Comprehensive History and Science of Dental restorative Materials

Ancient History and Early Dental Practices

Dental practices have been documented as far back as Ancient China in $2700\,A.C$, where acupuncture was utilized to treat various dental ailments. Furthermore, the Chinese were responsible for the creation of the first endoposts, which consisted of gold crowns supported by bamboo spikes. In the Mayan culture, early dentists employed rudimentary drills and used quartz powder as an abrasive. They were known for placing dental inlays in incisors and first molars using materials such as Jade, Hematite, Quartz, Cinnabar, and Iron Pyrite. The Mayans performed over $50$ different types of dental carvings, each likely holding specific ritualistic significance, including simple cuts, double cuts, removal of distal parts, and stippling. Notably, in Mayan archaeological findings, patients often did not present third molars.

The Aztec culture, spanning from $1168\,aC$ to $1520\,d.C$, made several contributions to dentistry. They performed dental extractions and placed a localized emphasis on halitosis (bad breath), often using herbal mixtures to combat it. They also utilized dental ornaments and practiced X-shaped incisions. The term "Tezcani o Tezoctezoani" refers to the Aztec dentist. In England, during the reign of Queen Elizabeth ($1533$-$1603$), the Queen was known to place pieces of cloth from her dresses in exposed dental cavities to obturate them.

Middle Ages to the Age of Enlightenment: From Monks to Professional Surgery

During the era of Monastic Medicine, monks were strictly prohibited from performing surgeries. Consequently, these tasks fell to barbers, who performed surgeries to treat cataracts, extracted bladder stones, opened abscesses, and extracted teeth. The first barber's guild was organized in Paris, France. Within this organization, barbers were forbidden from practicing surgery unless they were officially recognized members. Notable figures such as Roger Saleno and Rolando de Parma recommended avoiding extractions due to inherent risks, suggesting instead the application of leek seeds and henbane (beleño) along with cauterization.

In France, Ambroise Pare ($1509$-$1590$) became the first person to use lead or cork to obturate cavities. Later, Pierre Fouchard substituted missing teeth with ivory and bone and utilized wire for the correction of dental malformations. Fouchard also authored the comprehensive encyclopedia titled "Le chirurgien dentiste." Another Frenchman, Traveau, was the first to utilize "pasta blanca" (white paste), which was composed of silver coins and mercury. University medicine saw the foundation of the University of Salerno in the $X$ century, which integrated schools with hospitals for clinical practice.

The Foundations of Modern Dentistry and Educational Evolution

In the United States, several pivotal advancements occurred. Charles Goodyear invented vulcanized rubber, which allowed for the precise molding of dental prostheses. Greenee Vardiman Black (G.V. Black) conducted extensive research into dental fluorosis and the optimal composition for dental amalgam. He also investigated painless extractions using Nitrous Oxide. In $1919$, the North American Navy requested standardized regulations and evaluations for amalgams. The first dental university was established in the United States in $1840$, introducing the degree of "Doctor of Dental Surgery."

In the modern era, dentistry has diverged into numerous specialized fields known as Estomatological Specialties. These include Orthodontics, Pediatric Stomatology, Periodontics, Endodontics, Implantology, Maxillofacial Surgery, Oral Surgery, Dental Materials, Oral Pathology, and Oral Rehabilitation.

Fundamental Principles and Categorization of Dental Materials

Ideal dental materials must possess specific characteristics to be effective. They must be biocompatible, adhere permanently to bone or teeth, maintain a natural aesthetic appearance, and possess physical properties similar to enamel, dentin, and other dental tissues. Their primary goal is to restore or regenerate damaged tissues. Materials are classified according to their application:

1. Obturation Materials: Used for the sealing or closing of a cavity.

2. Restoration Materials: Used to seal a cavity while esthetically restoring the tooth's anatomy and permanent function.

3. Impression Materials: Used to create a negative copy of the oral tissues (the resulting gypsum model is the positive).

4. Prosthetic Materials: Used to replace teeth lost due to trauma or disease.

Standard requirements for obturation and restoration materials include good handling properties, high cavity-sealing capacity, low thermal conductivity, resistance to abrasion, resistance to dental fluids, and low solubility.

