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DEPTT OF PROSTHODONTICS CROWN AND BRIDGE
Mouth preparation for removable
partial denture
Seminar
Dr. Vikas Aggarwal
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CONTENTS
1. Introduction2. Pre prosthetic mouth preparation
Surgical preparation
Conditioning of abused and irritated tissues
Periodontal preparation
Treatment of muscular symptoms
Correction of occlusal plane
Conservative/endodontical preparation
Correction of malalignment
3. Prosthetic mouth preparation Developing guiding planes Changing height of contour Modifying retentive undercut. Rest seat preparation
4. Conclusion5. References
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INTRODUCTION
The preparation of the mouth is fundamental to a successful removable partial
denture service. Mouth preparation, perhaps more than any other single factor,
contributes to the philosophy by Devans that the prescribed prosthesis must
not only replace what is missing but also preserve the remaining tissue and
structures that will enhance the removable partial denture
"Mouth preparation" is a term intended to cover all types of changes
effected in the teeth, foundation ridges or oral structures which may be deemed
necessary to accomplish a better partial denture result. (Applegate 3rd ed)
Mouth preparation follows the preliminary diagnosis and the
development of a tentative treatment plan. Final treatment plan can be deferred
until the response to the preparatory procedures can be ascertained.
Mouth preparation can be generally classified as pre-prosthetic mouth
preparation that involves removal of any hindrances to prosthetic treatment and
prosthetic mouth preparation that involves mouth preparation done to facilitateprosthetic treatment.
1. Pre prosthetic mouth preparation Surgical preparation Conditioning of abused and irritated tissues Periodontal preparation Treatment of muscular symptoms Correction of occlusal plane Conservative/endodontic preparation Correction of malalignment
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2. Prosthetic mouth preparation Developing guiding planes Changing height of contour Modifying retentive undercut. Rest seat preparation
1) SURGICAL PREPARATION
They should be planned and completed well in advance. The longer the
interval between the surgery and the impression procedure, the more complete
the healing and consequently the more stable the denturebearing mucosa. The
important consideration is that the patient not be deprived of any treatment that
would enhance the success of the partial denture.
a) Extractionsb) Removal of residual rootsc) Removal of impacted teethd) Malposed teethe) Cysts and odontogenic tumorsf) Exostosis and torig) Hyperplastic tissuesh) Muscle attachment and frenii) Bony spines and knife edge ridges
j) Polyps, papilloma and traumatic haemangiomask) Hyperkeratosis,erythoplakia,and ulcerationl) Dentofacial deformitym)Ridge augmentationn) Osseointegrated devices
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a) Extractions
Planned extraction should be carried out after thorough evaluation of the
remaining teeth in the dental arch. The non-strategic teeth that present
complications or those whose presence may determine the design of the partial
denture should be extracted. Teeth with doubtful prognosis of which retention
would contribute little if anything, even if successfully treated and maintained,
are contraindicated.
b) Removal of residual roots
All retained roots or root fragments should be removed particularly if
they are in close proximity to the tissue surfaces or when they contribute to the
progression of periodontal pockets.
The removal of root tips can be carried out from the facial and palatal
surfaces without compromising the alveolar bone height or harming the adjacent
teeth.
c) Removal of impacted teeth
All impacted teeth are indicated for extraction because they can becomesource of spread of infection to the adjacent healthy teeth.
The skeletal structure of the body changes with age. Asymptomatic
impacted teeth covered with bone in elderly individuals with no evidence of
pathology should be left to preserve the arch morphology. This should be
documented in patients records. Age alterations that affect the jaws can result
in minute exposures of impacted teeth to the oral cavity through the sinus. Earlyelective removal of impactions can prevent later serious acute and chronic
infection with extensive bone loss.
d) Malposed teeth
The loss of individual or groups of teeth may lead to extrusion, mesial drifting,
or combinations of malpositioning of remaining teeth. In some cases the
alveolar bone will be carried occlusally along with the extruded teeth.
