enamel Flashcards
describe enamel composition
- almost entirely mineral
- 95% = organic HAP crystals
- 5% = unique enamel proteins, H2O, lipid material
- mature enamel = 1-3% organic matrix and 2% H20
- presence of H2O related to porosity of enamel
what does the organic matrix of enamel contribute
mechanical properties of enamel like fracture toughness
explain the physical properties of enamel
1) HARDEST biological tissue
2) highly mineralised and withstands both shearing + impact forces well
3) high abrasion resistance so wears down only slowly
4) low tensile strength
5) brittle
6) BUT high modulus of elasticity
what is the important of high abrasion resistance of enamel
means it wears down only slowly
imp bc enamel cannot undergo repair or replacement
what is the important of high modulus of elasticity of enamel
together w support of underlying dentine minimises possibility of fracture
explain the chemical properties of enamel
- calcium HAP crystallites containing impurities in tiny amounts (ie carbonates)
- larger crystallites than other mineralised tissues (30nm thick, 70nm wide = ADJ to tooth surface)
- pores (occupied by organic matrix +/or h2o) of <5nm between them
- NO CELLS in/on its surface
what is the origin of enamel
ECTODERMAL
- develops from IEE of tooth germ
what do enamel proteins do and what do they account for
- most imp components comprising organic matrix
- less than 1% of mature enamel weight BUT 25-30% of early developing enamel
what are the most abundant proteins in developing enamel and their %
1) amelogenin = 90-95%
2) ameloblastin = 5%
3) enamelin
what are the proteins in mature enamel and why
remaining 1% by weight of proteins is comprised of non-amelogenin proteins
bc amelogenins are selectively reabsorbed during enamel maturation (removal + modification of initially formed protein rich matrix)
- tuftelin and tuft proteins
explain the role of AMELOGENIN
- stabilisation of amorphous Ca-P phase
- controls crystal morphology + organisation
- control enamel thickness
explain the role of AMELOBLASTIN
- cell adhesion protein
- controls cell differentiation
- maintains enamel rod / prism integrity
explain the role of ENAMELIN
- controls mineral nucleation + elongated growth (possibly w amelogenin)
explain the role of TUFTELIN
- cell signalling
- enamel protein which persists in mature enamel
what is amelogenesis
process of enamel formation
secretory stage of amelogenesis:
all secretion + modification of matrix occurs via what
TOMES PROCESSES
secretory stage of amelogenesis:
what happens with the development of tomes processes
mineralising surface of enamel has pit like appearance (pits surrounded by inter-prismatic / interrod enamel)
secretory stage of amelogenesis:
why are BOUNDARY AREAS of prisms formed first
because crystallites are formed at surfaces of tomes processes at different rates
secretory stage of amelogenesis:
which areas secrete first and what does this mean
proximal regions between two processes (deep in junctional regions)
ALWAYS SECRETE AHEAD OF
more distal regions
forms walls that represent the periphery of the prism + interprismatic regions and outlines pits occupied by tomes processes
secretory stage of amelogenesis:
what happens to central pits
infilled by tomes processes as ameloblasts retreat to form main core of enamel prism
gives enamel its PRISMATIC STRUCTURE
secretory stage of amelogenesis:
what do ameloblasts have that accounts for variation in crystal orientation
2 secretory sites
secretory stage of amelogenesis:
what are inter-rod regions
regions between enamel rods and have different crystallite orientations and a higher organic content than them
what is the basic structural unit of enamel
this prism (rod)
why do we see prism boundaries and what happens at these boundaries
reflect sudden changes in crystallite orientation between prism core (head) and interprismatic enamel (tail) of ADJACENT prisms
crystallites deviate by 60 degrees from those in the prism core
more organic material + h20 (bc larger pores produced by abutment of HAP crystallites)
each prism (head) is formed by (a) but (b) may contribute to (c)
a) a single ameloblast
b) four
c) each interprismatic region (prism tail)
prism tail + prism head = prism rod
what is the crystallite orientation in the prism core / head
crystallites run PARALLEL to long axis of the prism
what is the crystallite orientation in the interprismic enamel / prism tail
crystallites gradually diverge from it to become angled 60 degrees to long axis (deviate by 60 degrees from those in prism cores)
explain the changes in a single prism
- gradual
- no clear division between head + tail of same prism
- but crystallites in tail of 1 show sudden divergence from those in head of an adjacent one
describe enamel prisms
- run from EDJ to surface
- 5-6 um diameter
- up to 2.5mm length
- each consists of several million crystallites
how do prisms travel when viewed in enamel sectioned parallel to long axis of tooth
in layers
in sinusoidal direction
from edj to surface
what do periodic changes in prism direction in alternating sheets result in
