Selection and choice of abutments / orthodontics training courses

SELECTION AND CHOICE OF
ABUTMENT
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Abutment Evaluation
Abutment teeth are called upon to withstand the
forces normally directed to the missing teeth, in
addition to those usually applied to the abutments.
Whenever possible, an abutment should be a vital
tooth. However, a tooth that has been
endodontically treated and is asymptomatic, with
radiographic evidence of good seal and complete
obturation of the canal, can be used as an abutment.
The tooth must have some sound, surviving coronal
tooth structure to insure longevity. However, some
compensation can be made through the use of a
dowel core, or a pin retained amalgam or a
composite core.

Teeth that have been pulp capped in the process of
preparing the tooth should not be used as an FPD
abutment unless they are endodontically treated.
The supporting tissues surrounding the abutment
teeth must be healthy and free from inflammation
before any prosthesis can be contemplated.
Normally, abutment teeth should not exhibit any
mobility, since they will be carrying an extra load.
The roots and the supporting tissues should be
evaluated for three factors:
• Crown-root ratio.
• Root configuration.
• Periodontal ligament area.www.indiandentalacademy.com

Crown-root ratio
This ratio is a measure of the length of tooth,
occlusal to the alveolar crest of bone compared with
the length of root embedded in the bone. As the
level of alveolar bone moves apically, the lever arm
of that portion out of bone increases, and the
chances for harmful lateral forces is increased. The
optimum crown-root ratio for a tooth to be utilized
as a FPD abutment is 2:3. A ratio of 1:1 is the
minimum ratio that is acceptable for a prospective
abutment under normal circumstances.

However, there are situations in which a crown
root ratio greater than 1:1 might be considered
adequate. If the occlusion opposing a FPD is
composed of artificial teeth, occlusal forces will be
diminished, with less stress on the abutment teeth.
The occlusal forces against prosthetic appliances
have been shown to be considerably less than that
against natural teeth: 26 lb for removable partial
dentures and 56 lb for fixed partial dentures versus
150 lb for natural teeth.

For the same reasons, an abutment tooth with a
less than desirable crown-root ratio is more
likely to successfully support FPD if the
opposing occlusion is composed of mobile,
periodontally involved teeth than if the teeth
are periodontally sound.

Root configuration
This is an important point in the assessment of an
abutment’s suitability from a periodontal standpoint.
Roots that are broader labiolingually than they are
mesiodistally are preferable to roots that are round
in cross-section.

Multirooted posterior teeth with widely separated roots will
offer better periodontal support than roots that converge,
fuse, or generally present a conical configuration. The tooth
with conical roots can be used as an abutment for a short
span FPD if all other factors are optimal. A single rooted
tooth with evidence of irregular configuration or with some
curvature in the apical third of the root is preferable to the
tooth that has a nearly perfect taper.

Periodontal ligament area / Ante’s law
When the normal complement of roots is not
available to distribute the forces of mastication,
pathologic stress concentrations may result in the
periodontal ligament and supporting bone. This
condition is the most fundamental problem with
which the dentist must contend each time a fixed
prosthesis is designed to replace a missing tooth.
Here we must take into accounts Ante’s law, which
states, “The total periodontal membrane area of the
abutment teeth should equal or exceed that of the
teeth to be replaced’’.

The essential feature of this clinical guideline is that
the actual area of the interface between tooth and
supporting structures must be of a certain minimal
amount to withstand and resist the forces that will
now be transmitted to those supporting structures.
Realistic determination of the area of good, healthy,
periodontal ligament available on a potential FPD
abutment is not an easy matter.

The combined root surface area of the second
premolar and the second molar (A2p+A2m) is
greater than that of the first molar being
replaced (A1m).

The combined root surface area of the first
premolar and the second molar abutment
(A1p+A2m) is approximately equal to that of
the teeth being replaced (A2p+A1m).

The combined root surface area of the canine
and the second molar (Ac+A2m) is exceeded
by that of the teeth being replaced
(A1p+A2p+A1m)

Also of extreme importance is the actual area of contact
between the periodontal structures and the root in question
as it relates to the normal amount of contact area for that
particular tooth in that particular mouth. One can assume
that in a given case, without bone loss, the area of this
contact is optimal. Therefore, any loss of bone support
compared with the optimal situation decreases the chances
of this root being an adequate bridge abutment. If one looks
at the problem in this manner, it becomes apparent that a
root that appears adequate in one situation may be
inadequate in another. The planning and design of a
restoration of this type must have the benefit of sound
clinical judgment and knowledge of basic biomechanical
principles.

