Tooth Preparation in FIXED PARTIAL DENTURE

Tooth Preparation in FIXED PARTIAL DENTURE FOR BDS STUDENTS
ADAL MEDICAL UNIVERSITY

1. Tooth Preparation in FPD Dr Anmol Asghar Ali BDS, PG Oral Implantol ADA Certified PG DIP:Orthodontics & Dentofacial Orthopadics USC Spain

2. We Will Study in this chapter Principles of Tooth Preparation Preparations for Full Veneer Crowns Preparation for Partial Veneer Crowns

3. PRINCIPLES OF TOOTH PREPARATION The basic principles on which tooth preparation is done are: • Preservation of tooth structure • Retention and resistance • Structural durability • Marginal integrity • Preservation of periodontium

4. Preservation of Tooth Structure “Muller De Van stated that the perpetual preservation of what remains is more important than the meticulous replacement of what is lost”. Care should be taken to prevent excessive tooth preparation.

5. Retention and Resistance Forms “the ability of the preparation to prevent displacement of the restoration in a direction opposite to the path of insertion.” One of the major sources of retention in a fixed partial denture is tensofriction . Retention can be of classified as follows : Primary Retention Secondary Retention

6. Primary retention —Sleeve retention provided by the opposing vertical surfaces of the tooth preparation. —Wedge type retention seen in intracoronal restorations.

7. Secondary retention: Retention obtained by retentive features like pins, boxes and grooves , etc. is known as secondary retention.

8. Resistance Resistance is the ability of the prosthesis to resist displacement by forces directed in an apical or oblique direction. This prevents any movement of the restoration under occlusal forces.

9. Retention Retention and resistance are reciprocative to one another; hence, an optimal balance between these factors should be established during any preparation .

10. The following features in a preparation should be designed to strike the balance between retention and resistance forms.

11. Taper The degree of taper is inversely proportional to the retention form. Zero degree taper is the most retentive but it is almost impossible to obtain. The sum of the degree of taper is called as degree of convergence. For optimum retention, 4°-10° convergence is sufficient

12. Cylinders with minimal taper produce excellent frictional resistance

13. Lingually tilted premolars should be prepared for a lingual path of insertion and not a vertical path of insertion

14. When a tapering fissure diamond is held parallel to the long axis of the tooth during preparation, the necessary taper (3°) is automatically produced

15. Freedom of Displacement Limiting the freedom of displacement from torquing and twisting forces aid to increase the resistance of the restoration

16. Length Greater the height of the crown, better the retention of the restoration. The length of crown improves retention in two ways : First, the height of the prepared tooth should be greater than the tipping arc of displacement. Second, increase in height increases the area of cementation there by improving retention

17. The height of the preparation (a) should be sufficient to produce interference to the arc of displacement (b

18. Substitution of Internal Features Internal features like proximal grooves, boxes and retention pinholes can be incorporated in the preparation to improve retention .

19. Indications Substitution of internal features should be done for the following cases where retention is difficult to achieve. • Over tapered preparations. • Partial veneer crowns. • Absence of two opposing walls (due to caries, etc.). Retention can be improved in faulty preparations by the placement of internal features, which are accurate and easy to prepare.

20. Path of Insertion It is an imaginary line along which the restoration will be placed onto or removed from the preparation.” It should be accurately determined using a surveyor as minor undercuts in the preparation tend to be hidden by the human binocular vision.

21. The path of insertion of the preparation should be parallel to the adjacent teeth

22. If both eyes are open when the preparation is viewed undercuts may remain undetected

23. Intraorally the preparation can be evaluated/surveyed by viewing the preparation on a mouth mirror using one eye, 30 cm (12 inches) away from the preparation. Intraorally, preparations are viewed through a mouth mirror using one eye

24. Structural Durability Durability comes with the thickness of the restoration . A restoration should contain sufficient bulk to withstand forces. The ability of the restoration to withstand destruction due to external forces is known as structural durability.

25. Occlusal Reduction Occlusal strength is the most vital as most of the forces affecting the restoration, act directly on the occlusal surface. Inadequate clearance may lead to a weaker restoration, which is prone to fracture or perforatio .