General Classification of Dental Materials

Materials for obturation and restoration are divided into two main categories:

Temporary Materials: These include Zinc Oxide, Plastic Cements, Provisit, and Cavit. They also involve fotopolymerizable materials and varnishes used for temporary shielding.

Permanent Materials: These include Amalgams, Gold Alloys, Permanent Cements, Porcelains, Composite Resins, and Glass Ionomers.

Prosthetic materials are utilized to replace teeth lost to caries, periodontal disease, or trauma, thereby restoring phonation (speech), deglutition (swallowing), and masticatory (chewing) functions. The choice of material depends on the prosthesis type: fixed, removable, partial, or complete. These prostheses may be supported by the tooth, the mucosa (gingiva), an implant, or a combination (e.g., mucosupported and tooth-supported in cases of total loss).

Common materials used for prostheses include Dental Composites, Acrylic Resins, Flexible Resins, Dental Porcelain, Dental Zirconia, Metals, Glass Fiber, and Carbon Fiber.

Impression Materials: Properties and Hydrocolloids

Impression materials are used to copy or reproduce in negative the hard and soft tissues of the oral cavity. Ideal traits include easy manipulation, adequate consistency and texture, proper wetting of oral tissues, high resistance to tearing or breaking, and elasticity to prevent permanent deformation after removal. They must also be compatible with modeling materials, accurate, have appropriate working and setting times, be easy to disinfect, and possess pleasant odors and flavors. They should not release gases during setting and must have a long shelf life.

Classification of impression materials includes Agar, Alginate, and Silicones (Condensation and Addition). Impressions are categorized as Primary or Secondary. A primary impression has a lower reproduction capacity and often requires a more fluid secondary material on top to achieve a faithful reproduction. Primary impressions do not capture small details but serve as a tray for the secondary material.

Hydrocolloids are coloidal states consisting of a solution (like sugar in water) where molecules are dispersed uniformly.

Reversible Hydrocolloid (Agar): These provide high-quality, resistant impressions but require the immediate creation of the positive model. They are mainly used for the construction of removable prostheses.

Irreversible Hydrocolloid (Alginate): An elastic material derived from marine algae. It emerged during World War II as a substitute for Agar and was further developed by Wilding in $1946$. Alginates are used for study models, fixed prostheses (improved alginates), working models, removable prostheses, Orthodontics, Orthopedics, and total Prosthodontics. They are non-toxic but can cause mechanical irritation if trapped. Handling involves adding powder to water in a rubber bowl, mixing for $45\,seg$ to $1\,min$ to achieve a creamy paste, and removing it from the mouth after at least $3\,min$.

Synthetic Polymers: Acrylic Resins

History and Classification: Acrylic resins began in $1940$ as denture bases and later expanded to include artificial teeth, veneers, impression trays, splints, night guards, and orthodontic appliances. The American Dental Association (ADA) classifies them under Specification $12$ (Denture bases), $13$ (Repair of resin appliances), and $15$ (Fabrication of teeth). They are categorized as Type I (Auto- or Quimiopolymerizable, powder-liquid) and Type II (Termopolymerizable, using powder, liquid, or plastic tablets).

Polymerization Stages:

1. Sandy Phase: The liquid contacts the powder.

2. Filamentous or Sticky Phase: The powder dissolves and polymerization begins.

3. Plastic Phase: The mass no longer sticks to fingers/spatula; this is the working stage.

4. Elastic Phase: Monomer evaporates, accompanied by an exothermic reaction.

5. Rigid Phase: The resin hardens.

Biological response to acrylics can include irritation from "Residual Monomer," vapor intoxication, contact dermatitis, and fungal growth (e.g., Candida) if there is excess monomer causing mucosal inflammation. Advantages include low cost, insolubility, and ease of use. Disadvantages include lower hardness than enamel and risk of fungal formation.

Rigid and Elastomeric Impression Materials

Modelina (Impression Compound): This is a rigid, thermoplastic material presented in tablets, bars, or cones. ADA Specification $3$ classifies it as thermoplastic: rigid when cold and plastic when heated. Type I is for oral cavity impressions (edentulous patients, primary impressions, or peripheral tracing), while Type II is for tray fabrication (now mostly obsolete). Manipulation involves softening in water or over a flame (for bars/cones).

