Pros 1 chapter 3

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Pros 1 chapter 3

  1. 1. PRINCIPLES OF TOOTH PREPARATION
  2. 2. 3 PRINCIPLES OF TOOTH PREPARATION The principles of tooth preparation may be divided into three broad categories: 1. Mechanical considerations, which affect the integrity and durability of the restoration 2. Biologic considerations, which affect the health of the oral tissues 3. Esthetic considerations, which affect the appearance of the patient
  3. 3. I. MECHANICAL PRINCIPLES
  4. 4. MECHANICAL PRINCIPLES Forces can be directed against the prosthesis during function. Significance?
  5. 5. MECHANICAL PRINCIPLES Types of Occlusal Forces 1. Tipping force 2. Twisting or rotational force 3. Path of insertion force
  6. 6. MECHANICAL PRINCIPLES Types of Occlusal Forces 1. Tipping force - Can occur in buccolingual or mesiodistal directions.
  7. 7. MECHANICAL PRINCIPLES Types of Occlusal Forces 1. Tipping force 2. Twisting or rotational force - May cause a restoration to start to move circumferentially around the prepared tooth
  8. 8. MECHANICAL PRINCIPLES Types of Occlusal Forces 1. Tipping force 2. Twisting or rotational force 3. Path of insertion force - can be apically or occlusally directed - depending on whether the mandible is closing into a bolus of food or opening with sticky food interposed between the prosthesis and opposing teeth.
  9. 9. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 1. Axial Wall Height Factors that will affect the length of occlusocervical height of the abutment 1.1. magnitude of occluding force 1.2. span length 1.3. type of preparation 1.4. length of the lever arm 1.5. bone support
  10. 10. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 1. Axial Wall Height - MINIMAL ACCEPTABLE HEIGHT Height which allows the tooth structure to interfere with the arc of rotation as tipping forces attempt to cause rotation a fulcrum located at the finish line on the opposite side of the tooth
  11. 11. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 1. Axial Wall Height On short teeth, adequate axial wall height may only be achieved by extending the finish line - subgingivally - onto the root surface - not desirable or advanageous Alternative - prepare tooth with less taper
  12. 12. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 2. Taper of the Preparation Opposing walls must converge occlusally Divergent walls produce undercuts and prevents seating of restoration
  13. 13. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 2. Taper of the Preparation Increased taper reduces the ability of the restoration to - resist occlusally directed dislodging forces - lessens its ability to interfere with the arc of rotation as tipping forces act to unseat the restoration
  14. 14. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 2. Taper of the Preparation A total convergence of 3-5 degrees is considered ideal
  15. 15. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 3. Ratio of Preparation Diameter to Axial Wall Height It is often mistakenly assumed that a large diameter tooth will yield a more retentive preparation than a smaller diameter ones.
  16. 16. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 3. Ratio of Preparation Diameter to Axial Wall Height If the axial wall height and taper are the same for both teeth, the smaller diameter tooth interferes more effectively with the arc of rotation because the smaller radius of curvature allows the preparation to better resist the dislodgement.
  17. 17. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 4. Circumferential Irregularity Circumference of the tooth is usually irregular therefore uniform reduction will create an irregular shaped abutment. - resists tipping and twisting forces.
  18. 18. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 4. Circumferential Irregularity In round, short and/or overtapered abutment - intentional placement irregularities are done - forms: boxes grooves - placement: middle of proximal surface
  19. 19. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 5. Occlusal Irregularity Occlusal reduction following the anatomic form produces an irregular surface which aids in retention. Irregularities can be used to enhance resistance to dislodgement - example: pinholes - occlusal, cingulum, incisal
  20. 20. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 6. Rigidity Prosthesis must be thick enough to resist flexure and loosening. Occlusal reduction – minimum of 1- 1.5 mm Axial reduction occlusal area- minimum of 1 mm cervical area – minimum of 0.3-0.5 mm
  21. 21. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 7. Adaptation Small amount of space is required between a restoration and the prepared tooth to allow complete seating during cementation. Excessive space reduces resistance to dislodgement by placing to much dependence on the physical properties of the luting agent.
  22. 22. MECHANICAL PRINCIPLES Factors Preventing Restoration Dislodgement 8. Surface area Increased surface area is most significant when the additional area results in greater axial wall length.
  23. 23. MECHANICAL PRINCIPLES Finish Line The point at which a preparation terminates on the tooth.
  24. 24. MECHANICAL PRINCIPLES Functions of a Finish Line 1. During visual evaluation of tooth preparation, it is a measure of the amount tooth structure already removed.
  25. 25. MECHANICAL PRINCIPLES Functions of a Finish Line 2. One of the features that can be used to evaluate the accuracy of the impression made for the indirect procedures.
  26. 26. MECHANICAL PRINCIPLES Functions of a Finish Line 3. On the die, a distinct finish line helps in the evaluation of the quality of the die and aids in trimming it accurately.
  27. 27. MECHANICAL PRINCIPLES Functions of a Finish Line 4. The correct marginal adaptation of the wax pattern depends on an obvious finish line.
  28. 28. MECHANICAL PRINCIPLES Functions of a Finish Line 5. The evaluation of the restoration is also aided by the proper finish line.
  29. 29. MECHANICAL PRINCIPLES Functions of a Finish Line 6. At cementation, a sharp finish line aids in determining whether the restoration is fully seated.
  30. 30. MECHANICAL PRINCIPLES Forms of Finish Line 1.Chamfer - preferred cervical finish line for fixed prosthodontics. - should be utilized whenever possible because it is easily developed and visually distinct.
