Bevels in dentistry

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CONTENTS

Introduction

Definition

Noy principles for ideal cavity walls

Function of bevel

Rationale

Gingival bevel

Occlusal bevel

Types of bevels

Bevels

Amalgam

Composite

Anterior restoration

Posterior restoration

Cast restoration

■ Inlays , Onlays

■ Full metal crowns and PFM crown o Flares

Controversies related to bevel

Conclusion

Reference

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BEVELS IN TOOTH PREPARATION

INTRODUCTION

3 requirements for successful restoration margins are

1. Fit as closely as possible to finish line of the preparation and minimize the width

of exposed cement

2. Must have sufficient strength to withstand forces of mastication

3. Located in areas where dentist can finish & inspect & patient can clean them

One of the objectives that helps in full filling these objectives is the bevel.

BEVEL:

Definition :

Bevels are "flexible extensions" of a cavity preparation, allowing the inclusion of surface

defects. Supplementary grooves, or other areas on the tooth surface bevels require

minimum tooth involvement, and do not sacrifice the resistance and retention for the

restoration”.-Marzouk

Historically bevels was used as compensation for the solidification shrinkage of

alloys used in fabricating cast restorations

Shillingburg : fundamentals of tooth preparation. Bevels-

i) Follows the contour of the tooth.

ii) Used on finish lines that are perpendicular to the path of insertion

Noy's principle for ideal cavity walls :

i) Enamel must be supported by sound dentin.

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ii) Enamel rods forming the cavosurface margin should be continuous

with sound dentin.

iii) Enamel rods forming the cavosurface margin should be covered with

restorative material.

iv) Angular cavosurface angles should be trimmed

Bevel angle: is defined as that formed by the bevel surface with the surface that has been

beveled.

Recommendation: 22-45 degree

45 degrees

45-60 degrees

65 degrees

Bevel height: 0.2m or less 0.5m

1mm upto 1.5mm

TYPES

Rosner : high angled long bevels

Osteland: low angled short

Rosner maintained that bevel allows for

Reduction of inherent defects in casting of cementation

Burnishing after cementation

Development of circumferential retention

Protection of enamel rods at the margin.

"Long high angled" perform all functions listed by "Rosner'.

"Short low angled” primarily to protect enamel wall at the preparation margin.

Types of bevel:

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A. Partial bevel: it involves part of the enamel wall, not exceeding two

third of its dimension. This is not used in cast restorations, except to trim

weak enamel rods from margin peripheries.

B. Short bevel: this includes entire enamel wall, but not dentin. This bevel

is used mostly with class I alloys especially for type 1 and 2.

C. Long bevel: All the enamel wall and up to one-half of the dentinal walls.

Most frequently used bevel for first three classes of cast restorations.

It preserves the internal "boxed-up" resistance and retention feature of the

Preparation.

D. Full bevel: include all the dentinal and enamel walls of the cavity wall

or floor. Although it is well reproduced by all four classes of cast alloys,

it deprives the preparation of its internal resistance and retention.

E. Counter bevel: when capping cusp to protect and support through this

type of bevel is used, opposite to an axial cavity wall, on the facial or

lingual surface of the tooth, and it will have a gingival inclination

facially or lingually.

F. Hollow ground (concave) bevel: bevel is prepared in a concave form.

This allows more space for cast material bulk, a design feature needed in special

preparations to improve material castability retention and better resistance to stress.

- Ideal for porcelain restorations and base metal alloys.

G. Inverted bevel - Shillingburg :

It is an incline in shoulder for metal ceramic crown labially.

Rationale :

Historically, the bevel was used as a device for compensation for the solidification

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shrinkage of alloys used in fabrication of cast restorations.

Metal margins should be acute in cross section rather than right-angled to facilitate a

close fit.

The shortest distance from the casting margin to tooth structure, it can be stated as

a function of D and the sin of angle m or the cosine of angle P, which is the angle

between the surface of the bevels and the path of insertion (180-m).

d = D sin m or

d = D cos P

An angle of 30 to 45 degree is considered optimal. If it is made more acute it

becomes weak, an acute edge of some kinds of alloys can also be burnished against the

tooth to further improve marginal fit.

