ACL injury: rehab, surgery or not, and how to prevent it

How it happens and how it's recognisedStrong evidence

Most ACL tears happen without contact with another player: changing direction fast, stopping suddenly or landing from a jump, often with the knee nearly straight and falling inwards. About 70% are non-contact. A direct blow to the outside of the knee can also tear it, sometimes with other parts of the knee.

Typical signs are a pop felt or heard, swelling within about 12 hours (bleeding into the joint), stopping play, and later a knee that gives way when you twist or turn. Female athletes are 3 to 8 times more likely to injure an ACL than male athletes, depending on the sport, and a previous ACL injury is the most consistent risk factor for another.

Sources 1, 2, 3, 4, 5

Diagnosis and scans

Examining a freshly injured knee is hard while it is painful and swollen, and is often easier a few weeks later. In skilled hands the Lachman test, which checks how far the shin slides forward, is accurate: it picked up 85% of tears and was negative in 94% of knees without one.

An MRI scan isn't needed to diagnose a torn ACL, but it shows damage to the meniscus, cartilage and other ligaments, which often affects the decision about surgery. An X-ray can show a small piece of bone pulled off with the ligament.

Sources 4, 1, 2

What happens over timeEmerging evidence

Not everyone with a torn ACL ends up with a problem: one NHS trust says the older and less sporty you are, the less likely it is to cause trouble, and for some people the looseness can be overcome with physiotherapy. In a Swedish trial (KANON), about half the people offered rehab first had a reconstruction within 11 years, and half never did.

Osteoarthritis is common later: 44% of the people in that trial had X-ray signs of it in the injured knee at 11 years, and a review of 41 studies found removal of a meniscus was the only consistent risk factor. After a reconstruction, about 15% have a second ACL injury, in the same or the other knee, rising to nearly 1 in 4 in athletes under 25 who return to sport.

Sources 4, 6, 7, 8

Can a torn ACL heal?Mixed evidence

Sometimes it looks healed on a scan. In the KANON trial, 30% of the people offered rehab first, and 53% of those who never had surgery, showed a continuous ligament on MRI 2 years after the injury. With a bracing protocol that holds the knee bent at 90° for 4 weeks and then frees it gradually over 12, 90% of 80 people showed signs of healing on a 3-month scan, and 14% injured the ACL again.

Whether that means a better knee is unclear. At 11 years, people in KANON whose ACL looked continuous at 5 years reported worse knee scores than those who had a reconstruction, and a 2-year study of 80 people in pivoting sports found the knee gave way again in 70% after the bracing protocol, against 2.5% after surgery. These are small, non-randomised comparisons and the bracing approach is contested, so discuss it with a specialist.

Sources 9, 10, 11, 12

Children and young teenagers

ACL injuries in children and young teenagers have their own International Olympic Committee consensus statement and are managed by specialist teams. This guide is written for adults and older teenagers.

Source 13

Rehab first is a reasonable choice for many peopleStrong evidence

In the KANON trial, 121 young active adults with a fresh ACL tear all had the same structured rehab; half were assigned early reconstruction and half rehab with the option of surgery later. Knee scores were the same at 2, 5 and 11 years, and about half of the rehab-first group never had an operation.

In a Dutch trial (COMPARE, 167 people), early surgery gave a small edge at 2 years that the authors judged not clinically relevant: the rehab group did better at 3 months, the surgery group was ahead from 9 months, and half the rehab group had a reconstruction. A Cochrane review rated the evidence low quality, because at the time it rested on a single trial.

Sources 14, 15, 6, 16, 17

When surgery is usually advisedEmerging evidence

The NHS says surgery is usually recommended if physiotherapy and lifestyle changes haven't helped, you plan to keep doing lots of sport or intense exercise, you have a very physical job, the knee gives way a lot, or other parts of the knee are damaged. One NHS trust adds your age, how you respond to physiotherapy and your sporting aims.

For people still unstable months after the injury, reconstruction beat further rehab at 18 months in a UK trial (ACL SNNAP, 316 people), and 41% of the rehab group went on to have surgery anyway. A 2025 meta-analysis found no clear difference in return-to-sport rates between reconstruction and rehab alone, at low certainty. Rehab is needed either way.

Sources 18, 4, 19, 20

Rehab is the core of every optionStrong evidence

The US physical therapy guideline for knee ligament injuries gives its top grade to strength exercises, both standing and seated or lying, with both lifting and lowering, started within 4 to 6 weeks of a reconstruction and done 2 to 3 times a week for 6 to 10 months, combined with balance and movement-control training. Its next grade covers moving the knee within the first week, ice for early pain, and supervised rehab with a home programme.

The Aspetar guideline, from a sports medicine hospital in Qatar, adds that quad sets and straight-leg raises are safe in the first 2 weeks, the leg press can start at about 3 weeks after a hamstring graft, and seated knee extensions from week 4 in a limited range. A Dutch guideline moves people through phases by criteria rather than dates, with rehab lasting 9 to 12 months. Much of this rests on low-certainty evidence and expert agreement.

