Dr. lama khaled El Banna Faculty of Oral and Dental Medicine Al – Azhar University operative dentistry

2. College of Dentistry
Operative Dentistry I
Tooth Histology, Form & Occlusion:Tooth Histology, Form & Occlusion:
Operative Considerations -1-Operative Considerations -1-
Dr. Hazem El Ajrami

3. • Success in clinical dentistry requires a
thorough understanding of the anatomic and
biologic nature of the tooth with its vital
components. A basic knowledge of the
histological structures of enamel and dentin, as
well as understanding proper tooth form and
occlusion, dictates vital considerations during
operative procedures.

4. • This knowledge is essential for:
Establishing an efficient treatment planning.
For example, the friability of enamel and type
of occlusion influences the selection of
restorative material.
Considerations during cavity preparation;
including the direction of enamel rods,
thickness of enamel and dentin, position and
size of the pulp and effect of cutting procedure
on these vital structures.

5. Attaining proper bonding of adhesive
restoratives.
Proper restoration of teeth because function
depends on establishing proper tooth form and
occlusion.

6. • Histology and structure of teeth:
The teeth are formed of enamel, dentin,
cementum and pulp. The relevance of each of
these structures to operative procedures will be
discussed individually.

7. Enamel
Enamel is hardest substance of the human
body. It provides a hard, durable protective
cover for vital dentin and pulp tissues. It is also
essential for function and esthetics. However,
enamel has a high elastic modulus and low
tensile strength which indicates a rigid but brittle
structure. It thus depends on dentin for
resiliency.

8. • In addition, much of the art of restorative
dentistry evolves from efforts to mimic the
color, texture, translucency and form of
enamel with synthetic dental materials such as
resin composite or porcelain.

10. • Physical Structure:
Human enamel has a physical unit structure
called enamel rods or prisms.
The rods are separated by interprismatic
substance which is less mineralized with
higher amounts of organic matrix. The enamel
thickness varies from tooth to tooth; generally
increasing in posterior teeth. It also varies
within the same tooth being thicker at the
incisal and occlusal areas and tapering until it
reaches a negligible thickness cervically at the
cement-enamel junction.

15. • Chemical composition:
Enamel is highly mineralized crystalline
structure containing 95 to 98% inorganic
hydroxyapatite by weight. The organic content
forms about 1 to 2%. Water content forms about 4%
by weight. Although enamel contains water in its
chemical composition, it is dry relative to dentin. Its
water content does not affect bonding to enamel,
however, it affect its ion exchange capacity and
permeability. This is of clinical relevance in
demineralization by acids (caries or during acid-
etching) and in remineralization and fluoride uptake
of enamel.

16. • When fluorides are incorporated in enamel,
solubility of enamel in acids is decreased owing
to conversion of hydroxyapatite to larger more
stable crystals of fluoroapatite.

17. • Clinical relevance to operative procedures:
I. During cavity preparation:
The enamel rod boundaries form natural
cleavage planes through which fracture might
occur. The enamel prisms are especially
weakened if the underlying dentinal support
is pathologically destroyed by caries. Enamel
unsupported by dentin is called undermined
enamel. Cleavage of undermined enamel with
hand or rotary instruments causes its fracture
along the interprismatic substance ensuring
that margin is formed of strong full-length
enamel rods.

18. Cleavage of undermined enamel

19. Undermined EnamelUndermined Enamel

20. • Enamel rods are generally aligned perpendicular
to the dentin-enamel junction. However, the rods
converge from the dentin-enamel junction
towards enamel outer surface in concave areas
(pits and fissures) and diverge in convex
surfaces (cusps and ridges). In addition, at the
cervical third, they are inclined gingivally. The
direction of enamel rods is one of the factors that
influence the direction of enamel walls during
cavity preparation, especially with brittle
restoratives that cannot support the tooth
structure.

21. Direction of enamel rods

22. II. During restoration:
With resin composite restorations, the
following should be considered during bonding
to enamel:
 The difference in the inorganic content of
prisms and interprismatic substance renders
them different in dissolution pattern during
acid-etching. Thus, etching the ends of enamel
rods at the margin (including both prisms and
interprismatic substance) produces a more
favorable etching pattern than etching their
sides only.

24. Before acid etchingBefore acid etching After acid etchingAfter acid etching

26. Uncut enamel surface is covered by aprismatic
(prismless) enamel in which the crystallites run
parallel to each other and perpendicular to the
underlying prismatic enamel. This layer should
be considered during bonding to uncut enamel.
In addition, enamel surface is covered by an
organic enamel pellicle formed of protein-fat-
carbohydrate complex. This layer should be
removed before bonding by thorough cleaning
of enamel surface.

