Aprismatic Enamel The outer surface layer of enamel (20-100m primary teeth and 20-70m secondary teeth) is aprismatic. The crystallites are aligned at right angles to the surface and parallel to each other. The surface layer is more highly mineralised than the rest of the enamel. This is attributed to the absence of prism boundaries where organic material is located.
Incremental lines Enamel is formed in increments: periods of activity alternating with periods of inactivity. This results in incremental lines: short period (cross striations) and long period (enamel striae).
Cross Striations Cross striations appear as lines crossing the enamel prisms at right angles to their long axes. They reflect a diurnal rhythm (daily increments of growth). They appear as lines 2.5-6m apart . Closer to each other near the enamel-dentine junction. May represent variations in the organic matrix. Crystal orientation and composition or prisms width
Enamel Striae Enamel striae run obliquely across the prisms. They represent incremental lines and are known as the striae of Retzius.
Enamel Striae In cross sections, the striae of Retzius run circumferentially like the rings of a tree. There are 7-10 cross striations between enamel adjacent stira. This suggests weekly intervals. Enamel stria are 25-35μm apart in the middle portion (cross striations 4μm apart), while they are 15-20μm apart cervically (cross striations 2μm apart).
Enamel Striae On the surface, the stria appear as fine grooves running circumferentially around the crown. The lines are termed Perikymata grooves, with perikymata ridges in between. Removed after eruption by attrition and abrasion
Neonatal line before birth stria are absent or less pronounced
Surface Enamel Physically and chemically, surface enamel differs from subsurface enamel. Surface enamel is harder, less porous, less soluble and more radio-opaque. Richer in trace elements esp F, Less carbonate. Aprismatic, therefore highly mineralised.
Surface Enamel On protected areas of enamel, small pits could be seen on the surface. The pits are within the perikymata. They mark the ends of ameloblasts (prism end markings. 1-1.5μm in depth.
Surface Enamel Enamel caps are small elevations 10-15μm across. they result from enamel deposition on top of debris late during tooth development.
Surface Enamel Focal holes are depressions on the surface. Loss of enamel caps with the underlying material. this happens through abrasion or attrition.
Surface Enamel Enamel brochs are elevations on the enamel surface. 30-50μm in diameter. Radiating groups of crystals More common in premolars.
Enamel - Dentine Junction Enamel-dentine junction has a scalloped pattern where shearing forces would be high (beneath cusps and incisal edges). The junction is smooth in lateral surfaces. Less minerlised than enamel and dentin
Enamel Spindles Narrow, round tubules 8μm in diameter. They extend up to 25m into the enamel. Direction is not aligned with prisms Most commonly beneath cusps as crowding of odontoblasts processes increases their. Odontoblastic processes among ameloblasts, remnants of dead odontoblasts, dentinal collagen.
Enamel Tufts Junctional structures in the inner third of enamel that resemble tufts of grass. They have the same direction as enamel prisms. Recur at 100μm intervals. They are hypomineralised and thought to be residual matrix protein at the prism boundaries. Tuft protein is a minor non-amelogenin protein.
Enamel Lamellae Structural faults, run the entire enamel thickness. Hypomineralised areas. Incomplete maturation of groups of prisms. Do not be confused with cracks produced during ground section preparation. Cracks disappear with demineralization
Enamel Microporosity Enamel pores are water filled spaces between the crystallites. Enamel porosity makes 3-5% by volume. Larger pores at prism boundaries.
Cement - Enamel Junction Three arrangements between cementum and enamel can be seen. Pattern 1: the cementum overlaps the enamel (60%). Pattern 2: the cementum and enamel meet at butt joint (30%). Pattern 3: cementum and enamel fail to meet and the dentine between them is exposed (10%). All these patterns may be present in a single tooth.
Age Changes Enamel wears slowly with age depending on diet and habits. It darkens in colour: Increased translucency of the tooth as secondary dentine forms and enamel thins Accumulation of surface coatings and stains. The composition of surface enamel changes as a result of exchanges with the oral fluids. There is a decrease in caries due to enhanced mineralisation.
Enamel Pearls Small droplet of enamel on the root, near the furcation. Budding of Hertwig’s root Sheath. Differentiation of ameloblasts.
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