How to Test for a Torn ACL: Signs, Exams, and Imaging

A torn anterior cruciate ligament is diagnosed through a combination of hands-on physical examination and, in most cases, an MRI scan to confirm the injury and check for additional damage inside the knee. No single test is definitive on its own. Clinicians typically start with a few specific manual maneuvers in the office, each designed to detect abnormal forward or rotational movement of the shinbone relative to the thighbone. What makes ACL diagnosis interesting is how much the accuracy of these bedside tests depends on timing, swelling, the examiner’s experience, and whether the tear is partial or complete.

What an ACL Tear Feels Like Before Any Test

Most people who tear their ACL know something serious has happened the moment it occurs. The classic story involves a sudden change of direction, a hard landing, or a collision, followed by an audible pop, immediate swelling, and a feeling that the knee has “shifted” or “given way.” Within a few hours the knee balloons with blood (a hemarthrosis), making it painful to straighten or bend fully. Not everyone hears a pop, but the rapid onset of swelling after a non-contact twist is one of the strongest clinical clues that the ACL is involved. If you can still walk but the knee feels unstable or loose, that pattern alone raises suspicion enough to warrant formal testing.

The Four Main Hands-On Tests

Orthopedic surgeons and sports medicine clinicians rely on a handful of physical examination maneuvers to assess whether the ACL is intact. Each one stresses the ligament in a slightly different way, and they work best when used together rather than in isolation.

Lachman Test

The Lachman test has long been considered the go-to bedside maneuver for ACL tears. You lie on your back with your knee bent slightly. The examiner stabilizes your thighbone with one hand and pulls the top of your shinbone forward with the other. In an intact knee the movement stops with a firm endpoint. When the ACL is torn, the shinbone slides forward with a soft or absent endpoint. A large meta-analysis pooling data from multiple studies found the Lachman test has a sensitivity of about 81% and a specificity of about 85% for detecting ACL tears overall.¹ That means it catches roughly four out of five tears and correctly clears a similar proportion of intact knees. Some individual studies have reported higher sensitivity values, above 90%, but the pooled evidence suggests earlier literature may have overestimated the test’s accuracy.

One practical wrinkle: the Lachman test can be difficult to perform on patients with large thighs, because the examiner needs to wrap a hand around the distal femur. A prone version of the test, where you lie face-down, has been studied as an alternative. One study found prone Lachman had 70% sensitivity but an impressive 97% specificity, meaning a positive result was highly reliable even if some tears were missed.²

Anterior Drawer Test

The anterior drawer test is older and more intuitive. You sit or lie with your knee bent to about 90 degrees while the examiner pulls your shinbone straight forward. It tests the same forward translation as the Lachman but at a deeper knee angle. Results in the literature vary widely. One pooled analysis reported a surprisingly low pooled sensitivity of 20%, though individual studies in the same review ranged from 18% to 92%.³ Other research has reported sensitivity above 90%.⁴ That gap largely reflects who is doing the test and when. The anterior drawer tends to perform poorly in the acute setting, when muscle guarding and swelling make the knee difficult to relax, and better in chronic cases when the initial inflammation has settled.

Pivot Shift Test

The pivot shift is different from the other tests because it checks for rotational instability rather than simple forward-backward movement. The examiner applies a combination of inward rotation and a sideways force to the knee while slowly bending it. In a torn ACL, you feel a sudden “clunk” as the shinbone subluxates and then snaps back into place. The experience is unmistakable when it happens, but the test is notoriously difficult to perform well. Its sensitivity is lower than the Lachman or anterior drawer, with pooled estimates around 55%, but its specificity is very high at roughly 94%.¹ In other words, it misses many tears, but when it is positive it is almost always right.

Part of the reason for the low sensitivity is that different examiners perform the maneuver in different ways, and some patients simply cannot relax enough for the subluxation to occur.⁵ The pivot shift is also influenced by injuries to other structures around the knee, so a strong positive result can signal damage beyond just the ACL.⁶

Lever Sign (Lelli’s Test)

The lever sign is a newer addition to the ACL exam toolkit. You lie flat on your back and the examiner places a fist under your upper calf, just below the knee. If the ACL is intact, pressing down on the thigh causes the foot to rise off the table like a seesaw. If the ACL is torn, the heel stays down. The appeal of this test is its simplicity, and early studies reported impressive sensitivity values in the range of 83–92%.⁷ However, those numbers have not held up universally. When medical students performed the test rather than experienced surgeons, sensitivity plummeted to as low as 12% in thin patients and 46% in overweight patients, suggesting that examiner skill and patient body habitus play a large role. A separate study found the lever sign had balanced sensitivity (about 85%) and specificity (about 86%) for complete tears.⁸ Another found it had the highest sensitivity and overall diagnostic accuracy compared with traditional tests.⁹ The evidence is still maturing, but the lever sign appears to be a useful addition rather than a replacement for the traditional three tests.

