ACID ETCHING
CONTENTS
Page no
INTRODUCTION 1
HISTORY 4
ENAMEL 10
Formation of enamel 10
Composition of enamel 11
Structure...
BIBLIOGRAPHY 105
INTRODUCTION
The possibility of bonding restorative materials to the hard
dental tissues intrigued the de...
The success of acid etching of enamel led Buonocore et al (1956)
to try to acid etch dentin using 7% HCL for one minute. U...
bacteria and their products. As these reactions are not seen following
acid etching of dentin bonding systems.
It is clear...
HISTORY
There are names in the lexicon of dental adhesion that those of
us in the field should always remember to acknowle...
the last thirty years and both the bond strength values and
biocompatibility to pulp dentin organ have tremendously improv...
constants with molecules that have linked groups similar to those of
NPG-GMA (Bowen 1978).
Furthermore, the oxalate would ...
of PMDM. This system was effective only if placed in the described
sequential order utilizing all three components. The ac...
oxalate and nitric acid were then applied, and no staining problems
occurred in animal trails (Blosser and others 1989).
T...
♦ An acidic NPG solution
♦ PMDM acetone solution
However, preparation and storage of the first solution was difficult
beca...
methods for preventive and restorative dentistry by way of adhesive
bonding to both dentin and enamel through the reliable...
This orientation results from their tendency to grow
perpendicularly to the surface on which they develop. The developing
...
isolated in several different fractions; they generally contain high
percentages of serine, glutamic acid and glycine.
Roe...
material. Changes in the orientation of the crystals, relative to each
other, mark the boundaries of the prisms. In the hu...
prism bodies, because the crystals meet at different angles and thus
cannot be packed as tightly together.
15
Fig 2. Compo...
The consistent arrangement of the inter prismatic enamel, with
its greater protein content, accounts for the fish scale ap...
coating of matrix. Matrix is more apparent in the tail region than in
the body region.
The material at prism junctions has...
lateral shear. The keyhole configuration results from the unique
shape of the typical pit produced on the development surf...
approximately transversely. However the transition between alternate
bands is gradual.
19
Fig 6. Key hole shaped enamel
ro...
Enamel Tufts
Enamel tufts are hypo-mineralized structures of enamel rods
and inter-rod substance that project between adja...
Enamel rods are formed linearly by successive opposition of
enamel in discrete increments (FIG. 16). The resulting variati...
In horizontal sections if the tooth, the Striae of Retzius appear
as concentric circles. In vertical sections, the lines t...
cross striations in inner enamel, but was unable to do so in outer
enamel, suggesting that the pores at most striations ar...
Pattern 2
The prisms have incomplete outlines and are arranged in rows.
Within each rows narrow bridges of inter prismatic...
Crystal Size and Morphology
The crystals of mature enamel appear to grow and fill the bulk
of the space available within t...
Dentino Enamel Junction
The interface of the enamel and dentin is called the
dentinoenamel junction (FIG 12). It is scallo...
27
DENTIN
28
Fig 16. Ground section of
enamel viewed under
transmitted light showing striae
of retzius
Fig 17. Photomicrograp...
Dentin provides the bulk and general form of the tooth and is
characterized as a hard tissue with tubules throughout the t...
It is light yellow in colour and becomes darker with age and less
translucent. It is harder than bone and cementum but sof...
They show two curvatures - primary curvature and secondary
curvatures (FIG. 22).
Primary curvature start at right angle fr...
Enamel Spindles
32
Near the dentino-enamel junction, the dentinal tubules divide
into several terminal and form an inter communicating and
an...
compared with a KHN of 52 for intertubular dentin. The presence of
this intertubular dentin narrows the lumen of the tubul...
dentin where it is housed in a fine canal, the dentinal tubules. The
odontoblastic processes are largest in diameter near ...
Secondary Dentin
36
Secondary dentin is a continuation of primary dentin that forms
at a slower rate as the tooth ages physiologically. It is ...
This line reflects the abrupt change in the environment that
occurs at birth. The dentin matrix formed prior to birth is u...
39
Fig 32. Contour lines of
Owen
Fig 31. Von Ebner’s lines
Fig 33. Neonatal line in
dentin
Fig 34. Ground section of denti...
AGE AND FUNCTIONAL CHANGES
Reparative Dentin
Reparative dentin is formed by the replacement (or) secondary
odontoblast in ...
Sclerotic Dentin
Sclerotic dentin results from aging or mild irritation (such as
slowly advancing caries) and causes a cha...
42
Fig 37. Types of reparative dentin
Fig 36. Reparative dentin
Fig 38. Dead tracts - ground section of
dentin viewed unde...
The amount of sclerosed dentin increases with age and is most
frequently encountered in the apical third of the root. Scle...
INNERVATIONS OF DENTIN
Intertubular nerves
Dentinal tubules contain numerous nerve endings in the
predentin and inner dent...
However, the true length of the processes in mature dentin, in
the absence of blood vessels or supporting cells, is an iss...
assessed (Kimberly and Byers 1988, Byers et al 1990, Byers 1996),
and results in the release of calcitonin gene related pe...
Fluid Flow
In clinical conditions there is an outward fluid flow across
exposed dentin in response to the low but positive...
The density of tubules per mm square varies from 15,000 at the
dentino-enamel junction to 65,000 at the pulp boundary be p...
Thus, within any location on the tooth peripheral dentin has a
lower permeability than deeper dentin. The permeability of
...
act as an integrated system. As soon as the tissues, which normally
cover dentin, are lost, then normal compartmentalizati...
ACID ETCHING ON ENAMEL
The developed materials that adhere or bond to tooth structure
would minimize removal of healthy ti...
Because there is concern for interference of flavoring oils,
glycerin and fluorides with the etching process, the use of w...
53
Fig 40
Pre operative - silver amalgam
restoration
After cavity preparation
Acid Etching Rinsing with water
Blot excess ...
It is important not to rub the enamel during acid application,
since burnishing the friable rods and their crystallites wi...
Washing
There is a significant increase in bond strength values when
enamel is washed for 60 secs compared to 15 secs. The...
Drying
Next, the teeth are thoroughly dried with a moisture-and-oil-free
air source to obtain the well-known dull, frosty ...
57
Fig 41. Etching pattern of enamel after acid etching
Fig 42a. Different types of etching pattern
Fig 42b. Acid etched e...
Type 1
Prism core material is preferentially removed, leaving the prism
peripheries relatively intact, resulting in a hone...
passage of only the smallest molecules comparable in size to that of
water. Acid etching enhances not only the size of the...
remineralization. Evidence also shows that topical fluorides alter the
oral bacterial flora, there by increasing resistanc...
not survive as long under conditions of oral moisture. It should be
noted that washing away the acid conditioning solution...
TIME
Increased Time Application
High fluoride content and primary teeth require longer etching
time. The increased etching...
Concentrations of phosphoric acid over 65% tend to show
minimal changes. The concentrations of acid, producing consistent,...
more or less evenly distributed relatively deep etch pattern, appear to
be in the range of 30 to 50%. Bond strengths are g...
Currently phosphoric acid is the acid of choice, but it is possible
that other acidic etching agents such as pyruvic acid ...
that has not been resolved is the optimal duration of etching with
phosphoric acid.
It is surprising that some authors rec...
placement with 85% phosphoric acid for 30 seconds. He noted that
the acid conditioning of the enamel resulted in on uncont...
Buonocore (1955) as an attempt to clean enamel, increase the
microscopic surface area for bonding, and infiltrate unfilled...
♦ Demineralize the superficial dentin matrix to permit resin
infiltration into surface.
♦ Uncover both intertubular and pe...
Nakamura and Yasuda 1991). Another danger in the etching step is
that the demineralized zone may extend, for instance, 5 µ...
Physical changes
Increases or decreases in the thickness and morphology of
smear layer changes in the shape of dentinal tu...
to and of the vital odontoblast processes to supply the calcium
phosphate from the pulp is essential for remineralization ...
100 microns at which point it diffuses laterally into the peri-tubular
dentin for up to 10 microns.
As in the previous cas...
impregnated dentinal layers. These changes will considerably improve
the bond strength as well as the tubule aperture seal...
