Enamel Flashcards
Composition of enamel by weight
95% hydroxyapatite, 4% water, 1% organic matrix
What forms the small (1%) organic component of enamel?
Proteins - amelogenins, enamelins, peptides, amino acids
What is the basic structural unit of enamel?
enamel prism/rod
Cross-sectional shape of enamel prisms
keyhole shaped
Longitudinal shape of enamel prisms
sinusoidal
How does the enamel rod/prism structure/boundary arise?
Sudden changes in crystallite orientation
Structure of amelodentinal junction
Scalloped - bowl appearance with edges pointing towards enamel (convexities) and the concavities on the dentinal surface.
Which cell type does the enamel originate from?
Ectodermal origin
Which dental tissues are of ectomesenchymal origin?
Dentine, cementum, pulp
Which sections are used to study enamel?
Ground sections - hard mineralised tissues remain intact but soft CT and epithelia are lost.
In which sectioning technique is enamel not visible?
Demineralised section - only soft CT and organic matrix of mineralised tissue remains.
How does the thickness of enamel vary?
Thickest over cusps and incisal edges, thinnest at the cervical margin.
Why is it important that enamel resists abrasion?
Wears down slowly - enamel cannot be repaired/replaced.
What is the hardest tissue in the body?
Enamel
How is the possibility of enamel fracture reduced despite being brittle?
Flexible support of underlying dentine
How does hardness / density of enamel vary?
Surface enamel is harder and more mineralised than deeper enamel. Hardness also decreases from the cusp tip / incisal edge to the cervical region.
How does translucency of enamel vary?
Translucency increases with age as mineralisation increases. This may lead to transmission of the yellow colour of underlying dentine.
What are enamel rods/prisms made up of?
Several million crystallites
What is the principal component of enamel?
calcium hydroxyapatite crystallites (95% weight, 90% volume of enamel) which run from the amelodentinal junction to the surface
How does the mineral content of the enamel rods/prisms vary from the ADJ to the surface?
Mineral content increases from ADJ to surface
Shape of hydroxyapatite crystals
Hexagonal
Molecular arrangement within each crystallite
Central hydroxyl group surrounded by 3 calcium ions which are surrounded by 3 phosphate ions that are evenly spaced. 6 calcium ions in a hexagonal shape enclose the 3 phosphate ions.
Possible ions which may substitute into the hydroxyapatite structure
Mg2+, CO3 2-, F-
Importance of water and distribution within enamel
Ions (e.g. F-) can travel through it
How are the regions of the keyhole-shaped enamel rods described?
Head and tail - tail lies between heads of adjacent prisms.
Which directions do crystals run in enamel prisms?
In the head of the prism, the crystals run parallel to the long axis of the prism. In the tail, crystals diverge to become angled 60-70 degrees to the long axis.
Why does the angles which the enamel prisms meet the surface vary?
Due to the shape of the amelodentinal junction and the outer surface
What are the different angles at which the enamel prisms meet the surface?
At right angles just above cervical margin, 60 degrees more occlusally, as acute as 20 degrees within fissures.
What are Hunter-Schrenger bands?
The banding pattern that arises due to periodic changes in prism direction.
Where is nonprismatic enamel located?
The outer layer of enamel of newly erupted deciduous and permanent teeth. The first enamel formed at the ADJ has also been reported to be nonprismatic.
Why is the outer layer of newly erupted teeth enamel nonprismatic?
The absence of Tomes processes in the ameloblasts in the first and final stages of enamel deposition, results in enamel crystals all aligned at right angles to the surface and parallel to each other.
2 main types of incremental lines found in enamel
cross-striations and striae of Retzius
What are cross-striations?
Lines that cross the enamel prisms at right angles to their long axis. Formed every 24 hrs
If enamel is formed more slowly, how will this affect the spacing of cross-striations?
Cross-striations will be closer together (less enamel formed per 24 hrs)
What are striae of Retzius?
prominent lines that run obliquely across enamel prisms to the surface (longitudinal sections). Seen as concentric rings in horizontal sections.
