Preparation of abutment teeth

1. Preparation Of
Abutment Teeth
PRESENTED BY :
DIPANWITA DAS
JR II

2. INTRODUCTION
After surgery, periodontal treatment, endodontic treatment, and tissue
conditioning of the arch involved, the abutment teeth may be prepared to
provide support, stabilization, reciprocation, and retention for the removable
partial denture. Rarely, if ever, is the situation encountered in which
alterations of the abutment are not indicated because teeth do not develop
with guiding planes, rests, and contours to accommodate clasp assemblies.
• A favorable response to any deep restorations, endodontic therapy, and
the results of periodontal treatment should be established before the
removable partial denture is fabricated.
• If the prognosis of a tooth under treatment becomes unfavorable, its
loss can be compensated for by a change in the removable partial
denture design.

3. If teeth are lost after the removable partial denture is fabricated,
then the removable partial denture must be added to or replaced.
When an original abutment is lost, it is extremely difficult to
effectively modify the removable partial denture to use the next
adjacent tooth as a retaining unit.
It is sometimes possible to design a removable partial denture so
that a single posterior abutment that is question- able can be
retained and used to support one end of a tooth- supported base.

4. Classification of abutment teeth
The subject of abutment preparations may be grouped as
follows:
(1)those abutment teeth that require only minor
modifications to their coronal portions
(2) those that are to have restorations other than
complete coverage crowns
(3) those that are to have crowns (complete coverage)

5. • Abutment teeth that require only minor modifications include teeth with sound enamel,
those with small restorations not involved in the removable partial denture design,
those with acceptable restorations that will be involved in the removable partial
denture design, and those that have existing crown restorations requiring minor
modification that will not jeopardize the integrity of the crown.
• The latter may exist as an individual crown or as the abutment of a fixed partial
denture.
complete coverage of all abutments may be desirable, it is not always possible or
practical.
• The decision to use unprotected abutments involves certain risks of which the patient

6. SEQUENCE OF ABUTMENT PREPARATIONS ON SOUND
ENAMEL OR EXISTING RESTORATIONS
Abutment preparations on sound enamel or on existing restorations that have
been judged as acceptable should be done in the following order:
1. Proximal surfaces parallel to the path of placement should
be prepared to provide guiding planes
Caption

7. 2. Tooth contours should be modified , lowering
the height of contour so that
(a) the origin of circumferential clasp arms may
be placed well below the occlusal surface,
preferably at the junction of the middle and
gingival thirds;
(b) retentive clasp terminals may be placed in
the gingival third of the crown for better
esthetics and better mechanical advantage; and
(c) reciprocal clasp arms may be placed on and
above a height of contour that is no higher than
the cervical portion of the middle third of the
crown of the abutment tooth. Caption

8. 3. After alterations of axial contours are accomplished and before rest seat
preparations are instituted, an impression of the arch should be made in
irreversible hydrocolloid and a cast formed in a fast-setting stone.
This cast can be returned to the surveyor to determine the adequacy of axial
alterations before proceeding with rest seat preparations.
If axial surfaces require additional axial recontouring, this can be performed
during the same appointment and without compromise.

9. • Occlusal rest areas should be prepared that will direct
occlusal forces along the long axis of the abutment
tooth .
• Mouth preparation should follow the removable
partial denture design that was outlined on the
diagnostic cast at the time the cast was surveyed and
the treatment plan confirmed.
• Proposed changes to abutment teeth should be made
on the diagnostic cast and outlined in colored pencil
to indicate the area, amount, and angulation of the
modification to be done .
• Although occlusal rest seats may also be prepared on
the diagnostic cast, indication of their location in
colored pencil is usually sufficient for the
experienced dentist because rest preparations follow a
definite pattern .

10. 4. ABUTMENT PREPARATIONS USING CONSERVATIVE
RESTORATIONS
• Conventional inlay preparations are permissible on the proximal surface of a tooth
not to be contacted by a minor connector of the removable partial denture.
• Proximal and occlusal surfaces that support minor connectors and occlusal rests
require somewhat different treatment.
• The extent of occlusal coverage (i.e., whether cusps are covered) will be governed
by the usual factors, such as the extent of caries, the presence of unsupported enamel
walls, and the extent of occlusal abrasion and attrition.
• When an inlay is the restoration of choice for an abutment tooth, certain
modifications of the outline form are necessary. To prevent the buccal and lingual
proximal margins from lying at or near the minor connector or the occlusal rest,
these margins must be extended well beyond the line angles of the tooth. This
additional extension may be accomplished by widening the conventional box
preparation.

