This On-Field Exam Offers Instant ACL Diagnosis as Accurately as an MRI

STACK investigates the Lachman Test, a time-tested and accurate assessment of whether a player has torn an ACL.

A simple hand-off up the middle. Three yards and a cloud of dust. The ref''s whistle blows the play dead. Several behemoths patiently untangle their limbs, escaping the pile of humanity. But one guy doesn't get up. He's writhing in pain, holding his leg. Teammates wave to the training staff. Trainers burst onto the field and exchange words with the downed athlete. It's his knee, he says, his knee. But instead of loading the player on a cart, the head trainer grabs the hurt leg. Now he's bending it! Doesn't that hurt? Now he's torquing it at the thigh! What is he doing?!

As it turns out, the trainer is administering a time-tested medical exam, the Lachman Test. It is used to assess ACL damage, and due to the prevalence of ACL injuries, you can see it on your TV screen nearly every weekend during football season.

Here's the lowdown on this "gold standard" of ACL injury detection.

What is the ACL and How Does it Tear?

Diagram of the Knee

Courtesy of American Academy of Orthopaedic Surgeons official website

ACL is the commonly used abbreviation for the Anterior Cruciate Ligament. The knee contains two major types of ligaments—collateral and cruciate. The collateral ligaments—specifically the lateral collateral ligament (LCL) and the medial collateral ligament (MCL)—control the sideways motion of the knee. The cruciate ligaments—specifically the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL)—control the forward and back motion of the knee.

According to the American Academy of Orthopedic Surgeons, the ACL has two primary duties. One, it prevents the tibia from sliding out in front of the femur, and two, it provides rotational stability to the knee.

There are 900 ligaments in the body, but the ACL is undoubtedly the best known. Why? Because it tears. A lot.

RELATED: The 3 Best Exercises to Prevent ACL Injuries

In fact, an estimated 100,000 ACL tears occur every year in the United States. Athletes suffer the majority of them, and it's not uncommon for an athlete to tear an ACL more than once during a playing career. Thomas Davis, a Pro Bowl linebacker for the Carolina Panthers, tore the same ACL three times in three years. ACL injuries often occur as a result of the tibia and femur twisting in opposite directions under full body weight.

Why is it Called the Lachman Test?

ACL Tear

The Lachman test is named for Dr. John Lachman, who was an orthopedic surgeon and Chairman of the Department of Orthopedic Surgery and Sports Medicine at Temple University. Lachman paid meticulous attention to detail in his work, and he noticed that patients with a torn ACL "demonstrated passive anterior subluxation of the proximal tibia in relationship to the femur that was apparent while lying supine."

We'll get into exactly what that means later, but he essentially discovered he could accurately identify a torn ACL by administering a test to determine how far the tibia can slide in front of the femur. Lachman soon began disseminating the technique, and doctors found it much more reliable than any of the other physical ACL examination techniques that were available.

How is the Lachman Test Administered?

Both those videos do a great job of explaining how to administer the Lachman Test. The second one illustrates the difference between a positive and a negative test.

The test can be broken down into five basic steps. The administrator of the test:

  1. Bends the injured knee to roughly 20 degrees of flexion and instructs the patient to relax his hamstring.
  2. Firmly grasps the femur with one hand.
  3. Firmly grasps the tibia with her other hand and places her thumb on the tibial tuberosity, the bony bulge at the top of the tibia.
  4. Holds the femur still while moving the tibia forward and back, noting how far it comes forward and whether there is a "hard endpoint."
  5. Repeats the process on the opposite (uninjured) leg for reference.

An ACL tear is indicated by a "soft endpoint" when pulling the tibia forward, since there is no longer a ligament preventing the tibia from sliding in front of the femur. This means the end of the tibia's forward movement feels loose or mushy, as opposed to an intact ACL, which provides a "hard endpoint." Think of the ACL as a tether. If it's intact, you can feel it firmly preventing the tibia from sliding forward. "It basically tests how loose the knee is after an injury. If the ACL is torn, it will be looser," says Mike Reinold, physical therapist and former head athletic trainer for the Boston Red Sox.

RELATED: Inside Carson Palmer's Grueling Recovery from a Torn ACL

Although the test is simple, it's extremely accurate when administered by someone with experience. "The test is very accurate. While there are other similar tests, the Lachman Test is considered a 'gold standard' test for ACL because it is so accurate," Reinold says.

Does it Hurt?

Lachman Test Painless

No. When it is performed correctly, the Lachman Test does not hurt. "The test is not painful," Reinold says. That's why you'll see trainers or team doctors performing it immediately after a suspected ACL injury. It's also a big reason it's become so popular since its invention. Not only is it accurate and easy to administer, but it also follows the idea of premium non nocere—above all, do no harm.

Why Use the Lachman Test Instead of an MRI?

MRI Machine

Unlike an MRI exam, the Lachman Test can be administered immediately after an injury, usually before the body's natural defenses like swelling or tightness set in. These defenses can make getting an accurate diagnosis more difficult, so that's an advantage. The short time it takes to administer the Lachman test also allows medical personnel to quickly draw up a plan of attack, instead of waiting for the time-consuming MRI. In addition, the test costs nothing to administer, whereas MRI exams are quite expensive. According to TIME, the average cost of an MRI in the U.S. is $2,611.

RELATED: Peyton Manning and 4 Other Stars Who Overcame a Stunning Number of Injuries

A positive Lachman Test is normally followed up with an MRI for confirmation, but there's speculation this doesn't always need to happen. Even though MRIs are time-consuming and expensive, they are not 100-percent accurate. A conclusive study has yet to be done comparing the accuracy of the MRI and the Lachman Test, but several experts have weighed in with their thoughts.

Dr. David Geier, an orthopedic surgeon and sports medicine specialist, covered the topic in a 2013 blog post. He wrote, "If the ACL tests clearly suggest an ACL injury, then an MRI isn't always needed. Obtaining an MRI to look at the other structures in the knee may or may not be necessary. The surgeon does look at the other structures in the knee arthroscopically when performing the ACL reconstruction."

The authors of a 2006 study published in the Journal of Athletic Training postulated that the Lachman Test, when combined with a knowledge of the patient's history, might one day eliminate the need for MRIs. They wrote, "Authors of future review studies should look at combining several clinical tests and even a thorough history to help improve the validity of the physical examination. Such studies may reveal that the results of the anterior drawer and Lachman Tests, combined with subjective history items, are as effective in diagnosing ACL ruptures as is magnetic resonance imaging. This premise is certainly attainable ... such a finding should reduce the need for this expensive and time-consuming test."

The Lachman Test is one of the best physical examination methods available. The next time you see a trainer bending an athlete's injured knee, rest easy knowing that he is probably performing a painless, time-honored, accurate exam. Maybe you'll even be able to tell whether the injured player has a torn ACL before Adam Schefter breaks the news.

Photo Credit: Getty Images // Thinkstock