Principles of preparation for cast restorations

Preparation features, modifications

1. PRINCIPLES OF PREPARATION FOR CAST RESTORATIONS DEEPTHI P.R. Ii year mds

2. CONTENTS Introduction Preparation path Apico occlusal taper of a preparation Preparation features of the circumferential tie Mechanical problems for cast restorations and preparation design solutions in general

3. INTRODUCTION Cast alloys & ceramics: intracoronal & extracoronal Intracoronal : ‘ Mortise’ shaped, definite walls & floors joined at line angles, point angles Extracoronal : Occlusal and axial surface reduction, ending gingivally with no definite flat floor General principles of tooth preparation- applicable Greater surface extension in outline form than amalgam  support for tooth structure  excellent marginal manipulation  compensates for cariously weak joints : self- cleansable & /or protection

4. PREPARATION PATH Single insertion path: opposite to the direction of occlusal loading- function will seat restoration rather than displace it Tooth reduction: oriented in one path, the path of withdrawal & insertion of the future wax pattern & restoration Parallel to the long axis of the tooth crown Enhanced retention & reduced micro movements during function

5. APICO-OCCLUSAL TAPER OF A PREPARATION Opposing walls & axial surfaces: perfectly parallel to each other- - maximum retention - materials inserted & withdrawn from preparation - permanent deformation: impractical technique/ misfitting restorations Exact parallelism: technical problems Slight divergence of opposing walls intracoronally & slight convergence of opposing axial surfaces extracoronally

6. APICO-OCCLUSAL TAPER OF A PREPARATION 2 to 5 degrees from the path of preparation – increased or decreased: 1. L ength of the preparation wall and/or axial surfaces The greater the wall length is, the more taper will b necessary: not> 10 degrees The less the wall length is, the less the taper will be, approaching 0 degrees: but should not be less in extremely short walls 2. D imensions and details of surface involvement and internal anatomy in the preparation The greater the surface involved The more detailed the internal anatomy Taper increased: not > 10 degrees- to diminish friction Frictional component between the preparation & the contacting materials: distortion in materials used

7. APICO-OCCLUSAL TAPER OF A PREPARATION 3. The need for retention The greater the need: the lesser the taper- approach parallelism Greater occlusal reduction and rounded internal & external anatomy for non noble alloys & cast ceramics - less taper to compensate for loss of retention Taper: equally at the expense of two opposing walls/ axial surfaces- one path of insertion Solely at the expense of one side only: opposing side absolutely parallel to insertion path

8. APICO-OCCLUSAL TAPER OF A PREPARATION Preparation should not be made with one side having more taper than the other: More than one path of insertion Micromovements of the final restoration If cariogenic & anatomical conditions dictate two different tapers for opposing walls: preferable to create two planes- inner & outer Inner planes: parallel to each other or of equal taper- assure single path of insertion Outer planes: satisfy the needs compelling different tapers

9. APICO-OCCLUSAL TAPER OF A PREPARATION Factors determining the choice of design Cariogenic & anatomical considerations Different lengths of opposing parts of the preparation Need for reciprocal retention Presence of more surrounding walls/ axial surfaces on one side than the other

10. PREPARATION FEATURES OF THE CIRCUMFERENTIAL TIE Tooth/ cement/ cast joint complex : weakest link Peripheral marginal anatomy: Circumferential tie Marginal peripheries: designed for the most favorable relationship between the restoring casting and the luting cement Margin ends on enamel: Fulfill Noy’s requirements for ideal cavity wall - enamel must be supported by sound dentin - enamel rods forming the cavosurface margin should be continuous with sound dentin - enamel rods forming the cavosurface margin should be covered with the restorative material - angular cavosurface angles should be trimmed

11. PREPARATION FEATURES OF THE CIRCUMFERENTIAL TIE: Intracoronal BEVEL A plane of a cavity wall or floor directed away from the cavity preparation Six types according to the shape and tissue involvement A. Partial bevel B. Short bevel C. Long bevel D. Full bevel E. Counterbevel F. Hollow ground / Concave bevel

