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Principles of tooth preparation in Fixed Partial Dentures

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principles of tooth preparation in FPD

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Principles of tooth preparation in Fixed Partial Dentures

  1. 1. Vinay Pavan Kumar K 2 nd year MDS student Department of Prosthodontics AECS Maaruti College of Dental Sciences
  2. 2. Principles of tooth preparation Preservation of tooth structure Retention & resistance form Structural durability Marginal integrity Geometry -taper -freedom of displacement -path of insertion -length -stress -preparation type Materials cemented Roughness of fitting surfaces Dislodging forces Luting agent used Occlusal reduction Axial reduction Preservation of periodontium
  3. 3.  Al-Fouzan etal quantified the volume of reduction of tooth structure associated with different commonly used preparation designs using microcomputed tomography  The all-ceramic crown preparation design for the mandibular central incisors had the highest percentage (65.26% ± 4.14%) of tooth structure reduction, while the lowest percentage of tooth structure reduction was associated with the ceramic veneer preparation design for maxillary central incisors (30.28% ± 5.54%) Al-Fouzan A.F Volumetric measurements of removed tooth structure associated with various preparation designs Int J Prosthodont 2013;26:545–8
  4. 4. Tooth preparation The process of removal of diseased and/or healthy enamel, dentin and cementum to shape a tooth to receive a restoration
  5. 5.  Biological -maintenance of pulp vitality, adjacent teeth & soft tissues -conservation of tooth structure  Mechanical - retention & resistance  Esthetic - minimal display of metal - adequate thickness of porcelain - proper shade matching
  6. 6.  Total occlusal convergence  Occlusocervical/incisocervical dimension  Ratio of OC and FL dimension  Circumferential form of the prepared tooth  Reduction uniformity  Reduction depths  Finish line location  Line angle form Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85.
  7. 7.  Preservation of tooth structure  Retention & Resistance  Structural durability  Marginal integrity  Preservation of the periodontium
  8. 8.  Preserve the remaining tooth structure  Conservation guidelines-  Coverage: Partial v/s complete  Margin: Supragingival v/s subgingival
  9. 9.  Preparation of teeth with the minimum practical convergence angle between axial walls  Occlusal surface reduction: follow anatomic planes  Axial surfaces : if necessary, teeth should be orthodontically repositioned.
  10. 10.  Retention prevents 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 and prevents any movement of the restoration under occlusal forces.
  11. 11.  Dislodging forces  Geometry of the tooth preparation  Roughness of the fitting surface of the restoration  Materials being cemented  Luting agent being used
  12. 12.  Forces that tend to remove a cemented restoration along its path of withdrawal  FPD subject to dislodging forces-  Flossing under the connectors  Sticky food
  13. 13.  Restrained movement (eg. Nut and bolt )  Sliding pair – two cylindrical surfaces constrained to slide along one another
  14. 14.  Taper / Total Occlusal Convergence (TOC)  Substitution of internal features  Path of insertion  Freedom of displacement  Length and Surface area  Stress concentration  Type of preparation
  15. 15.  Inclination - relationship of one wall of a preparation to the long axis of that preparation  Tapered diamond bur: 2-3° inclination  Opposing surfaces with 3° inclination= 6° taper External walls (converge) Internal walls (diverge)
  16. 16.  Parallel walls – maximum retention  Taper  visualize preparation walls  prevent undercuts  permit more nearly complete seating of restorations during cementation  Ideal taper: 6°
  17. 17.  More the taper, lesser the retention Retention Jorgenson KD. The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Scand 1955 Feb;59(2):94-8.
  18. 18.  Angle between two opposing prepared axial surfaces  Historically TOC : 2°-6°  Clinical goal : 10°-22°  TOC beyond 10-22° – auxilliary features needed  Resistance testing was found to be more sensitive to changes in the TOC than retention testing Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85.
  19. 19. Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85.
  20. 20. Esteves HJ, Costa N, Esteves IS, Clinical determination of angle convergence in a tooth preparation for a complete crown. Int J Prosthodont. 2014 Sep-Oct;27(5):472-4.
