8-Materials and Laboratory Protocols for Non-Metal Dental Crown Fabrication

INTRODUCTION TO NON-METAL DENTAL CROWN MATERIALS

Non-metal (metal-free) dental crowns are prosthetic restorations manufactured without a metal base or metal framework. These crowns are composed entirely of one of the following materials:

• Synthetic Plastic Material: Includes acrylic and composite resins.

• Mineral Ceramic Material: Includes various types of dental porcelains and glass-ceramics.

In clinical practice, dental crowns are generally categorized into three types based on the materials used in their construction:

1. Metal-Free Crown: Entirely made of synthetic resin or ceramic.

2. Full Metal Crown: Entirely composed of a metal alloy.

3. Veneered (Combined) Crown: A metal framework covered with a layer of ceramic or resin for aesthetics.

SYNTHETIC RESIN MATERIALS

Dental metal-free crowns made from synthetic resin materials are primarily utilized as temporary restorations. This limited application is due to three significant disadvantages:

• Insufficient Strength: They do not possess the required resistance to masticatory pressure.

• Rapid Abrasion: These materials are prone to wearing down relatively quickly during function.

• Lower Biocompatibility: The biological tolerance of surrounding soft tissues toward synthetic resin crowns is weaker compared to ceramics.

As of the current state of dental technology, the advancement of acrylic and composite materials is continuous. Improving material properties has led to a shifting trend where these crowns are gradually being indicated for permanent restorations rather than just temporary ones.

CERAMIC NON-METAL CROWNS

Ceramic non-metal crowns offer a distinct advantage over acrylic/composite versions. They are considered permanent restorations because they provide:

• Superior mechanical properties.

• Much higher biological compatibility.

• Exceptional aesthetic qualities that mimic natural tooth structure.

THE PORCELAIN JACKET CROWN

In $1895$, Lang introduced the first crown made entirely of porcelain. It was named the "jacket crown" because it was manufactured in a single piece and wrapped around the entire prepared tooth like a coat (jacket) to protect it.

Historical Technical Features

• Platinum Foil: The central feature was the use of a thin platinum foil measuring $0.01 - 0.02\,mm$. This foil was wrapped around the casted tooth die to act as a temporary metal base for the application and firing of the porcelain.

• Foil Removal: After firing and glazing were complete, the platinum foil was removed before cementation. Platinum was chosen because it does not form an oxide layer and therefore does not chemically bond with the porcelain.

Modifications and Evolution

• Reinforced Porcelain Jacket Crown: This modified version was fired on double platinum foil. For reinforcement, one layer of the foil remained permanently inside the crown and was cemented with it.

• Alumina Porcelain: In the $1960s$, the composition of dental porcelain transitioned from feldspar to aluminum oxide ($Al_2O_3$). This change approximately doubled the strength of the porcelain.

Preparation and Fabrication

• Finish Line: The tooth preparation always requires a shoulder finish line (either a rectangular shoulder or a deep rounded chamfer). This ensures the margins are thick enough to prevent fracture and allows masticatory pressure to be transferred through the shoulder for support.

• Foil Adaptations: The foil is cut into a rhomboid or semicircular shape (based on a VITA template). It is adapted to the die using a "tinsmith's seam" on either the palatal or proximal side. The foil must extend approximately $2\,mm$ below the shoulder.

• Cleaning: Before porcelain application, the foil is cleaned by heating it to red-hot or placing it in a ceramic furnace at $800\,^\circ C$ for $6\,minutes$.

LABORATORY PROTOCOL FOR PORCELAIN APPLICATION

Layering and Condensation

After cleaning the foil, porcelain powder is mixed with liquid or distilled water to a thick, creamy consistency. It is applied layer by layer using a brush or spatula.

• The First Base Layer: Contains a high percentage of aluminum oxide. It provides strength and prevents translucency, acting as a non-metal coping. It is shaped to a thickness of about half the width of the shoulder.

• Vibration and Condensation: Each applied layer (known as a "slip") is vibrated using a special instrument (Le Cron) passed along the die post. This brings excess liquid to the surface and ensures better condensation, which minimizes shrinkage during firing.

• Absorption: Excess liquid is absorbed with a paper tissue.

Firing and Shrinkage

• The First Firing: The applied layer and foil are placed on a refractory support, dried in the furnace pre-chamber, and then undergo vacuum firing.

