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A frame is an open structure--like the frame of a house before the siding is put up to make the walls.

A leg brace is something which helps to keep a leg in position, and to protect if from damage. Someone who had an injury, or a disease which weakened the legs, would use leg braces to help them keep their legs straight so that they could walk.

That sentence is actually somewhat nonsensical. If you encase the leg in a frame, it doesn't resemble a leg brace, it is a leg brace.