What Is a Lachman Test and What Does It Show?

The Lachman test is a hands-on clinical maneuver that checks whether the anterior cruciate ligament (ACL) in your knee is intact. A clinician stabilizes your thigh with one hand, then pulls your shin forward with the other while your knee is slightly bent. If the shinbone slides forward more than expected or without a firm stopping point, the ACL is likely torn. Across pooled studies, the test picks up about 81% of ACL tears and correctly rules them out about 85% of the time, though those numbers shift depending on factors like swelling, muscle guarding, and examiner experience.

How the Test Is Performed

You lie on your back with your knee bent to roughly 20 to 30 degrees. The examiner wraps one hand around the lower end of your thigh to hold it steady and places the other hand around the upper part of your shinbone. With a firm, quick pull, they try to slide the tibia forward relative to the femur. The examiner is feeling for two things: how far the tibia travels, and whether the motion ends with a solid stop (a “hard” endpoint) or fades out without one (a “soft” endpoint). A hard endpoint suggests the ACL is intact and snapping taut at the limit of motion. A soft or absent endpoint suggests the ligament is torn.

Hand placement matters more than most people realize. Research has shown that clinicians who grip the tibia closer to the knee joint (proximal placement) are more likely to perform and interpret the test correctly than those who grab farther down the shin.

One practical challenge is that the examiner needs hands large enough to stabilize the thigh and control the tibia at the same time. With a large or muscular leg, this can be difficult. A common workaround is the “stabilized” or “prone” Lachman, where the patient’s thigh rests on the examiner’s knee or a bolster, freeing up the examiner’s hand. This modification also helps the patient relax, which is a bigger deal than it sounds. In an acutely injured knee that is swollen and painful, fear and muscle spasm can tighten the joint enough to mask the abnormal motion entirely. Supporting the thigh on a stable surface gives the patient a sense of security, reducing apprehension and improving the test’s accuracy.

What the Test Is Actually Measuring

The ACL is the primary structure preventing your shinbone from sliding forward under your thighbone. It runs diagonally through the center of the knee, and its fibers bear a strikingly unequal share of the load. Biomechanical studies using cadaveric knees found that the central fibers of the ACL’s femoral attachment resist roughly 82% to 90% of the force that tries to push the tibia forward.

On the tibial side, the anteromedial fibers do most of the heavy lifting, carrying about 84% of the restraining force across a wide range of knee flexion angles.

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When the ACL is torn, the tibia has nothing to check its forward slide, and the Lachman test exposes exactly that gap. The 20-to-30-degree flexion angle is deliberate. At that angle the ACL is the dominant restraint, and other structures like the menisci and collateral ligaments contribute relatively little. Performing the test at 90 degrees of flexion (as in the anterior drawer test) recruits more of those secondary stabilizers, which can mask an ACL tear.

Diagnostic Accuracy in Numbers

A 2022 systematic review and meta-analysis pooled data from multiple studies and found the Lachman test has an overall sensitivity of about 81% and specificity of about 85%.

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That meta-analysis also noted the Lachman test has historically been given more credit than it deserves. Earlier literature sometimes reported sensitivity in the mid-90s, but those figures often came from individual studies with selected populations rather than broad pooled estimates. Some single-center studies do report sensitivity above 93%, which is encouraging but should be understood in context.

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The numbers also depend on what kind of tear you’re dealing with. For complete tears, sensitivity dropped to about 68% with specificity around 79% in the same meta-analysis. For injuries evaluated in the post-acute window (after the initial swelling has partly settled but before the knee has fully adapted), sensitivity was about 70% and specificity about 77%.

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How It Compares to the Anterior Drawer Test

The anterior drawer test is the other widely used hands-on check for the ACL. The patient sits with the knee bent to 90 degrees, and the examiner pulls the tibia forward from that position. It is older and more familiar to many clinicians, but its performance in the acute setting is weaker. A classic study found that within two weeks of injury, the anterior drawer test picked up only about 22% of ACL tears, compared with roughly 78% for the Lachman test. After two weeks, the anterior drawer’s sensitivity climbed to about 54%, but the Lachman still outperformed it at about 85%.

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The reason for the gap comes back to secondary restraints. At 90 degrees of flexion, the hamstrings are more engaged and the posterior horn of the meniscus acts as a wedge that blocks forward tibial slide. These structures can compensate for a missing ACL and produce a falsely reassuring result. At the Lachman angle of 20 to 30 degrees, those backup mechanisms contribute less, so a torn ACL is harder to hide. A separate pooled analysis found the Lachman test’s sensitivity at 0.86 with specificity of 0.91, compared with a pooled sensitivity of only 0.20 for the anterior drawer, though its specificity was a comparable 0.88.

