Biomechanical principles of TOOTH PREPARATION

Biomechanical principles of TOOTH PREPARATION FOR FIXED PARTIAL DENTURE.

1. Good morning

2. Biomechanical principles of tooth preparations PRESENTED BY: SAPAM SONIA BDS- REGIONAL DENTAL COLLEGE

3. CONTENTS OF THE SEMINAR INTRODUCTION PRINCIPLES OF TOOTH PREPARATION REQUIREMENTS OF PREPARATION RETENTION FORM GEOMETRY OF THE TOOTH PREPARATION RESISTANCE FORM STRUCTURAL DURABILITY MARGINAL INTEGRITY FINISH LINES CONCLUSION REFERENCES

4. INTRODUCTION Teeth do not possess the regenerative ability found in most other tissues. Therefore, once enamel or dentin is lost as a result of caries , trauma, or wear, restorative materials must be used to reestablish form and function. Teeth require preparation to receive restorations, and these preparations must be based on fundamental principles .

5. principles The design and preparation of a tooth are governed by five principles: 1)Preservation of tooth structure 2)Retention and Resistance form 3)Structural durability of the restoration 4)Marginal integrity 5)Preservation of the periodontium

6. REQUIREMENTS OF TOOOTH PREPARATION

7. 1.Preservation of tooth structure Excess removal of tooth structure can have many ill effects. If a tooth is overtapered or shortened too much, there will be unnecessary sacrifice of retention and resistance. Thermal hypersensitivity , pulpal inflammation and necrosis can result from approaching the pulp too closely. The tooth structure can be conserved by:

8. 1.USE OF PARTIAL COVERAGE RATHER THAN FULL COVERAGE RESTORATIONS

9. 2. Preparation of tooth with minimal practical convergence angle (taper) between axial walls

10. 3. Preparation of occlusal surface following the anatomical contour to give uniform reduction

11. 4. Reduction of axial walls with maximum thickness of remaining dentin surrounded the pulp 5. Selection of a conservative finish line compatible with the type of restorations. 6. Avoidance of unnecessary apical extension of the preparation

12. Also, we should be careful about; 1. Adjacent tooth- Iatrogenic damage to an adjacent tooth during tooth preparation will lead the enamel surface more prone to plaque retention. By using metal matrix band Use of proximal enamel of the tooth, adjacent tooth is protected. 2. Soft tisues 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.

13. 3.Pulp: Tooth preparation must not involve or irritate the pulp chamber. Pulp size is more in young and adolescent indivisuals and decrease with age. Tooth preparations must take into consideration the morphology of the dental pulp chamber.

14. Average pulp dimensions have been related to coronal contour and are presented in table.

15. Causes of pulp injury: 1.Increased depth of preparation- Use of depth orientation grooves, appropriate selection of size of diamond point(diameter), and use of finish lines as indicated, will prevent this.

16. 2.Temperature : Extreme temperature caused by excessive pressure and high rotational speeds, condition of cutting instrument and improper appliclation of coolants may all increase the generated heat. Use of high speed handpiece intermittently with a feather touch, sharp diamonds and directing water spray to the area of contact between bur and tooth , will prevent the thermal irritation to the pulp.

17. 3.Chemical Action: The chemical action of the certain dental materials(bases, restorative resins, solvents, and luting agents) can cause pulp damage, particularly when they are applied to freshly cut dentin. 4.Bacterial Action: Pulpal damage under restorations has been attributed to bacteria that either were left behind or gained access to the dentin because of microleakage . All carious dentin should be removed before placing a restoration.

18. 2.Retention form The feature of a tooth preparation that resists dislodgement of a crown in a vertical direction or along the path of placement(GPT9).

19. Retention prevents removal of the restoration along the path of insertion or long axis of the tooth preparation. The essential element of retention is two opposing vertical surfaces in the same preparation. These may be external surfaces, such as the buccal and lingual walls of a full veneer crown. It can also be internal,such as buccal and lingual walls of the proximal box of a proximo-occlusal inlay.

20. GEOMETRY OF THE TOOTH PREPARATION Degree of taper Total surface area of the cement film Area of cement under shear , and Roughness of the tooth surface.