Maxillary Maxillary Mandibular Mandibular
Tooth Area mm2 Ranking Area mm2 Ranking
Central 139 7 103 8
Lateral 112 8 124 7
Canine 204 3 159 4
First premolar 149 5 135 6
Second
premolar
140 6 135 5
First molar 335 1 352 1
Second molar 272 2 282 2
Third molar 197 4 190 3
Comparison of root surface areas of 16 teeth

The above table, which compares the root surface
areas of 16 teeth, may aid in visualizing root areas.
It is interesting to note that the addition of abutment
roots will provide a wide variation of additional
support. The addition of such support is not
necessarily proportional to the number of abutments
supporting prosthesis. There fore, the prosthodontist
should not expect a proportional increase in stress-
bearing ability, particularly when the amount of
periodontal ligament is reduced.

The area of the normal periodontal ligament (PDL)
for teeth to be replaced by pontics should be less
than the actual PDL area of the existing abutment
candidates.
The values given in the table are averages for the
various teeth in the mouth under ideal conditions.
Two problems are evident when one attempts to
arrive at useful interpretation of such data.

Degree and nature of bone loss
In clinical practice, the decision making process in
which root surface area information is to be used
does not always involve ideal situations. More
often than not, there has been some bone loss; thus,
a tooth with a moderate amount of bone loss may be
still a better candidate for use as a FPD abutment
than another tooth with no bone loss at all. To
determine as to which of the teeth in question has
adequate support for the anticipated loads, there are
few aids on which the dentist can rely, none of them
capable of giving all the information necessary to
make a decision. www.indiandentalacademy.com

These aids are
• Radiographs
• Periodontal probing, and
• Mobility tests.
The radiograph is unquestionably the most useful tool at
the disposal of the dentist in making a determination of
the integrity of the remaining periodontal supporting
structures as related to the loads anticipated. The
opportunity to make radiographs from different projection
angles should not be overlooked, since the primary areas
that can be visualized on radiographs are the mesial and
distal surfaces. Any chance for seeing even a little of the
facial or lingual surfaces should be taken advantage of.

Periodontal probing is the second tool at the disposal at the
dentist and should be used extensively when attempting to
determine the efficacy of using a given tooth as an
abutment. Periodontal probing is a particularly important
step as related to the facial and lingual surfaces, since these
areas of the tooth to supporting structures interface cannot
be viewed adequately on the radiograph.
Finally, standard clinical tests for mobility should be
employed. Any degree of mobility outside the normal
accepted physiologic range should be suspected. It means
either that the loss of supporting structure, whether or not
fully appreciated from the radiographs and probing, is
severe enough to alter physically the stability of the tooth or
that the occlusion has traumatically loosened the tooth.

It is very important to understand which of these processes
is at work in a given abutment situations. Occlusal trauma
is usually reversible, and given the fact that the
prosthodontist is going to construct a restoration on the
tooth in question, there is ample opportunity to correct the
situation. A periodontal defect, however, is not always
reversible and depending on its severity may require a
drastic alteration of the plan of treatment for the tooth in
question. The important concept to keep in mind is that the
prosthodontist must exercise a certain degree of good
judgement in the question of interpretation of the adequacy
of supporting structures in a given situations. Many aspects
of the final restoration and the chances for success are
basically technical and depend on the skill of a particular
prosthodontist. Not only good judgment but a conservative
approach must be hall marks of the thought processes of the
dentist in this all important consideration.www.indiandentalacademy.com

Ante’s law says that in a situation where the
preceding values are normal, prosthesis to replace
the maxillary first molar would need abutment teeth
with at least 335 mm2 of periodontal membrane.
This requirement is theoretically well satisfied in the
case because of the total average area of the second
molar and second premolar is 412mm2, however,
has there been enough loss of bone on these two
teeth to result in, for example, a total of only
300mm2, the law would not be satisfied.

Tilted molar abutment
Titled abutment teeth are a common problem that
must be addressed in construction of fixed partial
prostheses. The tooth to be replaced by the
restoration frequently has been missing for a long
time. Therefore, the tooth distal to the missing one
often will have tilted into the space. It is impossible
to prepare the abutment teeth for a fixed partial
denture along the long axis of the respective teeth
and achieve a common path of insertion. There is
further complication if the third molar is present. It
will usually have drifted and tilted with the second
molar. www.indiandentalacademy.com

Because the path of insertion of the fixed partial denture
will be dictated by the smaller premolar abutment, it is
probable that the path of insertion will be nearly parallel
to the former long axis of the molar abutment before it
tilted mesially. As a result, the mesial surface of the
tipped third molar will encroach upon the path of
insertion of the fixed partial denture, thereby preventing
it from seating completely.

