Principles of tooth preparation

Teeth don’t possess regenerative ability found in most other tissues. Therefore, once enamel & dentin are lost as a result of caries, trauma or wear, restorative material must be used, to reestablish form & function.
Teeth require preparation to receive restoration & these preparations must be based on fundamental principles, which are discussed in this presentation, from which basic criteria can be developed to help predict the success of prosthodontic treatment.

1. PRINCIPLES OF TOOTH PREPARATION PRESENTED BY: Dr. SONALI HARJANI III M.D.S.

2. Teeth don’t possess regenerative ability found in most other tissues. Therefore, once enamel & dentin are lost as a result of caries, trauma or wear, restorative material must be used , to reestablish form & function. Teeth require preparation to receive restoration & these preparations must be based on fundamental principles from which basic criteria can be developed to help predict the success of prosthodontic treatment .

3. The scope of fixed prosthetic treatment can range from the restoration of a single tooth to the rehabilitation of the entire occlusion. Restorations can help in achieving a full function and improved esthetic outcome. Missing teeth can be replaced with fixed prosthesis that will improve patient comfort and masticatory ability, maintain health and integrity of dental arches and also improve patient’s self-image.

4. TOOTH PREPARATION: Tooth preparation may be defined as the mechanical treatment of dental disease or injury to hard tissue that restores a tooth to the original form. ( Tylman ) The process of removal of diseased and/or healthy enamel, dentin and cementum to shape a tooth to receive a restoration. (GPT-9)

5. Crown: An artificial replacement that restores missing tooth structure by surrounding part or all of the remaining structure with a material such as a cast-metal alloy, metal-ceramics, resin or a combination of materials. (GPT-9) Fixed partial denture: Any prosthesis that is securely fixed to a natural tooth or teeth, or to one or more dental implants/implant abutments, that replaces one or more missing teeth, which cannot be removed by the patient . (GPT-9)

6. OBJECTIVES OF TOOTH PREPARATION: 1.Reduction of the tooth in miniature to provide retention. 2.Preservation of healthy tooth structure to secure resistance form. 3.Provision for acceptable finish lines. 4.Performing pragmatic axial tooth reduction to encourage favorable tissue responses from artificial crown contour.

7. GUIDELINES OF TOOTH PREPARATION: Total occlusal convergence Occluso -cervical/ inciso -cervical dimension Ratio of OC and FL dimension Circumferential form of the prepared tooth Reduction uniformity Reduction depths Finish line location Line angle form

8. PRINCIPLES OF TOOTH PREPARATION: Biologic considerations, which affect the health of the oral tissues Mechanical considerations, which affect the integrity and durability of the restoration Esthetic considerations, which affect the appearance of the patient REFERENCE: CONTEMPORARY FIXED PROSTHODONTICS, ROSENTIEL, 5 TH EDITION

9. PRINCIPLES OF TOOTH PREPARATION: REFERENCE: FUNDAMENTALS OF FIXED PROSTHODONTICS, SHILLINBURG, 3 rd EDITION

10. PRESERVATION OF TOOTH STRUCTURE 1) PRESERVATION OF TOOTH STRUCTURE Adjacent teeth Soft tissue P ulp

11. 1) ADJACENT TEETH: Damage to adjacent teeth is prevented by positioning the diamond so that a thin lip of enamel is retained between the bur and the adjacent tooth. The orientation of the diamond parallels the long axis . Metal matrix band around the tooth – can be perforated and the adjacent tooth will not be accidentally niched.

12. 2) SOFT TISSUES: Damage to the soft tissues of the tongue and cheeks can be prevented by careful retraction with an aspirator tip, mouth mirror , or flanged saliva ejector. Great care is needed to protect the tongue when the lingual surfaces of mandibular molars are being prepared.

13. 3) PULP: Great care also is needed to prevent pulpal injuries during fixed prosthodontic procedures, especially complete crown preparation. Pulpal degeneration can also occur many years after tooth preparation. Extreme temperatures, chemical irritation, or microorganisms can cause an irreversible pulpitis particularly when they occur on freshly sectioned dentinal tubules.

