Mouth preparation for removable partial dentures /certified fixed orthodontic courses by Indian dental academy

MOUTH PREPARATION FOR
REMOVABLE PARTIAL
DENTURES
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com


INTRODUCTION


DEFINITION
Mouth preparations are identified as those
procedures that are accomplished to prepare the
mouth for reception of prosthesis.
RENNER BOUCHER


More specifically they are the procedures that change
or modify existing oral structures of conditions to
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Facilitate placement and removal of prosthesis
Facilitate its efficient physiologic function
Enhance its long term success


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Mouth preparation follows preliminary diagnosis
and development of tentative treatment plan.


OBJECTIVES IN PLANNING MOUTH
PREPARATIONS FOR REMOVABLE
PARTIAL DENTURES
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To establish a state of health in the supporting and
contiguous tissues
To eliminate interferences or obstructions to the
placement, removal and function of prosthesis
To establish an acceptable occlusal scheme
To establish an acceptable occlusal plane
To alter natural tooth form to accommodate
requirements of form and function of prosthesis

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Mouth preparation procedures are classified into
Prosthodontic procedures which includes
Procedures related to Occlusion
Restorative dentistry (fixed partial dentures)
Non prosthodontic procedures which includes
Oral surgery
Orthodontics
Periodontics
endodontics

Classification


Mouth preparation

Non prosthodontic procedures

Prosthodontic procedures
•Procedures related to Occlusion
•Restorative dentistry
(fixed partial dentures)


•Oral surgery
•Orthodontics
•Periodontics
•Endodontics

PLANNING MOUTH
PREPARATIONS
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Thorough examination of patient including familial,
general health and dental histories
Thorough examination of oral structures including
vitality testing, mobility records and periodontal
evaluation
A complete roentgenographic survey
Making accurate diagnostic casts and mounting them
on a suitable dental articulator in centric relation
Diagnosis and evaluation of data gathered from
examinations
Surveying diagnostic casts

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The formation of an orderly, sequential treatment
outline to meet patients specific needs this outline
includes design of removable partial denture
As the final design of removable partial denture
evolves the need for specific mouth preparations is
identified and recorded in an appropriate manner


RECORDING MOUTH
PREPARATIONS
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As mouth preparations must be accomplished before
the impressions are made and removable partial
denture is constructed there is often a considerable
time lapse between the examination, diagnosis and
treatment planning actual commencement of mouth
preparations and construction of removable partial
denture.
Hence it is necessary to determine and record which
mouth preparations must be accomplished before and
during the removable partial denture phase of therapy

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Method of recording, listing or charting mouth
preparations will depends on needs and desires of
individual dentist
Record can be placed in patients chart and become
permanent part of patients record


RECORDING MOUTH PREPARATIONS
ON DIAGNOSTIC CAST
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Technique for recording mouth preparations on
diagnostic cast is usually executed in a red pencil to
identify the mouth preparations to be accomplished
Teeth to be extracted are marked with an X
Some dentists prefer to remove dental stone teeth from
diagnostic cast and identify the extraction site with an X
Areas of bony and soft tissue recontouring are outlined
with closely spaced parallel lines or shaded with a red
pencil

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Tooth structures to be altered are outlined or
shaded including those areas necessary to prepare
rest seats, guiding planes ,modification of survey
lines, occlusal refinements and removal of
interferences


CHARTING MOUTH PREPARATIONS
ON A PREPARED FORM


ADVANTAGES OF LISTING OR
CHARTING MOUTH PREPARATIONS
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It assures completeness
It provides a quick and convenient records of what
has to be accomplished to prepare patient for
reception of removable partial denture
When the record is properly prepared it serves as a
road map guiding the dentist through all of the
procedures that must be accomplished in proper
sequence and providing sufficient detail to enhance
the accuracy of those procedures that are
accomplished

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It serves as a legal record as to the thoroughness
of mouth preparations and of treatment plan
It virtually ensures that all procedures will be
executed in proper sequence before making
impression for master cast is made since it leaves
nothing to memory


TERM
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ABBREVATION

Survey line
Guiding plane
Interference
Occlusal rest
Incisal rest
Incisal hook rest
Cingulum rest
Facial
Lingual

SL
GP
INT
OR
IR
IHR
CR
F
L

TERM
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ABBREVATION

Mesial
Distal
Line angle
Raise survey line
Lower survey line
Reduce cusp tip
Selectively grind

M
D
LA
RSL
LSL
RCT
SG

SEQUENCE OF PROCEDURES TO BE
FOLLOWED DURING MOUTH
PREPARATIONS
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Oral surgical preparation
Periodontal preparation
Orthodontic considerations
Endodontic therapy
Restorative dentistry
Preparation of abutment teeth