Silicones (ADA Specification $19$): Used for study models, fixed prostheses, and total prosthodontics. They exhibit thixotropism, a property allowing them to flow under pressure into sulci. All polymers contract during polymerization: light and medium consistencies contract more, while heavy and extra-heavy (Putty) consistencies contract less. They provide accuracy up to $20\,\mu m$. Polysulfide rubbers offer the most working time.

Condensation Silicones: Require the positive model to be made within $1\,h$ of the impression. They have a short working time.

Addition Silicones: The most accurate with the highest dimensional stability. They can be hand-mixed (heavy) or used with cartridge mixing tips (light/medium). Mixing heavy consistencies with gloves is prohibited as it inhibits polymerization. These are the most expensive non-aqueous elastomers.

Dental Amalgams and Mercury Safety

G.V. Black standardized the proportions and placement of dental amalgams. ADA Specification $1$ governs these materials. Amalgams are indicated for restoring posterior teeth under occlusal load, provided the amalgam is surrounded by sound tooth tissue.

Physicochemical Properties: Amalgams conduct temperature and electricity and have high compressive strength. They can contract (preventing microfiltration) or expand (potentially causing tooth fracture). They exhibit "creep" (deformation over time) and oxidation.

Biological Response: They can irritate the pulp in deep cavities due to thermal changes and may cause galvanic shocks. Mercury content poses a risk of systemic contamination.

Manipulation:

1. Mix the capsule in an amalgamator for $10$-$15\,seg$.

2. Transfer to a cloth or dappen dish.

3. Carry to the cavity using an amalgam carrier and condense using a Mortenson condenser until overfilled.

4. Carve anatomy using a Cleoid-discoid instrument before crystallization.

5. Burnish and smooth with a ball burnisher.

6. Perform final polishing after $24\,h$ using stones and abrasive rubbers.

Pulp Protection Strategies and Materials

The pulp performs nutritive, sensory, protective, and formative functions. Pulp protectors aim to preserve vitality and prevent sensitivity by controlling bacteria, stimulating dentin formation, and providing a biocompatible seal.

Direct Pulp Capping: Used when the pulp is visibly exposed by caries, trauma, or mechanical action.

Indirect Pulp Capping: Used in deep cavities where the pulp is not exposed. Its function is to halt reversible pulp lesions and stimulate reparative dentin. Clinicians must distinguish between Infected Dentin (soft, bacteria-laden, non-remineralizable, must be removed) and Affected Dentin (demineralized but structural, lacks bacteria, can be remineralized, and is left to provide a $1\,mm$ buffer).

Materials for Protection:

• Calcium Hydroxide: High alkalinity ($\text{pH}$), antimicrobial, stimulates reparative dentin, but high solubility and low mechanical resistance. Used for both direct and indirect capping.

• Glass Ionomer: Direct bonding to dentin, fluoride release, but acidic $\text{pH}$ can be abrasive if used directly on exposed pulp. Used as an indirect protector.

• Mineral Trioxide Aggregate (MTA): High sealing capacity and biocompatibility due to alkaline $\text{pH}$. Best results for direct/indirect capping.

• Resin-modified Calcium Silicate: Fotocurable, high $\text{pH}$, stimulates hydroxyapatite and pulp regeneration.

Specialized Restorative Materials: Composites and Ionomers

Glass Ionomers (ADA $27$): Known for fluoride release and mechanical bonding to enamel and dentin. They are classified into Class A (occlusal restorations) and Class B (other uses). Nanoionomers combine glass ionomer with nanotechnology for better polish and strength. Compomers are composite/ionomer hybrids with lower fluoride release and higher wear.

Composite Resins: Aesthetic materials that suffer from polymerization shrinkage and can absorb pigments over time. Oxygen acts as a polymerization inhibitor, creating an "inhibited layer" that allows for the bonding of subsequent resin layers. Overheating or improper polymerization can lead to dental sensitivity or pulp necrosis. Handling quimiopolymerizable resins involves a $45\,seg$ mixing time and pressing with a matrix for $5\,min$.

Dental Cements: Chemical compounds that join two surfaces.

• Temporary Cements: Used for provisional restorations.

• Permanent Cements: High resistance to mastication.

• Specific variants: Zinc Oxide Eugenol (inhibits resin polymerization), Zinc Phosphate (high acidity), Resin Cements (foto/auto-curable for ceramics), and Glass Ionomer Cements (for metal/zirconia, releases fluoride).