ACL knee injury symptoms

If you’re looking up “ACL knee injury symptoms”, then chances are you’re worried about the dreaded ACL knee injury.

So what is an ACL, how does it happen and how can you tell if you’ve injured your ACL?

What is an ACL (Anterior Cruciate Ligament)?

The Anterior Cruciate Ligament is a thick band of connective tissue that joins the top of your shin bone (tibia) to your thigh bone (femur).

It is contained completely inside the knee joint and it has good blood supply – two important points for diagnosis.

The job of the ACL is to stop the tibia sliding forward off the femur and to resist rotation of the tibia as the foot twists outwards.

How does an ACL knee injury happen?

The ACL can be damaged in any situation when the movements it restrains are exposed to excessive loading.

The classic mechanism for ACL rupture (complete tear) is when a football boot digs in to the ground and the player twists against it.

ACL knee injury - Photo credit to AAP, via
Jack Bird (Brisbane Broncos NRL) ACL injury – March 2020

The same mechanism can occur during snow skiing, when a skier loses balance and twists over their ski as the ski grips and doesn’t twist with them.

Another mechanism is a deceleration movement. This is typically associated with basketball when a player hits the brakes coming in to the basket and the knee collapses under them.

In this circumstance, the strong quads muscle at the front of the thigh pulls the shin forward and the ACL fails to resist the force and ruptures.

The final mechanism for ACL knee injury is a hyperextension movement, or the knee forcefully and excessively straightening to the point of bending back on itself.

In this situation, the simple leverage of the thigh and shin bones rolling too far forward can create enough force to rupture an ACL.

What are the ACL injury symptoms? How can you tell if you’ve ruptured your ACL?

Firstly, you need a mechanism of injury. An ACL rupture can’t occur gradually over time, it needs an excessive force to overload it.

The knee will typically swell up rapidly, due to the ACL’s good blood supply. It bleeds into the knee joint and inflates it within a few minutes.

The knee is no longer trustworthy. The injured person often describes that they don’t want to load the knee because it feels like it’ll collapse. (This is different from not loading it because it hurts too much, which happens with many different injuries).

The person can often walk with a bent knee but won’t be able to walk on their heels. As heel walking forces the knee straight, they’ll refuse because of the feeling that it’ll give way on them.

Pain is quite variable, so don’t assume that all ACL knee injuries are painful as a criteria. Some isolated ACL tears are almost pain free while others have numerous other structures damaged, like MCL tears and meniscal injury. Pain levels are not a good guide of injury type or severity.

Should you go to hospital for ACL scans?

If you had an incident, the knee swelled quickly and doesn’t feel trustworthy, there’s a strong potential for an ACL knee injury.

An ACL rupture is not a medical emergency, so no need to panic and rush to a hospital emergency department. The damage is done and getting quick assessment won’t change anything.

That said, don’t keep loading it if it’s giving way on you as the collapsing movement can cause damage to other structures.

For the ACL knee injury alone, get medical assessment the next day to confirm the diagnosis. If the pain is unbearable when it happens, you may want immediate medical assessment so they can provide some pain relief.

If surgery is on the table (pun intended), they need to wait for the knee swelling and reaction to settle. So regardless of immediate or delayed assessment, surgery won’t be done for a few weeks anyway.