Structural Durability and Marginal Integrity / orthodontic courses by Indian dental academy
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Transcript of Structural Durability and Marginal Integrity / orthodontic courses by Indian dental academy
Introduction
When one considers critically and realistically, all the
advancements achieved in the overall area of restorative dentistry, a
gearing exception comes to the fore; that exception is the technique
for the preparation of teeth and the philosophy behind it. The
approach to preparations has not kept pace with other advances in
dentistry. Almost all dental literature, dental school, teaching,
essayists, programs and philosophies of dentists demonstrate this
traditional holdover.
Teeth require preparation to receive restorations, and these
preparations must be based on fundamental principles from which
basic criteria can be developed that help predict the success of
prosthodontic treatment.
The principles of tooth preparation may be divided into three
broad categories:
1) Biologic considerations.
2) Mechanical considerations.
3) Esthetic considerations.
1
Last week one of my colleague spoke on two mechanical
factors i.e. the retention and resistance form. Continuing the
principles of tooth preparation today I will be discussing one more
mechanical principle of tooth preparation i.e. structural durability
and one biologic principle i.e. marginal integrity.
So today’s topic of this seminar is structural durability and
marginal integrity.
Structure is any constructed thing here it refers the crown or
bridge.
Durability means “likely to last longer”.
Structures durability is the durability of the restoration so as
to how longer a crown or bridge lasts.
Features that contribute to the durability of the restoration are:
1) Alloy selection.
2) Adequate tooth reduction.
Occlusal reduction.
Axial reduction.
Provision for reinforcing struts.
3) Poor metal-ceramic framework design.
2
1) Alloy Selection:
The Type I and Type II gold alloys are satisfactory for
intracoronal cast restorations, they are too soft for crowns and fixed
partial dentures, for which Type III or Type IV gold alloys are
choosen. There are harder and their strength and hardness can be
increased by heat treatment.
High-noble metal content metal-ceramic alloys have a
hardness equivalent to Type IV gold whereas nickel-chromium
alloys are considerably harder. These may be indicated when large
forces are anticipated. Such as long span FPD, although their use
presents some problems.
2) Adequate Tooth Reduction:
Occlusal Reduction:
Enough tooth structure must be removed from the
occlusal surface of the preparation so that when the restoration
is built back to ideal occlusion it must be thick enough to
prevent wearing through or distorting.
3
Even the stronger alloys need sufficient bulk if they are
to withstand occlusal forces.
The occlusal thickness will vary with the restorative
material used. A gold crown requires approximately 1.5mm
clearance over the functional cusps and 1.0mm over the non-
functional cusps. Harder metals require slightly less. If a
porcelain veneer is extended onto the occlusal surface an
additional 0.5mm of space is needed.
The amount of occlusal reduction is not always the
same as the clearance needed. Often part of a tipped tooth is
already short of the occlusal plane and will require less
reduction than would a tooth in ideal occlusion.
Occlusal reductions of the posterior teeth can be
performed in three ways:
1) The occlusal surface of a tooth can be reduced entirely in a
flat plane.
4
2) It can be reduced uniformly following the geometric outline of
cusps or fossae.
3) Either a partial reduction in the form of a cavity or a cusp
reduction with a cavity can be made not including all the
surfaces.
The second one is a uniform occlusal reduction,
provides for an adequate thickness of metal without
jeopardizing the help, resists shearing stresses, and is more
rigid because of its “CORRUGATED EFFECT”.
The third type is the occlusal cavity reduction which
obliterates the occlusal groove for its outline. Its depth usually
extends into the dentin. The cavosurface may be a margin
unless adjacent cusps are also reduced.
Creating steep planes with sharp angles should be
avoided since these can increase the stress and hinder
complete seating of the casting. To diminish stress round the
angles and deep grooves in the center of the occlusal surface
5
keeping is avoided and the angulation of the occlusal plane
will be shallow.
Any necessary equilibration of the opposing teeth
should be done before the restorative procedure is begun.
Opposing cusps that are missing or short of their ideal
position should be replaced in a diagnostic wax up on a cast so
that the required amount of occlusal reduction can be
determined.
Functional Cusp Bevel
A wide bevel should be placed on the functional cusps
of posterior teeth to provide structural durability in this
critical area. It also rounds over the occlusal line angle, which
is the area of high stress concentrations. The angle on the non-
functional cusp is rounded over lightly.
Failure to place a functional bevel can result in thin,
weak areas in the restoration.
6
In crossbite occlusal relationship the functional cusps
are revered.
