The forces in a knee brace as a function of hinge design and placement

Customized knee braces for three normal subjects were instrumented to monitor the forces and moments across the hinges as the subjects performed various activities. The forces and moments were taken to represent a mismatch between actual knee motion and the motion the brace sought to impose. The different hinge designs studied were fixed axis, gear-on-gear, rack-and-pinion, and natural 3-D; they showed only moderate differences in forces. Much larger differences were seen if the hinges were offset 12 mm from the ideal placement. Posterior placement resulted in the least force and anterior placement the highest. The mismatch of knee motion to brace motion would probably lead to abnormal ligament lengths and tensions and other internal mechanical changes, as well as to pistoning and discomfort. The results of this study have implications on brace design, selection, and placement.