Abutment /orthodontic seminars

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

CONTENTS
- Introduction
- Definitions
- Ideal Requirements
- Contraindication
- Clinical examination of an abutment
- Diagnostic casts
- Radiographic examination.
- Evaluation of roots and their supporting tissues.
- Crown - root ratio
- Root configuration
- Periodontal ligament area
- Examination of crown of the tooth.
- Biomechanical consideration

Special considerations
- Pier abutment
- Tilted molar abutment
- Abutment for the cantilever FPD
-Endodontically treated teeth as abutments.
-Questionable abutments
-Detection of loose abutments.
Abutments for removable partial dentures
Abutments for immediate over denture
-Summary

INTRODUCTION:
The need for replacing missing teeth is obvious to the
patient when the edentulous space is in the anterior segment of
the mouth, but it is equally important in the posterior region too.
Missing teeth can be replaced by one of the following
prosthesis types.
1) Removable denture - Complete
- Partial denture
2) Tooth supported fixed Partial denture
3) Implant supported fixed Partial denture .
It is not uncommon to combine two types in the same
arch, such as a RPD and a FPD. When a missing tooth is to be
replaced, the majority of patients prefer a FPD. The usual
configuration for a FPD utilizes an abutment tooth on each end of
the edentulous space to support the prosthesis. If the abutment
teeth are periodontally sound, the edentulous span is short and
straight and the retainers are well designed and executed.

DEFINITION :
• Abutment : A tooth, portion of a tooth/root or an implant used
for support and retention of fixed or removable prosthesis.
• Intermediate abutment: An abutment located between
abutments, that form the end of the prosthesis.
Multiple abutments: Abutments splinted together as a unit to serve as
support and retention of a fixed prosthesis.

IDEAL REQUIREMENTS:
• An abutment should be a vital tooth.
• However a tooth that has been endodontically treated and is
asymptomatic, with radiographic evidence of a good seal and
complete obturation of canal, can be used as an abutment.
• The tooth must have some sound, surviving coronal tooth
structure to insure longevity.
• The supporting tissues surrounding the abutment teeth must
be healthy and free from inflammation.
• The optimum crown - root ratio for a tooth to be utilized as
a FPD abutment is 1:2. A ratio of 2:3 is considered adequate. A
ratio of 1:1 is the minimum ratio that is acceptable for a
prospective abutment.
• Abutment root should be broader labiolingually than
mesiodistally.

CONTRAINDICATION:
Teeth that have been pulp capped in the process of preparing
the tooth should not be used as FPD abutments, Unless they are
endodontically treated. Because there is a risk that they will require
endodontic treatment later, with the resultant destruction of the
retentive tooth structure.
CLINICAL EXAMINATION OF AN ABUTMENT:
Each abutment tooth should be examined for -
• Dental caries
• Decalcification, mobility, erosion
• Abrasion
• Attrition and
• Sensitivity or fractures

DIAGNOSTIC CASTS:
Articulated diagnostic casts can provide good
information for detecting and diagnosing the problems.
The length of the abutment tooth can be accurately
gauged to determine the preparation designs.
The true inclination of the abutment teeth will also
become evident.
Mesiodistal drifting, rotation and faciolingual
displacement of prospective abutment teeth can also be
clearly seen.

RADIOGRAPHIC EXAMINATION:
OPG and IOPA s are taken
• Radiographs should be examined carefully for signs of
caries, both on unrestored proximal surfaces and recurring around
previous restorations.
• Presence of periapical lesions and quality of previous
endodontic treatments can be evaluated.
• General alveolar bone levels, with particular emphasis on
prospective abutment teeth should be observed.
• The crown root ratio of the abutment can be calculated.
• The length, configuration and direction of those roots should
also be examined.
• Widening of PDL ligament can be detected.
• An evaluation can be made of the thickness of the cortical
plate of bone around the teeth and trabeculation of the bone.
• The presence of retained root tips beside the abutment tooth
can also be detected through radiograph.

