By cavity preparaition before inserting the inlay, the main condition is keeping its retention and stability during functional loadings. For this purpose the form of the corner, formed by lateral walls and the basis of a cavity, has great value. If this corner is clearly expressed and approaches to direct corner,the inlay will be steady, as the forces affecting a chewing surface of the inlay, are distributed, transforming into pressure upon the cement and firm layers of the tooth. If this corner tends to obtuse, the inlay is fixed worse, as the forces affecting a chewing surface, are partially transformed to a stretching, displacing the inlay.
The carried out mathematical calculation has shown that the magnitude of forces of wedging is directly proportional to the effort, applied to an inlay from the side of the occlusive plane, and inversely proportional to friction coefficient between an inlay and tooth layers, and also to a corner of divergency. The optimal angle of the divergency is considered to be an angle, that does not exceed 5,7º. On the basis of all the above mentioned, we suggest: to form the divergency 0 ° – 2 ° on the funktional wall, and on the nonfunctional 2 ° – 6 °.(The Patent of Ukraine for the utility model № 29617, The Patent of Ukraine for the utility model №33285, The Patent of Ukraine for the invention №84503).
For correct formation of divergency angles of the carious cavity walls, we have developed the set of burrs made by Komet (Germany) for precise divergency formation.
The adhesion of an inlay to a dentine is influenced by the speed of rotation and granularity of the diamond burrs. According to researches, the greatest value of adhesion is received at application of burrs with the particles size of 25 microns. Therefore, the granularity of 25 microns has been chosen for our burrs, assigned for finishing preparation.
The advantages of the given set:
- Thanks to the tool form (with insignificant processing on a cone: 2 °, 3 °, 4 °, 5 °, 6 °, small diameter and the rounded edges), the minimum quantity of tooth layers is deleted and the possibility of formation of the undercuts are excluded.
- With the increase of the depth of a cavity, the divergency angle can be increased proportionally also.
- The set provides the user with a complete and reliable technique of work in the course of finishing preparation of a cavity for micro prosthesis.
- By orienting the burr strictly in parallel to a long axis of tooth, the demanded divergency of the cavity walls is guaranteed.
- Finishing burrs form precise enamel edges without microcleavages and formations of new cracks, and also exclude formation of sharp internal and external corners.
- The minimum preparation of walls with the maximum preservation of firm layers of the tooth, allows to increase their firmness and stability to the influence of multydimentional forces.