How Do I Know If I Have Torn My ACL?

If you have just injured your knee on the field, it can be difficult to know immediately what has happened. But there are specific signs that point strongly towards an acute ACL tear — and understanding them can help you make the right decision quickly.

The Typical Story

Most ACL tears happen in pivoting sports — football, basketball, rugby, and similar activities. The injury often occurs during a sudden change of direction, a landing from a jump, or sometimes after contact with another player.

God has designed the ACL as a richly vascularised structure — meaning it has a good blood supply. This is actually what makes an ACL tear so distinctive in its presentation. When the ACL tears, blood immediately flows into the knee joint, producing what is called a haemarthrosis — an acute bleed within the joint.

Signs and Symptoms of an Acute ACL Tear

1. A pop sensation

Many patients describe feeling or hearing a pop at the moment of injury. This is one of the most telling signs of an ACL tear.

2. Immediate swelling

Because the ACL is a vascularised structure, swelling appears immediately — often within the first hour. This is different from a meniscus injury, where swelling is typically delayed. The swelling is inside the joint itself, not under the skin.

3. Inability to continue playing

This is one of the most important clinical clues. If you had to stop playing immediately after the injury, that is a significant warning sign. If a patient tells me they were able to return to the field and continue, I am actually less suspicious of a complete ACL tear — because the injury is serious enough that playing on is usually not possible.

4. Pain at the time of injury

There is usually a sudden onset of pain at the moment of injury. However, interestingly, the pain may settle somewhat after the acute episode — it is the swelling and instability that tend to persist.

5. No bruising

Unlike injuries to the skin or soft tissues around the knee, an ACL tear typically does not cause bruising. The bleeding is contained within the joint.

Why the Knee Collapses Inward — What the Research Shows

Understanding how ACL injuries happen helps explain what you may have felt on the pitch. A landmark study by Professor Stefano Zaffagnini and colleagues at the Rizzoli Orthopaedic Institute in Bologna — analysing 134 consecutive ACL injuries in professional male football across 10 seasons — found that knee valgus loading was the dominant injury pattern, observed in 81% of cases across all main situational patterns including pressing, tackling, and landing from a jump.

In plain terms: the knee collapses inward at the moment of injury. This is the position the ACL is most vulnerable in — and it is the reason why strengthening the hip and gluteal muscles to prevent that inward collapse is central to ACL injury prevention.

ACL Tear vs Meniscus Injury — A Quick Comparison

Patients often confuse an ACL tear with a meniscus injury. Here is a simple way to tell them apart:

	ACL Tear	Meniscus Injury
Swelling	Immediate (within 1 hour)	Delayed (hours)
Pop sensation	Common	Less common
Ability to play on	Usually not possible	Sometimes possible
Bruising	No	No
Cause	Pivoting, landing, contact	Twisting, deep squat

Note that both injuries can occur together — an ACL tear frequently involves a concurrent meniscus injury.

What Should You Do Next?

If you have experienced these symptoms after a knee injury, you should seek a formal assessment promptly. A sports surgeon will take a detailed history, examine the knee, and arrange an MRI scan to confirm the diagnosis.

Early assessment matters. The sooner the injury is properly diagnosed, the sooner a clear plan can be put in place — whether that involves surgery, rehabilitation, or both.

If you would like to arrange a consultation, you can find out more and get in touch at mokyingren.sg/acl/start.

References

Della Villa F, Buckthorpe M, Grassi A, et al. Systematic video analysis of ACL injuries in professional male football (soccer): injury mechanisms, situational patterns and biomechanics study on 134 consecutive cases. Br J Sports Med. 2020;54:1423–1432.