For some it’s excruciating, others just hear a “pop”. One thing is for sure: once someone has ruptured their ACL they’re unlikely to just walk away – it can be quite literally impossible to bear weight.

A-C-L. They’re the three letters sporting codes dread, signifying the injury that can strip teams of their most valuable players and leave professional careers in tatters.

When he was 20, Sydney Swans player Alex Johnson was on top of the world, having been key in carrying his team to a grand final win. But repeated ACL ruptures meant he did not make another senior appearance for the Swans until he was 26.

Then, two games into his return, he ruptured his previously undamaged knee. Now 29, Johnson has suffered a total of seven ACL ruptures. Unable to return to competitive play, he has instead embarked on a coaching career. More recently, NRL great Brett Morris announced his immediate retirement after suffering a freak ACL rupture in the 77th minute of the Sydney Roosters’ win over the Newcastle Knights in May.

ACL tears are a worry for recreational sportspeople, too. Knee troubles overall were the second-most common sports injury to land people in hospital in Australia between 2011-12 and 2018-19, according to the Australian Institute of Health and Welfare. The most common was concussion. Over that time, the number of soccer players ending up in hospital with an ACL injury doubled. ACLs accounted for one in 10 soccer injuries requiring hospital treatment. Nearly one in five touch football injuries were ACL ruptures. In Australian rules, ACL tears were the most common injury leading to a hospital spell for people aged between 20 and 29. Meanwhile, surgery rates on busted ACLs are higher in Australia than in any other OECD nation.

But what exactly is an ACL injury? How do they happen? And how are they repaired?

What’s an ACL?

Your knee joint is made up of three bones: the tibia (shinbone), the femur (thighbone) and the patella (kneecap). The ACL, or anterior cruciate ligament, is one of two cruciate ligaments that enable the knee to move forward and backwards: it and the posterior cruciate ligament form an X (cruciate means “cross-shaped”) with the anterior (meaning “in front of”) crossing in front of the posterior (posterior meaning “at the back of”).

Running diagonally in the middle of the knee, the ACL stops the tibia (shinbone) from sliding in front of the femur (above the knee) and gives rotational stability to the knee as a whole. The menisci (plural of meniscus) are two rubbery cushions of cartilage between the bones.

Two collateral ligaments, not shown in the image above, run vertically down the sides of the knee: the medial collateral ligament (or MCL) is on the inside and the lateral collateral ligament (or LCL) is on the outside. Together, they enable the safe sideways mobility of the knee.

Around half of the injuries to the ACL are incurred when structural knee damage already exists.

How do ACLs get injured?

The “danger zone” for ACL ruptures tends to be within the first three or four games of a season, when players are not yet match fit, says Dr Nathan Gibbs, a former captain of the South Sydney Rabbitohs and a sport physician across codes for more than 37 years. Gibbs is well acquainted with ACL injuries, having been head doctor for the Swans in the AFL, the NSW rugby league State of Origin side and now the Australian Wallabies.

Usually, “co-contraction” of the hamstrings and quadriceps stabilises the knee when players plant their feet or sidestep and change direction. This protects the tibia and femur bones from coming apart and slipping on one another. But when a player is unused to the stress of a game, this co-contraction is more likely to fail.

“Any sport that has twisting and turning will see ACL ruptures.”

“Any sport that has twisting and turning will see ACL ruptures,” says Gibbs. “If you look at the average football chain of 35 to 40 players, only one or two of those may sustain an ACL injury in any given year, which is a pretty good injury rate, but it’s catastrophic for the individual because it guarantees they’ll be out for that entire season, and they may do it again,” he says.

According to Gibbs, there are two main reasons that athletes incur ACL injuries: they have either a genetic predisposition due to weakened collagen, or their co-contracting muscles have failed to protect the knee when sidestepping or changing direction while playing. Those who have “inherent weakness” in the collagen that acts as the connective tissue of the ACL are at “high risk” of doing both knees, but it’s difficult to tell whether this is the case until the first injury is incurred.