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That said, individual studies vary widely. Some centers report anterior drawer sensitivity in the 90s for chronic injuries. The take-home point is that the anterior drawer test is unreliable in the days right after injury, and the Lachman is the better bedside choice when you need an answer early.

The Pivot-Shift Test and What It Adds

The pivot-shift test assesses a different dimension of instability. Rather than a straight forward-and-back slide, it reproduces the rotational “giving-way” that patients with a torn ACL describe during cutting or twisting movements. The examiner extends the knee while applying an inward rotation and a pushing force on the outside of the leg. If the ACL is torn, the tibia subluxates forward and then suddenly reduces with a visible or palpable clunk as the knee flexes past about 30 degrees.

Because the pivot shift depends on the patient being completely relaxed, it is notoriously difficult to perform on an awake patient who is guarding. Its sensitivity in the office is lower than the Lachman’s in many studies, though under anesthesia it becomes highly sensitive. An older study found the pivot shift reached about 89% sensitivity in acute injuries, similar to the Lachman, with specificity above 98%.

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The two tests are related but not interchangeable. Biomechanical work found that when the Lachman test shows three millimeters or less of increased forward translation, the pivot shift is essentially always negative.

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However, the correlation between the magnitude of the two tests is weak in ACL-deficient knees, meaning a patient can have a markedly positive Lachman but a subtle pivot shift, or vice versa.

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In practice, clinicians often use the Lachman as the first-line screening test and the pivot shift as a complementary test that can confirm the diagnosis and gauge functional instability.

Why Swelling and Muscle Guarding Change the Result

If you’ve ever had a badly swollen knee and someone tried to move it around, you know how hard it is to relax. Pain and effusion trigger reflexive muscle contraction that splints the joint and reduces the tibial translation the examiner is trying to feel. This is a real clinical problem, not a minor footnote. In one study, aspirating fluid from swollen knees of patients with acute ACL injuries significantly improved the sensitivity of the Lachman test, raising it from under half to over three-quarters in the group that received aspiration.

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This is also why examination under anesthesia (EUA) is considered a reference-standard scenario for the Lachman. Once the patient is fully anesthetized, there is no guarding at all, and the tibia slides freely if the ligament is gone. One study found the Lachman test was positive in 99% of confirmed ACL tears on initial awake examination and 100% under anesthesia.

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Instrumented measurements confirm the difference: the average side-to-side difference in forward tibial translation during the Lachman test was about 4.6 mm when awake and about 6.9 mm under anesthesia in ACL-deficient knees.

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Partial ACL Tears Are Harder to Catch

A fully torn ACL produces a dramatic, unmistakable Lachman result in most cases. Partial tears are trickier. Some fibers are still intact and provide enough restraint to produce a near-normal endpoint, which can fool even experienced examiners. Research using examination under anesthesia developed a grading system to help distinguish partial from complete tears. A Lachman graded as 1+ (mild laxity with a firm endpoint) was far more often associated with a partial tear, while a 3+ result (severe laxity without a firm stop) carried a relative risk over 50 times higher for a complete tear compared to 1+.

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Combining the Lachman result with patient history improves diagnostic accuracy for partial tears. A study of nearly 280 patients found that the Lachman test alone produced a very high positive likelihood ratio for identifying partial or complete ACL tears. When combined with a history of a pivoting injury and a “pop” at the time of injury, the combined clinical profile reached even stronger diagnostic validity, especially for complete tears.

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Lachman Test Versus MRI

MRI is often treated as the definitive answer for knee injuries, but the Lachman test holds its own surprisingly well in head-to-head comparisons. One study comparing diagnostic accuracy found that the Lachman test achieved an area under the curve (a summary measure of how well a test distinguishes between injured and uninjured knees) of 0.902 for ACL tears, outperforming MRI’s 0.870 for the same injury.

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This does not mean MRI is useless. MRI reveals the full picture of the knee: bone bruises, meniscal tears, cartilage damage, and other soft tissue injuries that the Lachman test cannot assess. But for the narrow question of whether the ACL is torn, a well-performed Lachman by an experienced examiner is at least as reliable as what the scanner shows, and it is faster, cheaper, and available in any exam room.

