Improved design for the Thompson dowel semiprecision intracoronal retainer

The new design of the dowel differs from the previous one in that the dowel is in contact with the inner shelf line at all times but is eased to enable it to rotate out from the well. The exact alignment of the two shelves is important. Incorporation of anything more than the slightest flare is discourged unless the shelves are at different heights. Finally, the lingual walls do not need to be parallel or at right angles to the axis of rotation, except where the shelves correspond perfectly and are horizontal to the tissue surface. The only requirement is that they do not diverge in the path of rotation.