EVALUATION OF THE ROOTS AND THEIR
SUPPORTING TISSUES:
The supporting structures around abutment teeth must be
healthy. Normally the abutment teeth should not exhibit
mobility since they will be carrying an extra load. Roots and
their supporting tissues can be evaluated for the following
factors.

1) The 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. The ideal crown root ratio for a
tooth to be utilized as a FPD abutment is 1:2, however 2:3
ratio is considered adequate.
However there are situations in which a crown root
ratio greater than 1:1 might be considered adequate. If the
opposing tooth is artificial tooth, occlusal force will be
diminished, with less stress on the abutment teeth.
For the same reasons, an abutment tooth with a less than
desirable crown root ratio is more likely to successfully
support a FPD if the opposing occlusion is composed of
mobile, periodontally involved teeth

2) Root configuration:
This is an important point in the assessment of
abutments suitability from a periodontal standpoint.
 • Roots that are broader labiolingually than they are
mesiodistlally 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 some curvature in the
apical 3rd of the root is preferable to the tooth that has a
nearly perfect taper.

Maxi. Central--------------------
Maxi. Lateral--------------------
Maxi. Cuspid--------------------
Maxi. 1st Bi cuspid-------------
Maxi. 2nd Bi cuspid------------
Maxi. 1st molar-----------------
Maxi. 2nd molar----------------
Mandibular central-------------
Mandibular lateral--------------
Mandibular cuspid--------------
Mandibular 1st Bi cuspid------
Mandibular 2nd Bi cuspid-----
Mandibular 1st molar-----------
Mandibular 2nd molar----------
Average surface area
in Sq mm
204
179
273
234
220
433
431
154
168
268
180
207
431
426
Type of tooth

Periodontal ligament area:
Another consideration in the evaluation of
prospective abutment teeth is the root surface area, or
the area of PDL attachment of the root to the bone.
Larger teeth have a greater surface area and
are better able to bear added stress.
Jepsen in 1963 conducted a study to measure the
root surface of the abutment and a method for X-Ray
determination of root surface area. He reported that
the average root surface areas of various teeth were
as follows.

Factors modifying Ante’s law:
Condition existing
1. Bone loss from PDL disease----
2. Medial or distal tipping or changes in
axial inclination
3. Migration of abutment teeth
decreasing mesiodistal length of
edentulous area
4. Less than favourable opposing arch
relationships producing increasing
occlusal loads
5. Endodontically restored teeth as
abutments with root resection
6.Arch from situations creasing greater
leverage factors
7.Tooth mobility created after osseous
surgery
Probable modification
Increase number of abutments
Decrease number of abutments
Increase number of abutments.

EXAMINATION OF CROWN OF THE TOOTH:
In this we should examine for -
• Crown condition
• Crown strength
• Crown size
• Crown shape
• Crown surface area
• Crown appearance
• Degree of eruption
• Pulp

i) Crown condition:
If the crown is carious and heavily filled it is always
desirable to remove the caries and an existing filling and then re-
restore.
ii) Crown strength:
Caries existing restorations or endodontic treatment may
have weakened abutment crown. So the extend of caries either
primary or secondary caries must be known before type of retainer
selection.
iii) Crown size :
Any tooth which has less than 4mm inter proximal height
from the marginal ridge to the gingival attachments is unsuitable
for extra coronal restorations. Pins and posts may be used for
extra retention in case of short crowns.
veneer. Full veneer crown retainers may overcome the problems of
discolored abutment crown.

vii) Degree of eruption :
This is the most important factor to determining the amount
of retention available. The preparation can be nearly ideal with
minimum convergence.
viii) Pulp :
The size of the pulp can be assessed by radiograph chance of
exposure of pulp is more particularly in lower first molar where the
mesiobuccal horn often remains large.
.