What do the striae of Retzius represent?
The successive positions of the enamel-forming front.
How many cross-striations are present between adjacent striae of Retzius?
about 7-9 (therefore striae are formed approximately weekly).
Name of the fine grooves that run circumferentially around the crown, parallel to cervical line
Perikymata grooves, separated by perikymata ridges
Cause of perikymata grooves
Due to striae of Retzius reaching surface
Towards the cervical line how does the space between perikymata grooves differ?
Perikymata grooves are closer near the cervical margin (slower growth of enamel)
What features of enamel can be used to age dental material?
Incremental markings - cross-striations, striae of Retzius, perikymata grooves
Which features contribute to the variable appearance of the enamel surface?
Perikymata, nonprismatic enamel, prism-end markings, cracks, pits, elevations/caps.
Where do most cracks extending inwards from the tooth surface terminate?
At the amelodentinal junction
What structures can be seen extending from the dentinal surface into the enamel at the ADJ?
enamel spindles, enamel tufts and enamel lamellae
What are enamel spindles?
Narrow, round, club-shaped tubules extending up to 25um into the enamel. Thought to be odontoblast process that insinuated between ameloblasts.
What are enamel tufts?
Junctional structures in the inner third of enamel that resemble tufts of grass, where protein content in enamel is the highest. Follow direction and undulation of sheets or prisms.
What are enamel lamellae?
Apparent structural faults that run along the entire thickness of enamel. Narrower, longer and less common than enamel tufts
What section of enamel is best to visualise enamel spindle, tufts and lamellae?
Transverse
Why do enamel lamellae form?
incomplete maturation of prisms (contain enamel proteins), or cracks due to loading of enamel (contain saliva and oral debris).
What are enamel pores?
Water-filled spaces between the crystallites.
What determines whether agents stain the tooth surface of not?
The attraction of the material to the enamel surface by electrostatic charge, London forces, and hydration and hydrogen bonds.
Example of whitening agent
Carbamide / hydrogen peroxide
How does tooth whitening work?
Whitening agents produce free radical that penetrate enamel pores and act as oxygenators and oxidants. This reduces the size of chromogenic molecules (substance that converts into a coloured compound) so that they can diffuse out of the enamel or absorb less light which gives a whiter appearance.
Why does enamel seemingly darken in colour as we age?
Enamel thins and secondary dentine forms which reduces the translucency of tooth. Surface coatings and stains also contribute.
How does fluoride affect porosity of enamel?
Fluoride reduces enamel porosity (and therefore also its susceptibility to caries)
Appearance of a carious lesion before cavitation occurs (in ground section, polarised light, microradiograph)
Intact surface layer with considerable subsurface demineralisation.
Factors that determine the rate/amount of hydroxyapatite dissolution
pH, concentration of calcium and phosphate ions (equilibrium), fluoride
How does fluoride reduce enamel dissolution?
By producing fluorapatite which is more stable than hydroxyapatite.
When is the maximum effect of fluoride obtained?
When about 50% of the hydroxyl groups are replaced with fluoride.
How are dairy products anticariogenic?
Casein is the predominant protein in milk and it can stabilise calcium and phosphate ions which improves remineralisation.
Bioactive definition
the ability of a material to interact with or cause a response when in contact with living tissue.
Example of a bioactive material
bioglass
Uses of bioglass
initially used for bone regeneration but potential use as a remineralising agent (bioglass gradually transforms into crystalline hydroxyapatite)
How can beta-sheet forming peptides promote remineralisation of carious lesions?
The peptides self-assemble into a 3D scaffold that can induce hydroxyapatite growth - “filling without drilling”
How has knowledge of enamel structure improved restorative dentistry?
By utilising remineralisation and adhesives which remove the need for mechanical retention. So less sound tissue is removed.
How do acid-etch techniques work?
Different concentrations of different acids lead to partial prism dissolution which provides a roughened surface for adherence of restorative materials.
What are enamel pearls?
Small, isolated spheres of enamel occasionally found on the root generally towards the cervical margin and root bifurcation region.
