Partial Coverage Crown preparation

www.caapidsimplified.com

• Introduction
• Difference between Inlay and Onlay Advantages and
• Disadvantages Indications and Contraindications Principles of tooth preparation
• Armamentarium
• Preparation steps for Inlay and Onlay Criteria for Amalgam Inlays and Onlays Nuances in Gold and Ceramic
• Tips and Tricks

Table of Contents

• An extra coronal restoration that restores all but one coronal surface of either a tooth or an abutment, usually not covering the facial surface
• The one that covers only the labial surface is called a veneer.

Introduction

⅞

3-quarter Gold crowns

th Crown Prep

Proximal half Prep

• They are made of composite resin(esthetic), gold (most strength and durable), porcelain in the dental laboratory, and then bonded to the tooth.

Inlays and Onlays

• Preservation of tooth structure
• Resistance and Retention form
• Structural Durability
• Marginal Integrity
• Preservation of periodontium

Principles of Crown Preparation

• Enamel margins support given by sound healthy dentin or filling material
• Smooth finish lines with rounded internal line angle given
• Preparation of axial walls should provide 10° to 15° of taper
• All proximal walls should be flared or diverged 5° to 15° (no undercuts)
• Shoulder, supragingival margins are a

necessity

• Finish lines should be prepared at 90° angles and should not exceed 110
• Avoid Beveled and feathered edges

Guidelines for Inlays/Onlays

• Gingival floor depth should be 1.0 mm to 1.5 mm to provide strength.
• Isthmus width should be 1.5 mm to 2.0 mm in the premolar region; molars require an isthmus width of 2.5 mm to 3.0 mm.
• Reduce Pulpal floor depth from 1.5 mm to 2.5 mm ideally to provide strength and ample space for aesthetic contours and characterization.
• For onlay restorations, all cusps should be covered with at least 1.5 mm to 2.0 mm of material while maintaining a minimum wall thickness of 1.0 mm to 1.5 mm for optimal strength.
• Occlusal margins should not coincide with occlusal contact points.

Guidelines for Inlays/Onlays

• Narrow (approximately 0.8 mm), round-tipped, tapered diamond (regular or coarse grit): 856-012

• Regular-size (approximately 1.2 mm), round-tipped, tapered diamond (fine grit): 856-016

• Football-shaped or wheel-shaped diamond (regular grit): 379-014

• Tapered and straight carbide fissure burs: 169L

• Small, round carbide bur: H1

• Finishing stones

• Diagnostic Instruments: Mirror, Explorer, and Probe

• Chisels, Hatchet, GMTs, Excavators, Articulating film

Armamentarium

• If the tapered fissure bur is held upright, it produces a desired minimal taper preparation.

• Straighten the bur to avoid encroaching the marginal ridges as extending the marginal ridge weakens the tooth.

• Make the initial cut in the defective central groove with a no. 330 bur to an approximate depth of 1.5mm.

Class I Inlay

• Flatten the pulpal floor with a no. 847-012 end cutting bur

• Extend the cut from the central groove to the other marginal ridge The width of the isthmus should be 1mm wide

• The occlusal view of the outline form reveals 0.5mm extensions into the facial and lingual grooves with a small dovetail at each end.
• These extensions provide additional retention and resistance form.

• Occlusal view of Class I inlay preparation

• The occlusal bevel is initially placed with a flame diamond. It extends one-third of the way down the side of isthmus walls and has a 15-20° inclination.

• Finishing the occlusal bevel with a flame carbide finishing bur

• Make Putty index

• Initial entry into the enamel is done with the tapered fissure bur 330/169L; once a cut is made, drag the bur through the central groove of the occlusal surface.

Class II Inlay

• Completed occlusal outline
• To provide maximum resistance, the pulpal floor should be flat, at an even depth, and perpendicular to the path of insertion of the restoration.

• Follow any developmental grooves making the isthmus approx. 1.5mm wide
• The penetration should end at least 1.0mm from the nearest occlusal contact.

• Begin the proximal box by running a 245/169L bur just inside the CEJ interproximally

• Sharp enamel chisel (hatchet) used to breakout undermined tooth structure and expedite the preparation of the proximal box.
• A hand instrument will break this enamel out very cleanly in the mouth.

• Use a no. 169L bur to extend the box facially and lingually to break contact with the adjacent tooth (keep it parallel)
• Create facial and lingual line angles to define the box

• The completed proximal wall should barely break contact with the adjacent tooth, with extension merging with the facial and lingual flares.

• Plane the gingival floor of the proximal box with an end cutting bur

• Give flare with flame diamond.
• Take care to blend the gingival bevel with facial and lingual bevels to avoid a scooped-out area, which would result in an undercut.

