Crown and bridge in dentistry ppt 3.pptx

Crown and bridge in dentistry

1. Crown and bridge Principles of tooth preparation Dental assistant – level 3

2. Principles : Biological considerations: 1- Preservation of tooth structure. 2- Structural durability. 3- Marginal integrity. 4- Preservation of periodontium . Mechanical considerations: - Retention and Resistance Form. Esthetic considerations.

3. Preservation of tooth structure : One of the basic tenets of restorative dentistry is to conserve as much tooth structure as possible consistent with the mechanical and esthetic principles of tooth preparation. Tooth structure is conserved by using the following guidelines: 1-use of partial coverage restoration rather than complete. 2-preparation of teeth with minimum taper between axial walls. 3-preparation of the occlusal surface so reduction follows the anatomic planes to give uniform thickness in the restoration.

4. 4-uprighting a tilted FPD abutment with orthodontic treatment. 5-selection of a conservative margin compatible with the other principles. 6-avoiding of unnecessary apical extension of the preparation .

5. Retention and resistance form: Retention form: it is the feature of the prepared tooth that prevent the movement of the final restoration along its path of insertion. Resistance form: it is the feature of the prepared tooth that tends to prevent the movement of the final restoration by forces directed in an apical or oblique direction [occlusal forces]. The axial walls of the preparation must be parallel or slightly tapered to permit the restoration to seat and to obtain good retention and resistance[the tapered angle is 2.5 – 6.5 .

6. The retention and resistance can be improved by : 1-limiting the paths of insertion. 2-boxes. 3-grooves. 4-pinhole .

7. Path of insertion: It is an imaginary line along which the restoration will be placed or removed or reseated on the prepared tooth without any stress. In case of tilted teeth: the path of insertion should be parallel to the adjacent proximal teeth. During preparation the abutments of an bridge , the path of insertion of all abutments must parallel each other. The path of insertion for posterior full partial veneer crowns is usually parallel with the long axis of the tooth, but of an anterior three quarter crown the preferred path of insertion is parallels the incisal two thirds of the labial surface.

8. Structural durability : The preparation must be designed to reduce minimum amount of the tooth structure and to have an adequate bulk of metal to allow the restoration to withstand the forces of occlusion. There should be a minimum alloy thickness of about 1.5 mm over centric cusps { buccal in mandible, lingual in maxillae }, while non centric cups can be protected wit h less metal 1 mm for a strong and long standing restoration.

9. Other features which increase the durability of the restoration: Offset. Grooves Occlusal shoulder Isthmus Proximal box

10. Marginal integrity: The margins of the final crown should be: extended to well-defined finishing line exactly without open, short, or over extend margins. Extended in an cleanable area by tooth-brushing and whenever possible. Placed on the sound tooth structure. Smoothed and well adapted to the finishing line.

11. Finishing line

12. Finishing line or margin of the preparation: It is the final margins, which separate between the prepared and the unprepared tooth structure. 1- feather-edge or knife-edge 2- chisel-edge 3- chamfer 4- heavy chamfer 5- shoulder 6- radial shoulder 7- shoulder with bevel

13. Chamfer shoulder Well defined, adequate bulk, easier to control. Less destruction of tooth structure. In full metal crown. Lingual margin of metal ceramic crown. Very distinct margin, bulk of restorative material, and provides better resistance to occlusal forces and minimizes stresses. More destruction of tooth structure than any other finishing line. The finish line of choice for the all ceramic crown.

14. Location or position of finishing line: The location of the finishing line is related to the gingiva: 1- supragingival finishing line ( above the gingival margin ). 2- subgingival finishing line ( below the gingival margin ). 3- with the gingival margin. this location depends on: The position of the tooth [anterior or posterior] 4) length of natural crown. Type of restoration. 5) degree of the oral hygiene. Presence or absent of caries and restoration as filling.

15. Preservation of the periodontium : Subgingival margins should be avoided as possible, because the subgingival restoration have been described as a major etiologic factor periodontitis. The finish line should be smooth placed in an area where the margins of the restoration can be finished by the dentist and kept clean by the patient. Finish lines should be placed in enamel when it is possible. The finish lines should be placed supragingival whenever possible. When the subgingival margins are unavoidable, the margins should be placed by a highly skilled dentist in the mouth of a motivated, cooperative patient.

16. Esthetic considerations : Gained by: minimal displaying of metal. Maximum thickness of the porcelain. Using a porcelain occlusal surfaces. Subgingival margin.

17. Clinical steps of crown construction : 1- Primary Impression for the production of primary cast ( diagnostic cast ) 2-Tooth Preparation ( removing suitable amount of tooth structure ; 3- Gingival Retraction . 4- Final Impression .5- Bite Registration 6-Temporary Restoration ( Provisional crowns ) 7- Try-In and cementation

18. Thank You