26. The amount of occlusal reduction required for commonly used materials is: Gold alloys require 1.5 mm clearance for the functional cusp and 1.0 mm clearance in the nonfunctional cusp. Metal ceramic restorations require 1.5 to 2.0 mm reduction in the functional cusp (2 mm if it is to be veneered with porcelain) and 1.0 to 1.5 mm reduction in the nonfunctional cusp. All ceramic restorations require a minimum of 2 mm reduction throughout.

27. The functional cusp bevel is an integral part of occlusal reduction

28. Axial Reduction Adequate axial reduction is necessary for structural durability. Inadequate axial reductio n may lead to over-contoured proximal surfaces , which can lead to periodontal problems Inadequate axial reduction will require over-contoured restorations

29. Marginal Integrity Marginal adaptation and the seating of the restoration affect marginal integrity . Poor marginal adaptation will lead to percolation of oral fluids (marginal leakage`) and secondary caries .

30. Sub-gingival margins may be required for certain restorations Sub gingival finish line

31. The indications for a sub-gingival margin are: For teeth with short clinical crowns. • Teeth affected by sub-gingival caries or cervical erosion where crown lengthening cannot be performed. • If the contact area is present at or below the gingival crest. • Where aesthetics is of concern (e.g. gummy smile). • For cases with unmanageable root sensitivity . • When the axial contours should be modified • When additional retention is required.

32. Finish Line Configurations A finish line should have the following characteristics: Shallow bevels nearly parallel to the cavosurface should be avoided because the restoration will be too thin at this area and may chip easily. The discrepancy decreases with the increase in angulation of the bevel The bevel should not produce a very acute margin, which can lead to fracture of the wax pattern during removal

33. Shallow cavosurface bevels may lead to chipping of the restoration

34. Chamfer This finish line possesses a curved slope from the axial wall till the margin It can be produced using a torpedo diamond point. The same diamond point when used to reduce more tooth structure will form a deep chamfer finish line. It is the finish line of choice for cast metal restorations

35. Chamfer finish line

36. Shoulder This finish line has a gingival finish wall perpendicular to the axial surfaces of the teeth If the marginal wall is at 120 degree to the axial walls, then it is termed a sloping shoulder.

37. Generally, a shoulder finish line is preferred for all ceramic restorations where sufficient thickness of the margin is required for structural durability. All anterior restorations are fabricated with a shoulder margin where aesthetics is the primary concern.

38. Shoulder finish line Sloping shoulder finish line

39. Radial Shoulder It is done with flat end tapered diamond and end cutting parallel side carbide-finishing bur Radial shoulder finish line

40. Shoulder with a Bevel It is similar to a shoulder finish line, but an external bevel is created on the gingival margin of the finish line. Shoulder with bevel finish line

41. Unbevelled preparations have an even perpendicular marginal discrepancy because the angle of the margin (θ) is around 90°

42. In beveled preparations, the marginal discrepancy (i.e. the effective distance between the tooth and the restoration) decreases from D1 to D2 because the angle of the margin (θ) is reduced. Theoretically, if θ decreases to 0°, the marginal discrepancy will also decrease to zero

43. Shoulder less or Feather Edge Preparation It is similar to a knife-edge preparation but is marginally thinner It has the same disadvantages as knife-edge preparations. Feather edge preparation

44. Knife-edge It is an extremely thin finish line. It is similar to a sloping shoulder with a very thin margin Knife edge preparation

45. Chisel-edge preparations are similar to feather-edge margins but it is marginally thicker.

46. Preservation of Periodontium The finish line should be atleast 3 mm away from the alveolar crest to preserve the periodontium

47. Instrumentation

48. The following instruments should be used during tooth preparation • Hand chisels can be used to accentuate the facial and lingual walls of proximal boxes • Torpedo diamonds followed by torpedo carbide burs can be used to prepare a chamfer finish line. • Flat end tapered diamonds followed by H158 carbide-finishing bur can be used for radial shoulders. • Flame diamonds followed by flame finishing burs can be used for gingival bevels and conservative proximal flares. • Abrasive paper discs can be used to smoothen the finish lines of vertical flares.

