Prophylactic knee braces are designed to prevent knee injuries in healthy athletes. These knee braces were popularized in the late 1970s when the use of prophylactic knee braces was tested in NFL players. Since that time, several studies have investigated the injury rates in athletes who wear prophylactic knee braces versus those who don't wear a knee brace.
Again, the evidence is cloudy, because the difference is very small. Studies do show that athletes in certain sports (football) have a lower rate of MCL injura when wearing a prophylactic knee brace.
However, the effect of the prophylactic knee brace should be placed in perspective. Much more important factors in determining the likelihood of an injury include: