Lecture 2: Porcelain Inlays/Onlays
Overview of Cascode Intracoronal Restorations (Inlays and Onlays)
A new and evolving class of indirect restorations, cascode intracoronal restorations, were introduced recently, encompassing cascode inlays and onlays. This approach emphasizes strong, predictable bonding to tooth structure.
Today’s discussion uses porcelain as the primary example, given its common aesthetic application. However, it's important to note that other aesthetic materials, such as the EMAC system (e.g., lithium disilicate), can also be used, with the core concepts of preparation and bonding remaining largely consistent.
Purpose: These restorations aim to provide a more durable and anatomically accurate alternative to direct restorations (like amalgam or composite fillings) while being more conservative than full coverage options (e.g., full crowns or traditional gold crowns). They help preserve healthy tooth structure by relying on adhesion rather than extensive mechanical preparation.
Indications
Fractured tooth: Ideal for teeth with significant structural compromise where a direct restoration would not provide adequate strength or longevity.
Fractured existing restorations: When an old, failing, or fractured restoration (e.g., amalgam) needs replacement, and the remaining tooth structure is still substantial enough to support a bonded restoration.
Moderate primary caries: Suitable for teeth with larger carious lesions where a tooth-colored, indirect restoration is desired for both aesthetics and enhanced structural integrity.
Typical treatment areas: Primarily recommended for premolars and first molars which can often achieve excellent isolation and have sufficient enamel for predictable bonding. Second molars are less frequently treated with this approach due to access and isolation challenges.
Rationale: The bonding-based nature of these restorations allows for maximum conservation of healthy tooth structure, which is a significant advantage over full coverage crowns, particularly when superior aesthetics are a key patient concern.
Contraindications
Bonding to tooth requires excellent isolation and a meticulously controlled operating field. The presence of saliva or blood contamination during bonding significantly compromises the bond strength and is an absolute requirement for clinical success.
Not recommended on second molars, or any tooth, where achieving and maintaining absolute isolation (e.g., with a rubber dam) is difficult or impossible. This is often due to their posterior position, limited mouth opening, or unusual anatomy.
Parafunctional habits (e.g., bruxism or clenching) can be a significant concern, especially when using porcelain against opposing natural enamel. The high hardness and abrasive nature of porcelain can lead to accelerated wear of the opposing dentition. Newer, less abrasive ceramic materials or specific occlusal adjustments may mitigate this risk.
Clinical success is highly dependent on achieving reliable enamel bonding. A lack of sufficient intact enamel, often due to very large existing restorations or extensive caries, severely reduces the available surface for bonding and therefore acts as a contraindication. Dentin bonding is less predictable and durable than enamel bonding.
Short clinical crowns (e.g., some second molars) reduce the available surface area for bonding and mechanical retention, making the long-term success of an inlay/onlay less predictable. Adequate surface area is crucial for both bonding and potential mechanical interlocking features.
Material considerations: porcelain vs other aesthetic materials
Porcelain advantages: Offers superior aesthetics, closely mimicking the natural appearance of tooth structure in terms of translucency, shade, and texture. This makes it highly desirable for visible teeth.
Bonding advantages: When properly bonded, the restoration becomes an integral part of the tooth, effectively reinforcing the remaining tooth structure and significantly reducing the risk of fracture for both the inlay/onlay and the prepared tooth itself. This 'monoblock' effect distributes stress more evenly.
Gold advantages (for comparison): Traditional gold inlays/onlays rely on mechanical retention principles, meaning they do not require bonding to enamel for primary retention. This offers predictability even in sub-optimal isolation and allows for different cavity designs focused on convergence/divergence and friction fit.
Porcelain drawbacks: The bonding process is extremely critical and technique-sensitive. If bonding fails at any stage ("all-or-nothing"), the restoration may entirely delaminate or experience marginal leakage, leading to recurrent decay or sensitivity. Porcelain also exhibits specific wear characteristics against opposing dentition, as mentioned, and may have relatively less long-term clinical data (e.g., 20+ years) compared with the extensive history of traditional cast gold restorations.