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acid etching/ rotary endodontic courses by indian dental academy

  1. 1. ACID ETCHING CONTENTS Page no INTRODUCTION 1 HISTORY 4 ENAMEL 10 Formation of enamel 10 Composition of enamel 11 Structure of enamel 12 DENTIN 23 Physical properties 24 Composition of dentin 24 Structure of dentin 24 ACID ETCHING OF ENAMEL 41 Steps in acid etching technique 41 Effects of etching on enamel 45 Role of fluorides in etching of enamel 47 Factors affecting etching of enamel 50 Variation in acid etching methodologies 52 ACID CONDITIONING Of DENTIN 55 Goals of acid conditioning of dentin 56 Effects of acid conditioning of dentin 58 Factors affecting dentin conditioning 68 Conditioners on dentin surface 77 BIOCOMPATIBILITY 79 VARIOUS ACID CONDITIONERS 86 SELF ETCHING PRIMERS 95 EFFECT OF ACID ETCHING ON PRIMARY TEETH 98 APPLICATIONS OF THE ACID CONDITIONERS 100 FACTORS TAKEN INTO CONSIDERATION 102 ACID CONDITIONERS FOR GI CEMENTS 103 1
  2. 2. BIBLIOGRAPHY 105 INTRODUCTION The possibility of bonding restorative materials to the hard dental tissues intrigued the dental professionals for many years. The development and regular use of adhesive materials has begun to revolutionize many aspects of restorative and preventive dentistry. Attitudes towards cavity preparation are altering since, with adhesive materials, it is no longer necessary to produce large undercuts in order to retain the filling. These techniques are therefore responsible for the conservation of large quantities of sound tooth structure, which would otherwise be victim of the dental bur. Buonocore (1955) was the first to report the positive effects of application of 85% phosphoric acid to enamel for the retention of acrylic resin restorations. Gwinnett, Matsui and Buonocore (1969), further explored the effect of acid solutions and this came to be accepted as an integral part of any direct tooth coloured restorative technique. Bonding of restorative materials to hard dental tissues would be impossible without the use of acid solutions. It is the effect of these various acid solutions and pretreatments that results in the hard dental tissues being characterized by numerous microscopic porosities, which allows the resin to readily wet the surface and penetrate into these micro porosities. Once the resin penetrates into these micro porosities it can be polymerized to a form a mechanical bond to the hard dental tissues. 2
  3. 3. The success of acid etching of enamel led Buonocore et al (1956) to try to acid etch dentin using 7% HCL for one minute. Unlike enamel when dentin is etched, surface becomes mineral poor, protein rich, and it tends to become wetter (Brannstrom and Norden Vall 1977). Unfortunately, success with dentin was never realized, because the relatively cured resin materials that were available at that time would not wet dentin very well. Buonocore, however, was very much aware of the requirements for good bonding. The term conditioner or etchant is used to describe agents that are washed off the dentin. The word “etchant” has, until recently, been taboo in western dentistry, for describing the action of various acidic materials on dentin. Etching of the dentin can be defined as “any alteration to the dentin done after the creation of dentin cutting debris, termed the smear layer” (Eick et al 1970). One of the objectives of dentin etching is to create a surface capable of micro-mechanical bonding to a dentin-bonding agent. Several acids have been researched as dentin etchants. These include hydrochloric acid, oxalic and pyruvic acid in addition to the better known acid such as phosphoric, maleic, citric and nitric acid. Fusayama et al (1979) were the first to report the successful use of Phosphoric acid to remove the smear layer, etch the dentin and restore with adhesive composite resin. Acid etching of dentin is used by many bonding systems to remove the smear layer and permit bonding directly to the dentin matrix. Although early animal studies indicated that acid etching caused moderate to severe pulpal reactions, there is a high probability that the pulpal irritation may have been due to micro leakage of 3
  4. 4. bacteria and their products. As these reactions are not seen following acid etching of dentin bonding systems. It is clear that one can acid etch dentin if, and only if, one can seal the dentin with subsequently placed bonding systems. Because acid etching increases dentin permeability and dentin wetness, successful bonding of adhesive resins to acid etched dentin requires the use of hydrophilic resins that bond equally well to both peri tubular and inter tubular dentin. The trend seems to be toward lowering both the concentration of acids and the time of etching of dentin. While all bonding systems should be carefully scrutinized prior to marketing, the future looks very promising for the use of adhesive resins on both enamel and dentin through the effective use of acid-etch technique. 4
  5. 5. HISTORY There are names in the lexicon of dental adhesion that those of us in the field should always remember to acknowledge because it was on their shoulders that we stood as we grappled with our own research problems. Dr. Michael Buonocore was certainly one of such best-known pioneers in adhesive bonding of resins to teeth. He found that lightly etching enamel created a micro porous surface into which direct filling liquid resins could flow polymerize and make a micro mechanical attachment. He thereby achieved his primary objective of bonding, a conservative means of sealing developmental pits and fissures. One of Dr. Buonocore’s contemporary Dr. George Newman developed similar methods to bond orthodontic brackets directly to the enamel of teeth. Another distinct advantage of effective acid etching technique and its resultant adhesive bonding to dentin is the prevention of removal of healthy dentin for mechanical retention of composite restorations, a process that is painful without an anesthetics. The mechanism of enamel bonding is well understood and involves a micro-mechanical union between enamel and the resin, which occupies tissue microspores enlarged by the action of an acidic conditioning agent. However, acid etching of the pulp dentin complex and its resultant bonding has undergone a number of changes over 5
  6. 6. the last thirty years and both the bond strength values and biocompatibility to pulp dentin organ have tremendously improved. Brannstrom et al (1984) suggested that on a number of occasions, they inadvertently acid etched teeth with small pulp exposures at the base of deep cavities. These were not often discovered until subsequent histo-pathological examination of the extracted teeth. Unless there was concomitant infection there was no particular damage or inflammation to the pulp. However, they noticed that when restorations leaked, and bacteria colonized the cavity surface, the teeth that had been etched exhibited more severe pulpal responses, than those that were not etched. Many attempts have been made to synthesize different coupling agents for tooth surfaces. One of the earliest successful compounds tested was NPG-GMA, the reaction product of N-phenyl-glycine and glycidyl methacrylate (Bowen 1965). The use of this surface-active co- monomer alone improved the water resistant bonding between resins and enamel and dentin to a degree that was statistically but not clinically significant (Bowen 1965). Removal of the structurally weak smeared layer, pellicle, or other superficial layers of the tooth surface by use of acidic (Fusayama & others, 1979, Fusayama 1980) or chelating agents might reduce the availability of calcium ions on dentin surfaces for interaction with a chelating surface active co-monomer like NPG-GMA or other coupling agent with, preferably, multiple-bonding ligand groups. To supplement calcium ion sites for improved bonding, certain appropriate metal cations were evaluated for use on tooth surfaces. Experiments indicated that the most effective agent might be ferric oxalate, primarily because of the iron ion’s high tendency to be bound strongly by denting and enamel and its high chelate stability 6
  7. 7. constants with molecules that have linked groups similar to those of NPG-GMA (Bowen 1978). Furthermore, the oxalate would form an insoluble precipitate with calcium ions, which, together with insoluble ferric phosphate, would seal the dental tubules to provide pulp protection and desensitization. Nakabayashi (1982) introduced the concept of hybridization. The technique consists of applying an acid, ranging in concentration from 10% to 30% to the surface of dentin. Within 15 minutes the acid selectively dissolves away the inorganic component of the dentin to a depth of 5 to 10 microns. It then flows in to the dentinal tubule for upto 100 microns at which point it diffuses laterally into the peritubular dentin for up to 10 microns. As in the previous case the calcium component is selectively eliminated. Then these spaces are replaced by an insoluble resin component that completely encapsulates all exposed collagenous fiber. It was then discovered that the additional use of a relatively hydrophilic monomer containing two free carboxyl groups in addition to two polymerizable groups on each molecule dramatically improved bond strengths to levels of clinical significance (Bowen and others, 1982). This monomer was called “PMDM” (the reaction products of pyromellitic dianhydride and hydroxyethyl-methacrylate). There was a synergistic interaction between the NPG-GMA and the PMDM (Bowen and others, 1984). The original adhesive system developed was a sequential application of aqueous acidic ferric oxalate, followed by an acetone solution of NPG-GMA or NTG-GMA (the reaction product of N- ptolyglycine and glycidyl methacrylate), and then an acetone solution 7
  8. 8. of PMDM. This system was effective only if placed in the described sequential order utilizing all three components. The acidic ferric oxalate solution was removing the original smear layer, the disturbed surface layer caused by mechanical abrasion in preparing a restoration site (Bowen and others, 1984), and laying down a layer of precipitation product that was plugging up the lumina of dentinal tubules. The latter function significantly reduced tooth sensitivity to the subsequent procedure. The NTG-GMA was necessary to induce polymerization of the PMM, but the exact mechanism of this free radial initiation is still not clear. During subsequent experimentation, it was discovered that the smear- removing capabilities of ferric oxalate were due primarily to the presence of small amounts of nitric acid left over from the synthesis of the oxalate (Cobb and others, 1989). Controlled additions of nitric acid to the aqueous oxalate solution were made to determine the optimum acid concentration for this solution (Blosser and Bowen, 1988). A small increase in the concentration of nitric acid to about 2.5% HNO3 by weight also improved the simultaneous etching of instrumented enamel. However, an adverse side effect of the application of the ferric oxalate solution was discovered; the occasional appearance of black staining at the adhesive interface in early animal trails (Stanley, Bowen and Cobb, 1988). This could be reproduced in the laboratory by applying a sodium sulfide solution to ferric oxalate treated dentin. The cause of this staining in vivo is probably (although not proven to be) the reduction of ferric to ferrous ions by sulfide forming anaerobic microorganisms resulting in the formation of black ferrous sulfide pigments. To eliminate this, acidic aluminum oxalate was substituted, and it produced no staining on dentin. Aqueous solution of aluminum 8
  9. 9. oxalate and nitric acid were then applied, and no staining problems occurred in animal trails (Blosser and others 1989). There was some evidence in vitro that aluminum oxalate did not produce as much of the reaction products plugging the dentin tubules, as has ferric oxalate. Eventually, the first successful transfer of the adhesion technology developed by scientists at the ADA Health Foundation’s Paffenbarger Research Center involved the development of a product that incorporated aluminum oxalate in a conditioning solution. In the continuing research, it was found that the aluminum oxalate could be eliminated entirely from the experimental system without loss in adhesion, if the dilute nitric acid solution was retained. None of the other acids evaluated, in a wide range of concentrations, were as good or better than the dilute nitric acid (which should be distinguished from concentrated nitric acid, a strong oxidizing agent). It was then surprisingly discovered that NPG (N- phenyl-glycine) could be substituted for NPG-GMA or NTG-GMA. The experimental system was then reduced to the three components of ♦ Dilute nitric acid ♦ NPG acetone solution ♦ PMDM acetone solution The three components still had to be applied individually in sequence to adhesive adhesion, and efforts were concentrated on ways to simplify application. It was then suspected and verified that NPG would be soluble in the dilute aqueous nitric acid solution. This permitted a simplification of the procedure to the application of two solutions. 9
  10. 10. ♦ An acidic NPG solution ♦ PMDM acetone solution However, preparation and storage of the first solution was difficult because of the reactivity of the NPG molecule to atmospheric oxygen. Storage times were very short if the acid NPG solution was exposed to air. If the solution was used shortly after mixing, adhesion was effective. Methods were then developed for preparing the solution under an inert atmosphere and protecting it from subsequent oxygen exposure. These protected solutions were solutions were stable under normal storage conditions. Some commercial products are currently based on this two-solution system. Current experimentation with the system is focusing on optimizing the individual components. Nitric acid concentrations will continue to be refined to yield optimal treatment of both dentin and enamel. Different analogues of the NPG molecules are being synthesized toward improving effectiveness, storage, stability and ease of synthesis (Johnson, Asmussen and Bowen 1989). PMDM is being investigated to isolate more effective linking agents between tooth surfaces and the overlying restorative resins. Many years of experience in etching enamel with phosphoric acid have shown bonding by this method to be most reliable clinically. However, it is noteworthy that the use of the chemically functional and more hydrophilic dentin bonding agents, significantly increases bond strengths to acid etched enamel at least in laboratory tests. A number of bonding systems already available to practioners are beneficial for increased versatility toward improving the performance of restorative materials. And, given the high tensile strength of dentin (Bowen and Rodriguez 1962), the progress made in the last decade, and the currently recognized need for dentin as well as enamel bonding, it is reasonable to expect that before the end of this decade the intensive and extensive research efforts will succeed in providing clinicians with completely satisfactory materials and 10
