onlays

1. ONLAYS
DR.MUKTA BANSAL
SR.LECTURER
DEPARTMENT OF CONSERVATIVE
DENTISTRY AND ENDODONTICS

2. CONTENTS :
 Introduction
 Indications
 General shape of onlay preparation
 Step wise tooth preparation part
 Features of onlay tooth preparation
 Modifications of onlay preparation
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3. INTRODUCTION :
 Full-The cast metal onlay by definition caps all of the cusps of a posterior tooth and
can be designed to help strengthen a tooth that has been weakened by caries or
previous restorative experiences.
 Partial onlay
-Studervant-6th
 Partly intracoronal partly extracoronal
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4.  distribute occlusal loads
 more conservative of tooth structure
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5. INDICATIONS
(MARZOUK)
1. considered, if the lesion is one-third to one-half the intercuspal distance.
2. mandatory, if the width of the lesion exceeds one-half the intercuspal distance.
3. if the length:width ratio of the cusp is more than 1:1, but not exceeding 2:1, cuspal
protection is to be considered.
4. if the length:width ratio of a cusp is more than 2:1, cuspal protection is mandated.
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6. 5. need to change the dimension, shape, and interrelationship of the occluding tooth
surfaces
6. for abutment teeth
7. supporting restorations for remaining tooth structure, combined with conservative
tooth involvement.
8. to include wear facets that exceeded the cusp tips and triangular ridge crests
facially and/or lingually.
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7. GENERAL SHAPE OF ONLAYS-
 Occlusally, onlays are dovetailed internally and follow cuspal anatomy externally.
 Proximally they appear either box- or cone-shaped.
 capping of the functional (and sometimes the non-functional) cusps, and the shoeing of the non-
functional cusps.
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dovetail
Box shaped

8. CAPPING V/S SHOEING
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 Cap the functional cusp-For
additional retention and
protection.
 1.5mm
 far enough gingivally to be
away from contact with the
opposing tooth surface
 margin should involve 1/4th to
1/3rd facial or lingual surface.
 Shoe the non-functional cusp.
 1mm
 just gingival to the tip & ridge
crests of the involved cusps and
away from occlusal contact
(static and functional).
 In cap & shoe, margins should
follow the contour of the cusp
tips & ridge crests.

9. STEPS OF THE PREPARATION OF MOD CAST METAL ONLAY
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10. INITIAL PREPARATION:
Cusp Reduction-
 cusps should be reduced-improves both the access and the visibility
 the efficiency of both the cutting instrument and the air-water cooling spray is
improved
 assess the height of the remaining clinical crown of the tooth, which determines the
degree of occlusal divergence necessary for adequate retention form.
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Preparation of pulpal floor
along the central groove
Facial and lingual groove
extension(no more than
2/3rd)

12. PREPARATION OF THE DEPTH GAUGE GROOVES:
 The depth cuts are usually placed on the:
i. crest of the triangular ridges
ii. facial and lingual groove regions.
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1.5 mm depth
gauge groove

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Completion of the cusp
reduction along the side of
the 271 bur.
• Completed reduction,should reflect the general topography of the
original occlusal surface.
• to avoid possibly hitting an adjacent tooth.
• the ridges will be reduced in a later step when the proximal boxes
are prepared

14.  single "draw" path, usually the long axis of the tooth crown, so that the completed
preparation will have no undercuts.
 gingival-to-occlusal divergence-2 to 5 degrees from the line of draw
 Cusp reduction appreciably decreases retention form because of decreasing the
height of the vertical walls, so minimal amount of divergence is often indicated
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15.  vertical walls are short, a minimum of 2 degrees occlusal divergence
 As the gingivoocclusal height of the vertical walls increases, the occlusal divergence
should increase, allowing as much as 5 degrees in the preparation
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16. OCCLUSAL STEP/TABLE-
 The “table” is the transitional area between the intracoronal
and extracoronal parts of the preparation, which is partly in
dentin and partly in enamel.
 Relieved from opposing cusps by atleast 1.5mm in both static
and functional contacts.
 major resistance forms of the entire structure
 flat, following cuspal anatomy in the M-D directions (i.e., three
tables for each cusp, each with different directions).
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17.  increase surface area,
 provide more locking retention,
 conserve tooth structure
 physiologically distribute occlusal loads.
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18.  0.5 mm deep occlusal step in the central groove region
between the reduced cuspal inclines and the pulpal floor.
 Extend the step along any remaining facial (and lingual)
occlusal fissures as far as they are faulty (fissured).
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 Occlusal step extended facially
and lingually,expose DEJ

