In general, there isn't much support for functional knee bracing after ACL injuries, but there is one study indicating that it helps significantly for skiing. See below.
Sterett W, Briggs K, Farley T, et al. Effect of functional
bracing on knee injury in skiers with anterior cruciate ligament
reconstruction. A prospective cohort study. Am J Sports Med.
Sterett et al recently conducted a prospective cohort
study to investigate the effect of functional braces on injury
rates of skiers after ACL reconstruction.14 This 6-year
study involved 820 skier-employees who had an ACL
reconstruction 2 or more years ago. In this study, 257
patients chose to wear a functional knee brace while skiing
and 563 patients opted against wearing a functional brace.
Injury data were recorded on all skiers. The results revealed
that the nonbraced skiers were 2.74 times more likely to
suffer a knee injury than a skier in the braced group. The
investigators report 61 subsequent knee injuries in all
patients. The injury incidence rate was 9% (51/563) in the
nonbraced group and 4% (10/257) in the braced group.
Surgical intervention was required for such injuries in 4%
(25/563) of the patients in the nonbrace group compared
with 1% (3/257) in the braced group. This study explains
that nonbraced skiers were at greater risk of injury and
subsequent surgery; therefore, Sterett et al advocate the use
of functional knee braces for patients who wish to return