Practical note: The bonding protocol for porcelain is distinct from direct composite bonding. It typically involves specific surface treatments for the porcelain itself (e.g., hydrofluoric acid etching and silanization) prior to the application of adhesive resin cement, in addition to the tooth conditioning steps.
Bonding, retention, and marginal considerations
Enamel bonding: The process initiates with etching the enamel surface with phosphoric acid for seconds to create a micro-retentive surface. This is followed by rinsing, drying, and the application of a dentin/enamel bonding agent to penetrate the etched enamel and prepare it for resin cement.
Porcelain bonding: The internal surface of the porcelain restoration is prepared by etching with hydrofluoric acid (typically for seconds for feldspathic porcelain) to create microscopic porosity. Subsequently, a silane coupling agent is applied, which chemically bonds to both the inorganic porcelain and the organic resin cement, acting as a crucial interface. Finally, an adhesive resin cement is used for luting.
Marginal adaptation: Historically, the marginal adaptation of gold restorations was enhanced by beveling the cavosurface margin, creating a larger area for burnishing the gold. For brittle materials like porcelain, however, cavosurface bevels are generally contraindicated because a thin, unsupported porcelain bevel is highly susceptible to fracture. Instead, the success of the porcelain margin relies critically on the strong, durable bond achieved between the porcelain, the resin cement, and the enamel, creating a sealed interface.
The bond-driven approach is a defining feature of porcelain cascode restorations, fundamentally distinguishing them from purely mechanically retentive designs often associated with gold or amalgam. This bond not only retains the restoration but also reinforces the tooth structure.
Cavity preparation: key design differences (porcelain vs gold)
Porcelain-specific considerations (due to its brittle nature):
Minimum occlusal porcelain thickness: A crucial of porcelain is required in all stress-bearing areas to provide adequate strength and resist fracture under occlusal forces. Inadequate thickness is a primary cause of porcelain fracture.
Functional and non-functional cusp reduction: Both functional and non-functional cusps must be reduced by at least to ensure sufficient porcelain thickness over these potentially high-stress areas and to create adequate occlusal clearance.
Shoulder placement: A 'shoulder' preparation, a flat ledge, is often required on the lingual surface of the posterior maxillary lingual cusp and on the buccal surface of the posterior mandibular buccal cusp. This provides a broad, flat seat for the porcelain, enhancing support and stress distribution.
Vertical overlap considerations: If there is significant vertical overlap between the opposing arches, a shoulder preparation may be necessary to reliably achieve adequate restorative material clearance on the functional cusp without over-reducing tooth structure. If vertical overlap is minimal, a slight bevel of approximately on the marginal areas can be used strategically to remove unsupported, weak enamel rods at the cavosurface, creating a smoother transition for the restoration.
Overall cavosurface margin design: Due to porcelain's brittleness, sharp cavosurface bevels are generally avoided. Instead, margins are typically prepared as butt joints (90-degree angle to the tooth surface) and rely on the strong bond between the porcelain, resin cement, and enamel for a clinically acceptable seal and structural integrity.
Outline form and internal angles: Proximal and occlusal walls typically require a controlled degree of divergence to allow for the draw of the indirect restoration. Crucially, all internal line angles (e.g., axiopulpal line angle) should be gently rounded to reduce stress concentrations within the porcelain and promote even cementation.
Gold-specific considerations (relying on mechanical retention):
Minimal occlusal thickness concepts differ significantly from porcelain, as gold is more ductile and forgiving. Retention primarily relies on the physical form of the preparation rather than bonding.
Proximal and occlusal walls are prepared with a precise divergence of about from the line of draw. This slight convergence towards the occlusal aspect creates a frictional fit and mechanical retention. Internal line angles are purposely kept sharp to enhance mechanical interlocking and resistance form.
General cavity design notes:
Cavo-surface margins for porcelain are designed as butt joints where the bond provides the seal; traditional bevels are not routinely used for porcelain margins to avoid creating weak, unsupported enamel or porcelain. The exception is the bevel for minimal overlap to remove weak enamel.
