An ACL injury is initially examined by going through the history of the injury with the patient, such the way the injury occurred, whether a sound was heard, or if the knee swelled up, etc. A common cause of ACL injury involves the patient suddenly twisting or changing directions with their foot planted on the ground, with a ‘pop’ sometimes heard. Swelling and pain usually follow due to a blood vessel being torn or ruptured, causing the knee to fill with blood. The patient may also report of instability in the knee, with sensations of the knee ‘giving out’, especially with cutting or pivoting movements. If an ACL injury is suspected then a physical examination is usually performed. There are three physical tests that could help diagnose an ACL injury.
Anterior Drawer Test
This test is used to determine the integrity of the ACL by feeling for laxity in the ligament. The test is performed with the patient lying on their back with the affected knee flexed to 90 degrees and foot flat on the table. The examiner grasps the head of the tibia (shin bone) with both hands and draws the tibia forwards (anteriorly) with a firm and quick force. An excessive amount forward translation or slide of the tibia relative to the femur (thigh bone) when compared to the non-affected knee or lack of a solid ‘end-feel’ indicates either a sprain or complete tear of the ACL.
Lachman Test
The Lachman Test also assesses ligamentous integrity and is regarded as the most sensitive and reliable physical test in determining an ACL sprain or tear. The test is similarly performed with the patient lying on their back but the knee only flexed 20 to 30 degrees. With one hand stabilising the end of the femur and the other hand grasping the head of the tibia, the examiner applies a quick and firm anterior force to the tibia. An intact ACL should prevent excessive forward movement of the tibia on the femur with a solid end-feel, whereas a soft or spongy or lack of end-feel compared to the non-affected knee indicates an ACL injury.
Pivot Shift Test
This test is sometimes performed as it is able determine injury to other structures in the knee in addition to the ACL, such as the lateral collateral ligament (LCL) and posterior joint capsule. The test is performed with the knee initially straight and the tibia slightly rotated inwards. The examiner then applies a slight pressure on the outside of the knee while slowly flexing the knee. If a shift or ‘clunk’ is felt when the knee is flexed approximately 20 to 40 degrees then this indicates an ACL deficiency. The pivot shift test is best performed under anaesthesia as its reliability is affected if the patient tenses their muscles due to pain.
To confirm an ACL injury, a magnetic resonance imaging (MRI) scan may be performed, which can show the degree of injury to the ACL and other soft tissue structures in the knee such as other ligaments, cartilage and meniscus. An X-ray may also be performed to rule out associate fractures and degenerative changes in the knee.
