Step-by-Step Lachman Test Guide for ACL Injury Diagnosis
The individual suffered an anterior cruciate ligament (ACL) tear about two weeks ago. The knee shows marked swelling and joint effusion, and it is naturally held in a slightly flexed position. These findings set the stage for a reliable Lachman examination.
Performing the Lachman Test
- Position the patient – Have the patient lie supine and ensure the leg is completely relaxed.
- Stabilize the femur – Grasp the distal thigh with one hand to prevent any movement of the femur during the maneuver.
- Set the knee angle – Flex the knee to approximately 20–25 degrees; this angle maximizes isolation of the tibia.
- Apply anterior force – With the other hand, grasp the proximal tibia just below the knee joint and pull it forward, creating an anterior drawer motion while the femur remains fixed.
Interpreting the Results
- Laxity detection – Observe the amount of forward translation of the tibia. Significant laxity suggests ACL compromise.
- End‑point assessment – A firm “snap” or clear end point indicates an intact ligament. Absence of this end point, described as a “mushy” feel, denotes a positive Lachman test and likely ACL deficiency.
Why the Test Works
The Lachman test isolates the tibia and applies an anterior force while the femur is stabilized. In a healthy knee, the ACL resists this motion, producing a distinct mechanical stop. When the ACL is torn, the restraint is lost, leading to increased tibial movement and a soft or missing end point.
Takeaways
- The Lachman test is performed with the patient supine, knee flexed 20–25 degrees, and the leg fully relaxed.
- Stabilizing the femur while pulling the tibia anteriorly isolates the ACL for assessment.
- Significant tibial laxity without a firm end point indicates a positive Lachman test and ACL deficiency.
- A healthy ACL produces a distinct mechanical stop, felt as a firm “snap” during the maneuver.
- The test is most reliable about two weeks after injury, when swelling has not yet obscured joint movement.
Frequently Asked Questions
What indicates a positive Lachman test?
A positive Lachman test is indicated by marked tibial laxity and the absence of a firm end point or “snap” when the examiner pulls the tibia forward. This lack of resistance reflects an ACL tear.
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Why the Test Works
The Lachman test isolates the tibia and applies an anterior force while the femur is stabilized. In a healthy knee, the ACL resists this motion, producing a distinct mechanical stop. When the ACL is torn, the restraint is lost, leading to increased tibial movement and a soft or missing end point.
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