Enamel

Chemical properties

- Mainly made of hydroxyapatite = calcium hydroxyapatite (Ca10(PO4)6(OH)2)- Main mineral component of enamel = 88-90% by tissue volume and 95-96% by tissue weight- Rest is organic material and water - Mineral content increases from EDJ outwards - HA present in crystal form, each = 75nm x 25nm- Crystallites for dentine, bone and cementum are much smaller than in enamel.- Crystallites when in cross section are in a hexagonal shape, but some distortion does occur

during development. - Core of crystallite is different from its periphery. The core is richer in magnesium- The arrangement of molecules is shown below, however it is subject to variation i.e.

phosphate and calcium can be replaced by carbonate; magnesium can replace calcium and any other ion; fluoride can substitute hydroxyl groups = increases resistance to acid demineralisation. Fluoride levels decline getting closer to the EDJ.

- Chloride, lad, zinc, sodium, strontium and aluminium ions can also sub in to the places in the hydroxyapatite molecule

- Small quantities of non-apatitic minerals may also be present in mature enamel e.g. octacalcium phosphate = possibly a HA precursor.

Water - makes up 2% by weight of tooth and 5-10% by volume.- Water present relates to the porosity of the tissue- Water may lie between organic materials, some trapped in defect on the crystalline

structure and the remainder forms a hydration layer coating the crystals.- Ions such as fluoride travel through water = clinically important.

Organic matrix- mature enamel = only 1-2% organic matrix- organic component of regions where the prismatic and crystallite arrangement is straight

and regular = only 0.05% organic and where crystal arrangement is regular – 3% organic.- Organic molecules = AA large unique portion complexes- Protein = amelogenin and non-amelogenin (enamelins)- 50-90% of matrix is composed of small molecules = peptides and free AA (including lots of

glycine and glutamic acid) - Larger molecules = components rich in carbohydrates - Protein concentration is highest in enamel tufts at the EDJ- Lipid content = same as protein content

Enamel Prisms- basic structural unit of enamel is the enamel prisms+ millions of hydroxyapatite crystallites

packed in to a long thin rod = 5-6um = diameter and 2.5mm length.- Run from EDJ to surface- Boundaries of prism = sudden changes in crystallite orientation- Increased micro porosity at prism boundaries = slightly more organic material can be

accommodated- In cross section, enamel prism can be in one of three patterns.

- All three are present in humans but the keyhole structure is more predominant. - Pattern I = near the EDJ, probably because enamel is formed slowly – prisms appear circular.- Enamel between prisms = interprismatic. Composition = similar to that of inside the prisms

but it has different optical effects. - Pattern III = keyhole shape = clear and tail region. Tail of one prisms = between two heads of

another. There is an abrupt change in crystal orientation, which is responsible for the refraction of light and the appearance of the prism boundary. In the head of the keyhole structure, the prisms are aligned longitudinally, and in the tail there is a gradual change in the orientation so that the prisms are about 60-70 degrees diverged from longitudinal prisms. It is difficult to distinguish between the tail and the interprismatic structure.

- Polarise light identifies changes in crystallite orientation = useful feature in enamel prisms. - When viewed in enamel fracture or a section parallel to the long axis, prisms appear to

travel in a straight line from EDJ to the surface - Prisms meet surface at varying angles i.e. at cervical end, prisms meet and a right angle from

the EDJ but closer to the occlusal surface they meet at an angle of 60 degrees. - Fractured/sectioned longitudinally prisms follow a parallel sinusoidal path.- About 10-13 blocks of prisms follow the same direction but blocks above and below this

follow a different direct: periodic changes in prism direction = Hunter Schreger bands - Parazones = bands of prisms cut longitudinally- Diazones = bands of prisms cut more transversely - Angle between parazones and diazones is 40 degrees. - Complex pattern of prisms = resistance to fracture- Outer quarter of enamel, prisms run in the same direction = no Hunter Scherger bands. - As prisms arranged in a spiral pattern, in some areas beneath the cusps and incisal edges the

changes in direction of the prisms appears more marked and irregular. Groups of prisms seem to spiral around others, giving the appearance of gnarled enamel.

Aprismatic Enamel- outer 20-100um of deciduous and outer 20-70 of permanent teeth = enamel is aprismatic =

enamel crystals are aligned at right angles to the surface and parallel to each other. - Surface layer is more mineralized than the rest of the enamel due to the absence of prism

boundaries where most organic material is located. - Variable thickness - It is a result of there being no tomes process on the ameloblasts in the final stages of enamel

deposition.

Incremental Lines - enamel is formed incrementally = activity period followed by periods of quiescence.- There are two types = short period = cross striation and long period = enamel striae.

