Dental Anatomy: Enamel Composition and Properties

Enamel

• Enamel is the outer layer covering the anatomical crown of teeth.
• It is the hardest tissue in the body and provides protection from masticatory forces.
• Enamel is acellular, inert, non-vital, and insensitive, meaning it cannot regenerate once damaged.

Chemical Composition

• Enamel is composed of 96% inorganic material and 4% organic material.
• The inorganic component is primarily hydroxyapatite, a crystalline calcium phosphate, with ions like fluoride, strontium, magnesium, lead, and water.
• The organic content consists of non-collagenous proteins.

Physical Properties

• Enamel is extremely hard but brittle.
• Its underlying layer of dentin provides resilience to maintain its integrity.
• Enamel is translucent, varying in color from light to yellow to grayish-white depending on the underlying dentin.
• Thickness varies from 2.5 mm on the working surfaces to a feather edge at the cervical line.
• Enamel functions as a semipermeable membrane.

Histological Structure

• Due to its highly crystalline nature, studying enamel in decalcified sections only reveals empty spaces where the minerals have been dissolved.
• Enamel prisms (rods) are the fundamental structural units of enamel.

Enamel Prisms

• Number: The lower incisor has approximately 5 million rods, while the upper first molar has up to 12 million.
• Direction: Enamel rods run perpendicular to the dentin surface.
  • In deciduous teeth, they are roughly vertical at the cusp tip or incisal ridge, then become oblique towards the occlusal surface in the middle third, and horizontal in the cervical region where the enamel ends as a thick layer.
  • In permanent teeth, the enamel rods are similar to deciduous teeth in the occlusal and middle thirds, but in the cervical region, they deviate from horizontal and become oblique towards the root. This is due to the enamel ending as a knife edge at the cervix.
• Course: Individual enamel rods exhibit a wavy course from the amelodentinal junction outward before straightening just prior to the outer enamel surface.
  • Under the cusp tips and incisal ridges, enamel rods become twisted and braided, forming "gnarled enamel" for increased strength in areas subjected to masticatory forces.
• Diameter: The diameter of the enamel rods varies between the amelodentinal junction (ADJ) and the outer enamel surface (outer surface diameter is twice that of ADJ diameter).
  • At the ADJ, diameter is 3-4 microns, while at the outer surface, it can reach 8 microns. This difference is attributed to the variation in surface area between the inner and outer enamel surfaces.

Enamel structure by the light microscope

• Under a light microscope, both longitudinal and transverse sections of enamel rods appear clear and structureless because of the tightly packed hydroxyapatite crystals allowing light transmission.
• The prism sheath forms an incomplete envelope around the prism and is less calcified (higher organic content) than the rod itself.
• The interprismatic substance separates enamel rods and is as highly calcified as the rods.
• Cross striation or short increments are less calcified dark lines appearing in longitudinally ground sections when treated with mild etching solutions, giving the enamel rods a striated appearance.

Hunter Schreger Bands

• Hunter Schreger bands are an optical phenomenon resulting from changes in rod direction.
• They are visible in longitudinal ground sections viewed by reflected light and are found in the inner two-thirds of the enamel.

Enamel Surface Structures

• Outer Structureless enamel (prismless enamel) is present on the outer layer of enamel where apatite crystals are arranged in parallel.
• The outer 15µm of enamel is highly mineralized.
• Outer structureless enamel is commonly found in the cervical area.
• Perikymata are shallow furrows on the enamel's outer surface.
• They are parallel to the CEJ and their number decreases towards the occlusal surface.
• Rod ends are concave depressions on the enamel surface.
• Cracks are fissure-like structures found on enamel surfaces that disappear with decalcification.

Hypocalcified Structures of Enamel

• Amelodentinal junction (ADJ) is the scalloped junction between enamel and dentin.
• Enamel tufts are less mineralized areas found in the inner third of enamel.
• They contain higher concentrations of enamel proteins and appear branched.
• Enamel spindles are short, straight, dark structures extending into enamel.
• They are residual tubules formed by odontoblastic processes during odontogenesis.
• Enamel lamellae are fissure-like defects extending from the enamel surface.
• True lamellae may contain enamel proteins or organic debris from the oral cavity.