11. However, the margin of a cast restoration produced for such a preparation may be
quite thin and may be damaged by the clasp during placement or removal of the
removable partial denture. This hazard may be avoided by extending the outline of
the box beyond the line angle, thus producing a strong restoration-to-tooth
junction.
In this type of preparation, the pulp is particularly vulnerable unless the axial wall
is curved to conform to the external proximal curvature of the tooth.
When caries is of minimal depth, the gingival seat should have an axial depth at all
points about the width of a No. 559 fissure bur.
It is of utmost importance that the gingival seat be placed where it can be easily
accessed to maintain good oral hygiene. The proximal contour necessary to
produce the proper guiding plane surface and the close proximity of the minor
connector render this area particularly vulnerable to future caries.

12. The first requisite can be satisfied by preparing opposing cavity walls 5
degrees or less from parallel and producing flat floors and sharp, clean
line angles.
It is sometimes necessary to use an inlay on a mandibular first premolar for
the support of an indirect retainer. The narrow occlusal width
buccolingually and the lingual inclination of the occlusal surface of such a
tooth often complicate the two-surface inlay preparation.

13. When multiple crowns are to be
restored as removable partial
denture abutments, it is best that all
wax patterns be made at the same
time. A cast of the arch with
removable dies may be used if they
are stable and sufficiently keyed for
accuracy. If preferred, contouring
wax patterns and making them
parallel may be done on a solid cast
of the arch , with individual dies
used to refine margins
Abutment preparation using crown

14. After the cast has been placed on the surveyor
to conform to the selected path of placement
and after the wax patterns have been
preliminarily carved for occlusion and contact,
proximal surfaces that are to act as guiding
planes are carved parallel to the path of
placement with a surveyor blade.
Guiding planes are extended from the marginal
ridge to the junction of the middle and gingival
thirds of the tooth surface involved .
One must be careful not to extend the guiding
plane to the gingival margin because the minor
connector must be relieved when it crosses the

15. • A guiding plane that includes the occlusal two thirds or even one third of
the proximal area is usually adequate without endangering gingival tissues.
• After the guiding planes are parallel and any other contouring to
accommodate the removable partial denture design is accomplished,
occlusal rest seats are carved in the wax pattern.
• It should be emphasized that critical areas prepared in wax should not be
destroyed by careless spruing or polishing. The wax pattern should be
sprued to preserve paralleled surfaces and rest areas.

16. o Rest seat areas should need only refining with round finishing burs.
If some interference by spruing is unavoidable, the casting must be
returned to the surveyor for proximal surface refinement.
o One of the advantages of making cast restorations for abutment teeth
is that mouth preparations that would otherwise have to be done in the
mouth may be done on the surveyor with far greater accuracy.

17. o The opportunity for contouring wax patterns and making them parallel
on the surveyor in relation to a path of placement should be used to its
full advantage whenever cast restorations are being made.
o The ideal crown restoration for a removable partial denture abutment is
the complete coverage crown, which can be carved, cast, and finished to
ideally satisfy all requirements for support, stabilization, and retention
without compromise for cosmetic reasons .

18. Porcelain veneer crowns can be made equally satisfactory but only by the
added step of contouring the veneered surface on the surveyor before the
final glaze. If this is not done, retentive contours may be excessive or
inadequate.
The three-quarter crown does not permit creation of retentive areas as does
the complete coverage crown.
However, if buccal or labial surfaces are sound and retentive areas are
acceptable or can be made so by slight modification of tooth surfaces, the
three-quarter crown is a conservative restoration of merit.

19. Regardless of the type of crown used, preparation should be made to provide the
appropriate depth for the occlusal rest seat.
This is best accomplished by altering the axial contours of the tooth to the ideal
before preparing the tooth and creating a depression in the prepared tooth at the
occlusal rest area (Figure).
Because the location of occlusal rests is established during treatment planning,
this information will be known in advance of any tooth preparations.

20. Metal-ceramic crown
preparation on tooth #21
shows mesial-occlusal rest
space provided in the
crown preparation at the
mesial. The inset picture
gives a perspective of the
vertical height this
provides for the rest to be
prepared in the wax pattern

21. If, for example, double occlusal rests are to be used, this will be known so that
the tooth can be prepared to accommodate the depth of both rests.
It is inexcusable when waxing a pattern to find that a rest seat has to be made
shallower than is desirable because of post-treatment planning. It can also create
serious problems when a rest seat has to be made shallow in an existing crown or
inlay because its thickness is not known.