Cross striations

- seen as lines transversing the enamel prisms at right angles along their long axes. - Cross striations represent diural rhythm. - Appearance relates to diurnal variation in the rate of secretion by ameloblasts. - Alternative theory = lines are a result of crystals within in the prisms following a spiral path. - At lower power, prisms appear 2.5 – 6 um apart = closer together at EDJ.

- Another theory = cross striations are the result of subtle changes in the nature of the organic matrix and/or crystallite orientation and/ore composition (especially in the carbonate component)

Enamel striae

- enamel cut on longitudinal axis = structural lines seen running obliquely across the prisms from near the EDJ to the surface = incremental lines = enamel striae (of Retzius)

- horizontal cut = striae appear as circumferential lines = like lines of a tree.- Routine demineralisation = all enamel structures lost because of low content of organic

material - Ground section = many structural features retained = main = keyhole pattern of prisms. This

structure is known to lack organic sheath but level of organic material and water is likely to be higher at the prisms boundaries of the larger pores produced by abutment of hydroxyapatite crystallites at this junction.

- Striae overlying cusps and incisal edges do not reach the surface because of the way the enamel has been deposited. Can only been seen to reach surface in enamel loss.

- Human teeth = 7-10 cross-striations between adjacent striae in any one individual.- Striae formed in weekly intervals - Average distance between two cross striations = 4um = enamel striae in middle portion of

enamel = 25-35 um apart.- Cervical enamel= enamel formed slowly = cross striations = 2um apart and striae = 15-20um

apart. - Accentuated striae = metabolic disturbances occurring during the time of mineralisation. - Lateral enamel = enamel striae reach the surface in a series of fine grooves = run

circumferentially around crown = perikymata grooves and are separated by perikymata ridges.

- Distance between perikymata = should be the same as ordinary enamel striae = cervical = 15-20um and middle enamel – 25-30um. But from cervical region, as they reach the surface, they can may become up to 100um apart towards the cusp of the tooth.

- Deciduous teeth , enamel striae and perikymata only seen clearly in the cervical enamel of deciduous second molars.

- Exaggeration of striae in different teeth forming at the same time = possible common systemic influence. One hypothesis = may be a rhythm with a 27 hour cycle in addition to a diurnal daily rhythm. The two rhythms would coincide every seven or eight days producing fault in the developing enamel

- In ground section, striae seen because of differential light scattering effects at this fault line, possibly due to a slight change in prism direction/thickness or slight differences in crystallite composition/orientation and/or differences in organic content

- Partial view of striate in demineralised sections could be due to the fact that there is a higher carbonate content, which causes greater solubility of the crystals and greater porosity.

- Possible to use the incremental lines in enamel = cross-striations, enamel striae and perikymata= to assess the time taken to form the crown of the tooth + help age material. Assuming adjacent perikymata are separated by 7-10 day intervals, the total number of perikymata on a crown indicates the time taken for the crown to be formed. If about 6-9

months are added for the 25-30 striae over the top of the crown that do not reach the surface, a specimens age can be calculated from their first molars.

- Enamel striae are less pronounces or absent from enamel formed before birth- Neonatal line = marked striae formed at birth = metabolic changes at birth. Prisms appear to

change direction and thickness at the time of this event

Surface enamel

- most clinically significant region- tooth comes into contact with food here, caries is initiated here, restorations are attached or

abutted, orthodontic, toothpaste action, bleaches and fluoride/remineralisation preparations applied.

- Surface enamel differs chemically and physically from subsurface enamel. - It is harder, less porous, less soluble, and more radio-opaque than subsurface enamel. - Richer in some trace elements e.g. fluoride, contains less carbonate. - Variable appearance, exhibiting features such as aprismatic enamel, perikymata, prism-end

markings, cracks, pits and elevations.- Unabraded outer enamel =very aprismatic = highly mineralised and resisitant to caries and

therefore acid etching may not always enhance adhesion.- Although surface enamel is aprismatic, the incremental lines of retzius reach the surface and

appear as perikymata grooves, wave like concentric surface rings parallel to the CEJ. They are separated by wave like perikymata ridges.

- Attrition and abrasion remove these surface features but may be present in protected cervical enamel.

- In some areas, particularly cervically where the reduced enamel epithelium persists for some time after eruption, small pits are seen within the perikymata. These are the impressions of the ends of ameloblasts and are 1-1.5um in depth

- Small cracks frequently found on surface, but it is difficult to know whether many of these were present in vivo or were induced by the procedures necessary to examine the tooth.

- Cracks show possible areas of weakness. - Small elevations, depressions (focal holes = from the loss of the cap and underlying material

by abrasion and attrition) are also found. Caps are thought to result from enamel deposition on top of small deposits of non-mineraliable debris late in development.

