The Lachman test is done to check for an anterior cruciate ligament (ACL) injury or tear. The ACL connects two of the three bones that form your knee joint:

When the ACL tears or gets injured, you may not be able to fully use or move your knee joint. ACL tears and injuries are common in athletes, especially soccer, basketball, and baseball players, who use their legs to run, kick, or tackle other players.

The test is named after John Lachman, an orthopedic surgeon at Temple University in Philadelphia who invented the technique.

The Lachman test has a few simple steps. It’s considered a reliable way to diagnose an ACL injury and decide what treatment is best for your injury.

Let’s take a closer look at how the test works, how it’s used to diagnose conditions related to your ACL, and what happens next based on your results.

Here’s a step-by-step guide on how a doctor performs the Lachman test:

  1. You lie down flat on your back, with your legs out straight and your muscles all relaxed, especially the hamstring muscles in your upper leg.
  2. Your doctor bends your knee slowly and gently to about a 20-degree angle. They may also rotate your leg so your knee points outward.
  3. Your doctor places one hand on your lower thigh and one hand on your lower leg just below where your leg bends.
  4. Your doctor gently but firmly pulls your lower leg forward, keeping your thigh stable with their other hand.

There are two main benchmarks that the Lachman test uses to assign a grade to your ACL injury:

  • Endpoint. How much do the shin bone and knee move during the test? The ACL responds to shin and knee movement by keeping them within a certain limited range of motion. If they move more than is normal, you might have an ACL injury. This can also help your doctor decide whether other tissues are injured and not properly stabilizing the joint.
  • Laxity. How firm does the ACL feel when it moves within its normal range of motion during the test? If the ACL doesn’t respond with a firm endpoint when it reaches the limit of its normal range of motion, it might be injured or torn.

Your doctor will likely conduct the Lachman test on your other leg, too, to compare its motion to that of your possibly injured leg.

Using observations of both of your legs with the two criteria above, your doctor grades your injury on this scale:

  • Normal. There’s no notable injury to your leg, especially in comparison to your other leg.
  • Mild (grade 1). The injured leg moves 2 to 5 millimeters (mm) more than is normal for its range of motion, compared to the other leg.
  • Moderate (grade 2). The injured leg moves 5 to 10 mm more than is normal for its range of motion, compared to the other leg.
  • Severe (grade 3). The injured leg moves 10 to 15 mm more than is normal for its range of motion, compared to the other leg.

Some doctors prefer to use an instrument known as the KT-1000 arthrometer to get a more accurate reading of the leg’s range of motion.

The KT-1000 might be preferred if your doctor thinks you have an especially severe ACL injury or if you’ve had a long-term injury that may not be noticeable right away. This could be the case because the ACL can develop scar tissue that then limits your leg’s range of motion.

The Lachman test is most commonly used to diagnose ACL injuries.

ACL injuries usually involve tears that happen from repetitive or violent motions that wear away at the ligament over time. With enough repetitive strain or a sudden enough motion, the ACL can snap into two pieces and make it painful or impossible to move the knee.

An anterior drawer test (ADT) is commonly done at the same time as the Lachman test to help confirm the diagnosis of an ACL injury.

This test is done by bending the hip 45 degrees and the knee 90 degrees, then pulling the knee forward with a sudden jerk to test the leg’s range of motion. If it moves 6 mm beyond its normal range of motion, then you may have an ACL tear or injury.

Some studies show that the ADT is slightly more accurate in diagnosing an ACL injury than the Lachman test. However, the ADT isn’t always thought to be as accurate as the Lachman test, especially on its own.

Doing both tests usually produces much more accurate results than either test by itself.

Many studies have shown that the Lachman test is highly accurate in diagnosing ACL injuries, especially when it’s used along with an ADT or other diagnostic tool.

A 1986 study of 85 people tested under anesthesia with knee injuries found that this test had nearly a 77.7 percent success rate in helping diagnose ACL injuries that happened less than two weeks before the test was done.

However, there’s some subjectivity. A 2015 study found that two doctors testing the same patient agreed 91 percent of the time. This means there’s some margin of error between doctors as to whether they interpret the results correctly.

A 2013 study that looked at 653 people with ACL ruptures found that the Lachman test had a 93.5 percent success rate, only 1 percent less accurate than the ADT. The 2015 study noted a similar success rate of about 93 percent.

Scar tissue formation on the ACL can result in a false positive. This makes the leg look like it’s limited to the normal range of motion when it’s really just scar tissue holding it back.

Lastly, studies have found that being under general anesthesia makes it more likely for your doctor to make an accurate diagnosis.

Based on your results, your doctor may recommend one or more of the following treatments:

  • Using the RICE method (rest, ice, compression, elevation) relieves swelling right after you get injured.
  • Wearing a knee brace keeps your knee stable and relieves pressure on the ACL.
  • Physical therapy or rehabilitation for a strained, scarred, or recently repaired ACL that was previously torn can help you regain strength or motion in your knee.
  • Undergoing ligament restoration surgery to replace or restore the tissue that was torn or damaged with a graft, either with tissue taken from a nearby ligament or from a donor.

ACL injuries can be painful and limit your ability to use your knees or legs to their full capabilities.

If you think you have an ACL injury, the Lachman test can be used along with several other tests to confirm the injury and help you figure out what to do next.

With the proper treatment for your injury or tear, you can get back most, if not all, of the strength and movement that your ACL provides for your leg.