20. REVERSE BEVEL OR COUNTER BEVEL-
 generous width
 prepared on the facial (lingual) margin of a reduced cusp
 flame-shaped, fine-grit diamond instrument.
 It should be relieved from opposing cuspal elements by atleast 1mm in both static
and functional occlusal contact.
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21. REVERSE BEVEL OR COUNTER BEVEL-
 two hollow ground bevels occlusally:
occluso-distally
occluso-mesially
 angulation is not as standard.
 It is measured by the embracing angle, i.e., the angle between the bevel and the
long axis of the crown. (Marzouk)
 On the average, this angulation is from 30-70 degrees, but it varies according to:
a. The amount of indicated involvement for the facial or lingual surface.
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b. The amount of needed
retention.
c. The type of cast alloy.
Embracing angle

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Fissure
included

24. FUNCTIONS OF COUNTERBEVEL/ HOLLOWGROUND BEVEL
i. bulk
ii. distribute forces physiologically on underlying tooth structures.
iii. increase the immobilization of the restoration.
iv. retention and/or support.
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25. Proximal box-
Continuing with the No. 271 carbide bur held parallel to the long axis of the tooth
crown, prepare the proximal boxes.
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26. FINAL PREPARATION:
Removal of Infected Carious Dentin and Defective Restorative Materials and Pulp
Protection
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27. Preparation of Bevels and Flares-
 flame-shaped, fine-grit diamond instrument
 counterbevels on the reduced cusps,
 to apply the gingival bevels
 to create secondary flares on the facial and lingual walls of
the proximal boxes.
 The fine-grit diamond instrument also is used to bevel the
axiopulpal line angles lightly.
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 metal angle at the margins of onlays is 40 degrees
 except at the gingivally directed margins, where the metal angle should be 30
degrees.
 After beveling and flaring, any sharp junctions between the counterbevels and the
secondary flares are rounded slightly to produce an uninterrupted blending of all
junctions.

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Counter
beveling on
facial/lingual
surface +
depening of
counter bevel
in fissured
area
Rounding of the
junctions
Beveling of the
axiopulpal line angle
Cutting
proximal
grooves(retens
ion)
Completed MOD onlay preparation

30. VERIFICATION OF OCCLUSAL CLEARANCE-
 wax interocclusal record
 wax "bite
 Light transmission
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31. FEATURES OF CAST METAL ONLAY :
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32. MODIFICATIONS IN ONLAY TOOTH PREPARATIONS
i. Facial or Lingual Surface Groove Extension.
ii. Inclusion of Portions of the Facial and Lingual Smooth Surfaces affected by
Caries, Fractured Cusps, or Other Injury.
iii. Enhancement of Resistance and Retention Forms.
iv. Modifications for Esthetics on Maxillary Premolars and First Molars.
v. Endodontically Treated Teeth.
vi. Restoring the Occlusal Plane of a Tilted Molar.
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33. i. FACIAL OR LINGUAL SURFACE GROOVE EXTENSION
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 facial surface fissure
(mandibular molar) or a
lingual surface fissure
(maxillary molar) is included
 additional retention form

34. ii. INCLUSION OF PORTIONS OF THE FACIAL AND LINGUAL SMOOTH
SURFACES AFFECTED BY CARIES, FRACTURED CUSPS, OR OTHER
INJURY.
 cut a shoulder perpendicular to occlusal force by extending
the proximal gingival floor (adjacent to the fracture) to include
the affected surface.
 desired resistance form by being perpendicular to gingivally
directed occlusal force.
 vertical wall in the remaining lingual enamel.
 The linguogingival bevel is continuous with the gingival bevel
on the mesial surface.
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Fractured mesiolingual
cusp
Preparing gingival
shoulders and
vertical lingual walls
+ cusp reduction+
facial groove
extension
Bevelling margins
Completed
preparation Completed onlay

36.  These preparation features:
a) improve retention form,
b) resist forces normally opposed by the missing mesiolingual wall, and
c) help protect the restored tooth from further fracture injury.
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37. iii. ENHANCEMENT OF RESISTANCE AND RETENTION FORMS
 When the tooth crown is short (which is often the case when all cusps are reduced),
the operator must strive to maximize retention and resistance form in the
preparation.
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38. PROXIMAL RETENTION GROOVES
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 Shallow(0.3mm deep) retention grooves may
be cut in the facioaxial and the linguoaxial
line angles with the No. 169L carbide bur.
 The long axis of the bur must be held parallel
to the line of draw and the tip of the bur must
be positioned in the gingival box internal
point angles.