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All preprosthetic surgical treatments for removable
partial denture patient should be completed as early as
possible
When possible necessary endodontic surgery,
periodontal surgery and oral surgery should be
planned so that they can be accomplished during same
time frame
The longer the interval between surgery and
impression procedure the more complete the healing
and consequently more stable the denture bearing
area

ORAL SURGICAL PREPARATION
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EXTRACTIONS
Extraction of non strategic teeth that would present
complications or those that may be detrimental to
design of partial denture is a necessary part of
overall treatment plan


REMOVAL OF RESIDUAL ROOTS
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All retained roots or root fragments should be
removed particularly if they are in close proximity to
the tissue surface or if there is evidence of
associated pathologic finding
Residual roots adjacent to abutment teeth may
contribute to progression of periodontal pockets


IMPACTED TEETH
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All impacted teeth including those in edentulous areas
as well as those adjacent to abutment teeth should be
removed
Asymptomatic impacted teeth in elderly that are
covered with bone with no evidence of pathologic
condition should be left to preserve arch morphology
If an impacted tooth is left it should be recorded in
patients record and patient should be informed of its
presence.`


TISSUE REACTIONS TO WEARING OF
PROSTHESIS
PALATAL PAPILLARY HYPER PLASIA
 It is a lesion of mucosa that occurs more often on hard
palate but may extend onto residual ridges
 It is associated with poorly fitting prosthesis that has
been worn for prolonged periods generally 24 hours
per day
 It is also associated with inadequate oral and
prosthesis hygiene
 Tissue conditioning and tissue rest may help to resolve
some of the edema and inflammation but only surgery
will eliminate the papillae

EPULIS FISSURATUM
 It is a tumour like hyperplastic growth caused by an
ill fitting or over extended border of a removable
prosthesis
 A relatively soft epulis may resolve if irritation is
removed
 The offending border should be adjusted until it is
completely out of contact with lesion
 Fibrosed epulis should be removed surgically


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DENTURE STOMATITIS
It is characterised by generalised erythema
including all the tissues covered by prosthesis
Oral mucosa is swollen and inflammed
Patient complains of burning or itching and pain
It is caused by trauma from occlusion, poor fit of
prosthesis, poor oral hygiene and continuous
wearing of prosthesis
Complete tissue rest and tissue conditioning
procedures are effective in treating this condition

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CYSTS AND ODONTOGENIC
TUMOURS
Panoramic roentgenograms of jaws are recommended
to survey jaws for unsuspected condition
When suspicious area appears on survey film
periapical radiograph should be taken to conform or
deny the presence of lesion
All radiolucencies and radioopacities observed in the
jaws should be investigated
Dentist should confirm the diagnosis by performing the
biopsy of the area and submit the specimens to oral
pathologist for microscopic study

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EXOSTOSIS, TORI ,UNDERCUTS
Exostosis and undercuts in residual ridge areas that
prevents proper extension of denture borders should
be surgically corrected.
Torus palatinus is a benign slowly
growing
protruberance of palatine process of maxilla
Removal of torus palatinus is not necessary unless it
is so large that it interferes with design and
construction of prosthesis


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Torus mandibularis is an exostoses occuring bilaterally
on lingual surface of body of mandible
Mandibular tori should be removed if patient is to wear
removable partial denture with comfort
Modification of denture design to accommodate for
exostoses results in additional stress to supporting
elements and compromised function hence their
removal is recommended
Mucosa covering bony protruberances is extremely thin
and friable partial denture components in close
proximity to this type of tissue may cause irritation and
chronic ulceration

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Undercut areas may be minimized by changing
path of insertion of removable partial denture
Undercuts that would seriously compromise
prognosis should be surgically removed
Surgical correction of under cuts should be
accomplished if relieving denture base or reducing
length of denture border would
Significantly reduce support and stability of
prosthesis
Create a bothersome food impaction area
Cause a denture border to be so far away from
underlying tissues that it may affect function,
compromise esthetics or cause discomfort for the
patient

BONY SPINES AND KNIFE LIKE RIDGES
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Sharp bony spicules should be removed and knife like
crest gently rounded
These procedures should be carried out with minimum
bone loss
If correction of knife edge alveolar crest results in
insufficient ridge support for denture base dentist
should consider vestibuloplasty


MUSCLE ATTACHMENTS AND FRENA
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As a result of alveolar bone height muscle
attachments may insert on or near alveolar crest
Mylohyoid,
buccinator,
mentalis,genioglossus
muscles are likely to introduce these problems
Repositioning muscle attachments will enhance
comfort and function of removable partial denture


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Maxillary labial frenum presents problem when
anterior teeth are replaced with a removable partial
denture
If the frenum is attached near crest of ridge or if it is
hypertrophic notch that must be placed in denture
base to accommodate frenum it may be unsightly
Hypertrophic
lingual
frenum
can
greatly
compromise the rigidity and placement of major
connector
Frenectomy is done to correct these conditions

