Do knee braces prevent injuries?

Short answer

What braces prevented was pain at the front of the knee: in 6 weeks of training it fell from 37% to 18.5%. Ligament injuries in contact sports dropped only among defensive players in a trial of 1,396 people, and gym knee sleeves added 5 kg to the squat but showed no drop in injuries.

Do knee braces prevent injuries?
'Brace → knee held in place → fewer injuries' — does this formula hold?
Squat sleeves and hinged sports braces are both sold as knee protection. We checked whether people wearing them actually got hurt less.
In contact sports?
Fewer injuries in a trial of 1,396 people, but only on defense
1,396 cadets at the US Military Academy played two seasons of tackle football with or without braces. The braced group had fewer knee injuries, especially to the inner knee ligament. But the drop was only among defensive players; offensive players showed no difference. Injuries were not less severe either.
Other studies found more injuries
In 580 people, more knee and ankle injuries
Following 580 high school football players for two seasons, those in single-hinged braces had more knee injuries than those without, and ankle and foot injuries rose sharply. In an analysis pooling 7 college football studies, braced players had more injuries in 4 of them.
Front-of-knee pain was different
In 6 weeks of training, pain was cut in half
167 people went through 6 weeks of tough basic military training, and 54 wore a brace that holds the kneecap. Pain at the front of the knee hit 18.5% of the braced group and 37% of the rest. A Cochrane analysis pooling 2 trials (227 people) found the risk was 0.41 times as high.
What about gym knee sleeves?
Squat max went up by 5 kg
15 people tested their squat max in different sleeves. With neoprene sleeves they lifted 166 kg; with a barely snug sleeve, 161 kg. Jumps, leg extension max and reps showed no difference.
Do sleeves change how the knee moves?
The feel changed, the movement barely did
17 people did front squats and box jumps with and without sleeves. 94% felt more stable and 100% believed sleeves would prevent pain, but knee movement and load dropped slightly in just two measures. No trial tracked whether wearing sleeves meant fewer injuries.
Bottom line: braces prevented front-of-knee pain
Gym sleeves are gear for lifting more
Kneecap braces cut front-of-knee pain from 37% to 18.5% in 6 weeks of training. Ligament injuries in contact sports dropped only among defensive players in the 1,396-person trial and rose in the 580-person study. Sleeves added 5 kg to the squat, but showed no drop in injuries.