Bevel is placed on the facial cusps of the maxillary teeth
and the lingual cusps of mandibular teeth.
Axial Reduction
A second prerequisite for structural durability is
adequate axial reduction.
When axial reduction is sufficient, restoration walls can
have satisfactorily thick.
Therefore the more common result of inadequate axial
reduction is an overcontoured restoration.
3) Reinforcing Struts
Structural durability at the margins of full veneer crown is
relatively easy to achieve because of high tensile strength of
the metal.
7
The circumferential axial walls of a restoration hold
themselves together much as a barrel is held together by its
hoops.
When one or more surfaces of a tooth are left uncovered in a
partial veneer crown, the circumferential binding is lost. The
restoration margins can distort during fabrication, cementation
unless features are added to reinforce the structure.
In a three-quarter crown it is a connecting rib of metal that
joins the grooves to provide a “trussing effect”. This
reinforcing rib takes the form of an offset on a maxillary
preparation and an occlusal shoulder on a mandibular one.
Marginal Integrity
The restoration can survive in the biological environment of
the oral cavity only if the margins are closely adapted to the
cavosurface finish line of the preparation. Marginal integrity
includes the location, configuration and marginal fit of the crown.
The configuration dictates the shape and bulk of the restoration. If
8
can also affect both the marginal adaptation and the degree of
seating of the restoration.
9
Margin Placement:
The placement of finish lines has a direct bearing on the ease
of fabricating a restoration and on the ultimate success of the
restoration. The best results can be expected from margins that are
as smooth as possible and are fully exposed to a cleansing action.
Finish lines should be placed in enamel when it is possible to do so.
There are three locations in which to prepare crown margin:
1. Supragingival.
2. At the crest of the gingival or equigingival.
3. Subgingival.
In the past, the traditional concept has been to place the
finishlines as far subgingivally as possible based on the mistaken
concept given by G.V. Black, that the subgingival sulcus is caries
free. The preponderance of recently published literature, however,
strongly supports and have advocated the use of the supragingival
margin. In early 1941 Orban proposed supragingival margin for
improved periodontal health.
10
Larato showed that the most crowns with subgingival margins
exhibited gingival inflammation compared with supragingival.
Silness used plaque index, an index of gingival health, and pocket
depth to compare crowns that had subgingival and supragingival
margins. He showed an increased accumulation of plaque when
compared with crowns with supragingival margins.
In a study on dogs, MARCUM found that crown margins
located at the crest of the gingival caused less inflammation than
either those below or above the gingival crest. HARRISON
supported this view and pointed out that possibly the reason for
successful crown margins located at the gingival crest was the
presence of keratinized epithelium in this region, unlike the
epithelium deeper in the gingival sulcus.
Ritcher and Veno reported that there was no difference
between subgingival and supragingival margins in a 3 year clinical
study. ESSMANN et al and KOTH made a similar recommendation.
These studies merely demonstrate that margin location is not as
11
crucial when placed properly. Crown fit and finish may be more
significant to gingival health than the location of the margin.
However whenever possible margins are placed
supragingivally on the enamel of the anatomic crown.
Advantages of Supragingival Placed Margins
- Favourable reaction of gingiva.
- Wider shoulder tooth preparation can accommodate an
adequate bulk of porcelain without-pulpal injury.
- Metal margin finishing is easy.
There are some situations which require intracrevicular
margin placement they are:
- Esthetic demands.
- Caries removal.
- Subgingival tooth fracture.
- Cover existing subgingival restorations.
- To gain needed crown length.
- To provide a more favourable crown contour that is
furcation involvement.
12
Berman has given a method of placing the margins
subgingivally with a collar of metal.
First step is to prepare the tooth to the crest of the gingiva.
Gingival retraction is obtained with a chord or electrosurgery. A
diamond point with an angled tip of calibrated length is introduced
to prepare the bevel. This instrument eliminates the sharp edge of
the shoulder and the undercut which extends apically from the
shoulder.
Margin Configurations:
There are four basic types of finish lines:
- Shoulder – PARDO in 1982 horizontal.
- Beveled shoulder.
- Chamfer – PARDO in 1982 Inclined vertical.
- Knife edge or feather edge.
The question seems to be whether to use shoulders or
chamfers with or without bevels or whether which of these
acceptable forms should be used in which situation.
13
Miller and Belsky in 1965 advocated a full-shoulder
preparation.
Stein in 1977 recommended a uniform chamfer with bevel.
Preston in 1977 advocated a multiple approach using chamfers
with or without bevels in non-porcelain bearing areas shoulders with
bevels in porcelain bearing areas.
