The Lachman test is a precise, manual assessment performed by trained clinicians to evaluate the integrity of the anterior cruciate ligament (ACL) within the knee joint. This diagnostic maneuver is widely regarded as the most sensitive and reliable clinical method for assessing ACL stability in both acute injury settings and in chronic instability cases. It is a non-invasive procedure that directly measures the amount of forward movement, or anterior translation, of the shinbone (tibia) relative to the thighbone (femur) under controlled stress. The information gathered from this test provides immediate insight into the ligament’s condition, helping to guide decisions about further imaging and subsequent treatment planning.
Context: Why the Lachman Test is Essential
The ACL is a thick band of connective tissue deep within the knee, primarily preventing the tibia from sliding too far forward beneath the femur. It also limits excessive rotation of the knee joint. A deficient ligament can lead to chronic knee instability and further damage to the meniscus and cartilage over time, making accurate diagnosis essential. The Lachman test is highly valued because it isolates the function of the ACL more effectively than older clinical methods.
The test achieves greater diagnostic accuracy by positioning the knee in slight flexion, typically between 20 and 30 degrees. At this angle, surrounding structures like the menisci and the posterior capsule are less taut, reducing the chance they will mask an injury. This subtle flexion also encourages the hamstring muscles to relax, minimizing the patient’s ability to guard the joint and unintentionally stabilize a torn ACL. Due to its high sensitivity, the Lachman test is the preferred physical examination technique for suspected ACL injuries.
Proper Patient and Examiner Setup
For the Lachman test to be successful, the patient must be completely relaxed, lying face-up (supine) on the examination table. The injured leg must be accessible, and the patient’s hip is often placed in slight external rotation to promote muscle relaxation. Maintaining 20 to 30 degrees of knee flexion is crucial, often achieved using a small bolster, towel roll, or the examiner’s thigh beneath the knee.
The examiner is typically positioned on the side of the leg being tested. One hand is placed firmly around the distal femur just above the kneecap to stabilize the thigh. This stabilization ensures that any observed movement is the tibia translating forward on the femur, not the entire leg shifting. The second hand grasps the proximal tibia just below the joint line, with the thumb placed anteriorly for optimal control and feedback.
Step-by-Step Execution of the Test
Once positioned, the examiner maintains a secure hold on the thigh and upper shin. The hand stabilizing the distal femur must maintain a constant, firm counter-force. The other hand, holding the proximal tibia, then applies a controlled, anteriorly directed force to the shinbone.
This force is applied swiftly but gently, aiming to translate the tibia forward relative to the stationary femur. Avoiding sudden, jerky motions prevents patient guarding and invalidates the result. The examiner focuses on feeling the joint movement and the quality of the “endpoint” as the tibia moves forward. The action is often repeated to overcome muscle spasm or apprehension, allowing for a true assessment.
The controlled anterior translation must be maintained precisely along the axis of the tibia while the knee remains in 20-to-30-degree flexion. This technique minimizes the risk of false results caused by secondary knee stabilizers. The test is purely an action of applying force; the interpretation relies on the clinician’s tactile sense. The entire sequence is then repeated on the uninjured, opposite leg to establish a baseline for the patient’s normal joint laxity.
Interpreting the Test Results
Interpretation centers on assessing two primary factors: the amount of anterior translation and the quality of the joint’s “endpoint.” The endpoint is the sensation the examiner feels when the tibia reaches its maximum forward travel. A normal, intact ACL abruptly stops the forward motion, perceived as a distinct, “firm endpoint.”
A positive Lachman test, suggesting an ACL tear, is characterized by increased anterior translation accompanied by a “soft” or “mushy endpoint,” or no discernible stop. Clinicians grade the laxity based on the degree of anterior movement compared to the uninjured knee.
Grading Laxity
Clinicians use a grading system based on the degree of anterior movement:
- Grade 1: Mild instability, showing 0 to 5 millimeters of translation, often with some resistance.
- Grade 2: Moderate instability, indicated by 6 to 10 millimeters of forward movement, typically associated with a soft endpoint.
- Grade 3: Severe laxity, involving 11 to 15 millimeters of translation with no firm endpoint, strongly suggesting a complete ACL rupture.
The comparison to the contralateral, uninjured knee is essential because normal joint laxity varies significantly among individuals. A difference of 3 millimeters between the knees is often considered a positive indication of injury.