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Orthodontics can be used to correct such occlusal discrepancies. Otherwise
surgical repositioning of the malposed teeth and the supporting bone can be
done.
e) Cysts and odontogenic tumors
All radiolucencies or radiopacities observed in the jaws should be
investigated. Panoramic radiographs are recommended to survey the jaws for
unusual pathologies.
Cysts, odontogenic tumors, should be removed because their presence
may compromise the design of the removal partial dentures.
f) Exostoses and tori
The presence of abnormal bony enlargements should not be allowed to
compromise the design of the partial denture. Although modification in design
to accommodate for exostoses can be done, this will place additional stresses to
supporting elements and compromise the function.
The mucosa covering these enlargements is thin and friable. Partial
denture components in proximity to this type of tissue can cause irritation andchronic ulceration. Also exostoses close to gingival margin lay complicate
maintenance of periodontal health and lead to eventual loss of strategic
abutment teeth.
g) Hyperplastic tissue
They are seen in form of fibrous tuberosities, soft flabby ridges, folds of
redundant tissue in the vestibule or floor of the mouth and palatal papillomatosisAll these forms of excess tissue should be removed to form a firm base for the
partial dentures.
Hyperplastic tissue can be removed with any preferred combination such
as scalpel, curette, electrosurgery, or by laser. Some form of surgical stent
should be considered for such patients for a comfortable and enhanced healing.
All such excised tissues should be sent to oral pathologist for microscopic
study.
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h) Muscle attachments and frena
Loss of alveolar bone height renders the muscle attachments near the
alveolar crest making the designing of the partial dentures difficult. Mylohyoid,
buccinator, mentalis and genioglossus muscles are those which can cause
problems. Sometimes some muscles (mentalis, genioglossus) produce bony
protuberances at their attachments which interfere with design of partial
denture.
Repositioning of these supra-placed muscles by ridge extension is
necessary in such condition to enhance comfort and function. Mylohyoid can be
easily repositioned but genioglossus is much more difficult to reposition.
The maxillary labial and mandibular lingual freni may interfere in partial
denture design. These should be modified with surgical interventions.
i) Bony spines and knife- edge ridges
Sharp bony spicules should be removed and knife-edge ridges rounded to
facilitate easy designing of the partial dentures. These procedures should becarried out with minimal bone loss. Vestibular deepening or ridge augmentation
procedures can be considered.
j) Polyps, papilloma and traumatic haemangiomas
All abnormal soft tissue lesions should be excised and submitted for pathologic
examination. New or additional stimulation to the tissue may producediscomfort or even malignant changes.
k) Hyperkeratosis, erythoplakia, and ulceration
All abnormal red, white and ulcerative patches should be investigated and
treated accordingly. A biopsy of areas larger than 5 mm should be completed,
and if the lesions are large (more than 2 cm in diameter), multiple biopsies
should be taken. The biopsy report will determine whether the margins of the
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These conditions are usually associated with ill-fitting or poorly occluding
removable partial dentures. However, nutritional deficiencies, endocrine
imbalances, severe health problems (diabetes or blood dyscrasias), and bruxism
must be considered in a differential diagnosis.
If a new removable partial denture or the relining of a present denture is
attempted without first correcting these conditions, the chances for successful
treatment will be compromised because the same old problems will be
perpetuated. The patient must be made to realize that fabrication of a new
prosthesis should be delayed until the oral tissues can be returned to a healthy
state.
The first treatment procedure should be an immediate institution of a
good home care program. A suggested home care program includes rinsing the
mouth three times a day with a prescribed saline solution; massaging the
residual ridge areas, palate, and tongue with a soft toothbrush; removing the
prosthesis at night; and using a prescribed therapeutic multiple vitamin along
with a prescribed high- protein, low-carbohydrate diet. Some inflammatory oralconditions caused by ill-fitting dentures can be resolved by removing the
dentures for extended periods.
Use of tissue conditioning materials
The tissue conditioning materials are elastopolymers that continue to flow
for an extended period, permitting distorted tissues to rebound and assume theirnormal form. These soft materials apparently have a soothing effect on irritated
mucosa, and because they are soft, occlusal forces are probably more evenly
distributed.