According to studies by Mclean and Wilson reports the superiorly of bevels when

a ceramic level is employed. They state that margin must reach a angle of 10 to 20 degree

before it can fulfill its intended role.

Gavelis et al found that knife-edge margins produced the best seal, while

shoulders allowed the most complete sealing of any of the margin configuration they

tested.

BEVELLING OF AMALGAM

The design of the cavosurface angle is dependent of the restorative material being

used. Because of the low edge strength or friablility of amalgam a tooth preparation

cavosurface angle of 90 degrees produces maximal strength for both the amalgam and the

tooth . thus no bevels are placed at the cavosurface margin. When amalgam is

used bevelling is also contraindicated except on the gingival floor of a posterior class II

preparation when enamel is still present. In these instances it is usually necessary to

place a slight bevel (app. 15-20degrees) only on the enamel portion of the wall to remove

unsupported enamel rode. This is necessary because of the gingival orientation of enamel

rode in the cervical area of tooth crown.

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This minimal bevel may be placed with an appropriate gingival margin trimmer

hand instrument and once placed results in a 90 degree amalgam marginal angle. If the

angle of the marginal amalgam is less than 90 degree (90degrees is best), it is likely to

fracture because it has low edge strength. Such a fracture leaves a crevice at at the

interface. Thus the external walls of the amalgam must be designed to result in an

approximately 90 degree amalgam material.

Bevels in composites

Bevelling enamel margin in composite preparation is primarily indicated or large

restorations that have increased retention need. The use of a beveled margin form with a

composite tooth preparation may be advocated because the potential of retention

increased by increasing the surface are a of the enamel available for etching

Enamel margin: bonding to enamel changes cavity preparation design because less tooth

reduction is necessary. Enamel disking:

Freshly cut enamel provides better bonding surface than nonprepared enamel.

Aprismatic enamel - outer layer of one deciduous tooth and 70% of permanent molars.

This layer is 30um thick; disking off 0.1mm of the enamel removes this layer.

More perpendicular enamel rods are exposed.

90-degree exit angle: this angle does not expose the ends of the enamel rods and is less

retentive.

45-degree bevel: most commonly used finish lines.

Conserve much of the original tooth structure and expose the enamel rod ends.

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Provide a superior seal for enamel, particularly at the gingival margins.

Smooth transition and gradual shade change from composite to enamel.

Inlay

Occlusal bevel, long bevel constituting almost i/3 of the facial and lingual

(proximal ) walls.

Beveled outer plane of walls will have average angulations of 30-45 degree to the

long axis of the crown (wall proper).

These angulations should increase as the width of the cavity preparation increases,

in order to accommodate more bulk of cast alloy and to be able to resist increased stress

near the cusp of the inclined planes.

This increase angulation is necessary to bevel enamel rods which are inclined

towards the cusp in the occlusal 1/3 of the inclined plane.

Angulations of the bevel should decrease with increased steepness of the cusp.

Sometime bevel are not needed at all in very steep cusps (especially in a very

narrow preparations) as occlusal extension & enamel involvement of the wall proper

will automatically bevel enamel rods in the inner 1/3 of the inclined planes.

An increased bevel angulation is necessary for a direct wax pattern as compared

to cavity preparation for an indirect wax pattern. This is because more marginal bulk is

required for direct wax pattern technique.

In addition bevel angulation should be increased to include remotely laocated

defects, supplemental groove or the decalcification on the occlusal surface.

Beveling- gingival

Proximal half should be beveled in the form of a ling bevel inclining gingivally.

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This bevel is usually angulated on the average of 30-450 to wall proper. However

this angulation can be increased by an increase in the gingival extent and length of the

surrounding walls. This will serve to minimize the marginal discrepancy.

Marginal symptoms of internal spacing are inversely proportional to these bevel

angulations. Also in cases where there is a limited axioproximal dimension of the

gingival floor, increasing the angulations of bevel can minimize the part of floor use d up

by the bevel.

