The first clue is the way the injury happened, because ACL tears tend to follow a very specific mechanism.

Most of them are non-contact, and that genuinely surprises people. You don’t need someone to hit you or tackle you. The classic ACL injury is a sudden change of direction, a pivot, a twist, or an awkward landing from a jump — typically where the foot stays planted on the ground while the body keeps turning. That rotational force, with the foot fixed, is exactly what the ACL is least able to cope with.

It’s why you see these injuries so often in sports like football, netball, rugby, basketball and skiing — anything involving quick stops, turns and changes of direction. If your injury happened in that kind of moment, with the foot planted and the knee twisting while your momentum carried you on, that’s the textbook setup for an ACL tear, and it’s the first thing worth paying attention to.

Once you’ve considered how it happened, there are several signs of a torn ACL to look for in the moments and hours afterwards. Here are the main ones.

The pop. A lot of people genuinely hear or feel a distinct pop or snap deep in the knee at the moment it goes. Sometimes it’s loud enough that people standing nearby hear it too. Now, not everyone experiences this, so you shouldn’t rule out an ACL tear just because you didn’t feel a pop — but if you did feel one right as the injury happened, that’s a strong pointer towards the ligament.

Fast swelling. This is one of the most useful clues of the lot, so it’s worth understanding properly. With an ACL tear, the knee usually swells up quickly — within the first few hours. The reason is that the ACL has a good blood supply, so when it tears it bleeds into the joint, and the knee balloons up fast. That rapid swelling is a real giveaway. If your knee blew up within a couple of hours, take it seriously. By contrast, swelling that creeps in slowly over a day or two is more typical of other injuries, such as a meniscus tear — which is one of the main ways the two can be told apart.

Instability and giving way. The ACL’s job is to stop the shin bone sliding forward on the thigh bone and to give the knee its rotational stability. So when it’s torn, the knee often feels unstable — like it might give way or buckle underneath you, especially when you try to turn, pivot or change direction. People describe it in different ways: the knee feeling loose, wobbly, or like it’s about to come apart. If your knee is giving way on you, that instability is one of the most classic signs of a torn ACL there is.

Significant pain and having to stop. At the moment of injury there’s usually significant pain, and most people simply can’t carry on. You typically have to stop playing or come off there and then. In the hours afterwards you might also struggle to fully straighten or bend the knee, partly because of all that swelling filling the joint.

Run through those four alongside the mechanism, and you’ll usually get a clear sense of how likely an ACL tear is. The more of them that match, the stronger the suspicion.

As useful as these signs are, it’s important to be honest about their limits — recognising the pattern tells you an ACL tear is likely, but it doesn’t confirm it. For that, you need a proper assessment.

A skilled clinician can carry out specific hands-on tests to check how the knee is moving and whether the ACL is doing its job. The main one is called the Lachman test, where the clinician gently checks how much the shin bone moves forward on the thigh — an ACL-deficient knee tends to show more movement than a healthy one. There are other tests too, but the principle is the same: they’re assessing the stability the ACL is supposed to provide.

The definitive confirmation usually comes from an MRI scan. As well as confirming the ACL tear itself, an MRI shows whether anything else has been injured at the same time — and that matters, because ACL injuries very commonly come with damage to the meniscus or the cartilage. Knowing the full picture, not just the ACL, is what allows the right decisions to be made about treatment.

So while you can get a strong sense from the signs, the assessment and scan are what turn a suspicion into a clear answer.

If your injury matches that pattern — the twist, the pop, the fast swelling, the instability — the most important thing is not to sit on it. It’s tempting to wait and hope it settles, especially because, as the swelling eases over the following weeks, the knee can start to feel deceptively okay. But feeling better is not the same as being healed, and an unstable knee that keeps giving way can do further damage over time.

The better approach is to get it properly assessed sooner rather than later. The sooner you know exactly what you’re dealing with, the better every decision after that becomes — whether that turns out to be rehabilitation, surgery, or simply reassurance that it’s less serious than you feared.

If you’ve had a knee injury and several of these signs of a torn ACL ring true, treat that as your cue to get it looked at by someone who can examine it properly and arrange a scan if needed. Every knee is different, and a proper assessment is the only way to know for certain what’s going on inside yours — but knowing what to look for puts you in a far stronger position to take the right next step.

Joe Sharp
BSc (Hons) Physiotherapy