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Maximum benefit from using tissue-conditioning materials may be obtained by:
Eliminating deflective or interfering occlusal contacts of old dentures (byremounting on an articulator if necessary)
Extending denture bases to proper form to enhance support, retention, andstability
Relieving the tissue side of denture bases sufficiently (2 mm) to providespace
Applying the material in amounts sufficient to provide support and acushioning effect
Following the manufacturer's directions.The conditioning procedure should be repeated until the supporting tissues
display an undistorted and healthy appearance. Usually intervals of 3 to 4 days
between changes of the conditioning material are clinically acceptable. An
improvement in irritated and distorted tissues is usually noted within a few
visits, and in some patients a dramatic improvement will be seen. Usually three
or four changes of the conditioning material are adequate, but in some instances
more changes are required. If positive results are not seen within 3 to 4 weeks,
one should suspect more serious health problems and request consultation from
a physician.
3) PERIODONTAL PREPARATIONS
The periodontal procedures follows surgical procedures and done
simultaneously along with tissue conditioning procedures. The periodontal
procedures are necessary to restore the mouth to the state of health required for
definite treatment. The periodontal health of the remaining teeth especially the
abutment teeth is evaluated carefully and corrective measures are instituted
before fabricating the removable partial denture.
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Objectivesof periodontaltherapy:
Removal of all etiologic factors contributing to periodontaldiseases.
Elimination or reduction of all pockets with the establishment ofgingival sulci free of inflammation.
Establishment of functional atraumatic occlusal relationships andtooth stability
Development of a personalized plaque control programme anddefinite maintenance schedule.
Periodontal diagnosis and treatment planing
Diagnosis
The diagnosis of periodontium is based on systematic and careful
observation of the periodontium. It follows the procurement of health history of
patient. It is performed using direct vision, palpation, periodontal probe, mouth
mirror, and other auxiliary aids such as curved explorers, furcations probes,
diagnostic casts and roentgenograms.
Most important is careful exploration of the gingival sulcus and recording
the probing pocket depth. The probe is inserted gently but firmly between the
gingival margin and the tooth surface, and the depth of gingival sulcus is
determined circumferentially around each tooth. A critical assessment of the
sulcular health can be done by judging the amount of bleeding on probing. This
along with the pocket depth is an excellent indicator of health and disease.
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Dental roentgenograms
They are used to supplement clinical examination but cannot substitute it.
The extent and pattern of bone loss can be estimated. They also provide
information regarding the following:
Type location and severity of bone loss Location, severity and distribution of furcation involvement. Alteration of periodontal ligament space. Alterations of the lamina dura Calcified deposits Location and conformity of restoration margins Evaluation of crown and root morphologies. Root proximity Caries Evaluation of other associated anatomic structures, such as mandibular
canal or sinus proximity.
Mobility
Each tooth should be evaluated for mobility. It is graded according to ease and
extent of tooth movement. Normal mobility is in order of 0.05 to 0.10 mm.
Grade Imobility slightly more than normal.
Grade IImoderately more than normal.
Grade IIIsevere mobility with vertical displacement.
Mobility is assessed with ends of two instruments. If fingers are used the
movement of soft tissue may mask accurate determination of mobility
If etiologic factors are removed most Grade I and II mobile teeth will become
stable and can be used to support the partial denture. Mobility in itself is not an
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indication for extraction. Grade III cannot be reversed and thus cannot be used
for support of the partial denture.
Treatment planning
Depending on the severity of periodontal changes a series of simple to complex
procedures may be indicated. The treatment planning can be divided into the
following phases.
Phases in Treatment
Disease control therapy phase-phase 1
Definitive periodontal surgery phase-phase 2
Maintenance phase- phase 3
Disease Control Therapy Phase-Phase 1
This phase consists of:Oral hygiene instructions
The patient should be instructed in the use of disclosingwafers, soft nylon toothbrush, and unwaxed dental floss.