  31. 31. MECHANICAL PRINCIPLES Forms of Finish Line 2. Knife edge or chisel edge finish - not as well defined as chamfer. - often used on tipped teeth when formation of a chamfer would result in excessive tooth reduction.
  32. 32. MECHANICAL PRINCIPLES Forms of Finish Line 3. Feather edge - unacceptable because – - not sufficiently distinct - results in so little cervical tooth reduction - restoration must be over contoured to possess adequate rigidity. - since a feather edge is difficult to see visually, rregularities in the finish line are more likely to be present, making it more difficult to fabricate a restoration that fits accurately.
  33. 33. MECHANICAL PRINCIPLES Forms of Finish Line 4. Shoulder and beveled shoulder - difficult to form - produces a greatest depth of tooth reduction - required with ceramic restorations because proper color is achievable only through material thickness.
  34. 34. MECHANICAL PRINCIPLES INSTRUMENTATION
  35. 35. MECHANICAL PRINCIPLES Instrumentation Rotary instruments must be selected that allow the tooth to be reduced 1. according to the requirements of proper retention and resistance form and 2. finish line development.
  36. 36. MECHANICAL PRINCIPLES Instrumentation Kinds of Rotary Instruments 1. Diamond cutting instruments. - Have diamond particles attached to a concentric metal shaft. - Available in coarse, medium and fine grit 2. Dental burs possessing carbide cutting blades.
  37. 37. MECHANICAL PRINCIPLES Visibility Ways on achieving good visual access 1. Use of fiberoptic handpiece lights and numerous lights aimed at the oral cavity from different directions greatly aids in the visibility. 2. Removing of excess oral fluids. 3. Retraction of soft issues that interfere with vision.
  38. 38. II. BIOLOGIC PRINCIPLES
  39. 39. BIOLOGIC PRINCIPLES Teeth must be prepared in a manner that creates the least amount of trauma to the pulp. - retain as much tooth structure as possible - proper use of rotary instruments and the application of surface coolants
  40. 40. Pulpal Consideration
  41. 41. BIOLOGIC PRINCIPLES Pulpal Considerations 1. Conservation of tooth structure. 2. Depth of reduction. 3. Speed of reduction. 4. Instrument age and use of pressure.use of coolants.
  42. 42. BIOLOGIC PRINCIPLES Pulpal Considerations 1. Conservation of tooth structure. Retain as much as tooth structure as practicable
  43. 43. BIOLOGIC PRINCIPLES Pulpal Considerations 1. Conservation of tooth structure. 2. Depth of reduction. WAYS ON CONTROLLING THE AMOUNT OF REDUCTION 1.Best provided by placing strategically located depth cuts in the unprepared tooth surfaces and placed to the desired depth.
  44. 44. BIOLOGIC PRINCIPLES Pulpal Considerations 1. Conservation of tooth structure. 2. Depth of reduction. WAYS ON CONTROLLING THE AMOUNT OF REDUCTION 2. The intervening tooth structure is removed by using the base of the depth cut as a guide to proper reduction.
  45. 45. BIOLOGIC PRINCIPLES Pulpal Considerations 1. Conservation of tooth structure . 2. Depth of reduction. WAYS ON CONTROLLING THE AMOUNT OF REDUCTION 3. If caries removal will make the preparation excessively deep, place an insulating base material (minimum of 0.5mm) over the area in proximity to the pulp. * For adequate protection against thermal shock
  46. 46. BIOLOGIC PRINCIPLES Pulpal Considerations 1. Conservation of tooth structure. 2. Depth of reduction. 3. Speed of reduction. Rapid continuous removal of tooth structure causes rapid heat build-up that may cause irreversible pulpitis. - reduction must be performed INTERMITTENTLY in a STEADY and CONTROLLED MANNER to avoid EXCESSIVE HEAT BUILD-UP. 1. Reducing the tooth for a period of 5-10 seconds 2. Then remove the instrument from the surface for a few seconds
  47. 47. BIOLOGIC PRINCIPLES 1. Pulpal Considerations Conservation of tooth structure. 2. Depth of reduction. 3. Speed of reduction. 4. Instrument age and use of pressure. Only sharp instruments should be used for bulk tooth reduction. - Dull instruments create more friction , thus more heat is produced. - Use of excessive pressure should be avoided because this will cause undue heat generation. - Accentuated if worn instruments are used.
  48. 48. BIOLOGIC PRINCIPLES Pulpal Considerations 1. Conservation of tooth structure. 2. Depth of reduction. 3. Speed of reduction. 4. Instrument age and use of pressure . 5. Use of coolants. Delivery of water stream from handpieces during reduction. Disadvantage: interferes with vision
  49. 49. Periodontal Consideration
  50. 50. Periodontal Consideration  Supragingival location for the finish line: - allows good visual access for evaluating finish line forms. - facilitates accurate impression of prepared tooth. - allows more accurate assessment of prosthesis fit and contour. - Provides access for marginal refinement and polishing. - permits more accurate long term post insertion evaluation of marginal integrity
  51. 51. Periodontal Consideration - MOST important reason relates to the preservation of periodontal health. NOTE: There is no junction of any restorative material and the tooth that is as smooth as intact tooth structure. - marginal plaque acculmulation is inevitable and when this occurs subjingivally, it is not easy for the patient to remove it and the likelihood of adverse periodontal changes increases. - if tooth preparation extended beyond the gingival margin for some reasons, care must be exercised to avoid excessive tissue trauma.
  52. 52. III. ESTHETIC PRINCIPLES
  53. 53. Achieving a color that matches the surrounding teeth necessitates a certain minimal thickness in the ceramic material. This is accomplished by adequate and uniform reduction of the facial surface. Use of Depth Guides 1. Prevents excessive depth reduction. 2. Ensures adequate uniform reduction.
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