Materials Science Hardness and Wear Testing

Hardness Testing Methods

• Different methods of hardness testing exist depending on the indenter's shape.
• Common tests include Vickers, Knoop, Brinnell, and Rockwell.

Fatigue Wear

• Characterized by crack initiation, crack propagation, and catastrophic failure.
• Crack initiation takes place at a high stress concentration point.
• Crack propagation occurs gradually with each stress cycle.
• Catastrophic failure occurs quickly when the crack reaches a critical size.

Fatigue Failure

• Structures can fail at stresses below the material's tensile strength when exposed to repeated stress cycles.
• Fatigue failure occurs at low loads with no outward visible signs.

Corrosion Wear

• Occurs primarily in oral environments due to combined mechanical and chemical attacks.
• Abrasion-erosion lesions are a common example.

Creep

• Occurs when materials undergo permanent deformation under sustained or cyclic loading for a period of time.
• Factors impacting creep include applied load, temperature, and time.

Surface Contact Angle

• The angle between the surface of a material and a liquid droplet in contact with it.
• Influences the wetting and adhesion properties of materials.

Elastic and Plastic Behaviour

• Elastic behaviour exhibits instantaneous and full recovery after load removal.
• Plastic behavior retains deformation after load removal.

Viscoelastic Behaviour

• Deformation in materials, particularly polymers, can be time-dependent.
• Recovery after loading can be complete (elastic) or incomplete (viscous).

Stress Relaxation

• The reduction of stress in a material under constant strain.
• The rate of stress relaxation is crucial in orthodontic elastic bands.
• Latex bands have a constant relaxation rate while plastic bands exhibit variable relaxation.

Viscoelasticity in Dental Materials

• Viscoelastic behavior is important when removing elastic impression materials from undercuts.
• Permanent distortion during removal depends on force magnitude and duration.
• Force magnitude depends on elastic modulus, material thickness, and undercut depth.
• Distortion can be represented as time x force.

Rheological Properties

• Describe the deformation behavior of materials under stress.
• Elasticity and viscoelasticity theory explains rheology in solids and elastomers.
• Viscosity, the resistance of a fluid to flow, explains the rheology of liquids and pastes.

Rheological Behaviour and Shear Rate

• Dilatant: Viscosity increases with increasing shear rate (shear thickening).
• Newtonian: Shear stress is directly proportional to shear rate, and viscosity remains constant.
• Pseudoplastic: Viscosity decreases with increasing shear rate (shear thinning).

Rheological Behaviour of Dental Materials

• Non-setting materials generally experience reduced viscosity with increasing temperature.
• Conversely, setting materials exhibit increased viscosity with rising temperature.
• Viscosity increases during the setting process until manipulation becomes impossible.
• Working time is the time from mixing to reaching the point of unmanageability.
• Setting time is the time from mixing until the desired initial properties are achieved.

Damage to Teeth

• Caries (Preventable): Biofilm-mediated, chronic disease caused by dietary carbohydrates and oral microbial imbalance.
• Non-Carious: Irreversible loss of tooth surface not caused by caries or trauma.

Non-Carious Damage

• Tooth Wear: Attrition (tooth-to-tooth contact), Abrasion (tooth-to-non-tooth contact), Abfraction (cervical v-shaped enamel-dentin defect).
• Developmental Defects: Hereditary (Amelogenesis imperfecta, Dentinogenesis imperfecta) and Non-hereditary (Hypoplasia, Hypomineralization).
• Trauma: Accidental and non-accidental.

Causes of Erosion

• Intrinsic acid (gastric acid) - involuntary.
• Extrinsic acid (dietary-acidic foods and drinks).
• Environmental (acid fumes).

Mechanical Wear: Tooth to Tooth Contact

• Functional: Normal chewing.
• Parafunctional: Habits like bruxism.
• Proximal wear: Resulting from physiological tooth movements.

Abrasion

• Mechanical/frictional wear caused by contact between teeth and non-tooth surfaces.
• Caused by habits like nail biting, bottle opening, stripping wire, and chewing abrasive materials.

Abfraction

• Cervical wedge-shaped enamel or dentin wear, attributed to masticatory stress concentrated in the cervical area.
• Likely a multifactorial process involving erosion and toothbrush abrasion.

Stephan's Curve

• Shows a sudden reduction in plaque pH following glucose rinse, returning to normal within 30-60 minutes.
• Net demineralization occurs below a critical pH of 5.5.
• Mature plaque metabolizes fermentable carbohydrates, producing acid.

Progression of Caries/Caries Balance

• A dynamic equilibrium between demineralization and remineralization processes.
• Affected by factors like diet, oral hygiene, and fluoride exposure.

Rampant Caries

• Accelerated caries with multiple carious lesions.
• Occurs in infants (nursing bottle caries), teenagers/youth (cariogenic diet, recreational drugs), and adults (xerostomia).

Arrested Caries

• Caries lesions that have stopped progressing.
• Enamel lesions: White lesions can be arrested and potentially reversed with proper oral hygiene.
• Dentin lesions: Can be arrested but not reversed.