Some of the possible solutions to these problems
are:
Preparation modifications: The design of the
preparation could be modified to be in harmony
with the line of draw requirements of the other
abutment and adjacent teeth while at the same time
satisfying all other preparation criteria, such as
retention and protection of the pulp. A proximal half
crown can be used as a retainer on the distal
abutment. This retainer can be used only if the distal
surface itself is untouched by caries or
decalcification and if there is very low incidence of
proximal caries throughout the mouth.www.indiandentalacademy.com

If there is a severe marginal ridge height
discrepancy between the distal of the second molar
and the mesial of the third molar as a result of
tipping, the proximal half crown is
contraindicated.

Telescopic crown designs: A two piece restorations is
constructed whereby the line of the draw of one
component (seated on the tipped tooth preparations) is
such that it favors the tooth. The line of the draw of the
component is then in harmony with the other abutment
preparation.

Broken connectors: In these situations it is
desirable to connect units of fixed bridges in
some manner that will allow the various
components of the prosthesis to be seated
separately.

Pre-prosthetic orthodontics: Some degree
of uprighting the tooth may be accomplished
orthodontically.

Composite resin bonded prosthesis
The most recent innovation in multiple unit
restorations is the composite resin bonded
prosthesis. Utilization and popularization of this
technique is based on the ability to etch certain high
modulus, non precious alloys. After etching, the
metal can be placed after only a minimum of tooth
reduction. To accomplish the goals of this
conservative restoration, one must make the metal
frame work thin and in-conspicuous which has led
to FPD’s with minimal structural integrity.

The essential features of this type of restoration
have included:
• Minimal axial reduction lingually at the height of
contour.
• 1 mm deep occlusal rests inclined toward the
center of the abutment teeth.
• 180-degree proximal wraparounds approximately
0.4mm thick.
• A distinct path of insertion.
• For anterior abutments, bonded cingulum rests
have been advocated.

When these composite resin bonded prosthesis are
subjected to occlusal loadings, very high complex
stresses are generated at the connector areas and
extend into the high flexure of the wraparound arms.
These high flexural stresses are transmitted to the
resin adhesive. During function, the bridge is
subjected to a large number of chewing cycles,
which may be translated into fatigue failure of the
adhesive layer

When the thickness is increased, a substantial
decrease in the level of stress concentration results.
Another means to substantially reduce the level of
stresses within the frame work is to include
occlusogingival extensions adjacent to the
extraction site.
The occlusal rests are also important structural
elements in the transmitting of forces from the
pontic to the abutment teeth. A similar structural
support may be obtained by preparing a ledge on
which the occlusogingival extension rests. This
support is, in essence, a very minor box preparation.
There are pros and cons to both approaches, but one
of these two rest concepts should be used.

Structural considerations for the success of this
technique should include:
• Wraparound arms as thick as possible consistent
with reasonable tooth contour.
• Occlusogingival proximal extensions and
• A sound rest, whether it is on the occlusal surface
or in the form of a gingival box.

Canine replacement fixed partial dentures
Fixed partial dentures replacing canines can be difficult
because the canine often lies outside the interabutment axis.
The prospective abutments are the lateral incisors, usually
the weakest tooth in the entire arch, and the premolar, the
weakest posterior tooth. A fixed partial denture replacing a
maxillary canine is subjected to more stresses than that
replacing a mandibular canine, since forces are transmitted
outward (labially) on the maxillary arch, against the inside
of the curve (its weakest point).

On the mandibular canine, the forces are directed
inward (lingually), against the outside of the curve
(its strongest point).
Any fixed partial denture replacing a canine should be
considered a complex a fixed partial denture. No fixed partial
denture replacing a canine should replace more than one
additional tooth. An edentulous space created by the loss of a
canine and any two contiguous teeth is best restored with a
removable fixed partial denture.www.indiandentalacademy.com

Cantilever fixed partial dentures
A cantilever fixed partial denture is one that has an
abutment or abutments at one end only, with the other end
of the pontic remaining unattached. This is a potentially
destructive design with the lever arm created by the pontic.
In a routine three-unit fixed partial denture, force that is
applied to the pontic is distributed equally to the abutment
teeth. If there is only one pontic and it is near the
interabutment axis line, less leverage is applied to the
abutment teeth or to the retainers than with a cantilever.
When a cantilever pontic is employed to replace a missing
tooth, forces applied to the pontic have an entirely different
effect on the abutment tooth. The pontic acts as a lever that
tends to be depressed under forces with a strong occlusal
vector.