  21. 21.  Basic unit of retention-opposing walls with minimal taper  Opposing walls not available for use-  Destroyed previously (severe attrition)  Partial veneer restorations  Greater than desirable inclination Groove Box Pinhole
  22. 22.  Imaginary line along which the restoration will be placed onto and removed from the preparation  Paths of all FPD abutments must parallel each other
  23. 23.  Visual survey - ensures preparation is neither undercut or overtapered  Center of the occlusal surface of the preparation is viewed with one eye from a distance of 30 cm (12”)  Binocular vision avoided- undercut preparation can appear to have an acceptable taper
  24. 24.  In patient’s mouth – mouth mirror is held at an angle approximately ½ inch above the preparation  Image viewed with one eye
  25. 25.  FPD abutments– common path of insertion  Firm finger rest established – mirror maneuvered until one preparation is centered– mirror moved by pivoting on the finger rest without change in angulation till the 2nd preparation is centered
  26. 26.  Path of insertion considered in 2 dimensions- mesiodistally and faciolingually.  Mesiodistal inclination - parallel to contact areas of adjacent teeth
  27. 27.  Faciolingual orientation - affects esthetics of metal ceramic and partial veneer crowns Facially inclined path of insertion prominent facio-occlusal line angle overcontouring or opaque show-through  For full veneer crowns  parallel to long axis of the tooth
  28. 28.  Posterior ¾ crown  parallel to long axis of the tooth  Anterior ¾ crown  parallel to incisal ½ of the labial surface
  29. 29.  Numbers of paths along which a restoration can be removed from the tooth preparation  Only one path – maximum retention
  30. 30.  Longer preparation – more surface area – more retentive  Length must be great enough to interfere with the arc of the casting pivoting about a point on margin on opposite side of restoration  Short preparations – inclination critical
  31. 31.  Smaller tooth - short rotation radius  Grooves in the axial walls- reduce the rotation radius
  32. 32.  Retentive failure occurs - cohesive failure in cement  Stress concentration- around the junction of axial and occlusal surfaces  Rounding the internal line angles
  33. 33.  Complete crown> partial coverage crowns  Adding groove/ boxes increases retention
  34. 34. Potts RG, Shillingburg HT Jr, Duncanson MG Jr,Retention and resistance of preparations for cast restorations. J Prosthet Dent. 1980 Mar;43(3):303-8
  35. 35.  Roughening/grooving the restoration - retention increased  Prepared by air-abrading the fitting surface with 50 µm of alumina  Airborne particle abrasion - increase in vitro retention by 64%  Roughening the tooth preparation- not recommended
  36. 36.  Retention affected both by the casting alloy and the core build-up material  The more reactive the alloy is, the more adhesion there will be with certain luting agents  Type I and II gold alloys- intracoronal restorations  Type III and IV gold alloys- crowns and FPD  Ni-Cr alloys- long span FPD
  37. 37.  Adhesive cements- most retentive  Film thickness of luting agent- effect not certain  Adhesive resin> Glass ionomer> Zinc Phosphate= Polycarboxylate> ZnO-E
  38. 38.  Dislodging forces  Luting agent being used  Geometry of the tooth preparation
  39. 39.  Mastication and parafunctional activity - substantial horizontal or oblique forces  Lateral forces displace the restoration by causing rotation around the gingival margin
  40. 40.  Resistance to deformation affected by compressive strength and modulus of elasticity  Adhesive resin> Glass ionomer> Zinc Phosphate> Polycarboxylate> ZnO-E
  41. 41.  Type of preparation  Freedom of displacement  Occlusocervical/incisocervical dimension  Ratio of OC and FL dimension  Circumferential form of the prepared tooth
  42. 42.  Partial coverage restoration< complete crown (no buccal resistance areas in partial coverage)  Adding groove/ boxes increases resistance (greatest if walls are perpendicular to direction of force)
  43. 43. GROOVE  Lingual wall perpendicular to the direction of force Oblique angle V-shaped groove PROXIMAL BOX  Buccal and lingual walls must meet the pulpal wall at 90° Oblique angle
  44. 44. Minimal OC dimension:  Anteriors - 3mm  Premolars - 3mm  Molars - 4mm Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85. Occlusocervical dimension Total occlusal convergence 1mm <6° 2mm <12° 3mm <17°
  45. 45.  Should be 0.4 or higher for all teeth Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85. OC/FL ratio Total occlusal convergence 0.1 <6° 0.2 <12° 0.3 <18° 0.4 <24°
  46. 46.  Should possess circumferential irregularity  Maxillary molars – rhomboidal form  Mandibular molars – rectangular form  Premolars and anteriors – oval form Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85.