22. Ledges on Abutment Crowns
In addition to providing abutment protection, more ideal retentive
contours, definite guiding planes, and optimum occlusal rest support,
complete coverage restorations on teeth used as removable partial
denture abutments offer still another advantage not obtainable on
natural teeth.
This is the crown ledge or shoulder, which provides effective
stabilization and reciprocation.

23. Any rigid reciprocal arm may provide horizontal stabilization if it is located
on axial surfaces parallel to the path of placement.
To a large extent, because it is placed at the height of convexity, a rigid
reciprocal arm may also act as an auxiliary indirect retainer. However, its
function as a reciprocating arm against the action of the retentive clasp arm is
limited to stabilization against possible orthodontic movement when the
denture framework is in its terminal position.
Such reciprocation is needed when the retentive clasp produces an active
orthodontic force because of accidental distortion or improper design.
Reciprocation, to prevent transient horizontal forces that may be detrimental
to abutment stability, is most needed when the restoration is placed or when a
dislodging force is applied.

24. A, Incorrect relationship of retentive and reciprocal clasp arms to each other when the removable
partial denture
framework is fully seated. As the retentive clasp arm flexes over the height of contour during
placement and removal, the reciprocal clasp arm cannot be effective because it is not in contact with
the tooth until the denture framework is fully seated.
B, Horizontal forces applied to the abutment tooth as the retentive clasp flexes over the height of
contour during placement and removal.
Open circles at the top and bottom illustrate that the retentive clasp is passive only at its first contact
with the tooth during placement and when in its terminal position with the denture fully seated.
During placement and removal, a rigid clasp arm placed on the opposite side of the tooth cannot

25. True reciprocation can be obtained only by creating a path of placement for the
reciprocal clasp arm that is parallel to other guiding planes. In this manner, the
inferior border of the reciprocal clasp makes contact with its guiding surface
before the retentive clasp on the other side of the tooth begins to flex . Thus,
reciprocation exists during the entire path of placement and removal.
A ledge on the abutment crown acts as a terminal stop for the reciprocal clasp
arm. It also augments the occlusal rest and provides indirect retention for a distal
extension removable partial denture

26. A, Preparation of the ledge in a wax pattern with a surveyor blade parallel to the path of placement. B,
Refinement of the ledge on casting, using a suitable stone or milling device in a handpiece attached to
the dental surveyor or a specialized drill press for the same purpose. C, Approximate width and depth
of the ledge formed on the abutment crown, which will permit the reciprocal clasp arm to be inlaid
within the normal contours of the tooth. D, True reciprocation throughout the full path of placement
and removal is possible when the reciprocal clasp arm is inlaid onto the ledge on the abutment crown.
E, Direct retainer assembly is fully seated. The reciprocal arm restores the lingual contour of the

27. A ledge on an abutment crown has still another advantage.
The usual reciprocal clasp arm is half-round, and therefore convex, and is
superimposed on and increases the bulk of an already convex surface. A reciprocal
clasp
arm built on a crown ledge is actually inlayed into the crown and reproduces more
normal crown contours.
The patient’s tongue then contacts a continuously convex surface rather than the
projection of a clasp arm. Unfortunately, the enamel is not thick enough nor
the tooth so shaped that an effective ledge can be created on an
unrestored tooth. Narrow enamel shoulders are sometimes
used as rest seats on anterior teeth, but these do not
provide the parallelism that is essential to reciprocation
during placement and removal.

28. The crown ledge may be used on any complete or three-quarter crown restored
surface that is opposite the retentive side of an abutment tooth. It is used most
frequently
on premolars and molars but also may be used on canine restorations. It is not
ordinarily used on buccal surfaces for reciprocation against lingual retention
because of the excessive display of metal, but it may be used just as effectively on
posterior abutments when
esthetics is not a factor.

29. In forming the crown ledge, which is usually located on the lingual surface, the
wax pattern of the crown is completed except for refinement of the margins before
the ledge is carved. After the proximal guiding planes and the occlusal rests and
retentive contours are formed, the ledge is carved with the surveyor blade so that
the surface above is parallel to the path of placement.
Thus, a continuous guiding plane surface will exist from the proximal surface
around the lingual surface.
The full effectiveness of the crown ledge can be achieved only when the crown is
returned to the surveyorfor refinement after casting. To afford true reciprocation,
the crown casting must have a surface above the ledge that is parallel to the path of
placement. This can be accomplished with precision only by machining the casting
parallel to the path of placement with a handpiece holder in the surveyor or some
other suitable machining device.

