Enamel Dr Firas Alsoleihat, BDS, PhD Department of Conservative Dentistry

Outline 14 lecture 40% mid 60% final attendance.

Lecture Outline Physical properties Chemical composition Histological structure of enamel Age changes in enamel

Physical Properties The hardest tissue. Withstands shearing and impact forces and has a high resistance to abrasion. Thickness varies from up to 2.5mm (1.3mm in primary) over cusps to feather edge at cervical margins. It cannot be repaired or replaced.

Physical Properties Low tensile strength (It is brittle, requires the support of the resilient dentine) but has high modulus of elasticity ( resist elastic deformation). Surface enamel is harder, denser and less porous than subsurface enamel.

Physical Properties Hardness and density decrease from the cusp tips to the cervical margins. Young enamel appears white turning to a more yellow appearance as translucency increases with age.

Chemical Composition Enamel is composed of 96% inorganic components, 2% organic component and 2% water by weight. Inorganic composition: Calcium hydroxyapatite Ca10(PO4)6(OH)2 is the principal mineral component of enamel. It is present in the form of crystallites. Organic composition: Free amino acids, small molecules, peptides and large protein complexes (amelogenins and non-amelogenins)

Hydroxyapatite Crystals Enamel hydroxyapatite crystals are about 70nm in width, 25nm thick and of great length (almost the full thickness of enamel). Most crystallites are hexagonal in cross section. The cores of the crystals are richer in magnesium and carbonate in comparison to the peripheries. 68nm 26nm ?

Hydroxyapatite Crystals Each crystal unit has a hydroxyl group surrounded by 3 calcium ions which are surrounded by 3 phosphate ions. Six calcium ions in a hexagon enclose the phosphate ions. The crystal is made of a repetition of those planes of ions side by side in stacked layers.

Substitutions in the Hydroxyapatite Crystals The main substituents of human apatite are: 1- HPO4 and CO3 for PO4 2- Sr, Ba, Pb, Na, K and Mg for Ca 3- F, Cl, Br and I for OH The ions present in enamel may influence dental caries by affecting the dissolution of the apatite crystals and/or affecting remineralisation. Fluoride’s incorporation in the crystal inhibits caries. Carbonate’s incorporation in the crystal promotes the carious attack.

Water 2% by weight, 5-10% by volume. Water presence is related to the porosity of the tissue. Might be present between the crystals surrounding the organic component. Might be trapped within crystalline defects forming a hydration layer. Ions such F travel through the water component.

Organic Matrix Mature enamel 1-2%; varies from 0.05% to 3% depending on the regularity of the crystals. Consists of proteins that are exclusively found in enamel Amelogenin 90%, non amelogenins 10% They do not contribute to the enamel structure

Organic Matrix The highest concentration of proteins in enamel is in tufts at the dentine-enamel junction. Lipid content 1% by weight of enamel. May represent remnants of cell membranes.

Organic Matrix Amolegenins Hydrophobic, low molecular weight and tend to aggregate into clumps. Produced by ameloblasts They spread throughout the whole developing enamel resulting in a gel matrix through which molecules and ions spread readily. This helps in the formation of large crystals.

Organic Matrix Non-amelogenins (such as tuftelin, ameloblastin and enamelin) Low molecular weight May be derived from plasma albumin Contain distinct components secreted by ameloblasts. They may have a role in mineralisation along with amelogenins.

Histology Due to its high mineral content (96%) enamel is totally lost in demineralized sections. Enamel structure is mainly studied in ground sections. Immature enamel can be studied in demineralized sections due to its high protein content (25-30%)

Enamel Prisms Prisms (rods) are the basic structural units of enamel. Each prism consists of several million hydroxyapatite crystals packed into a long thin rod 5-6m in diameter and up to 2.5mm in length.

Enamel Prisms Prisms are separated by inter-rod substance where crystals change direction and deviate by 40-60° leaving some space for organic material to accomodate Prism boundaries reflect a sudden change in crystallite orientation. Slightly undulating course that reflects ameloblast path during secretion.

Enamel Prisms in Cross Section Prisms in cross section appear in different patterns, but the keyhole pattern (pattern III) predominates. Prisms have head and tail regions. The tail of one prism lies between the heads of the two adjacent prisms. An abrupt change of crystal orientation at the prism boundary is responsible for the optical appearance of the boundary.

Enamel Prisms in Cross Section The crystals in the head of the prism run parallel to the long axis of the prism. In the tail, the crystals diverge gradually to become angled at 65-70° to the long axis. The change from head to tail is gradual in each prism, however a tail of one prism shows a sudden divergence from the head of an adjacent prism.

Enamel Prisms A variation of prism shape might be encountered in cross sections. This is dependent on the angle at which the section was cut.

Enamel Prisms Every 10-13 layers of prisms follow the same direction, but blocks above and below follow paths in different directions. This gives rise to a banding pattern called the Hunter-Schreger bands. They are approximately 50m in width and are visible due to light reflection in different directions. In the outer ¼ of enamel, all prisms run in the same direction and so there is no banding.

Longitudinally cut prism bands are dark (parazones) while transverse cut bands are light (diazones) Angle between them 40 degrees This pattern strengthen enamel against fracture

Enamel Prisms Prisms over the cusps appear twisted around each other in a complex arrangement known as gnarled enamel.