.
BIOMECHANICAL CONSIDERATIONS :
Bending or deflection of the FPD varies directly with
the cube of the length and inversely with the cube of the
occlusogingival thickness of the pontic

Compared with a FPD having a single tooth pontic span, a 2
tooth pontic span will bend 8 times as much. A 3-teeth
pontic will bend 27 times as much as a single pontic.
Double abutments are sometimes used as a means of
overcoming problems created by unfavorable crown root
ratios and long spans. There are several criteria for the
secondary abutments.

A secondary abutment must have at least as much root surface area
and as favorable a crown root ratio as the primary abutment.
A canine can be used as a secondary abutment to a first
premolar primary abutment, but it would be unwise to use a lateral
incisor as a secondary abutment to a canine primary abutment.
When the pontic flexes, tensile forces will be applied to the
retainers on the secondary abutments. Also there should be
sufficient crown length and space between adjacent abutments to
prevent impingement on the gingiva under the connector

When pontics lie outside the interabutment axis line, the pontics act
as a lever arm, which can produce a torquing movement. This is a
common problem in replacing 4 maxillary incisors with a FPD. This
can be best accomplished by gaining additional retention. i.e., the
first pre molars sometimes are used as secondary abutments for a
maxi. 4 pontic canine to canine FPD. Because of the tensile forces
that will be applied to the premolar retainers, they must have
excellent retention.

Special considerations :
1)Pier abutments :
An edentulous space can occur on both sides of a tooth,
creating a bone, free standing abutment called as pier abutment.

Studies in periodontometry have shown that the faciolingual
movement ranges from 56-108 µm and intrusion is 28 µm. Teeth in
different segments of the arch move in different directions. These
movements can create stresses in a long span bridge that will be
transferred to the abutments.

It has been stated that, forces are transmitted to the terminal
retainers as a result of the middle abutment acting as a fulcrum,
causing failure of the weaker retainer.
In this situation rigid restoration is not indicated. The non
rigid connector has been suggested as a solution to this problem.
A non-rigid FPD transfers shear stress to supporting bone
rather than concentrating it in the connectors. It minimizes
mesiodistal torquing of the abutments while permitting them to
more independently.

The location of the stress breaking device in the fine unit fix
abutment restoration usually is placed on the middle abutment,
since placement of it on either of the terminal abutments could
result in the pontic acting as a lever arm.

2. Tilted molar abutments:
Another problem that occurs with some frequency is the mandibular 2nd molar
abutment that has tilted mesially. It is impossible to prepare the abutment teeth for a FPD along
the long axes of the respective teeth and achieve a common path of insertion.
A helical up righting spring is inserted into a tube on the banded molar and activated by hooking
it over the wire on the anterior segment. The average treatment time required is 3 months.

If the tilting is slight, the problem can be solved by recontouring
the mesial surface of the 3rd molar.
If the tilting is severe, the treatment of choice is the up
righting of the molar by orthodontic treatment.
Up righting is best accomplished by the use of a fixed
appliance. Both premolars and the canine are banded and tied to a
passive stabilizing wire.

There are other treatment options.
• A proximal half crown sometimes can be used as a retainer on
the distal abutment.
•

A telescope crown and coping can also be used as a retainer on
the distal abutment. A full crown preparation with heavy
reduction is made to follow the long axis of the tilted molar. An
inner coping is made to fit the tooth preparation and the proximal
half crown that will serve as the retainer for the FPD is fitted
over the coping.
·

The non rigid connector is another solution to the problem
of the tilted FPD abutment.

3 Abutment for the cantilever FPD:
A cantilever FPD is the one that has an abutment or
abutments at one end only, with the other end of the
pontic remaining unattached
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.

Prospective abutment teeth for cantilever FPDs should be evaluated
with an eye toward lengthy roots with a favorable configuration,
long clinical crowns, good crown root ratios and healthy
periodontium.
A cantilever can be used for replacing a maxillary lateral
incisor. There should be no occlusal contact on the pontic in either
centric or lateral excursions. The canine must be used as abutment,
and it can serve in the role of solo abutment only if it has a
long root and good bone support.