Incremental Lines of Enamel

• Cross striations, also known as short increments, can be seen running along the length of each enamel rod.
• They are thought to represent daily enamel deposition by ameloblasts.
• Incremental lines of Retzius are dark bands in enamel caused by variations in organic material deposition.
• In longitudinal section, they appear as transverse depressions called perikymata.
• In cross section, they appear as concentric rings.
• Neonatal line is a dark line in enamel formed at birth.
• It marks the transition between enamel formed before and after birth.
• Prenatal enamel is generally of better quality than postnatal enamel.

Enamel Structure

• Enamel is made up of enamel rods
• The number of enamel rods varies depending on the tooth: 5 million in lower central incisors and 12 million in upper first molars.
• The course of enamel rods is affected by Hunter-Schreger Bands, which are absent in the region of gnarled enamel.

Submicroscopic Structure of Enamel Rods

• Enamel rods have a keyhole-like, fish scale, oval-rounded, hexagonal, or cylindrical shape.
• Enamel rods are separated by less calcified dark lines called cross striations.

Keyhole Pattern

• Enamel prisms are highly calcified and have a keyhole or fish scale shape.
• Interprismatic regions are highly calcified and surround the prism.
• Prism sheath is less calcified than the prism and forms an incomplete envelope around it.

Electron Microscopy

• Enamel prisms are cylindrical and have crystals with their long axes parallel to the prism's long axis in the central part.
• At the surface, the crystals are twisted under the cusp tip and incisal edge, forming gnarled enamel.
• The diameter of the enamel prisms increases towards the surface, with a ratio of 1:2.
• The longitudinal section of the prism is cylindrical and shows cross striations。
• The cross section of the prism is hexagonal, fish scales, or keyhole pattern.

Brown Striae of Retzius

• Brown striae of Retzius are rhythmic deposition of enamel associated with the weekly rest period of ameloblasts.
• Striae are about 3-4 μm apart, representing one day of enamel deposition.
• Enamel lamellae are classified into three types: A, B, and C.
• Type A: Occurs during enamel formation but before calcification, with less common occurrence and limited to enamel.
• Type B: Occurs during enamel formation but before eruption, with less common occurrence and may cross the amelodentinal junction (ADJ) to reach the dentine.
• Type C: Occurs after enamel formation but before eruption, with more common occurrence and may cross the ADJ to reach the dentine; contains organic materials from saliva.

Age Change in Enamel

• The text mentions age changes in enamel, but does not specify what they are.

Dentin

• Interglobular dentin is a form of hypomineralized dentin.
  • Size: Large
  • Cause: Areas of unmineralized or hypomineralized dentin.
  • Site: Appears in the crown just below mantle dentin.
• Tomes' Granular layer is a constant feature of root dentin, found adjacent to the cementum.
  • Size: Small, granular in appearance.
  • Cause: It is a result of the looping of the terminal portions of dentinal tubules.
  • Site: Appears in the root adjacent to the cementum.
• Incremental lines of dentin are the result of the rhythmic deposition of dentin.
  • Incremental lines of Von-Ebner: Reflect normal rhythmic linear pattern of dentin deposition.
    • Rate: 4-8 microns in crown, less in root.
    • Appearance: Run at right angles to the dentinal tubules.
  • Contour lines of Owen: Are accentuated incremental lines of Von-Ebner.
    • Cause: Disturbance in matrix mineralization process.
    • Appearance: Easily seen in longitudinal ground section.
  • Neonatal line: A line that marks the change in dentin formation at birth.
    • Appearance: Seen in primary teeth and first permanent molars.
    • Formation: Dentin nearer the ADJ was formed prior to birth, dentin nearer to the pulp was formed after birth.

Dentin Junctions

- **Dentino-enamel junction:**  A smooth, wavy junction that marks the boundary between the enamel and the dentin. 
- **Dentino-cemental junction:**  A smooth junction in permanent teeth and scalloped in deciduous teeth.
    - **Appearance:** Often masked by the granular layer of Tomes.

Peri-tubular dentin

- **Location:** Immediately surrounds the dentinal tubules. 
- **Characteristics:**  Highly calcified matrix with little collagen fibrils.
- **Appearance:** Translucent rings around odontoblastic processes spaces with inter-tubular dentin in between.

Periodontoblastic space

- **Location:** Space between the wall of the tubules and odontoblastic processes.   
- **Content:** Contains “tissue fluid”, dentinal lymph.
- **Function:** Plays an important role in tissue changes within the dentin.