12. BEVELS PARTIAL BEVEL Part of the enamel wall not > 2/3 rd of its dimension Not used usually Trims weak enamel rods from margin peripheries SHORT BEVEL The entire enamel wall, but not dentin Used mostly with Class I alloys especially for type 1 and 2

13. BEVELS LONG BEVEL Enamel wall and upto one half of the dentinal wall Most frequently used for the first 3 classes of materials Preserves the internal “boxed up” retention and resistance features FULL BEVEL All of the enamel and dentinal walls of the cavity wall or floor Well reproduced by all 4 classes of alloys Deprives the preparation of the internal resistance & retention Used only when any other bevel is impossible to b used

14. BEVELS COUNTERBEVEL Capping cusps to protect and support them Bevel used opposite to a an axial wall- on the facial or lingual surface Gingival inclination facially or lingually HOLLOW GROUND/ CONCAVE BEVEL Any bevel especially long, full or counter bevels when prepared in a concave form More space for cast material: material’s castability, retention and better resistance to stresses Ideal for Class IV and V

15. BEVELS- Functions Bevel portion: specific angulation relative to the remaining portion of the wall/ floor, the long axis of the crowns or a specific landmark Extend to certain limits Create obtuse- angled marginal tooth structure: bulkiest & strongest configuration Acute angled marginal cast alloy substance: most amenable to burnishing Eliminates the cement line by bringing the cast alloy closer to tooth structure

16. BEVELS- Functions Only circumferential tie with possibility of a direct retentive frictional component between the casting & tooth Resistance form of tooth- restoration complex by encompassing cusps Compensate for problems in the castability of alloys- produce better details for retention “ Flexible extensions” of a cavity preparation, allowing the inclusion of surface defects, supplementary grooves, or other areas on the tooth surface. Minimum tooth involvement and no sacrifice of the resistance & retention Gingival bevels: Gingival margins to cleansable or protected areas Facial & lingual proximal walls:

17. FLARES Flat or concave peripheral portions of the facial and lingual walls Facial and lingual proximal walls in intracoronal cavity preparations for castings 2 types: Primary & Secondary PRIMARY FLARE : The conventional and basic part of the circumferential tie facially & lingually for an intracoronal preparation Similar to long bevel Specific angulation: 45 to the inner dentinal wall proper Hollow ground: cast ceramics/ non noble alloys

18. FLARES Functions: same as bevels The most peripheral part of proximal preparation: if they fulfill the objectives of a circumferential tie SECONDARY FLARE A flat plane superimposed peripherally to a primary flare Sometimes, prepared in hollow ground form to accommodate materials with low castability Solely in enamel, with some dentin in all or parts of its surfaces Different angulations, involvement & extent, depending on function

19. FLARES: Functions & Indications - Bucco - lingual w ide extensions- primary flare ending with acute angled margins Secondary flare at correct angulation- the needed obtuse angle of marginal tooth structure - Broad/ malposed contact areas Finishable cleansable areas without changing the fixed angulation of the primary flare - Ovoid teeth: peripheral marginal undercuts occlusoapically on the facial &/or lingual peripheries of a cavity Secondary flare at correct angulation-eliminate these undercuts with minimal sacrifice of tooth structure - S urface defects/ decalcifications: involved in preparation without extending the primary flare

20. CIRCUMFERENTIAL TIE: Extracoronal CHAMFER FINISHING LINE Class I, II, III materials Bulk & definite termination for the preparation marginally, with little tooth involvement – 0.5mm maximal depth Placed gingivally on any involved axial surface: if finishable - cleansable / protected Subgingival extracoronal preparations if gingiva floor considerations are precluded Contraindicated : Class IV & V materials Disadvantages Liability of transitional continuation of a circumferentia tie with adjacent bevel ties Limited burnishability of marginal cast alloy

21. CIRCUMFERENTIAL TIE: Extracoronal KNIFE EDGE/ FEATHER EDGE FINISHING LINE: The least tooth structure involvement Involves enamel only: when the margin is on enamel Very castable - burnishable type of alloy- Type II Gold alloy Accessible areas of tooth surface for proper finishing Minimal axial depth required for biologic / anatomic purposes Easy & efficient blending with beveled constituents

