Biomedical Engineering Reference
In-Depth Information
It is suggested that the rate of secondary dentin deposition depends upon the
individual's diet and occlusal forces [ 4 ]. Secondary dentin deposition results in
gradual narrowing of the pulp chamber.
Tertiary dentin, also referred to as reparative, irregular secondary, irritation,
response, or reactionary dentin, is formed in response to an insult such as caries or
abrasion [ 29 ]. This tissue, which appears to represent a protective response, has a
less regular structure with fewer and less well-aligned tubules as compared to
primary dentin [ 35 ]. Reparative dentin is formed by new odontoblast-like cells,
while reactionary dentin is formed by surviving odontoblasts subjacent to damaged
or diseased dentin [ 22 ]. For example, pulpal injury leads to the proliferation,
migration, and differentiation of odontoblast-like cells from the pulp, giving rise
to the secretion of reparative dentin [ 36 ]. In comparison, mild injuries stimulate the
surviving postmitotic odontoblasts at the site of the injury to secrete reactionary
dentin [ 37 ]. It has been suggested that fibroblasts or undifferentiated cells in the
pulpal tissue are the likely progenitor cells of the new odontoblast-like cells [ 38 ].
The terms sclerotic and transparent dentin are often used interchangeably to
describe dentin that has altered mineralization. As an example, sclerotic dentin
describes tissue that exhibits obliteration of the dentin tubule as a result of progres-
sive deposition of peritubular dentin [ 29 ]. This type of dentin is generally found in
the roots, especially near the apex and the amount of sclerosed dentin increases with
age [ 4 , 35 ].
Carious dentin is characteristically described as consisting of infected and
affected layers. The infected layer is removed prior to reparative procedures with
synthetic materials. The affected layer is generally not removed during treatment
and, based on structural features, this layer is subdivided into the following: turbid
or discolored layer, transparent zone, and subtransparent zone [ 11 , 39 ]. The trans-
parent zone occupies the largest proportion of the carious dentin [ 40 ]. Transparent
dentin has been characterized as hypermineralized, but results from a recent study
suggest that only a limited number of carious lesions with transparent dentin
develop hypermineralized intertubular regions [ 31 ]. However, investigators agree
that, in contrast to normal, healthy dentin, the tubules within the caries-affected
dentin are frequently occluded by acid-resistant mineral deposits [ 11 , 31 , 39 ].
7.3 Failure of Posterior Composite Restorations
and Dentin/Adhesive Bonding
The primary factor in the premature failure of moderate-to-large composite
restorations is secondary decay at the margins of the restorations [ 7 ]. For exam-
ple, in a study of radiographs from 459 adults, age 18-19 years, the investigators
reported that among 650 interproximal restorations the failure rate as a result of
secondary or recurrent decay was 43% for composite compared to 8% for
amalgam [ 6 ]. In a separate study of amalgam and composite restorations
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