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All abnormal soft tissue lesions like polyps papillomas
should be excised and submitted for pathologic
examination before fabrication of removable partial
denture
All abnormal white, red or ulcerative lesions should be
recognized and properly evaluated through biopsy


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Augmentation of alveolar bone with use of
autogenous or alloplastic materials
Use of osseointegrated devices (implants)
Implants are placed using clean and controlled
surgical procedures and are allowed to heal before
surgical exposure and fabrication of dental
prosthesis


PERIODONTAL PREPARATION
OBJECTIVES OF PERIODONTAL THERAPY
 Removal and control of all etiologic factors
contributing to periodontal disease
 Elimination
or removal of all pockets with
establishment of healthy gingival sulci
 Establishment of functional and non traumatic
occlusal relationships and tooth stability
 Development
of personalized plaque control
programme and definitive maintainence schedule

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After a thorough examination of periodontium
complete periodontal charting that include pocket
depths, assessment of attachment levels,
furcations, mucogingival problems and tooth
mobility should be performed
Extent of periodontal destruction must be
determined by use of radiographs


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EXAMINATION
FINDINGS`THAT
INDICATE
POSSIBLE NEED FOR PERIODONTAL TREATMENT
INCLUDE THE FOLLOWING
Pocket depth In excess of 3mm
Furcation involvement
Deviations from normal color and contours in gingiva
indicating gingivitis
Marginal exudate
Potential abutment teeth with less than 2mm of gingiva

TREATMENT PLANNING
There are three phases
 PHASE 1:INITIAL DISEASE CONTROL THERAPY
 PHASE 2:DEFINITIVE PERIODONTAL SURGERY
 PHASE 3:RECALL MAINTAINENCE


INITIAL DISEASE CONTRL
THERAPY
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Oral hygiene instructions
Scaling and root planing is done for removal of
calculus and plaque deposits from coronal and root
surfaces of teeth
Elimination of local irritating factors other than calculus
like overhanging margins of amalgam alloy and inlay
restorations
Overhanging crown margins
Open contacts leading to food impaction

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Elimination of gross occlusal interferences
Selective grinding is procedure generally applied and
coronal reshaping of teeth is done to produce
simultaneous occlusal contacts or harmonizing cuspal
relations
Deflective contacts in centric path of closure are
removed
Balancing side or non chewing side interferences
which are usually most destructive should be
eliminated


CLINICAL SYMPTOMS OF TRAUMATIC
OCCLUSION ARE
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Excessive wear of teeth which may include chipping or
fracture of teeth
A change in loss of supporting structures which may
include increased tooth mobility ,tooth migration and
pain during and after occlusal contact
Involvement of neuromuscular mechanism of
temporomandibular joint which may include muscle
spasm, muscle pain and joint symptoms


RADIOGRAPHIC SIGNS OF
TRAUMATIC OCCLUSION ARE
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Widening of periodontal ligament space with either
thickening or loss of lamina dura
Periapical or furcation radioluscency
Resorption of alveolar bone
Root resorption


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Extensive occlusal equilibration should never be
initiated on a patient with temporomandibular joint
dysfunction the symptoms and muscle spasm should
be eliminated through the use of occlusal splint before
occlusal adjustment is initiated
The mere presence of occlusal abnormalities in the
absence
of demonstrable
pathologic
change
associated with occlusion does not contribute
indication for selective grinding
Indication of occlusal adjustment is based on presence
of pathologic condition

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If decision is made to equilibrate the occlusion it
should be done before any definitive restorative
procedures
It is beneficial to perform occlusal equilibration on
duplicate set of diagnostic casts to determine
whether equilibration is feasible this serves as a
blue print for selective grinding in mouth if the
sequence of grinding is recorded


SPLINTING OF PERIODONTALLY
WEAKENED TEETH
REMOVABLE SPLINTING
 Most patients who have periodontally weakened teeth
are in their fourth, fifth or sixth decade of life and have
major medical problems that contraindicate extensive
treatment necessary for multiple fixed prosthesis are
considered for removable splinting
 Splint type guide plane removable partial denture
continuous loop removable splint can be used


FIXED SPLINTING
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It is done when an individual tooth or two adjoining
teeth may have lost some periodontal support as a
result of local conditions
Fixed splinting must be accomplished with full or partial
coverage crowns soldered together
Splinting of periodontally weakened teeth in partially
edentulous arch maintains continuity of arch avoids
additional modification spaces simplifying construction
and fitting of partial denture and improving prognosis


NIGHT GUARD
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It is removable acrylic resin splint designed as an
aid in eliminating deleterious effects of nocturnal
clenching and grinding
They may act as temporary splints if worn at night
when partial denture has been removed
Night guard is useful before fabrication of partial
denture when one of abutment teeth has been
unopposed for an extended period as night guard
returns some functional stimulation to tooth
periodontal ligament changes are reversed