22. CIRCUMFERENTIAL TIE: Extracoronal Disadvantages : Indefinite termination for the casting Margins not being covered with a casting: less wetting alloys Risk of fracture: burnishing-finishing-polishing Definite contraindication: Class III, IV, V materials Thin cross- section Ease of over- strain- hardening

23. CIRCUMFERENTIAL TIE: Extracoronal BEVELLED SHOULDER FINISHING LINE: The most tooth structure involvement Definite gingival floor: wall proper + bevel Maximum bulk of the cast is needed marginally for materials that are limited in their castability & burnishability Easy blending with the bevelled constituents Maximum reduction of marginal problems of internal spacing Ideal for subgingivally located margins- maximum predictability of the casting termination gingivally

24. CIRCUMFERENTIAL TIE: Extracoronal HOLLOW GROUND/ CONCAVE BEVEL: Exaggerated chamfer or a concave beveled shoulder Tooth involvement : Chamfer< Hollow ground < Bevelled shoulder Termination: not as predictable, but mechanically comparable to a beveled shoulder S uperior to chamfer Care: no residual frail enamel/ thinned tooth structure

25. CIRCUMFERENTIAL TIE: Extracoronal Advantages: Good transitional continuity with the beveled portion of the circumferential tie Helps the casting to seat preferentially Aids in stabilizing the casting Ideal for Class IV & V cast materials Used successfully for materials with limited castability

26. CIRCUMFERENTIAL TIE: Extracoronal Functions similar to bevels & gingival floors Minimize symptoms of internal spacing marginally: chamfer & hollow ground Combinations used: cariogenic & anatomic needs; castability & finishability of restorative material Avoid sharp angles/ interruptions at the junction of the gingival & occlusal corners of the tie Round the junctions at the level of the circumferential tie, not internally Avoid stress concentration Non coverage of the margin by the casting Possible minor undercuts

27. CIRCUMFERENTIAL TIE: Extracoronal Maximal depth at the junction of the tie with the wall/ floor/ axial surface  Complete reproduction and coverage of the details in the casting Best resist stresses No element in occlusal contact Peripheral margin: linear - paralleling the periphery/ curvature of anatomy Smoothest & continuous: impression materials, die materials, waxes, investment, alloy & ceramic melts can wet details- precise reproduction

28. AUXILLARY MEANS OF RETENTION Compound & complex tooth involvement Formability of cast restorations: myriad retention & resistance means

29. LUTING CEMENTS Locking the cast to tooth structure Auxillary : can’t substitute frictional retention Susceptible to dissolution & weaker than casting or tooth structure The less the cement thickness- the less the possibility of clinically recognizable failure at tooth- cement-casting interface Fill the space between them Fill the vacancies or irregularities Wet the details of both casting & tooth preparation

30. GROOVES Completely in dentin Internal grooves : as internal as possible, adjacent to the axial wall Prevents lateral displacement of mesial, distal, facial &/or lingual parts Stepped to preserve anatomical landmarks, increase locking capabilities Indicated: dimensions are limited & ocking mechanism needed proximally Dentinal portion of the facial/ lingual walls Gingival floors proximally Mesial or distal walls Gingival floors of the facial or lingual portion

31. GROOVES Contraindicated : - impinge on the pulp chamber or root canal system - undermine/ involve axial angle of the tooth - undermine adjacent enamel External grooves: Preparations lacking retention Short/ severe taper/ extreme width Placed in areas of sufficient dentin bulk Periphery of surface extension Cut in a step form inwards or outwards- increases locking

32. GROOVES Reciprocal means of retention : improves seating of the restoration, minimize marginal discrepancies Standardized tapered fissure bur: No. 168/ 699 Maximum 2mm depth- at the expense of the side walls or axial surfaces Accentuated: Wedelstaedt/ Hatchet All types of cast materials: ceramics- no definite internal line angles

33. REVERSE BEVEL Expense of the gingival floor- internal dentinal plane inclining gingivally - axially Locks the restoration & prevents proximal displacement Flat dentinal transition with the gingival bevel Gingival floor with sufficient dimension without decreasing the resistance Gingival marginal trimmers Class I, II, III materials- rarely class IV