Dentinal tubules

- **Description:** Closely packed, transverse units that make up the structure of dentin.
- **Content:**  Cytoplasmic processes of odontoblasts (Tomes' fibers).
- **Appearance:**  More closely packed towards the pulpal surface than the outer surface.
- **Diameter:** Varies along the length of the tubule:
    - 2.5 µm near the pulpal surface.
    - 1.2 µm in the mid root portion
    - 900 nm near the ADJ.
- **Lateral branches:** Side branches that originate at a right angle to the dentinal tubules.
- **Terminal branches:**  Branches that divide into two at the outer dentin surface near the ADJ.

Dentin: Curvatures of dentinal tubules

- **Primary curvature:** S-shaped curve of tubules with the convex side facing the root.
- **Secondary curvature:** Spiral tracks taken by odontoblasts during their course from the outer dentin to the pulp. 

Inter-tubular dentin

- **Location:**  Dentin located between the dentinal tubules.
- **Characteristics:**  Contains tightly interwoven network of type I collagen fibrils.
- **Position of apatite crystals:**   Deposited in a random, roughly right angle orientation to the dentinal tubules.
- **Function:**    Makes up the main bulk of dentin.

Dentinal tubules:

- **Profuse branching:** Makes dentin permeable, possibly providing a pathway for the invasion of caries.
- **Occupants:**  Filled with tissue fluid and odontoblastic processes. 
- **Adult tissue:** The extent of odontoblastic processes varies. 

Tubular Dentin

• Tubules are less regular and twisted compared to atubular dentin
• Atubular dentin has irregularly scattered and twisted tubules, some areas may lack tubules
• Odontoblastic processes are exposed and odontoblasts are damaged due to severe stimulation
• This can lead to the formation of reactive dentin or reparative dentin
  • Reactive dentin is formed by injured odontoblasts continuing to form irregular dentin
  • Reparative dentin is formed by degenerated odontoblasts being replaced by undifferentiated cells, which then form pulp

Osteodentin

• Occurs when dentin-forming cells are embedded in rapidly forming matrix
• These cells degenerate and leave empty spaces in the matrix

Vasodentin

• Entrapment of blood vessels in dentin

Clinical Considerations of Dentin

• Dentin sensitivity is not a symptom of caries unless the pulp is inflamed
• Air-driven cutting instruments can damage odontoblasts at the pulp periphery
• Indirect pulp capping using calcium hydroxide (Ca(OH)2) stimulates reparative dentin formation
• Dentin tubules allow substances applied to the surface to reach and affect the pulp
• Marginal seal quality of restorative materials is important to prevent microleakage and bacterial contamination
• Tooth colour is affected by enamel translucency and dentin thickness
• Dentin thickness increases with age, contributing to tooth yellowing and making vitality testing more difficult
• Deposition of tertiary dentin reduces pulp chamber volume and alters its shape, making endodontic procedures more challenging
• Exposure of 1 mm3 of dentin damages approximately 30,000 odontoblasts, emphasizing the importance of conservative cavity preparation and non-irritant sealing materials
• The smear layer, composed of dentin instrumentation debris, reduces dentinal tubule permeability
• Smear layer entry into tubules is dependent on the size of smear particles and the diameter of the tubules

Advantages and Disadvantages of the Smear Layer

• Advantages
  • Reduces permeability
  • Minimizes infiltration of harmful agents
• Disadvantages
  • Interferes with dentin bonding
  • Difficult to remove by irrigation
  • Can be removed or modified by acid conditioning, but this can damage the pulp

Dentin Sensitivity

• Enamel and cementum are not sensitive, while dentin exhibits sensitivity
• Sensitivity is highest at the dentinoenamel junction (DEJ) and near the pulp
• Theories explaining dentin sensitivity:
  • Direct neural stimulation: Nerve endings in dentin respond directly to stimuli
  • Odontoblastic transduction theory: Stimuli excite odontoblastic processes, transmitting impulses to nerve endings
  • Fluid or hydrodynamic theory: Stimuli affect fluid movement in dentinal tubules, stimulating nerve endings in the plexus of Rashkow

Age Changes in Dentin

• Changes in primary dentin:
  • Transparent/translucent/sclerotic dentin: Calcification of dentinal tubules with fatty degeneration and calcification of odontoblastic processes
  • Dead tracts: Completely destroyed or damaged odontoblastic processes, appearing black in transmitted light and white in reflected light
• Regular secondary dentin: Normally occurring dentin formed throughout life
• Tertiary dentin: Dentin formed in response to injury or irritation

