The aim of dental clinicians, prosthodontists, and manufacturers throughout the history of dentistry has been to achieve esthetically and functionally ideal restorations. Ceramic crowns have an impressive natural appearance and biocompatibility, and their clinical application has now been expanded to the complete arch (1). The success of a dental restoration is determined by 3 main
factors: esthetic value, resistance to fracture, and marginal
adaptation (2). Inadequate
fit results in plaque accumulation, which increases the risk of carious
lesions and can cause microleakage and inflammation of the endodontics.
Plaque accumulation, especially with subgingival margins, can also cause
periodontal diseases. Finally, misfit
may lead to cement
dissolution, particularly when using
zinc phosphate cements (3).
Until the late 1980s , researchers who
measured marginal fit did not always measure the same distances. In
1989 Holmes et al proposed a simple terminology. In
general, marginal
fit is
assessed by measuring the marginal gap or absolute marginal discrepance.
In Specification No. 8, the American Dental Association specifies that the
film thickness of zinc phosphate cement does not exceed 25 mm for Type
1 cements and 40 mm for Type 2 cements. However, experiments that analyzed
marginal adaptation have shown that this goal was rarely achieved.(4)
Gardner3 found that it was difficult to achieve this degree of accuracy
even under ideal conditions.
After a 5-year analysis of more than 1000
restorations, McLean and von Fraunhaufer concluded that 120 mm was the highest
tolerable marginal opening. Ceramic crown manufacturing may require such
different methods as
conventional slip casting, hot pressing, or computer-aided design /
computer-aided manufacturing (CAD / CAM). The most esthetic
restoration was considered to be porcelain jacket crown. However,
the inherent weaknesses and brittleness of dental
porcelain along with the high sintering shrinkage were noted as serious
drawbacks to its universal acceptance as a complete veneer restoration.
Despite the many available
techniques, the growing desire for better esthetics and
improved biocompatibility has resulted in the development of a large
number of all-ceramic restorative systems. Some use a single layer
glass-ceramic material
such as Dicor and IPS Empress, while others use a dual layer
design such
as In-Ceram and Procera method. With the advent of computer-aided design
and milling (CAD / CAM) systems (5) further advances were made in highstrength
all-ceramic technology.
In order to allow the restoration
to survive in the oral cavity, the margins should be closely adapted to
the preparation's cavosurface finish line. The type of reparation finish line
has the main role in dictating the shape and bulk of restorative material
in the restoration margin. It also has a main effect on the marginal
adaptation and the degree of seating of the restoration and hence
the clinical longevity of orcelain jacket crowns (6).
Types
of marginal finish lines and their connection to ceramic restoration
The chamfer finish line is an obtuse-angled finish line that exhibits the least stress concentration and most conservation (Fig. 1). It is indicated for a complete metal crown or the lingual aspect of metal-ceramic restoration.
the obtuse
angle produces shearing forces which are not well tolerated by porcelain (8).
Shoulder
finish is a
right-angled finish line that can resist compressive occlusal forces but
it requires more preparation, so it is not conservative. A shoulder finish
line is recommended for allceramic as it minimizes the stresses and preventing
the possibility of porcelain fracture (Fig.
2) (8). However, shoulder or chamfer
finish lines can be selected for all-ceramic crowns that are bonded to
prepared teeth. In a laboratory study, when all-ceramic crowns were internally
etched
and
cemented on natural teeth with a resinous cement, there was no significant
strength reduction or in a longitudinal retrospective clinical evaluation
of all-ceramic crowns (9).
A bevel can
be made on the external edge of the shoulder finish line to reduce the
marginal discrepancy of the restoration as it can be burnished (Fig. 3). It also protects the edge of the finish line
preventing chipping. Shoulder with bevel is primarily indicated for the facial
finish line of metal-ceramic restorations to hide the supragingival facial
metal margin of the restorations. It is also used as the gingival finish
line on the proximal box of inlays and onlays, and as occlusal finish line
for onlays and partial veneer crowns (8).
On
the other hand,
the knife-edge is the more conservative finishing line, but it is
difficult to wax and cast and susceptible to distortion and may results in
an over-contoured restoration (8). In lower anterior teeth, periodontically
treated teeth, very convex
axial surfaces, and tilted teeth where preparation is minimal, the knife-edge
finish line appears to be the treatment of choice (7,8). Similar to
knife-edge finish line, Chisel edge and feather edge finish lines, and
they are rarely used (8).