14. THERMAL ACTION CHEMICAL ACTION BACTERIAL ACTION FRICTION MATERIALS- ON FRESHLY CUT DENTIN ALL CARIOUS DENTIN NOT REMOVED WATER SPRAY CAVITY VARNISH OR DENTIN BONDING AGENTS REMOVAL OF CARIOUS DENTIN FOLLOWED BY RESTORATION IF NEEDED RETENTION FEATURES- AT SLOW SPEEDS CLEANING AND DEGREASING AGENTS AVOIDED ZINC PHOSPHATE CEMENT- ANTIBACTERIAL

15. THERMAL ACTION:

16. Zach L, Cohen G: Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 19:515, 1965

17. Pulpal temperature rise during tooth preparation Group I:  air turbine, water cooled   Group II:  air turbine, dry Group III:  low speed, water cooled Group IV:  low speed, dry Zach L, Cohen G: Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 19:515, 1965 Acc. To Zach and Cohen: Rise of 5.5 °C – 15% necrosis Rise of 11.1 ° C – 60% necrosis Rise of 16.6 ° C – 100% necrosis

18. CONSERVATION OF TOOTH STRUCTURE: Extensive amount of reduction – every dentinal tubule exposed – communicated directly with the dental pulp. Any damage to the odontoblastic process would adversely affect the cell nucleus at the dentin-pulp interface 

19. Use of partial coverage rather than full coverage restorations Preparation of tooth with minimal practical convergence angle (Taper) between axial walls Preparation of occlusal surface following the anatomical contour to give uniform reduction Avoidance of unnecessary apical extension of the preparation Selection of marginal geometry which is conservative and compatible with other principles Reduction of axial walls with maximal thickness of remaining dentin surrounding the pulp

20. CONSIDERATIONS AFFECTING FUTURE DENTAL HEALTH:

21. RETENTION AND RESISTANCE Taper Freedom of displacement Length Substitution of internal features Path of insertion

22. RETENTION FORM: Retention prevents removal of the restoration along the path of insertion or long axis of the tooth preparation RESISTANCE FORM: 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.

23. i ) Taper: • Taper is defined as the convergence of two opposite-facing external walls of a crown preparation as viewed in a given plane. The axial walls of the preparation must taper slightly. • T wo opposing external walls must gradually converge or • T wo opposing internal surfaces of tooth structure must diverge occlusally . • The terms angle of convergence and angle of divergence – state relationships between the two opposing walls of a preparation. Nearly parallel sides give maximum retention

24. Recommended convergence angle is 6 degrees. For illustrative purposes, one can visualize the hands of a clock at 12:01, which produces an angle of 5.5° To produce 6° taper, each opposing axial wall must have an inclination of 3°

25. Tilting the bur away from the tooth creates an undercut Tilting the bur towards the tooth results in over reduction and excessive taper

26. Jørgensen KD: The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Scand 13:35, 1955 As the degree of taper of a preparation increases, its ability to retain a restoration decreases

27. ii) Freedom of Displacement:  • M ore is the freedom of displacement, less is the retention. Retention is enhanced by restricting the paths of withdrawal. The excessively tapered cone has infinite paths of withdrawal– Fig. A Addition of parallel sides grooves limits the path to one thus reducing possibility of dislodgement– Fig. B

28. In an anterior three quarter crown, retentive grooves are made, lingual walls of the grooves must be perpendicular to the path of displacement

29. iii) Length:  Longer preparations – more surface area – more retention Length must be enough to interfere with the arc of the casting pivoting about a point on margin on opposite side of restoration.

30. iv) Substitution of internal features: The basic unit of retention is two opposing walls with a minimum taper. But in cases such as, when one or more than one walls are lost due to decay, it may not be possible to use opposing walls for retention. In such cases, internal features such as grooves, box forms or pinholes can be substituted for an axial wall. The addition of a groove limits the paths of placement, thus the retention is increased.

31. Resistance of a short preparation can be improved by addition of grooves

32. v) Path of Insertion: Path of insertion is an imaginary line along which the restoration will be placed into or removed from the preparation. This is important in preparing bridge abutments, because multiple paths of insertion must be parallel. Path of insertion should be such that it allows the margins of the retainers to fit against their respective finish line with the removal of minimum sound tooth structure.

33. The path of insertion for a posterior full and partial veneer crown is usually parallel to the long axis of the tooth. However, the path of insertion of an anterior three-quarter crown is parallel to the incisal two-thirds of the facial surface (Fig. B), to prevent unaesthetic display of the metal on the facial surface (Fig. A).

34. Path of insertion of a full veneer crown parallels the long axis of tooth. A tipped tooth. If the path of insertion of a tipped tooth is kept parallel to its long axis, the seating will be prevented by adjacent teeth which protrude into path of insertion. Correct path of insertion for such a tooth is perpendicular to the occlusal plane.