21. 1)Taper and Retention Taper is the convergence of two opposing external walls of a tooth preparation as viewed in a given plane. Taper is the angle, measured in degrees as viewed in a given plane, formed between an external wall and the path of placement of a tooth preparation or machined surfaces on a metal or ceramic material when prepared for fixed dental prosthesis.(GPT9)

22. More nearly parallel the opposing walls of a preparation, the greater will be the retention. This has been verified experimentally by Jorgensen, who found that retention decreases as taper increases. The most retentive preparation would be one with parallel walls. To allow the complete seating of restoration during cementation, the range of 2 to 6.5 degrees taper has been considered to be optimal.

23. As the degree of taper of a preparation increases , its ability to retain a restoration decreases

24. To produce an optimal 6- degree taper, or convergence angle, each opposing axial wall should have an inclination of 3 degrees to the path of insertion

25. 2)Surface area The greater the area of the cement film bound to the preparation and to the internal detail of the casting,the greater the retention of the casting will be. Therefore, the greater the surface area of the preparation, the greater the retention of its restoration.

26. The total surface area is influenced by: The size of the tooth The extent of the coverage by the restoration Features such as grooves and boxes that are placed in the preparation.

27. 3)Area under shear Most important for retention than the total surface area is the area of cement that will experience shearing rather than tensile stress. So, to decrease the failure potential , it is essential to minimize tensile stress. To utilized the shear strength of the cement , the preparation must have opposing walls,i.e.,two surfaces of the preparation in separate planes must be nearly parallel with each other and the line of draw. The opposing surfaces may be internal or external.

28. (A)An inlay depends on internal retention to hold it within its preparation. (B)Internal retention is created by the close adaptation of a restoration to 2 or more opposing, slightly divergent internal walls. (A) A CROWN DEPENDS PRIMARILY ON EXTERNAL RETENTION TO RESIST REMOVAL.(B) EXTERNAL RETENTION IS PROVIDED BY APPROXIMATION OF THE RESTORATION TO THE OPPOSING EXTERNAL AXIAL WALLS OF THE PREPARATION .

29. (B)The addition of parallel-sided grooves limits the path of withdrawal to one direction Path of insertion and retention A severely overtapered preparation has many paths along which a tensile force could remove a crown. Retention is enhanced by restricting the possible paths of withdrawal or paths of insertion.

30. However, if one of the four axial walls is missing or left uncovered, the potential paths of withdrawal are greatly increased and retention is compromised A full veneer crown preparation has excellent retention because the mesial , distal, lingual, and facial walls limit the possible paths of insertion to a narrow range.

31. A)Making the path of insertion of an anterior three-quarter crown parallel with the long axis of the tooth would result in an unnecessary display of metal. B)The preferred path of insertion for an anterior preparation parallels the incisal 2/3 rd of the labial surface. This will minimize the display of metal and allow the grooves to be longer and more retentive

32. When an axial wall is left unveneered , retention is achieved by substituting grooves(A),Boxes(B),or Pinholes(C) for the missing wall.

33. Length of the preparation is an important factor in retention. A long preparation has greater retention than does a short preparation. This is due to , greater the surface area of cement film, greater the retention.

34. A restoration on long preparation(A) can withstand a force that could remove a restoration from a shorter preparation of equal diameter(B)

35. A preparation with a larger diameter(and circumference ) will have greater retention than with a narrow preparation of the same length.

36. 4.Surface roughness a)The adhesion of dental cements depends primarily on projections of the cement into microscopic irregularities and recesses on the surface being joined, the prepared tooth surface should not be highly polished. b) Oilo and Jorgensen found retention of castings cemented with zinc phosphate cement on test dies with 10degrees taper to be twice as great on preparations with 40micrometer scratches as on those with 10micrometer scratches.

37. Resistance form The features of a tooth preparation that enhance the stability of a restoration and resists dislodgement along an axis other than the path of placement(GPT9)

38. Resistance prevents dislodgement of a restoration by forces directed in an apical, oblique, or horizontal direction . Leverage and resistance The strongest forces encountered in function are apically directed and can produce tension and shear in the cemented film only through leverage.

