Human Biology Group I - Enamel and Micro-Mechanical Retention in Clinical Dentistry
E-facilitator: Cathy Snelling
Learning Outcomes
  1. Describe the microscopic features of enamel with a particular focus on occlusal fissures.
  2. Integrate this information on enamel histology with the clinical considerations involved in applying fissure sealants
  3. Explain and visually demonstrate the enamel-sealant interface.

1.Describe the microscopic features of enamel with a particular focus on occlusal fissures.


Enamel rods
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external image
Enamel is composed of millions of calcified enamel prisms, which are know as enamel rods. These follow the entire length of enamel, extending from the dentio-enamel junction (DEJ), to the surface of the tooth. There are more per square millimetre at the DEJ, and fan out towards the tooth surface (Bath-Balogh & Fehrenbach, 2006).The rods are shortest at the cemento-enamel junction and fissures. The enamel rods are grouped in rows organized around the circumference of the long axis of the tooth. The rods within each row follow a course that is roughly perpendicular to the surface of the tooth; this is a result of the path that the ameloblasts take during amelogenesis (Bath-Balogh & Fehrenbach, 2006). The diameter of a rod is approximately 4micrometres wide, depending on its location and influenced by the width of the tomes process. Orientation of the rods in relation to one another, is determined by the shape of the Tomes process (6 sided pyramidal).(Bath-Balogh & Fehrenbach, 2006).
(image from http://www.heididds.com/images/servtemp/decay.html )

Aprismic areas
>parts of enamel that lack the characteristic rods
>formed after regular enamel, on the surface of enamel
>as such it is usually worn away fairly early in life, exceptions include cervical areas and in pits or fissures.
>clinically significant in that it cannot be etched in the same way as rodded enamel

Lines of Retizius
The Line of Retizius and the Perikymata are line that run more or less at right angles to the enamel rods and reflect successive incremental deposition of mineraliased enamel. They can represent planes of weakness in the enamel, particulary in the cervical area of the tooth, where the enamel is the thinnest. In relation to fissures sealants, the areas that are sealed could include these features and help to protect these weaker areas.

Neonatal line
Prenatal enamel is formed in a relatively stable environment; it tends to be more regular, more opaque and generally contains few disruptions or defects. Conversely postnatal enamel is formed in a less stable environment; it tends to be less regular, more opaque and contains relatively more disruptions and defects than prenatal enamel. The line separating the pre- and post- natal enamel is called the neonatal line (Bath-Balogh & Fehrenbach, 2006). The neonatal line is an incremental line of Retziu. The accentuated incremental line indicates an abrupt change in certain body processes of the fetus during birth. The change presents dentally as a neonatal ring, as a result of the disturbance (Pinkham et al, 2005). Affected teeth are formed at birth (read upper 1's and 6's). Because of this the neonatal line may be visible in the occlusal fissures of first molars.

Spindles
Enamel spindles are short dentinal tubules, that have cross over into the enamel during the enamel mineralization process. This occurs when an odontoblast moves in the wrong direction during amelogensis. The tip of the odontoblast process, becomes trapped in the enamel. Spindles originate at the DEJ, extending to the enamel, mainly at the incisal edges or cusps of teeth. Unknown clinical significance, since they are not surface features of enamel (Bath-Balogh & Fehrenbach, 2006). They could conceivably form in the enamel around occlusal pits and fissures

Tufts
Enamel tufts originate from the DEJ, appearing in rows as small dark brushes. They are named because of their similarity in appearance to tufts of grass. They are formed during the development of the Tomes process, representing protein rich areas in the enamel matrix that has failed to mature. Enamel tufts extend through 1/3-1/2 of thickness of enamel. Unknown clinical significance, could conceivably form in the enamel around occlusal pits and fissures (Bath-Balogh & Fehrenbach, 2006).

Lamellae
As with enamel tufts, enamel lamellae are lines of less mineralized enamel originating from the enamel surface in varying degrees towards the DEJ and visa vera . Lamellae consists of thin sheets of organised material that extends throughout the enamel (Bath-Balogh & Fehrenbach, 2006). Due to the high content they can be seen in sections of demineralized enamel. They run vertical from the incisial or cusp areas, towards the cervix of the tooth. They tend to be longer and thinner than enamel tufts.They are often associtated with an increased susceptibility to caries as they as they are considered weak spots. They are most widespread defect in enamel. If suspected, can be a consideration for placement of a fissure sealant as they can lead to further areas of deminerlization.