Deep
chamfer finish line (also known as a heavy chamfer) is similar to chamfer but prepared
with a diamond of greater. It is indicated for all-ceramic crowns and
provides better support for a ceramic crown than does a conventional
chamfer, but it is not as good as a shoulder. A deep chamfer can produce
an unsupported lip of enamel that is susceptible to fractured during or
after cementation of the restoration (Fig. 4) (6).
The
radial shoulder is a modified form of shoulder finish line with a rounded
internal line angle that is rounded using an end-cutting diamond and
finished with a bin-angle chisel. There is less stress concentration in
the tooth structure than with a classic shoulder, so it provides
good support for ceramic restoration walls. However, the amount of tooth
structure destruction required for this configuration is not significantly
less than that required for a classic shoulder (6).
The
effect of different margins on the final esthetics of ceramic
restorations (10) In restoring
extensively damaged or previously restored teeth, the contemporary dentist
has a wide range of options to utilize. All-ceramic crowns are also used
to restore both esthetics and function. One of the essential to success
with either option is proper tooth preparation, which
includes proper selection and preparation of the preparation cervical
margin (10).Regardless of the margin geometry, it is imperative
to properly place the prepared gingival margin in relation to the free
gingival margin, the epithelial attachment, and the alveolar housing. Wherever
the
esthetic demands allow, there should be margins in a supra-gingival
location.
Margins
must be positioned in an intracrevicular position in many patients in
order to cover those margins with healthy gingival tissue and thus have an
appropriate esthetic result. Clinicians need to realize that placing a
cervical margin in an intracrevicular position is a precision exercise. If the
margin is placed within a short distance of the free gingival margin, a
small amount of gingival recession can result in margin exposure and
esthetic failure. A margin placed too deep in the sulcus risks violation
of biological width and concomitant chronic inflammation of the gingiva
There are two potentially useful landmarks to guide the clinician in precisely
placing margins. A generally useful guideline is to place 0.5 mm
apical cervical margins on the healthy free gingival margin. A
more accurate method is to sound through the attachment to probe
the alveolar bone crest and place at least 4 mm of coronal cervical
margins on that alveolar crest. With either landmark, it is critical that
the prepared cervical margin follow the alveolar bone scalloped anatomy,
the attachment, and the gingival tissue. Clinicians make the most
common error of flattening the cervical margin in the inter-proximal
regions, violating biological width and eliciting a chronic inflammatory
response. The clinician has three options in circumstances
where esthetics is important: The first is to use an all-ceramic crown. While in recent
years , the overall lifetime of most all-ceramic options has increased,
metal-ceramic crowns can provide longer service. However, several
all-ceramic systems provide excellent longevity-enhancing esthetics
and can be used on anterior teeth with relative confidence. All-ceramic
alternatives on posterior teeth should be avoided. Though many
all-ceramic systems are available, the design of the cervical margin is
identical for all the systems.
A shoulder margin with a rounded inner angle should be ready to end at about 90 to the outer angle of the labial or buccal surface. A slight slant of no more than 5 is acceptable. The margin should be as smooth as possible, and should be between 1.2 and 1.5 mm in depth to provide maximum esthetics and strength (Fig.7). This design provides an adequate volume for esthetics and strength, and places the cervical margin during function in compression. It also allows glazing of the restore without rounding the ceramic margin of the terminal. Tensile forces occur in function when a more pronounced slant is produced, which can result in half-moon fractures in the cervical region.