reflecting bands on the cut surface
diff sheets inhibit diff crystal orientation thus diff degrees of polarisation
what can be seen in the outer 1/4 of enamel
all prisms run in same direction
NO BANDS
what can be seen in longitudinal sections of enamel
sinusoidal direction of enamel prisms in alternating sheets
prisms perpendicular to ADJ
alternating regions with groups of prisms sectioned more transversely (DIAZONES - lines) or longitudinally (PARAZONES - dots) reflect/transmit light in diff directions that give rise to…
HUNTER-SCHREGER BANDS (50um wide)
- outer ¼ of enamel = ALL prisms become parallel + hunter-schreger bands no longer evident
how does the arrangment of crystallites + prisms confer strength + fracture resistance
prisms appear to travel in alternating sheets in sinusoidal direction from edj to surface w layers of prisms BUT blocks above and below are following paths in different directions
most common pattern in human enamel = prisms arranged in staggered rows so theres interlocking + offsetting of prism heads and tails of adjacent rows = keyhole shape (prism boundary incomplete so continuum between interlocking of head in 1 row w interprismatic enamel tails in row above)
= breaks up lines of shear in enamel + dissipate forces applied to the tissue
why isnt acid etching prior to bonding always effective
- unabraded surface enamel aprismatic in most areas so more highly mineralised (due to absence of prism boundaries where more organic material is located)
- SO not effective unless carried out for long enough to penetrate the prismatic enamel
- or enamel margin bevelled
what is the result of etching prismatic enamel with 37% orthophosphoric acid gel
- classic etch pattern due to variation in degrees of acid dissolution between prism cores + prism boundary regions = reveals intrinsic enamel architecture
- produces micro-porosities in prismatic enamel surface
why can multiple types of etch pattern be observed and explain the 2 types we may see
depending on rod/interrod orientation
1) dissolution of prism core = leaves the prism periphery relatively well preserved
2) reverse pattern = peripheral interrod enamel is dissolved leaving rods less corroded, prisms persist
why is enamel acid etching a good approach to tooth prep
- more conservative approach
- enables retention for a restoration to be achieved with reduced need to cut further vital tooth
what is the issue with enamel structure relating to caries
- acids produced by plaque bacteria can penetrate into micro-porosities of surface enamel and initiate early sub-surface lesions
- prism boundaries provide path of least resistance to penetration of bacterial products SO mineral loss begins at prism periphery
- early caries legion in enamel appears as ‘white spot’ (subsurface mineral loss w partial dissolution of crystals = enhanced spacing between crystals)
explain how occlusal fissure caries develop
follow direction of enamel prisms on the lateral walls of the fissure
may cavitate to expose dentine
what does knowledge of enamel prism orientation at surface of the crown enable
cavity margin prep to avoid leaving unsupported enamel (prisms sectioned / cracked /pulled apart during tooth prep whose bases don’t reach ADJ intact) which may fracture under masticatory loading = fracture would cause microleakage + early failure
how will failure of cavity margin prep arise and how is this overcome
1) weak enamel margins / weak amalgam margins
- requires proper direction to the proximal walls resulting in full length enamel rods + 90 degrees amalgam at prep margin
use of rotary bur in interproximal box for cavity prep may…
leave proximal wall with an acute enamel angle and undermine enamel at the cavity margin (marginal defect arising from improper cavity margin prep)
cervical cavity margins require
20 degree angle
what is the enamel-dentine junction (EDJ)
AKA AMELODENTINAL JUNCTION (ADJ) + DENTINOENAMEL JUNCTION (DEJ)
interface between enamel + dentine (marks site of their initial deposition)
pattern of 25-100um scallops (particularly evident beneath cusps + incisal edges bc face high shearing masticatory forces)
smoother on lateral surface of crown
crucial to aiding retention of enamel on dentine surface, limiting propagation of cracks through tooth and providing resistance to shearing forces
HIGH MAGNIFICATION = microscallops 2–5 μm
NANOSTRUCTURAL LEVEL = ends of fine collagen fibrils from dentine mingle w initial crystals of enamel
what 3 lines are found in enamel
1) incremental
2) cross striations
3) striae of retzius
explain incremental lines
displayed by enamel for short + long periods due to its phasic development
explain cross striations
- short period incremental lines due to rhythmic recurrent deposition of enamel, cross enamel prisms at right angles to their long axis
- transverse to prisms
- throughout enamel
- 4um apart
- represent DAILY increments (bc of circadian / diurnal rhythm of enamel production)
- prism varies in width
explain striae of retzius
- long period, prominent incremental lines, run obliquely across enamel prisms to the surface