At subsequent appointments oral hygiene can beevaluated carefully, & other oral hygiene aids added,
such as a rubber tip stimulator. Without good oral hygiene any dental procedure,
regardless of how well it is performed, is ultimately
doomed to failure.
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Scaling & root planing
Ultra sonic instruments are used for gross calculus removal.
This is followed by root planning with sharp curettes.
Elimination of other local irritating factors
Overhanging margins of amalgam & inlay restoration. Overhanging crown margins. Open contacts leading to food impactions. Deep carious lesions should be eliminated before the start of
definitive prosthetic treatment .
Elimination of occlusal interferences
Poor occlusal relationship may act as a factor thatcontributes to more rapid loss of periodontal attachment.
Selective grinding procedure is generally applied at thisstage. Traumatic cuspal interferences are removed by
judicious grinding procedures. Deflective contacts in the
centric path of closure are removed, eliminating mandibular
displacement from the closing pattern.
The indication for occlusal adjustment is based on thepresence of pathology rather than on a preconceived
articulation pattern.
Occlusion on natural teeth needs to be perfected only to apoint at which cuspal interference within the patients
functional range of contact is eliminated and normal
physiologic function can occur.
Guide to Occlusal Adjustment (Schuyler)
Accurately mounted diagnostic casts are extremely helpful in
determining static cusp to fossa contacts of opposing teeth and as
guide in the correction of occlusion anomalies.
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1) A static coordinated occlusal contact of the maximum number
of teeth when the mandible is in centric relation to the maxillae
should be the first objective. The procedure is as follows:-
a) A prematurely contacting cusp should be reduced only if the
cusp point is in premature contact in both centric and eccentric
relations. If a cusp point is in premature contact in centric
relation only, the opposing sulcus should be deepened.
b) When anterior teeth are in premature contact in centric
relations, or in both centric and eccentric relations, corrections
should be made by grinding the incisal edge of the lower teeth.
c) Usually, premature contacts in centric relation are relieved by
grinding the buccal cusps of the lower teeth, the lingual cusp of
upper teeth, and the incisal edges of the lower anterior teeth.
Deepening the sulcus of the posterior tooth or the lingual contact
area in centric relation of an upper anterior tooth changes and
increases the steepness of the eccentric guiding inclines of the
tooth; although this relieves trauma in centric relation, it may
predispose the tooth to trauma in eccentric relations.
2) After establishing a static, even distribution of stress over the
maximum number of teeth in centric relation, evaluate opposing
tooth contact or lack of contact in eccentric functional relations.
First balancing side contacts are seen. Subluxation, pain, lack of
normal functional movement of the joint, or loss of alveolar
support of the teeth involved may be evidence of excessive
balancing contacts. Balancing side contacts receive less
frictional wear than working side contacts, and premature
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should never be ground to bring the posterior teeth into contact in
either protrusive position or on the balancing side. In the
elimination of premature protrusive contacts of posterior teeth,
neither the upper lingual cusps nor the lower buccal cusps should
be ground. Corrective grinding should be done on the surface of
the opposing teeth on which these cusps function in the eccentric
position, leaving the centric contact undisturbed.
5) Any sharp edges left by grinding should be rounded off
Splinting
In many partially edentulous mouths some of all the remaining teeth lose
their periodontal and bone support rendering them mobile and not suitable to
provide support to the partial dentures. In order to use such teeth as abutments
additional support for these teeth by splinting them together is necessary.
The cause of mobility must be assessed and the causative factors should be
eliminated. Secondary mobility resulting from presence of inflammatory lesion
may be reversible.
Teeth may be immobilized during periodontal treatment by acid etching
teeth with composite resin, with fiber reinforced resin, with cast removable
splint, or with intracoronal attachments.
Splinting can be achieved by a removable restoration or by fixed restoration
which becomes a permanent splint.