• Shrinkage: During firing, the porcelain mass contracts by approximately $15 \,$. This causes the margin to pull away slightly from the finish line shoulder.

• Corrections: After the first firing, freshly mixed mass is applied to correct the contraction. This is followed by definitive layering (cervical, body, and incisal shades) and a second firing.

Finalizing the Restoration

• Try-in: The crown is finished and sent for a clinical trial in the patient's mouth.

• Glazing: Necessary corrections (often at proximal contact points) are made, and the crown is glazed. Glazing involves melting the surface layer at high temperatures to form a smooth, glass-like surface that seals pores.

• Final Delivery: After cooling, the platinum foil is removed from the internal surface, and the crown is ready for cementation.

VITA IN-CERAM PORCELAIN CROWN

The VITA In-Ceram system is a non-metal (all-ceramic) crown featuring a framework made of oxide-ceramic material rather than metal.

• Core Composition: A stronger oxide-ceramic core made of aluminum oxide (alumina) particles bonded with a thin glass layer.

• Indications: Suitable for single crowns and small bridges (specifically consisting of $2\,crowns$ with $1\,pontic$ in between).

Fabrication Process

1. Model Duplication: The working model is duplicated using a highly heat-resistant investment material instead of regular gypsum.

2. Sintering: An oxide-ceramic slurry (slip) is applied to the die and sintered in a Vita In-Ceram furnace to create a porous coping.

3. Infiltration Firing: The porous sintered framework is covered with glass powder and fired again at $1100\,^\circ C$. The molten glass infiltrates the spaces between the particles. This process vitrifies the framework (making it glass-like).

4. Final Layering: The full anatomical shape is achieved by layering standard cervical, dentin, and incisal porcelain (Vitadur-Alpha).

IVOCLAR PORCELAIN SYSTEM: IPS EMPRESS

IPS Empress is a significant system that introduced the protocol for melting and casting (pressing) porcelain. It is used for crowns, inlays, onlays, and vestibular veneers (laminates).

• Material: Ceramic blocks (pellets or ingots) reinforced with leucite.

• The Pressing Technique: The blocks are melted at $1180\,^\circ C$ and injected into a silicone flask mold under a pressure of $3.5\,bar$.

Processing Methods

1. Layering Method (Schicht Technique): The crown is modeled in wax as a reduced form or shell. Once the shell is pressed, ceramic is applied over it in layers to achieve final color and anatomy.

2. Staining Method (Mal Technique): Used for posterior inlays, onlays, and crowns. The restoration is modeled in wax to its full anatomical form. After pressing, it is directly colored/stained and glazed.

DICOR CROWN SYSTEM

Introduced by DeTrey Dentsply, this system is based on glass-ceramic technology utilizing the mineral MICA.

• Material Characteristics: Mica is a natural mineral of crystalline structure (oxides of $SiO_2 \cdot K_2O \cdot MgO \cdot Al_2O_3 \cdot ZrO_2$ reinforced with fluorine). It produces a "chameleon effect," mimicking natural tooth light reflection. It also offers superior plaque resistance.

• Fabrication: The restoration is cast like a metal alloy from "Dicor thermo-cones" at a temperature of $1380\,^\circ C$.

• Ceramization (Devitrification): The initial cast crown is transparent (like frosted glass). A second thermal treatment initiates controlled crystal formation, transforming the glass into a glass-ceramic.

• Final Aesthetics: Can be finished via staining or by building a core and layering with "Dicor Plus" ceramics (which includes $9\,dentin$, $2\,incisal$, and $2\,transparent$ shades).

SUMMARY OF ALL-CERAMIC SYSTEM CHARACTERISTICS

CONCLUSION ON METAL-FREE CONSTRUCTIONS

While traditional metal-based restorations are strong, they face modern disadvantages including potential corrosion (especially with base-metal alloys) and inferior aesthetics due to the blockage of light translucency by the metal coping.

Currently, there are three primary methods for fabricating non-metal crowns:

1. Layering and Firing: Traditional buildup techniques.

2. Casting (Press Ceramics): Melting and injecting material into a mold.

3. Milled Ceramics (CAD-CAM): Computer-controlled milling of prefabricated ceramic blocks.

All-ceramic constructions offer superior aesthetics and biocompatibility but require specialized equipment and are generally more expensive and complex to fabricate than metal-ceramic options