For porcelain onlays/inlays, it is always paramount to preserve healthy enamel where possible, as enamel provides the most predictable and strongest bond for long-term success of the restoration.
If there is a wide isthmus (the occlusal portion of the preparation), porcelain onlays may be a preferable and more conservative alternative to full crowns. Onlays can effectively cap and protect cusps while preserving more coronal tooth structure, relying on the robust bond for retention and reinforcing the remaining tooth.
Clinical case concepts (illustrative cases from the transcript)
Case A: Mandibular first premolar with existing DO amalgam and fractured lingual cusp; deep pulpal floor ( deep).
Plan: A porcelain inlay is chosen as an aesthetic and conservative alternative to replacing the direct restoration. The decision considers the remaining tooth structure and the need for cusp protection.
Preparation: Initial steps involve the careful removal of the existing amalgam. The walls are then modified to achieve the necessary divergence for the inlay, and the occlusal depth is already adequately met by the inherently deep pulpal floor from the previous restoration, requiring minimal additional reduction in that area.
Case B: Maxillary first molar MOD with buccal amalgam and deep pulpal floor ( deep).
Plan: The clinician considers either a full crown or a porcelain onlay. The extensive nature of the existing restoration and subsequent tooth loss necessitates substantial coverage or robust reinforcement.
Full crown issues: A full crown would typically require a foundation restoration (e.g., a core build-up) due to significant proximal and buccal wall loss. However, the presence of considerable remaining enamel suggests that a bonding-based porcelain onlay could be a viable option, potentially avoiding the need for a full foundation restoration if sufficient enamel is available for a strong bond.
Concrete steps: The preparation would involve creating two distinct proximal boxes, carefully reducing the disto-lingual cusp for strength and clearance, and finally shaping the tooth for a porcelain onlay, meticulously preserving as much enamel as possible to maximize bonding potential and reinforce the remaining tooth structure.
Case C: Class I amalgam with incomplete fracture of distal buccal cusp.
Plan: A porcelain inlay is planned. The initial step would be to remove approximately of the existing amalgam. The distal buccal cusp, being fractured, would then be reduced by approximately down to the pulpal floor to ensure adequate porcelain thickness for strength. Occlusal walls would be prepared with the necessary divergence.
Question raised: The discussion point with the instructor revolved around why a shoulder was not placed on the buccal surface of the distal buccal cusp, prompting consideration of occlusal forces, cusp strength, and specific anatomical requirements for a given case. This highlights the nuanced decision-making in clinical practice.
Case D: Maxillary premolar with MOD amalgam and fractured lingual cusp.
Plan: A porcelain onlay is indicated due to the fractured lingual cusp requiring coverage and protection. The treatment involves removing the existing amalgam, modifying the cavity walls to the appropriate divergent form, and crucially, placing a distinct shoulder in the area of the fractured lingual cusp to ensure optimal support and thickness for the onlay in this high-stress region.
Seating and cementation sequence (bonded porcelain restorations)
Ideal isolation: Cementation simply must occur under absolute isolation, typically achieved with a rubber dam. This is paramount for maximum success, as the performance of the bond is highly sensitive to moisture contamination (saliva, gingival crevicular fluid) during the entire cementation process. Without proper isolation, bond strength will be significantly compromised.
Cementation sequence:
Remove temporary restoration: Carefully remove the provisional restoration, ensuring all temporary cement is thoroughly cleaned from the tooth surface and margins. Residual temporary cement can interfere with the bond.
Check proximal contacts first: Before fully seating the restoration, carefully verify proximal contacts with dental floss or articulating paper. Adjustments to the restoration's interproximal areas should be made at this stage to ensure proper contact and prevent open contacts or excessive pressure, as adjustments become much harder after cementation.
Check marginal fit and adjust as needed: Visually inspect the marginal adaptation of the restoration to the prepared tooth using an explorer. Ensure there are no open margins, discrepancies, or ledges. Minor adjustments can be carefully made with fine diamonds at this point if necessary.