• Use a sharp gingival margin trimmer to create a V-shaped groove at the junction of the axial and gingival floor of the box (Minnesota Ditch), enhancing resistance to displacement forces

Flame bur produces the most consistent bevel

• Bevel placed around the entire periphery of the occlusal portion of the preparation.
• Ingraham et al. recommended a 15-20° bevel beginning at the junction of occlusal 1/3rd and the pulpal 2/3rd of the isthmus wall.

• Has to be 0.5mm wide
• Increases enamel strength
• Improves marginal seal
• Burnishable margins

What do bevels do?

• Class III inlay has drastically diminished in use.
• Conzett (1910) stated that Class III inlays were contraindicated for use on any tooth because of the amount of tooth destruction required for its cavity preparation.
• Redfern recommends teeth with extensive caries or for replacing failed resin restorations.
• It is an excellent restoration for the distal surface of canines.

Class III Inlay

Inlays vs Onlays

Inlays vs Onlays

• Capping refers to the complete coverage of the cusps of a tooth with sufficient extension of the bevel onto the buccal and lingual surfaces to carry the margins of the restoration into areas where stresses cannot be brought to bear directly into them. Done on functional cusps.

• Shoeing refers to veneer coverage of the tooth's cusp with only a slight finishing bevel on the crest of the cusp. The bevel is extended either to the right angle to the long axis of the tooth or in a little reverse direction.
• Done on the non-functional cusps.

Onlays

Why 2mm thickness?

• Some studies suggest that very thin restorative material becomes weaker and more likely to fracture, although other studies show that strong materials such as lithium disilicate and zirconia can perform well when thinner, or when bonded with resin cement.

​

• Thus a 2 mm space or reduction should be the goal, on the understanding that sometimes, during occlusal adjustment, porcelain will be removed and the final thickness will be less.

• Planar Occlusal with Round end tapered diamond

• Depth orientation grooves at the crest of each triangular fossa and one in each developmental groove
• 1.5 mm over functional cusps and 1.0 mm over non-functional cusps
• 0.5 mm depth at facio-occlusal line angle as maxillary non-
• functional side is visible

Steps

Round-off and follow the original contour of the cusps, reproducing basic geometry of inclined planes to the occlusal surface in the process

​

Depth orientation cuts made on the outward-facing inclines of

the functional cusp

1.5 mm at the cusp tip, fades away to the occlusal shoulder

​

Reduction is completed by removing inter-groove enamel

Functional cusp Bevelling to approximate the opposite cuspal

incline angle by holding the bur at a 45o angle to the occlusal

surface

Smoohen both the planes: occlusal and bevel to ensure better

seating of the restoration

Occlusal Shoulder by No.245 carbide bur

Check for the occlusal clearance by:

Visual Inspection for labial to occlusal

Utility wax/Thickness gauge for lingual cusps

Place the shoulder roughly to eliminate any sharp line angle or point angle

Shoulder with bevel:

• Easier to prepare

Chamfer/Shoulder:

• Ensures acute edge in the restoration margin with a nearby bulk of metal

• Follow the termination line of the functional cusp bevel on the axial surface of the functional cusp
• Create 1mm wide shoulder from mesial to distal

• 1mm shallower than an inlay as the occlusal surface is already reduced
• Narrow Isthmus is created to provide for retention and resistance

• Isthmus and Proximal box by No. 245/169L bur

• Barely break contact with the adjacent teeth with a smaller preparation on both sides. 1mm wide gingival floor
• Accentuate facio-axial and linguo-axial line angles with 169L
• Less conservative Facial extension

• Contacts are broken with a 1mm Enamel hatchet Smoothen the walls to ensure resistance form

• Ensure occlusal divergence of facial and lingual walls and occlusal convergence of the axial walls

• End cutting carbide bur or no. 957 to plane the isthmus and occlusal shoulder of functional cusp bevel of 1mm width
• Finish the gingival floor to resist displacement of restoration against compressive occlusal forces

• Flame shaped bur for proximal flare

• Enter the restricted embrasure space without adjacent tooth damage
• Follow with a hatchet to shape mesiofacial flare to ensure pleasing aesthetics
• Follow with sandpaper disk to polish the proximal flare
• Gingival bevel 0.5mm to 0.7mm given along the gingival floor
• Bend into the flares of facial and lingual walls of the proximal box
• without any undercut
• Distinct finish line with smoothened bevel and flares

• Facial and Lingual bevel is produced with 169L (occlusal finishing bevel): 0.5mm wide
• Perpendicular to the long axis of the tooth

• Lingual: The occlusal shoulder of 0.5mm blends well with the lingual flare
• Do not extend far gingivally

• Facial: Outer edge of the occlusal bevel continues with the outer edge of the facial flare as a smooth finish line in the transitional area
• Avoid any sharp projections to ensure a gapless casting margin.