49. It should be used alongwith a rubber dam to avoid damage to soft tissues. • Grooves, boxes, offsets and isthmus can be smoothened with No: 169L, 170C and 171L non-dentate tapered burs. • Cross cut or dentate burs are used to remove old restorations but they produce horizontal ridges on the tooth surface. Hence, they cannot be used for planing tooth surfaces. • Small diamond points with air-water spray in a high-speed hand piece can precisely remove tooth structure and also produce a smooth finish

50. PREPARATIONS FOR FULL VENEER CROWNS Indications It is indicated when: • The abutment tooth is small • The edentulous span is long • When the partial veneer crown lacks in retention, resistance, coverage or esthetics • When the abutment is extensively decayed or decalcified or previously restored • For endodontically treated teeth.

51. Contraindications A full crown is not given for patients with uncontrolled caries

52. Procedure The steps to be followed for preparing a posterior tooth are as follows: • Occlusal reduction • Axial reduction —Buccal reduction —Lingual reduction —Proximal reduction • Establishing the Finish lines The above-mentioned procedures vary accor ding to the choice of material for the restoration.

53. All Metal Full Veneer Crowns It is usually given for posterior teeth. Armamentarium • Handpiece • No: 171L bur • Round-end tapered diamond • Short needle diamond • Torpedo bur • Red utility wax Note: The term “bur” is usually used to denote a (TC) Tungsten Carbide rotary cutting instrument. Diamond rotary cutting instruments are called “Diamond points” or “Diamonds”.

54. Occlusal Reduction • The principle of occlusal reduction is to achieve an occlusal clearance of 1.5 mm for the functional cusps and 1.0 mm for the non functional cusps.

55. Fractured or malposed teeth may require lesser tooth reduction. • Depth orientation grooves should be placed on the occlusal surface to act as a reference guide for the required amount of occlusal reduction Depth orientation grooves placed along the cuspal inclines to guide reduction

56. A functional cusp bevel is placed using a round-end tapered diamond, on —The buccal inclines of mandibular buccal cusps —The palatal inclines of maxillary palatal cusps (a) Functional cusp bevel of the mandibular buccal cusps (b) Functional cusp bevel of the maxillary palatal cusps

57. The angulation of the bevel should be parallel to the inner inclination of the opposing cusps The occlusal clearance is verified by asking the patient to close on a 2.0 mm thick red utility wax. Insufficient clearance can be detected as a thin spot on the wax and it is reduced accordingly until there are no spots on the wax

58. Occlusal clearance is evaluated using red utility wax

59. Axial Reduction The buccal and lingual walls are reduced with a torpedo diamond as it has the advantage of forming the chamfer finish line alongwith the axial reduction thereby avoiding the need of changing the diamonds for establishing the finish line

60. The chamfer finish line provides adequate bulk of metal along with a good adaptation. Completed preparation with an axial seating groove • The seating groove will prevent the rotation of the crown during cementation and acts as a guide during insertion of the crown. • Buccal and lingual grooves can be placed for preparations involving long span fixed partial dentures . Features • Planar occlusal reduction: • Axial reduction: • The proximal reduction is started with a short needle diamond. This diamond point is used in a sawing motion to break the contact point . The sawing motion can be done in an occluso gingival or bucco -lingual direction. Care should be taken to avoid damage to the adjacent teeth Proximal reduction should be started by carefully breaking the contact using a thin tapering diamond point

61. Seating Groove A seating groove is made on the buccal surface of mandibular teeth and on the palatal surface of maxillary teeth. The seating groove will prevent the rotation of the crown during cementation and acts as a guide during insertion of the crown.

62. Metal-Ceramic Full Veneer Crowns It is also known as Porcelain-Fused-to-Metal restoration . It has a thin metal coping with a facial ceramic layer. This crown has the strength and accurate fit of a metal crown and esthetics of a ceramic crown. Metal ceramic crowns are stronger than all ceramic crowns and can be used as a fixed partial denture retainer.

