Dental Restorations: Inlay and Onlay

Inlay vs. Onlay

• Inlay: an intracoronal indirect restoration that does not involve cuspal coverage
• Onlay: a partial extracoronal indirect restoration that involves one or more cuspal coverage

Indications for Inlay and Onlay

• Inlay: indicated for extensive preparations that require strength greater than a composite or amalgam restoration but do not require cuspal coverage
• Onlay: indicated for compromised teeth in high stress areas, and for large restorative defects in low stress areas where esthetics are involved

Advantages of Inlay and Onlay

• Improved esthetics
• Cover and reinforce cusps without removal of healthy tooth structure
• Supragingival margins for easier maintenance

Preparation for Inlay and Onlay

• Removal of caries or defect
• Initial preparation similar to direct restoration
• Cavosurface margins are beveled, including the proximal box
• All walls are divergent
• Occlusal dovetail is placed if mesial or distal proximal box is not prepared
• Secondary flaring of the proximal facial and lingual cavosurfaces joins the gingival bevel uninterrupted

Occlusal Reduction for Onlay

• Functional cusps: reduced 1.5 mm
• Nonfunctional cusps: reduced 0.75-1.0 mm with minimal extension onto the axial wall
• Shoulder following the lingual or facial axial walls of the cusp has a depth of 0.8 mm and occlusogingival height of 2.0-3.0 mm

Conservative Preparation

• Remove only compromised tooth structure
• Convenience access is provided for the restoration
• Affected dentin should be evaluated
• Dentin discolored by caries or amalgam may need to be removed or covered by an opaquer

Internal Line Angles and Isthmus Width

• Rounded internal line and point angles
• Flat ended diamonds work well to define internal line angles
• Isthmus width: 1.5-2.0 mm

Pulpal Floor and Proximal Extensions

• Pulpal floor can vary according to the planned restored anatomy
• Should be 1.5-2.0 mm for adequate bulk for fabrication
• Proximal extensions are determined by the extent of caries or previous restoration

Gingival Floor Depth and Cuspal Protection

• Gingival floor depth: 1.0-1.5 mm
• Should be maintained in enamel whenever possible
• Cuspal protection or replacement: extensions of the coverage are a clinical judgment, considering factors such as amount of enamel, occlusal forces, size of functional contact, esthetics, and mesial-distal width of affected area

Cuspal Reduction for Bulk of Restorations

• Functional cusp: 2.0-2.5 mm
• Nonfunctional cusp: 1.5-2.0 mm

Finish Line and Axial Wall

• Finish line should NOT be beveled
• Two schools of thought for the finish line:
  1. A well-defined, smooth butt joint (shoulder)
  2. A hollow-ground chamfer (deep chamfer, or modified butt joint)
• Axial wall should be divergent, more than what is required of a gold restoration

Temporization and Cementation

• After final impression, the preparation(s) need to be temporized to avoid shifting of the teeth and sensitivity
• Indirectly using a preoperative matrix, poured model, temporary fabrication with cementation using a non-eugenol temporary cement
• Directly with composite with NO etching or bonding

Try-In and Cementation

• Try-in of the restoration with a light transmitting medium like glycerine, water, or try-in paste
• Check for proximal contact, marginal integrity, and color
• Bonding of the ceramic restoration involves a series of individual links, including tooth interface, restoration interface, and hybrid resin luting cement

Bonding and Cementation

• Clean the restoration of any debris
• Coat the restoration with a silane coupling agent
• Apply a thin layer of unfilled resin or enamel bonding agent
• Light cure
• Apply composite resin luting agent to the preparation and restoration
• Seat the restoration into the preparation, cement should extrude from the margins
• Clean off excess cement and floss

Onlay/Inlay Preparation and Cementation

• The coupling agent is volatile and allowed to vaporize, leaving a reactive silane on the surface (Crispin, 1994)
• A thin layer of unfilled resin or enamel bonding agent is applied to the etched surface of the restoration
• The cavity preparation is cleaned using a wet slurry of flour of pumice, soft bristle brush, or micro air abrasion

Luting Agent Application

• A thin layer of luting agent is applied to the etched surface of the restoration
• The luting agent should be of dual cure capacity due to the thickness and opacity of the onlay/inlay
• The restoration is seated into the preparation, and the cement should extrude from the margins