39. (A) When the line of action of an applied force passes within the margins of the restoration , no secondary lifting forces are produced because the margin on all sides of the restoration is supported by the preparation. (B)When the line of action passes outside the margin of the restoration , a torque is produced that will tend to tip or rotate the crown around a point on the margin.

40. Preparation length and resistance Shortening a preparation will produce a proportionally greater diminution of the resisting area. Decreasing the length of a tapered preparation causes a disproportionate decrease in the resisting area.

41. A crown with relatively long axial walls can resist a strong tipping force.

42. If two crowns of unequal length on two preparations of equal length are subjected to identical forces ,the longer crown is more likely to fail because the force on it acts through a longer lever arm.

43. The resistance of this preparation is adequate to prevent the crown from tipping under the applied force. Although the preparation and the applied force in (B) are identical to those in (A) , the crown fails because of the greater height of the restoration .

44. The length of the primary lever arm is the shortest distance from the line of action of the force to the nearest margin. On the short crown the lifting force is small because the primary lever arm is short. With a long crown , the same force produces a greater torque because its line of action passes farther from the point of rotation.

45. Resistance and tooth width A crown on the narrow tooth can have greater resistance to tipping than one on the wider tooth. This occurs because the crown on the narrower tooth has a shorter radius of rotation resulting in a lower tangent line and a larger resisting area.

46. A)Because of the small diameter, the tangent line of this narrow preparation falls low on the wall opposite the axis of rotation, resulting in a large resisting area. B)Because of the much greater radius of the arc of rotation, its resisting area is smaller than that of the narrower preparation.

47. The weak resistance of a short, wide preparation(A) can be enhanced by the addition of vertical grooves(B). From the side(C), it can be seen that the arc of radius r2 is effectively blocked by the resisting area of the groove walls, while the arc of radius r1 encounters little or no resistance on the far axial wall

48. Taper and resistance The resisting area decreases as the preparation taper increases. For a cylinder with no taper, the resisting area would cover half the axial walls For a ideally- tapered tooth preparation the resisting area covers somewhat less than half the axial walls An over-tapered(20 degrees) preparation has only a small resisting area near the occlusal surface .

49. The inclination of the walls of a preparation that will still provide effective resistance is dependent on the height/width ratio. A) A preparation with height/width ratio of 1:1 can have a wall inclination of 15 degrees and still have effective resistance. B) A preparation with a ratio of 1:2 can have a indivisual wall inclination no greater than 7degrees without seriously compromising resistance

50. Rotation around vertical axis When a crown is subjected to an eccentric horizontal force, moments of torque occur around a vertical as well as horizontal axis

51. A partial veneer crown which has no grooves has little resistance to rotation around a vertical axis When grooves are present, their lingual walls provide resistance by blocking the arc of rotation

52. A)The axial symmetry of a full veneer crown preparation may allow rotation of the restoration around the preparation B)Resistance can be gained by forming vertical planes(wings) which are perpendicular to the arc of rotation .

53. PATH OF INSERTION The specific direction in which prosthesis is placed on the abutment teeth(GPT8) The specific direction in which a prosthesis is placed on the residual alveolar ridge, abutment teeth, dental implant abutment(s), or attatchments .(GPT9) Also known as Path of placement.

54. A path must be selected that will allow the margins of the retainers to fit against their respective preparation finish lines with the removal of minimum sound tooth structure.

55. The ideal path of insertion for a posterior full or partial veneer crown is parallel with the long axis of the tooth.

56. A)For a full crown to have structural durability with proper contours ,its path of insertion would parallel the long axis of the tooth.

57. B) If the tooth is tilted C) A path of insertion paralleling the long axis of the tooth may be blocked by the proximal contours of the adjacent teeth. D)In this case , the path of insertion is made perpendicular to the occlusal plane

58. When viewed with one eye from a distance of 30cm or 12 inch ,all the axial surfaces of a preparation with an ideal taper or angle of convergence of 6degees can be seen. All negative taper, or undercut must be eliminated or it will prevent the restoration from seating .

59. Binocular vision should never be employed to evaluate a preparation for correct taper. With both eyes open, a preparation that is undercut can appear to have an acceptable degree of taper

60. A mirror is used to evaluate a preparation where direct vision is not possible. An obstructed view of the entire finish line barely outside the circumference of the occlusal surface indicates correct taper.

