How the Lachman Test Is Used to Diagnose ACL Tears

The Lachman test is a hands-on clinical examination used to check whether the anterior cruciate ligament (ACL) in your knee is intact. A clinician bends the knee slightly, stabilizes the thighbone with one hand, and pulls the shinbone forward with the other, feeling for how much the tibia shifts and whether it comes to a firm stop. It remains one of the most widely taught and relied-upon physical tests in orthopedic and sports medicine, though its real-world accuracy depends on several factors that clinicians and patients should understand.

How the Test Is Performed

You lie on your back with the injured knee bent to roughly 20 to 30 degrees of flexion. The examiner wraps one hand around the lower thigh just above the knee to hold the femur steady, and places the other hand behind the upper shinbone. They then apply a firm forward pull on the tibia. What the examiner is feeling for are two things: how far the shinbone slides forward relative to the thighbone, and the quality of the endpoint, meaning whether it stops with a firm “clunk” or trails off into a soft, mushy finish. A firm endpoint suggests the ACL is intact. A soft or absent endpoint raises suspicion that the ligament is torn.

Research on endpoint quality shows it can be more diagnostically useful than simply measuring how many millimeters the tibia moves. In one study, agreement between examiners on endpoint quality (firm versus soft) reached 91%, whereas agreement on the amount of translation was only 65%. The sensitivity of the endpoint assessment alone was 81% with perfect specificity, giving an overall accuracy of 93%.¹ In practice, this means a well-trained examiner may learn more from how the motion stops than from trying to estimate the exact distance traveled.

Hand placement matters more than many clinicians realize. A study looking at clinician grip found that those who positioned their hand on the proximal (upper) part of the tibia were more likely to perform and interpret the test correctly than those who gripped farther down the shin.² Gripping too far distally changes the lever arm and can make it harder to isolate the anterior translation you are looking for.

Why It Works at That Knee Angle

The reason the knee is bent only 20 to 30 degrees, rather than the 90 degrees used in the older anterior drawer test, is rooted in how the ACL behaves at different positions. An in vivo study measuring ligament tension in normal knees found that baseline ACL tension was greater at 15 degrees of flexion than at 90 degrees, and that a Lachman maneuver produced maximal tension in the majority of the ligaments tested.³ At low flexion angles, the ACL is the primary restraint to forward tibial motion. At 90 degrees, other structures like the hamstrings and the posterior horn of the meniscus contribute more, which can mask a torn ACL by providing a false sense of stability.

The medial meniscus also plays a stabilizing role. Cadaver testing has shown that when an anterior load is applied to an ACL-deficient knee, the medial meniscus acts as a secondary restraint against forward tibial translation.⁴ This means a patient who has torn their ACL but still has an intact medial meniscus may show less translation on the Lachman test than someone who has damaged both structures. Conversely, a combined ACL and meniscal tear typically produces a more obvious positive result.

Diagnostic Accuracy

The Lachman test has traditionally been considered the most accurate bedside test for ACL tears, but a recent systematic review and meta-analysis found the picture is more nuanced than textbooks suggest. That review reported that for complete ACL tears, the Lachman test had a sensitivity of 68% and specificity of 79%, and concluded that its diagnostic accuracy has been previously overestimated compared to other clinical tests like the anterior drawer and pivot shift.⁵

That said, individual studies have reported much higher numbers. One prospective study comparing the Lachman test against arthroscopic confirmation found a sensitivity of 90%, specificity of about 88%, and an overall diagnostic accuracy near 90%.⁶ Another reported even higher values: sensitivity of 92% and specificity of 99%, with overall accuracy approaching 97%.⁷

The wide spread in reported accuracy numbers is telling. It reflects differences in who is performing the test, how acutely injured the patient is, whether the patient is awake or under anesthesia, and what gold standard the study uses. A single “accuracy percentage” for the Lachman test is misleading without knowing the context behind it.

How It Compares to Other ACL Tests

The three main clinical tests for ACL tears are the Lachman test, the anterior drawer test, and the pivot shift test. Each stresses the knee differently, and each has strengths in different clinical situations.

A study comparing all three found that for acute injuries (within two weeks of injury), the pivot shift test was actually the most sensitive at about 89%, with the Lachman at 78% and the anterior drawer trailing at only 22%. For injuries older than two weeks, the Lachman and pivot shift were tied at about 85% sensitivity, while the anterior drawer improved to 54%.⁸ This highlights an important nuance: the anterior drawer test is weakest when the injury is fresh, probably because acute pain and swelling cause the hamstrings to guard reflexively at 90 degrees of flexion, masking the instability.

