A review of nine studies of prophylactic knee braces in American tackle football found some support for the use of double-hinge braces in high school but little support for their use at the collegiate level. Problems with bias and confounding make it necessary that caution be exercised in the interpretation of the results of these studies, however,. To which groups these results might apply must also be considered. The two studies that assigned braces randomly found lower injury rates among high school and high-school-sized players for knee injuries and knee ligament injuries. Conversely a large, multiteam collegiate study found a significantly higher rate of knee injuries among brace users, a difference that remained when controlled for position, skill, and previous injury.