Cementation: Apply the chosen adhesive resin cement uniformly to the internal surface of the dried, conditioned restoration and/or the prepared tooth. Carefully seat the restoration with firm pressure, ensuring that it is fully seated and that the margins are impeccably sealed during the setting process. Light-curing protocols (if applicable) must be strictly followed.
Occlusion considerations after cementation: This is a critical point – occlusal contacts should always be adjusted after the permanent cementation and complete setting of the restoration. Adjusting before cementation is unreliable because the restoration's position might shift, or the cement layer could alter the occlusal relationship. Adjusting post-cementation avoids post-placement occlusal interferences, which can cause sensitivity, fracture of the restoration or tooth, or even symptoms of temporomandibular joint dysfunction.
Excess cement management: Removing excess cement before it completely polymerizes and hardens is essential. Once set, especially with resin cements, removal becomes extremely difficult, often requiring aggressive instrumentation that can damage the restoration margins or gingival tissues. Techniques suggested include using cotton-tipped applicators or microbrushes immediately after seating to wipe away excess cement from buccal and lingual surfaces, and using a sharp explorer or specialized instruments (e.g., fine scalers or interproximal files) to meticulously clean proximal and gingival margins before the final light cure, if applicable.
Post-cementation steps:
Remove the dental dam: Once cementation is complete and all excess cement is removed, the rubber dam can be carefully removed.
Use a fine diamond bur to adjust occlusion as needed: With the patient's natural bite, use articulating paper to identify any premature contacts or high spots. Carefully adjust the occlusal surface of the restoration with a fine diamond bur, polishing after adjustment, until harmonious occlusion is achieved, ensuring even distribution of occlusal forces across the restoration and natural dentition.
Laboratory exercise (porcelain inlay preparation on tooth #19)
Key starting principle: Achieving adequate occlusal bulk for the porcelain is the single most important feature for its strength and longevity. Therefore, the preparation begins with a precise depth cut into the central pit of the occlusal surface, serving as a critical reference point for subsequent reductions.
General criteria for occlusal and proximal extensions:
Occlusal extension: The width of the occlusal portion of the preparation should extend approximately one-third of the intercuspal distance. This is a guideline to ensure adequate width for the restoration without unnecessarily removing healthy tooth structure, while still addressing the carious lesion or fracture.
Proximal extension: A minimum of clearance is required on both the buccal and lingual proximal walls to ensure adequate space for the restorative material and cement, as well as to facilitate proper contouring of the restoration and ease of cement removal. This clearance prevents the restoration from having overhanging margins.
Gingival clearance on the gingival seat: Approximately of gingival clearance is needed on the gingival seat (the floor of the proximal box). This ensures that the margin of the restoration is supragingival or at least equigingival, allowing for good oral hygiene and easier cement removal, which is crucial for periodontal health.
Wall divergence: Occlusal and proximal walls should be prepared with a divergence of about from the path of insertion. This provides a slight taper, allowing the indirect restoration to be seated without binding, while still offering sufficient resistance and retention form when combined with bonding.
Internal line angles: All internal line angles (e.g., axiopulpal, axiobuccal, axiolingual) must be gently rounded. Sharp internal angles act as stress concentrators, which can lead to fracture of the porcelain restoration or the tooth itself under masticatory forces. Rounding these angles promotes better stress distribution and easier cement flow.
Finishing and wall smoothing technique: Meticulous smoothing of all proximal walls and cavosurface margins is absolutely critical for the success of porcelain restorations. A smooth, continuous surface is essential for optimal bonding and marginal adaptation.
A fine garnet-disc is typically used to smooth the prepared walls. It is imperative to ensure that the disc covers the entire wall surface, from the axial wall to the cavosurface margin, in one continuous motion. A common error to avoid is creating a 'biplane wall,' where the wall has two distinct planes or angles, which can compromise seating and marginal integrity.
If you have questions about the proper application and technique of the garnet-disc, it is essential to ask your instructor for clarification and guidance to master this crucial step.
Practical implications and takeaways
The success of porcelain cascode restorations is directly contingent upon meticulous isolation throughout the entire procedure, precise and scrupulous bonding to intact enamel, and the application of appropriate material-specific cavity designs. Any deviation in these areas significantly jeopardizes the longevity and performance of the restoration.
