Enamel

Dr. Syed Sadatullah

Introduction

Enamel is an epithelially derived hard, protective covering of teeth

Fully formed enamel is the most highly mineralized extracellular matrix known

It is highly brittle yet exhibits certain degree of resistance to withstand fracture

1 Physical characteristics of enamel

Enamel is the hardest substance of the body, its hardness is comparable to mild steel

Average knoop hardness number for enamel is approximately 343

Surface of enamel is more mineralized and hard than deeper enamel

Unlike other calcified structures in the body enamel is unique as it is totally acellular

2 Composition of enamel

Enamel consists of approximately 96% of inorganic material and 4% of organic material and water by weight

It consists of approximately 12% of inorganic material and 88% of organic material and water by volume

The organic component forms the matrix and the inorganic component comprises of various minerals

Tissue Inorganic Content

Organic Content + Water

Enamel 96% 4%

Dentin 70% 30%

Cementum 45-50% 50-55%

Percentage of dental tissue components by weight

The organic matrix of enamel is made from non-collagenous proteins and enzymes

Of the enamel proteins 90% are amelogenins and 10% are non-amelogenins

The different types of nonamelogenins associated with formation of enamel are ameloblastin, enamelin and tuftelin

The primary function of the organic material is to direct the growth of enamel crystals

The inorganic component of enamel is comprised almost entirely of hydroxyapetite crystals

Enamel hydroxyapetite crystals are the largest hydroxyapetite crystals of all the calcified tissues in the body

In addition to hydroxyapetite crystals enamel also contains carbonates and trace elements

These crystals are susceptible to dissolution by acids and hence provides the basis for dental caries

Enamel is translucent and varies in colour from light yellow to whitish

It varies in thickness, with maximum over cusps (2.5 mm) to a feather edge at the cervical line

Thickness of enamel in primary teeth is nearly half than that in permanent teeth

Although enamel is an extremely hard tissue it is partially permeable to some fluids, bacteria and other products of the oral cavity

The permeability of enamel is due to the presence of cracks and microscopic spaces on the surface of enamel which allows penetration of fluids

The permeability of enamel decreases and hardness increases with age

3 Structure of enamelRod and interrod enamel

The fundamental units of enamel are rods and interrod enamel

The rod and interrod enamel is built from closely packed and long ribbon like hydroxyapetite crystals

The rod is shaped like a cylinder with a wide head portion, a neck and a thinner tail portion

Each rod is formed by four ameloblasts

Enamel rods are connected to one another in such a way that they appear like ‘keyholes’. Head of one rod nestling against the necks of two neighbouring rods

Enamel rods appear like keyholes

Rods are formed nearly perpendicular to DEJ and curve slightly towards the cusp tip

The follow a wavy course as the traverse from the DEJ to the surface of the crown

The length of most rods is much longer than the thickness of enamel

The diameter of the rod at the outer surface is double the diameter at DEJ

Crystals that surround each rod are called interrod enamel

Rod and interrod enamel is formed from the Tomes process of Ameloblasts

Enamel rod and Interrod enamel

The crystals making up the rod and interrod enamel have same composition but are oriented in different direction

Immunocytochemical preparation showing rodsheath

The boundary between rod and interrod enamel is marked by a narrow space filled with organic materials known as rod sheath

Dentino-enamel junction (DEJ)

DEJ represents the interface between dentine and enamel

It appears scalloped which increases the surface area and enable the two dissimilar matrices to interlock

DEJ

Scalloped nature of DEJ as seen with SEM

3 Histological features of enamel

Enamel spindles

Enamel spindles originate from the DEJ

Before enamel forms, some developing odontoblasts process extend into the ameloblast layer, and when enamel formation begins become trapped to form enamel spindles

Enamel Spindles

Enamel tufts

Enamel tufts also originate from the DEJ, run a short distance in the enamel or sometimes to one half of the thickness

They represent protein (enamelin) rich areas in the enamel matrix that fail to mature

They are formed during the formative stages of enamel

They are considered to be ‘faults’ by some researchers while others consider them to be necessary to anchor dentine to enamel

Enamel Tufts

Enamel lamellae

Enamel lamellae extend from the surface to varying depths of the enamel

They are faults that develop as a result of failure of maturation process

They are filled with organic material and water

There are three types of lamellae

Type A – composed of poorly calcified rod segments

Type B – filled with degenerated epithelial cells and formed before tooth eruption

Type C – filled with organic matter and formed after eruption

Enamel Lamella

Cross striations

Cross striations are periodic bands that appear along the full length of enamel rod . Because of this the enamel rod appears like a ladder with cross striations being the rungs of the ladder

They appear at regular intervals that is in agreement with the rate of enamel deposition (which is approximately 4μm per day)

Cross Striations

Striae of Retzuis

Striae of Retzuis also represent incremental growth

In ground cross sections they appear like concentric growth rings similar to those found in trees

In ground longitudinal sections they appear to be dark line extending from the DEJ to the tooth surface

Along the Retzuis striae fewer enamel crystals are found and this is related to physiologic disturbances in the body

Neonatal line is a Striae of Retzuis that forms at birth

Striae of Retzuis

Striae of Retzuis often extend from the DEJ to the outer surface of the enamel, where they end in shallow furrows know as perikymata (or imbrication lines)

Perikymata

Gnarled enamel

Most enamel rods follow an undulating pathway from DEJ to the tooth surface

But in the cusps tips of molars groups of enamel rods twist about one another. This twisting pattern of enamel rod is known as Gnarled enamel

Gnarled enamel makes the enamel strong and more resistant to fracture

Hunter-Schreger bands

Hunter-Schreger bands are an optical phenomena and are seen in reflected light

They can be seen in ground longitudinal sections as alternating dark and light bands

Hunter Shcreger bands

Hunter Shcreger bands

The dark bands correspond to the cross sectional enamel rods (diazones) and the light bands represent the longitudnally sectioned interrod enamel (parazones)

Age changes in enamel

With age enamel becomes worn out because of masticatory attrition

Age also causes a decrease in the permeability of enamel

Other characteristics of aging of enamel are discoloration and a change in the surface layer

Enamel attrition and discoloration

Defects of enamel formation

Generally three conditions effect enamel during its formative stages

Defects caused by febrile disease

Defects caused by tetracycline

Finally defects caused by excess fluoride

Dental Fluorosis (mottled enamel)

Clinical Implications of enamel

Fluoridation

If fluoride ion is incorporated into the hydroxyapetite crystals then it becomes more resistant to acid dissolution

The amount of fluoride must be controlled because high fluoride can cause mottled enamel (in excess of 5ppm)

Acid etching

Acid etching of enamel is a very important technique for conditioning enamel for many clinical procedures

Acid etching is used when doing fissure sealants, restoration, cementing orthodontic bands etc.

It is carried out by using a mild acid like orthophosphoric acid on the enamel surface for a controlled period of time

Applying acid on tooth surface for etching

Enamel before acid etching

Enamel after acid etching

Acid etching results in three predominant patterns

Type 1

Type 1 is characterized by removal of rods

Type 2

Type 2 is characterized by removal of inter rod enamel and the rods remain intact

Type 3

This type is characterized by irregular and random removal of enamel. Type 3 is less frequent