MASTERING THE LACHMAN TEST: DETECTING ACL TEARS

This ligament provides stability to the knee joint, and its injury is common in sports that involve sudden stops, pivots, or direct impacts. The Lachman test is particularly valuable because it can be more sensitive and specific than other tests like the anterior drawer test, especially in the acute setting when the knee is swollen.

Perform the Lachman test when:

• There is a suspicion of an ACL tear based on history and mechanism of injury.
• The patient has acute knee pain after a twisting or direct blow injury.
• You need to differentiate between an ACL tear and other knee injuries.

1. Patient Positioning: The patient should be lying supine on the examination table. The knee to be examined should be flexed to about 20-30 degrees.

2. Examiner Position: Stand or sit beside the knee being tested. Your hands should be free, and the patient should be relaxed.

3. Stabilize the Femur: With one hand (typically your non-dominant hand), grasp the distal femur, just above the knee joint.

4. Grasp the Tibia: With your other hand, hold the proximal tibia just below the knee joint.

5. Apply an Anterior Force: With the hand that's on the tibia, attempt to translate the tibia anteriorly (forward).

6. Assess Movement and Feel: As you apply the anterior force, evaluate the quality of the endpoint when pulling on the tibia. A firm stop suggests an intact ACL, while a soft or absent endpoint suggests a torn ACL.

7. Compare with the Opposite Knee: Always compare the findings to the opposite, uninjured knee. This helps identify subtle differences and confirms if the ligament is compromised.

• Intact ACL: A distinct and firm end-feel when attempting to translate the tibia forward.
• Partial ACL Tear: A softer endpoint than the opposite, uninjured knee, but with some resistance.
• Complete ACL Tear: A significantly soft or even absent endpoint, indicating the tibia can move forward with little resistance.

• Ensure that the patient is comfortable and relaxed. Tensing of the muscles can give a false sense of stability.
• If there's significant swelling or pain, the test might be difficult to perform and interpret.
• Other knee injuries can co-exist with an ACL injury. Comprehensive examination of the knee is essential.

The Lachman test is a valuable tool in assessing ACL integrity. However, in clinical settings, especially when findings are ambiguous or when quantification of the anterior tibial translation is desired, the use of a knee arthrometer can be beneficial. A knee arthrometer is a device that quantitatively measures the amount of anterior-posterior movement (translation) of the tibia relative to the femur. It provides an objective measurement of knee laxity, which can be especially useful in research settings, post-surgical evaluations, or when monitoring rehabilitation progress.

While a positive Lachman test strongly suggests an ACL tear, definitive diagnosis often requires further imaging studies, such as an MRI or Arthrometers. Arthrometers have indeed been proven to be more effective at diagnosing partial ACL ruptures than MRI (see study attached below the article). Additionally, the objective measurements from a knee arthrometer can further validate clinical findings. If you suspect an ACL injury or any other significant knee injury, it is essential to refer the patient to a specialist for further evaluation and management.