DEFINITIVE PERIODONTAL
SURGERY
GINGIVECTOMY :
 It is indicated to eliminate supra bony pockets
 Pocket depth confined to band of attached gingiva
 PERIODONTAL FLAP PROCEDURES :
 They may be used to perform osseous recontouring
 Osseous recontouring may be indicated for pocket
elimination, when crown lengthening is needed


MUCOGINGIVAL SURGICAL PROCEDURES :
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They are considered when an abutment tooth for a
removable partial denture lacks adequate attached
keratinized gingiva and requires root coverage to
facilitate
partial
denture
construction
and
maintenance


RECALL MAINTANENCE
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Frequency of recall appointments depends on
susceptibility and severity of periodontal disease
Patients with previous moderate to severe
periodontitis should be placed on 3 to 4 months
recall system


ADVANTAGES OF PERIODONTAL
THERAPY
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Elimination of periodontal disease removes primary
etiologic factor in tooth loss
Periodontium free of disease presents a much
better environment for restorative correction
Response of teeth to periodontal therapy provides
an important opportunity for reevaluating their
prognosis before final decision is made to include
or exclude them in partial denture design

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Through
periodontal
surgical
techniques
environment of potential abutment teeth may be
altered to point of making an otherwise
unacceptable tooth to most satisfactory retainer for
a partial denture


ORTHODONTIC
CONSIDERATIONS
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Loss of individual tooth or groups of teeth may lead
to extrusion, mesial drifting orthodontic appliances
are used for correction


CORRECTION OF OCCLUSAL
PLANE
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Occlusal plane in most partially edentulous patients
will be uneven
Teeth that have been unopposed for a time tend to
over erupt
Maxillary molars if not opposed tend to migrate
downward carrying bony tuberosity along in this
condition surgery is indicated to reduce height of bone
If space is extremely limited between overerupted teeth
and opposing ridge a thin metal casting may be
designed to cover the ridge in place of acrylic resin
denturebase

CLASSIFICATION OF SUPER
ERUPTED TEETH
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A class 1 super erupted teeth poses no appreciable
problems in positioning the prosthetic replacement
teeth in opposing dental arch and has no potential for
creating occlusal trauma hence no treatment is needed
A class 2 extruded posterior teeth poses definite
problems of moderate magnitude that can be
successfully managed by enameloplasty
A class 3 extruded posterior teeth poses moderately
severe problems that cannot be successfully managed
without altering tooth to degree that enamel is
penetrated thus requiring placement of restoration

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In class 3 (E) the degree of tooth reduction needed will cause
pulpal exposure necessitating endodontic therapy in addition
to complete cast crown restoration
In class 4 tooth is severely extruded if tooth is considered
non essential to success of removable partial denture it may
be extracted
A class 4 (E) extruded tooth is considered nonessential for
retention but essential for support in eliminating a distal
extension situation it may be treated endodontically and used
as an overdenture abutment
In class 4 (O) when the extruded teeth are considered to be
essential for bracing ,retention and support of removable
partial denture surgical orthodontics might be considered for
repositioning teeth and alveolus

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In class 5 extruded teeth poses moderate to
moderately severe problems relative to ideal plane of
occlusion and space relation but cause no occlusal
trauma because of their location in dental arch
shortening of cusp tips by means of selective
grinding is satisfactory to provide acceptable
interarch space


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In case of infra erupted teeth lengthening of clinical
crown is done to restore occlusal plane it is done by
orthodontic treatment or placement of cast onlays
or crowns
Tipped molars also present problems in
establishing harmonious occlusal plane treatment is
to upright the teeth orthodontically if it is not
possible occlusal plane may be reestablished by
using crowns or inlays

ENDODONTIC THERAPY
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A tooth with pulpal involvement or root end pathology may
be considered a candidate for endodontic therapy
USE OF PULPLESS TEETH AS AN ABUTMENT
It is considered when pulpless teeth that has been treated
endodontically is presented as a potential abutment in
mouth of patient for whom a removable partial denture is to
be made
A potential abutment with an infected pulp is present in
mouth of a candidate for partial denture

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A tooth that has been serving as an abutment for
prosthesis that has developed pulpitis must be treated
either endodontically or extracted
TREATED PULPLESS TEETH
To use them as abutment teeth they should satisfy
same criteria used for teeth with normal healthy pulp
Canals have been filled to apex with what appears
radiographically to be well condensed filling material
There is no radioluscency at apex tooth has been
asymptomatic clinically since the therapy was
accomplished

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INFECTED TEETH
When this teeth are important to design of partial denture
feasibility of endodontic therapy should be considered this
should satisfy certain criteria like
Access to canals
If apicoectomy is needed if its apex is in maxillary sinus
then tooth becomes poor candidate for endodontic
therapy
If apicoectomy is performed will it create an unfavorable
crown root ratio