Other authors:
Rosner D., Sozio R.B., Berman and Preston strongly
advocated a beveled finishline, pointing out as superior marginal
closing through the “SLIP-JOINT” effect.
There are studies either to support or reject the commonly
held opinions on marginal configurations. Two areas have been
tested.
El-Ebrashi et al studied experimental stress analysis on
photoelastic models as it relates to differing marginal geometry.
They found that the shoulder with a rounded internal line angle and
the chamfer showed the least stress concentration and that shoulders
14
with bevels and feather edges showed the most stress concentration.
In general they showed that margins with relatively large bulk and
no sharp line angles were superior to margins with either sharp or
acute line angles. FARAH and CRAIG found the similar results.
They showed that the chamfer was the optimum marginal
configuration.
Other area of research tested has been the effect of porcelain
firing distortion on different marginal shapes. SHILLINGBURG et
al found that under routine firing cycles used to condition and apply
porcelain to metals, the shoulder configuration with or without bevel
showed significantly less distortion than did a shamfer with or
without a bevel. This is because of metal found at the internal line
angle of a shoulder when compared with a chamfer.
The consensus on the matter of marginal configurations seem
to be that feather edge or knife edge margins are usually not
desirable. But this does not imply that we should throw reason and
experience to the wind and just stick to the shoulder and chamfer.
15
There can be a situation where knife edge margin can have a distinct
advantage.
Chamfer margin: Jacobsen and Robinson defined chamfer as width
greater than 0.3mm at its cervical termination precluded any margin
being called a chamfer. A chamfer margin is particularly suitable for
cast metal veneers. It is an obtuse angled gingival termination. It is a
concave extracoronal finishline that possesses greater angulation
than a knife edge with less width than a shoulder. This finish line
has been shown experimentally to exhibits the least stresses so that
the cement underlying it will have less likelihood of failure.
The most suitable instrument for making a chamfer margin is
the tapered diamond with a rounded tip, the margin formed is the
exact image of the instrument.
Tilting the bur away from the tooth will create an undercut,
angling it toward the tooth will lead to overreduction.
A heavy chamfer is used to provide a 90° cavosurface angle
with a large radius rounded intraoral angle.
16
Sometimes it may create an undesirable fragile “lip” of
enamel at the cavosurface. The heavy chamfer provides better
support for a ceramic crown.
Shoulder Margin: Defined using marginal geometry, where the
discriminating features are an external cavosurface angle of 90° and
a corresponding butt joint of restoration / tooth at the margin.
The shoulder has long been the finish line of choice for the
all-ceramic crown. The wide ledge provides resistance to
occlusal forces and minimizes stresses that might lead to
fracture of porcelain.
It produces the space for healthy restoration contours and
maximum esthetics.
It requires the destruction of more tooth structure than any
other finish line.
The sharp 90° line angle associated with shoulder concentrates
stress in the tooth and is conducive to coronal fracture.
17
To overcome this a modified shoulder line is used with the
shoulder width being slightly lessened by the rounded internal
angle. So that the stress concentration is less. This type is also
called as RADIAL SHOULDER.
Full shoulder usage was increased after the introduction of
injectable ceramics such as dicor and cerestore, hiceram, IPS
empress.
The shoulder with a beveled margin is often recommended for
the facial surface of a metal-ceramic restoration where a metal
collar is to be used.
It is also utilized as the gingival finish line on the proximal
box of inlays and onlays, and for the occlusal shoulder of
onlays and mandibular three quarter crowns.
It can be used in those situations where a shoulder is already
present, either because of destruction by caries or the presence
of previous restorations.
18
The beveling removes unsupported enamel and may allow
finishing of the metal.
Beveled shoulder is referred to as a biologic and esthetic
finish line. Esthetic, because the metal margin can be thinned
to a knife edge and hidden in the sulcus without the need to
position the margin closer to the epithelial attachment.
It is a good finish line for preparations with extremely short
walls, since it facilitates axial walls that are nearly parallel.
According to Giboe and Thayer an advantage of beveled
shoulder preparation were that they allowed the incorporation
of physiologic contours in tooth the temporary and final
crown.
KNIFE EDGE MARGIN
The ultimate in finish lines that permits an acute margin of
metal is the KNIFE EDGE.
19
Unfortunately its use can create problems unless it is cut
carefully, the axial reduction may slide out instead of
terminating in a definite finish line.
The thin margin of the restoration that fits this finish line may
be difficult to accurately wax and cast resulting in
overcontoured restorations when an attempt is made to obtain
adequate bulk.
Inspite of its drawbacks in some situations knife edge margins
has a distinct advantage.