Prospective abutment teeth for cantilever fixed partial
dentures should be evaluated with an eye towards lengthy
roots with a favorable configuration, long clinical crowns,
good crown-root ratios, and healthy periodontium.
Generally, cantilever fixed partial dentures should replace
only one tooth and have at least two abutments.
A cantilever can be used for replacing a maxillary lateral
incisor. There should be no occlusal contact in either centric
or lateral excursions. The canine must be used as an
abutment, and it can serve in the role of solo abutment only
if it has a long root and good bone support. There should be
a rest on the mesial of the pontic against a rest seat
preparation in an inlay or other metallic restoration on the
distal of the central incisor to prevent rotation of the pontic
and the abutment. www.indiandentalacademy.com

The mesial side of the pontic can be little ‘wrapped around’
the distal portion of the uninvolved central incisor to
stabilize the pontic faciolingually. The root configuration of
the central incisor does not make it a desirable cantilever
abutment.

A cantilever pontic can also be used to replace a missing
first premolar. This scheme will best work if occlusal
contact is limited to the distal fossa. Full veneer retainers
are required on both the second premolar and first molar.
These teeth must exhibit excellent bone support. This design
is acceptable if the canine is unmarred and if a full veneer
restoration is required for the first molar in any event.

Cantilever fixed partial denture can also be used to replace
molars when there is no distal abutment present. When used
judiciously, it is possible to avoid the insertion of a
unilateral removable partial denture. Most commonly, this
type of fixed partial denture is used to replace the first
molar, although occasionally it is used to replace a second
molar to prevent supereruption of opposing teeth. When
pontic is loaded occlusally, the adjacent abutment tends to
act as a fulcrum, with a lifting tendency on the farthest
retainer. To minimize the leverage effect, the pontic should
be kept as small as possible, more nearly representing a
premolar than a molar. There should be a light occlusal
contact with absolutely no contact in any excursion. The
pontic should possess maximum occlusogingival height to
ensure a rigid prosthesis.www.indiandentalacademy.com

A posterior cantilever pontic places maximum demands on the
retentive capacity of the retainers. Its use, therefore, should be
reserved for those situations in which there is adequate clinical
crown length on the abutment teeth to permit preparations of
maximum length and retention. The success of cantilevers in the
restoration of the periodontally compromised dentition is
probably due, at least by part, to the fact that periodontally
involved abutments do have extremely long clinical crowns.
While cantilever fixed partial dentures appears to be a
conservative restoration, the potential for damage to the
abutment teeth requires that they be used sparingly.

Double abutment
Many clinical situations require the use of double
abutments in the fixed bridges. The term as used here
refers to the use of two adjacent teeth at one or both ends
of a fixed prosthesis joined by a solid connector. The
usual reasons for use of double abutment are:
• Increase retention of the restorations as a whole
• Splint and stabilize periodontally compromised teeth and
• Increase the area of the supporting PDL and bone.
Improvement of the retentive aspects of the restoration
would seem to be a reasonable justification for including
an extra abutment. This rationale is not always true. As
seen, the second premolar has insufficient coronal dentin
to provide the necessary retention for use as an abutment.
Th assumption was made that adding the extra premolar
abutment would give the bridge adequate retention of the
anterior end.

This abutment would allow retention of the second
premolar root to reduce future bone loss, which
would occur if this tooth were extracted. This latter
point would certainly add credibility to the rationale,
but at least two other more conservative methods
could be considered to render the second premolar a
sound abutment. First, a pin retained intra coronal
casting or build up could be made for the second
premolar if maintenance of the vitality of this tooth
is a prime concern. Second, endodontic therapy and
a retentive post and core could be done on the
second premolar. The latter method would usually
be the method of choice in this situation due to the
greater chance of long term success compared with
the pin buildup. www.indiandentalacademy.com

Either of these options, particularly the post and core, could
obviate the need for double abutting this restoration. The
reason being that by correcting the problem involving the
second premolar, which is lack of retention, the operator has
created a typical three unit prosthesis situation. The preceding
example considered the use of double abutment strictly on the
basis of lack of retention of the primary abutment choice. A
discussion of other reasons for the use of multiple abutments
follows. However, before proceeding, it is advisable to
consider some of the ramifications of using double abutment
as a solution for lack of abutment retention.

During function, the case treated as shown, often
develops a cement failure at the second premolar
because of poor retention characteristics. The other
units will often be retained adequately. Breakdown
of the cement layer of this abutment tooth leads to
slow destruction by action of the saliva and its
acidic components. Had this same loss of retention
occurred in the case of a single unit restoration, it
would have simply come away from the preparation
and the patient would have sought treatment for an
obvious problem. Dislodgement of the restoration
does not occur, however, when other retainers of a
multiple unit restoration remain in place on their
respective abutment teeth.