Why Timing Matters for Physical Exam Accuracy

A freshly injured knee is swollen, painful, and hard to examine. Muscles involuntarily tighten to protect the joint, which can mask the laxity that these tests are designed to detect. Research on the lever sign found it to be more accurate in acute injuries, while the Lachman test performed better in chronic ACL-deficient knees.¹⁰ The meta-analysis mentioned earlier reported that the Lachman test’s sensitivity dropped to around 70% in post-acute injuries, compared with its overall pooled figure of 81%.¹ Clinicians sometimes examine an acutely injured knee under anesthesia before surgery, which removes the muscle-guarding problem and generally improves test accuracy.

MRI and What It Reveals

MRI is the imaging standard for confirming an ACL tear. It can visualize the ligament directly, show whether the tear is partial or complete, and reveal damage to other structures in the same scan. Studies comparing MRI findings with what surgeons actually see during arthroscopy have reported sensitivity above 90% and specificity above 95% for ACL tears.¹¹

Beyond the ligament itself, MRI picks up secondary clues. A characteristic pattern of bone bruising, with swelling on the back of the outer shinbone plateau and the front of the outer thighbone condyle, is strongly associated with ACL tears and can confirm the diagnosis even when the ligament image is ambiguous.¹² MRI also shows meniscus tears, cartilage damage, and collateral ligament injuries, all of which commonly accompany ACL ruptures and affect treatment planning.

Do X-Rays Play Any Role?

Plain X-rays cannot show a torn ligament, but they are still commonly ordered after acute knee injuries, mainly to rule out fractures. One specific finding on X-ray, the Segond fracture, is a small chip of bone pulled off the outer edge of the tibia. This fracture is strongly associated with ACL rupture and can be the first clue pointing toward the ligament injury on an image that would otherwise look unremarkable.¹³ These avulsion fractures can be subtle and easy to miss on standard films.¹⁴ An X-ray that looks normal does not rule out an ACL tear, but one showing a Segond fracture should prompt an MRI.

Ultrasound as a Diagnostic Tool

Ultrasound is not the first thing most people associate with knee ligament injuries, but research is building a case for its use, particularly as a rapid bedside option. A retrospective study of 247 patients found that dynamic ultrasound detected about 88% of arthroscopically confirmed ACL tears, with 82% specificity.¹⁵ The technique involves watching the knee in real time while the examiner applies stress, looking for abnormal ligament movement, swelling at the ligament’s attachment points, and measurable instability. It performed better for complete tears than partial ones, where sensitivity dropped to around 52%.

More recent work has focused on standardizing the ultrasound protocol so that different examiners can reproduce the findings reliably, examining parameters like the angle of the ACL, swelling patterns, and the degree of dynamic instability.¹⁶ Ultrasound won’t replace MRI for detailed pre-surgical planning, but it could become a valuable tool in settings where MRI access is limited or where a quick confirmation is needed before deciding whether to order more expensive imaging.

The Partial Tear Problem

Partial ACL tears are the diagnostic headache of this entire process. When only some of the ligament fibers are torn, the knee may not feel as loose on examination, and imaging findings can be ambiguous. A systematic review found that Lachman test sensitivity dropped from about 96% for complete tears to 68% for partial ones.¹⁷ The pivot shift showed a similar pattern, falling from 86% to 67%. MRI results for partial tears also show overlap with complete tears, making it hard to distinguish between the two on imaging alone.¹⁸

This is where clinical judgment becomes especially important. A surgeon may combine physical exam findings, MRI results, and the patient’s functional complaints to decide whether a partial tear is likely to progress, stay stable, or respond to rehabilitation without surgery. In some cases, diagnostic arthroscopy, where a small camera is inserted into the joint, remains the only way to know for sure whether a partial tear is present and how significant it is.