61. 3.Structural durability of the restoration It is the ability of the restoration to last long without damage, under occlusal forces. There are three preparation features that contribute to the durability of the restoration: A) occlusal reduction B)axial reduction C)provision for reinforcing struts.

62. Occlusal reduction Enough tooth structure must be removed from the occlusal surface of the preparation so that when the restoration is built back to ideal occlusion it will be thick enough to prevent wearing through or distorting. A gold crown requires approximately 1.5mm clearance over the functional cusps and 1.0mm over the non-functional cusps. If a porcelain veneer is extended onto the occlusal surface, an additional 0.5mm of space is needed.

63. Occlusal reduction should reflect the geometric inclined planes underlying the morphology of the finished crown and follow the major planes of the opposing facial and lingual cusps as well. Avoid creating steep planes with sharp angles, since these can increase stress and hinder complete seating of the casting. To diminish stress, round the angles and avoid deep grooves in the center of the occlusal surface.

64. A)Correct occlusal reduction parallels the major planes of the tooth B)Flat ,single-plane occlusal reduction may result in insufficient thickness of the restoration over the grooves and fossae . C)An attempt to avoid this problem by lowering the entire occlusal table will cause excessive destruction of tooth structure,and the axial walls will be overshortened wth a resultant loss of retention and resistance

65. Functional cusp bevel : A wide bevel should be placed on the functional cusps of posterior teeth to provide structural durability in this critical area.

66. A) A functional cusp bevel in a plane paralleling that of the opposing cusp allows for adequate restoration thickness without undue sacrifice of tooth structure. B)If the functional cusp bevel is omitted, the restoraton is likely to be too thin in this stress bearing area

67. C)If restoration thickness is achieved by overtapering the axial wall, retention will be compromised D)Frequently , in the absence of a functional cusp bevel, the technician will overbulk the crown. This can result in superocclusion of the restoration,which could only be corrected by occlusal reduction of the opposing tooth

68. When teeth are in a crossbite relationship, the functional cusp bevel is placed on the buccal cusps of maxillary teeth and the lingual cusps of mandibular teeth.

69. 2) Axial reduction A)When axial reduction is suffucient , restoration walls can have satisfactory thicknesses without overcontouring . B)If axial reduction is inadequate, a restoration with normal contours would be thin and flexible. C)The more common result of inadequate axial reduction is an overcontoured crown

70. 4.Marginal integrity Margin: The outer edge of a crown,inlay,onlay , or other restoration(GPT9) Finish line: Terminal portion or peripheral extension of the prepared tooth(GPT8) A boundary surface of a tooth preparation is termed the finish line or finish curve.(GPT9)

71. There are 3 requirements for successful restoration margins: a)They must fit as closely as possible against the finish line of the preparation to minimize the width of exposed cement. b)They must have sufficient strength to withstand the forces of mastication and c)Whenever possible they should be located in areas where the dentist can finish and inspect them and the patient can clean them.

72. Margin can be placed by: Supragingival (at or above the gingival crest) Subgingival (below the gingival crest) Margins should be placed supragingivally whenever possible.

73. Supragingival margins: Advantages- Placed on enamel Easy to prepare without trauma to soft tissue Can be easily finished Impression making is also less traumatic to soft tissues and easy to reproduce. Fit of the restorations can be easily evaluated Can be maintained easily by the patient

74. Subgingival margins : Placed at least 2mm above the alveolar crest so that the biologic width is not encroached. Indications: a)caries, erosion or restorations extending subgingivally b)Aesthetics-when metal-ceramic restorations are used. c)Additional retention- in case of short crown d)Root sensitivity e)Proximal contact extending to gingival crest

75. bevel The process of slanting the finish line and curve of a tooth preparation.(GPT9) Roughness of the tooth surface under margins can prevent close adaptaion , therefore all bevels and flares should be given a smooth finish line. Metal margins should be acute in cross-section rather than right-angled to facilitate a closer fit.

