Risk
Second ACL injury risk, in numbers
The rates by age, by sport and by knee, what raises them, what lowers them, and why the other knee is at least as much of a problem as the graft.
This is the page the rest of the site exists to serve. Every threshold, every criterion and every argument about month nine is downstream of one fact: a young athlete who returns to pivoting sport after an ACL reconstruction has a risk of tearing an ACL again that has no equivalent anywhere else in orthopedics.
The headline rates
The 23% figure is roughly 30 to 40 times the risk of an ACL injury in an uninjured adolescent. In the cohort that reported 29.5%, 20.5% injured the other knee and 9.0% retore the graft, and the contralateral knee of female athletes was at greatest risk.
The other knee is not a footnote
In the meta-analysis, contralateral injuries (8%) slightly outnumbered ipsilateral ones (7%). In a 316-patient series, graft ruptures affected 18% of patients and contralateral ACL injuries 17.7%, with 35% of patients sustaining at least one further ACL injury after the primary surgery. Rehabilitation that treats the healthy leg as a reference standard rather than as a knee at risk is missing half the problem, and it also quietly corrupts every limb symmetry measurement you take.
When it happens
| Finding | Figure | Source cohort |
|---|---|---|
| Average time from surgery to graft rupture | 1.8 years | 316 patients, 89% follow-up |
| Graft ruptures in the first postoperative year | 47% | same series |
| Graft ruptures within two years | 74% | same series |
| Average time from return to sport to graft rupture | 105 days | 158 male professional athletes |
| Second injuries within 24 months of return | 29.5% | American cohort with controls |
The first season back is the dangerous one
Not month nine specifically. The concentration of ruptures in the first year after surgery and within about three months of returning to sport is the reason a graded return matters as much as the clearance decision itself.
What raises the risk
- Youth. Odds of graft rupture rose 2.3 times per 10-year decrease in age in the MOON cohort. The highest rate in one series, 28.3%, was in males under 18.
- Returning to level I sport. A 4.32 times higher reinjury rate in the Delaware-Oslo cohort.
- Returning before 9 months. Hazard ratio 6.7 in a Swedish registry cohort of 159 athletes aged 15 to 30.
- Returning without meeting discharge criteria. Hazard ratio 4.1 in 158 professional athletes.
- Allograft in a young athlete. Four times the rupture odds of an autograft.
- A high preinjury activity level. Hazard ratio 2.1 for a higher Tegner score.
- Low hamstring to quadriceps ratio. Hazard ratio 10.6 per 10% difference at 60 degrees per second.
- Low psychological readiness in patients 20 or younger. An ACL-RSI under 76.7 at 12 months identified 90% of those who went on to a second injury in that age group.
Sport matters, and the numbers are uncomfortable
Across 350,416 athlete-exposures in NCAA sports, with 1,105 ACL ruptures of which 126 were recurrent, the highest recurrent rupture rates per 10,000 athlete-exposures were in men's football (15), women's gymnastics (8.2) and women's soccer (5.2). Women's soccer carried 3.8 times the recurrent rupture rate of men's soccer.
What lowers it, honestly
Time is the most consistent finding: reinjury fell 51% for each month return was delayed up to nine months. Quadriceps symmetry before return significantly reduced reinjury in the same cohort. Passing a criteria battery was associated with 5.6% versus 38.2% reinjury there.
And the counter-evidence, because it belongs on this page: a meta-analysis of four studies found the pooled risk difference for passing criteria was -3% and not statistically significant, with 14.4% of those who passed still sustaining a second injury, at very low quality of evidence. A registry cohort found symmetrical strength at return was not associated with second injury. And in a cohort of high-level female athletes, the nine who got hurt again had higher ACL-RSI scores and met criteria sooner. Testing is the best instrument available. It is not a shield.
What to do with all of this
Take the time
It is the finding that survives across cohorts, and it is the one entirely under your control.
Measure, then fix what is measured
A quadriceps deficit is worth closing whatever the pooled hazard ratio says, because it persists for years and rewrites how you land.
Train the other knee like it is at risk
Because it is, and because it is your measurement baseline.
Keep training after you return
The first year back is where the ruptures concentrate, and neuromuscular work is the part everybody drops the week they get cleared.