  11. 11. methods for preventive and restorative dentistry by way of adhesive bonding to both dentin and enamel through the reliable use of acid treatments on both enamel and dentin. ENAMEL Enamel is the most highly calcified and hardest tissue of the body. Enamel contains 96% inorganic portion and 4% organic portion. Unlike dentin, cementum and bone, cells of ectodermal origin. In the human tooth, the enamel normally forms a covering layer for the whole of the crown, but varies considerably in thickness in different parts of the crown (FIG .1). Enamel is a composite material consisting of two phases: 1: Mineral. 2: Organic. The mineral phase, an apatite calcium phosphate, is the major component and accounts for the hardness of the tissue. The properties of the mineral phase are modulated dramatically because it is divided into microscopic whiskers or fibers known as crystals. The crystals are cemented together by the organic phase, which is a matrix of protein polymer. The composite resists brittle fracture far better than does crystalline apatite alone. Formation of Enamel The long, thin, lathe like crystals that compose enamel are oriented roughly perpendicular to its surface. These crystals grow in a gel of protein matrix, which disappears to a large extent as the crystals grow within it. Eventually, the protein matrix takes the form of extremely thin layers, which both glue and separate the enamel crystals. The basic orientation of the enamel crystals is perpendicular to the tooth surface. 11
  12. 12. This orientation results from their tendency to grow perpendicularly to the surface on which they develop. The developing surface is not simply flat, but is pitted by the secretory poles of the ameloblasts. A good three-dimensional picture of the sub-microscopic structure of enamel can be obtained by visualizing crystals perpendicular to this peculiarly shaped, pitted surface. However, it is probably of more significance and greater interest to understand the discontinuities in the enamel structure, which develop at the sharp concavities of the boundaries, or floors and walls, of these pits. It is the arrangement of the crystals at the developing surface that causes the discontinuities in crystal orientation, which we know as the prism boundaries or junctions. These locations acquire a more concentrated organic matrix during maturation and in the adult tissue are distinguished by the name “prism sheaths”. Composition of Enamel The enamel consists mainly inorganic material (96%) and only a small amount of organic substance and water (4%). The inorganic material is apatite. The nature of the organic constituents of enamel is incompletely understood. In development and histological staining reactions the enamel matrix resembles keratinizing epidermis. More specific methods have revealed sulfydryl groups and other reactions suggestive of keratin. (FIG. 2) However chemical analysis of the matrix of mature enamel indicate that the amino acid composition is not closely related to the keratin and is distinctly different from collagen. Proteins can be 12
  13. 13. isolated in several different fractions; they generally contain high percentages of serine, glutamic acid and glycine. Roentgen-ray diffraction studies reveal that the molecular structure is typical of the group of proteins called cross-beta proteins. In addition histochemical reactions have suggested that the enamel- forming cells of developing teeth also contains a polysaccharide- protein complex and that an acid mucopolysaccharide enters the enamel itself at the time when calcification becomes a prominent feature. Tracer studies have indicated that the enamel of erupted teeth of rhesus monkeys can transmit and exchange radioactive isotopes originating from the saliva and the pulp. Considerable investigation is still required to determine the normal physiologic characteristics and the age changes that occur in the enamel. Brudevold et al (1960) reported the inorganic components inorganic components of enamel are principally apatite in its hydroxy, fluoride or carbonate ions. Minor variations occur in composition in which aluminium, barium, magnesium, strontium, radium and vanadium among others can be found in the lattice. Structures of Enamel Enamel Prism or Rod The prism or rod is the fundamental structural unit of enamel, each prism extends from its site of origin at the DEJ to the outer enamel surface crystals of hydroxyapatite (FIG. 3). All enamel with few exceptions (eg: very thin enamel) is made up of super assemblies of these structures, combined with varying amounts of interprismatic 13
  14. 14. material. Changes in the orientation of the crystals, relative to each other, mark the boundaries of the prisms. In the human enamel, the boundary of the prism body is incomplete cervically. Here the prism is continuous with a wedge-shaped ‘tail’, which comparative studies (BOYDE 1965) show to be interprismatic enamel. The combined shape of the prism body and the tail is that of a keyhole (FIG. 4 and 5). The body of the prism is approximately 5micron meter wide and the prism plus tail keyhole is approximately 9micron meter long (FIG. 6). The apatite crystals are most closely packed in the prism bodies, which occupy 60-65% v/v of enamel (Shellis 1984) when considered (FIG. 7). The configuration of enamel crystals is related to the organization of the ameloblast and it’s tome’s processes. The forming surface of enamel consists of pits, each defined by a wall made up of newly formed interprismatic enamel. During active secretion, each of these walled pits is occupied is occupied by a tomes processes. The inter prismatic walls are formed slightly earlier than the prism enamel, which constitutes the floors of the pits and are formed by secretion sites at the ameloblasts peripheries. The presumptive prism boundary is defined by the position of the junction between the pit wall and the floor. In human enamel the pit is at it’s deepest occlusally, and rises to become confluent with the wall cervically, thus eliminating the boundary in this region. Each wall (inter prismatic region) is formed as a cooperative effort by adjacent secretory ameloblasts. Based on current knowledge of enamel formation, it is clear that each ameloblast is responsible for the formation of one prism at its central secretary site and a portion of the surrounding inter prismatic region at its cooperative peripheral sites. Inter prismatic enamel contains more enamel protein than the 14
  15. 15. prism bodies, because the crystals meet at different angles and thus cannot be packed as tightly together. 15 Fig 2. Composition of enamel by volume percentage Fig 1. Distribution of enamel (A. Dental enamel covering anatomical crown, B. Dentinoenamel junction, C. Cemento enamel junction) Fig 3. Individual enamel rods inter digitizing with neighboring rods Fig 4. Orientation of crystals in forming rod head & tail Fig 5. Orientation of enamel rods
  16. 16. The consistent arrangement of the inter prismatic enamel, with its greater protein content, accounts for the fish scale appearance observed in ground sections. Due to its ultra-structural organization, enamel despite it’s hardness and density- has appreciable porosity. The pore affects the mechanical and optical properties of enamel; the formation of carious lesions is strongly influenced by the pathways for diffusion and by electro chemical effects arising from the charge on the pore wall. The prism junctions or boundaries, which are the sites where crystals of the tail region of one prism meet with those in the body of another, are sites where there is an abrupt change in the crystal orientation. Consequently, prism junctions have enlarged pores, filled with matrix and hence increased porosity. (Hamilton et al 1973). In human enamel the incomplete prism junctions form laminar pores with curved cross-section running from the dentinoenamel junction to the outer surface. In outer enamel the prism junctions tend to separate, and thus exist as independent channels, whilst those in inner enamel (especially in molars) interconnect to form a three-dimensional network of laminar spores (Boyde 1989, Shellis 1996). Enamel mineral is composed of relatively small crystals, the arrangement of which results in internal pores that are small and variable in form, orientation and distribution. Chromium soleplate demineralization has been used to provide ultra structural information on the distribution of matrix (Sundstrom and Zelander 1968) used this technique, and reported individual crystals with a 16
  17. 17. coating of matrix. Matrix is more apparent in the tail region than in the body region. The material at prism junctions has a raised solubility (Shellis 1996), which may be due to the deposition of the mineral with increased magnesium and carbonate content during amelogenesis, leading to the formation of sites with defective, more soluble apatite (Shellis 1996). The increased solubility at the prism junctions, combined with faster diffusion in this region, accounts for the demineralization pattern observed in advancing carious lesions. At such lesions sites demineralization occurs preferentially via these prism junctions and then spreads laterally into the inter prismatic regions. While the largest pores in enamel are associated with the prism junctions, they only contribute in a small way to the total porosity, most of which is associated with prism bodies and tails. Here, the pores exist as very narrow gaps between closely packed crystals but some, while small, are elongated and tubule like and may communicate with the prism junction pores only through narrow inter crystalline pores. Enamel rods follow a wavy, spiraling course, producing an alternating arrangement for each group or layer of rods as they change direction in progressing from the dentin toward the enamel surface where they end a few micrometers short of tooth surface (FIG. 8) Enamel rods rarely run a straight radial course because it appears there is an alternating clockwise and counterclockwise deviation of the rods from the radial course at all levels of the crown. They initially follow a curving path through one third of the enamel next to the dentino-enamel junction. After that, the rods usually follow a more direct path through the remaining two thirds of the enamel to the enamel surface. Boyde (1976) stated that the keyholes shape of the prisms in cross section tends to prevent slip across prism boundaries under 17
  18. 18. lateral shear. The keyhole configuration results from the unique shape of the typical pit produced on the development surface by ameloblast. Gnarled Enamel There are groups of enamel rods that may entwine with adjacent groups of rods, and they follow a curving irregular path towards the tooth surface. These comprise gnarled enamel, which occurs near the cervical regions and the incisal and occlusal areas (FIG. 5) Gnarled enamel is not subject to cleavage as is regular enamel. This type of enamel formation does not yield readily to pressure of bladed, hand cutting instruments in tooth preparation (FIG. 9) Hunter Schreger Bands The changes in the direction of the enamel prisms that minimize cleavage in the axial direction produce an optical appearance called Hunter Schreger bands (FIG.10 and 11). These bands appear to be composed of alternate light and dark zones of varying widths that are slightly different permeability and organic content. These bands are found in different areas of each class of teeth. Since the enamel rod orientation varies in each tooth, Hunter – Schreger bands also have a variation in the number present in each tooth. In the anterior they are located near the incisal surface. They increase in the number and areas of the teeth from, the canines to the premolars in the molars the bands occur from near the cervical region to the cusp tips. The orientation of the enamel rod heads and tails and gnarling of the enamel rods provide strength by resisting, distributing, and dissipating impact forces. In the inner one-half to two thirds of the enamel, curvature of the prisms is responsible for the formation of HUNTER-SCHREGER BANDS. Each band consists of 10-13 prisms, which in alternate bands are sectioned approximately longitudinally or 18
  19. 19. approximately transversely. However the transition between alternate bands is gradual. 19 Fig 6. Key hole shaped enamel rods Fig 7. Enamel rods in cross section Fig 8. Enamel rods appear wavy in section of enamel Fig 9. Gnarled enamel Fig 10. Photomicrograph of enamel illustrating phenomenon of light & dark bands (Hunter Shregar Bands) Fig 11. Hunter – Shregar Bands when enamel is viewed under polarized light
