An ACL injury is usually caused by a sudden twisting of the knee whilst the foot is planted on the ground, or a direct impact or trauma to the knee, often on the outside of the joint.
In order to diagnose an ACL injury, the clinician or therapist will use a number of tests to assess the stability of the knee and the integrity of the ACL, two of which we highlight and explain below.
Symptoms include sudden pain in the knee joint at the time of injury, sometimes with an audible pop or crack. The athlete may have a feeling of instability and rapid swelling may occur from bleeding within the joint which will feel warm to touch.
Often an athlete will seek professional advice at a clinic the next day or two after injury. By that time there will be considerable swelling making it very difficult to diagnose exactly what is wrong with the knee. After a few days when the swelling has gone down it will be easier to examine the knee joint.
There will be restricted range of movement with particular difficulty straightening the leg. Tenderness or pain may be felt all around the knee joint and there will be positive signs in the anterior drawer test and Lachman's test.
It is important at this stage to assess all the structures within the knee joint and in the surrounding area as complete ACL tears are sometimes associated with concurrent injuries to the cartilage and collateral ligaments. This is for information purposes only and these tests should always be carried out by a doctor or professional therapist.
Anterior Draw Test
The patient lies on their back and bends the injured knee to 90 degrees with the foot flat on the table (the practitioner may stabilize the foot by sitting on it). The practitioner grasps the upper tibia (shin bone) with both hands, just below the knee joint. The tibia is then pulled forwards and the degree of movement is assessed in conjunction with the end feel of the test. A positive result is indicated if the tibia moves excessively forwards on the femur. Every patient is different so in order to assess whether the movement is excessive, the injured knee should always be compared to the healthy knee which hopefully shows the patient’s normal 'normal' movement.
The Lachman's Test
The patient lies on their back with the knee flexed between 15 and 30 degrees. The practitioner grasps the outside of the lower femur (thigh bone) with one hand, just above the knee joint, and the inside of the upper tibia with the other hand. The femur is then stabilised with the upper hand while the lower hand pulls the tibia forwards. Again, similar to the anterior draw test, a positive result is seen if the tibia moves excessively forward compared to the other (“healthy”) knee.
In addition to the tests described above, the practitioner may also test the range of motion of the knee, the strength of the surrounding muscles and use a number of tests to assess for possible associated injuries such as meniscus tears.
An MRI scan can confirm the diagnosis and an X-ray can eliminate an avulsion fracture where the ligament pulls a piece of bone away.