Instrumental Arthrometers and Objective Measurement

Because the Lachman test relies on the examiner’s subjective feel, attempts have been made to quantify it using mechanical or digital devices called arthrometers. The KT-1000 is the best-known model. It straps to the leg and applies a standardized force to the tibia while a dial measures how far it moves. Research has confirmed that arthrometer readings at certain force levels correlate well with the clinical Lachman test.

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Newer digital arthrometers apply a continuous, controlled load and record displacement throughout the range. One such device showed strong ability to distinguish ACL-injured knees from healthy ones, with the best discrimination occurring at moderate loading forces.

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The appeal of these devices is objectivity and standardization, but they have an underappreciated weakness: inter-rater reliability is poor. A study evaluating four different arthrometers found that while a single examiner could get consistent results across sessions, two different examiners testing the same knee often disagreed substantially.

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This limits their usefulness for comparing results between clinics or before and after surgery when different providers do the testing. Manual inter-observer reliability for the Lachman test itself faces a similar challenge, with one study finding satisfactory test-retest reliability within a single examiner but unsatisfactory agreement between different observers.

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The Role of Secondary Restraints

The ACL is not the only structure holding the tibia in place. Surrounding ligaments, the menisci, and the joint capsule all contribute to knee stability, and their influence explains some of the variability in Lachman test results. When the ACL is torn, these secondary structures take on a larger share of the restraining load. Biomechanical research found that after ACL sectioning, the anterolateral ligament (a small band on the outside of the knee) bore up to 55% of the load the intact ACL would normally carry, though it only engaged after the tibia had already moved beyond its normal limits.

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The medial meniscus also acts as a secondary stabilizer. A study found that the degree of increased forward laxity after ACL cutting was strongly associated with how early the secondary ligaments and the medial meniscus engaged to resist the load. Knees where the meniscus carried a higher in situ force when the ACL was still intact tended to show smaller increases in laxity after the ACL was removed.

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Tibial rotation also plays a role. Research has shown that secondary structures like the iliotibial band can tighten in certain rotational positions and create a mechanical block that dampens forward translation, potentially producing a false-negative Lachman result.

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This is one reason experienced examiners pay close attention to how the foot and tibia are positioned during the test.

Testing Children and Adolescents

ACL tears in young athletes have become more common as youth sports have intensified, and the Lachman test works in this population too, with a few caveats. Children are often more anxious and less cooperative on the exam table, and smaller body proportions can make hand placement awkward. A prospective study of children and adolescents with suspected ACL injuries found that when the examiner specifically assessed the endpoint feel, the Lachman test had a positive predictive value of 95% and a negative predictive value of 82%.

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The bigger concern in pediatric patients is not whether the test works but what happens after a positive result. Children who are still growing have open growth plates at the ends of their bones, and standard ACL reconstruction techniques that drill tunnels through these areas risk growth disturbance. Surgical planning in skeletally immature patients requires careful balancing of restoring stability against protecting the growth plate, which is why the accuracy of the initial diagnosis matters so much.

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Examiner Agreement and the Endpoint Question

One underappreciated aspect of the Lachman test is that the endpoint assessment, whether the motion ends with a hard stop or a soft fade, is arguably more diagnostically useful than how many millimeters the tibia moves. A study that had multiple examiners grade the endpoint on the same patients found 91% agreement, with a kappa coefficient of 0.72, indicating substantial agreement.

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This is a helpful finding because measuring absolute translation by hand is unreliable. Two examiners can easily disagree on whether the tibia moved five or eight millimeters. But asking “did the motion stop abruptly or trail off?” turns out to be a question humans can answer more consistently. This is why many teaching protocols now emphasize endpoint quality over translation distance as the key interpretive feature of the Lachman test. If you have a positive Lachman with a soft endpoint, the odds heavily favor a torn ACL regardless of how much motion the examiner thinks they felt.

Telehealth and Remote Knee Evaluation

The rise of telehealth during and after the COVID-19 pandemic raised an obvious question: can you perform a Lachman test over video? The short answer is no, not in the traditional sense. The test fundamentally requires the examiner’s hands on the patient’s leg to apply force and feel the endpoint. However, sports medicine specialists have developed virtual knee examination protocols that guide patients through self-assessment maneuvers on camera, covering range of motion, swelling, and functional tests like single-leg squats. These cannot replicate the Lachman test, but they can help triage which patients need an urgent in-person exam and which can safely wait. A self-performed anterior drawer or pivot-shift equivalent does not exist, and virtual exams acknowledge this gap rather than trying to work around it.