  20. 20. Enamel Tufts Enamel tufts are hypo-mineralized structures of enamel rods and inter-rod substance that project between adjacent groups of enamel rods from the dentino-enamel junction (FIG. 12 and 13) these projections arise in the dentin, extend into the enamel in the direction of the long axis of the crown, and may play a role in the spread of dental caries. These regions are of high porosity, as they cut across the prism structure, in which crystals are small and dispersed and protein abundant (Orams et al 1976). Enamel Lamellae They are thin leaf faults between enamel rod groups that extend from the enamel surface towards the dentino-enamel junction, sometimes extending into the dentin (FIG. 12). They contain mostly organic material, which is a weak area predisposing a tooth to the entry of bacteria and dental caries (FIG. 14). Enamel Spindles Odontoblastic processes sometimes cross the dentino-enamel junction into the enamel; these are termed enamel spindles when their ends are thickened (FIG. 12). They may serve as pain receptors, there by explaining the enamel sensitivity experienced by some patients during tooth preparation (FIG. 15). Incremental Lines of Enamel - Striae of Retzius 20
  21. 21. Enamel rods are formed linearly by successive opposition of enamel in discrete increments (FIG. 16). The resulting variations in structure and mineralization are called the Incremental Striae of retzius and can be considered growth rings (FIG. 12). 21 Fig 12. Photomicrograph exhibiting enamel tuft, enamel lamellae, enamel spindle, striae of retzius, Dentino enamel junction Fig 13. Transmitted light micrograph of DE junction showing enamel tufts Fig 14. Enamel lamellae Fig 15. Enamel spindles
  22. 22. In horizontal sections if the tooth, the Striae of Retzius appear as concentric circles. In vertical sections, the lines transverse the cuspal and incisal areas in a symmetric arc pattern descending obliquely to the cervical region and terminating at the dentino-enamel junction. When these circles are incomplete at the enamel surface, a series of alternating grooves, called the imbrication lines of Pickerill, are formed. The elevations between the groves are called Perikymata; these are continuous around the tooth and usually lie parallel to the cemento-enamel junction and each other. The enamel of deciduous teeth develops partly before and partly after birth. The boundary between the two portions of enamel in the deciduous tooth is marked by an accentuated incremental line of retzius, the neonatal line or neonatal ring (FIG. 17). It appears to be the result of abrupt change in the environment and nutrition of the newborn infant. The prenatal line is usually well developed than the postnatal enamel. This is explained by the fact that the foetus develops in a well-protected environment with an adequate supply of all the essential materials, even at the expense of the mother. In addition, it has been reported that there is locally increased porosity at the incremental growth lines (Newman and Poole 1974). As a result, enamel structure is altered along these lines and electron microscopy has reveled a possible decrease in the number of crystals in the striae. There is also increased porosity on the cross striations (Boyde 1989), which are a pattern of periodic banding noted at 2-6 micron meter intervals along the length of the prisms, and which represent the circadian variation in secretory activity of the ameloblast. Shellis (1996) produced methacrylate replicas of some 22
  23. 23. cross striations in inner enamel, but was unable to do so in outer enamel, suggesting that the pores at most striations are very small or inaccessible. In cuspal enamel the prism curvature gives rise to a related but often apparently more complicated appearance of gnarled enamel. Bands in which the prisms run parallel with the section plane reflect the light to a different degree compared with those in which the prisms are perpendicular to the section plane (Silverstone 1982). Because of the deviations in prism orientation, inner enamel is relatively porous. It is thought that the relatively complicated prism arrangement within the Hunter-Schreger bands to reduce the propagation of fractures (Osborn 1968, Boyde 1989). In the outer enamel, the prisms are straight and parallel in the cuspal and lateral regions; so do not show Hunter-Shreger banding. The angle at which prisms reach the surface varies with the anatomical location on the tooth. At the cervical margin, the prisms follow an undulating course and approach the surface at very variable times acute angles (Boyde 1989). Occlusally different orientation is noted, with prisms on the lateral surface of the crown being angled at approximately 70°, whilst on the cuspal surface the angle returns to approximately 90°. Prism Shape and Crystal Orientation The cross sectional appearance of prisms is by the inter- relationship of prismatic and inters prismatic enamel (FIG. 18). Three classical prism patterns have been defined, termed (1-3) (Boyde 1989). Pattern 1 Is characterized by prisms with complete boundaries, separated by well defined inter prismatic regions. 23
  24. 24. Pattern 2 The prisms have incomplete outlines and are arranged in rows. Within each rows narrow bridges of inter prismatic enamel separate the rows. Pattern 3 Is the structure observed in human enamel, containing alternating prisms with horseshoe shaped boundaries. Although pattern 3 is predominant in human enamel (Boyde 1989), the other patterns can be found in restricted areas. In particular, pattern 1 enamel, occurs close to the dentinoenamel junction and also near the outer surface i.e., in the enamel formed at the beginning and end of the ameloblast life cycle. Comparative studies show that there is no correlation between prism pattern and incremental rate. In all the three patterns, crystals in the inter prismatic regions are oriented approximately perpendicular to the general forming surface (i.e., perpendicular to the plane of the retzius lines), while the crystals within the prisms form perpendicular to the floor of the Tome’s process pit. In human enamel, this results in a gradual divergence of the crystals in the tail region from the parallel intra prismatic arrangement by angles of about 15°-45° in the cervical direction (Poole and Brookes 1961). In pattern 2 enamel it results in a large angle between the interprismatic crystals and those in the prism sheets. This distinction between pattern 2 and pattern 3 is important because of the widespread use of rodent and bovine enamel (pattern 2) in dental research. 24
  25. 25. Crystal Size and Morphology The crystals of mature enamel appear to grow and fill the bulk of the space available within the prism. The apatite crystals characteristically exhibit considerable irregularity of outline, but are roughly hexagonal in cross-section, with a mean width of 68.3 nm and mean thickness of 26.3nm. Many of the crystals in mature enamel show evidence of crystallographic defects (Ichijo et al. 1993). Aprismatic Enamel Aprismatic enamel, up to 100-micron meter thick, has been reported to be present at the surface of both permanent and deciduous human enamel (Boyde 1989, Kodaka et al. 1989) (FIG. 19). The thickness of aprismatic enamel varies both within and between tooth types. Within aprismatic surface enamel, the crystals are arranged parallel to each other and perpendicular to the surface, although some deviation in crystal orientation, due to the presence of remnants of prism boundaries, may be detectable in some areas (Kodaka et al. 1989). Because of the parallel alignment of crystals and the absence of prism boundaries, the surface layer is generally more highly mineralized than the subsurface enamel (Robinson et al 1971). This relatively featureless layer is thought to be result from the loss of the tomes ‘process by the ameloblast; thus the structural feature which directs the deposition of crystal into prisms and interprismatic material is lost, altering enamel structure as a consequence. 25
  26. 26. Dentino Enamel Junction The interface of the enamel and dentin is called the dentinoenamel junction (FIG 12). It is scalloped or wavy in outline, with the crest of waves penetrating toward the enamel. The rounded projections of the enamel fit into the shallow depressions of the dentin. This inter digitations seems to contribute to affirm attachment between dentin and enamel. The dentino-enamel junction is also a hypo-mineralized zone about 30 micrometer thick (FIG. 20). Enamel is incapable of repairing itself once destroyed because the ameloblast cell degenerates following formation of the enamel rod. The final act of the ameloblast cell is secretion of a membrane covering the end of the enamel rod. This layer is referred to as Nasmyth membrane, or the primary enamel cuticle. This membrane covers the newly erupted tooth and is worn away by mastication and cleaning. The membrane is replaced by an organic deposit called a pellicle, which is a precipitate of salivary proteins. Microorganisms may invade the pellicle to form bacterial plaque, a potential precursor to dental disease. 26
  27. 27. 27
  28. 28. DENTIN 28 Fig 16. Ground section of enamel viewed under transmitted light showing striae of retzius Fig 17. Photomicrograph showing prenatal and post natal enamel in primary teeth Fig 18. Different prism patterns in transverse section Fig 19. Aprismatic enamel Fig 20. The Scalloped appearance of dentino enamel junction
  29. 29. Dentin provides the bulk and general form of the tooth and is characterized as a hard tissue with tubules throughout the thickness. It forms slightly before the enamel; it determines the shape of the crown, including the cusps and ridges and the number and size of the roots (FIG. 21). Along the crown, the dentin is covered by enamel, along the root by cementum. It encloses the dental pulp, with which it shares a common origin from the dental papilla. The dentin and pulp can be considered as a single development and functional unit, often described as pulpodentinal complex. Dentin can be defined as porous biological composite composed of apatite crystal filler particles in a collagen matrix (Pashley 1996). The apatite crystallites are thought to provide strength, where as the collagen matrix provides toughness. Dentin contains dentinal tubules surrounded by highly mineralized (95% volume mineral phase) intratubular dentin embedded within a partially mineralized (30% volume mineral phase) collagen matrix (inter tubular dentin) (Marshall et al. 1997). The majority of tooth structure is composed of dentin, which is the vital component of the tooth. When compared with the enamel (Knoop hardness number KHN 343), dentin is much softer (KHN 68) (Craig 1993), a characteristic explains why dentin exhibits much faster wear. In addition the modulus of elasticity of enamel is approximately 84 Gpa (Craig 1993) compared with a value of 13-17 Gpa reported for dentin. Physical Properties 29
  30. 30. It is light yellow in colour and becomes darker with age and less translucent. It is harder than bone and cementum but softer and less brittle than enamel. Dentin has greater compressive strength and tensile strength than enamel because it is traversed by tubules. The dentin is readily permeable. Specific gravity – 2 .1g/ml. Dentin is elastic and subject to slight deformation and acts as a shock absorber to overlying enamel. The lower mineral salt content in dentin renders it more radiolucent than enamel. Compressive strength of dentin - 40 – 50,000 PSI. Modules of resilience vital dentin – 100-140 LBS/Inch. Modules of vital dentin – 1,90,000 psi. Composition of Dentin 70% - In organic material 20% - Organic Materials 10% - Water The inorganic substance consists of hydroxyapatite crystals and small amount of phosphate, carbonates and sulfates (FIG. 23). The organic substance consists of type-1, collagen containing 20% of matrix with proteoglycans between the fibres. Structure of Dentin Dentinal Tubules The dentinal matrix contains tubules, each or which ranges from about 1 to 2micro meter in diameter at its outer end and 3 to 4micrometer at is pulpal side. The number of tubules are about 15,000 /mm2 near the dentinoenamel junction and it is 65,000mm2 near the pulpal surface. The dentinal tubules are fine canals that extend across entire width of the dentin. They contain odontoblastic process. The course of the dentinal tubules follows a gentle curve, which is “S” Shaped. 30
  31. 31. They show two curvatures - primary curvature and secondary curvatures (FIG. 22). Primary curvature start at right angle from the pulpal surface, the convexity of this curved course is directed towards the apex of the root and the curvature in the outer half is directed towards the occlusal or incisal surface. These tubules end perpendicular to the dentino-enamel junction and cemento-dentinal junction. It is almost straight at the root apex, incisal edges and cusps. Over their entire, length, the tubules exhibit minute relatively regular secondary curvatures (FIG. 24). The fore most morphological characteristic of dentin is it’s tubular branched structure the pulp to the dentino-enamel junction. Under normal conditions the tubules are filled with fluid, may be important in hydraulically transferring and relieving stresses imparted to dentin through the supporting structures of the periodontium and the enamel. Indeed this may explain why endodontically treated teeth are more brittle than vital teeth. When isolated from the dentin, each individual dentinal tubule would have the appearance of an inverted cone; with the smallest dimension being recorded at the dentino- enamel junction end the largest dimension adjacent to the cell body in the pulp. Canaliculi or Microtubules The dentinal tubules have lateral branches throughout the dentin termed as canaliculi. These canaliculi are1micrometer or less in diameter and originate more or less at right angle to the main tubule. 31 Fig 21. Structures seen in dentin Fig 22. S-shaped dentinal tubules Fig 23. Composition of dentin by volume percentage Fig 24. Dentinal tubules seen in longitudinal ground section showing primary and secondary curvatures
  32. 32. Enamel Spindles 32