  47. 47.  Preserve corners of a tooth preparation  No axial grooves, boxes should be provided in corners Chewing and parafunctional habits Dislodging forces largely faciolingual So, grooves and boxes on the proximal surfaces Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85.
  48. 48.  A restoration must contain a bulk of material that is adequate to withstand the forces of occlusion  Bulk should be confined to the space created by the tooth preparation  To provide adequate bulk: ▪ Occlusal reduction ▪ Functional cusp bevel ▪ Axial reduction
  49. 49.  Full metal restoration:  1.5 mm – functional cusp  1mm – non functional cusp  Metal-ceramic crowns :  1.5 to 2mm – functional cusp  1 to 1.5mm – non functional cusp  All ceramic crowns :  2mm over all
  50. 50. Adequate reduction Inadequate clearance Overpreparation
  51. 51.  Wide bevel on-  Lingual inclines of the maxillary lingual cusps  Buccal inclines of mandibular buccal cusps  Adequate bulk of metal in area of heavy occlusal contact
  52. 52.  Lack of functional cusp bevel: Thin area in casting Overcontouring Overinclination
  53. 53.  Thin walls of casting– subject to distortion  Overcontouring- disastrous effect on the periodontium
  54. 54.  Closely adapted margins to finish lines of preparation- survival of restoration in the oral environment  Configuration of finish line-  dictates the shape and bulk of metal at the margins  affects the marginal adaptation  affects degree of seating
  55. 55.  Chamfer  Heavy chamfer  Shoulder  Sloped shoulder  Radial shoulder  Shoulder with a bevel  Knife edge
  56. 56.  Indications-  Cast metal crowns  Metal-only portion of PFM crowns  Distinct, easily identified  Least stress  Round end tapered diamond  Half the tip of the diamond
  57. 57.  Indicated for all-ceramic crowns  90 degree cavosurface angle with a large radius rounded internal angle  Round end tapered diamond  Better than conventional chamfer but not shoulder  Bevel added - to use with metal restoration
  58. 58.  All-ceramic crowns  Facial margin of PFM crowns  Wide ledge-  resistance to occlusal forces  minimizes stresses which leads to fracture of porcelain  Flat-end tapered bur  Healthy contours  Maximum esthetics
  59. 59.  Destruction of more tooth structure Sharp 90° internal line angle concentrates stress on tooth Coronal fracture  Not used for cast metal restorations
  60. 60.  120° sloped shoulder margin  Facial margin of a metal-ceramic crown  No unsupported enamel, yet sufficient bulk to allow thinning of the metal framework to a knife-edge for acceptable esthetics
  61. 61.  Modified shoulder  Cavosurface 90°  Shoulder width lessened with rounded internal angles  Lesser stress concentration  Good support for porcelain
  62. 62.  Indications:  Proximal box of inlays, onlays  Occlusal shoulder of onlays and mandibular ¾ crowns  Facial finish line of metal-ceramic restorations (gingival esthetics not critical)  Situations where a shoulder is already present (destruction by caries, previous restorations)
  63. 63.  Bevel:  allows the cast metal margin to be bent or burnished against the prepared tooth structure  minimizes the marginal discrepancy  removes unsupported enamel
  64. 64.  Permit acute margin of metal  Axial reduction may fade out  Thin margin - difficult to wax and cast  Susceptible to distortion  Indications:  Mandibular posterior teeth with very convex axial surfaces  Lingually tilted lower molars
  65. 65.  All metal crowns –  Chamfer depth: 0.3-0.5 mm  Axial surface reduction: 0.5 -0.8 mm  Occlusal reduction: 1- 1.5 mm  Metal ceramic crowns –  Finish line depth: 1-1.5 mm  Occlusal reduction: 2mm  All ceramic crowns–  Finish line and facial reduction depth: 1mm  Incisal/occlusal reduction: 2mm Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85.