Flame shaped fine grit diamond instrument is used to bevel the occulasal &

gingival margin and to apply the secondary flare on the distolingial and the distofacial

walls. This should result in 30 to 40 degree marginal metal on the inlay. This

cavosurface design helps seal and protects the margins & results in a strim enamel margin

with an angle of 140-150 degree.

A cavosurface enamel angle of more than 150 degrees is incorrect because it

result in a les defined enamel margin (finish line) and the marginal cast metal alloy is too

thin and weak if its angle is less than 30 degree.

If enamel margin is 140 degree or less , the metal is too bulky and difficult to

burnish when its angle is greater than 40 degree.

Bevel for Onlay

Onlay – are the most indicated an universally used cast restorations for the

individual teeth. It is partly intracoronal and partly extracoronal type of restoration which

has cuspal protection as the main feature.

The main feature of these designs of tooth preparations are capping of functional

cusps and shoeing of nonfunctional cusps. On the shoed side which is the nonfunctional

side the facial or the lingual margin will be located just gingival to the tip & ridge crests

of the involved cusps and away from occlusal contact.

The counterbevel - a bevel of generous width is prepared on the facial (lingual)

margin of a reduced cusp with aflame shaped fine grit diamond instrument which is the

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exrtacoronal feature of this part of preparation.

Usually it is found in areas of enamel and dentin or it may be completely in either.

It should be relieved form opposing cuspal elements by atleast 1 mm in both static

and functional occlusal contact. It is hollow ground, inclined away from the

intracoraonal parts of the preparation gingivally – facially or lingually and follows the

cuspal anatomy. So each cusp on the capped side will have 4 different hollow ground

beel each corresponding to an inclined plane on the cusp.

In addition to the advantage of those features with occlusal bevel, counter bevel

will embrace the cuspal element for retention and & or support.

Angulations of the counter bevel is not as standard as that of the occlusal bevel .

it is measured by the embracing angle that is the angle between the bevel and the long

axis of the crown. On the average this angulations is 30-70degree but it varies accoding

to :

1. Amount of indicated involvement for the facial or the lingual surface

2. Amount of needed retention

3. Type of the casting alloy

Occlusal bevel

Placed 1.0m away from centric occlusal contacts.

Acute edge of metal and obtuse angle of enamel created by a bevel can withstand

impact much better than can a square edge of enamel.

Lap joints:With approx 35 degree metal margin

Sliding lap joint: with approx 25-degree metal margin

Enhances adaptation by taking advantage of the elongation and proportional limit

characteristics of cast metal.

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Closure occurs when the wedge-shaped margin is burnished beyond its

proportional limit.

Cavity design should permit attainment of margin closure in all areas of the

casting.

Gingival finish lines:

Beveled shoulder:

0.4 to 0.5mm bevel is placed to form recommended obtuse finish line.

PFM crown where esthetics is not critical.

Extremely short walls, since it permits critical portion of the axial walls just

coronal to the finish line to the formed nearly parallel with the path of insertion.

Nearly parallel walls enhance retention, while providing adequate reduction to

prevent over contouring.

Full shoulder with bevel preparation:

> Ideal for porcelain fused-to-metal restorations.

Advantage:

i) Adequate room is created in the gingival one third for proper containing

of the restorations to maintain periodontal health and straight emergence

profile is achieved.

ii) Room is created in the gingival one third for proper porcelain application

and esthetics.

iii) A buttress of metal is created in the gingival area to prevent distortion of

the metallic framework during the baking of porcelain and sealing of

casting.

iv)A more parallel, less tapered preparation is created, which enhances

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Retention and have the restoration.

45 degree facial bevel:

to avoid displaying a labial metal collar, a facial 45-degree bevel and a full

shoulder can be prepared, proximally and lingually 80-degree bevel is cut.

Because the vertical amount of marginal metal is narrow, the opaque is rarely

visible in the finished restoration.

45-degree bevel is like a sloped shoulder and allows creation of the desired

esthetic results.

45-degree bevel with porcelain over the metal collar has greater esthetic potential

and the some marginal adaptation as the 80-degree bevel with an all-metal collar.

