Second ACL injury: how common and what lowers the risk

The short answer: in a pooled analysis of 19 studies, 15 per cent of people tore an ACL again after reconstruction, 7 per cent in the operated knee and 8 per cent in the other one. The risk is not spread evenly. It is highest in people under 25 who go back to sport, where it was 23 per cent, and it falls with age, with a lower activity level, and with a stronger quadriceps and more time before return, though in the youngest patients time alone did not help. Whether your own knee is ready for sport is for your surgeon and physiotherapist to decide; this article describes what the studies found, not what will happen to you.

How common is a second ACL injury?

“Second ACL injury” covers two different events: a graft rupture (the reconstructed ligament tears) and a contralateral injury (the ACL of the other knee tears). Studies often count them together, which is why the figures differ.

Wiggins and colleagues pooled 19 studies in 2016. The overall rate was 15 per cent, split 7 per cent graft and 8 per cent other knee. For patients younger than 25 it was 21 per cent, for athletes who returned to sport 20 per cent, and for athletes younger than 25 who returned to sport 23 per cent, nearly one in four. The authors note that these injuries tend to happen early in the return to play.

The MOON cohort, a large American study that Kaeding and colleagues analysed, shows what a more mixed group looks like. Among more than 2,600 patients with an average age of 27, 4.4 per cent tore the graft and 3.5 per cent tore the other ACL. The pooled figure and this one are both right; they describe different people.

Who is most at risk?

Age and activity are the factors that keep appearing. In the MOON cohort the odds of both a graft tear and an other-knee tear went down with every extra year of age and went up with every point on the Marx activity score, a questionnaire about how often someone runs, cuts, decelerates and pivots. Neither sex, smoking, nor the sport played changed the odds significantly. Donor tissue from another person (an allograft) carried 5.2 times the odds of a graft tear compared with a patellar tendon graft from the patient’s own knee, while the difference between patellar tendon and hamstring grafts was not significant. Graft choice is a surgeon’s decision, and the comparison of hamstring and patellar tendon recovery covers what it changes.

Paterno and colleagues followed 78 young athletes, average age 17, who were cleared for cutting and pivoting sports, and 47 teammates with no ACL history. Over 24 months 29.5 per cent of the reconstructed group tore an ACL again, 20.5 per cent in the other knee and 9.0 per cent in the graft, against four injuries in the comparison group. Counted per game or practice played, the reconstructed athletes’ injury rate was nearly six times higher. Among the young women, the other knee was hurt about twice as often as the graft, though in a group this small that was a trend, not a firm result.

Does waiting longer help?

The strongest evidence that timing matters comes from Grindem and colleagues’ Delaware-Oslo cohort of 106 people in pivoting sports, followed for two years. People who returned to the highest-level sports had a reinjury rate 4.3 times higher than those who did not. Each month that return was delayed, up to nine months after surgery, was associated with a 51 per cent lower reinjury rate, and there was no further drop after nine months. This is where the nine months in many guidelines comes from, and it is a calendar standing in for criteria, not a finish line; how long recovery takes puts it in context.

The finding has limits. Webster and colleagues advised 142 patients under 20 to wait 12 months and compared them with 299 who were not told to. Subsequent ACL injury was just as frequent in the delayed group, 33 per cent, as in the other, 31 per cent, and similar again between those who returned before and after 12 months. In the very young, time alone did not protect them. What was happening in those months seems to matter more than the number of them.

Strength and test results before return

Grindem’s group also looked at what the knee could do. Those who failed the return-to-sport criteria, which were strength, hop symmetry and questionnaire scores above 90 per cent, had a reinjury rate of 38.2 per cent against 5.6 per cent for those who passed; the difference did not reach statistical significance in a cohort this size, but more symmetrical quadriceps strength before return did significantly reduce the reinjury rate.

Kyritsis and colleagues tested 158 male professional athletes before they returned to team training, with isokinetic strength, a running test and three hop tests. Twenty-six, 16.5 per cent, tore the graft, on average 105 days after their return. Those who had not met all six discharge criteria had about four times the risk of graft rupture, and a weak hamstring-to-quadriceps ratio in the operated leg added to it. How the symmetry number is worked out and what the hop tests measure are covered separately. How clearance and a staged return usually work is described in return to sport after ACL reconstruction.

What these studies cannot tell you

All of this is about groups, and mostly about people who play pivoting sports. A rate of 23 per cent does not mean any one person has a 23 per cent chance; it means about one in four of that kind of athlete, followed over a few years. These are observational studies: people who passed the tests may also have been the more careful, better supervised or less exposed. Most of the data come from young athletes, so they say much less about someone in their forties who walks, cycles and hikes. For Noyes’ Knee Disorders, the established structure is progressive phases that finish with running, jumping and cutting before sport, and that is the shape the studies above support, not a list of promises.

Where the app fits

Second-injury risk belongs to the end of the programme, Levels 8 to 10 in ACL Recovery Tracker, our free ACL rehab app for iOS and Android, where running, landing and cutting are built up. Level 10’s checklist lists hop tests at 90 per cent of the other leg, a minimum of nine months since surgery and clearance from your clinician, and you tick it yourself; the app compares nothing against your logs and does not tell you that you are ready. That verdict, and the testing behind it, come from your physiotherapist. You can use it for what the studies keep returning to: the record of sessions completed month by month, the healthy-leg baseline for comparing the two knees, and the pain diary that shows whether the knee objected after the harder sessions.

The figures are sobering, but they come from groups that mostly returned early and to the hardest sports. The same studies point to what is within reach: more months, a stronger quadriceps and test results that are looked at before the first match.