  33. 33. Near the dentino-enamel junction, the dentinal tubules divide into several terminal and form an inter communicating and anastomosing network. Some dentinal tubules extend into the enamel for several millimeters. These are formed as enamel spindles (FIG. 15). Peritubular Dentin The dentin that immediately surrounds the dentinal tubules is called peritubular dentin. This dentin forms the walls of the tubules. It is more highly mineralized about 9% than the intertubular dentin. It is completely broken down and disappears on being subjected to routine decalcification methods. Intertubular dentin The main body of the dentin is composed of intertubular dentin. It is located between the dentinal tubules or between the zones of peritubular dentin. Although it is highly mineralized this matrix, like bone and cementum is retained after decalcification. About one half of its volume is organic matrix, specially collagen fibres which are randomly oriented around the dentinal tubules. The fibres have a lattice like arrangement coursing in gentle curves between the tubules and their peri-tubular zones. The fibres also exhibit cross-bonding. Hydroxyapatite crystals are formed along the fibres. Within each tubule is a collagen-deficient, hyper mineralized layer of dentin, which has been termed as peritubular dentin, and which may be more accurately termed periluminal (Pashley 1996) or intratubular dentin, which is calcium deficient carbonate rich hydroxyapatite. The small crystals present have a higher crystallinity and are five times harder than the intertubular dentin, with KHN of 250 33
  34. 34. compared with a KHN of 52 for intertubular dentin. The presence of this intertubular dentin narrows the lumen of the tubule from its original 3-µm to as little as 0.6- 0.8 µm in superficial dentin near the dentino-enamel junction. The width of intratubular dentin decreases in a pulpward direction, where there is a zone in which there is no intratubular dentin present and the tubule (luminal) diameter is approximately 3µm (Garberoglio and Brannstrom 1976). There is little published information on the biological control of intra tubular apposition, but it is known to be a slow process, slower than the incremental formation of secondary dentin in the pulp chamber. Pre dentin Predentin is located adjacent to the pulpal tissue and is 2µm to 6µm wide. It is the first formed dentin and is not mineralized. As the collagen fibres undergo mineralization at the pre-dentin front, the predentin then becomes dentin and a new layer of predentin forms circumpulpally (FIG. 25). Odontoblasts The cells, which are related to the deposition of dentin, are the odontoblasts. The odontoblasts are a layer of specialized cells, which lie on the surface of the pulp against the internal surface of the dentin. In a fully formed tooth, the odontoblasts are arranged at a single layer of closely packed cells, which are pyriform, in shape. As the cells are the cells are at different levels in the layer, on erroneous impression of stratification results. Each odontoblast possesses a long process (Tome’s Fibres), which passes from the distal end of the cell into the substance of the 34
  35. 35. dentin where it is housed in a fine canal, the dentinal tubules. The odontoblastic processes are largest in diameter near the pulp (3 to 4µm) and taper upto 1mm further into dentin (FIG. 26, 27 and 28). Primary Dentin The dentin that forms the initial shape of the tooth is called primary dentin. It is usually completed three years after tooth eruption. It consists of mantle and circumpulpal dentin (FIG. 29). Mantle Dentin Mantle dentin is the name of the first formed dentin in the crown underlying the dentino-enamel junction. It is thus the outer (or) most peripheral part of the primary dentin and it is about 20µm thick. The fibrils formed in this zone are perpendicular to the dentino- enamel junction and the organic matrix is composed of the collagen fibrils (FIG. 30). Circumpulpal Dentin Circumpulpal dentin forms the remaining primary dentin or bulk of the tooth. It is circumpulpal dentin that represents all of the dentin formed prior to root completion. The fibrils in circumpulpal dentin are much smaller in diameter and are more closely packed together. The circumpulpal dentin may contain slightly more mineral than mantle dentin. 35 Fig 25. Predentine Fig 28. Extension of odontoblast process in dentinal tubule Fig 26. Odontogenic zone comprising odontoblasts, cell rich zone, cell free zone Fig 27. SEM of deep dentin showing odontoblastic process Fig 29. Primary dentin and secondary dentin Fig 30. Histology of mantle
  36. 36. Secondary Dentin 36
  37. 37. Secondary dentin is a continuation of primary dentin that forms at a slower rate as the tooth ages physiologically. It is a narrow band of dentin bordering the pulp and represents the dentin formed after the root completion. Secondary dentin formation takes place without any external stimuli. In secondary dentin, the tubules take a different directional pattern in contrast to primary dentin (FIG. 29). Incremental Lines The incremental lines von ebner or imbrication lines appear as fine lines (or) striations in dentin (FIG. 31). They run at right angles to the dentinal tubules and correspond to the incremental lines in enamel (or) bone. These lines reflect the daily rhythmic, recurrent deposition of dentin matrix as well as hesitation in the daily formative process. The distance between lines varies form 4 to 8µm. In the crown to much less in the root. The course of the lines indicates the growth pattern of the dentin. Contour lines of Owen Occasionally some of the incremental lines are accentuated because of the disturbances in the matrix and mineralization process. Such lines are readily demonstrated in ground sections and are known as contour lines. The most consistently seen contour lines is at the junction of the primary and secondary dentin (FIG. 32). Neonatal Lines In the deciduous teeth and in the first permanent molars, where dentin is formed partly before and partly after birth, the prenatal and postnatal are separated by an accentuated contour line. This is termed as neonatal line and is seen in enamel and as well as dentin. 37
  38. 38. This line reflects the abrupt change in the environment that occurs at birth. The dentin matrix formed prior to birth is usually of better quality than that formed after birth and neonatal line may be a zone of hypo-calcification (FIG. 33). Inter Globular Dentin Some times mineralization of dentin begins in small globular areas that fail to fuse into a homogenous mass. This results in zone of hypo-mineralization between the globules. These zones are known as interglobular dentin. Inter globular dentin forms in the crown of teeth in the circumpulpal dentin just below the mantle dentin, and it follows the incremental pattern (FIG. 34). The dentinal tubules pass un-interruptedly through interglobular dentin, thus demonstrating defects of mineralization and not of matrix formation. In dry ground sections some of the interglobular dentin black in transmitted light. However, spaces in interglobular dentin are not believed to occur naturally. Granular Layer When dry ground section of the root dentin is visualized in transmitted light, there is a zone adjacent to the cementum that appears granular. This is known as tomes (or) granular layer (FIG. 35). This zone increases slightly in amount from the Cementoenamel junction to the root apex and is believed to be caused by a coalescing and looping of the terminal portions of the dentinal tubules. The cause of development of this zone is probably similar to the branching and beveling of the tubules at the dentinoenamel junctions. 38
  39. 39. 39 Fig 32. Contour lines of Owen Fig 31. Von Ebner’s lines Fig 33. Neonatal line in dentin Fig 34. Ground section of dentin viewed under transmitted light showing interglobular dentin Fig 35. Ground section of dentin, viewed under polarized light showing granular layer
  40. 40. AGE AND FUNCTIONAL CHANGES Reparative Dentin Reparative dentin is formed by the replacement (or) secondary odontoblast in response to irritation caused by attrition, abrasion, erosion, trauma, dental caries, some operative procedures and other irritants (FIG. 36 and 37). Reparative dentin is formed when Tomes Process are cut within 1.5 mm from the pulp. The cut fibres die along with the corresponding odontoblasts leaving dead tracts. New odontoblasts are differentiated from mesenchymal cells of the pulp in about 15 days and these replacement odontoblasts lay down the reparative dentin. Dead Tracts This is a type of reaction dentin, which appears to result from irritation of greater severity. The odontoblast process in the whole length of the injured tubule degenerates and at the same time is sealed off at the pulpal end by a deposit of reactionary dentin (FIG. 38). In dried ground section of normal dentin the odontoblast processes disintegrate and the empty tubules are filled with air. They appear black in transmitted and white in reflected light. Loss of odontoblast process may also occur in teeth containing vital pulp as a result of caries, attrition, abrasion, cavity preparation (or) erosion, (When the tomes process are cut more than 1.5mm). These areas demonstrate decreased sensitivity and appear to a greater extent in older teeth. 40
  41. 41. Sclerotic Dentin Sclerotic dentin results from aging or mild irritation (such as slowly advancing caries) and causes a change in the composition of the primary dentin. The peritubular dentin becomes wider, gradually filling the tubules with calcified material, progressing from the D.E. Junction pulpally. These areas are harder, denser, less sensitive, and more protective of the pulp against subsequent irritations (FIG. 39). The deposition of intratubular dentin, as a result of ageing or in response to attrition, results in a progressive reduction in the tubule lumen, and if continued, obliterates the tubule. If this occurs in several tubules in adjacent areas, the dentin assumes a glassy appearance. The term used to describe this progressive deposition and obliteration of the tubule is SCLEROSIS, resulting in sclerotic dentin. This process begins in root dentin of 18 – year old premolars without any external influence. It can therefore be assumed that this is a physiological response and the occlusion of the tubules is achieved by continued intratubular deposition. The mechanism by which intratubular dentin is formed are poorly understood and three possible mechanisms have been suggested (Torneck 1994). Firstly, it has been suggested that there may be a passive redistribution of mineral from inter tubular dentin into the tubules around the pre-existing components of the tubule. Secondly there may be an active response on the part of the odontoblast process, resulting in an organic matrix that is actively mineralized as a result of odontoblast activity. Finally, it has been suggested that the odontoblast may produce an organic matrix that becomes mineralized by redistribution of mineral from intertubular dentin, as in the first case. In which ever way it is formed, the net result is that intratubular dentin is deposited at the expense of the odontoblast process, which is either retracted or shortened by the loss of it’s distal extremity. 41
  42. 42. 42 Fig 37. Types of reparative dentin Fig 36. Reparative dentin Fig 38. Dead tracts - ground section of dentin viewed under transmitted light Fig 39. Sclerotic dentin
  43. 43. The amount of sclerosed dentin increases with age and is most frequently encountered in the apical third of the root. Sclerosis reduces the permeability of dentin and thus may help prolong pulp vitality. Processes, which contribute to sclerotic dentin in the crown in response to attrition and caries, may differ from the physiological deposition of sclerotic (translucent) dentin in the root, which is age- dependent and whose rate of deposition is not altered by attrition. Although there is a little evidence in the literature, it is thought sclerosis resulting from aging is physiological dentin sclerosis and that resulting from mild irritation is reactive dentin sclerosis. Eburnating dentin is a term referring to the outward portion of reactive sclerotic dentin where slow caries has destroyed formerly overlying tooth structure, leaving, a hard, darkened, cleanable surface. The refractive indices of dentin in which the tubules are occluded are equalized and such areas become transparent. Transparent (or) sclerotic dentin can be observed in the teeth of the elderly people, especially in the roots. Sclerotic dentin may also be found under slowly progressing caries. Mineral density is greater in this area of dentin as shown both by radiography and permeability studies. It appears transparent or light in transmitted light and dark in reflected light. 43
  44. 44. INNERVATIONS OF DENTIN Intertubular nerves Dentinal tubules contain numerous nerve endings in the predentin and inner dentin no further than 100 to 150µm from the pulp. Most of these small vesiculates endings are located in the tubules in the coronal zone, specifically in the pulp horns. The nerves and their terminals are found in close association with the odontoblast process within the tubule (FIG. 28). Nerve grows into the papilla in the bell stage of tooth development (Byers 1980) both afferent neurons and efferent automatic nerves that innervate pulpal blood vessels are present. The number of myelinated axons in permanent teeth increases with age and /or tooth development, reaching a plateau value of about 500 myelinated axons per human premolar at age 15, which remains constant upto 60 years. There may be single terminals or several dilated and constricted portion. In either case, the nerve endings are packed with small vesicles, either electron dense or lucent, which probably depends on whether there as been discharge of their neuro transmitter substance. In any case, they interdigitate with the odontoblast process, indicating an intimate relationship to this cell. It is believed that most of these are terminal processes of the myelinated nerve fibres of the dental pulp. The primary afferent somato sensory nerves of the dentin and pulp project to the main sensory nucleolus of the midbrain. Extent of Odontoblastic Process During tooth development, at the bell stage, odontoblast processes extend from the odontoblast cell body through predentin to the dentino-enamel junction. As the thickness of dentin increases, the cellular processes must elongate. 44