Transparent/translucent/sclerotic dentin

• Increases the mineralization of the tooth, leading to:
  • Decreased conductivity of odontoblastic processes
  • Slowed carious process (defensive)
  • Prolonged pulp vitality
  • Decreased dentin permeability
  • Increased dentin brittleness

Dead Tracts

• Sever stimulation causes complete destruction or damage to odontoblastic processes in the affected area
• Appear black in transmitted light and white in reflected light due to the difference in refractive index between affected and normal tubules
• Dead tracts are bounded by a narrow zone of sclerotic dentin
• Dried ground sections of normal dentin may show empty tubules with an appearance similar to dead tracts due to the crowding of odontoblasts.

Transparent/Translucent Sclerotic Dentin

• A regressive alteration in the tooth hard substance characterized by calcification of dentinal tubules.
• Occurs due to fatty degeneration and then calcification of odontoblastic processes.
• The most likely source of calcium salts is the fluid of dentinal lymph within tubules.
• Prevalent in older individuals as a manifestation of normal aging.
• Can result from dentin injury by slow caries or abrasion.
• Has a uniform refractive index because dentinal tubules are occluded, widening the peritubular zones and reducing odontoblastic processes in diameter.
• Appears translucent when viewed with transmitted light, and dark when viewed with reflected light.
• Leads to occlusion of the dentinal tubules by calcified deposits.
• Increased mineralization leads to:
  • Decreased conductivity of the odontoblastic processes.
  • Slower carious process (defensive).
  • Prolonged pulp vitality.
  • Decreased permeability of dentin.
  • Increased brittleness of dentin.

Dried Ground Dentin

• Odontoblasts in the narrow pulpal horn die.
• This leaves empty tubules.
• These tubules may be filled with air.
• The tubules resemble a dead tract.

Secondary Dentin

• Develops after root formation is complete
• Deposited by odontoblasts, but much slower than primary dentin
• Tubular structure, but less regular than primary dentin
• Tubules change direction and have a wavy course
• Fewer dentinal tubules per unit area than primary dentin
• Continuous deposition reduces pulp surface area, crowding odontoblasts
• Crowding leads to odontoblast degeneration and rearrangement
• Formation is stimulated by mild stimuli such as slow attrition and caries
• Leads to pulp reduction in size ("pulp recession"), which reduces the risk of pulp exposure during cavity preparation
• A darkly staining line (line of demarcation) marks the transition between primary and secondary dentin
• This line shows changes in:
  • Dentinal tubule direction
  • Wavier course of dentinal tubules
  • Reduced number of dentinal tubules
• Fewer dentinal tubules per unit area in secondary dentin due to the gradual reduction in pulpal wall surface area
• Secondary dentin tubules sclerose (fill with calcified material) more readily than primary dentin, reducing permeability and protecting the pulp

Dentin Types

• Tubular dentin: Tubules are present, but fewer in number and less regularly arranged than in regular dentin.
• Atubular dentin: Tubules are irregularly scattered and twisted. Some areas may lack tubules altogether.
• Reactive dentin: Forms in response to injury or irritation. In severe cases, odontoblastic processes are exposed, leading to odontoblast cell damage. Damaged cells can either continue forming irregular dentin (reactive dentin) or degenerate and be replaced by undifferentiated cells from the pulp, which then form reparative dentin.
• Osteodentin: Forms when dentin-forming cells are trapped within the rapidly forming matrix. These cells degenerate and leave spaces within the dentin.
• Vasodentin: Contains entrapped blood vessels.