39. SKIRT PREPARATION
 Definition- Skirts are thin extensions of the facial or lingual
proximal margins of the cast metal onlay that extend from the
primary flare to a termination just past the transitional line
angle of the tooth.
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40.  Concept-
1. conservative method
2. retention and the resistance form.
3. eliminates the chance of postrestorative fracture of the tooth
4. an extracoronal restoration that encompasses and braces the tooth against forces
that might otherwise split the tooth.
5. atraumatic –only on enamel-removing very little (if any) dentin.
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42.  When lingual wall of proximal boxing is inadequate or missing,
retention form can be improved by facial skirt extension.
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43. Technique-
 the slender, flame-shaped, fine-grit diamond instrument.
 Usually prepared entirely in enamel.
 Step 1: long axis of the instrument parallel to the line of draw,
 translate the rotating instrument into the tooth to create a definite vertical margin,
just beyond the line angle of the tooth, providing at the same time a 140-degree
cavosurface enamel angle (40-degree metal angle).
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45.  Step 2: Extending into the gingival third of the anatomic crown is usually necessary
for effective resistance form.
 Step 3: Translate the instrument from the entrance cut toward the proximal box to
blend the skirt into the primary flare and the proximal gingival margin.
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46.  Step 4: round the junction between the skirt and the counterbevel.
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47. ADVANTAGES OF SKIRTING-
 successfully for many teeth that exhibit the split-tooth syndrome.
 when the proximal surface contour and contact are to be extended more than the
normal dimension to develop a proximal contact.
 when splinting posterior teeth together with onlays.
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48. DISADVANTAGE
 increases the display of metal on the facial and lingual surfaces of the tooth.
 For this reason, skirts are not placed on the mesiofacial margin of maxillary
premolars and first molars.
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49. COLLAR PREPARATION
 To increase the retention and resistance forms when preparing a weakened tooth
for a MOD onlay capping all cusps,
 To reduce the display of metal, however, facial surfaces of maxillary premolars and
first molars are not usually prepared for a collar.
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50. TECHNIQUE-
 Step 1: Use a No. 271 carbide bur at high speed
 0.8-mm deep shoulder around the lingual (or facial) surface to provide
 2 to 3 mm high occlusogingivally.
 Step 2: To provide for a uniform thickness of metal, the occlusal 1 mm of reduction
should be prepared to follow the original contour of the tooth.
 Step 3: Round any undesirable sharp line angle.
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52. Point to remember-
 The lingual skirt extension, the lingual collar or the lingual surface groove
extension on a maxillary molar protects the facial cusps from fracture.
 The facial skirt extension, the facial collar or the facial surface groove extension on
a mandibular molar protects the lingual cusp from fracture.
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53. SLOT PREPARATION
 retention form.
 Such a slot is preferred over preparing a box in the distal surface because:
1. more conserving of tooth structure and of strength of the tooth crown,
2. the linear extent of marginal outline is less.
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54. TECHNIQUE-
 use a No. 169L carbide bur whose long axis should parallel the line of draw.
 The slot is cut in dentin so that if it were to be extended gingivally, it would pass
midway between the pulp and the DEJ.
ADVANTAGES
 Such a position and direction of the slot prevents:
1. exposure of the pulp,
2. removal of the dentin supporting the distal enamel,
3. perforation of the distal surface of the tooth at the gingival termination of the slot.
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56.  The slot should have the following approximate dimensions:
1. mesiodistally, the width (diameter) of the bur-0.5mm
2. Faciolingually- 2 mm
3. depth,-2 mm gingival of the normally positioned pulpal wall.
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58. iv. MODIFICATIONS FOR ESTHETICS ON MAXILLARY
PREMOLARS AND FIRST MOLARS.
i. Occlusal reduction should only be 0.75 to 1 mm
ii. cusps do not receive a counterbevel but are "stubbed" or blunted by the
application of a sandpaper disc or the fine-grit diamond instrument
iii. blunting -0.5 mm in width.
iv. Secondary flare omitted
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60. v. ENDODONTICALLY TREATED TEETH
 more conservative for the health of the facial and lingual gingival tissues not to
prepare the tooth for a full crown but for a MOD onlay
 skirt extensions and collar preparations.
 better able to resist lateral forces that might otherwise fracture the tooth.
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61. vi. RESTORING THE OCCLUSAL PLANE OF A TILTED MOLAR
 When the unprepared occlusal surface is less than the desired occlusal plane, a
corresponding decrease in occlusal surface reduction is indicated.
 The mesiofacial and mesiolingual margins should be well extended onto respective
facial and lingual surfaces.
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