knee cap.skp (196.4 KB)
If you mean the arcs that run from the bottom circle up to the plate at the top, they can’t possibly form a surface because they aren’t in the same plane. And without a closed surface, they can’t be a solid. To begin with, create components or groups from everything except the arcs and the base circle so that things don’t stick to each other and get distorted. Then the best way to deal with this sort of situation would be to complete an arc into a face by drawing a horizontal edge to the center, a vertical edge up to the top, and another horizontal edge out to the top of the arc. Select the circle at the bottom, activate the follow-me tool, and click on the new face to create a surface.