Enamel Caries

• Subsurface demineralization causes enamel prism porosities (white spot lesions).
• White spot lesions can be remineralized with proper oral hygiene and fluoride exposure.
• Enamel caries progresses as an inverted cone towards the dentino-enamel junction (DEJ).
• Active white spot lesions are smooth, frosty white, and non-cavitated.
• Arrested lesions have smooth, shiny surfaces.
• Porous lesions can become stained, forming brown spot lesions.
• Advanced lesions become rough and prone to cavitation.

Caries at the DEJ

• Caries histologically reaches the DEJ before cavitation.
• The pulp responds with a translucent dentin zone and tertiary dentin formation.
• The lesion extends laterally and penetrates the dentinal tubules.

Caries in Dentin

• Visibly creates a gray shadow/opacity.
• Triggers pulpal response to hot, cold, and sweet stimuli.
• Active caries: Dentin appears moist, matte, rough, and soft.
• Arrested caries: Dentin appears shiny, hard, and scratchy.

Soft Dentin/Infected Dentin

• Outer carious dentin, characterized by bacterial contamination.
• Low mineral content and irreversibly denatured collagen.
• Does not self-repair.
• Easily excavated with hand and rotary instruments.

Firm Dentin/Affected Dentin

• Inner carious dentin with demineralized intertubular dentin.
• Transparent appearance.
• Softer than normal dentin.
• Can be remineralized.
• Resistant to hand excavation and only removed by exerting pressure.
• Transition zone between soft and firm dentin has a leathery texture.

Hard Dentin

• Deepest zone of a caries lesion, including sound normal dentin.
• Hard, resistant to penetration by a blunt explorer.
• Removed only by a bur or a sharp cutting instrument.

Minimal Invasive Dentistry (MID)

• Focuses on early detection and minimal intervention to restore damaged tooth structure.
• Emphasizes management of risk factors rather than solely relying on restorative treatment.

Caries Detection and Diagnostic Aids

• Visual and clinical methods: Mirror, probe, magnification (loupes, headlight), wet/dry field, caries detecting dyes.
• Radiographs: Intraoral (bitewing, periapical), extraoral (OPG).
• Optical/trans illumination: Intraoral cameras, LED cameras, FOTI, laser, QLF systems.
• Tooth separators.

Principles of Caries Management in MID

• Disease control by reducing cariogenic bacteria.
• Remineralization of early lesions.
• Minimize tooth structure removal.
• Use of dental materials to preserve tooth structure.

International Caries Detection and Assessment System (ICDAS)

• A standardized system for classifying and assessing caries lesions.
• Uses visual and tactile criteria for diagnosis.

ICCMS Risk Factors

• Factors that contribute to an individual's risk of developing caries.
• Include dietary habits, oral hygiene practices, fluoride exposure, and genetics.

Caries Treatment

• Non-invasive: Oral hygiene instruction, topical fluoride, fissure sealants, varnish.
• Micro-invasive: Pit and fissure sealants.
• Selectively invasive: Partial caries removal.
• Invasive: Conventional preparations and restorations.

Prevention

• Non-invasive and preventive approaches.
• Fluoride: Promotes enamel remineralization.
• Casein phosphopeptide-amorphous calcium phosphate (CPP-ACP): Effective for remineralization.
• Xylitol: Suppresses cariogenic bacteria and promotes beneficial bacteria.
• Dental ozone: Antibacterial and remineralization properties.

Dental Ozone

• Indications: Primary root carious lesions and primary pit and fissure carious lesions.
• Advantages: Kills microorganisms, oxidizes caries, speeds up remineralization, whitens discolored caries, reduces treatment time, painless, noiseless, no allergic reactions, removes organic debris, minimal resistance development.
• Contraindications: Highly irritating to eyes and lungs.

MID: Alternatives to Rotary Instruments

• Minimally invasive techniques for caries removal.
• Air abrasion: Impacts abrasive particles in a stream of air or air-water.
• Hard tissue laser: Precision tool for accessing enamel and dentin caries.
• Chemo-mechanical caries removal: Uses chemical agents to dissolve carious dentin.

Pit and Fissure Sealants

• Low viscosity plastic materials applied to occlude pits and fissures, preventing caries development.
• Act as a physical barrier and eliminate the cariogenic environment.

Pit and Fissure Sealant Application Techniques

• Acid etch: Removes plaque and creates a microsurface for better sealant adhesion.
• Field isolation: Essential to prevent contamination during application.
• Pit and fissure preparation: Not typically needed for complete occlusion.
• Complete sealing: Critical for optimal protection.
• Minimal sealant application: Avoid bulky overfilled sealants.

GIC-Based Pit and Fissure Sealants

• Contain fluoride for enhanced caries protection.
• Adhere under moist conditions.
• Lower retention rate compared to composite resins.

Resin Sealants

• Flow readily, adhere strongly, and withstand masticatory forces.
• Offer good resistance to wear and tear.
• Available in both clear and opaque versions.

Steps in Pit and Fissure Sealant Restoration

• Clean and dry the tooth surface.
• Acid etch the enamel for 15-30 seconds.
• Rinse and dry thoroughly.
• Apply sealant material without trapping air.
• Light cure the sealant for 20 seconds per application.
• Check for complete sealing and smooth surfaces.

Light-Cured Resin Sealant Clinical Procedure

• Clean the tooth with pumice or prophy paste.
• Rinse and dry.
• Acid etch for 15 seconds.
• Rinse and dry.
• Apply the sealant.
• Light cure for 20 seconds.
• Inspect for complete sealing.