  66. 66.  Uniformly reduced :  normal crown form  improved aesthetic  Makes easier for laboratory technician to create esthetic restorations  Best achieved by placing depth grooves Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85.
  67. 67.  Should be rounded (increases crown strength)  Sharp line angles – stress concentration  Facilitates laboratory fabrication and fit  Ease to pour impressions Goodacre C J. Designing tooth preparations for optimal success. Dent Clin NAm 2004; 48: 359-85.
  68. 68. Margin placement  Direct effect on ultimate success of restoration  Margins should be as smooth as possible  Placed in area that can be finished well by the dentist and kept clean by the patient  Placed in enamel whenever possible  Should be supragingival whenever possible
  69. 69.  Supragingival margins  Less potential for soft tissue damage  Easily prepared and finished  More easily kept clean  Impressions are more easily made  Restorations easily evaluated at recall appointments
  70. 70.  Subgingival margins:  Esthetics  Existing caries, cervical erosion, or restorations extend subgingivally, and crown-lengthening is not indicated  Proximal contact area extends to the gingival crest  Additional retention is needed  Margin of a metal-ceramic crown is to be hidden behind the labiogingival crest  Root sensitivity cannot be controlled by more conservative procedures, such as the application of dentin bonding agents
  71. 71.  Finish line should not be closer than 2mm to the alveolar crest  Placement in this area –  gingival inflammation  loss of alveolar crest height  pocket formation
  72. 72.  Junction between a cemented restoration and the tooth - potential site for recurrent caries  Casting- fits within 10 µm  Porcelain margin- 50 µm  Stepped irregular margin- poor adaptation
  73. 73.  Adjacent teeth :  Iatrogenic damage  Metal matrix band  Leave a slight lip or fin of proximal enamel  Soft tissues:  Careful retraction of lips, cheeks  Care to protect tongue when lingual surfaces of mandibular molars prepared  Pulp  Temperature  Chemical action of cements  Bacterial action (microleakage)
  74. 74. Borelli etal In vitro analysis of residual tooth structure of maxillary anterior teeth after different prosthetic finish line preparations for full-coverage single crowns Journal of Oral Science, Vol. 55, No. 1, 79-84, 2013
  75. 75.  Different preparation depths  With/without coolants  Rise in temperature was noted without coolants  1mm depth – 0.540 C  2mm depth – 10 C  3 mm depth - 1.840 C  Drop in temperature was noted with coolants  1mm depth – 0.400 C  2mm depth – 0.820 C  3mm depth – 1.130 C Chhatwal N. Effect of tooth preparation and coolants on temperature within the pulp chamber. TPDI 2010;1(2):45-48.
  76. 76.  Shillingburg HT, Fundamentals of Fixed Prosthodontics, 4th edition, USA, Quintessence publications,2012, pp119-137.  Rosenstiel SF, Contemporary Fixed Prosthodontics, 4th edition, USA, Mosby, 2006, pp 166-201.  Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48: 359-85.  Borelli etal In vitro analysis of residual tooth structure of maxillary anterior teeth after different prosthetic finish line preparations for full-coverage single crowns Journal of Oral Science, Vol. 55, No. 1, 79-84, 2013  Al-Fouzan A.F Volumetric measurements of removed tooth structureassociated with various preparation designs Int J Prosthodont 2013;26:545–8
  77. 77.  Parker MH. Resistance form in tooth preparations. Dent Clin N Am 2004; 48: 387-96.  Owen CP, Retention and resistance in preparations for extracoronal restorations. Part II: Practical and clinical studies, J Prosthet Dent 1986;56(2):148-153.  Gilboe DB, Teteruck WR. Fundamentals of extracoronal tooth preparation. Part I-Retention and resistance form. J Prosthet Dent 2005;94:105-7.  Chhatwal N. Effect of tooth preparation and coolants on temperature within the pulp chamber. TPDI 2010;1(2):45-48.

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