A cantilever pontic can also be used to replace a missing first premolar
For this purpose, full veneer retainers are required on both the
second premolar and the first molar. These teeth must exhibit
excellent bone support.

4. Endodontically treated teeth as abutments:
Endodontically treated teeth should not be used as
abutments for distal extension RPDs. They are more than 4 times
as likely to fail than pulp less teeth not serving as abutments. Pulp
less FPD abutment teeth fail nearly twice as often as single teeth.
There is no contra-indication to use pulp less tooth as a part of
bridge if there is a satisfactory root filling.

QUESTIONABLE ABUTMENTS :
Classification of questionable abutments in FPD :
The following outline is presented as a guideline for
identification of teeth that are difficult to use as an abutment for
FPD.
CLASSIFICATION :
I)GENERAL DISORDERS:
A) Mineralization
1) Amelogenisis imperfecta
2) Dentinogenisis imperfecta
3) Hypo calcification
4) Ectodermal dysplasia
5) Discoloration due to drugs like tetracycline
6) Flouridosis
7) Internal resorption.

Skeletal B) Congenital and growth deformities
1) Malformed dentition
2) Malposed teeth
3) disparities of maxillomandibular relationships
4) Oligodontia
II) LOCAL PROBLEMS ASSOCIATED WITH
QUESTIONABLE ABUTMENTS:
A) Poly carious tooth
B) Periodontally involved teeth
C) Occlusal plane correction
D) Endodontically treated teeth
1) Previously treated teeth
2) Currently treated teeth
E) Tilted teeth
F) Attrition, abrasion, or erosion

Detection of loose abutment :
A loose abutment can be detected by pulling occlusally on
the splint or bridge, then drying the gingival margins and pressing
the appliance into place. If saliva comes out, that shows that the
abutment is loose.
Another clinical sign is a foul odor about which the patient
complains or with the operator detects in an otherwise clean mouth.
Abutments for removable partial dentures :
The requirements of an abutment for a RPD are not as
strong as those for a FPD abutment. Tipped teeth adjoining
edentulous spaces and prospective abutments with divergent
alignments may tend themselves more readily to utilization as RPD
rather than FPD abutments.

Periodontally weakened primary abutments may serve
better in retaining a well-designed RPD than in bearing the
load of a FPD.
Teeth with short clinical crowns or teeth that are just
generally short usually will not be good FPD abutments.
An insufficient number of abutments may also be a
reason for selecting a removable rather than a FPD

ABUTMENTS FOR IMMEDIATE OVERDENTURE:
Preparing cast abutments:
Procedure:
• Mark the abutment on the cast indicating the amount of the
tooth to be reduced.
• Shorten the cast abutment with a bur.
• The abutment should be shortened so that a minimum space
of 2-3 mm exists between the abutment preparation and the
opposing tooth.
The abutment is prepared on the cast, removing stone from
the facial, proximal and lingual surfaces. Approximately 60% of
the reduction should come from the facial surface and about 40%
from the lingual surface. The basic purpose of the prepared cast
abutment is to form an indentation in the Overdenture that will be
occupied by the natural abutment tooth. Then the Overdenture is
constructed in conventional manner

Effect of abutment mobility, site, angle of impact on
retention of fixed partial denture:
By Richard Jacobi , T. Shillinburg JPD. 1985; 54: 178/183.
The study included three positions of impacting dowel in relation
to fixed partial denture. They were as follows.
Position A : Force was directed apically at an angle of 900
to
occlusal plane into fossa farthest from pontic area and centered
3mm distal to long axis of abutment die.
Position B : Force was at 900
to occlusal plane, but directed in
to the fossa closest to pontic area and centered 1.5 mm mesial
to long axis of die.
Position C : Force was directed 450
to occlusal plane and
centered on lingual wall of fossa far from pontic area.www.indiandentalacademy.com

Each group was submitted to a mobility of
0.04mm, 0.08 mm and 0.16 mm. 0.08mm was
considered normal by the authors. They observed
that retention of fixed partial denture decreased
when abutment teeth were mobile.
The authors concluded that :
1) Crowns that anchor rigid prosthesis to mobile
teeth require greater retentive ability.
2) Occlusal impacts are best with stood when they
fall on the areas of fixed partial denture over and
between center of rotation of abutments.

