Biomedical Engineering Reference
In-Depth Information
amalgam restorations [ 9 ]. At 5 years, the need for additional treatment was 50%
greater in children receiving composite restorations as compared to children treated
with dental amalgam [ 5 ]. Based on a review of dental records from 3,071 subjects,
Simecek and colleagues reported in 2009 a significantly higher risk of replacement
for posterior composite restorations as compared to amalgam [ 4 ]. In a study of
amalgam and composite restorations placed by 243 Norwegian dentists, the mean
age of failed amalgam was ~11 years, while the mean age for failed composite was
significantly lower at 6 years [ 7 ]. Indeed, after nearly five decades of research, the
clinical lifetime of large-to-moderate posterior composite restorations continues to
be approximately one-half of that of dental amalgam [ 10 ].
The reduced clinical lifetime of moderate-to-large class II composite
restorations can be particularly detrimental for patients because removal of these
restorations can lead to extensive loss of sound tooth structure. For example, the
removal of composite restorations produced significantly greater increases in cavity
volume in comparison to the removal of amalgam [ 11 ]. The increase in cavity
volume and increased frequency of replacement means that significantly greater
amounts of tooth structure will be lost with treatment and re-treatment of class II
composite restorations [ 11 ]. Over the lifetime of the patient, the additional loss of
tooth structure will translate to more complex restorations and eventually total tooth
loss. The reduced longevity, increased frequency of replacement, and the need for a
more complex restoration mean increased costs to the patient in terms of both time
and money [ 12 ].
The premature failure of moderate-to-large composite restorations can be traced
to a breakdown of the bond at the tooth surface/composite material interface [ 9 , 10 ,
13 - 17 ] and increased levels of the cariogenic bacteria, Streptococcus mutans, at the
perimeter of these materials [ 18 - 22 ]. The composite is too viscous to bond directly
to the tooth and thus, a low viscosity adhesive must be used to form a bond between
the tooth and composite. The breakdown of the composite/tooth bond has been
linked to the failure of current adhesives to consistently seal and adhere to the
dentin [ 2 , 17 - 26 ]. Acid etching provides effective mechanical bonding between
enamel and adhesive, but bonding to dentin has been fraught with problems.
7.2 Dental Substrate
Dentin is the hydrated composite structure that constitutes the body of each tooth,
providing both a protective covering for the pulp and serving as a support for the
overlying enamel. Enamel, with its exceptionally high mineral content, is a very
brittle tissue. Without the support of the more resilient dentin structure, enamel
would fracture when exposed to the forces of mastication. Dentin supports, as well
as compensates, for the brittle nature of the enamel.
Dentin is composed of approximately 50% inorganic material, 30% organic
material, and 20% water by volume [ 27 ]. Dentin mineral is a carbonate rich,
calcium deficient apatite [ 28 , 29 ]. The organic component is predominantly type I
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