ABUTMENT TOOTH WITH PULPITIS
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If tooth develops pulpitis while serving actively as an
abutment for prosthesis several factors must be
considered with regard to treatment
Endodontic treatment should be considered only when
abutment tooth with pulpitis is healthy from periodontal
stand point
Crown root ratio is favorable
When prosthesis itself is satisfactory
When mouth as a whole is in a state of good health and
repair

RESTORATIVE DENTISTRY
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It should be integrated with endodontic treatment
when this type of therapy is part of treatment plan
All restorative work including crowns , inlays and
onlays should be programmed to contribute to
restoration of best possible occlusal plane


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Caries lesions should be treated with suitable
restorations
Any defective restorations (restorations with broken
contacts ,overhanging margins) should be repaired
and replaced


PREPARATION OF ABUTMENT
TEETH
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Abutment is a tooth ,a portion of a tooth or that
portion of implant that serves to support and retain
a prosthesis
After surgery ,periodontal treatment, endodontic
treatment and tissue conditioning of arch involved
abutment teeth
may be prepared to provide
support, stabilization and retention for partial
denture

OBJECTIVES OF ABUTMENT TEETH
PREPARATION
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To prepare teeth that are to be clasped so that occlusal
rest directs stress along long axis of tooth
Recontouring of teeth when an altered contour will
eliminate an interference or otherwise contribute to
better design
To create retention by simple alteration procedure
To allow placement and removal of prosthesis without
having it transmitting wedging or torsional types of
stress against teeth with which it comes in contact

CLASSIFICATION OF ABUTMENT
TEETH
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Abutment teeth that require only minor
modifications to their coronal portions
Abutment teeth that are to have restorations other
than complete coverage crowns
Abutment teeth that are to have crowns


SEQUENCE OF ABUTMENT
PREPARATIONS ON SOUND ENAMEL
OR EXISTING RESTORATIONS
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Proximal surfaces parallel to path of placement
should be prepared to provide guiding planes
Tooth contours should be modified lowering height
of contour so that
Origin of clasp arms may be placed well below
occlusal surface preferably at the junction of middle
and gingival third

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Retentive clasp terminals may be placed in gingival
third of crown for better esthetics and better
mechanical advantage
Reciprocal clasp arms may be placed on or above
height of contour
Occlusal rest areas should be prepared that will
direct occlusal forces along long axis of abutment
tooth


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Proposed changes to abutment teeth should be
made on diagnostic cast and outlined in red pencil
to indicate area, amount of modification to be done


PREPARATION OF GUIDING
PLANES

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Guiding planes are vertically parallel surfaces on
abutment teeth oriented so as to contribute to the
direction of the path of placement and removal of a
removable partial denture


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The diagnostic cast mounted on surveying table at
the tilt at which design of removable partial denture
was drawn should be available at mouth preparation
appointment
It should be placed on table in front of patient and
hand piece with appropriate diamond instrument in
place positioned over the cast so that relation ship
of hand piece and diamond stone to tooth can be
visualized
This same relationship can then be duplicated in
patients mouth this ensures that guiding plane will
be parallel to planned path of insertion

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A cylindric diamond point is generally the instrument
used to make preparation
A gentle light sweeping stroke from buccal line
angle to lingual line angle should be used
Flat surface created should be 2 to 4mm in
occlusogingival height the reduction should follow
curvature of surface so that uniform amounts of
enamel are removed throughout buccolingual width
of preparation
All prepared tooth surfaces must be polished when
contouring is complete

GUIDING PLANES ON ABUTMENT TEETH
ADJACENT TO DISTAL EXTENSION
EDENTULOUS SPACES
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In these cases occluso gingival height of guiding
plane is reduced to 1.5 to 2mm to permit partial
denture to rotate slightly around distal occlusal rest
as downward force occurs on artificial teeth
The slight movement allows release of denture
from guiding plane there by avoiding torquing or
twisting forces on abutment tooth

GUIDING PLANES ON LINGUAL
SURFACES OF ABUTMENT TEETH
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The purpose is to provide maximum resistance to
lateral stress
Occluso gingival height of guiding plane is 2 to
4mm the plane ideally should be located in the
middle third of clinical crown of tooth


GUIDING PLANES ON ANTERIOR
ABUTMENT TEETH
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Guiding planes on anterior teeth adjacent to
edentulous spaces provides parallelism needed to
ensure stabilization, minimize wedging action between
teeth ,decrease undesirable space between denture
and abutment teeth
Special purpose is to increase or restore normal width
of edentulous space as anterior teeth are lost and
replacement teeth not provided immediately teeth
adjacent to space will drift and tip into space
Both actions reduce the size of space and make the
esthetic replacement of missing teeth difficult