76. A bevel allows closer approximation of crown margin to the tooth. The distance,D by which a crown fails to seat( A ), is reflected in the marginal opening,d ,in the butt joint( B ). However, in the presence of a bevel , the shortest distance from the margin to tooth structure is less than D. And is a function of the sine of the acute angle of the margin,m or the cosine of the obtuse angle of the finish line,p ( C )

77. It is explained by the formula d=D sin m, or d=D cos p. As the angle m is reduced its sine becomes smaller and so does d. The more obtuse the angle of tooth structure at a horizontal finish line, and therefore, the more acute the restoration margin, the shorter the distance between the restoration margin and the tooth . d =the shortest distance from the casting margin to tooth structure D=the distance by which the crown fails to seat .

78. The smaller the angle between the prepared tooth surface at the finish line and the path of insertion ,the less the marginal opening for the same amount of incomplete seating The margin angle must become quite acute before the actual distance is diminished to a great extent. An angle of 30 to 45 degrees is considered optimal. If it is made much more acute it becomes weak .

79. To impart strength and rigidity to a margin , there should be a nearby bulk of metal. This can be provided by an: occlusal offset or shoulder an isthmus a vertical groove a box or a gingival shoulder.

80. Occlusal bevels The cavosurface line angles of the preparation in the occlusal portion of the tooth should be finished with a bevel to avoid a right angle “butt joint” which would leave a brittle, easily fractured edge of tooth structure. ( left) Impacts striking enamel next to a bevel can be withstood without damage. (right) Where there is no bevel, the unprotected enamel rods near the restoration can be more easily fractured

81. Flares The vertical finish line of the inlay, onlay or partial veneer crown preparation is finished with a flare, which forms an acute edge of metal in the casting and extends it into an accessible area. A flare differs from a bevel in that a flare is a geometric plane inclined slightly to the path of insertion and cutting through the contour of the tooth whereas bevel follows the contour of the tooth.

82. Finish line configurations

83. 1.chamfer It is an obtuse angled finish line. It is distinct. Exhibits least stress. Most conservative. Indications: Cast metal restorations, lingual margins of metal ceramic crowns. Round end tapering diamond.

84. 2.Heavy chamfer Similar to chamfer but prepared with a diamond of greater diameter Indicated for all ceramic crowns Distinct margin, adequate bulk, easy to control Can produce an unsupported lip of enamel.

85. 3.Shoulder It is a right angled finish line. Produce a wide ledge which resists compressive occlusal forces. Not conservative Indicated for ceramic restorations- all ceramic and metal ceramic Flat end tapering diamond.

86. a.Shoulder with bevel b.facial margin hidden in metal ceramic restoration 4.Shoulder with bevel Protects the edge of finish line preventing chipping Recommended for extremely short axial walls to create maximum retention and resistance form. Primarily indicated to hide the supragingival facial metal margin of metal-ceramic restorations.

87. 5.Radial shoulder It is a shoulder finish line with rounded line angle. All ceramic crowns

88. 6.Sloped shoulder Similar to shoulder but with obtuse angle Indicated for facial margin of metal ceramic crown

89. 7.Knife edge It is a thin finish line Advantages :Highly conservative Disadvantages: Not distinct May be used for metal restoration.

90. 5.Preservation of the periodontium Several studies have determined that the deeper the restoration margin extends into the gingival sulcus , the more severe the inflammatory response. A crown margin should not be placed any closer than 2.0 mm away from the alveolar crest, or bone resorption will occur.

91. The distance from the epithelial attatchment to the crest of the alveolar bone has been described as biologic width . It is normally about 2mm width, including the epithelial attatchment and connective tissue attatchment

92. When the margin of a restoration intrudes into the biologic width, inflammation and osteoclastic activity are stimulated

93. Bone resorption will continue until the alveolar crest is at least 2.0mm from the restoration margin. This can result in an infrabony pocket.

94. CONCLUSION Biomechanics is of prime consideration in achieving an aesthetic, functional and durable fixed prosthesis so every dentist should have a thorough knowledge about the boimechanics of tooth preparation as every step in this plays a major role in the fixed prosthesis success.

95. REFERENCES Fundamentals of tooth preparations- Shillingburg /Jacobi/Brackett Contemporary Fixed Prosthodontics-3 rd edition Rosenstiel -Land-Fujimoto Textbook of Prosthodontics - V Rangarajan -TV Padmanabhan

96. Thank you