REF-1665 · Grindem et al., 2016 · British Journal of Sports Medicine, · Paper · PubMed

In 106 pivoting-sport athletes followed for 2 years after ACL reconstruction, returning to level I sports went with a 4.32 times higher knee reinjury rate; each month return was delayed cut the reinjury rate by 51% up to 9 months after surgery, with no further fall after that, and more symmetrical quadriceps strength before return lowered it; passing all return criteria (38.2% reinjured among those who failed against 5.6% among those who passed, HR 0.16) fell short of significance (p = 0.075)

Main caveat: An observational cohort of 106; the number of reinjuries, adjustment for other factors and confidence intervals are not in the abstract.
limited evidenceMeasured on a mixed groupRead from the abstract only

Grindem and colleagues asked how knee reinjury after ACL reconstruction relates to three things: returning to level I sports, the timing of the return, and knee function before it. They followed 106 patients who took part in pivoting sports for 2 years, recording sports participation and reinjuries each month.

Knee function was tested with the Knee Outcome Survey-Activities of Daily Living Scale, a global rating of function, and symmetry of quadriceps strength and hop tests; passing the return-to-sport criteria meant a score above 90 on every test, and failing any one test was a fail.

Patients who returned to level I sports had a reinjury rate 4.32 times that of those who did not (p = 0.048). The reinjury rate fell significantly, by 51% for each month the return was delayed, up to 9 months after surgery; beyond that no further reduction was seen. Of those who failed the criteria, 38.2% were reinjured, against 5.6% among those who passed (HR 0.16, p = 0.075).

Quadriceps strength that was more even between legs before return went with a significantly lower reinjury rate. The authors conclude that a return to level I sports raises reinjury more than fourfold over 2 years, and that returning at 9 months or later, with more symmetrical quadriceps strength, substantially lowers it.

Participants

n = 106 patients after ACL reconstruction who took part in pivoting sports. Both sexes and ages from adolescence to adulthood by the PubMed indexing (the abstract states neither). The sports, competition level, graft type and number of reinjuries are not in the abstract.

Protocol

Prospective cohort with 2-year follow-up; sports participation and knee reinjury recorded monthly.
Knee function before return: the Knee Outcome Survey-Activities of Daily Living Scale; a global rating of function; symmetry of quadriceps strength; symmetry of hop tests.
Pass: scores above 90 on all tests; fail: failing any test.
Exposures: return to level I sports (not defined in the abstract), timing of return after surgery, knee function before return.
Not in the abstract: the rehabilitation programme, which hop tests were used, how symmetry was expressed, adjustment for other factors, and the number of reinjuries.

Key Results

FactorResultp
Return to level I sports, against no return to level IReinjury rate 4.32 times higher0.048
Each month return was delayed, up to 9 months after surgeryReinjury rate 51% lower per monthsignificant (value not in the abstract)
Delay beyond 9 monthsNo further reduction—
Failed against passed the return criteria38.2% against 5.6% reinjured; HR 0.160.075 (not significant)
More symmetrical quadriceps strength before returnLower reinjury ratesignificant (value not in the abstract)
The 84% in the paper's title matches the HR of 0.16 for passing the criteria (1 − 0.16), the result with p = 0.075.

Practical Application

After ACL reconstruction, a later return and more even quadriceps strength went with fewer reinjuries: in this cohort each month of waiting, up to 9 months after surgery, cut the reinjury rate by about half (51%). The surgeon and physiotherapist set the return date and run the strength and hop tests; the assistant plans around their decision and does not bring it forward.
Quadriceps symmetry is the measurable target this cohort backs. More symmetrical quadriceps strength before return significantly lowered reinjury; the abstract gives no threshold for it, and the pass mark of above 90 on every test belongs to the combined criteria, whose result was not significant (p = 0.075).
The "84%" headline is not an established effect. Passing every criterion went with 5.6% against 38.2% reinjured, but the hazard ratio of 0.16 was not significant (p = 0.075); the timing and strength findings carry more weight than the pass/fail rule.
Inference, not tested here: running and cycling are not pivoting sports, so the 9-month figure transfers loosely to an endurance athlete; return to running follows the physiotherapist's criteria (REF-1663, REF-1664), while an athlete who also plays a pivoting sport carries the risk measured here.
Not settled by this design / not from this paper: whether waiting itself lowers the risk (in an observational cohort, those who return later may differ in other ways), the risk for endurance-only athletes, and differences by graft or sex.

Limitations

An observational cohort of 106; the number of reinjuries, adjustment for other factors and confidence intervals are not in the abstract.
The pass/fail result (HR 0.16) is not significant (p = 0.075), despite the title.
Pivoting-sport athletes; level I is not defined in the abstract; endurance sports were not studied.
Follow-up of 2 years only.
Written from the abstract and the PubMed record; full text not read.
How this entry was checked and changed
Why this level

a cohort of 106, below the rubric's 200 for B+ (small cohort); observational, and the pass/fail criteria result is not significant

Content correction 2026-11-07
2026-11-07: added from the abstract and the PubMed record; level B (prospective cohort, n = 106).