- Used on lingual surfaces of mandibular posterior teeth.
- On teeth with very convex axial surfaces and on the
surfaces toward which a tooth may have tilted.
- In younger patients.
- On cementum.
- Privilege preparations and outline of partial veneer
crowns.
20
Historically their main advantage was that they facilitated the
making of impressions with rigid modeling compound in
copper bands (a technique which is severely used today),
because there was no ledge on which band covered catch.
MARGINAL FIT
According to ADA specification No. 8 the marginal adaptation
of cemented castings should be in the range of 25µm. This
range is below the range of visual acuity.
Recently Gavelis et al used an experimental design intended to
eliminate all casting error and measure only the marginal
opening.
They found that feather edges and shoulders and chamfers
with parallel bevels had the least opening. This showed that
‘slip joint’ margins provide the least marginal discrepancy.
Rosner showed a mathematical trigonometric analysis of
marginal discrepancy. He said that bevels have been
advocated as a means of diminishing marginal discrepancy.
21
If the vertical discrepancy in fit is designated as D, the closest
distance between the margin and the surface of the preparation
is a line d that is perpendicular to the surface of the tooth.
Figure - Refer Shillingburg
Now it can be stated as a function of D
d = D sin µ -------i
d = D cos ------ii
As µ becomes smaller (more acute). The sine of µ becomes
smaller or becomes larger (more obtuse) cosine becomes
smaller.
By either computation d diminishes by the same amount.
The more acute the angle of margin or more obtuse the angle
of finish line. The shorter the distance between finishline and
margin.
However this is true only if there is no cement between the
restoration and the preparation.
22
The presence of cement changes the scenario completely as
suggested by osteoid.
The film thickness of the cement will prevent the complete
seating of a casting with bevels that are nearly parallel with
the path of insertion.
Film thickness imposes a limit on the reduction of the
perpendicular distance from the margin to the tooth d. The
distance d, therefore becomes a constant and the previous
equation is solved for D instead of d.
D = d/sinµ -----iii D = d/cos ------- iv
As angle of margin bevel more acute since –smaller on as
angle of finish line become more obtuse –eosine-smaller and
D become larger.
The more nearly the bevel pallels the path of insertion, the
greater the distance by which the restoration fails to seat.
23
If a bevel of 45° is added to a shoulder, the crown will be
prevented from seating by a factor of 1.4.
30° - the crown is displaced twice.
15° - factor 3.9.
5° - factor 11.5
Mclean and Wilson have disputed the use of bevels for metal-
ceramic crowns because the bevel margin must be 10-20° to
noticeably improve adaptation.
Panno and associates reported no better adaptability of crowns
with highly acute 80° bevels than those with less acute 45°
bevels.
Pascoe in 1978 showed the trigonometric analysis of
relationship between internal casting discrepancy and
marginal opening.
Taking Rosner’s equation as a basic theorem Pasco applied it
to the internal discrepancy.
24
As Rosner showed the marginal opening of a vertically
displaced casting (d) is related to the displacement (D) by the
formula:
d = D cos -------(i)
Where = angle of the bevel
From the figure it can be seen that this theorem is valid only
when there is:
i) Initial adaptation of the
casting is exact.
ii) There is a purely vertical
displacement of the casting such that point Y 2 is directly
above Y1.
Now this situation can arise as a result of 1) Cement film
thickness, 2) defect on the internal surface of the casting.
Distance between the axial wall of the tooth and casting is
designated as f.
Now we can derive the equations for internal discrepancy.
25
D = f/cos ------------(ii)
Equating (i) and (ii)
D = d/cos = f/cos
= f cos = d cos
d = f. cos /cos -----------(iii)
Thus a direct relationship is shown between the internal
discrepancy and marginal opening.
Rosner said that a 5 degrees is the minimum acceptable taper.
Most recommended marginal bevel is 45°.
According to ADA specification No. 8 the cement should meet
the thickness of 25µm.
By substituting these values in equation (iii).
f = 25µm
= 85°
= 45°
d = f.cos /cos = 25.cos 45/cos 85°
= 25 x 0.707/0.087
= 203µm
26
So the marginal opening would be 203µ which is considerably
more than the 40µ that has been advocated as acceptable
value.
Similarly by compacting for undersized cemented casting –
277µ.
Oversized cemented casting = 25µ.
So Pascoe demonstrated that slightly oversized casting with
shoulder exhibit the least marginal discrepancy.
Functions of a bevel:- Bulk of material.
- Protection of enamel rods at the margin.
- Allowance for burnishing and cementation.
