Dr. Recep Uzgur Department of Prosthodontics Tooth Preparation Dr. Recep Uzgur Department of Prosthodontics

Basic Principles The basic principles on which tooth preparation is done are: • Preservation of tooth structure • Retention and resistance • Structural durability • Marginal integrity • Preservation of periodontium

1. Preservation of Tooth Structure It is stated that the preservation of what remains is more important than the meticulous replacement of what is lost. Whenever possible, partial crowns should be done (Indications). There should be minimal possible reduction for all surfaces (Material Selection). 1.

2. Retention and Resistance Forms Path of Insertion; “It is an imaginary line along which the restoration will be placed onto or removed from the preparation. The path of insertion of the preparation should be parallel to the adjacent teeth. Must be single.

How can Retention provide that? Retention prevents the removal of the restoration along the path of insertion on the long axis of the tooth. It can be defined as, “the ability of the preparation to prevent displacement of the restoration in a direction opposite to the path of insertion. How can Retention provide that?

Types of Retention Primary Retention; Provided by the opposing vertical surfaces of the tooth preparation. Secondary retention: Retention obtained by retentive features like pins, boxes and grooves, etc. is known as secondary retention. For primary retention, degree of convergence in preparated tooth is used. It is called Taper.

Zero degree taper is the most retentive but it is almost impossible to obtain, For optimum retention, 6° -10° convergence is sufficient. A tapering fissure diamond is ideal to produce the required taper for any preparation. This diamond is designed with a three-degree taper. When a tapering fissure diamond is held parallel to the long axis of the tooth during preparation, the necessary taper (3°) is automatically produced.

Length Greater the height of the crown, better the retention of the restoration. Height increases the area of cementation, in this way retention is increased.

3. Structural Durability Durability comes with the thickness of the restoration. A restoration should contain sufficient bulk to withstand forces. The amount of reduction required for structural durability depends on the type of restorative material being used and the design of the restoration. Occlusal Reduction; Gold alloys require 1.5 mm clearance for the functional cusp and 1.0 mm clearance in the nonfunctional cusp.

Metal ceramic restorations require 1. 5 to 2 Metal ceramic restorations require 1.5 to 2.0 mm reduction in the functional cusp and 1.0 to 1.5 mm reduction in the nonfunctional cusp. All ceramic restorations require a minimum of 2 mm reduction throughout. Functional Cusp Bevel provides to increase the thickness.

Axial Reduction; Adequate axial reduction is necessary for structural durability. Inadequate axial reduction may lead to over-contoured proximal surfaces, which can lead to periodontal problems. The required taper should be obtained, during axial reduction. It is 1.0- 1.5 mm reduction.

4. Marginal Integrity Marginal adaptation and the seating of the restoration affect marginal integrity. Poor marginal adaptation will lead to marginal leakage and secondary caries. The margin of a restoration should be preferably placed supra-gingivally because it has the following advantages: It can be easily finished. Easy to maintain. Easy to identify and reproduce during impression making. Easy to examine during future visits.

The indications for a sub-gingival margin are; Where aesthetics is concern For teeth with short clinical crowns. Teeth affected by sub-gingival caries Chamfer Finish Line; This finish line possesses a curved slope from the axial wall till the margin. It can be produced using a torpedo diamond point.

A chamfer provides less stress and good success rate. Shoulder Finish Line; This finish line has a gingival finish wall perpendicular to the axial surfaces of the teeth.

Shoulder with a Bevel is similar to a shoulder finish line, but an external bevel is created on the gingival margin of the finish.

5. Preservation of Periodontium The placement of finish lines influences the fabrication of the restoration and the final outcome of the treatment. The finish lines should be placed in an accessible region so that the margins of the restoration can be easily finished by the dentist and effectively cleaned by the patient. The finish line should be in enamel whenever possible. Sub-gingival finish lines predispose to periodontitis. If the distance between the finish line and the alveolar crest (combined width of epithelial and connective tissue attachments) is less than 2.0 mm, the restoration may lead to gingival inflammation, loss of alveolar crest and pocket formation

Burs And Rotary Attachments A Bur has three components. 1. Head: Part of bur that cuts, polishes, or finishes. Available in a variety of shapes and sizes. 2. Neck: Part of bur that tapers to connect shank to head. 3. Shank: Part of bur that is inserted into the handpiece.

Different styles of burs Length and style of shank may vary depending to handpiece used.

1.Round Bur To remove caries from tooth structure, To open tooth for endodontic treatment To place retention in cavity preparation Commonly used sizes: No. 1 / 4 to No. 10.

2.Pear-Shaped Bur To open tooth for a restoration To remove caries Frequently used in preparation of composite restorations Commonly used sizes: No. 330 to No. 333.

3. Inverted Cone Bur To remove caries To establish retention in tooth for cavity preparation Commonly used sizes: No. 33 1 / 2 , No. 34, No. 37, No. 39

3. Straight Fissure Burs- Plain Cut or Crosscut To cut cavity preparation To form inner walls of cavity preparation To place retention grooves in walls of cavity preparation Commonly used sizes: No. 56, No. 57, No. 58/ No. 556, No. 557, No. 558

4. Tapered Fissure Burs- Plain Cut or Crosscut To cut cavity preparation To form angles in walls of cavity preparation To place retention grooves in walls of cavity preparation Commonly used sizes: No. 168, No. 169, No. 170, No. 171/ No. 699, No. 700, No. 701, No. 702, No. 703

5. Finishing Bur To nish composite restoration To nish restoration by restoring anatomy in tooth To equilibrate or adjust occlusion

6. Diamond Bur—Flat-End Taper To reduce a tooth for crown preparation when a square shoulder is needed

7. Diamond Bur—Flat-End Cylinder To reduce a tooth for crown preparation when parallel walls and flat floors are needed

8. Diamond Bur—Flame To reduce a tooth at crown preparation for subgingival margins

9. Diamond Bur—Wheel To reduce a tooth for crown preparation on lingual aspect of anterior teeth To reduce bulk of incisal edges

10. Mandrels—Snap On or Screw On To attach discs to Mandrel for nishing and polishing inside or outside oral cavity 1. Long shank—For straight slow-speed handpiece 2. Short latch-type shank—For slow-speed handpiece 3. Friction grip shank—For high-speed handpiece

11. Sandpaper Disc With Screw-Type and Snap-On Mandrel To contour restorations To polish restorative material

12. Composite Discs To contour restorations To polish or smooth restorative material

13. Rubber Points To polish restorations, amalgam, composite, and gold

14. Laboratory Bur—Acrylic Bur To cut models or trim acrylic in laboratory

14. Laboratory Bur—Diamond Disc To contour or cut models in the laboratory