6.2 Dental Materials Science, Biomaterials, and Impression Systems

2.4 Dental Materials Science, Biomaterials, and Impression Systems

INBDE Core Concept: Understanding physical, chemical, and mechanical properties of dental biomaterials allows clinicians to select appropriate restorative materials, manage polymerization stress, execute proper adhesive bonding protocols, and achieve accurate clinical impressions.

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Restorative Biomaterials: Dental Amalgam

Dental amalgam is an alloy created by mixing liquid mercury with silver-tin alloy particles.

Alloy Phases & High-Copper Evolution

• Low-Copper Amalgams (Historical):
  • $\gamma$ (Gamma) phase: $Ag_3Sn$ (strongest phase).
  • $\gamma_1$ (Gamma-1) phase: $Ag_2Hg_3$ (matrix phase).
  • $\gamma_2$ (Gamma-2) phase: $Sn_8Hg$ (weakest, most corrosion-prone phase that causes marginal breakdown and breakdown creep).
• High-Copper Amalgams (>12% Copper - Modern Standard):
  • Added copper reacts preferentially with tin to form the $\eta$ (eta) phase ($Cu_6Sn_5$).
  • This reaction completely eliminates the corrosion-prone $\gamma_2$ phase ($Sn_8Hg$), dramatically reducing marginal breakdown, ditching, and creep.

$\text{High-Copper Reaction:} \quad Sn + Cu \longrightarrow Cu_6Sn_5 \quad (\text{Eliminates } Sn_8Hg / \gamma_2 \text{ phase})$

Particle Types

1. Lathe-cut: Irregular, jagged particles; requires higher condensation force and provides good resistance to matrix band overhangs.
2. Spherical: Smooth spheres; requires lower condensation force, sets faster, but can be difficult to establish tight interproximal contacts.
3. Admixed: Blend of lathe-cut and spherical particles; combines ease of condensation with excellent proximal contact placement.

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Resin Composites & Polymerization Kinetics

Direct composite resins consist of an organic polymer matrix, inorganic filler particles, a coupling agent, and an initiator system.

Key Components

• Resin Matrix: Dimethacrylate monomers such as Bis-GMA (bisphenol A-glycidyl methacrylate) or UDMA (urethane dimethacrylate).
• Diluent Monomer: TEGDMA (triethylene glycol dimethacrylate) added to reduce viscosity and allow high filler loading.
• Inorganic Fillers: Silica, quartz, or barium glass. Higher filler loading increases flexural strength and wear resistance while decreasing thermal expansion and polymerization shrinkage.
• Coupling Agent: Silane (organosilane), which chemically bonds inorganic filler particles to the organic resin matrix.
• Photoinitiator: Camphorquinone (CQ), activated by blue light at a wavelength of 465–470 nm.

Polymerization Shrinkage & C-Factor

All resin composites undergo volumetric polymerization shrinkage of 2% to 5% during curing.

Cavity Configuration Factor (C-Factor): Defined as the ratio of bonded surfaces to unbonded (free) surfaces in a cavity preparation.

$\text{C-Factor} = \frac{\text{Number of Bonded Surfaces}}{\text{Number of Unbonded Surfaces}}$

Clinical Mitigation: Apply composite in incremental layers (< 2 mm thickness) to minimize the C-factor, ensure complete light penetration, and decrease overall polymerization stress.

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Dental Cements & Luting Agents

Luting cements secure indirect restorations (crowns, bridges, inlays) to prepared tooth structure.

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Impression Materials & Elastomeric Kinetics

VPS Hydrogen Gas Release: Some VPS formulations produce secondary hydrogen gas. Immediate pouring can cause small gas bubbles on the stone model surface. Clinicians should wait 30 minutes before pouring unless palladium gas scavengers are added to the formulation.

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Restorative Ceramics: PFM, Lithium Disilicate, and Zirconia

Modern indirect ceramics are classified into glass-based ceramics and high-strength polycrystalline ceramics.

Ceramic Strength Spectrum:
Feldspathic Porcelain (~70-90 MPa) ◄── Lithium Disilicate (~400 MPa) ◄── Monolithic Zirconia (900-1200 MPa)

1. Porcelain-Fused-to-Metal (PFM)

• Consists of a metal coping overlaid with feldspathic porcelain.
• Chemical bonding occurs via an oxide layer formed on the metal substrate during degassing.

2. Lithium Disilicate ($Li_2Si_2O_5$ - e.g., IPS e.max)

• Glass-ceramic containing ~70% needle-like lithium disilicate crystals embedded in a glass matrix.
• Flexural strength: ~400 MPa.
• Etching Protocol: Etched with hydrofluoric acid (HF) for 20 seconds, followed by application of a silane coupling agent for micromechanical and chemical resin bonding.

3. Zirconia ($ZrO_2$)

• Monolithic polycrystalline ceramic containing no glass phase. Stabilized with 3 mol% yttria (3Y-TZP).
• Flexural strength: 900–1200 MPa (highest strength ceramic).
• Transformation Toughening Mechanism: Under mechanical stress at a crack tip, zirconia undergoes a phase transformation from the tetragonal phase to the monoclinic phase. This transformation is accompanied by a 3% to 5% volume expansion that exerts compressive forces on the crack, effectively pinching the crack shut and halting crack propagation!