Instrumented Laxity Testing

Between the physical exam and MRI sits another category of testing: arthrometers, which are devices that measure exactly how far the shinbone translates forward under a controlled force. The KT-1000, the most well-known device, was shown in early research to produce results suggestive or diagnostic of abnormal anterior laxity in the vast majority of patients with confirmed complete ACL tears.¹⁹ The advantage is objectivity: instead of relying on the examiner’s feel for “firm” versus “soft” endpoint, the device gives a number in millimeters. A side-to-side difference of three millimeters or more between the injured and uninjured knee is generally considered significant. Arthrometers are more common in research settings and large sports medicine clinics than in general orthopedic offices, and they are also used during follow-up after ACL reconstruction to track whether the graft is holding.

Checking for Injuries That Tag Along

An ACL tear rarely happens in isolation. The forces that rupture the ligament frequently damage other structures in the knee at the same time. Meniscal tears are the most common companion injury. A large case-control study found that about 41% of patients with isolated ACL tears had a meniscal injury, and that rate rose to over 53% in patients who also tore their medial collateral ligament.²⁰ Lateral meniscal tears were especially common when both the ACL and MCL were involved. This is why MRI is so valuable: it shows the full picture in one scan, and knowing about meniscal or collateral ligament damage before surgery changes the operative plan.

Testing ACL Injuries in Kids and Teens

ACL tears in young athletes are becoming more common, and diagnosing them requires some adjustment. Children and adolescents with open growth plates can sustain tibial spine avulsions (where the bone the ACL attaches to fractures away) rather than a classic mid-substance ligament tear, and these look different on imaging. Physical examination is also more challenging in younger patients, who may be anxious and difficult to relax during testing. Research has shown that the standard battery of tests, including Lachman, anterior drawer, and pivot shift, can still achieve high predictive values in children and adolescents when performed by an experienced orthopedic surgeon.²¹ However, the threshold for ordering MRI tends to be lower in pediatric patients, both to confirm the diagnosis and to assess growth plate status before deciding on treatment.

Artificial Intelligence Reading Your MRI

One of the more intriguing developments in ACL diagnosis is the use of AI to interpret MRI scans. A systematic review and meta-analysis of AI models applied to ACL tear detection found that these algorithms achieved about 87% accuracy, with sensitivity around 91% and specificity around 91%.²² Those numbers matched or slightly exceeded the performance of clinicians in some comparisons. Deep learning models trained on large databases of knee MRIs have shown particular promise, with one study reporting an area under the curve of 0.89 for a CNN-based system using arthroscopy as the reference standard.²³

These tools are not replacing radiologists, but they could serve as a second set of eyes, flagging tears that might be missed during a busy reading session or assisting in clinics where a specialized musculoskeletal radiologist is not available. The technology is still largely in the research and validation phase.

Virtual Exams and Telehealth Limitations

Telehealth expanded rapidly during the COVID-19 pandemic, and sports medicine was no exception. Researchers have developed protocols for virtual knee physical examinations, providing patients with verbal instructions in plain language so they can perform simplified versions of tests on camera.²⁴ The obvious limitation is that the examiner cannot apply force to the knee through a screen, so tests like the Lachman, anterior drawer, and pivot shift simply cannot be replicated at home. A virtual visit can still be useful for gathering a detailed injury history, observing swelling and range of motion, and deciding whether an in-person exam and imaging are warranted. It is a triage tool, not a diagnostic one.

Functional Testing After Reconstruction

Testing does not end once the ACL tear is confirmed and treated. After surgical reconstruction, a separate battery of functional tests determines when it is safe to return to sport. These typically include single-leg hop tests, strength measurements of the quadriceps and hamstrings, and psychological readiness questionnaires. Research has recommended that return-to-sport protocols include at least two dynamic functional tests, strength testing, psychological readiness assessment, and a minimum of eight months since surgery.²⁵

A common metric used in hop testing is the limb symmetry index, which compares performance between the surgical and non-surgical legs. However, recent work has shown that symmetry in jump distance does not necessarily reflect good movement quality during the landing phase, and suboptimal landing mechanics are a known risk factor for re-injury.²⁶ The current trend is to evaluate not just how far you can jump, but how well you land. Clinicians are increasingly looking at knee control, trunk stability, and asymmetries in landing mechanics as part of a more comprehensive clearance process before athletes return to cutting, pivoting, and full competition.