- Development of circumferential retention.
27
SUMMARY & CONCLUSION
Principles of tooth preparation can be categorized into
biologic, mechanical and esthetic consideration. If too much
emphasis is given for any one of the principle then the success of the
procedure may be limited by a lack of considerations of other
factors.
The structure of a restoration must be sound and have
sufficient strength to prevent it from being permanently deformed
during function.
The margin is one of the components of the cast restoration
must susceptible to failure, both biologically and mechanically. If all
the principles of tooth preparation are achieved in a restoration
excluding marginal integrity the prosthesis may become detrimental
to the dental tissues, finally leading to the failure of the prosthesis.
The quality of margin may be of as much importance to gingival
health as location.
28
As long as fixed prosthodontics must rely on the cemented
castings, the search for more knowledge about an innocuous,
esthetic, indestructible margin must continue.
29
REFERENCES:
1. Abrahams E.J. : Combination of shoulder-feather edge veneer
crown preparation. JPD, 13: 901, 1963.
2. Becker M.C. et al : Current theories of crown contours margin
placement and pontic design. JPD, 45: 268, 1981.
3. Behrand D. : Cerammometal restoration with supragingival
margins. JPD, 47: 625, 1982.
4. Bridger D.V. : Distortion of ceramometal FPD during firing
cycle. JPD, 45: 507, 1979.
5. Bryant R.A. : Measurement of distortions in FPD resulting
from degassing. JPD, 42: 515, 1979.
6. Donovan T. : An analysis of margin configuration for metal-
ceramic crowns. JPD, 53: 153, 1985.
7. Faucher R.R. : Distortion related to margin design in porcelain
fused to metal restoration.
8. Gardiner F.M. Margins of complete crowns – A review. JPD,
48: 396, 1982.
30
9. Grajower R. : A mathematical treatise on the fit of crown.
JPD, 49: 663, 1980.
10.Hunter A.J. :Gingival crown margins configurations – A
review. JPD, 64: 548, 1990.
11.Jacobson P.H. et al : Basic techniques and materials for
conservative dentistry. J Dent, 9: 101, 1981.
12.Kashani H.G. et al : The effect of bevel angulation on
marginal integrity. JADA, 103: 882, 1981.
13.Panno: Evaluation of the 45° labial bevel with a shoulder
preparation. JPD, 56: 655, 1986.
14.Pardo G.I. : A full cast restoration design of firing superior
maginal characteristics. JPD, 48: 539, 1982.
15.Pascoe D.F. : Analysis of the geometry of finishing liner for
full crown restorations. JPD, 40: 151, 1978.
16.Perel M.L. : Axial crown contours. JPD, 25: 642, 1971.
17.Prince J. : The all porcelain labial margins JPD, 50: 185,
1983.
31
18.Prince J. : The all porcelain labial margin for ceramo-metal
restoration – A new concept. JPD, 50: 793, 1983.
19.Rosensteil : Contemporary fixed prosthodontics, 2 nd edition.
20.Rosner D. : Function, placement and reproduction of bevels
for gold castings. JPD, 13: 1161, 1963.
21.Samuel E.G. : Multiple preparations for fixed prosthodontics.
JPD, 23: 529, 1970.
22.Schweikert E.O. : Feather edged or knife edged and
impression techniques. JPD, 52: 243, 1984.
23.Schwartz I.S. : A review of methods and techniques to
improve the fit of cast restorations. JPD, 56: 219, 1986.
24.Shillingberg H.T. :Preparation design and margin distortion in
PFM restoration. JPD, 29: 276, 1973.
25.Shillingburg H.T. : Fundamentals of tooth preparations.
26.Shillingburg H.T. : Fundamentals of FPD 3 rd edition.
27.Smyd E.S. : The role of torque, torsion, and bending in
prosthetic failures. JPD, 11: 95, 1961.
28.Tylman: Theory and practice of FPD 8 th Edition.
32
29.Watson: Margin placement of esthetic veneers crowns. JPD,
45: 499, 1981.
30.Willis C.M. :Distortion in dental soldering as affected by gap
distance. JPD, 43: 272, 1980.
33
STRUCTURAL DURABILITY AND MARGINAL INTEGRITY
CONTENTS
Introduction
Structural Durability
- Alloy Selection
- Adequate Tooth Reduction
o Occlusal Reduction
o Axial Reduction
o Reinforcing Struts
- Poor Metal-Ceramic Framework Design
Marginal Integrity
- Margin Placement
- Margin Configurations
- Margin Fit
Summary & Conclusion
Bibliography
34