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Teeth that have tipped towards an edentulous space
will exhibit a large undercut area below height of
contour on proximal surface
If height of contour is not reduced as guiding planes are
established the undercut will appear as a large
unsightly space between artificial tooth and restored
tooth
The space not only detracts from esthetic value of
denture but also traps food
If sufficient tooth structure cannot be removed to
restore the space and reduce the undercut without
penetrating the enamel layer a restoration must be
planned

ENAMELOPLASTY TO CHANGE
HEIGHT OF CONTOUR
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Height of contour is changed most frequently to
provide better positions for clasp arms or lingual
plating
Retentive clasp arm should be located no higher on
crown of abutment tooth than function of gingival
and middle thirds
This position not only enhances esthetic quality of
clasp but also provides mechanical advantage

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In maxillary arch molars and premolars if unsupported
tend to tip in buccal direction this causes height of
contour to be near occlusal surface on facial side of
abutment tooth
In mandibular arch molars and premolars if unsupported
tip lingually this causes problems with positioning of
reciprocal clasps and lingual plating
If tipping is severe it causes problems in placement of
lingual bar major connector
Height of contour is best lowered by tapered diamond
stones

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The amount of correction that can be accomplished
by recontouring the enamel surface is limited by
thickness of enamel
If dentin is exposed restoration must be placed to
protect the tooth


ENAMELOPLASTY TO MODIFY
RETENTIVE UNDERCUTS
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Contouring the enamel surface to produce retentive
undercut to be successful buccal and lingual
surfaces of teeth must be nearly vertical
`if both surfaces have a pronounced slope
procedure is contraindicated
If surface to receive undercut is sloped indentation
would have to be excessively deep to be effective


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If opposing surface is sloped reciprocal arm could not
brace the tooth sufficiently to prevent retentive clasp
tip from being dislodged from undercut
Retentive undercut must be created in the form of
gentle depression it is prepared by using small round
end tapered diamond stone
The end of stone is moved in anteroposterior direction
near line angle of tooth
Preparation is made parallel to and as close as
possible to gingival margin without encroaching on
gingival crevice
Depression should be 4mm in mesiodistal length and
2mm in occlusogingival height

PREPARATION OF REST SEAT
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Rest seat is prepared recess in a tooth or
restoration created to receive the occlusal ,incisal,
cingulum or lingual rest
Rest is component of partial denture that is placed
on an abutment tooth ideally in prepared rest seat
so that it limits movement of denture in a gingival
direction and transmits functional forces to tooth

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Outline form of occlusal rest seat is triangular with
base of triangle at marginal ridge and apex towards
centre of tooth
An occlusal rest must be at least 1mm thick at its
thinnest point if chrome alloy is used 1.5mm if gold
is to be used
Extension of occlusal rest seat preparation should
vary from one third to one half the mesiodistal
diameter of tooth
Bucco lingual extent should be half the distance
between buccal and lingual cusp tips

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Floor of occlusal rest must be inclined towards centre of tooth
must be spoon shaped
The enclosed angle formed by inclination of floor of rest and
vertical projection of proximal surface of tooth must be less
than 90 degrees so occlusal forces can be directed along
vertical axis
Occlusal rest seats in enamel should be prepared with round
diamond stone
First a channel of correct depth and at desired outline of
preparation is created by using small round diamond stone to
lower marginal ridge at either buccal or lingual extent of rest
seat to continue inward towards centre of tooth and to return
to marginal ridge

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Island of enamel that remains with in outline form can be
removed with same diamond stone so that sufficient tooth
structure is removed to provide the thickness of metal
required for strength of rest
Deepest portion of rest seat is towards centre of tooth
preparation raises gradually towards marginal ridge it is
called positive rest located in fossa area away from
marginal ridge it is concave area it should be 0.5 to
1.0mm deeper than general base of rest seat.

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Adequacy of occlusal rest seats can be checked
before impression of master cast is made by
Visual inspection
Direct tactile contact
By making imprints in red utility wax
By making impression to create a diagnostic cast


OCCLUSAL REST SEAT
PREPARATON IN EXISTING GOLD
RESTORATION
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It should always be placed in wax pattern after
establishment of guiding planes
After preparation for restoration is complete it is
helpful to add a depression to preparation to
accommodate depth of occlusal rest
Rest seat can be carved with suitable wax
instruments
The anatomy of rest seat must not be destroyed
during polishing procedure

OCCLUSAL REST SEAT
PREPARATION IN EXISTING GOLD
RESTORATION
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If existing restoration display marginal integrity and
occlusalharmony attempt should be made to
contour them to satisfy requirement of proposed
prosthesis


OCCLUSAL REST SEAT
PREPARATION IN AMALGAM
RESTORATION
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Occlusal rest seat preparation in a multi surface
amalgam restoration is less desirable as amalgam
alloy tends to flow when placed under constant
pressure
Care must be taken not to weaken proximal portion
of amalgam restoration at isthmus during
preparation
This may result in fracture during function
Rest seats are prepared using no.4 round bur.