Enamel Composition & Structure

• 96% of enamel is inorganic material.
• Enamel is the hardest tissue in the human body, highly mineralized and resistant to abrasion.
• Decalcification removes the mineral content, leaving a porous, weakened enamel framework.
• Enamel's thickness can range from 2.0 mm to 2.5 mm at its thickest point.
• In permanent teeth, enamel rods are generally directed towards the occlusal surface.
• The primary mineral content of enamel is hydroxyapatite.
• Enamel's color can vary from light yellow to grayish white.
• Sodium is not part of enamel's inorganic content.
• Enamel rods exhibit wave-like patterns before reaching the outer surface.
• The diameter of enamel rods is smaller at the outer surface than at the inner surface.
• Dark lines in longitudinally ground enamel represent Hunter-Schreger bands, formed by varying directions of enamel rods.
• The peripheral part of the enamel rod is called the rod sheath.
• Hunter-Schreger bands are a result of changing rod directions, creating an optical illusion.
• Hunter-Schreger bands are most clearly observed in the middle third of enamel.
• Etching longitudinally ground enamel with a mild solution reveals Hunter-Schreger bands.
• Gnarled enamel is found under cusp tips and incisal ridges, providing extra strength.
• Perikymata are wave-like ridges on the enamel surface, formed during enamel development.
• The neonatal line is typically located near the DEJ (dentino-enamel junction).
• Incremental lines of Retzius are formed due to periodic changes in enamel formation.
• Enamel tufts are finger-like extensions of hypocalcified enamel extending from the DEJ.
• Carefully decalcifying enamel will make perikymata disappear.
• Striae of Retzius are associated with periodic changes in enamel formation during development.
• The neonatal line is more pronounced in deciduous teeth.
• Striae of Retzius appear as **concentric rings ** in cross-section.
• The neonatal line marks a distinctive change in enamel due to birth.
• Perikymata are surface irregularities caused by changes in enamel formation rate.
• Enamel tufts are hypocalcified areas branching from the DEJ.
• Enamel spindles are elongated invaginations of dentin into enamel, formed during ameloblast migration.
• Striae of Retzius appear as dark lines when viewed longitudinally.
• The firm attachment between enamel and dentin at the DEJ is primarily due to a complex interlocking of enamel and dentin.
• Enamel tufts are associated with hypocalcified zones.
• Enamel spindles are formed from odontoblastic processes that become entrapped within the enamel.
• True enamel lamellae are thin, plate-like structures running from the DEJ to the enamel surface.
• Enamel tufts are primarily located near the DEJ.
• Enamel spindles appear as small, dark, spindle-shaped structures in ground sections.
• The scalloped profile at the DEJ is marked by irregularities due to interlocking enamel rods anddentin.
• Abrupt changes in the direction of enamel rods contribute to structural weakness.
• Enamel lamellae often consist of organic material and mineral deposits.
• Type B enamel lamellae are associated with traumatic stimuli.
• Perikymata typically form on the enamel surface near the gingival margin.
• Type A enamel lamellae are primarily located near the DEJ.
• Cracks that reach the enamel surface are more common in erupted teeth.
• The outer structureless enamel layer provides a protective surface by covering enamel rods.
• Perikymata are **surface indentations ** on the enamel that are more pronounced in deciduous teeth.
• The rod end structure in enamel is more irregular at the DEJ.
• Enamel lamellae are thin, plate-like structures extending from the DEJ to the enamel surface.
• True enamel lamellae are deeper and more extensive than other structures.
• Outer structureless enamel can be best described as a layer with no visible enamel rods.
• Perikymata contribute to the overall surface texture and irregularity of enamel.
• Microscopic cracks in dental enamel can be initiated by mechanical stress, thermal changes, or chemical agents.
• The rod end of an enamel prism represents the terminal portion of the enamel rod at the DEJ.
• Cracks within enamel are often characterized by irregular, branching patterns.