Sometimes the reason for an ACL injury can be hard to pinpoint. Rugby league great Andrew Johns ruptured his ACL when he was 29, just three years before he retired from the sport in 2007. The injury still leaves Gibbs perplexed. “He would have done 5 trillion sidesteps in the lead-up to that moment and why, on that particular day, did he not do a co-contraction properly and tear his ACL?”

It’s little wonder, then, that non-professionals are also at risk of tearing an ACL in the heat of contest. In under-25s, the rate of ACL injuries overall rose by 75 per cent between 2000 and 2015, according to research in the Medical Journal of Australia. In males, ACL injuries peaked among 20- to 24-year-olds; in females, among 15- to 19-year-olds. Researchers attributed the rise to earlier specialisation in sports, longer seasons, more intense training and increasingly high levels of competition.

ACL ruptures are the sporting injury most likely to land you in hospital if you are aged between 20 and 29.

Statistics from the Australian Institute of Health and Welfare on both rugby codes, Australian rules, soccer and touch football tell a similar story. Soccer had the highest rate of ACL injuries requiring hospital, from 435 cases in 2011-12 (11.5 per cent of all soccer injury hospital stays in that year) to more than 800 in 2018-19 (16.3 per cent). Concussion, by comparison, led to a trip to the emergency department (and admission to hospital) for about 200 soccer players in 2018-19.

The AFL brought in a “no contact below knees” rule to protect players who were standing over the ball from being injured by other players diving or sliding in. But among recreational Australian rules players, ACL ruptures are far from uncommon, too. They are the sporting injury most likely to land you in hospital if you are aged between 20 and 29, and one of the most common among the 15- to 19-year-old age bracket, after concussion, followed by thumb and finger fractures. In touch football, ACLs account for 18.4 per cent of all injuries needing a hospital stay.

Where a sport involves changing direction rapidly, landing poorly after a jump, colliding with another person or suddenly stopping because of a fall or other high-stress activity, the potential to do an ACL is there. The most common group of patients that orthopedic surgeon and knee specialist Dr David Cossetto sees are younger than 25 and have ruptured their ACL while playing contact sports such as soccer or rugby. First-time skiers are prone to ACL trouble too.

“If you were to look at the number of ACL ruptures per head of population it’s higher in females than males,” said Cossetto, who had just finished operating on a skier in her early 30s. While the gender imbalance is influenced by a difference in hormones, it’s not yet known whether it’s also caused by the difference in strength in men and women or the physical differences in their lower extremity alignment. AFL Women’s players are nine times more likely to rupture an ACL than their male counterparts, says the director of the La Trobe sport and exercise medicine research centre, Professor Kay Crossley. The centre estimates 5 per cent of the 500,000 female Aussie rules players are likely to sustain the injury each year. Netball Australia estimates ACL ruptures make up a quarter of serious injuries in its sport.

While knee or hip replacements are not so uncommon among older people, ACL ruptures are rare in that age group. It’s not a wear-and-tear injury, Cossetto points out. Older people who rupture their ACLs generally do so because of a fall.

How bad is tearing an ACL?

Gibbs says tearing the ACL can take an average of nine to 12 months to recover from, which can mean it “ruins careers” for athletes.

“If you’re a football player on the way up and you have to take 12 months off because of an ACL injury, you may miss your window to play professionally. If you’re lucky enough to return to play, you may never return with the same confidence you had before the injury,” he says.

In team sports, ACL ruptures are worse than shoulder reconstructions or some other serious injuries because they prevent athletes from training entirely and cannot be put off until the end of the season. “Worst-case scenario for shoulders is you’re out for four to five months. ACL injuries rule you out for a year,” says Gibbs.

Those who are genetically predisposed often do their other knee after they’ve recovered from their first injury.

Then there’s re-rupturing your ACL. Those who are genetically predisposed often do their other knee after they’ve recovered from their first injury. Those who aren’t genetically predisposed still risk re-rupture if they continue to engage in the activity that caused the injury.

