principles of preparation for cast restorations
TRANSCRIPT
PRINCIPLES OF PREPARATION FOR CAST RESTORATIONSDEEPTHI P.R.II YEAR MDS
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
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
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
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- materials inserted & withdrawn from preparation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
APICO-OCCLUSAL TAPER OF A PREPARATION
2 to 5 degrees from the path of preparation – increased or decreased:
1. Length 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. Dimensions 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
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
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
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
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
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
BEVELS PARTIAL BEVEL
Part of the enamel wall not > 2/3rd 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
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
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
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
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:
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: 450 to the inner dentinal wall proper
Hollow ground: cast ceramics/ non noble alloys
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
FLARES: Functions & Indications -Bucco- lingual wide 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
- Surface defects/ decalcifications: involved in preparation without extending the primary flare
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
DisadvantagesLiability of transitional
continuation of a circumferentia tie with
adjacent bevel ties
Limited burnishability of marginal cast alloy
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
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
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
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
Superior to chamfer
Care: no residual frail enamel/ thinned tooth structure
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
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
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
AUXILLARY MEANS OF RETENTION
Compound & complex tooth involvement
Formability of cast restorations: myriad retention & resistance means
Luting cementsGroovesReverse bevelInternal box External boxesSlotPins
CollarSkirtsCapping of cuspsCapping of the marginal ridgePostsGross roughening
Reciprocal retentionCapping of occluso- proximo- facial or lingual corners Precementation groovingElectrolytic etching
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
GROOVESCompletely in dentinInternal 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 capabilitiesIndicated: 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
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
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
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
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
INTERNAL BOXMinimum 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
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
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
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
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
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
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
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.
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
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 HNO3 ,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
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
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- shaped 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
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
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
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
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
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
INLAY- Class I, II & III materials GENERAL SHAPE:
Occlusal portion: dove- tailed
Proximal portion: boxed
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 1/3rd 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
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
INLAY- Class I, II & III materials INTERNAL ANATOMY:
Occlusal portion: Facial/ lingual walls and sometimes proximal walls- Wall proper & Occlusal bevel
Wall proper:
Pulpal 2/3rd of the walls
Formed completely of dentin
Taper from each other : 2-50 or parallel
Walls parallel to the long axis of the crown
Right angle/ slightly obtuse angle with the pulpal floor
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
Width of the preparation- Accommodate more bulk of the alloy- Resist increased stresses- Bevels the enamel rods inclined towards the cusps
Direct wax pattern
- More marginal bulk required
Extensions:
Remotely located defects,
supplementary grooves,
decalcifications on occlusal surface
INLAY- Class I, II & III materials
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
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
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
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 450 to the wall proper
Third plane: Secondary flare- enamel peripherally
Simplify impressions & wax pattern manipulations
Not used if direct wax pattern is to be fabricated
Variable angulation & extent
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- 450 to the wall proper
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- Occlusally
Capping corners of cusps
Rounded
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
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
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