After ACL surgery, when is it safe to return to sport?

Short answer

Passing the tests matters more than the date. Up to 9 months, each month of delay cut reinjury by 51%, but among people who passed strength and hop tests, returning before or after 9 months made no difference.

After ACL surgery, when is it safe to return to sport?
'9 months have passed → you are safe' — does this formula hold?
After surgery on the knee's anterior cruciate ligament (ACL), '9 months' is often treated as the return date. We checked whether the date alone is enough, and what separated people who got hurt again from those who did not.
Reinjury is more common than you think
Over 10 years: 11% in the operated knee, 12% in the other
An analysis that pooled studies following 5,735 people for about 10 years found that 11% re-tore the repaired ligament and 12% injured the ACL in the other knee. Another analysis found that 23% of people who returned to sport at a young age got hurt again.
Up to 9 months, waiting lowered the risk
Each month of delay cut reinjury by 51%
Researchers followed 106 people from cutting-and-pivoting sports like soccer and basketball for 2 years. Those who went back to such sports had 4.32 times the risk of reinjuring the knee. Each month of delay up to 9 months cut reinjury by 51%, with no further drop after 9 months.
Passing the tests mattered more than the date
Failing any of 6 tests meant 4.1 times the re-tear risk
158 people took 6 tests before returning, including strength, running and single-leg hops. Those who returned without passing all of them had 4.1 times the risk of re-tearing the graft. In the 106-person study, 38.2% of those who failed the tests got hurt again, versus 5.6% of those who passed.
Once the tests were passed, 9 months made no difference
530 people, no difference in new injuries
Researchers followed 530 people for 2 years after surgery. Among those who finished rehab and passed the test criteria, returning before or after 9 months made no difference to the risk of a new knee or ACL injury. Passing the criteria also made a return to sport 5.71 times more likely.
But the tests protect only the operated knee
Re-tear risk about halved, the other knee unchanged
An analysis that pooled 9 studies with 1,410 people found that passing return-to-sport tests cut the risk of re-tearing the repaired ligament to about half (0.49 times). Injury to the other knee's ACL and knee injuries overall were no different whether people passed or not.
Bottom line: 9 months is just a date, the real bar is passing the tests
The formula breaks at the 'date → safety' link
Waiting up to 9 months lowered reinjury, but among people who passed the tests, returning before or after 9 months made no difference. The bar used in the studies was leg strength and single-leg hop scores above 90% of the other leg. The tests reduced re-tears in the operated knee, while the other knee kept a 12% risk over 10 years.