Looking at sensitivity and specificity together, one review concluded that the Lachman test is a better overall test at both ruling in and ruling out ACL ruptures than the other two.⁹ Another comparative study, however, found the Lachman and anterior drawer were roughly equivalent when a decision to reconstruct needed to be made.¹⁰ In chronic ACL injuries specifically, the Lachman test showed a sensitivity around 92% and accuracy of 86%.¹¹

The takeaway is that no single test is definitive on its own. Most experienced clinicians use a combination of all three, along with the patient’s history and mechanism of injury, to form their clinical impression before ordering imaging.

Why Muscle Guarding and Anesthesia Matter

One of the biggest practical challenges with the Lachman test is that a patient in pain will involuntarily tighten their muscles, particularly the hamstrings, which resist the forward pull on the tibia and can make a torn ACL look intact. This is why the same test performed in a relaxed patient can yield a dramatically different result than one done in the emergency room on someone with a swollen, painful knee.

Research directly measuring tibial translation in the same patients awake and under anesthesia found that the average side-to-side difference during the Lachman test was 4.6 mm when awake but 6.9 mm under anesthesia, a statistically significant increase.¹² This gap illustrates just how much muscle guarding conceals. A study on the influence of anesthesia and other patient factors found that the Lachman test performed under anesthesia reached sensitivity values of 96.7% to 100%.¹³

This is why surgeons commonly perform an examination under anesthesia right before ACL reconstruction surgery. With the muscles completely relaxed, the clinical tests become far more revealing. If you had a Lachman test in a clinic that came back equivocal, it does not necessarily mean your ACL is fine; it may simply mean your muscles were doing a good job of compensating.

The Prone Lachman Variation

For clinicians with smaller hands or when examining a large, muscular thigh that is hard to stabilize, a prone variation exists. The patient lies face-down with the knee hanging off the edge of the table at about 30 degrees of flexion. The examiner then uses gravity and one hand to apply the anterior tibial force. This version can be easier to perform physically and may give a clearer endpoint on large patients.

A study assessing the prone Lachman found a sensitivity of 70% and a specificity of 97%. The positive likelihood ratio was high at about 20, meaning a positive result is very strong evidence of a tear, though the sensitivity indicates some tears will be missed.¹⁴ The prone version trades some sensitivity for practicality, and some clinicians use it as a supplement rather than a replacement.

Examiner Experience and Reliability

How much the examiner matters is not an abstract concern. A study comparing inter-examiner agreement between experienced and inexperienced clinicians found that agreement on the Lachman test between two experienced examiners was almost perfect, with a kappa of 0.92. Between an experienced and an inexperienced examiner, kappa dropped to 0.42, which is considered weak.¹⁵ The Lachman test still fared better than both the anterior drawer and the lever sign test in this comparison, but the gap between novice and expert performance is striking.

This partly explains the wide range of sensitivity figures in the literature. A study from a specialized sports medicine center with fellowship-trained surgeons will naturally produce higher accuracy than a study conducted in a general emergency department. If you are concerned about your knee, being examined by someone who performs ACL assessments regularly makes a real difference.

Partial ACL Tears and Diagnostic Difficulty

The Lachman test is designed to detect a ligament that is not doing its job. When the ACL is completely torn, the tibia slides forward with a soft endpoint, and an experienced examiner usually picks this up. Partial tears are much harder. A partially torn ACL retains some fibers that still check forward motion, often producing enough of a firm endpoint to pass the Lachman test even though the ligament is structurally compromised.

A study looking at patients thought to have partial ACL tears found that among 95 patients, 42% had examination-under-anesthesia findings consistent with a partial tear, and 23% had MRI findings suggesting the same. But arthroscopic examination, the gold standard, confirmed a true partial tear in only 12% of those patients. MRI was 91% sensitive and 86% specific for partial tears, while the clinical exam under anesthesia was 100% sensitive but only 65% specific, meaning it frequently overcalled partial tears.¹⁶ The clinical picture here gets murky: partial tears occupy a gray zone where physical exam findings and even MRI may overestimate or underestimate the real damage.

Using the Lachman Test After ACL Reconstruction

Once an ACL has been surgically reconstructed, the Lachman test takes on a different role. Instead of diagnosing an initial tear, it is used to monitor the graft’s integrity over time. The test still works in principle the same way, but the numbers shift. One study comparing Lachman sensitivity in native ACL injuries versus graft re-injuries found sensitivity was 95% for native tears but dropped to 85% in patients who had previously undergone reconstruction.¹⁷ The graft behaves differently from the original ligament, and scar tissue around the reconstruction can partially constrain tibial translation even when the graft itself is failing.