27. Aprismatic enamel
Enamel pellicle

28.  In addition, the following should be considered
during shade selection of esthetic restorations:
A. Upon tooth isolation or dehydration, the tooth
appears whiter in color. This change in color
is explained by the temporary loss of loosely
bound water present in enamel. So, proper
shade selection should be determined under
wet not dry conditions.

30. B. The natural color of the enamel by itself is
grayish or whitish blue. However, it is
translucent, i.e. the yellowish color of dentin
shows through it. Thus, the thickness of enamel
affects the overall color of the tooth. The
tapering of enamel thickness cervically results
in three color zones. The incisal third (w)
(mostly enamel) is lighter and more translucent
than the gingival third (y) (thin enamel and
mostly dentin), whereas the middle third (x) is
a blend of both.

31. Cross section of anterior tooth showing three color zones

32. C. The degree of mineralization influences the
appearance of enamel. In hypomineralization,
enamel appears more opaque and milky white
than normally mineralized translucent enamel.

34. Dentin
Dentin provides an elastic foundation for
enamel. It is more elastic than enamel thus
supporting the brittle non-resilient enamel. Dentin
forms the bulk of the tooth and it encompasses the
pulp. Dentin is formed by cells called
odontoblasts.

35. • These cells are considered part of both dentin and
pulp tissues because their cell bodies lie in the
pulp cavity and their long, slender cytoplasmic
processes (odontoblastic processes) extend into
the tubules of dentin. It is because of these
odontoblastic processes that dentin is considered a
living tissue with the capability to react to
physiologic and pathologic stimuli.

37. • Physical structure:
Dentin is composed of inorganic apatite
crystals embedded in a cross linked organic
matrix of collagen fibrils. It is traversed by
dentinal tubules. Each tubule contains the
cytoplasmic cell process of an odontoblast in
addition to dentinal fluid.

38. Odontoblastic process (star)
Dentinal tubules

40. • Each dentinal tubule is lined by a layer of
peritubular dentin which is much more
mineralized than the surrounding intertubular
dentin. The number of tubules per unit area varies
from DEJ to pulp. At the pulpal side, the number
of tubules is large, while their density decreases
toward DEJ. In addition, the diameter of tubules
increases towards the pulp.

41. Cross section of dentinal tubules;
P = Peritubular dentin, I = Intertubular dentin

42. (OP) odontoblastic process.(OP) odontoblastic process.
(DTS) dentinal tubule space.(DTS) dentinal tubule space.
(LL) lamina limitans.(LL) lamina limitans.
(PTD) peritubular dentin.(PTD) peritubular dentin.
(ITD) intertubular dentin.(ITD) intertubular dentin.
ITDITD

43. • Chemical composition:
Dentin is formed of 75% inorganic material,
20% organic material and 5% water by weight. A
large fraction of water content of dentin is
confined to dentinal tubules, and because number
and diameter of tubules increases with dentinal
depth, the water content of dentin is greater in
deep dentin relative to superficial dentin. This
inherent wetness complicates bonding to dentin.

44. • Dentin sensitivity:
Dentin is sensitive to stimuli. The
most accepted theory for pain transmission is the
hydrodynamic theory which states that pain is
transmitted through small rapid movements of
fluid that occur within dentinal tubules. The
extensive branching of dentinal tubules near DEJ
may account for the increased sensitivity at this
area.

45. Lateral branching of dentinal tubules at DEJ

47. • Dentin permeability:
Dentinal tubules are filled with dentinal fluid
providing wetness to dentin surface. The pulpal
fluid has slight positive pressure (25-30 mmHg)
that forces dentinal fluid outward. However, the
movement of fluid within dentinal tubules in
response to stimuli causes dentin hypersensitivity.
In addition, irritants could diffuse through open
dentinal tubules which are thus considered a direct
pathway to the pulp. If the remaining dentin
thickness is decreased, diffusion of irritants
increase, since the tubules are shorter, become
more numerous and increase in diameter in deep
dentin.

48. Dentin Permeability

49. • Smear layer:
The smear layer is cutting debris that is
smeared over enamel and dentin surfaces when
tooth surface is instrumented with rotary or
manual instruments during cavity preparation. Its
thickness varies according to the depth of cavity
and conditions of cutting (2-5 microns). It is
attached to the underlying surface in a manner
that prevents it from being rinsed off by air-water
spray. It is formed of tooth debris, mixed with
saliva and bacteria.