Dentin Structure

• Dentinal tubules, the primary structures of dentin, allow for permeability.
• Dentinal tubules near the pulp typically range from 1.5 to 3.0 µm in diameter.
• Dentinal tubules take a spiral path as they travel through dentin.
• Longitudinal extensions of odontoblasts, called odontoblastic processes, contribute to dentin's sensitivity.
• Inter-tubular dentin surrounds the dentinal tubules.
• The density of dentinal tubules increases towards the pulp due to odontoblast activity.
• The spiral track taken by odontoblasts in dentin is termed the Schreger’s curves.
• Lateral branches of dentinal tubules are responsible for interconnecting tubules and contributing to dentin's sensitivity
• Interglobular dentin is more porous and irregular than Tomes' granular layer.
• Incremental lines of von Ebner are formed due to daily variations in dentin formation and contribute to the contour lines of Owen.
• The neonatal line in dentin is more pronounced and located closer to the pulp.
• Tomes' granular layer appears finer and more granular than interglobular dentin.
• Incremental lines of von Ebner indicate periodic changes in dentin formation.
• In deciduous teeth, the dentino-cemental junction is less defined compared to permanent teeth.
• The small, granular appearance of Tomes' granular layer is caused by incomplete calcification.
• Interglobular dentin is larger than Tomes' granular layer.
• Peri-tubular dentin surrounds the dentinal tubules with increased mineral density, while inter-tubular dentin fills the spaces between the tubules.
• The periodontoblastic space is located between the odontoblast and dentin.
• Interglobular dentin is most commonly found in the crown of teeth.
• The Granular Layer of Tomes is a thin, structureless layer found near the dentin-enamel junction.
• Dentinal tubules are completely absent within interglobular dentin.
• Vitamin D deficiency can result in impaired dentin formation and **increased ** interglobular dentin.
• The Granular Layer of Tomes is contiguous with dentinal tubules.
• Cytoplasmic vacuoles in the periodontoblastic space are primarily responsible for fluid transport and dentin sensitivity.
• Sclerotic dentin is characterized by increased mineralization and occluded dentinal tubules.
• Tertiary dentin is irregularly formed due to external stimuli, whereas primary dentin is formed during normal tooth development.
• Increased mineralization reduces dentin's permeability.
• Dead tracts in dentin exhibit a dark, non-reflective appearance due to occluded tubules.
• The smear layer reduces dentinal permeability.
• Dentin sensitivity is increased in inflamed pulp.
• Indirect pulp capping aims to protect the pulp by placing materials over a partial exposure of pulp.
• Applying calcium hydroxide during indirect pulp capping can induce a protective barrier by promoting tertiary dentin formation.
• Dentin permeability affects the performance of restorative materials by influencing adhesion and sensitivity.
• Occluding dentinal tubules through calcification can reduce dentin sensitivity.
• The smear layer can inhibit bonding between dentin and restorative materials.
• In dead tracts, odontoblastic processes are degenerated or absent.
• Reactive dentin is characterized by dense mineralization and altered tubule morphology.
• Dentin thickness affects tooth color, where thicker dentin results in yellower appearance.
• Severe stimulation effects on odontoblasts can be treated with desensitization.
• When odontoblasts are affected and degenerate, reparative dentin is formed.
• The hydrodynamic theory is the most widely accepted for explaining dentin sensitivity.
• The smear layer poses a challenge for dentinal bonding because it blocks adhesion.
• Exposure of dentin can lead to a reduction in living odontoblasts due to inflammatory reactions.
• Acid conditioning can lead to increased sensitivity by removing the smear layer.
• The outer layer of dentin, peritubular dentin, is highly sensitive.
• Smear particles can block and occlude dentinal tubules, reducing sensitivity.
• Dentin sensitivity theories propose that fluid movement within dentinal tubules is the primary cause of sensitivity sensation
• The smear layer is not a uniform layer, it is irregularly formed and variable in thickness.

Dentin Sensitivity

• Enamel and cementum are not sensitive to stimuli.
• Dentin is sensitive, but not uniformly.
• Dentin sensitivity is greatest near the dentin-enamel junction (DEJ) and close to the pulp.
• Three main theories explain dentin sensitivity:
  • Direct neural stimulation: Dentin has nerve endings which are directly stimulated by stimuli. This is the oldest theory.
  • Odontoblastic transduction theory: Stimuli excite the odontoblast process, which then transmits impulses from the DEJ to nerve endings in contact with the odontoblast cell body.
  • Fluid or hydrodynamic theory: This is the most accepted and popular theory. It proposes that stimuli like heat, cold, and air affect fluid movement within the dentinal tubules. This movement stimulates nerve endings in the plexus of Raschkow (a network of nerves in the pulp).
• Supporting evidence for the hydrodynamic theory:
  • When dentin is exposed, small blebs of fluid can be seen on the cavity floor.
  • Drying the cavity with air or cotton wool causes a greater fluid loss, leading to more movement and greater pain.

Cementum Function

• Cementum is a mineralized connective tissue that covers the roots of teeth
• Provides attachment for collagen fibers of the periodontal ligament to the alveolar bone
• Maintains this attachment by continuous deposition on the superficial layers
• Compensates for lost tooth structure due to wear or fracture by deposition of new cementum
• Repairs damaged areas of the root caused by resorption

Age Changes in Cementum

• Surface becomes irregular due to calcification of ligament fiber bundles
• Continuous cementum deposition occurs with age, maintaining tooth length and potentially obstructing the apical foramen
• Cementum resorption occurs, followed by deposition, creating reversal lines
• Root dentin resorption occurs with aging, covered by cemental repair
• Cementum permeability decreases gradually, with permeability from the periodontal side lost except in the newest layer, while permeability from the dentin side remains only in the apical region
• Cementicles are calcified nodules in the periodontal ligament, appearing with aging or trauma, composed of calcium phosphate and collagen

