The Lachman test is a hands-on physical exam that checks whether your anterior cruciate ligament (ACL) is intact. It’s the single most sensitive manual test for detecting an ACL tear, with specificity above 95%. If you’ve injured your knee and a doctor or physical therapist performs this test, they’re looking for abnormal forward movement of your shinbone, which signals that the ACL isn’t doing its job.
How the Test Works
You lie on your back with your injured knee bent to about 20 to 30 degrees. The examiner places one hand on your thigh to hold it steady and the other hand behind the top of your shinbone, with their thumb on the bony bump just below your kneecap. From that position, they pull your shinbone forward while keeping your thigh locked in place.
A healthy ACL acts like a restraint cable inside the knee, preventing the shinbone from sliding forward relative to the thighbone. If the ligament is torn, the shinbone shifts forward more than it should. The examiner is feeling for two things: how far the bone moves and whether there’s a definite stopping point at the end of that movement.
What the Results Mean
Results are graded by how many millimeters the shinbone shifts forward compared to your uninjured knee:
- Grade 1: 3 to 5 mm of forward movement. This suggests a partial tear or a mild sprain of the ACL.
- Grade 2: 5 to 10 mm of movement. This points to a more significant, possibly complete tear.
- Grade 3: More than 10 mm of movement. This typically indicates a complete rupture.
Beyond the distance, the examiner notes whether the movement has a firm endpoint (labeled “A”) or no clear endpoint (labeled “B”). A firm stop means some intact fibers are still catching, which can happen with a partial tear. When there’s no endpoint at all, and the shinbone just keeps gliding forward, the ligament is likely torn completely through. So a Grade 2A result tells a different story than a Grade 2B, even though the amount of movement is similar.
Why It’s Preferred Over Other Knee Tests
The anterior drawer test is an older exam that checks the same ligament but positions the knee at 90 degrees of flexion. At that angle, the hamstring muscles on the back of the thigh tend to tighten up, especially when the knee is swollen and painful. That involuntary muscle guarding can hold the shinbone in place and mask a torn ACL, giving a falsely normal result.
The Lachman test avoids this problem by keeping the knee only slightly bent. At 20 to 30 degrees, the hamstrings are more relaxed, so they can’t compensate for a damaged ACL. This is a major reason the Lachman test is considered the go-to physical exam for ACL injuries, particularly in the acute phase right after an injury when swelling and muscle spasm are at their worst.
Accuracy in Acute and Chronic Injuries
Timing affects how well the test performs. In acute injuries (examined within two weeks of the event), the Lachman test has a sensitivity of about 78% and specificity above 95%. When the injury is older than two weeks, sensitivity actually improves to roughly 85%, likely because swelling has decreased and the examiner can feel the movement more clearly. The specificity stays above 95% regardless of timing.
What those numbers mean in practical terms: the test is excellent at confirming a healthy ACL. If the result is negative, you can be quite confident the ligament is intact. It occasionally misses a tear, particularly in the first few days when the knee is very swollen and painful, which is why an MRI is often ordered if clinical suspicion remains high despite a normal exam.
When the Test Can Be Misleading
A posterior cruciate ligament (PCL) injury can produce a false positive. The PCL sits behind the ACL and prevents the shinbone from sliding backward. When the PCL is torn, the shinbone naturally sags backward when the knee is positioned for the Lachman test. The examiner then pulls it forward, and the bone moves from its abnormally posterior position back to its normal resting spot. That forward translation feels identical to what happens with an ACL tear, even though the ACL is perfectly fine.
Experienced examiners check for this by looking at the resting position of the shinbone before performing the test. If the bone appears to be sitting farther back than normal compared to the other knee, a PCL injury is the more likely explanation. This distinction matters because the treatment and rehabilitation path for a PCL injury is quite different from an ACL tear.
Modified Versions of the Test
The standard Lachman test requires the examiner to stabilize the thigh with one hand while pulling the shinbone forward with the other. This can be physically difficult when the patient has large, muscular legs or when the examiner has smaller hands. A prone version addresses this: you lie face down, and the examiner stabilizes your lower leg using their own thigh, freeing both hands to apply the forward force on the back of your upper shinbone. The mechanics are the same, just repositioned for a better grip.
Some clinicians also use a stabilization roll, placing a firm bolster under the thigh so gravity helps hold the leg in place. These modifications don’t change what the test is measuring. They simply make it easier to perform accurately across different body types.
What Happens After a Positive Result
A positive Lachman test strongly suggests an ACL injury, but it’s rarely the final word. Most orthopedic specialists will order an MRI to confirm the diagnosis, assess whether the tear is partial or complete, and check for damage to other structures like the meniscus or cartilage. The Lachman test tells the clinician what to look for; the MRI fills in the details that guide treatment decisions.
If you’re having this test done, the examiner may also perform a pivot shift test, which checks for rotational instability, and palpate along the joint line for meniscus tenderness. Together, these physical exams build a picture of the overall damage before any imaging is ordered.