 mm drawer sign in a knee with a significant tear of the ACL (left) compared to a normal drawer of 5 mm (right).

The arthrogram: this is an x-ray taken following the injection in the knee of a dye. This dye coats the menisci and thus helps detect meniscal tears.

The MRI (Magnetic Resonance Imaging): this test does not require x-rays. It provides information on the soft tissues in addition to the bone. The soft tissues include the ligaments, the meniscus and the articular cartilage. Despite its use, it is not absolutely necessary to obtain an MRI in all cases of a torn ACL.

 

Arthroscopic surgical examination of the knee for the purpose of diagnosing an ACL tear

Such a surgical procedure is not indicated when done strictly for diagnostic purposes. The physical examination combined when necessary with the above mentioned imaging studies, largely suffices to make a diagnosis of an ACL tear.