  45. 45. However, the true length of the processes in mature dentin, in the absence of blood vessels or supporting cells, is an issue that is open to debate (FIG. 28) In human teeth, the thickness of dentin is about 3-3.5mm Such that if an odontoblastic process were to pass the entire distance from the pulpal border to the DEJ, then the volume of the cellular process would be four fold larger than that of the cell body (Pashley 1996). This difference in volume between the cell body and the process is even greater if the situation with cuboidal or flattened odontoblasts is considered, as seen in the root towards the apex. It is generally agreed that the process of most odontoblasts is between 0.1 and 1.0mm (Byers 1996). The question of how far the odontoblast process penetrates dentin is of vital importance when considering dentin sensitivity. If odontoblasts were to participate directly in the sensitivity of dentin to surface stimuli, then the stimuli must interact directly with the process, which is unlikely to be the case. Normally dentin is covered coronally with and on the root surface by cementum. When these surface coverings are lost, dentin is subjected to a variety of stimuli, including mechanical, chemical, thermal and smaller mechanical stimuli to which intact teeth are responsive. When exposed, it is proposed that the fluid filled tubules allow minute fluid shifts across the dentin when exposed to thermal, tactile, evaporative or osmotic stimuli. The effect of this is that mechanoreceptors in the pulp are stimulated (Pashley 1996). These fluid shifts can directly irritate odontoblasts, pulpal nerves and sub odontoblastic blood vessels by applying large sheer forces on their surface as the fluid streams through narrow spaces. The effect of fluid shift on the release of neuro peptides has been 45
  46. 46. assessed (Kimberly and Byers 1988, Byers et al 1990, Byers 1996), and results in the release of calcitonin gene related peptide (CGRP) or substance p (SP) from the pulpal nerves to generate a local neurogenic inflammatory condition. Dentin Characteristics Change With Depth Both primary and secondary dentin contains tubules. The circumference of the dentin at the most peripheral part of the crown or root is much greater than that of the final circumference of the pulp chamber or root canal space this results in the odontoblasts being much more crowded as they approach their final position, thus leading to the appearance of a columnar layer of odontoblasts, especially over the pulp horns. The convergence of odontoblasts towards the pulp creates a unique structural organization, with functional consequences. The convergence has been estimated to be 4:1. The number of tubules per unit area and the radius of the tubules increases in the direction from the dentino-enamel junction to the pulp, thus the area occupied by tubule lumina also increases. Pashley (1984) calculated the area occupied by tubule lumina at the dentino-enamel junction to be approximately 1% of the total surface area of the dentino-enamel junction and 22% of the pulp. As this area is occupied by dentinal fluid, which is 95% water. (Pashley 1996), the surface area figures are also approximately equal to the tubule water content of these regions. Therefore, the water content or wetness of dentin increases 20 fold from superficial to deep dentin. This factor has clinical implications; in terms of dentin bonding of restorative materials to deep dentin the water competes with resin monomers for surface collagen fibrils (Pashley and Carvalho 1997). 46
  47. 47. Fluid Flow In clinical conditions there is an outward fluid flow across exposed dentin in response to the low but positive pulpal tissue pressure. The composition of this fluid is uncertain, but must have an ion product of calcium and phosphate, which is above or near the solubility product constants for a number of forms of calcium phosphate (Pashley 1996). This would in turn lead to the formation of mineral deposits in dentinal tubules which have many forms (Mjor 1985), as the dentinal fluid moves outwards, larger amounts of mineral ions are presented to the walls of tubules than would occur in sealed tubules. Indeed, Shellis (1994) used this principle to reduce the depth of demineralization in vitro under stimulated caries forming conditions, by using a supersaturated surrogate dentinal fluid, which was perfused through the pulp chamber. When examined microscopically, translucent bands resembling sclerotic dentin were sometimes observed. Clinically, patients who complain of dentin sensitivity report that a cold stimulus elicits a greater response than evaporative, tactile or osmotic stimulation (Orchardson and Collins 1987). Outward direct fluid movement (in response to cold) is far more effective at activation pulpal mechanoreceptors than is the inward movement of fluid (seen following a hot stimulus). Dentin Permeability The structure of dentin is tubular, as previously stated, and it is this characteristic that provides the channels for the permeation of solutes and channels for the permeation of solutes and solvents across dentin. 47
  48. 48. The density of tubules per mm square varies from 15,000 at the dentino-enamel junction to 65,000 at the pulp boundary be predicted from tubule density and diameters, due to the presence of intra tubular material such as collagen fibrils and mineralized constrictions of the tubules (Pashley 1996). Dentin permeability can be subdivided into two broad categories (Pashley 1996): Transdentinal movements of substances through the entire thickness of dentin via dentinal tubules (such as fluid shifts in response to hydro dynamic stimuli). Intradentinal movement of exogenous substances into the infiltration of hydrophilic adhesive resins into demineralized dentin surfaces during resin bonding or demineralization of inter tubular dentin by bacterially derived acids (Kinney et al 1995), where the material enters the tubules but does not travel across the tubules. The presence of the smear plugs and / or intra tubular deposits (i.e. sclerotic dentin) is thought to lower intratubular permeability to minimal values (Pashley et al 1991). Dentin permeability (Transdentinal or intratubular) is not uniform across the tooth. Coronal dentin permeability is much higher than that of the root. This can be attributed to the convergence of tubules towards the pulp chamber, the tubule density increases about four fold in coronal dentin, but only two fold in root dentin. 48
  49. 49. Thus, within any location on the tooth peripheral dentin has a lower permeability than deeper dentin. The permeability of intertubular dentin has never been quantified, but it must be very low and limited to patent lateral canals that branch off from tubules (Chappell et al 1994, Mjor and Nordahl 1996). Numerous methods have been used to assess dentin permeability (Pashley 1990). The easiest method of measuring trans dentinal permeability is to quantify its hydraulic conductance. This measures the ease with which fluid can filter across a unit surface area of dentin in a unit time under a unit pressure gradient (Pashley 1990). It has been reported, in unobstructed dentin, that the hydraulic conductance increases as dentin thickness decreases. However, the presence of intratubular dentin and hence lowers its permeability (Pashley 1996). The structure of dentin makes it act both as a barrier and a permeable structure, depending on its thickness, age and other variables (Pashley and Pashley 1991). Dentin is very porous because of its tubular structure and the minimum porosity of normal peripheral coronal dentin is about 15000 tubules per square. If the dentin is uncovered, then the tubules provide a diffusion channel from the surface to the pulp. The rate at which diffusional flux of exogenous material crosses dentin to the pulp is highly dependent on dentin thickness and upon the hydraulic conductance of dentin (Pashley 1985, 1990). The Pulpo-Dentinal Complex Dentin and pulp are embroyologically, histologically and functionally united and there is much evidence to support the concept of viewing the dentin and pulp as a functionally coupled unit, which 49
  50. 50. act as an integrated system. As soon as the tissues, which normally cover dentin, are lost, then normal compartmentalization between the tissues is lost (Pashley 1996) and they become functionally continuous. The pulp responds to the stimuli generated by the loss of dentinal covering, in the short term, by mounting an outward movement of fluid (Vongsavan 1994, Mathews 1996) and macromolecules (Byers 1996). The long-term response to the stimulus is the production of tertiary dentin, which is a biological response to reduce the permeability of the dentin of the dentin –pulp complex. 50
  51. 51. ACID ETCHING ON ENAMEL The developed materials that adhere or bond to tooth structure would minimize removal of healthy tissue, thus allowing a more conservative preparation and providing for an impenetrable seal at the margin between the tissue and restoration. Criteria for Bonding Three basic criteria necessary for bonding. The surface with which the bonding is to occur should be: 1: Similar to the surface. 2: Free of contamination. 3: Smooth and uniform. Steps In Acid Etch Technique Enamel Prophylaxis The mechanical cleaning of the enamel is an important first step clinically in the bonding procedure. Maximum bond strength was developed only when an oral prophylaxis was done before etching. An examination of etched enamel surfaces not receiving an oral prophylaxis shows pellicular remnants and microorganisms contaminating the enamel. Clearly, acid alone cannot remove all contaminants. This especially true of calculus and a careful inspection should be made for the presence of this accretion which should be removed by scaling. 51
  52. 52. Because there is concern for interference of flavoring oils, glycerin and fluorides with the etching process, the use of watery slurry of flow pumice has been recommended. There is no significant difference in the retention rate of sealants with or with out pre-etch pumice prophylaxis (Donnan and Ball 1988). However, no clinical or laboratory evidence has been presented to preclude the use of commercial pastes, even those containing fluoride. Studies in 1980’s have showed no difference in the clinical performance of a sealant whether fluoridated or non- fluoridated toothpaste was used for the prophylaxis. Further research is indicated. Pellicle Removal An oral prophylaxis should remove all gross deposits and accretions from enamel, but it may not remove all integuments such as subsurface pellicle. In addition, some protein may become smeared over the surface during the prophylaxis. Some of this pertinacious constituent may go into the solution in the acid while the remainder may be floated away mechanically as the phase of enamel is solubilized. Application of Etchant (FIG. 40) In the next step, with the teeth dried and properly isolated from saliva, the acid is applied by one of several means including a cotton pellet, brush or minisponge. The object is to gently agitate the acid for a minute for maximal effect. This can be achieved using a gentle swabbing motion. Clinical reports have suggested extending the etching time upto 2 minutes in relatively high fluoride areas and highly calcified mature enamel as for an adult. 52
  53. 53. 53 Fig 40 Pre operative - silver amalgam restoration After cavity preparation Acid Etching Rinsing with water Blot excess water using mini sponge or cotton Application of bonding agent Placement of composite restoration Finished composite restoration
  54. 54. It is important not to rub the enamel during acid application, since burnishing the friable rods and their crystallites will reduce the surface area available for bonding. This has been shown to reduce bond strength. Scrubbing or rubbing may push the decalcified material back into the pores that are being formed. No apparent difference exists in the degree of irregularity after etching acid solution compared with an acid gel. Gels provide better control for restricting the etch area but may require more through rinsing afterward. The most popular enamel / dentin etchant in general dentistry is phosphoric acid blue gel. This gel is syringe dispensed, as adequate colour contrast, smooth consistency and almost ideal viscosity for application and rinsing off cleanly, and provides and even, nicely demarcated white frosted appearance. This etchant is recommended whenever extra good etching of enamel is desired, such as deciduous teeth. Studies and clinical experience indicate the 15 seconds is probably adequate for etching most young permanent teeth. However, individual variation exists in enamel solubility between patients, between teeth, and with in the same tooth, and 30 to 60 seconds may recommended for molars and adult teeth. Longer periods provide no more, but actually less, retention because of loss of surface structure. Caution should be exercised when etching over acquired and developmental demineralization. It is best to avoid it. If this is impossible a short etching time the applicant of the sealant, and the use of direct bonding with extra attention to not having areas of adhesive deficiency are important. The presence of avoids, together the poor oral hygiene, can lead to indelible staining of underlying developmental white spots. 54
  55. 55. Washing There is a significant increase in bond strength values when enamel is washed for 60 secs compared to 15 secs. These observations were made using phosphoric acid in concentration of 30% and lower. The chemical composition of the rinsing solution did not affect the bond strength. 1% potassium Chloride solution was found to improve bond strength. The presence of contaminants in the post etch rinsing solutions could adversely affect the composite bond strength. Given the size of dentinal tubules any contaminant that is small enough to penetrate or obstruct the flow of monomers into the dentinal tubules may influence the process of polymerization and ultimately affect the development of hybrid layer and potential bond strength. Significant reduction in the bond strength was demonstrated when saline was used as rinsing solution, due to the presence of ions, which interfered with the formation of hybrid layer (Eric C. Sung et al 2002). In clinical procedure involving the etching of dentin with phosphoric acid, it requires complete removal of etchant and reaction products that are formed on the etched dentin surface, as in complete removal of reaction products will intervene with bond strength (Bates et al 1982). At the end of etching period the etchant is rinsed off the teeth with abatement water spray. A high-speed evacuator is strongly recommended for increased efficiency in collecting the etchant - water rinse and to reduce moisture contamination on teeth and Dri-Angles. Salivary contamination of the etch must not be allowed (If it occur, rinse with the water spray or re-etch for few seconds; the patient must not rinse). 55
