ANNOTATION
The shoulder is a platform in the cervical area for an artificial crown, which is designed to evenly distribute the load through the artificial crown to the tooth root. The ledge, as a rule, should be uniform in width. The form of the ledge may be:
- straight 90̊
- straight with beveled edge (with a 45ос bevel)
- at an angle of 135̊
- gutter
- the symbol of the shoulder (at the level of the edge of the gums create a groove in the form of a groove of 0.2 - 0.3 mm)
Features of preparation for CAD-CAM crowns: - The angle of the cone of the side walls of the stump should be at least 6 degrees, better than 10-15. This is important for obtaining an “optical” impression.
- At least 2 mm of tissue should be removed from the occlusal surface. The height of the stump should be at least 3 mm. Occlusal concavity of the posterior teeth should not be expressed.
- The tip of the stump at the front teeth should be not less than 1 mm thick, preferably 1.2 - 1.5 mm, so that the cutter has a stroke.
- All sharp corners and edges should be smoothed.
- For the preparation of all-ceramic crowns with a frame made by the method of milling, only two types of ledge are suitable. The shoulder ledge - with an angle of 90 to 120 degrees and a rounded inner corner (the transition of the shoulder in the side wall), and pronounced rounded ledge. There should be no protruding sharp edges along the edge of the ledge (as well as for preparation under the ceramic-metal crown); the ledge should not be in the form of a groove. The shoulder with a beveled preparation edge is not suitable for all-ceramic crowns, as This bevel is difficult to scan and mill on the frame, it can only be blocked off with a metal in the standard molding method of making the frame.
Dissection of the abutment teeth under the ceramic crown is carried out according to the same rules as under the ceramic-metal crown, with the only difference that the step for the metal-free ceramic crowns is created by the gum, ie at the level of the gums, and does not start under the gums. The inclination of the walls of the prepared tooth should not exceed 4-8 °. The preparation is carried out first with coarse-grained diamond burs with water cooling and intermittent movements. The surface of the tooth stump at the final stage of preparation is smoothed out by fine-grained diamond burs.
Stages of laboratory technology for manufacturing metal-free ceramic crowns:
- getting a collapsible model;
- modeling of the tooth crown or skeleton;
- pressing of the tooth crown or skeleton;
- layering cladding on the frame;
- final processing;
- glazing;