34. INTERNAL BOX One of the most efficient immobilizing retention resistance means In dentin 4 vertical surrounding walls joining the floor at definite line and point angles Next to a marginal ridge with intact proximal wall as one of the walls Anywhere in the preparation floor with sufficient dentin bulk Peripheries : ideal location for even immobilization Reciprocated at the opposite ends of the preparation Shallow cavities/ short preparations- dovetail cannot be prepared

35. INTERNAL BOX Minimum 2mm in three dimensions, but not equal length, width, depth Stepped apically or occlusally with different levels in th epulpal floor No. 168/ 699 Surrounding walls shaved to be parallel & have definite angles: Hatchet, Monangle chisel, angle former Contraindicated: Class IV & V materials

36. EXTERNAL BOX Box shaped preparations opening to the axial tooth surface 3,4,5 surrounding walls & floors- proximal, facial or lingual Stepped occlusally or gingivally or not stepped Accommodate grooves in their surrounding walls Peripheral portions: beveled or flared Armamentaria: similar to internal box Contraindicated for cast ceramics

37. SLOT Internal cavity- floor of a preparation Continuous surrounding wall – non defined angles & floor, junction between floor and walls very rounded Less locking, but more readily reproduced in casting Indication: same as internal box Suitable size round bur & tapered fissure bur 2-3 mm depth – compatible dimensions with the size of restoration All five classes of materials

38. PINS Cemented & threaded/ Parallel & non- parallel/ Vertical & horizontal/ Cast & wrought Used for Pinlay design of cast restoration I. According to the mode of incorporating in a casting A. Wrought pins: Cast on/ Soldered. Threaded in B. Cast pins II. According to their relationship to the ong axis of the tooth A. Vertical B. Horizontal

39. COLLAR, SKIRTS, CAPS, POSTS COLLAR : Surface extension completely surrounding a cusp or a surface of a tooth SKIRT : A Specific extension involving a part of the axial angle of a tooth Part of surface extension preparation designs CAPPING OF CUSPS: Very efficient if the cusp is not shortened to the extent that it will not effective in locking the restoration Hollow- ground bevel/ hollow- ground bevel collar CAPPING OF MARGINAL RIDGES: Finishable cleansable finish line created Intact marginal ridge, sound adjacent proximal surface & pronounced occlusal embrasure Collars, skirts, caps: all classes of materials with modifications

40. CAPPING OF THE OCCLUSO-PROXIMO-FACIAL OR LINGUAL CORNERS , POSTS Protects thinned corners due to over preparation or wide preparation of ovoid teeth Adds to retention by locking the restoration A bevel extension added facially or lingually at the corners- added to the preparation in a knife edge or chamfer finish line Filamentous diamond stone Contraindicated: Class IV & V materials POSTS : not used as retention means for final cast restoration Used for cast core foundation or substructure restoration

41. GROSSLY ROUGHENING, IRREGULARIZING OR MULTIPLE LEVELLING THE SURFACE Pulpal floor or occlusal surface: Preserving present irregularities or creating intentional irregularities More retention & lateral locking Adequate dimension- leaving bulky tooth structure between them for resistance Smooth surfaced- with no undercuts, no frail or undermined enamel Flat, dished up or sloped gingival or pulpal floors: different levels created Fissure burs, chisels All types of cast materials

42. RECIPROCAL RETENTION Placing retention modes at every end of the preparation or parts of the preparation: Basic principle Every retention mode – opposite retention mode for complete immobilization of the restoration Otherwise: Substantial stresses in the locked side, especially at the interface Opposite grooves or internal box to oppose an internal box/ dovetail to oppose a proximal external box etc.

43. PRECEMENTATION GROOVING OF THE CASTING & THE ADJACENT TOOTH SURFACE/ WALLS An inverted truncated cone groove cut on one or more sides of the casting & similar groove cut opposite it on the preparation Sufficient dimension: bulky amount of cementing agent Sufficient amount of cement mixed & flown into the grooves when cementing the casting Enhances the retention provided by the cement Class V cavity preparations: little frictional retention form

