three-point pressure system

Think about how you straighten a bent green stick: you push down in the middle while your two hands pull up at the ends. That trio of forces is exactly how a brace controls a joint. The three-point pressure system is the basic mechanical principle behind almost every orthosis: a single force pushing one way at the middle of a segment, balanced by two opposing forces near the ends.

In an orthosis the three points are pads or shells that press the body. To stop a knee from bowing outward, a brace presses inward at the knee (the central force) and presses outward against the thigh above and the calf below (the two counter-forces). The forces must balance, or the brace would simply push the limb away rather than control it. The further apart the two end forces sit, the gentler each one can be for the same correcting effect, which is why a longer brace can use lower, more comfortable pressures than a short one doing the same job.

This principle is why every orthosis must touch the body at carefully chosen places and why fit is everything. Get the points right and a brace corrects alignment with comfortable, spread-out pressure. Get them wrong, or let a single point carry too much load over a bony prominence, and you risk a pressure sore — a real and sometimes serious complication, especially in people with reduced sensation who cannot feel the warning discomfort. Understanding the three points lets you look at any brace and predict where it pushes and why.