- over lateral surface of crown enamel striae reach surface in a series of grooves running circumferentially around the crown PERIKYMATA GROOVES separated by the perikymata RIDGES (result from termination of striae on enamel surface + are rings)
- throughout enamel
- 25-40um apart
- represents weekly increments
- reach surface of perikymatas
describe the basics of enamel
- anatomical crown larger than clinical crown in healthy teeth
- colour of healthy teeth determined by dentine and modified by colour, translucency and thickness of enamel (young teeth white but yellow w age)
describe basics of where enamel is found and its structure
- thickest over cusps + incisal edges
- thinnest at cervical margin
- unworn permanent teeth = 2.5mm thick over cusps + 1.3mm on lateral surfaces
- thickness declines to become thin layer at cervical margin
- thickness varies between individuals + teeth
- enamel surface smooth but w small developmental surface irregularities (gives teeth their morphology)
how do mature enamel crystals form the interprismic / interrod enamel
- irregular outline bc press against each other during final part of growth
- contain impurities ie carbonate + Mg2+ = make these mineral phases more soluble in acid
what prismatic structure of enamel is less freq near ADJ and surface (bc enamel here is formed slowly)
2) prisms = roughly circular
3) rows offset relative to ones above + below them
4) prisms have complete boundaries
5) clear distinction between prisms + interprismatic enamel
the keyhole structure is a modified form of this
what is gnarled enamel
- groups of prisms ‘spiral’ around others (regular changes in direction more marked beneath cusps + incisal edges) giving appearance of ‘gnarled’ enamel
- so enamel over cusps + incisal edges better resists occlusal masticatory forces
- breaks up lines of shear directly where max force will impinge on the crown
explain enamel spindles
1) arise at ADJ (esp round cusps = most crowding of odontoblasts)
2) dark, club shaped
3) penetrate <25μm into enamel
4) seen in longitudinal sections of enamel
5) not aligned with prisms
6) odontoblast processes = elongate into forming enamel + disrupt it leaving a void during early stages of development + enamel forms around these forming tubules
7) in erupted tooth these tubules DON’T contain cell processes
explain enamel tufts
1) hypomineralised structures
2) radiate from ADJ 1/3 of enamel thickness
3) appear like ‘tufts’ of grass in transverse sections - appearance represents areas between prisms where enamel protein has collected (residual organic enamel matrix)
4) contain TUFTELIN ENAMEL PROTEIN
5) regions of highest protein content of mature enamel
6) tufts travel in same direction as sheets of prisms SO this is the superimposition of changing prism orientations in thick ground sections that produces the tuft appearance
explain enamel lamellae
1) fine cracks in enamel surface visible to naked eye
3) histologically = sheet like faults that extend from surface of enamel toward ADJ + may run through entire thickness of enamel
4) hypomineralised
5) best visualised in transverse enamel sections
6) some form during enamel development bc incomplete maturation of groups of prisms (in this case = contain enamel proteins)
7) another example of lamellae = spaces between groups of prisms (caused by stress cracks that occur bc of loading of enamel AND contain saliva + oral debris)
describe the composition of surface enamel
- outer 20–70 μm of newly erupted permanent teeth is nonprismatic (ie enamel crystallites at right angles to surface + parallel to each other)
- more highly mineralised than rest of enamel bc of absence of prism boundaries
- more F so harder, denser, less soluble - reduces susceptibility to caries
- aprismatic = areas of surface enamel not subject to abrasion are less porous + less susceptible to acid dissolution
- nonprismatic = limit diffusion from surface through prismatic enamel
what is the neonatal incremental line
- marked line in enamel
- formed at birth + reflects metabolic changes @ birth
- prisms change direction + thickness @ time of this event
- systemic disturbances (ie fervers) that affect amelogenesis produce accentuated incremental lines
- oblique to prisms
- single line
- 20-40um thick
- prisms vary in width + direction
what happens to enamel with age
- progressively thinner bc of tooth surface loss
- reduced translucency of tooth as secondary dentine forms
- enamel wear facets
- portions of crown (enamel + dentine) lost bc of mechanical wear or chemical dissolution caused by dietary acids
- less permeable as microporosity reduces + spaces between enamel crystallites diminish as crystals acquire more ions + INC in size
explain bleaching and its implications
- H2O2 produces free radicals that penetrate enamel pores and produce oxidation SO break down large molecules in enamel matrix to smaller which diffuse out of enamel or absorb less light + give whiter appearance)
- results = reduction of discolouration
- enamel + dentine change colour (result of passage of bleaching agents through microporous enamel)
- porosity of 3-5% by vol