44. ELECTROLYTIC ETCHING OF TOOTH SURFACE OF THE CASTING Internal/ tooth surface of Class IV cast material : electrolytic etching Margins & external surface : covered with sticky wax Electrolytic cell: 0.5 N HNO 3 ,Anodic electrode of the casting, Cathode of another non- noble alloy higher in the electromotive force table Low voltage current passed – selective etching of the casting, dissolution of grain boundaries & less noble elements Microscopic irregularities similar to acid etching of enamel 10- 15 minutes

45. ELECTROLYTIC ETCHING OF TOOTH SURFACE OF THE CASTING Suitable cementing agent – wets irregularities on both casting & tooth surface increase retention Principal means of retention if microscopic retaining irregularities on the alloy reciprocated to similar ones on the tooth Tooth preparation: confined to enamel Several techniques use the phenomenon as a sole retaining device for fixed prosthesis & cast restorations based on few principles: * The luting bonding agent is always a composite resin * Tooth should have adequate enamel thickness to be etched by phosphoric acid * Class IV cast alloy- for better selective etching

46. ELECTROLYTIC ETCHING OF TOOTH SURFACE OF THE CASTING * Minimal tooth preparation- requiring only enough reduction to accommodate the metal, with no attempt to create mortise, cone or box- sha p ed preparations * Cast alloy should be in contact with the maximum surface area of available enamel * If the casting is an attachment for a fixed prosthesis, the pontic should be an indexing tooth for lateral movement of the mandible Failures: cohesive in the luting agent, so a minimum thickness should be used

47. ELECTROLYTIC ETCHING OF TOOTH SURFACE OF THE CASTING - The tooth surface is reduced to allow 1- 1.1 mm cast thickness - Casting is fabricated in the conventional way - The casting is tried in the mouth - The internal (tooth) surface of the casting is etched - The involved enamel surface is etched - The composite is flowed into both the enamel and the casting and allowed to cue under pressure - Excess composite is flaked off - The exposed surface of the composite resin is polished Similar procedure with cast ceramic restorations: etching with HF 5-10% for 15 minutes

48. OTHER PREPARATION FEATURES TO MINIMISE MECHANICAL PROBLEMS All line and point angles should be definite, but not angular - easily reproduced in casting - avoid stress concentration - substantial roundness: Class V materials Axial wall slant towards the pulpal floor + Rounding of the axiopulpal line angle : reduce stresses at the isthmus area Reduction of tooth structure should follow the original anatomy of the tooth - create even reduction - minimum tooth involvement - even physiologic distribution of forces

49. OTHER PREPARATION FEATURES TO MINIMISE MECHANICAL PROBLEMS Maximum reduction – occluding surfaces - average 1mm cleared for metallic casting in inclined planes of cusps - 1.5 mm for cast ceramics - reduction in concave form: to accommodate bulk of casting where stresses are maximum Internal parts of the preparation- mortised to preserve retention & resistance features - boxed up portion occupy maximum dimensions of the cavity preparation - necessitates the walls be prepared in different planes - internal planes be fixed at almost right angles with the adjacent walls

50. DESIGNS OF PREPARATIONS FOR CAST RESTORATIONS 5 general designs: 1. Inlays 2. Onlays 3. Cast restorations with surface extensions 4. Pin-lays 5. Full veneer cast or cast based restorations

51. INLAY INDICATIONS: Width of the cavity does not exceed one- third the intercuspal distance Strong, self- resistant cusps remain Minimal or no occlusal facets, and if present, are confined to the occlusal surfaces The tooth is not to be an abutment for a fixed or removable prosthesis Occlusion or occluding surfaces are not to be changed by restorative procedure

52. INLAY- Class I, II & III materials GENERAL SHAPE: Occlusal portion: dove- tailed Proximal portion: boxed

53. INLAY- Class I, II & III materials LOCATION OF MARGINS: OCCLUSAL PORTION- Facial, lingual & sometimes proximal margins :inclined planes of the corresponding cusps/ triangular ridges/ marginal ridges Bucco -lingual width does not exceed 1/3 rd intercuspal distance The most peripheral margins: away from the contact with the opposing tooth surfaces Wear facets, supplementary grooves, areas of decalcification, any defects in the adjacent parts of the occlusal surface- beveled portion of the preparation

