Can rehab alone work after a torn ACL?

Short answer

For about half, yes. Starting with rehab gave knee scores at 2 and 5 years that were no worse than early surgery, and half never had surgery. But when the knee kept giving way, surgery gave better results.

Can rehab alone work after a torn ACL?
'Torn means surgery, no exceptions' — does this formula hold?
The ACL is the ligament that keeps your knee from sliding forward or twisting. When it tears, surgery to put in a new ligament is often seen as a given. We checked trials that compared this with people who started with rehab instead.
Starting with rehab did not leave knee scores behind
No difference at 2 years or at 5 years
121 people were split into surgery plus rehab from the start, or rehab first (with surgery later if needed). On a 100-point knee score, the gap in improvement was 0.2 points at 2 years and 2.0 points at 5 years, which was no difference. Arthritis and meniscus surgery at 5 years did not differ either.
Half of the rehab group never had surgery
Their results were no different from operated knees
Of the 59 people in the rehab-first group, 30 (51%) had surgery within 5 years. The other 29 got by with rehab alone. At 5 years, knees operated on early, operated on late, and treated with rehab alone did not differ.
A trial in another country was similar
A 5.3-point gap at 2 years, short of a meaningful level
In a Dutch trial of 167 people, knee scores at 2 years were 84.7 for early surgery and 79.4 for rehab first. The researchers judged the 5.3-point gap too small to be felt in daily life. 50% of the 82 people in the rehab group never had surgery.
But when the knee kept giving way, it was different
In 316 people with lasting instability, surgery scored 7.9 points higher
In a UK trial of 316 people whose knee still kept giving way some time after the injury, the surgery group's knee score was 7.9 points higher at 18 months. 41% of the 160 people in the rehab group ended up having surgery.
A torn ligament sometimes reconnected
16 of the 30 (53%) who had rehab alone
In the 121-person trial, 16 of the 30 (53%) who had rehab alone showed a reconnected ligament on MRI at 2 years. They scored 25.1 points higher on sports function than the 14 whose ligament did not reconnect. Among 80 people whose knee was braced in a bent position within 4 weeks of injury, 90% had reconnected at 3 months.
Bottom line: starting with rehab did not leave results behind
The formula breaks at 'no exceptions'
Rehab first, with surgery only when the knee could not hold up, gave knee scores no worse than early surgery, and half never had surgery. But knees with lasting instability scored higher with surgery. Whether surgery was needed came down to whether the knee held up after rehab.