The
marginal fit of different recently introduced dental ceramics to the dental
field
One
of the important factors used in clinically assessing their long term
performance is the optimal fit of all-ceramic restorations. In a
restauration the presence of marginal discrepancies exposes the luting
agent to the oral environment. The marginal opening allows for more
plaque accumulation that can cause inflammatory gingival reactions and may
result in degradation of the soft tissue due to periodontal disease. It
also causes recurrent caries and bone loss. Many studies have assessed the
marginal fit of various all-ceramic crowns.(11) The results show great variations within
a single crown system. The evaluation of the marginal discrepancy of the crowns
depends on several factors: the measurement of cemented or noncemented
crowns, the type of abutment used for the measurements, the type of microscope,
the factor of enlargement used for the measurement location and the
quantity of single measurements. Variations exist on what is a clinically
acceptable margin. McLean and von Fraunhofer30 suggested a restore if marginal
gaps and cement thicknesses of < 120 mm can be achieved. Tuntiprawon
and Wilson31 found that ceramic crowns at the axial wall with smaller gap
dimensions and marginal opening showed the best compressive strengths when
loaded onto the dies. In fixed prosthodontics marginal accuracy is an
important quality criterion. Many studies examined the marginal fit of
crowns,(12)and in many of those studies the authors used steel or resin
dies to measure the marginal precision. Natural teeth show great
variations due to their age, individual structures and post-extraction storage
time, thus creating difficulties in obtaining standardized abutments. The
benefit of this approach is the potential for all crown systems to achieve
a consistent planning. The master steel die remained clear and damage-free,
which was an added benefit. The cementation of crowns before
determining marginal fit is an important factor in the literature.
Most
authors studied cemented crowns.(13) The marginal distance usually
decreases after luting, which is clinically important. Moreover, due to
cementation, additional systemic and statistical faults are introduced.4 For
this reason, cementing procedures have been avoided in this review. To
avoid an increase in statistical error, all measurements were carried out
by the same investigator. Marginal fit measurements are often done by
sectioning a crown and measuring it with a light microscope or a scanning
electron microscope. This method however has the potential for artifact
distortion, provides only limited numbers and measurement positions, and
is destructive. In the present case, a direct measuring technique with
image analysis software under a microscope allowed for
non-destructive quantification and multiple measurements.
Nonetheless
this research has had many limitations. Several researchers measured the
internal fit of the crowns, but in this study it was difficult
to calculate the internal fit of the crowns, since measuring the internal
fit of the crowns involves cementing the crowns and sectioning the
specimens. Another limitation of this study was that the marginal gap was
perpendicularly and orthoradially oriented on the computer monitor, prior to
the microscopic measurement. Nevertheless, precise placement of a specimen
was difficult, since the researcher could only visually monitor the
measurements. In addition , this study did not completely simulate
clinical conditions by examining the marginal fit without cementation
of crowns. Important factors are sample size and number of measurements
per specimen. For each group some authors selected six24 or eight
specimens. Many authors conceived studies using 10 samples per test. Just
4-2 positions at the margin of each crown specimen / abutment
were measured in most earlier studies. (14) Some authors, however, made
measurements at multiple eference points, each with small distances,
resulting in 150 measurements for a single specimen / abutment. Other
authors randomly selected points along the crown's margin without
precise positions, and determined the mean of all measurements for the crown
's marginal fit. Groten et al suggested that ideally, it would require 50
or at least measurements to obtain clinically relevant information on the
size of the gap.
In
this analysis, 20 crowns were prepared for each group, and 80 measurements
per crown were made to improve the accuracy of the statistics. Only if
internal relief is sufficient to accommodate the luting agent can an
improvement in the seating of a crown. The internal relief must
accommodate the coating of cement and any defects on the surface of
the tooth and the inner crown. In this analysis, die spacers were applied
as per the instructions of the manufacturer. Yeo et al6 reported that the
marginal openings of the In Ceram and IPS Empress 2 crowns were
respectively 112 and 46 mm, which is consistent with the results of this study.
The marginal
distance of II Ceram crowns was measured by Sulaiman et al8 and found to be
160,66 mm. Those results are also consistent with this study. However for
In Ceram crowns, Rinke et al2 registered a marginal gap of 33.5 mm. The
differences in the methodology used by various investigators researching
marginal accuracy may be an explanation for the lack of
consensus. Sulaiman et al8 suggested the use of different measurement
instruments may be the cause. An important consideration is also the skill
of a dental technician who performed the restorations. The explanation the
In-Ceram crowns demonstrated the largest marginal gap in this study may
have been correlated with two factors. First, the glass mixture appears to
settle during glass infiltration firing, which produces an unnecessary
bulk at the edge of the coping after the shooting is finished, and must be
trimmed using a rotary tool. Careless removal of excess material at the margin
may theoretically lead to increased marginal difference. The second
explanation could be the need for a second additional impression
from gypsum dies to prepare a slip-cast. Because of these procedures a
dimensional change could have occurred.
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