How to tell if you may have torn your ACL

Written by Dr Sean Leo, a consultant orthopaedic surgeon whose clinical focus is the knee, sports injuries and the lower limb.

Last updated: 9 October 2026.

Some people feel or hear a pop when they tear the anterior cruciate ligament (ACL), followed by swelling and a sense that the knee cannot be trusted. These signs also occur with other knee injuries, so they are a reason to have the knee assessed, not a way to diagnose yourself.

What does an ACL tear feel like?

• A pop can happen. It can be a sign, but it is not a diagnosis, and not everyone notices it.
• Swelling, pain and reduced movement can follow.
• Afterwards the knee may feel unreliable, as if it could buckle.

More on what people notice

People often describe a loss of strength, pain when they try to carry on with an activity, or a sense of instability when turning or landing. None of these is a test on its own. They are the kind of story a clinician listens for.

What does this page cover?

• What does an ACL tear feel like?
• What usually causes an ACL tear?
• ACL tear or another knee injury?
• When should a knee injury be checked urgently?
• What should I do next?

What usually causes an ACL tear?

An ACL tear often happens during a sudden change of direction, an awkward landing, or a direct blow to the knee.

• These are common ways the injury happens. They are not the only ways.
• The same movements can injure other parts of the knee, which is why the cause on its own does not tell you what is torn.

More on how the ACL works

The ACL is one of the major ligaments inside the knee. It helps control forward and twisting movement of the shin bone relative to the thigh bone, which is why landing, cutting and pivoting are the moments it is most stressed.

ACL tear or another knee injury?

The signs of an ACL tear are not unique to it. Other structures in the knee may be injured at the same time, so the history, the examination and any imaging findings need to be read together.

• The knee has four main ligaments. The MCL (medial collateral ligament) sits on the inner side, and the ACL crosses through the middle of the joint with the posterior cruciate ligament.
• A clinician may check whether the knee opens inward even when held straight, which can point to a second ligament, most often the ACL, alongside an MCL injury. You do not need to test this yourself.
• Not every ACL tear needs an operation ^[1]. Rehabilitation-led care is an option for some people ^[1], and the right choice depends on the knee, other injuries and the person’s goals.

More on how an assessment helps

An assessment starts with the person’s story: what happened, when the swelling came, whether the knee gives way, and what they want to get back to. Then the whole knee is examined. Pain and guarding soon after an injury can limit what can be tested, so an MRI (a detailed scan of the knee) can add useful information, especially early on. MRI does not decide treatment by itself. It is read together with your history and examination.

For a fuller look at how MCL injuries are recognised and graded, see the MCL page.

When should a knee injury be checked urgently?

What should I do next?

If you have hurt your knee and recognise some of these signs, an assessment is a sensible next step. It brings together what happened, an examination and any tests that help, so that you and your clinician can decide what to do next. An appointment supports an informed decision. It is not a commitment to surgery.

• Read how an ACL injury is assessed and how the treatment options compare on the ACL reconstruction surgery page.
• Browse the Knee Hub for other knee problems.
• Read about three things to know about ACL injuries.
• Read about MCL tear symptoms.
• Read about whether a torn ACL can heal without surgery.
• A short video on hearing a pop at the time of injury covers one common story.

References

1. Frobell RB, Roos HP, Roos EM, Roemer FW, Ranstam J, Lohmander LS. Treatment for acute anterior cruciate ligament tear: five year outcome of randomised trial. BMJ. 2013;346:f232. doi:10.1136/bmj.f232. PMID:23349407. PMCID:PMC3553934.