Sources 1, 21, 22

Going back to pivoting sport: time and testsEmerging evidence

Waiting at least 9 months after a reconstruction before going back to pivoting sport went with fewer reinjuries in two observational studies. In 106 athletes followed for 2 years, 39.5% of those who went back before 9 months injured the knee again, against 19.4% of those who waited, and the rate fell by about half for each month of delay up to 9 months. In 159 young athletes, going back before 9 months went with about 7 times the rate of a second ACL injury.

These studies can't separate time from readiness: people who go back early may also be less ready. Passing return-to-sport tests matters too: professional athletes who hadn't met all 6 discharge criteria had 4 times the rate of graft rupture. But no test battery has been shown to predict who is ready, and reviews disagree on whether passing one lowers the risk of a second injury.

Sources 23, 24, 25, 21, 1

How many get back to sportEmerging evidence

After a reconstruction, 81% returned to some sport, 65% to their pre-injury level and 55% to competitive sport (69 studies, 7,556 people). About 15% had a second ACL injury, and 23% of athletes under 25 who returned to sport. The NHS says it can take up to a year to play sport again.

Sources 26, 8, 27

ACL injuries can be preventedStrong evidence

Warm-up programmes of strength, jumping, landing and balance exercises cut ACL injuries by about half across 8 meta-analyses, and non-contact ACL injuries in female athletes by about two-thirds. In 18 studies of 27,231 female athletes, this kind of training cut the risk from 1 in 54 to 1 in 111, with bigger effects in school-age players.

The US prevention guideline (top grade) recommends programmes with several components, done several times a week in sessions of over 20 minutes, from pre-season through the season, for all young athletes in high-risk sports. An Australian position statement from 2026 adds that the evidence in male, elite and many other athletes is more limited, and that no single test can yet pick out who is at high risk.

Sources 28, 29, 30, 31, 3

The FIFA 11+ warm-upStrong evidence

The 11+ is a 20-minute warm-up of running drills and strength, balance and jumping exercises. Across 6 cluster trials it cut football injuries by 39%. In one season of US college men's football (61 teams), there were 3 ACL injuries in teams doing the programme against 16 in teams that weren't. In its first trial, in 1,892 Norwegian girls aged 13 to 17, it reduced injuries overall and severe injuries, though the fall in leg injuries could have been chance.

Sources 32, 33, 34

Braces: optional, and only if your team gives you oneMixed evidence

After a reconstruction, the evidence on braces goes both ways, so the US guideline leaves it to the patient's preference; in a review of 11 studies, a brace made no clinically important difference to looseness, strength, movement, pain or function. The NHS says wear one only if your surgeon or physiotherapist gives you one. Without surgery, an NHS trust says a brace can help in sports where the knee tends to give way, such as tennis, squash and skiing, though it is too cumbersome to wear every day.

Sources 1, 27, 4

After surgery: protect the knee early

The NHS advises against resting with a pillow under the knee and against keeping the leg straight for long when you sit, and says to use crutches until your physiotherapist says you can stop. It lists the signs of an infection or a clot after ACL surgery, which are in the warning box at the top of this page. After a hamstring graft, resisted hamstring exercise usually waits about 12 weeks, and seated knee extensions start from about week 4 in a limited range, as your team allows.

Don't drive until you can brake safely in an emergency. That usually returns about 4 to 6 weeks after a right-knee reconstruction and 2 to 3 weeks after a left one. Wait until your medical team says you're fit to drive, and tell your insurer.

Sources 27, 36, 21, 35

Prevention: the 11+ warm-up, part by part

In its first trial, the 11+ replaced the usual warm-up at every training session, its running part was used before matches, and it took about 20 minutes. The US prevention guideline asks for several sessions a week, each over 20 minutes. The knee control warm-up on this site trains the same skills in a shorter session. The parts, with the trial's doses:

• Running drills, 8 minutes: straight ahead, hip out, hip in, circling, running and jumping, and quick runs, 2 runs of each. To finish, 2 minutes of running across the pitch, bounding and running with sharp cuts.
• Plank: 3 holds of 20 to 30 s, then with alternate leg lifts, then holding one leg up.
• Side plank: 3 holds of 20 to 30 s each side, static, then dynamic, then with a leg lift.
• Nordic hamstring lowers: 3 to 5, then 7 to 10, then 12 to 15.
• Single-leg balance: 2 holds of 30 s each leg, holding a ball, then throwing it with a partner, then with a partner nudging you.
• Squats: 2 sets of 30 s with the heels raised, then walking lunges, 2 sets of 30 s, then one-leg squats, 2 sets of 10 each leg.
• Jumps: 2 sets of 30 s, vertical, then lateral, then box jumps.

Sources 33, 31