  56. 56. Drying Next, the teeth are thoroughly dried with a moisture-and-oil-free air source to obtain the well-known dull, frosty appearance. Teeth that do not appear dull and frosty white should be re-etched. Cervical enamel, because of its different morphology, usually looks somewhat different from the centre and incisal portions of a sufficiently etched tooth. It should not be re-etched in attempts to produce a uniform appearance over the entire enamel surface. Effects Of Etching On Enamel A routine etching removes from 3 to 10µm of surface enamel. Another 25µm reveals subtle histologic alterations, creating the necessary mechanical interlocks. Deeper localized dissolution will generally cause penetration to a depth of 100µm or more. Although laboratory studies indicate that enamel alterations are largely (though not completely) reversible, it can be stated that the overall effect of applying etchant to healthy enamel is not detrimental. This is augmented by the fact that normally enamel is from 1000 to 2000µm thick. (except as it tapers toward the cervical margin), abrasive wear of facial enamel is normal and proceeds at the rate of upto 2µm per year, and facial surfaces are self-cleaning and not prone to caries. On the other hand, caution should be exercised when etching damaged teeth with exposed dentin, deep enamel cracks or external or internal demineralization. Pattern of Etching Silverstone et al (1975) studied the morphological changes produced on the acid etched enamel surface scanning electron microscope. Exposure of human enamel to conditioning solutions produces three basic etching patterns (FIG. 42a). 56
  57. 57. 57 Fig 41. Etching pattern of enamel after acid etching Fig 42a. Different types of etching pattern Fig 42b. Acid etched enamel rod core dissolved to greater extent than rod sheath Type I Type II Type III
  58. 58. Type 1 Prism core material is preferentially removed, leaving the prism peripheries relatively intact, resulting in a honeycomb appearance (FIG. 42b). The average diameter of the hollowed prism cores measures about 3µm. This pattern is most common of the three types observed. Type 2 The peripheral regions of the prism are dissolved preferentially, leaving the prism cores relatively intact, resulting in a cobblestone appearance. Type 3 Etching pattern contains areas, which resembles both type 1 and type 2 along with some distinct areas where the pattern of etching appears to be unrelated to the enamel prism morphology. Studies with polarized light microscope showed that sound enamel etched with phosphoric acid to be affected at 3 distinct levels and may be described in terms of three specific zones (Silverstone 1974). A superficial etched zone, which is a narrow zone of enamel of about 10 microns in depth that is removed by etching. A Qualitative porous zone of about 20 microns in depth. It is rendered porous by the acid attack and may be identified qualitatively using polarized light. A Quantitative porous zone of about 20 microns depth that qualitatively indistinguishable form adjacent enamel. Enhancement of Enamel Porosity Enamel is a porous tissue that contains approximately 0.1% to 0.2% by volume of space. Many of the pores communicate to allow for transport of tissue fluid and ions in solution. Poole and his coworkers (1961) showed that enamel behave like a molecular sieve, allowing 58
  59. 59. passage of only the smallest molecules comparable in size to that of water. Acid etching enhances not only the size of the pores to permit access of relatively large resin molecules, but does so far distances approximately 20 to 30 micrometer in from the tissue surface. Decreased concentration of phosphoric acid enhances porosity to greater depths in the enamel. This observation holds significance for the depth to which resin may penetrate into the tissue. Antimicrobial Property of Etchants Lsettembrine et al (1997) at the university college of dentistry New York concluded that all phosphoric acid etchant materials tested demonstrated antimicrobial activity against several bacteria commonly found in the oral cavity. They also reiterated that addition of antimicrobial agents to etchant or cavity preparation may not be necessary given the antimicrobial activity of the etchant, if the current bonding systems can provide and sustain sealed tooth restorative interface. Role Of Flourides In Etching Of Enamel Enamel is soluble when exposed to an acid medium, but the dissolution is not uniform. Solubility of enamel increases from, the enamel surface to the dentino-enamel junction. When fluorides are present during enamel formation or are topically applied to the enamel surface, the solubility of the surface enamel is decreased. Flouride concentration decrease towards the dentino-enamel junction. Flouride additions can affect the chemical and physical properties of the apatite mineral and influence the hardness, chemical reactivity, and stability of enamel while preserving the apatite structures. Trace amount of fluorides stabilize enamel by lowering acid solubility, decreasing the rate of demineralization and enhancing the rate of 59
  60. 60. remineralization. Evidence also shows that topical fluorides alter the oral bacterial flora, there by increasing resistance to dental caries. It has been accepted to etch apparently normal enamel for 15 secs and enamel that shows signs of fluoridation for double that time or more. The use of prophylaxis pastes containing fluorides and topical fluoride treatments prior to etching is slowly diminishing. There is virtually no evidence that the fluoride incorporated in enamel prior to etching will significantly interfere with etching or will significantly affect bond strengths. It is well known that even the fluoride-acquired from acidulated topical fluoride solutions is poorly retained and easily removed under oral conditions in a short time. There is no contraindication to the use prior to etching because the fluoride from these agents including acidulated or nonacidulated sodium fluoride and stannous fluoride, will most likely find its way into the deep recesses of the pits and fissures and benefit in sealing them. This solid fluoride may conceivably be retained in the pits and fissures even after etching. Once sealed in the fissure by a sealant, the fluoride may gradually react with enamel (and perhaps with dentin, which in accessible at the base of some fissures) to produce a resistant tooth structure that can afford protection against caries even when sealant application is no longer provided. Fluorides should be avoided as part of the etching solution or immediately prior to regular bonding. Studies have shown that fluorides react with etched surface to produce reaction products that may interfere with bonding. These reaction products appear to interfere with optimal adhesive penetration resulting in weaker bond and/or bonds that will 60
  61. 61. not survive as long under conditions of oral moisture. It should be noted that washing away the acid conditioning solution with water containing 1or 2 parts per million of fluoride is not expected to interface with achieving high bonding strengths. Uptake of Fluorides in Etched Enamel Most important use of fluoride is after bonding procedures of all types. During etching more enamel surface usually is intentionally or unintentionally etched than is subsequently covered by adhesive, such as inter proximal areas etched by acid spillover. Etched enamel is highly reactive and readily combines with and better retains many times more fluoride than a natural unetched enamel surface. The large amount of fluoride thus acquired by etched but uncovered enamel from a topical fluoride application, may confer on the etched enamel surface a greater resistance to cavities, normally have this capacity. Infact, it has been suggested that a mild acid etch (independent of bonding procedures) be employed prior to application of acidulated sodium fluoride in order to enhance the acquisitions and retention of fluoride from this source. Retief et al (1986) has stated that fluoride is not evenly distributed through out enamel, the fluoride concentration follows a negative exponential distribution being highest in the surface enamel. The loss of fluoride rich surface enamel during the etching procedure may make the enamel more caries susceptible in the oral environment. Factors Affecting Etching On Enamel 61
  62. 62. TIME Increased Time Application High fluoride content and primary teeth require longer etching time. The increased etching time is needed to enhance the etching pattern on enamel that is more a prismatic than that of permanent enamel. Currently 15 sec, a sufficient time to produce a bond equivalent to that produce by a 60 sec etching time is used routinely. Shorter Etching Time C.J. Guba et al (1994) highlighted that etching times and etchant consistency were not critical to enamel bond strengths. It yields acceptable bond strength. It conserves enamel and saves time. They also found that on microscopic examination of a 10 secs etch versus a 60 sec etch showed that etching the enamel for 10 sec produced a very superficial etch compared to a very deep etch with a 60 sec etch. However this did not have significant impact on the tensile strength. Though some researchers suggested that the etching effect is reduced when the etching viscosity of the acid is high, this study showed no significant difference as related to their viscosities. ACID CONCENTRATION An interesting and important phenomenon is the existence of an inverse relationship between the etching effect of phosphoric acid and it’s concentration. The phenomenon was first observed and reported in 1965 and subsequently confirmed by others. The same etch time lower concentrations of acid tend to be more destructive of the enamel than higher concentrations (FIG 43a to f). 62
  63. 63. Concentrations of phosphoric acid over 65% tend to show minimal changes. The concentrations of acid, producing consistent, 63 Fig 43a. 10% H3PO4 Fig 43e. 36% H3PO4 for 30 secs Fig 43f. 37% H3PO4 Fig 43b. 32% H3PO4 Fig 43c. 35% H3PO4 Fig 43d. 35% H3PO4 for 2 minutes Etching pattern with various concentration of phosphoric acid (SEM)
  64. 64. more or less evenly distributed relatively deep etch pattern, appear to be in the range of 30 to 50%. Bond strengths are greater with 30 to 50% acid concentration, the difference between their values and those obtained on surfaces etched with 10 to 70% acid were not as great. The higher concentration of acid may not produce a sufficient in depth etch to provide adequate resin penetration (tag formation) and / or sufficient bonding area to resist repeated long-term masticatory and other dislodging stresses encountered in the oral environment. In an in vitro study carried out on bovine substrate by M.J.Shingi et al (2000) it was concluded that milder concentrations of phosphoric acid or less aggressive acids could be used to pretreated enamel for orthodontics adhesive systems and sealants if the diffusion potential of applied monomers is high enough. According to Unos (1996) depths of demineralization increased by both acid concentration and conditioning times following a logarithmic relationship. Chow and Brown (1973) demonstrated that the application of phosphoric acid solutions greater than 27% Phosphoric acid resulted in the formation of monocalcium phosphate monohydrate while dicalcium phosphate dehydrate was formed with phosphoric acid concentrations less than 27%. The former product is readily soluble and would be completely washed away in the clinical situations. VARIATIONS IN ACID ETCHING METHODOLOGIES 64
  65. 65. Currently phosphoric acid is the acid of choice, but it is possible that other acidic etching agents such as pyruvic acid may be used in the future. A controversial issue, however, is the optimal concentration of phosphoric acid. The most widely used concentrations of phosphoric acid used in clinical practice exceed 30% phosphoric acid. This is partly based on the findings the phase diagram of the phosphoric acid ± calcium hydroxide ± water ternary system. They demonstrated that the application of phosphoric acid solutions greater than 27% phosphoric acid resulted in the formation of monocalcium phosphate monohydrate. While dicalcium phosphate dihydrate was formed with phosphoric acid concentrations less than 27% phosphoric acid. The former product is readily soluble and would be completely washed away in the clinical situation, while the latter product is less soluble. The reaction products, if not completely removed after the etching procedure, may interfere with the bonding of composite resins to etched enamel surfaces. The effect of phosphoric acid concentration on the tensile bond strength of a conventional composite resin to enamel surfaces etched with 10, 20, 30, 40, 50, 60, and 70% phosphoric acid was determined. The tensile bond strength to enamel surfaces etched with 70% phosphoric acid was significantly lower than the bond strengths recorded to enamel surfaces etched with other phosphoric acid concentrations. The application of a phosphoric acid etching solution to freshly cut dentin may elicit a pulpal response. To prevent the flow of phosphoric acid applied to the enamel walls of preparations to the freshly exposed dentin at the floors of the preparations phosphoric acid gels were recently introduced. The objective was to confine the acid-etching agent to the intended site of application. It is recommended that the etching agent should be applied to the enamel surface using a dabbing action as opposed to rubbing. Another issue 65
