These calculations were done by Michael Frind on July 17, 2007.
Exertion of anterior-drawer-counteraction forcing by a functional knee brace.
Only the horizontal forces enter into these calculations. The system is symmetrical about the centre.
The knee is assumed to be at full extension, and the system is stationary. There is no movement. (The brace hinges do not move, as this calculation makes clear.)
Please note that here were are only looking at the forces germane to exerting anterior-drawer-counteraction forcing on the knee. We do not need to worry about the forces involved solely in anchoring the brace to the leg.
We want to find the magnitudes of the forces X1 and X2 so that the system is balanced and the brace does not accelerate off the leg.
General notes: note that the closer to the knee joint the forces X1 and X2 are, the more effective they are at exerting the desired anterior-drawer-counteraction forcing.
The tighter the knee-region straps are, the greater the forces at the top and bottom of the brace become.
For knees with PCL deficiency, the forces exerted by the brace would be for purposes of posterior-drawer counteraction. The system of forces would merely be a mirror image of that depicted above. (If a given knee harbours both ACL and PCL problems, then it is inherently inappropriate to have the brace exert drawer-counteraction forcing of any type.)