A biomechanical study explored this further by looking at different degrees of graft slackening. Instrumented Lachman testing showed that a side-to-side increase in anterior tibial translation of 3 mm or less was 100% predictive of a stable graft with a negative pivot shift. Once slackening reached 5 mm, most knees still functioned well, but 7% converted to a positive pivot shift, indicating the graft was no longer doing its job.¹⁸

Perhaps most sobering is a study that performed arthroscopy on 24 patients after ACL reconstruction and found that 13 had an insufficient graft, but only 5 of those were detected by physical examination or the KT-1000 arthrometer. The remaining 8 had clinically stable knees despite lacking a functioning graft.¹⁹ Scar tissue, secondary stabilizers, and neuromuscular adaptation can all conspire to make a knee feel stable on exam even when the graft has failed. If you have had ACL surgery and notice recurrent instability episodes despite a “normal” clinical exam, further investigation may be warranted.

Instrumented Measurement with the KT-1000

The KT-1000 arthrometer is a device that straps to the leg and measures anterior tibial translation in millimeters under a standardized force, removing much of the subjectivity from the Lachman test. It is used in research settings, postoperative follow-up, and sometimes in clinical decision-making when the manual exam is equivocal.

A study evaluating the relationship between the KT-1000 and clinical tests found a significant correlation between the arthrometer readings at a standardized force and both the Lachman and anterior drawer tests.²⁰ However, the relationship is not as tight as you might expect. Another study found that one year after ACL reconstruction, Lachman results correlated modestly with KT-1000 measurements from the involved knee (r = 0.39) but not at all with side-to-side KT-1000 differences (r = 0.15).²¹ This disconnect suggests that the subjective feel of the Lachman test and the objective number from the arthrometer are measuring overlapping but not identical things. The manual test integrates endpoint quality and tibial rotation in a way that a device measuring purely anterior-posterior translation in one plane cannot fully capture.

Cadaver work has also shown that a manual Lachman test tends to underestimate tibial translation compared to a standardized force applied through a mechanical setup.²² This makes sense: a human hand does not generate the same consistent force every time, and the amount of force varies between examiners. But in clinical practice, few surgeons rely on the KT-1000 alone. It supplements rather than replaces the manual exam.

The Test in Children and Adolescents

ACL injuries in children and adolescents are increasingly common, and performing a reliable Lachman test in this population presents unique challenges. Smaller limbs, open growth plates, and sometimes significant anxiety about being examined can complicate the assessment. A prospective study in children and adolescents found that when the Lachman test was performed by an experienced orthopedic surgeon and endpoint feel was assessed, it achieved a positive predictive value of 95% and a negative predictive value of 82%.²³ The anterior drawer test actually showed slightly better negative predictive value in this group, at 90%, suggesting it may be a useful complement in younger patients whose smaller legs are easier to position at 90 degrees of flexion.

The key qualifier in that study was “experienced orthopedic surgeon.” In younger patients, where the consequences of a missed diagnosis include potential growth-plate damage and long-term joint degeneration, referral to a specialist rather than relying on a screening exam in a primary-care setting is particularly important.

The Lachman Test in Veterinary Medicine

Dogs, especially larger breeds, commonly tear their cranial cruciate ligament, the anatomic equivalent of the human ACL. Veterinary surgeons use analogous tests to detect this injury, and the biomechanics are surprisingly similar despite the four-legged anatomy. An ex vivo study evaluating three manual laxity tests in canine cadaver limbs, including a cranial tibial thrust test (a test that parallels the Lachman principle), found 100% sensitivity and specificity when the cranial cruciate ligament was sectioned.²⁴ Intra- and interobserver agreement for translation measurements was excellent, though agreement on rotation and force variables was more inconsistent.

If your dog has been diagnosed with a cruciate tear through a “drawer test” or similar manual exam, the underlying logic is the same as when your own knee was examined: the examiner is feeling for forward tibial shift and endpoint quality. The cadaver study numbers are ideal-case scenarios, but the clinical principle translates cleanly across species. Treatment decisions in dogs, just as in humans, typically combine the physical exam with imaging and the patient’s (or pet’s) functional history.

Origin of the Test

The test is named after John Lachman, an orthopedic surgeon who chaired the Department of Orthopaedic Surgery and Sports Medicine at Temple University in Philadelphia. The original published description was submitted by one of his colleagues, Joseph Torg, in the 1970s. Before the Lachman test gained widespread acceptance, the anterior drawer test at 90 degrees of flexion was the standard clinical examination for ACL injuries. The shift toward testing at low flexion angles represented a meaningful improvement in diagnostic thinking, grounded in the biomechanical reality that the ACL is under greatest tension near full extension. The test’s enduring presence in clinical practice, more than four decades later, speaks to how well that basic insight holds up, even as the field continues to refine the details around who should perform it, when, and how the results should be interpreted alongside imaging and instrumented testing.