35. RESISTANCE FORM

36. STRUCTURAL DURABILITY Occlusal Reduction Functional Cusp Bevel Axial Reduction

37. A restoration should contain bulk of material that is adequate to withstand the forces of occlusion. Sufficient tooth structure must be removed to create space for an adequate bulk of restorative material to accomplish this without departing from the normal contours of tooth. Occlusal surface should be prepared anatomically to aid in adequate clearance and prevent excessive amount of reduction. i ) Occlusal Reduction:

38. Recommended occlusal reduction for restorations with different alloys:

39. ii) Functional Cusp Bevel: Integral part of occlusal reduction. Wide bevel on functional cusps provides space for adequate bulk of metal in area of heavy occlusal loads.

40. Function bevel allows for adequate bulk of restoration. If bevel not given, restoration is likely to be too thin in the stress-bearing area If restoration thickness is achieved by over-tapering axial wall, retention is compromised. In absence of bevel, to provide sufficient bulk, lab technicians may over-contour the restoration which may result in super-occlusion of the restoration

41. iii) Axial Reduction: Important for securing space for an adequate thickness of restorative material. Inadequate axial reduction will have thin walls which will be subject to distortion (Fig. B). To compensate that, lab technicians may try to provide the necessary bulk for the restorative material by overcontouring the axial surfaces. This in turn has disastrous effect n the perodontium (Fig. C).

42. MARGINAL INTEGRITY Bevels Finish line configuration

43. i ) Bevels: Even the restorations with high degree of precision in fitting show some discrepancy between the margin and the restoration d

44. Bevel allows closer approximation of a crown margin to the tooth. More acute the angle of the margin-m, or more obtuse the angle of finish line-p, less is the discrepancy at the margin, m.

45. Smaller the angle between the prepared tooth surface and the path of insertion, less is the marginal discrepancy. However, angles less than 25 degrees produce a margin which is too thin and weak.

46. Acute margin should be continued to be used for metal restorations and the angle should be kept in 30- 45 degree range. Tapered edge in a wax pattern produced by beveling is more readily adaptable to a die than butt joint.

47. ii) Finish Line Configuration: • Prosthesis should fit as closely to the finish line to minimize exposed cement. • Sufficient strength to withstand the forces of mastication. • Should be located where the dentist can inspect them and the patient can clean them.

48. FUNCTIONS OF FINISH LINE: Measure of tooth structure that has been removed Used to measure the accuracy of an impression Helps to evaluate a die and trim accurately Proper fabrication of a wax pattern Evaluation of a restoration Helps in determining if the restoration is seated completely 

49. Easily finished w/o soft tissue trauma Easily kept plaque free Impressions easily made, with less tissue damage Restorations can be easily evaluated Dental caries, cervical erosion,or restoration extend subgingivally Crown lengthening procedure is contraindicated Proximal contact area extends apically to level of gingival crest Margin of an aesthetic restoration is to be hidden For control of severe root sensitivity ADVANTAGES OF SUPRAGINGIVAL MARGIN: INDICATIONS OF SUBGINGIVAL MARGIN:

50. CHAMFER MARGIN: Least stress • Torpedo – less likely to produce a butt joint F ormed as a negative image of a round-ended tapered diamond. Indicated: all metal restorations, lingual margins of ceramic facing restorations

51. HEAVY CHAMFER: • Rounded internal line angle • Better support than Chamfer • Bevel can be added to provide an even better support  

52. SHOULDER MARGIN: Healthy restoration contours and maximum esthetics • Minimizes stress that may lead to fracture of porcelain Sharp line angle – stress concentration – coronal fracture Indicated: facial margins of metal-ceramic, margins of all ceramic restorations

53. RADIAL SHOULDER:   Essentially same as the shoulder Internal line angle rounded Cavo surface margin – 90 degrees Stress concentration less than in classic shoulder

54. KNIFE EDGE MARGIN: Permits an acute margin of metal Thin margin – difficult to wax up Susceptible to distortion when subjected to occlusal forces May result in over contoured restoration to compensate for bulk Not indicated.

55. PRESERVATION OF THE PERIODONTIUM Placement of margins Preservation of Biological width

56. i ) Margin placement: Margin placement has direct effect on ultimate success of the restoration. M argins should be as smooth as possible. T hey should be placed in areas that can be finished well by the dentist and kept clean by the patient. T hey should be placed in enamel wherever possible. S hould be kept s upragingival whenever possible.

57. ii) Preservation of biological width: Biologic width  describes the combined heights of that the healthy gingival tissue occupies above the alveolar crest i.e. the connective tissue and epithelial attachments to a tooth. T he connective tissue attachment having an average height of 1 mm, and the epithelial attachment also having an average height of 1 mm, leading to the 2 mm dimension. Biologic width is essential for — the preservation of periodontium and removal of irritation that might damage the periodontium .