REST SEAT PREPARATION FOR
EMBRASSURE CLASP
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This preparation extends over occlusal embrassure
of two approximating posterior teeth from mesial
fossa of one tooth to distal fossa of other tooth
Main problem with this preparation is failure to
remove sufficient tooth structure over buccal slopes
of preparation it leads to occlusal interferences
between metal of clasp opposing cusps
Relieving metal to gain occlusal freedom leads to
breakage of clasp during function

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Small round diamond stone is used to establish outline
form for normal occlusal rest in each of approximating
fossae
The contact point between the teeth should not be broken
because a wedging action and food impaction between
teeth may take place
The same diamond stone is used to carry buccal and
lingual extensions of occlusal rests over buccal and
lingual embrassures
Cylindrical diamond stone held horizontally from buccal
surfaces of teeth pointing towards lingual surface

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Stone is held against distal incline of buccal cusp of one
tooth and mesial incline of buccal cusp of other tooth for
creating occlusal clearance
The patient should be able to close without contacting
metal
As preparation passes over buccal and lingual
embrassures it should be 1.5-2mm wide and 1 -1.5mm
deep
Buccal inclines of preparation must be rounded after
preparation

LINGUAL REST SEAT
PREPARATION
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Outline form of lingual rest seat is half moon shaped
It should form a smooth curve from one marginal ridge
to other crossing centre of tooth incisally to cingulum
Rest seat is v shaped
Labial incline of lingual surface of tooth makes one wall
other wall of v shaped notch starts at top of cingulum
and inclines linguo gingivally towards centre of tooth to
meet other wall of preparation
Sharp lines and angles must be avoided because they
will interfere with fit of framework of partial denture

LINGUAL REST SEAT PREPARATION
IN CAST RESTORATION
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Lingual rest seat should be carved in wax pattern
LINGUAL REST SEAT PREPARATION IN
ENAMEL
Lingual rest seat may be prepared in enamel
surface of an anterior tooth if tooth is sound
Cingulum also should be prominent to present a
gradual slope to lingual surface rather than steep
vertical slope

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This is why mandibular canines are poor candidates for
lingual rest
A safe side ¼ inch diamond disk should be used it must
be held so that it is parallel or slightly inclined labial to
path of insertion
The cut with disk should start low on one marginal ridge
pass over cingulum and then pass gingivally to contour
opposite marginal ridge this will produce half moon shape
When space is not available to permit use of disk flat end
large diamond cylinder is best choice
Rest seat must always be gingival to contact level of
opposing tooth

INCISAL REST SEAT
PREPARATION
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It should be done on only enamel surfaces
Incisal rest seat is usually placed near one of
incisal angles of canines
If incisal rest is used with circumferential clasp rest
should be placed at distal incisal angle
If rest is used in conjunction with bar clasp rest
should be placed at mesio incisal angle
Incisal rests are least desirable rests for anterior
teeth

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On incisor teeth an incisal rest is used as last resort to
stabilize removable partial denture the prognosis for these
teeth is usually poor
An incisal rest seat is begun with small safe side diamond
disk held parallel to path of insertion
First cut is made vertically 1.5 -2mm deep in form of notch
and approximately 2 -3mm inside proximal angle of tooth
A small flame shaped diamond point is used to complete
the preparation
The notch created by disk is rounded slightly but not to level
of base of notch
The enamel wall created by disk toward centre of tooth
must be rounded with flame shaped diamond point

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Base of notch is also rounded with tip of flame shaped
diamond the groove that results after notch has been
completely rounded must be carried slightly over onto labial
surface
This projection onto facial surface provides a locking device
to prevent the tooth from being tipped or moved facially
The groove should be continued part way down the lingual
surface as an indentation this indentation will help
accommodate minor connector
Incisal hook rest seat is prepared as a modification of incisal
rest seat by extending preparation 1.5 -2mm onto labial
surface of tooth as concave depression advantage of it is
greater stability

ABUTMENT TEETH PREPARATIONS
USING CONSERVATIVE
RESTORATIONS
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Proximal and occlusal surface that support minor
connectors and occlusal rests require modification
in restoration when an inlay is restoration of choice
for an abutment tooth
The extent of occlusal coverage depends on extent
of caries ,presence of unsupported enamel walls
,extent of occlusal abrasion and attrition

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To prevent buccal and lingual proximal margins from lying at
or near minor connector or occlusal rests these margins must
be extended well beyond line angles of tooth this may be
accomplished by widening the conventional box preparation
The margin of cast restoration produced may be quite thin
and may be damaged by the clasp during placement and
removal of partial denture
This hazard may be avoided by extending outline of box
beyond line angle
Pulp is particularly vulnerable unless the axial wall is curved
to confirm with external proximal surface of tooth

