Biomechanics of tooth preparation/ orthodontics website


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Biomechanics of tooth preparation/ orthodontics website

  1. 1. BIOMECHANICS OF TOOTH PREPARATION INDIAN DENTAL ACADEMY Leader in continuing dental education
  2. 2. The design of a preparation for a cast restoration & the execution of the design are governed by five principles:- 1) Preservation of tooth structure 2) Retention & Resistance 3) Structural durability 4) Marginal integrity 5) Preservation of the periodontium
  3. 3. 1. Preservation of Tooth Structure While replacing a lost tooth structure, a restoration must preserve the remaining tooth structure as much as possible.
  4. 4. Tooth structure is conserved by using the following guidelines:- - 1. Use of partial coverage rather than complete coverage restorations. 2. Preparation of teeth with minimum practical - convergence angle (taper) between axial walls. -
  5. 5. 3. Preparation of the occlusal surface so reduction follows the anatomic planes to give uniform thickness in the restoration.
  6. 6. 4. Preparation of the axial surfaces so that the tooth structure is removed evenly: if necessary, teeth should be orthodontically repositioned.
  7. 7. 5. Selection of a conservative margin compatible with the other principles of tooth preparation. 6. Avoidance of unnecessary apical extension of the preparation.
  8. 8. 2. Retention & Resistance For a restoration to accomplish its purpose, it must stay in place on the tooth. RETENTION:- prevents the removal of the restoration along the path of insertion or long axis of the tooth preparation. RESISTANCE:- prevents dislodgment of the restoration by forces directed in an apical or oblique direction & prevents any movement of the restoration under occlusal forces.
  9. 9. Essential elements of retention:- 2 opposing vertical surfaces in same preparation. 1) External surfaces:- eg. Buccal & lingual walls of full veneer crown. An extracoronal restoration is an example of veneer, or sleeve retention. Scan0001.jpg
  10. 10. 2) Internal surfaces:- eg. Buccal & lingual walls of the proximal box of a proximo-occlusal inlay. An intracoronal restoration resists displacement by wedge retention.
  11. 11. TAPER:- The axial walls of the preparation must taper slightly to permit the restoration to seat. i.e. 2 opposing external walls must gradually converge – ANGLE OF CONVERGENCE. 2 opposing internal surfaces of the tooth structure must diverge occlusally – ANGLE OF DIVERGENCE. The relationship of one wall of a preparation to the long axis of that preparation is the INCLINATION of that wall.
  12. 12. The more nearly parallel the opposing walls of a preparation, the greater should be the retention. Most retentive preparation should be one with parallel walls, but the parallel walls are impossible to create in the mouth without producing preparation undercuts. An UNDERCUT is defined as a divergence between opposing axial walls, or wall segments, in a cervical- occlusal direction.
  13. 13. Tooth preparation taper should be kept minimal because of its adverse effects on retention. ed taper ed retention ed taper ed retention. Greater the surface area of a preparation, greater is its retention.
  14. 14. FREEDOM OF DISPLACEMENT:- Retention is improved by geometrically limiting the numbers of paths along which a restoration can be removed from the tooth preparation. Maximum retention is achieved when there is only one path.
  15. 15. Limiting the freedom of displacement from torquing or twisting forces in a horizontal plane increases the resistance of a restoration.
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  17. 17. LENGTH:- Occlusogingival length is an important factor in both retention & resistance. Longer preparations will have more surface area & therefore will be more retentive . The length must be great enough to interfere with the arc of casting pivoting about a point on the margin on the opposite side of the restoration.
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  19. 19. SUBSTITUTION OF INTERNAL FEATURES:- The basic unit of retention for a cemented restoration is two opposing axial walls with a minimal taper. It may not be possible always to use opposing walls for retention. Therefore, internal features such as the groove, the box form, & the pin hole can be substituted for an axial wall or for each other.
  20. 20. PATH OF INSERTION:- It is an imaginary line along which the restoration will be placed onto or removed from the preparation. It is of special importance when preparing teeth to be fixed partial denture abutments, since the paths of all the abutment preparations must parallel each other. Surveying visually, since it is the primary means of insuring that the preparation is neither undercut nor over-tapered.
  21. 21. The path of insertion must be considered in two dimensions:- - Faciolingually - Mesiodistally The faciolingual orientation of the path can affect the esthetics of metal-ceramic or partial veneer crowns.
  22. 22. The mesiodistal inclination of the path must parallel the contact areas of adjacent teeth. So if path is inclined mesially or distally, the restoration will be held up at the proximal contact areas & be “locked out”.