58. Violation of the biologic width in case of subgingival restoration margins which are less than 2mm away from the alveolar crest — lead to ultimate failure of the restoration Bone loss and gingival recession occur as the body attempts to recreate room between the alveolar bone and the margin to allow space for tissue reattachment.

59. SUMMARY

60. PRESERVATION OF TOOTH STRUCTURE RETENTION AND RESISTANCE FORM STRUCTURAL DURABILITY Adjacent teeth Soft tissue P ulp Taper Freedom of displacement Length Substitution of internal features Path of insertion Occlusal Reduction Functional Cusp Bevel Axial Reduction

61. MARGINAL INTEGRITY PRESERVATION OF THE PERIODONTIUM Bevels Finish line configuration Placement of margins Preservation of Biological width

62. Current Concepts of Tooth Preparation: Tooth preparations for complete crowns: An art form based on scientific principles Charles J. Goodacre , Wayne V. Campagni , and Steven A. Aquilino , DDS. The Journal Of Prosthetic Dentistry, April 2001. 1) The total occlusal convergence, formed between 2 opposing prepared axial surfaces, ideally should range between 10 and 20 degrees. 2) Three millimeters should be the minimal occlusocervical / incisocervical dimension of incisors and premolars prepared within the recommended 10 to 20 degrees of total occlusal convergence.

63. 4) The ratio of the occlusocervical / incisocervical dimension of a prepared tooth to the faciolingual dimension should be at least 0.4 or higher for all teeth. 5 ) Whenever possible, teeth should be prepared so that the facioproximal and linguoproximal corners are preserved , thereby sustaining variation in the circumferential morphology that enhances resistance form. 6) Teeth without natural circumferential morphology after tooth preparation (round teeth) or teeth that lack adequate resistance form should be modified with the creation of grooves / boxes . 3) The minimal occlusocervical dimension of molars should be 4 mm when prepared with 10 to 20 degrees total occlusal convergence.

64. 7) Many molars need auxiliary grooves or boxes to enhance resistance form because of their short occlusocervical dimensions. 8) Axial grooves / boxes should be used routinely when mandibular molars , and they should be located on the proximal surfaces. 9) When tooth conditions and esthetics permit, finish lines should be located supragingivally . 10) When subgingival finish lines are required, they should not be extended to the epithelial attachment. 11) Chamfer finish lines approximately 0.3 mm deep are well suited for all-metal crowns.

65. 12 ) Type of finish line for metal-ceramic crowns should not be based on marginal fit but on personal preference, esthetics, ease of formation . 13) Both shoulder and chamfer finish lines can be used with all-ceramic crowns if the crowns are bonded to the prepared teeth. 14) Axial and occlusal reductions for all-metal crowns should be at least 0.5 mm deep and 1.0 mm deep , respectively . For metal-ceramic crowns , facial/axial reduction of 1.5 mm is recommended , whereas 2.0 mm of occlusal reduction is commonly advised. Two millimeters incisal/occlusal reduction is recommended for all ceramic crowns.

66. 15 ) Line angles should be rounded on the preparations to reduce stress in the definitive restoration. 16) Smooth tooth preparation appears to enhance the fit of restorations. Hence the preparations should be finished with a finishing bur.

67. CONCLUSION

68. The current focus is on conservative tooth preparation that is non-invasive and that minimally involves dentin . This trend is rational in the light of the reduction of the caries rate by fluorides, nutritional counseling and oral hygiene programs. Principles of tooth preparation are presently being modified to accommodate modern approaches i.e., acid etching with minimum reduction . Dentistry is changing from macro tooth preparation to an environment of molecular chemistry i.e., esthetic bonding.

69. Despite these advances, traditional crowns are still indicated for majority of patients. The classic design of the preparation must be visualized so that modifications, if required, can be instituted. Diagnosis and disciplined tooth preparation are highly essential to successful fixed prosthetics.

70. REFERENCES: Textbook of Fundamentals of Fixed Prosthodontics by Herbert T. Shillinburg . T hird edition. Textbook of Contemporary Fixed Prosthodontics by Rosentiel . Fifth edition. Laforgia PD, et al: Temperature change in the pulp chamber during complete crown preparation. J Prosthet Dent 65:56, 1991. Proussaefs P, et al: The effectiveness of auxiliary features on a tooth preparation with inadequate resistance form. J Prosthet Dent 91:33, 2004.

71. REFERENCES: Tooth preparations for complete crowns: An art form based on scientific principles Charles J. Goodacre , Wayne V. Campagni , and Steven A. Aquilino , DDS. The Journal Of Prosthetic Dentistry, April 2001. Zach L, Cohen G: Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 19:515, 1965 Jørgensen KD: The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Scand 13:35, 1955

72. Thank you!!