How Is an ACL Tear Diagnosed? Lachman Test, Exam and MRI

How Is an ACL Tear Diagnosed? Lachman Test, Exam and MRI

How is an ACL tear diagnosed? The typical symptoms, how the Lachman, anterior drawer and pivot shift tests work, what MRI adds, and how partial and complete tears are told apart.

An ACL tear is usually recognised from how the injury happened, the symptoms in the first hours and the examination findings: a "pop" in the knee during a sudden twist or an awkward landing, marked swelling within a few hours and, later, episodes of the knee giving way. In the clinic, tests such as the Lachman test check how well the ligament holds, while MRI supports the diagnosis and shows associated damage to the meniscus or cartilage.

This article explains what the anterior cruciate ligament does, which symptoms suggest a tear, how the Lachman and other tests are performed, what an MRI report looks for, and how partial and complete tears are told apart.

What is the ACL and how does it tear?

The anterior cruciate ligament runs through the centre of the knee from the thigh bone to the shin bone. It stops the shin from sliding forwards and controls rotation. Behind it, crossing it, lies the posterior cruciate ligament; when people talk about a "cruciate ligament tear", they usually mean the ACL.

Most ACL tears happen without contact: a sudden change of direction, the body turning over a planted foot, or landing from a jump with the knee collapsing inwards. Football, basketball, handball, volleyball and skiing are classic settings. A blow to the side of the knee can injure the ACL together with other ligaments. Female athletes are known to be at higher risk.

What are the symptoms of an ACL tear?

The combination of the following findings points towards an ACL tear:

• A pop: many people hear or feel something snap or shift in the knee.
• Rapid swelling: marked swelling within the first few hours often means bleeding inside the joint.
• Unable to continue: pain and a sense of instability force most athletes off the field.
• Restricted movement: the swollen knee will not fully bend or straighten.
• Giving way: once swelling settles, the knee may buckle or shift, especially when turning or going downstairs.

Symptoms often calm down within a few weeks and walking on level ground may feel normal. That does not mean the ligament has healed; a torn ACL generally does not reconnect on its own. The real problem is the knee giving way during pivoting movements, and each episode can put the meniscus and cartilage at further risk.

For what to do on the field and in the first days, see our article on knee twist injuries in sport.

How is an ACL tear diagnosed on examination?

The clinical examination is the cornerstone of diagnosis. The doctor asks how the injury happened, how quickly the knee swelled and whether it gives way, then performs specific tests comparing both knees.

Lachman test

You lie on your back with the knee bent about 20–30 degrees. The doctor steadies the thigh with one hand and draws the top of the shin forwards with the other. With an intact ligament, the shin moves a little and then stops firmly. With a torn ACL, it slides further and the end point feels soft or vague. Because it can be done even in the early painful phase, the Lachman test is one of the most widely used ACL tests.

Anterior drawer test

With the knee bent to 90 degrees and the foot resting on the couch, the shin is pulled forwards. Increased movement compared with the other side suggests ligament deficiency, although muscle guarding can make the test misleading in the acute phase.

Pivot shift test

This test reproduces the shifting sensation patients feel when the knee gives way. A positive result is highly meaningful, but in a painful, swollen knee protective muscle spasm often makes it impossible to perform while awake.

Pain and swelling straight after the injury can limit the examination, so a repeat assessment once the swelling has eased often gives clearer information. The collateral ligaments, menisci and kneecap are checked at the same time.

MRI and other tests: when and why?

• X-ray: does not show ligaments, but rules out fractures and can reveal small bone fragments pulled off with a ligament.
• MRI: shows the ACL as well as the menisci, cartilage, collateral ligaments and bone bruising. ACL tears often leave a typical bruising pattern on the outer side of the thigh and shin bones.
• Ultrasound: of limited value for the ACL itself, but can show fluid in the joint and superficial structures.

MRI is a valuable aid, but it does not make the decision on its own. With partial tears in particular, images and examination findings do not always match. A firm diagnosis comes from combining symptoms, examination and imaging.

Partial tear or complete rupture?

A "partial tear" on an MRI report means some fibres are damaged but the ligament is not completely divided; in a complete rupture it is split into two ends. What matters most is how unstable the knee is on examination, not the wording of the report.

• If there is clear forward and rotational instability, the ligament may have lost most of its function even if the report says "partial".
• If the knee is stable and does not give way, partial tears can often be managed with physiotherapy and strengthening.
• Associated meniscus or cartilage injuries are factored into the treatment plan.

How is treatment planned after diagnosis?

Not everyone with an ACL tear needs surgery. The decision depends on age, sport and goals, how often the knee gives way, any meniscus or cartilage damage, and expectations for daily life. Young, active people who want to return to pivoting and jumping sports are often advised to consider arthroscopic reconstruction, while less active people whose knees do not give way may do well with a structured physiotherapy programme.

Our article on ACL surgery techniques and grafts explains the surgical options in detail. In Prof. Dr. Gökhan Polat's clinic, the plan is built on the examination, the imaging and the patient's own goals; response to treatment and recovery time vary from person to person.

When should you see an orthopedic surgeon?

• A pop in the knee during a twist or landing, followed by swelling within hours
• The knee giving way, shifting or feeling unreliable
• The knee locking or failing to straighten fully, which may point to an associated meniscus tear
• Inability to bear weight or a visible deformity
• Swelling and stiffness that have not settled after several weeks

Fever, redness and warmth of the knee, or one-sided calf swelling and pain, need urgent assessment. To learn more about diagnosis and treatment options, visit our anterior cruciate ligament injury page.

In summary

• A pop during a twist or landing, swelling within hours and later giving way suggest an ACL tear.
• The Lachman, anterior drawer and pivot shift tests assess ligament stability; a repeat examination may be needed once swelling settles.
• MRI supports the diagnosis and reveals associated injuries, but decisions rely on the examination too.
• Surgery is not automatic; age, activity level and instability guide the choice.

This article is for general information only; diagnosis and treatment decisions are made individually after examination and the necessary tests.