80-degree facial bevel with porcelain, covering metal collar:

Complete bevel prepared with a plug finishing bur, has a convergence angle of 80

degree or greater.

Porcelain baked onto this labial approx will fracture off as a result of flexure of

the metal in the open area.

80-degree facial bevel with metal collar:

360-degree encirclement by a buccal is required for closure of the termination of

the preparation.

The bevel should extend onto about 0.5mm into the labial sulcus area. The metal

collar is not a limiting factor in the fabrication of fixed prosthesis.

It can be used in a single full coverage restoration as for fixed partial prosthesis.

Full shoulder with a bevel and a facial half joint preparation:

The labial surface terminates in a half joint shoulder finishing line to avoid

showing a labial metal collar.

In addition, for porcelain to be esthetically acceptable, 1.5mm of thickness must

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be obtained for the opaque layer and body porcelain.

Instruments used:

Diamond: remove tooth structure more effectively, but they leave in desirable rough

surface and irregular cavosurface finish lines.

Tungsten carbide: produce smooth finish lines and precise internal flatness, but they cut

more slowly.

Occlusal bevel:

Bevel with non-dented taper fissure line, revealed a markedly

smoother bevel and more distinct finish line than a bevel made with a

fine flame diamond.

Gingival bevel: must be creator by tip of instrument

Gingival marginal trimmer: not suitable for cast restorations, they produce a

rough, irregular bevel.

Long 12-blade flame-shaped carbide finishing bur, produce distinct finish line

with a smooth surface.

Full veneer crowns / full cast crown:

Functional cusp bevel:

Made with round-end tapered diamond.

Depth -> 1.5mm

Angle -> 45 degree with the axial wall.

Placed on: Facial cusp of mandibular teeth, Lingual cusp of maxillary teeth.

Provides structural durability.

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Failure to place a functional cusp bevel can result in thin, weak area, in the

restoration.

Compensation for lack of a bevel by overbuilding the crown will cause a

premature occlusal contact.

Maxillary posterior three quarter crown:

Occlusal finish bevel: along the occlusofacial line angle, keep it perpendicular to the path

of insertion.

0.5mm wide

Mandibular post three-quarter crowns

Class II inlay:

Occlusal bevel : Ingeaham et al recommended using a bevel of 15 to 20 degree,

beginning at the junction of the occlusal one third and pulpal two-third of the isthmus

wall. Gingival bevel:

Metzler and Chandler: have confirmed bevel as the finish line of choice for inlays.

Marginal bevel should lie between 30 and 45 degree to provide optimum blend of

strength and marginal fit.

Flame diamond is used.

Gingival marginal trimmer can also be used likely to produce ragged finish line.

> Flame carbide bur is used for finishing bevels. MOD onlays:

Functional cusp bevel:

Round end tapered diamond on 171 burs. Depth is 1.5mm deep at the cusp tip.

Occlusal and gingival bevel

Concave bevel (Chamfer):

Exposing max surface area of the enamel rod ends.

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Most retentive finish line.

> Chamfer also allows a 90-degree angle of exist for the restoration, making it a

more durable margin, particularly at occlusal contact points.

Convex bevel (adhesive preparation): seals better than the three conventional designs.

> Rounded areas of unsupported enamel provide excellent exit for bonded resins but

make it more difficult to remove carious dentin from the DEJ.

Design works only with stiff composite because the composite must support the

enamel.

Marginal discrepancy: 25 um - explores can feel.

80 um - difficult to detect under clinical condition Less than 120 um for the success of

restoration Clinical significance of marginal discrepancies:

> Defective margins are responsible for most of failed units.

CONCLUSION :

Proper preparation and extension of bevel is important for proper cavosurface margin and

preparation of leakage along circumferential tie.

References:

Operative dentistry- Marzouk

Operative dentistry- Sturdevant

Fundamentals of tooth preparation- Shillinburgh

Operative dentistry- Shillingburgh

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Critical appraisal. To bevel or not in anterior composites. Jol of Est. Rest.

Dentistry- 2005:17(4)264-9

F.