54. INLAY- Class I, II & III materials PROXIMAL PORTION- Facial & lingual margins: embrasures Full length of an explorer passes freely occluso-gingivally All undermined enamel, surface defects & peripheral marginal undercuts eliminated Proximal extension increased to improve accessibility, but terminated short of the axial angle of the tooth Extensions: flare portion & not the wall proper Gingival margins: extended to include any surface defects & concavities and eradicate marginal undercuts Extensions: bevels & not wall proper

55. INLAY- Class I, II & III materials INTERNAL ANATOMY: Occlusal portion: Facial/ lingual walls and sometimes proximal walls- Wall proper & Occlusal bevel Wall proper: Pulpal 2/3 rd of the walls Formed completely of dentin Taper from each other : 2-5 or parallel Walls parallel to the long axis of the crown Right angle/ slightly obtuse angle with the pulpal floor

56. INLAY- Class I, II & III materials Occlusal bevel: Long bevel constituting almost one third of the walls Bevelled outer plane: 30- 45 degrees to the long axis of the crown width of the preparation - Bevels not needed in very steep cusps - Narrow preparations - Occlusal extension & enamel involvement of wall proper: bevel enamel rods in the inner one third of the inclined planes Steepness of the cusps

57. INLAY- Class I, II & III materials

58. INLAY- Class I, II & III materials Bevel: usually half of the wall proper Extended : wear facets, decalcifications, occlusal defects, supplementary grooves Move the margin away from occlusal contacts Wider & deeper cavities: bevels extended to improve the taper & reduce frictional components for easier material manipulation

59. INLAY- Class I, II & III materials Pulpal floor: Flat over most of its extent or at least the peripheral portions Conventional depth > amalgam for increased length of the surrounding walls 1- 1.5mm from the DEJ Definite line angle with all the walls except the axial wall- very rounded

60. INLAY- Class I, II & III materials Axial wall : Bucco - lingually: Flat or slightly rounded Gingivo - occlusally : vertical or slightly divergent to the pulpal foor - 5 to 10 degrees Divergence: taper Creating bulk: stress concentration areas Extreme rounded junction with pulpal floor: prevents stress concentration Ideal depth: 1- 1.5 mm from the DEJ

61. INLAY- Class I, II & III materials Facial & Lingual walls: 2 or 3 planes Axial half : Wall proper- completely dentin& meets axial wall at a right angle relationship Main resistance & retention feature Proximal half : Primary flare- enamel & dentin with an unchanged 45 to the wall proper Third plane : Secondary flare- enamel peripherally S implify impressions & wax pattern manipulations Not used if direct wax pattern is to be fabricated Variable angulation & extent

62. INLAY- Class I, II & III materials Gingival floor: Flat in the bucco - lingual direction; slightly obtuse angle with the buccal & lingual walls 2 planes in axio - proximal direction Axial half: Gingival floor proper Perfectly flat, formed of dentin & making a right angle/ obtuse angle with the axial wall Proximal half: Long bevel inclining gingivally Angulated at 30- 45 to the wall proper

63. INLAY- Class I, II & III materials Occlusal bevel & primary or secondary flare proximally Primary or secondary flares proximally & the gingival bevel Additional retention means: Grooves- Facial, lingual or gingival proximally Internal boxes/ slots- O cclusally Capping corners of cusps Rounded

64. MODIFICATIONS FOR CLASS IV AND SOME CLASS III ALLOYS More rounded internal line and point angles Less tapered & more parallel surrounding walls All circumferential tie constituents hollow ground to improve the capability of these alloys to replicate marginal details during casting Tooth preparation deeper: axially & pulpally to compensate for poor castability Extent & angulation of circumferential tie constituents governed similarly as earlier

65. MODIFICATIONS FOR CLASS V CAST MATERIALS In addition to the changes for Class IV alloys: Definitely flat pulpal & axial walls meet surrounding walls in very rounded line angles No decisive differentiation between surrounding walls & the circumferential tie constituents Both – exaggerated hollow ground bevels/ very rounded shoulderss

66. REFERENCES Sturdevant’s Art & Science of Operative Dentistry Operative Dentistry- Marzouk Principles and Practice of Operative Dentistry- Charbeneau GT Cast Restorations- Shillingburg Fundamentals of Operative Dentistry- Summit