This problem is difficult to diagnose because “loose”
retainer is still held in its correct position in relation to the
abutment tooth, though no longer cemented. The patient
complains of pain. Since the retainer is still held in its
correct position relative to the abutment tooth preparation,
no marginal opening can be detected, nor can any looseness
or movement.
As can be easily seen, diagnosis of the patient’s complaint
can be difficult, if not impossible, without removal of the
entire restoration. Due to these problems, it is imperative
that precautions be taken in the design and construction of
multiple unit restorations to preclude the loss of retention on
any abutment. Further it is strongly recommended that the
use of double abutments to compensate for lack of retention
on one of the abutment teeth of a fixed prosthesis be
discouraged.

The procedure may be justified from the view point of
maintaining bone, but it is less justifiable when considered
in the light of resistance to the forces to which the
restoration will ultimately be subjected. The alternative of
pins or posts will usually be found to be the treatment of
choice to permit saving of the root.
Splinting and stabilization of a periodontally compromised
tooth can be more valid reasons for the use of double
abutments on a fixed bridge. However, a fundamental
decision must be made early in the planning of the case; is
the mobility the result of a continuing process of
periodontitis, or occlusal trauma. If the mobility of the tooth
is only the result of occlusal trauma, stabilization of such a
tooth in this manner may be perfectly justified, providing
that the trauma can be eliminated in the occlusal scheme of
the restorations.

When a tooth is subjected to occlusal forces that
cannot be controlled, the adjacent tooth might be
added to the restoration as a double abutment to
provide the needed resistance to lateral forces. A
classic example on this situation would be a bridge
replacing a missing maxillary canine. In such a
case, the lateral occlusal forces generated on the
canine pontic are such that the lateral incisor is
seldom an adequate abutment due to its short root
form. It is then justified to add the central incisor to
such design. It has been shown that mobility
resulting from occlusal trauma is reversible once the
cause for the trauma is removed.www.indiandentalacademy.com

On the other hand, if the lack of bone support is due
to periodontal disease, and if the disease is not
totally controlled, using this tooth as part of double
abutment is contraindicated. In such a situation,
bone loss on the affected abutment tooth continues,
with the end result being that this tooth eventually
becomes simply another pontic in the bridge. Also,
pockets become less cleanable after the placement
of the restoration due to poorer access,
compounding the problem.
Finally, the best justification, for using double
abutments is to satisfy Ante’s law. If there are not
enough periodontal ligaments for a given number of
missing teeth, there is no better solution than to add
one or more teeth that do have sound support.www.indiandentalacademy.com

When many missing teeth are replaced by a fixed
restoration using a limited number of abutments,
most of which do not even possess the normal
amount of bone support, failure is assured. One
must make decision whether the addition of more
abutments in the design of the restoration is more
important than satisfying the concomitant
requirement for conservatism. There may be no
choice if the restoration is to be made at all. If it is
not possible to satisfy Ante’s law in this regard, a
removable partial denture should be considered so
that occlusal forces may be distributed cross arch
and to the edentulous ridges.www.indiandentalacademy.com

From the viewpoint of mechanical principles, the advantage
of adding a second abutment at one end of a fixed prosthesis
is that in so doing, we are better able to distribute the forces
that would be applied to the prosthesis. Nothing would be
gained if a crown were placed on the added abutment were
it not connected rigidly to the remainder of the prosthesis.
When the added tooth is made an integral part of the
prosthesis, its periodontal ligaments provide resistance to
forces transmitted by the other abutment at this end of the
bridge. This shared load-bearing responsibility is the
essence of Ante’s law. An additional abutment tooth, or
teeth, is used to replace the missing tooth. Other wise, only
two abutment teeth would be performing the function of
resisting forces applied to three occlusal surfaces.

There is a common problem in replacing all four
maxillary incisors with a fixed partial denture and
the problem is more pronounced in the arch that is
pointed in the anterior. This occurs because the
pontics lie outside the interabutment axis line and
thus acts as a lever arm, which can produce a
torquing movement. In order to offset the torque,
additional retention is obtained in the opposite
direction of the lever arm and at a distance from the
interabutment axis equal to the length of the lever
arm.

The first premolars sometimes are used as secondary
abutments for a maxillary four-pontic canine to
canine fixed partial denture. Because of the tensile
forces that will be applied to the premolar retainers,
they must have excellent retention.

Selection and choice of abutments / orthodontics training courses

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