Depth orientation cuts

Occlusal step after FCB

Onlay on Molars

Cuspal reduction

Functional cusp shoulder

Non-functional cusp bevel

• Facial/Lingual surface groove extension Enhancement of Retention and Resistance form Collars
• Slots

Modifications of Onlay tooth preparations

Gingival bevel: 15-20 degrees

The occlusal cavosurface angle is 90 degrees (Butt joint)

The isthmus is 1/4th of the intercuspal distance.

Outline form is narrow as no surface involvement and depth is 1.5-2mm.

​

The walls converge

occlusally: 169L or 245

Outline form is wide as the surface involvement is more and depth can be less.

​

The walls diverge or parallel

occlusally: 245 or 169L

​

The isthmus may increase

upto 1/3rd of intercuspal

distance

The occlusal cavosurface

angle is 135-145 degrees to

achieve the sliding fit

Gingival bevel is steeper: 20- 30 degrees

The occlusal bevel blends in another plane to occlusal shoulder

Gingival bevel is steeper and blends with proximal flare

Outline form covers the involved cusps and depth remains as shallow as ~0.5mm

​

The walls diverge by 6-10

degrees except the axial wall

that converge occlusally

The isthmus is narrower to

maintain cuspal integrity

Class II Amalgam

Amalgam Inlay

Amalgam Onlay

Proximal clearance <0.5mm

The gingival seat should be placed supra-gingivally.

Gingival cavosurface margin is beveled to remove unsupported enamel.

Undercuts improve retention of the restoration

Proximal clearance can exceed >0.5mm.

No undercuts should be present.

Subgingival gingival seat is acceptable

​

Gingival and occlusal

cavosurface margin beveled

to provide frictional retention

Supragingival only

Proximal clearance >0.5mm

No undercuts should be present. Overall flaring prep

Gingival, Occlusal, Proximal margins beveled

Class II Amalgam

Amalgam Inlay

Amalgam Onlay

Secondary retention: Grooves, slots, pins

The reverse curve may be present

Internal line angles rounded beveled Axiopulpal line angle.

Proximal walls have a primary flare

Reverse curve absent

Secondary retention: Grooves, slots, pins, internal boxes, collars, reverse bevel, skirts.

The proximal wall has both

primary and secondary

bevels.

Internal line angles are well- defined but rounded Axiopulpal line angles.

Internal line angles are well- defined but beveled Axiopulpal line angle.

​

Reverse curve absent

Secondary retention: Grooves, slots, pins, internal boxes, collars, reverse bevel, skirts.

​

The proximal wall has both primary

and secondary flares.

Class II Amalgam

Amalgam Inlay

Amalgam Onlay

• Secondary Flare:
• Bevelling of cavosurface wall peripheral to the primary flare
• Mandatory to leave walls in a self-cleansable area interdentally

• Primary Flare:
• Divergence of the buccal and lingual proximalbox at an angle of 45 degrees

Margins are chamfer

Parallel or straight axial wall

Gingival bevel placed subgingivally.

Parallel cavity walls for primary retention

1mm pulpal depth, 1-1.5mm cuspal reduction

Gingival bevel are supragingival.

Margins be deep chamfer or shoulder.

10-12 degrees of axial wall convergence and is beveled

0.5 mm of pulpal depth, 2mm cuspal reduction

Over-tapered cavity walls for secondary retention; Primary by cement lute (Greater than 10 degrees divergence)

Gold Partial Restorations

Ceramic Partial Restorations

Tips and Tricks

• Give orientation cuts 0.5mm lesser than the required dimensions to leave space for finishing
• Always prepare cusps first in an onlay, and then move to the occlusal and the proximal segment
• Use an enamel hatchet to break the enamel shell
• Occlusal divergence is best given by no. 169L bur
• If you damage the adjacent tooth, polish it with composite strips
• Bevels are best given by flame-shaped bur kept at 45 degrees
• Use no.245 bur to keep line angles and point angles well-defined

Summary

Talent you have naturally.

Skill is only developed by hours and hours and hours of beating on your craft!

-Will Smith

Marzouk MA, Simonton AL, Gross RD. Operative Dentistry-

Modern theory & practice, 1itionst edition

Roberson ™, Heymann HO, Swift EJ.Sturdevant’s Art & Science of Operative Dentistry, 5th

Edition

Anusavice, Shen, Rawls. Phillips’ Science of Dental Materials, 12th edition

Summit JB, Robbins JW, Schwartz RS. Fundamentals of Operative dentistry. A

contemporary approach, 2nd edition

Schulein ™. Significant events in the history of Operative dentistry. Journal of history of

dentistry. Vol 53. No 2.2005.-63-72

References

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