Porcelain, while providing superior aesthetics, demands extremely careful preparation. This includes ensuring adequate bulk (the minimum thickness), strategic cusp reduction, and specific marginal considerations (e.g., butt joints over bevels for most areas) to manage its inherent brittleness and to effectively rely on the adhesive bond for retention and reinforcement.
Gold remains a highly reliable, mechanically retentive alternative, but it employs entirely different design principles, such as specific divergence angles () and sharp internal line angles, tailored to its ductile properties and mechanical retention mechanism.
Clinical decisions regarding the choice between an inlay, an onlay, or a full crown are complex and highly depend on the amount of remaining sound tooth structure, the availability of sufficient enamel for predictable bonding, the extent of carious involvement, and the overarching desire to preserve as much natural tooth structure as possible.
Margin design, the selective use of bevels, and the detailed occlusal preparation must be meticulously tailored to the specific material properties and the chosen bonding or retention strategy to ensure clinical success and reduce complications.
Real-world practice necessitates the ability to translate these fundamental principles into case-specific treatment plans. This involves carefully considering patient-specific occlusal schemes, the strength and integrity of individual cusps, and their aesthetic goals to achieve optimal, functional, and durable outcomes.
Key terms and concepts
Cascode intracoronal restorations: A modern class of indirect restorations, including inlays and onlays, designed to fit within the crown structure, leveraging advanced bonding concepts to reinforce and restore the tooth.
Inlay vs Onlay: An inlay restores the occlusal and sometimes proximal surfaces, fitting within the cusps. An onlay extends to cover one or more cusps, providing greater protection and reinforcement, particularly in cases of compromised cuspal integrity. Their retention is primarily bonding-based, contrasting with older mechanical retention methods.
Bonding protocol for enamel vs porcelain:
Enamel: Phosphoric acid etching to create micro-porosities, followed by the application of an adhesive bonding agent to penetrate the etched surface.
Porcelain: Hydrofluoric acid etching to roughen the surface, application of a silane coupling agent (a bifunctional molecule that bonds to both porcelain and resin), followed by an adhesive resin cement.
Marginal adaptation: The precise fit of the restoration at its interface with the tooth structure. For porcelain, this is primarily achieved and sealed through effective bonding techniques, as opposed to bevels often used for gold.
Occlusal bulk and cusp reduction: The requirement for a minimum thickness of restorative material ( for porcelain) over occlusal surfaces and cusps to provide adequate strength against masticatory forces and prevent fracture, ensuring the longevity of the restoration.
Proximal and gingival clearances: The necessary space (typically ) established between the preparation walls and adjacent teeth or gingival tissues. This clearance is crucial for proper seating of the restoration, accurate contouring, efficient cementation, and ease of excess cement removal.
Garnet-disc smoothing technique: A specific laboratory technique utilizing a fine abrasive disc to meticulously smooth the prepared cavity walls and cavosurface margins. This creates a highly uniform surface essential for optimal bonding and prevents the formation of unsupported enamel or inconsistent bonding areas (e.g., biplane walls).
Summary of numerical references (quick recap)
Porcelain thickness: Minimum required for strength in all stress-bearing areas.
Cusp reduction: reduction specified for both functional and non-functional cusps to accommodate porcelain bulk.
Vertical overlap considerations: Significant overlap may necessitate a shoulder; minimal overlap might allow for a bevel to remove weak enamel.
Proximal/gingival clearances: for proximal buccal/lingual walls and the gingival seat to facilitate cementation and ideal restoration contours.
Cavity walls (porcelain): Optimal divergence of for occlusal and proximal walls with all internal angles rounded.
Gold retention: Proximal and occlusal walls prepared with a divergence, featuring sharp internal line angles for mechanical retention.
Lab exercise depth: Initial central pit cut of ; occlusal extension approximately one-third of the intercuspal distance.
Margins and bevels: Cavo-surface bevels are typically contraindicated for porcelain to avoid fracture; bevels are commonly used on gold margins to aid in mechanical retention and burnishing.