Understanding ACL Tears: What Happens and Why

Springbok rugby has one of the sport's most remarkable ACL tear comeback stories. Pieter-Steph du Toit tore his ACL in 2014, then tore it again in the same knee the following year — reconstructed the second time using a hamstring tendon donated by his own father. He came back to be named World Rugby Player of the Year in 2019 and lift the Rugby World Cup with the Springboks. But while rugby puts ACL tears in the spotlight, the injury isn't limited to the sport — it shows up just as often in soccer, netball, and other pivot-heavy sports.

Let's look at what the ACL is, how it usually tears, and the symptoms to look out for.

What does the ACL do?

The ACL (anterior cruciate ligament) sits deep inside the knee, connecting the thigh bone to the shin bone. Its job is to stop the shin from sliding too far forward relative to the thigh and to control knee rotation. In practice, that means it's doing a lot of work whenever you cut, pivot, decelerate suddenly, or change direction, which is exactly why it comes up so often in sports like rugby, soccer, and netball.

How does the ACL tear?

ACL tears happen in one of two ways:

• Contact: A direct blow to the knee, usually from another player or an impact during play, forcing the joint into a position it wasn't built for.

• Non-contact: This happens when someone plants their foot and twists, or lands awkwardly from a jump, putting more rotational force through the knee than the ligament can handle. So your ACL can tear without any collision at all — in fact, most tears happen this way.

In both cases, people often describe hearing or feeling a distinct "pop" at the moment of injury, followed by swelling that sets in fairly quickly, sometimes within minutes, sometimes over a few hours.

Interestingly, ACL tears aren't evenly split between men and women. A 2025 study found men were hospitalised for ACL ruptures at roughly 2.6 times the rate of women overall. But that gap largely comes down to how many men versus women play high-risk sports in the first place. When you compare male and female athletes in the same sport, the pattern flips. In fact, research (Orthop, 2016) has shown that women experience two to eight tmes more of these injuries than men, likely due to a combination of anatomical, biomechanical, and hormonal factors.

Partial tear or complete tear?

Not all ACL tears are the same. Sometimes the ligament is just stretched, sometimes it's partially torn, and sometimes it's torn all the way through. A partial tear means some of the ligament's fibres have given way, but it's still holding together to some degree, so the knee might still feel reasonably stable. A complete tear means it's torn clean through, which is what happens in most sports-related ACL injuries, and it usually comes with much more noticeable instability.

What to look out for

Not every knee twinge is an ACL tear, but a few signs are worth taking seriously:

• A pop or snap felt at the time of injury
• Swelling that comes on rapidly, often within the first few hours
• A feeling of instability, or the knee "giving way" underneath you
• Pain when trying to put weight on the leg
• Reduced range of motion in the knee afterwards

Any one of these on its own is reason enough to get the knee checked. A physiotherapist doesn't just come into the picture after surgery — an early assessment helps establish what's going on and what the right next step is, whether that's a structured rehab programme or a referral to a doctor for further investigation.

At Hein Viljoen Physiotherapy, we're happy to help you figure out which path is right for you. Click here to book your appointment with one of our Cape Town physios.

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