2. Integrate this information* on enamel histology with the clinical considerations involved in applying fissure sealants


Technique Considerations:

When placing sealants, the enamel surface is etched to remove some organic portions of the enamel crystals. The removal of the crystals, exposes the enamel rods. The orientation of the crystals in the rods and interrod enamel contributes to the etching pattern, creating porosities. The created porosities create a larger surface area and greater adherence when the sealant is applied.

Adequate surface preparation, may cause improper cleaning, prior to the placement of the etch. Untreated enamel is covered with plaque and pellicle. These form a barrier to etching, if not removed prior to etching, the enamel rods are not exposed. Making the surface mechanically unsuitable for bonding (Pinkham, et al, 2005).

During the application of the etch solution is applied with an acid-etch brush. When placing the etch, a dabbing motion is advised. If a rubbing motion is used, the fragile enamel lattice work thats formed during the etching process will break. If the lattice work breaks then the porosities needed for bonding will not be present and increases the chances of failure (Pinkham, et al, 2005).

The etching process dehydrates the tooth surface. The tooth is subject to injury and bacteria for approximately 24 hours after application. Fluoride provides the necessary protection for the tooth during this period of time and reduced the risk of continual demineralization (Harris & Garcia-Godoy, 2004).

Inadequate isolation is critical to the placement of a fissure sealant. If the enamel porosities created by the etching process are filled with any kind of liquid, the formation of the enamel tags are blocked or reduced causing poor retention (Harris & Garcia-Godoy, 2004).

When the tooth enamel is etched, the enamel surface becomes demineralized. The demineralized enamel surface needs to be protected from contamination and remineralization during the sealant placement (Pinkham, et al, 2005). Saliva contamination causes the etched surface to remineralise, the porosities are no longer present and the etch process needs to be repeated. If the surface isn't protected, these is an increase in the risk of failure of the sealant Pinkham, et al, 2005).

After the application of sealant, all margins on the tooth surface should be checked that they are flushed and application was successful. If the sealant margins extends beyond the tooth structure , those of extended areas will increase the risk of micro-leakage beneath the sealant and/or fracture the sealant. Micro-leakage increases the risk of the etched enamel rods being exposed (Pinkham, et al, 2005). The organization of the enamel rods into rows, is clinically important because caries follows the path of the rods (Bath-Balogh & Fehrenbach, 2006).

Tooth Morphology

especially in relation to the depth, width and shape of the pits and fissures
DW_wide_shallow_and_self_cleansing.JPG
wide&shallow fissure

Some fissures are relatively thin and wide; they may self cleanse and tooth brush bristles will easily fit into them. There is likely to be no clinical benefit of a fissure sealant in this type of a fissure.




DW_thin_deep_and_difficult_to_clean.JPG
a thin deep fissure
Fissure may also be very deep and so thin that the bristles of a toothbrush are physically unable to fit in between the gap. The depth of such a fissure may also result in very little space between the surface of the enamel and the DEJ; this is of clinical significance in that in such an instance it would take the caries far less time to enter the dentine that if it originated from one of the cusps.



1DW_i_shaped.JPG
an i-shaped fissure

Fissures may become very thin at one point, before opening up into a larger space. Mechanical removal of plaque in this bottom area may be impossible and plaque may be able to mature relatively unimpeded.



the caries history of the tooth/other teeth in the mouth; if other, similar teeth have been prone to caries, then it is likely that other teeth in the mouth will also be prone
Age of the tooth; generally newly erupted teeth are seen as better candidates for sealants than teeth that have been in the mouth for a long time.

*I'm having trouble with the whole integration thing ;)

3. Explain and visually demonstrate the enamel-sealant interface.


The enamel surface interface describes the relationship between the enamel rods and sealant after placement. After placement, the relationship between the enamel and resin shows no detectable micro-spaces between the two (Pinkham, et al, 2005).

This occurs as a result of the etching process. After etching, the surface enamel appears chalky and rough. The roughness allows for better adherence to the etched surface, than it would a smooth non-etched surface. When comparing non-etched and etched surfaces, the porosities created in the enamel surface, increase the area of retention, which results in increased adhesive potential (Harris & Franklin, 2004).