30. A milling machine used to prepare parallel surfaces, internal rest seats, lingual grooves, and
ledges in cast restorations. Such a device permits more precise milling than is possible with a
dental handpiece attached to the dental surveyor. To be effective, the cast must be positioned
on the drill in such a manner that the previously established path of placement is maintained.
A movable stage or base therefore should be adjustable until the relation of the cast to the
axis of the drill has been made the same as that obtained when the cast was on the dental

31. The reciprocal clasp arm is ultimately waxed on the investment cast so that it is
continuous with the ledge inferiorly and contoured superiorly to restore the crown
contour, including the tip of the cusp. It is obvious that polishing must be
controlled so as not to destroy the form of the shoulder that was prepared in wax
or the parallelism of the guiding plane surface. It is equally vital that the
removable partial denture casting be finished with great care so that the accuracy
of the counterpart is not destroyed.
Modern investments, casting alloys, and polishing techniques make this degree of
accuracy possible

32. Veneer Crowns for Support of
Clasp Arms
For cosmetic reasons, resin and porcelain veneer crowns are used
on abutment teeth that would otherwise display an objectionable
amount of metal.
The development of abrasion-resistant composites offers materials
suitable for veneering that can withstand clasp contact, thereby
eliminating an undesirable display of metal.
Veneer crowns must be contoured to provide suitable retention.
This means that the veneer must be slightly over contoured and
then shaped to provide the desired undercut for the location of
the retentive clasp arm

33. A porcelain veneer crown is resurveyed following
adjustment, glazing, and polishing. It is important to
survey crowns returned from the laboratory before
cementation. The best time to ensure control of all
abutment contours for a removable partial denture is
when surveyed crowns are used and they are
resurveyed before permanent placement.

34. If the veneer is of porcelain, this procedure must precede glazing, and if it is of
resin, it must precede final polishing. If this important step in making veneered
abutments is neglected or omitted, excessive or inadequate retentive contours may
result.
The flat underside of the cast clasp makes sufficient contact with the surface of the
veneer so that abrasion of a resin veneer may result.
Although the underside of the clasp may be polished (with some loss in accuracy
of fit), abrasion results from the trapping and holding of food debris against the
tooth surface as the clasp moves during function. Therefore, unless the retentive
clasp terminal rests on metal, glazed porcelain should be used to ensure the future
retentiveness of the veneered surface.

35. Often, a tooth is considered too weak to use alone as a removable partial
denture abutment because of the short length or excessive taper of a single
root, or because of bone loss resulting in an unfavorable crown-to-root
ratio. In such instances, splinting to the adjacent tooth or teeth can be used
as a means of improving abutment support. Thus, two singlerooted teeth
serve as a multi-rooted abutment.
Splinting should not be used to retain a tooth that would otherwise be
condemned for periodontal reasons.

36. The most common application of the use of multiple abutments is the splinting of
two premolars or a first premolar and a canine (Figure).
First premolars and canines have been splinted in this Class I, modification 1,
partially edentulous arch. The splint bar was added to provide cross-arch stabilization
for splinted abutments and to support and retain the anterior segment of the removable
restoration. The prospective longevity of the abutments has been enhanced.

37. Mandibular premolars generally have round and tapered roots, which are
easily loosened by rotational, as well as by tipping, forces. They are the
weakest of the posterior abutments.
Maxillary premolars also often have tapered roots, which may make them poor
risks as abutments, particularly when they will be called on to resist the
leverage of a distal extension base.
Such teeth are best splinted by casting or soldering two crowns together. When
a first premolar to be used as an abutment has poor root form or support, it is
best that it be splinted to the stronger canine.

38. Anterior teeth on which lingual rests are to be placed often must be
splinted together to avoid orthodontic movement of individual teeth.
Mandibular anterior teeth are seldom used for support, but if they are,
splinting of the teeth involved is advisable. When splinting is
impossible, individual lingual rests on cast restorations may be slightly
inclined apically to avoid possible tooth displacement, or lingual rests
may be used in conjunction with incisal rests, slightly engaging the
labial surface of the teeth.