50. Smear layer on cut dentin surface

51. • On the prepared dentinal surfaces it is packed
into the dentinal tubules forming smear plugs.
The smear layer is reported to reduce dentinal
permeability. However, it interferes with
bonding between the tooth structure and
adhesive restorative materials.

52. Longitudinal section in cut dentin showing
smear layer (SL) and smear plug (SP)

53. • Types of dentin:
I. Primary dentin: is type of dentin formed
before and shortly after eruption.
II. Secondary dentin: is a continuation of
primary dentin that forms at slower rate
as the tooth ages even without obvious
external stimuli.
III. Reparative dentin (tertiary dentin): is
formed in response to irritation such as
abrasion, erosion, attrition or caries. It is
usually localized at the area of the tooth that
has been irritated by the lesion.

54. IV. Sclerotic dentin: this type of dentin is
characterized by obstruction of dentinal tubules
by precipitation of mineral crystals. The dentin
could undergo physiologic dentinal sclerosis as
an aging process or reactive sclerosis in
response to mild irritations such as slowly
advancing caries. Sclerotic dentin is less
sensitive and less permeable than virgin
unaffected dentin. It seals off irritants from
entering the pulp. However, it is less receptive
to resin bonding.

57. • Clinical relevance to operative procedures:
A. During cavity preparation:
 The efficiency of cutting is greater in
dentin than in harder enamel.
 During cutting of dentin by rotary
instruments, heat generation should be
minimized as much as possible to avoid
thermal irritation to the pulp.

58. B. During restorative procedures:
 In deep cavity preparations, the dentinal
tubules should be sealed to avoid any
irritation from restorative material to the
pulp.
 During bonding to tooth structure, the
structural variation of dentin from
enamel should be considered to attain proper
bonding.

59.  How to distinguish between enamel and
dentin during tooth preparation:
1. Color: dentin is normally yellowish white and
darker than the grayish white translucent
enamel.
2. Reflectance: dentin surface is more opaque
and dull, being less reflective to light than
enamel surfaces which appear shiny.
3. Sound: dull sound of dentin as opposed to the
sharp higher pitch of enamel when an
explorer is moved across the surface.
4. Tactile sensation: dentin has greater yield to
pressure than enamel.

60. Pulp
The dental pulp is a specialized connective
tissue that occupies the pulp cavity in the
tooth. The pulp is circumscribed by dentin and
is lined peripherally by a cellular layer of
odontoblasts.

61. • The pulp serves four functions:
1) Formative: is the production of primary
and secondary dentin.
2) Nutritive: the pulp supplies nutrients and
moisture to the dentin through the dentinal
fluid.

62. 3) Sensory: the pulp comprises sensory nerve
fibers to mediate the sensation of pain.
However, the pulp does not differentiate
between heat, touch, pressure or chemicals.
4) Defensive: the pulp response to stimuli differs
according to the severity of the irritant.
Reparative dentin may be formed following
mild irritation as a defensive action. Sclerotic
dentin and dead tracts are other forms of pulp
response to an irritant while in severe cases the
pulp responds by an inflammatory reaction.

63. • Effect of aging:
The size of the pulp chamber decreases with
continuous deposition of dentin. The degree of
vascularity also decreases and so does the
capacity of the pulp to react to various insults.
The imaginary lines through which the pulp
recedes during formation of secondary dentin is
called the recessional lines of the pulp.

64. Size of pulp cavity (arrows) in young age,
compared to older age.

65. Cementum
Cementum is the hard dental tissue covering
the roots of teeth. It is slightly softer than dentin
and consists of about 45 to 50% inorganic
material (hydroxy-apatite) by weight and 50 to
55% organic material and water. The attachment
of cementum to the dentin is firm. The
cementum joins the enamel at cemento-
enamel junction which is referred to as the
cervical line.

66. Cemento-enamel junctionCemento-enamel junction

67. • Clinical relevance to operative procedures:
In about 10% of teeth, enamel and cementum do
not meet, and this can result in a sensitive area
due to exposed dentin.
Abrasion, erosion, caries and the procedures of
finishing and polishing may result in denuding
the dentin of its cementum covering which can
cause the dentin to be sensitive to various
stimuli.
Morphologically, the surface of intact cementum
and the cemento-enamel junctions are rough
compared to enamel surface. This could enhance
plaque retention and caries formation.

69. Thank You