Designs of removable partial dentures that are
appropriate to aid in supporting teeth with secondary
occlusal traumatism.
JPD 1947:6; 587 / 584.
1) BAR RESTS
A bar rest is basically an occlusal rest that contacts
the prepared occlusal central fossa of a tooth or group of
teeth. When a bar rest is seated on a prepared tooth, it
provides resistance to movement of contacted tooth from
lateral and or vertical forces. This is best suited to stabilize
mobile teeth when there is no distal extension base.

2) Multiple I-bar stabilization
These are useful for lateral stabilization of mobile
teeth. If there is no distal-extension base, then I-bar
stabilizers can be positioned to contact a 0.01 inch
infrabulge undercut. If there is adequate retention
from other clasps, then I bar stabilizers may be
positioned occlusally to the supra bulge.
If there is a distal-extension base, then only those I-
bar stabilizers that are distal to fulcrum line
(occlusal rest) can engage the 0.01 inch undercut.
Once adequate retention is achieved all other I bar
stabilizers need only contact on suprabulge of
mobile teeth. Regardless of whether I bar stabilizer
engages an undercut or not, it should be plate.

3) Swing lock removable partial dentures
This is alternative treatment for a partially
edentulous patient designated for full-mouth extractions
and complete dentures or for over dentures.
After extensive periodontal therapy, certain
situations require splinting or some other form of
stabilization. In these cases, the swing lock removable
partial denture can provide stabilization through control of
posterior occlusal forces and through anterior and
sometimes posterior splinting.

The removable partial denture as a periodontal
prosthesis.

   DCNA vol. 28 No.2 April 1984.
Components parts of a removable partial denture
that may affect the periodontal condition directly or
indirectly are the following:
Major connector, minor connector, extra coronal direct
retainer (rest, retentive), proximal plate, indirect
retainers, denture base.

Major connector
1) They should be rigid and should not damage the
periodontal support of abutment teeth.
2) They should not impinge on free marginal tissue and
must never depend on gingival margin for support.
3) Highly polished major connectors are more desirable,
to decrease plaque accumulation
4) When periodontally compromised anterior teeth require
stabilization, a special design of major connector should be
used for splinting teeth together. A lingual plate should
extend to the middle third of the lingual surface at
mandibular anterior teeth and coronal border should follow
the natural curvature of the cingulam surface.

Minor connector

By contacting guiding planes, these aids in
distributing, forces to the abutments and in immobilizing the
prosthesis against lateral movement.
Any space less than 5mm between two vertical minor
connectors will have a tendency to accumulate food and
plaque in the area.
Minor connectors associated with bar clasp arms
should be located over keratinized gingiva and should not
interfere with movement of alveolar mucosa and frena. If
adequate attached gingiva does not exist on respective,
mucogingival procedures such as free grafts or pedicle
grafts should be instituted to prevent gingival irritation
and possible future loss of periodontal attachment.

Direct retainers
The circumferential clasp changes the contour of the tooth
and interferes with the normal flow of food over surfaces of
tooth. Thus allowing marginal gingiva to lose physiologic
stimulation.
1) Wrought wire clasps reduces stress on abutment teeth
as compared with a cast circumferential clasp.
2) A periodontally acceptable clasp arm should cover a
minimum of tooth surface.
3) A retentive arm should be tapered uniformly from its
point of attachment at the clasp body to its tip, to minimize
damage to periodontal ligaments of abutment tooth.