63. For ceramic veneering, a cutback design is provided to on the facial surface of the metal coping in order to provide space for the ceramic layer and the lingual reduction is similar to a full metal crown Features of an anterior metal-ceramic preparation

64. Labial Reduction Depth-orientation grooves are placed on the labial and incisal surfaces using a flat-end tapered diamond point. The depth of reduction can be measured using the uncut outer surface of the remaining tooth structure as a guide . The labial grooves should be atleast 1.2 mm in depth to provide adequate thickness of ceramic . The labial grooves are prepared as two sets in order to obtain a two-plane reduction

65. One set should be parallel with the gingi val half of the labial surface Other set should be parallel with the incisal half of the labial surface Placing depth orientation grooves in two planes to produce a two plane labial surface

66. Completed two plane labial reduction If the reduction is done following the gingival plane, it will result in a protruded incisal edge leading to an over-contoured restoration or poor aesthetics . —If the reduction is done following the incisal plane, the labial surface will be over tapered and will be in close proximity to the pulp.

67. (labial reduction is 1.2-2.0 mm deep and the lingual surface is 0.7-1.0 mm deep), is described as a wing preparation

68. Incisal Reduction After the labial reduction, incisal grooves ( 2.0 mm deep) should be made across the incisal edge. The bur should be held at a direction perpendicular to the incisal half of the labial reduction

69. Lingual Reduction The cingulum should be reduced with a small wheel diamond to obtain a minimum clearance of 0.7 mm with the opposing teeth . 1.0 mm clearance is needed if a ceramic veneer is given.

70. Proximal Reduction A long needle diamond point is used to break the contact and gain access into the proximal region without damaging the adjacent teeth . The axial surfaces and the chamfer finish line are smoothened with a torpedo bur.

71. An H158-012 radial fissure bur is used to smoothen the labial surface. All the angles and edges are rounded with the sides of the bur and the end of the bur is used to form a radial shoulder finish line. Axial shoulder finishing is done using Radial fissure bur

72. Preparation of Posterior Metal Ceramic Full Veneer Crowns It is used as an aesthetic replacement of missing teeth in the appearance zone namely maxillary premolars, maxillary first molars and mandibular first premolars.

73. Disadvantages Additional tooth reduction is essential to provide adequate thickness for ceramic facing More expensive than full metal crowns Increased risk of failure as the ceramic veneer may fracture easily.

74. Occlusal Reduction Depth orientation grooves are made with a round end tapered diamond point along the cuspal inclines Depth orientation grooves placed along the cuspal inclines

75. Planar occlusal reduction done using; Round-end tapered diamond and no. 17IL bur

76. There should be at least 1.5 to 2.0 mm reduction for ceramic coverage. The reduction is completed by joining the strips of tooth structure left between the dept orientation grooves. The reduction should follow the occlusal morphology

77. A functional cusp bevel is prepared on the palatal inclines of maxillary palatal cusps and on the buccal inclines of mandibular buccal cusps The bevel should be 1.5 mm deep for metal coverage and 2.0 mm in deep for metal with ceramic veneer.

78. Facial Reduction It is also done in two planes similar to anterior teeth . One plane should be parallel to the cuspal inclines and the other plane should be parallel to the long axis of the tooth

79. Three vertical depth orientation grooves should be made on the occlusal third of the facial surface. These grooves should fade out gingivally Depth orientation grooves on the occlusal half of the buccal surface

80. The facial reduction should be atleast 1.2 m for base metal ceramic crown and 1.4 mm for noble metal ceramic crowns to have an ideal contour and adequate opacity.

81. Proximal Reduction The bur can be moved either in an up-down motion on the facial aspect of the inter proximal region or in a facio -lingual motion on the occlusal aspect of the inter-proximal region Breaking the proximal contact using a thin tapering diamond (a) Using up down motion (b) Using horizontal sawing motion

82. Lingual Reduction It is done with a torpedo diamond. Tooth reduction is done on the lingual and proximal walls to attain a chamfer finish line. The preparation to be covered with metal is smoothened with a torpedo carbide-finishing bur. A shoulder with a gingival bevel may be given. A narrow bevel about 0.3 mm wide is made on the shoulder with the tip of a flame-shaped diamond. The bevel should be as narrow as possible and it is finished with an H48L-010 flame-shaped carbide-finishing bur

83. Shoulder with gingival bevel done using a Flame diamond and finishing bur