For regular people, though, the impact of the injury is slightly different, says Cossetto. An operation such as a shoulder reconstruction can be worse. “You can’t dress yourself. You can’t drive. You become reliant on other people, and it’s often more painful at night,” he says.

How do you fix a ruptured ACL?

Netballer Liz Ellis fell to the ground after an ACL rupture caused “incredible pain” in her right knee in a match against New Zealand in 2005. At first, she was in total denial, she later recounted to Netball Scoop, but a scan the following day made the problem clear. “[I] said, ‘I guess I’ve done my knee then.’ It was very emotional ... I went to the airport, headed home and started my rehab.” Ellis defied the naysayers by recovering after surgery and returning to the court just five months after her ACL injury.

ACL surgery is the surgical reconstruction or replacement of the ligament only, although according to the Clinical Journal of Sports Medicine those who have an ACL reconstruction are five times more likely to later require a total knee replacement than the general population.

There are three types of surgical options available after rupturing an ACL, only two of which are endorsed in Australia.

• An autograft is where a doctor uses a tendon from somewhere else in your body, such as your other knee, hamstring or thigh. The patellar tendon graft, a kind of autograft, was historically seen as the “gold standard” for ACL reconstruction, but surgeons are now likely to recommend a hamstring graft from the same injured leg for young patients or professional athletes, to lessen recovery time and pain levels.
• Older patients or athletes may be advised to have an allograft. This is also known as a cadaver graft and is when a surgeon takes tissue from a deceased donor to repair the torn ACL. Patients often recover more quickly after cadaver grafts as their body is not impeded beyond the initial ACL tear. For this reason, it’s often used by athletes who need to recover for a specific event, such as the Olympics. In 2003, world champion skier Alisa Camplin snapped her ACL in a training accident and was out of action for 10 months after an autograft. Once recovered, she tore the same ligament just months out from the Torino 2006 Winter Olympics. She decided to have allograft surgery to repair the knee, was skiing six weeks out from the opening ceremony, and took home a bronze medal for Australia.
• The third type of ACL surgery is a synthetic graft, where artificial materials replace the affected tendon. It’s not usually recommended by orthopedic surgeons in Australia due to “the risk of debris” and often short-lived repair, says Cossetto.

“Australia is the mecca for ACL reconstructions.”

Non-athletes may decide not to have an operation if they’re relatively sedentary and happy to discard rotation-based activities. Cossetto says: “If the patient is happy to get rid of high-risk activities such as contact sport, rehabilitation may be enough to get the strength back into the knee without reconstruction.” But there are still plenty of takers. “Australia is the mecca for ACL reconstructions,” he says.

For every 100,000 people in Australia, 77.4 will have ACL surgery at some time in their life, according to research in The Medical Journal of Australia, compared with 52 in the United States and 37 in New Zealand. Cossetto puts this down to the way Australian doctors are trained. He also suspects that the average Australian plays more contact sport than many of their OECD peers. The same research shows that 72 per cent of ACL reconstructions in Australia are sport-related.

Are there ways to avoid an ACL tear?

Professional sport codes use agility and mobility exercises to strengthen athletes’ quadriceps and hamstrings as well as co-ordination drills for hopping, jumping, landing and twisting and rotating to improve the biomechanics of players who are regularly required to jump, land and sidestep.

“Prehabilitation and implementing co-ordination drills as part of a team’s normal strength training regime has been shown across clubs to lower the rate of ACL ruptures,” says Gibbs. “It may not help those with the predisposed tissue type, but it’ll lower the overall rate in a team.”

For athletes and non-athletes alike, stretching and agility exercises help avoid ACL tears, says Cossetto.

“If you go skiing once or twice a year, it’s important you do a bit of strength work ahead of your trip. It’s the same with getting into social sport; it’s important to include [agility training] if you want to avoid injury.”

With Damien Ractliffe, Craig Butt