39. Lingual rests should always be placed as low on the cingulum as possible, and
single anterior teeth, other than canines, should not be used for occlusal
support. Where lingual rests are used on central and lateral incisors, as many
teeth as possible
should be included to distribute the load, thereby minimizing the force on any
one tooth.
Even so, some movement of individual teeth is likely to occur, particularly
when they
are subjected to the forces of indirect retention or when bone support is
compromised. This is best avoided by splinting several teeth with united cast
restorations.

40. Splinting of molar teeth for multiple abutment support is less frequently used
because they are generally multi-rooted.
A two- or three-rooted tooth that is not strong enough alone is probably a poor
abutment risk. However, there may be notable exceptions when a molar abutment
would benefit from the effect of splinting, as in a hemi-sected molar root.
A, Periodontal disease required removal of 46 distal and 47 mesial roots. B, The first premolar and
hemi-sected roots were splinted using a five-unit fixed partial denture. C, Fixed prosthesis provided
cross-arch support, stability, and retention to a Kennedy Class II removable partial denture.

41. The isolated abutment tooth is subjected to mesial tipping caused by lack of
proximal contact. Despite indirect retention, some lifting of the distal extension
base is inevitable, causing torque to the abutment.
In a tooth-supported prosthesis, an isolated tooth may be used as an abutment by
including a fifth abutment for additional support.
Thus, rotational and horizontal forces are resisted by the additional stabilization
obtained from the fifth abutment. When two such isolated abutments exist, a sixth
abutment should be included for the same reason.
Thus, the two canines, the two isolated premolars, and two posterior teeth are used
as abutments.

42. In contrast, an isolated anterior abutment adjacent to a distal extension base usually
should be splinted to the nearest tooth by means of a fixed partial denture.
The effect is two fold:
(1) the anterior edentulous segment is eliminated, thereby creating an intact
dental arch anterior to the edentulous space; and
(2) the isolated tooth is splinted to the other abutment of the fixed partial
denture, thereby providing multiple abutment support.
Splinting should be used here only to gain multiple abutment support rather
than to support an otherwise weak abutment tooth

43. Splinting is advocated for abutment teeth that are considered too weak to risk
being used alone, a single abutment standing alone in the dental arch anterior to
a distal extension basal seat generally requires the splinting effect of a fixed
partial denture.
A, Isolated abutments have been splinted using splint bars.
B, The removable partial denture is more adequately supported
by the splinting mechanism shown in A than could be realized with isolated abutments.

44. Even though the form and length of the root and the supporting bone seem to be
adequate for an ordinary abutment, the fact that the tooth lacks proximal
contact endangers the tooth when it is used to support a distal extension base
removable partial denture.

45. When a removable partial denture is used to replace missing posterior teeth,
especially in the absence of distal abutments, any additional missing anterior
teeth are best replaced by means of fixed restorations rather than included in
the removable partial denture.
The best esthetic result can be obtained by replacing missing anterior teeth
and tissues with the removable partial denture rather than with a fixed
restoration.
The decision to include an anterior segment on the denture depends largely on
the support available for that part of the removable partial denture,The greater
the number of natural anterior teeth remaining, the better is the available
support for the edentulous segment.

46. If definite rest seats can be prepared on multiple abutments, the anterior
segment may be treated as any other tooth-bounded modification space.
Inclined tooth surfaces should not be used for occlusal support, nor should rests
be placed on unprepared lingual surfaces.
The best possible support for an anterior segment is
multiple support extending, if possible, posteriorly
across prepared lingual rest seats on the canine teeth
to mesio-occlusal rest seats on the first premolars.
Such support would permit the missing anterior
teeth to be included in the removable partial
denture, often with some cosmetic advantages over
fixed restorations

47. Without adequate tooth support, any such prosthesis lacks the stability that
results from replacing only the posterior teeth with the removable partial
denture and the anterior teeth with fixed restorations.
In cases where anterior teeth have been lost through accident or have
been missing for some time, resorption of the anterior residual ridge
may progress to the point that neither fixed nor removable pontics
may be butted to the residual ridge, the missing teeth must be
replaced with a denture base supporting teeth that are more nearly in
their original position, considerably forward from the residual ridge.

48. Such a removable partial denture, both from an esthetic and a biomechanical
standpoint, is one of the most difficult of all prosthetic restorations.
However, a splint bar, connected by abutments on both sides of the
edentulous space, will provide much-needed support and retention to the
anterior segment of the removable partial denture.
The splint bar will provide vertical support, rest seats on abutments adjacent
to the edentulous area need not be prepared, thus simplifying an anterior
restoration to
some extent.

49. Temporary Crowns When A Removable Partial Denture Is
Being Worn