Hypercementosis

• Abnormal thickening of cementum, affecting one or multiple teeth
• Can be limited to a small area of the root or extend across the entire root length

Cementum Structure

• Incremental lines of Salter: both cellular and acellular cementum are separated into layers indicating periodic formation
• Cellular cementum forms faster than acellular cementum, resulting in incremental lines spaced further apart

Cemento-enamel Junction (CEJ)

• Relation between enamel and cementum edges at the tooth cervix
• Three types:
  • Edge to edge: enamel and cementum meet in a sharp line
  • Denuded area of dentin: a zone of root dentin devoid of cementum
  • Cement overlap enamel: cementum covers the cervical enamel surface
• Incidence:
  • Edge to edge: 30%
  • Denuded area: 10%
  • Cement overlap: 60%

Cementum Classification

• Development:
  • Primary cementum: formed during root formation
  • Secondary cementum: formed after root completion
• Cellularity:
  • Acellular cementum: covers the cervical two-thirds of the root, providing insertion for Sharpey's fibers
  • Cellular cementum: contains embedded cells (cementocytes)
• Origin of Collagen Fibers:
  • Extrinsic: fibers originate from the periodontal ligament
  • Intrinsic: fibers originate from the cementum itself
  • Mixed: combination of extrinsic and intrinsic fibers

Age-Related Changes in Cementum

• The surface of cementum becomes rough with age due to calcification of ligament fiber bundles where they attach to cementum.

• Cementum deposition continues throughout life in the apical region. This process helps maintain tooth length but can also obstruct the apical foramen.

• Cementum resorption is active for a period before being replaced by cementum deposition, creating reversal lines.

• Root dentin resorption occurs with aging and is subsequently covered by cemental repair.

• The permeability of cementum decreases with age. Permeability from the periodontal side is lost except in the most recently formed layer of cementum. Permeability from the dentin side remains only in the apical region.

Cementicles

• Cementicles are calcified, ovoid, or rounded nodules found in the periodontal ligaments.

• They can be single or multiple, free, attached to, or embedded in cementum.

• They appear with age or at sites of trauma.

• Cementicles contain a nidus of epithelial cells and are composed of calcium phosphate and collagen in equal amounts (45% to 50% inorganic, 50% to 55% organic).

Hypercementosis

• Abnormal thickening of cementum
• Can affect one tooth or all teeth
• Limited to a small area or the entire root length

Cementum Hypertrophy

• Physiologic thickening of cementum
• Occurs in functioning teeth
• Response to increased function
• Increases root surface area
• Allows for more periodontal fiber attachment

Cementum Hyperplasia

• Pathologic overgrowth of cementum
• Occurs in non-functioning or embedded teeth
• Characterized by absence of Sharpey's fibers

Clinical Considerations

• Cellular cementum is similar to bone but lacks nerves
• Not sensitive to pain
• Scaling procedures do not cause pain
• Removing cementum exposes dentin, causing sensitivity
• Cementum is resistant to resorption, especially in younger patients
• Orthodontic treatment causes alveolar bone resorption, not root loss
• Cementum has a high fluoride content
• High fluoride content contributes to its resistance to resorption

Hypercementosis

• Abnormal thickening of cementum
• Can affect one tooth or all teeth
• Limited to a small area of the root or through the whole root length
• Two types:
  • Cementum hypertrophy: Physiologic thickening of cementum in good functioning teeth
  • Cementum hyperplasia: Pathologic overgrowth of cementum in non-functioning or embedded teeth

Cementum Hypertrophy

• Increase in root surface area
• Allows more periodontal fibers to attach to the tooth
• Improves functional quality

Cementum Hyperplasia

• Absence of Sharpey's fibers

Clinical Considerations

• Cellular cementum is similar to bone but has no nerves
• Not sensitive to pain during scaling procedures
• Removal of cementum exposes dentin, which can cause sensitivity during scaling
• Cementum is resistant to resorption, especially in younger patients
• Orthodontic tooth movement causes alveolar bone resorption, but not tooth root loss
• Higher fluoride content in cementum contributes to its resistance to resorption

This quiz explores the structure, composition, and physical properties of enamel, the hardest tissue in the human body. Understand the chemical makeup and functional importance of enamel, as well as its interaction with dentin. Test your knowledge of dental anatomy and enamel characteristics.