There are a variety of acids used for etching, and they may be specific to the enamel rods or the inter-prismic regions. The type of etch is labelled according to the part of the enamel that is worn away.During etching, about 2-3 microns of enamel is removed, the average thickness of enamel is 1500microns. These are three types of etching patterns that occur in the enamel after exposure to acid-etch (Bath-Balogh & Fehrenbach, 2006).

a Type I etch is achieved when the acid targets the enamel rods, this leaves an array of small, regular pits with raised areas consisting of the inter-rod parts of the enamel lattice. This is the most commonly used variety of etch used for fissure sealants (Bath-Balogh & Fehrenbach, 2006).

type_1_colour.JPG
external image f1t0191.jpg
[http://www.forp.usp.br/bdj/Bdj9(1)/t0191/t0191.html]

A Type II etch occurs when the acid predominantly erodes the inter-rod areas, leaving behind raised bumps (the enamel rods) (Bath-Balogh & Fehrenbach, 2006).
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type_II_etch_pattern.JPGtype_2_colour.JPG















A Type III etch is a result of neither the rods nor the inter-rod parts of the enamel being selectively eroded by the acid. This can be a result of using an inappropriate acid or trying to etch an inappropriate area of enamel (i.e. aprismatic enamel) (Bath-Balogh & Fehrenbach, 2006). A Type III does not have a regular pattern and as such is less suited to most dental materials than type I or II etches. It is of clinical significance to fissure sealants as the target location (pits and fissures) are often one of the last places on the tooth to have aprismatic enamel (Bath-Balogh & Fehrenbach, 2006). .

type_III_etch.jpg
(http://www.forp.usp.br/bdj/Bdj9(2)/t0592/f01t0592.html)
The type of etching pattern has not been found to be associated with the increase or decrease in the sealant retention rates (Pinkham, et al. 2005, p. 552)

The sealant material penetrated in the porosities created in the surface enamel during the etching process (Harris & Franklin, 2004). As the sealant material flows into the porosities, tag like structures are created, which provide mechanical retention for bonding. Resin tags have a number of functions, the major being the the mechanical means of retention (Pinkham, et al, 2005).

Micro-mechanical retention occurs on the treated enamel surface, as the foreign material e.g. plaque are removed.When etch is applied, it removes superficial plaque and debris that act as a barrier to expose inorganic crystalline components of enamel. The polar phosphate groups are created for potential bonding sites(Pinkham, et al, 2005). The uniformity and roughness of the etched enamel surface allows the low viscosity fissure sealant material to flow into and between the porosities, resulting in an interlocking interface between the etched enamel and the sealant (Pinkham, et al, 2005). When the material is set (often with a light curing device) it hardens in tag-like projections. The material and the etched enamel surface are physically unable to move apart without breaking at least part of the enamel or the sealant. This is known as micro-mechanical retention (Bath-Balogh & Fehrenbach, 2006).

2.JPG

microscopic image of an etched enamel surface
A schematic diagram showing the inter-locking nature of micro-mechanical retention.

Finally the sealant material changes the surface morphology. The enamel-resin interface creates a protective barrier against bacteria in a sealed fissure and does not allow nutrients into the fissure (Pinkham, et.al, 2005).


Resources to View:

My Uni - CLINICAL PRACTICE IOH PART 2 > COURSE MATERIALS > FISSURE SEALANTS > resource notes fissure sealants 2007.pdf - pg12 there is some info and pictures on enamel etching for fissure sealants - definitely worth the read.

Links to journal articles/sites with pictures of etched enamel:
http://www.forp.usp.br/bdj/Bdj9(1)/t0191/t0191.html - shows how different types of etching compounds impact on the type of etch achieved. (need to click on the text to bring the images up)

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-64402003000300005&lng=es&nrm=iso&tlng=es - shows different types of etching materials on primary teeth, has a good pic of a type III (non defined) etch.

http://www.medicinaoral.com/medoralfree01/v11i1/medoralv11i1p40.pdf - "Acid-etching effects in hypomineralised amelogenesis imperfecta". A microscopic and microanalytical study. Has some very clear pictures

References:

Bath-Balogh, M and Fehrenbach, M 2006 Dental Embryology, Histology and Anatomy, 2nd edn, Elsevier Saunders, St Louis, Missouri.

Harris N & Garcia-Godoy 2004 Primary Preventative Dentistry, 6th edn Pearson Prentice Hall, New Jersey.

Pinkham, J.R, Casamassimo, P.S, Fields, H.W, McTigue, D.J, Norwak, A.J, 2005, Pediatric Dentistry: Infancy through Adolescence, Elsevier Saunders, Missouri.

Sánchez-Quevedo, C, Ceballos , G, Ángel Rodríguez, I, Manuel García , J & Alaminos, M 2005, 'Acid-etching effects in hypomineralized amelogenesis imperfecta. A microscopic and microanalytical study' Medicina Patologia Oral, vol. 11 pp 40-43 viewed 20 September 2007
http://www.medicinaoral.com/medoralfree01/v11i1/medoralv11i1p40.pdf

Silverstone, L & Dogon, I 1974, The Acid Etch Technique, North Central Publishing, Minnesota.