Cementation and Occlusal Reduction

• Clean off excess cement and floss to check for any interproximal overhangs
• Occlusal reduction should be uniform and of sufficient thickness for ceramic to provide optimal strength
• Thickness of ceramic should be at least 1.5 mm in the central fossa and over nonfunctional cusps
• Marginal integrity: A sharp explorer should travel from the restoration to tooth structure, and vice versa, smoothly and without catching

Bonding of Ceramic Restoration

• Tooth interface: etched enamel micro mechanically bonds with the bonding agent
• Restoration interface: The etched porcelain is made reactive by a silane, causing it to mechanically and chemically bond to an unfilled resin layer
• Both interfaces are linked together by a dual cured hybrid resin luting cement

Ceramic Restoration Preparation

• Cavosurface margins should be placed to avoid contact with opposing teeth
• Margins should be smooth and well-defined with no beveling
• Thin areas of the ceramic are prone to fracture
• Onlay/Inlays rely on adhesion of the resin cement to the dentin and enamel for retention

Preparation Requirements

• Prep is nonmechanically retentive
• Floors and walls should avoid concavities and divots
• All line angles are rounded
• Isthmus is at least 2.0 mm to avoid onlay/inlay fracture

Final Thoughts

• Occlusal contacts should be checked AFTER the restoration has been seated
• Centric and excursive contacts should be checked and adjusted as needed
• Bonding of the ceramic restoration involves a series of individual links

Inlays and Onlays

• Inlays: intracoronal indirect restorations, not involving cuspal coverage
• Onlays: partial extracoronal indirect restorations, involving one or more cuspal coverage

Indications and Contraindications

• Inlays: indicated for extensive preparations requiring strength greater than a composite or amalgam restoration, but not requiring cuspal coverage
• Onlays: indicated for compromised teeth in high-stress areas, large restorative defects in low-stress areas, and when esthetics are involved

Advantages

• Conservative, as only compromised tooth structure is removed
• Improved esthetics
• Cover and reinforce cusps without removing healthy tooth structure
• Supragingival margins, easier for patients to maintain

Preparation

• Initial preparation similar to direct restoration
• Cavosurface margins are beveled, including the proximal box
• All walls are divergent
• Occlusal dovetail is placed if mesial or distal proximal box is not prepared
• Secondary flaring of the proximal facial and lingual cavosurfaces joins the gingival bevel uninterrupted

Onlay Preparation

• Similar to inlay preparation, but with one or more cuspal coverage
• Occlusal reduction of nonfunctional cusps is 0.75-1.0 mm with minimal extension onto the axial wall
• Functional cusps are reduced 1.5 mm
• Shoulder following the lingual or facial axial walls of the cusp has a depth of 0.8 mm and occlusogingival height of 2.0-3.0 mm

Cuspal Reduction

• Functional cusps: 2.0-2.5 mm, capped with a butt joint shoulder
• Nonfunctional cusps: 1.5-2.0 mm, shoed just beyond the tip and ridge of the cusp
• In both capping and shoeing, the margins should follow the contour of the cusp tip and ridges

Finish Line

• Two schools of thought: well-defined, smooth butt joint (shoulder) or hollow-ground chamfer (deep chamfer, or modified butt joint)
• Axial wall should be divergent, more than what is required of a gold restoration
• Increased taper, without unnecessary removal of tooth structure, allows for easier placement and removal when trying the restoration in

Temporization and Cementation

• After final impression, the preparation(s) need to be temporized to avoid shifting of the teeth and sensitivity
• Indirectly using a preoperative matrix, poured model, temporary fabrication with cementation using a non-eugenol temporary cement
• Directly with composite with NO etching or bonding
• Light-cured temporary resin material

Try-in and Cementation

• Check for residual provisional material, interproximal overhangs, and marginal integrity
• Color is dependent on shade selection, opacity versus translucency of the ceramic restoration, and the resin luting cement
• Occlusal contacts should be checked AFTER the restoration has been seated
• Bonding of the ceramic restoration involves a series of individual links: tooth interface, restoration interface, and a dual-cured hybrid resin luting cement