  66. 66. that has not been resolved is the optimal duration of etching with phosphoric acid. It is surprising that some authors recommend that the etchant should remain on the tooth surface for at least 60seconds to develop an appropriate etched pattern. The etch duration is of particular importance in acid etching enamel prior to the direct bonding of orthodontic attachments, as it is practically impossible to confine the bonding site. Fluoride is not evenly distributed in enamel but allows a negative exponential distribution with fluoride concentration being in the surface enamel. The loss of fluoride rich enamel surface during prolonged etching may make the adjacent enamel more susceptible to enamel decalcification during orthodontic treatment. The reaction products that are formed on the enamel surface after phosphoric acid etching should be removed completely, as incomplete removal may interfere with bond strength. Etched surface should be washed for at least 15 secs to remove the reaction products. The tooth to be restored should be isolated with a rubber dam to prevent saliva contamination prior to acid etching and the placement of composite resin. It is generally recommended that saliva contaminated etched enamel should be washed and retched. O’Brien and others showed, however that it was not necessary to re-etch an enamel surface contaminated briefly with saliva, as a thorough washing of such a surface did not have a detrimental effect on bond strength. Buonocore M (1955) introduced a simple conservative technique for bonding restorative resins to enamel. He placed a drop of self- curing acrylic resin on the labial enamel surface of upper central incisor of ten subjects. One surface was treated prior to resin 66
  67. 67. placement with 85% phosphoric acid for 30 seconds. He noted that the acid conditioning of the enamel resulted in on uncontioned control surfaces lasted less than 12 hours. After three decades of laboratory and clinical research, Buonocore’s method is widely adopted and has added a new and exciting technical dimension to the practice of dentistry. Gwinnett, Matsui and Buonocore (1969) suggested that formation of resin tags was the primary attachment mechanism of resin to phosphoric acid. Acid etching removes about 10 microns of the enamel surface and creates a porous layer ranging from 5-50 microns deep. When a low viscosity resin is applied, it flows into the micro porosities and channels to this layer and polymerizes to form a micro mechanical bond with the enamel. Fusayama et al (1979) introduced an etching technique for both the enamel and the dentin cavity wall using 37% phosphoric acid followed by a dentin-bonding agent containing methacryloxyethyl hydrogen phenyl phosphate (phenyl-P). This improved bond strength greater extent and dentinal etching has become fairly common practice in Japan. However, the concept of total etching only recently has gained acceptance in the United States. ACID CONDITIONING OF DENTIN Any discussion of the effects of acid conditioning of dentin must begin with the acid etching of enamel. This was first proposed by 67
  68. 68. Buonocore (1955) as an attempt to clean enamel, increase the microscopic surface area for bonding, and infiltrate unfilled resins into enamel porosities. Many investigators were alarmed at what was then regarded as an unconventional and even reckless approach to the problem. Buonocore, Wilernan and Brudevold (1956) not only introduced the acid etching of enamel to dentistry, they also were among the first to attempt to bond resins to acid-etched (7% HCL, one minute) dentin. Their success with acid etching of enamel led them to try to acid etch dentin. Unlike enamel, when dentin is etched, its surface becomes mineral -poor protein rich; and it tends to become wetter (Brannstrom and Nordenvall 1977). Unfortunately, Buonocore and his colleagues' success with dentin was never realized, because the relatively crude resin materials that were available at that time would not wet dentin very well. Buonocore, however, was very much aware or the requirements for good bonding. For clinical success, the conditioned dentin must be sealed to prevent sensitivity and to prevent the pathology (Brannstrom, 1981) associated with the increased permeability of the dentinal tubules. Conditioning of dentin will be defined as any alteration of dentin done after the creation of dentin cutting debris, termed the smear layer. The objective of dentin conditioning is to create a surface capable of micro-mechanical and possible chemical bonding to a dentin-bonding agent. Goals of acid conditioning of dentin ♦ Remove the intrinsic weakness of the smear layer to permit bonding to underlying dentin. 68
  69. 69. ♦ Demineralize the superficial dentin matrix to permit resin infiltration into surface. ♦ Uncover both intertubular and peritubular dentin. ♦ Clean the dentin surface free of any biofilms. It is important to define the purpose of the acid etching of dentin so that once identified, these goals can be tested in a systematic scientific manner. As the smear layer is intrinsically weak, the first goal is to loosen it or remove it so that subsequently placed adhesive resins can interact with solid dentin adhesive resins can interact with solid dentin matrix. Most smear layers are 1-2 µm thick; they are composed of the cutting debris of the materialized tissue on which they lie. (Ruse and Smith 1991) The reason for acid etching is to demineralize the solid dentin matrix (both intertubular and peritubular dentin) to increase the porosity of the dentin. While this is analogous to why enamel is etched, the porosities that are produced are of the order of 0.05 – 1-3 µm in peritubular dentin rather than the 5-7 µm diameter of enamel prisms. Further, acid-etched enamel can be thoroughly dried, while that foal is much more difficult in vital, normal dentin. Enamel contains little protein that is at risk of being denatured by acid treatment. Dissolving away hydroxyapatite mineral crystallites from the collagen component of dentin matrix creates dentin porosities. The crystals tend to stabilize collagen and prevent its denaturation. There is a risk that the acid used to demineralize the dentin may denature or weaken the collagen. As denatured proteins generally change their dimensions, the pores may become smaller if the collagen is denatured. This may interfere with subsequent resin infiltration and prevent the formation of a hybrid layer (Nakabayashi, 69
  70. 70. Nakamura and Yasuda 1991). Another danger in the etching step is that the demineralized zone may extend, for instance, 5 µm into the dentin, while the resin infiltration may only extend 4 µm, leaving a 1 µm demineralized zone at the base of the hybrid layer that is unpro- tected by mineral or resin and that may be structurally weak. If the pulpodentin complex can re-mineralize this unprotected basal 1 µm of demineralized dentin (Tatsumi, 1989; Tatsumi and others 1992), then the layer may become as strong as normal dentin, rather than be a zone of debonding that has been seen in vitro (Nakabayashi and others 1991). Another purpose of acid etching dentin is to clean the dentin surface. Often dentin is inadvertently contaminated with blood during the cavity preparation. Acid etchant, by dissolving most of the smear layer, tend to float these biofilms on the dentin when it is rinsed. The low pH of the etchant may also denature the plasma proteins and hemoglobin. The purpose of acid etching may vary depending upon the material. If the intention is simply to remove the smear layer but leave the smear plugs in place, as when one uses glass-ionomer cements, then short etching times with dilute acids would seem to be indicated (Bowen 1978; Pashley and Others 1981; Hamlin and Others 1990a). However, if one wants to create a resin hybrid layer (Nakabayashi and Others 1991) in the dentin rather than on the dentin, then one must demineralized more deeply and in the process, removes smear plugs. This can still be accomplished using dilute acids, but the etching time may have to be extended. Effects Of Conditioning Of Dentin The principal effects of conditioning of dentin may be classified as a) Physical changes b) Chemical changes 70
  71. 71. Physical changes Increases or decreases in the thickness and morphology of smear layer changes in the shape of dentinal tubules. Chemical changes a) Modification of the fraction of organic matter b) Decalcification of the inorganic portion Conditioning of dentin may be done by several means 1) Chemical a) Acids b) Calcium chelators 2) Thermal a) Lasers 3) Mechanical a) Abrasion When dentin is cut for cavity preparation, the wrenched cutting debris of the dentin forms a thin smear layer on the surface. It is also driven into the dentinal tubule apertures displacing the odontoblast process and forming a smear plug at a depth of less than 10-micron meter. Etching dissolves the smear layer and part of the peritubular dentin, leaving tapered cylindrical holes of that depth. In an experiment on monkey, dentin wall demineralized with a phosphoric acid jelly etchant for 60 sec was completely re-mineralized after 4 months. This results indicates that etching did not result in deleterious effect upon either the collagen fibers or the odontoblast processes, because the presence of collagen fibers maintaining their proper cross bonded structure as a base for apatite crystals to attach 71
  72. 72. to and of the vital odontoblast processes to supply the calcium phosphate from the pulp is essential for remineralization of dentin. A.J.Gwinnett and M.D.Jendresen (1978) have concluded from their experiments and observations that the surface of acid conditioned eroded dentin is significantly different from that of acid conditioned normal dentin. They further observed the depth of penetration of resin is also less in acid treated eroded dentin where many tubules remain partially occluded by intratubular insoluble deposits. Ruse and Smith (1991) found when common conditioning agents were used, it has been found by X-ray photo electron microscopy that the outermost surface contains only 10% or less of the calcium and phosphorus initially present. They concluded that the treatment of dentin with acidic conditioners leaves the surface so depleted of calcium and enriched by organic residues that subsequently placed bonding systems should be based upon agents able to interact with organic components of dentin. Bonding agents that rely on chelation to calcium are unlikely to be successful when applied to acid etched dentin unless they penetrate into the demineralized matrix to reach normal, mineralized dentin. Acid etching of dentin is not harmless but represents one more source of acute irritation to the pulpodentin complex in addition to the vibratory, thermal, mechanical and evaporative stimuli that accompany cavity a preparation. However, it is not as irritating as has been previously thought. Nakabayashi (1982) introduced the concept of hybridization. The technique consists of applying an acid, ranging in concentration from 10% to 30% to the surface of dentin. Within 15 minutes the acid selectively dissolves away the inorganic component of the dentin to a depth of 5 to 10 microns. It then flows in the dentinal tubule for up to 72
  73. 73. 100 microns at which point it diffuses laterally into the peri-tubular dentin for up to 10 microns. As in the previous case the calcium component is selectively eliminated. Then these spaces are replaced by an insoluble resin component that completely encapsulates all exposed collagenous fiber. He also reported that dentin conditioning by citric acid containing ferric chloride followed by a dentin bonding agent containing 4 META (methacryloxyethyl trimellitate anhydride) was effective method of dentinal bonding. Concerning the bonding mechanism, he proposed that diffusion and impregnation of monomers into the subsurface of pretreated dentinal substrate and their polymerization, creating a hybrid layer of resin reinforced dentin. This newly formed hybrid layer may be thought of as an admixture of polymer and dentinal components, creating a resin dentin composite. This technique not only enhances the shear bond strength of the resin to the dentin but also increase the potential against micro leakage and postoperative sensitivity. Nakabayashi (1985) suggested that the acidic treatment partially demineralized a zone of the dentin near the surface, facilitating an infiltration process of compatible monomers. The polymerized resin forms a reinforced zone of dentin on which a resin based restorative material can be bonded. The bond strength is not dependent upon interlocking at the dentinal tubules. Kurosaki et al (1987) found that etching of dentin of the clinical cavity floor allows the chemically adhesive composite resin to produce resin tags of tapered, cylindrical or tubular form as well as 73
  74. 74. impregnated dentinal layers. These changes will considerably improve the bond strength as well as the tubule aperture seal. Surface Interactions Of Dentin Conditioners Smear layer removal One purpose of a dentin conditioner is removal of the smear layer to provide a surface that is more suitable for adhesion; however this does not necessarily apply to all systems. For example, the All- Bond system can be used with the SA-HEMA conditioner, which is weakly acidic and probably modifies the smear layer without removing it, except where the smear layer is quite thin. Dentin permeability changes Removal of the smear plugs results in increased permeability of the dentin, and the rate of removal by conditioner can be examined by measurements of permeability increase for different application times. This is controlled by the strength of the acid, it’s concentration, and whether there are modifying components in the conditioning solution (FIG. 44). Because acid etching increases dentin permeability and dentin wetness, successful bonding at adhesive resins to acid etched dentin requires the use of hydrophilic resins that bond equally well to both peritubular and intratubular dentin. Future trends seem to be toward lowering both the concentration of acids and the time of etching dentin. Conditioner Application Time To Achieve Maximum Permeability CONDITIONER Time (sec) 74

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