- Large surface elevations, enamel brochs 30-50um in diameter also occur occasionally and consist of radiating groups of crystals. These are more common in premolars.

Enamel dentine junction

- boundary between enamel and dentine has a scalloped pattern – areas where shearing force would be high e.g. beneath cusps and incisal edge, but it is smooth on lateral surfaces.

- Convex enamel and concave dentine. - Dentine crystals are much smaller than enamel crystals and the transition from one to the

other at the junction of the tissues is generally clear. - Aka amelodentinal junction

Features from EDJ to enamel surface

Enamel spindles

- narrow = 8um diameter, round, sometimes club-shaped tubules, extend up to 25um in to the enamel.

- Not aligned with the prisms - Thought to be the result of some odontoblasts processed that, during early stages of enamel

development insinuated themselves between the ameloblasts.- Size of spindles may exceed ameloblasts- May be dentinal collagen or remnants of dead odontoblasts- Most common beneath cusps where most crowding of odontoblasts would occur.- Best seen in longitudinal sections of enamel.

Enamel tufts

- junctional structures in the inner third of the enamel that, in ground section resembles tufts of grass

- appear to travel in the same direction as the prisms. They appear to undulate with sheets of prisms.

- Hypomineralised and recur at approximately 100um intervals along the junction- Each tuft is several prisms wide - Tufts are best visualised in transverse sections of enamel.- Appearance results from proteins = residual matrix at the prism boundaries of

hypomineralised prisms. - Tuft protein is not the highly abundant amelogenin but the minor non-amelogenin.

Enamel lamellae

- are sheet-like apparent structural faults that run through the entire thickness of enamel. They are hypomineralised and narrower, longer and less common than enamel tufts, but like tufts there are best visualised in transverse section.

- Can easily be mistaken for cracks but cracks will disappear in a demineralised section- Lamellae have developed due to incomplete maturation of groups of prisms or after

eruption cracks during function – contains saliva and debris.

Microporosity of enamel

- in vivo = pores in enamel = water-filled spaces between crystallites- enamel has porosity of about 3-5% by volume. - In prisms most pores exist as very narrow gaps between closely packed crystallites, some

appear as elongated and tube-like- Most pores are accessible only to small molecules such as water - Larger pores that can let through molecules bigger than water are located at prism

boundaries and those that let through smaller molecules than water are located throughout the enamel.

- Therefore prism boundaries = main highway through enamel.- The pathways for diffusion and, to a lesser extent, electrochemical effects arising from the

charge on the pore walls have an important influence on the formation of a carious lesion.

Age changes in enamel

- wears away slowly, depending on diet and masticatory habits - seem darker in colour = reduced translucency = secondary dentine forms and reduced

enamel, also partly due to change in surface enamel coatings and stains- fluoride can be incorporated in to enamel = reduces porosity and susceptibility to caries

Clinical considerations

Defects in enamel

- present in 68-95% population- may due to environment or genetic origin - hypoplasia = disturbance in original enamel formation, manifests as pits and grooves on

enamel surface , induced by infectious disease in childhood = measles.- Hypomineralisation – disturbances in the maturation of enamel = surface is generally intact

but opaque rather than translucent, delay in the removal of amelogenins during maturation- Also caused by birthing difficulties and dietary deficiencies. - Mottled-enamel = high levels of fluoride = fluorosis = opacity of enamel and surfacing pitting

occurs.

Enamel structures and dental caries

- tooth decay = acid by plaque bacteria = dissolve enamel mineral. - Carious lesion seen in a ground section in polarised light.- Before cavitations = surface shows little change =, but beneath = 20-50% mineral is lost.- Mineral is dissolved = loss begins at periphery of prism- Mineral not totally lost as remineralisation can occur by saliva and fluoride. - Prevent caries and treat it by promoting remineralisation more than demineralisation =

minimise plaque formation = starve plaque biofilm and make enamel less soluble by adding fluoride to it.

Enamel structure and restorative dentistry

- Different acids at different concentrations can produce a variety of patterns of partial prism dissolution to provide a roughened surface suitable for adherence of restorative materials = reduces or eliminates the need for mechanical retention cut in to sound tissue

- Agents mechanically binding to enamel = need microporistiy in the surface by acid-etch technique = microscopic tags can be seen invaginating in to the roughened surface.

- Preparing cavity = need to know microanatomy of enamel + prism orientation = conserve as much of original strength as possible

- Cutting cavities into enamel with rotary instruments = lead to subsurface cracking but some of the adhesive materials are capable of reinforcing this weakened substrate.

Enamel Pearls

- Small isolated spheres of enamel that are occasionally found on the root towards the cervical margin

- Common in the root bifurcation region- Predispose to plaque aggregation following gingival recession