  23. 23. 3. STRUCTURAL DURABILITY A restoration must contain a bulk of material that is adequate to withstand the forces of occlusion. The bulk must be confined to the space created by the tooth preparation.
  24. 24. OCCLUSAL REDUCTION:- An important feature for providing adequate bulk of metal & strength to the restoration is occlusal clearance. Gold alloys – 1.5mm (FC) & 1mm(NFC) Metal-ceramic crowns- 1.5-2mm(FC)&1-1.5mm(NFC) All-ceramic crowns – 2mm of clearance on preparation
  25. 25. The occlusal reduction should reproduce the basic inclined plane pattern of the occlusal surface without overshortening the preparation rather than being cut as one flat plane.
  26. 26. FUNCTIONAL CUSP BEVEL:- It is an integral part of the occlusal reduction. A wide bevel on the lingual inclines of the maxillary lingual cusps & the buccal inclines of the mandibular buccal cusps provides space for an adequate bulk of metal in an area of heavy occusal contact.
  27. 27. If a wide bevel is not placed on the functional cusp, several problems may occur : - If the crown is waxed & cast to normal contour it can cause a thin area or perforation in the casting. - To prevent this the crown may be waxed to optimal thickness resulting in overcontouring & poor occlusion.
  28. 28. - If an attempt is made to obtain space for an adequate bulk in a normally contoured casting without a bevel, it will result in over inclination of the buccal surface which will destroy excessive tooth structure while lessening retention.
  29. 29. AXIAL REDUCTION:- It plays an important role in securing space for an adequate thickness of restorative material. Inadequate axial reduction can cause thin walls & a weak restoration subjected to distortion or a bulbous, overcontoured restoration which will strengthen the restoration but may have a disastrous effect on periodontium.
  30. 30. Other features that will provide space for metal that will improve the rigidity & durability of the restoration are: The offset, the occlusal shoulder, the isthmus, the proximal groove & the box.
  31. 31. 4. MARGINAL INTEGRITY The restoration can survive in the biological environment of the oral cavity only if the margins are closely adapted to the cavosurface finish line of the preparation.
  32. 32. MARGIN PLACEMENT:- Whenever possible the margin of the preparation should be supragingival. Advantages of supragingival margins are:- -   T                       1) They can be easily finished. -   2) They are more easily kept clean -   3) Easy to make impressions with less damage to soft tissue. R 4) Restorations –easily evaluated –at recall.
  33. 33. Subgingival margins of cemented restorations are a major factor in periodontal disease. A subgingival margin is justified if any of the following pertains:- 1)Dental caries, cervical erosion extend subgingivally. 2)Proximal contact area extends to the gingival crest 3)Additional retention is needed 4)Margin of a metal-ceramic crown is to be hidden behind the labiogingival crest. 5)Root sensitivity not controlled by more, conservative means like the application of dentin bonding agents. 6)Modification of axial contour is indicated.
  34. 34. MARGIN ADAPTATION:- The junction between a cemented restoration and the tooth is always a potential site for recurrent caries because of the dissolution of luting agent and inherent roughness. The more accurately the restoration is adapted to the tooth, the lesser the chance of recurrent caries or periodontal disease.
  35. 35. MARGIN GEOMETRY:- For the evaluation of the margin the following guidelines for margin design should be considered. 1)  Ease of preparation without overextension or unsupported enamel.     2)  Readily identifiable in impression & on die.     3) A distinct boundary to which the wax pattern can be finished.     4) Provide sufficient bulk of material. 5) Conservation of tooth structure.
  36. 36. MARGIN DESIGNS:-       Feather Edge Chisel Edge Bevel Chamfer Shoulder Sloped Shoulder Shoulder with Advantages Conservation of tooth structure Conservation Removes unsupported enamel, allows finishing of metal Distinct margin, adequate bulk easier to control. Bulk of restorative Material Bulk of material + advantage of bevel Bulk of material + Adv. Of bevel Disadvantages Does not provide sufficient bulk Location of margin difficult to control Extends preparation into sulcus Care is needed to remove unsupported lip of enamel Less conservative Less conservative Less conservative apically extends Indications Not recommended Occasionally on tilted tooth Facial margin of maxillary partial coverage restoration Cast metal restoration Facial margin of metal ceramic Facial margins of metal ceramic crowns Facial margin of posterior metal ceramic crowns
  37. 37. 5. Preservation of periodontium The placement of finish lines has a direct bearing on the ease of fabricating a restoration & on the ultimate success of the restoration. Margins that are smooth & fully exposed to cleansing action give best results. Finish lines should be placed in enamel when it is possible to do so. Subgingival restorations are described as a major etiologic factor in periodontitis. The deeper the restoration margin resides in the gingival sulcus the greater the inflammatory response.
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