Clayton – Jaslow 1971, showed that circumferential clasps
even with wrought wire retentive clasp arms sometimes exert
more force on abutment teeth beyond that required for
orthodontic movement. This force causes increased mobility
of abutment teeth after initial placement of the removable
partial denture.
A rest on mesial side of abutment teeth in distal extension
removable partial dentures will transfer the chewing forces
more perpendicular to ridges than distal occlusal rests. The
gingival mucosa of abutment tooth will be better protected
when mesial occlusal rests are used. The abutment tooth has
more tendencies to rotate mesially, which will be protected by
other teeth in front of abutment tooth.

Movement of abutment teeth and damage to the periodontal
ligament is related to many elements such as
1) Location of rests.
2) Extension of removable partial denture base.
3) Contour and rigidity of direct retainers.

TEBROCK et al 1979 (JPD 41: 511, 1979).
studied three clasping systems – circumferential with
distal rests, 18 gauge wrought wire clasp and the distal
rest, and a buccal I-bar retentive clasp arm. They
concluded that there was no significant mobility of
abutment tooth during 4-week test period with each
clasping system. However, any mobility during increases
were in a buccal direction only. There was never a change
in lingual mobility of abutment.

SHOKET 1969 (JPD 21: 267, 1969),
studied four types of retainers and reported that greatest
degree of destructive distal stress on abutments occurred
with removable partial dentures with circumferential
clasps and precision attachments.

The role of occlusion for the stability of fixed bridges in
patients with reduced periodontal tissue support.
Suture Nyman, Jan Lindhe. J.Clin. Perio. 1975; 2; 53/66
The present investigation reports how occlusion
may be utilized to establish and maintain stability of fixed
bridges in patients with markedly reduced periodontal
tissue support.
The material consisted of 20 adult patients, age 27-69,
with advanced periodontal breakdown often in
combination with extensive loss of teeth. After periodontal
treatment, patients were rehabilitated with fixed bridges
whose stability was evaluated once a year for 2-6 years.

The results show that permanent stability of bridge work can
be obtained in patients where there is a minimum of remaining
periodontal tissue support even in combination with marked
hyper mobility of individual abutment teeth.
The stability was achieved by proper treatment
of diseased periodontal tissues, and establishment of stable
occlusion in the intercuspal position. When there was a risk of
bridge mobility on excursive movements of mandible,
balancing contacts were established for prevention of
migration, tilting and increasing mobility.

SUMMARY
• The success of prosthesis depends on the many foundational
steps taken to prepare it. The proper handling, of abutment teeth is
one of these important foundational steps that either enhances or
detracts from the eventual value of the prosthesis.
• When the conditions are proper like, crown contour, retention
and criteria of good preparation techniques and design are met, sound
abutment considerations will also be a strong link in the success of the
prosthesis.
Deevan stated that preservation is most important than
replacement. In daily practice when we come across abutments which
are mobile or have the history of periodontitis they should not be
advised for extraction. Treatment planning should be done in such a
way so that these mobile teeth can be used as abutments with all
precautionary measures to reduce the amount of occlusal forces acting
on these mobile abutment teeth.

REFERENCES
1) Tylman’s theory and practice of fixed prosthodontics. 8th
ed.
2) Shillinburg. Fundamentals of Fixed prosthodontics. 3rd
ed.
3) Rosenstiel, Land, Fujimoto.Contemporary Fixed
prosthodontics. 3rd
ed.
4) The removable partial denture as a periodontal prosthesis.
DCNA vol. 28 No.2 April 1984.
5) BDJ 2001: vol 191 No. 11, 597-604
6) Fixed bridge prosthodontics. D H .Roberts-2rd
ed

7) JPD 41: 511, 1979.
8) JPD 21: 267, 1969.
9) The role of occlusion for the stability of fixed bridges in
patients with reduced periodontal tissue support.
Suture Nyman, Jan Lindhe.. J.Clin. Perio. 1975; 2; 53/66.
10) Designs of removable partial dentures that are
appropriate to aid in supporting teeth with secondary
occlusal traumatism. JPD 47:6; 587 / 584
11) Effect of abutment mobility, site, angle of impact on
retention of fixed partial denture: By Richard Jacobi ,
T. Shillinburg JPD. 1985; 54: 178/183.

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