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Gingival rest should be placed where it can be easily
accessed to maintain oral hygiene
The proximal contour necessary to produce proper guiding
plane surface and close proximity of minor connector render
this area vulnerable to future caries attack
Every effort should be made to provide restoration with
maximum retention and resistence which can be obtained by
preparing opposing cavity walls 5 degrees or less from
parallel and producing flat floors and sharp clean restorations


ABUTMENT PREPARATIONS USING
CROWNS
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Crowns may be in the form of three quarter
complete coverage cast crowns, porcelain veneer
crowns
Ideal crown restoration for a partial denture
abutment is complete coverage crown which can
be carved ,cast and finished to ideally satisfy all
requirements for support , stabilization and
retention

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Three quarter crown does not permit creating retentive
areas as does complete coverage crown
If buccal or labial surfaces are sound and retentive areas
are acceptable or can be made so by slight modification
of tooth surfaces three quarter crown is conservative
restoration of merit
Regardless of type of crown used preparation should be
made to provide appropriate depth for occlusal rest seat
this is best accomplished by creating depression in
prepared tooth at occlusal rest area

CONTOURING WAX PATTERNS
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To contour wax pattern to desired configuration die
of tooth preparation in a cast of remainder of arch
must be analyzed on surveyor
Working cast should be mounted on surveyor at the
same tilt that diagnostic cast was mounted once
correct tilt has been established wax knife is
substituted for analyzing rod and guiding plane is
carved in wax pattern by shaving the wax

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Height of contour of crown can also be determined by use
of analyzing rod
Pattern must be hand carved to place the height of
contour on middle third of lingual surface if tooth is to
receive a reciprocal clasp and at junction of gingival and
middle third if retentive clasp has been planned
Position and depth of retentive under cut can be verified
by means of undercut gauge
0.010 inch undercut gauge will be used for most cast
chrome clasps
The measured undercut should fall at distal or mesial line
angle of tooth depending on type of clasp that has been
designed

REFINING CAST RESTORATIONS
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After casting has been made it should be finished carefully
so that contour that was carved in wax pattern must be
maintained
Working cast with die and casting in position should be
returned to surveyor before final polish of restoration is
complete
Guiding planes on casting and any changes in contouring
of axial surfaces should be refined using surveyor as
machining device by attaching hand piece holder to the
vertical arm of surveyor and straight hand piece holder
A straight cylinder mounted stone is used in hand piece to
accomplish machining procedure

LEDGES ON ABUTMENT CROWNS
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Complete coverage restorations on teeth used as
partial denture abutments offers an advantage that
is not obtainable on natural teeth this is crown
ledge or shoulder which provides effective
stabilization and reciprocation
True reciprocation is not possible with a clasp arm
that is placed on occlusally inclined tooth surface
because it does not become effective until
prosthesis is fully seated

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When a dislodging force is applied reciprocal clasp
arm along with occlusal rest breaks contact with
supporting tooth surfaces they are no longer effective
As the retentive clasp flexes over height of contour
and exerts horizontal force on abutment reciprocation
is nonexistent just when it is needed most
True reciprocation can be obtained only by creating
path of placement for reciprocal clasp arm that is
parallel to other guiding planes


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Here inferior border of reciprocal clasp makes contact
with its guiding surface before retentive clasp on other
side of tooth begins to flex
Thus reciprocation exists during entire path of
placement and removal
Presence of ledge on abutment crown acts as a
terminal stop for reciprocal clasp arm
Ledge on abutment crown has still another advantage
usual reciprocal clasp arm is half round and therefore
convex when superimposed on tooth increases bulk of
already convex surface reciprocal clasp arm built on a
crown ledge is inlayed into crown and reproduces more
normal crown contours

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The patients tongue then contacts continuously convex
surface rather than projection of clasp arm
Crown ledge may be used on any full or three quarter crown
restored surface that is opposite the retentive side of
abutment teeth it is used most frequently on premolars and
molars
Ledge should be placed at junction of gingival and middle
thirds of tooth curving slightly to follow curvature of gingival
tissues
In forming crown ledge wax pattern is completed after
proximal guiding planes,occlusal rests and retentive contours
are formed ledge is carved with surveyor blade

 Full

effectiveness of crown ledge can only be
achieved when crown is returned to surveyor for
refinement after casting


SHAPING VENEER CROWNS
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Resin and porcelain veneered crowns are used on
abutment teeth that would otherwise display an
objectionable amount of metal
Veneer crowns must be contoured to provide
suitable retention this means veneer must be
slightly over contoured and then shaped to provide
desired undercut for location of retentive clasp arm


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If veneer is of porcelain this procedure must
precede final glazing
If it is of resin it must precede final polishing
If this step is neglected excessive or inadequate
retentive contours may result


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Mouth preparation for removable partial dentures /certified fixed orthodontic courses by Indian dental academy

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