Splinting of weakened teeth in partially edentulous arch located in a position
where the partial denture will not require an unusual amount of support, is
achieved by using fixed splinting, this maintains the continuity of the arch,
avoids additional modification spaces, thus simplifying the construction and
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fitting of partial dentures and improving prognosis. Fixed splinting must be
accomplished with full or partial coverage crowns soldered together; this gives
additional resistance to antero-posterior stresses. To offer resistant to lateral
forces, the splint must be extended anteriorly to include canine teeth and also
include the lateral plane of the posterior teeth.
Periodontal Surgery Phase 2
It is a definitive periodontal surgery phase. If oral hygiene
is optimal, yet pockets with inflammation and osseous defect are
present, various surgical techniques like gingivectomy,
periodontal flap should be considered to improve periodontal
health.
Gingivectomy
Gingivectomy is indicated when there are supra bony
pockets of fibrotic tissue, absence of deformi ties in the
underlying bony tissue & pocket depth confined to attached
gingiva. If osseous deformities are present or if pocket depth
traverses mucogingival junction gingivectomy is not the treatment
of choice.
Periodontal Flap
The flap is widely employed for the treatment of periodontal
diseases. It may be used to gain access for root planing, osseous
recontouring for pocket elimination or crown lengthening and
also for osseous grafts.
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Maintenance Phase
This is phase 3 of the periodontal procedures. It includes
reinforcement of plaque control measures, thorough debridement of
root surfaces of subgingival & supra gingival plaque. Frequency of
recall is according to patients requirements. In moderate to severe
periodontitis, 3-4 months recall system is followed.
4) TREATMENT OF MUSCULAR SYMPTOMS
Prior to adjustment of the occlusion of the teeth the muscular symptoms
should be analyzed. Patients with partially edentulous arches often show
symptoms of muscle spasm. Therefore the first objective of the operator is to
eliminate this muscle spasm. This can be achieved by giving the patient an
acrylic overlay splint with a flat occlusal surface which will eliminate premature
tooth contacts causing deviation of the mandible leading to spasm. Adjunct
therapies like short-wave therapy, infra-red radiation, and light massage are
designed to increase the volume of the blood flowing through the muscles and
thereby removing the offending metabolites. The use of muscle relaxant drugs
like Diazepam 5-10 mg B.D is effective in relaxing the symptoms.
5) CORRECTION OF OCCLUSAL PLANE
The occlusal plane in most partially edentulous mouths will be uneven.
The severity of this irregularity will determine the treatment necessary to
correct the condition. Teeth that have been unopposed for a long time tend to
overerupt, e.g. the maxillary molars if unopposed will migrate downwards
carrying the maxillary tuberosity with them creating a problem to reestablish the
occlusal plane. This is because surgery to reduce the bone height may encroach
upon the maxillary sinus.
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Normally the occlusal plane is corrected by reduction of the height of
overerupted teeth.
Methods undertaken are:
Orthodontic tooth movement Enameloplasty Onlay Crowns Endodontics with crown or coping Extraction Surgery
Orthodontics is the ideal treatment to upright the tilted teeth and re- establish
the occlusal plane. If this is not possible other methods are employed.
Enameloplasty word used to describe the removal of a portion of enamel
surface of tooth to accomplish specified purpose. It consists of reduction of cusp
height in order to level the curve of spee. Penetration of enamel layer should be
avoided, However in older individuals with wear and subsequent secondary
dentine formation slightly more tooth structure can be removed. Care to be
taken not to mutilate the anatomic contours such as accessory grooves, and
sluiceways must be restored. Reduction is done with tapered diamond cylinder
or stones in high speed hand piece. The cut enamel surface is smoothened
with carborundum containing rubber wheels and fluoride gels.
Onlays were common previously but now rarely used. The occlusal surface can
be covered with onlay rest free of pits and fissures. The use of chrome- cobalt
can cause extreme wear of natural teeth. Tooth colored resin may be processed
over the metal, however this will wear rapidly. The simplest method is the use
of cast gold onlays. One of main advantages of onlay is maintain the natural
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contours of enamel surfaces of tooth. However full crowns,are replacing onlays
presently because of esthetic and retentive concerns.
Crowns are indicated when the facial and lingual surfaces need to be altered, to
produce desirable height of contour, a guiding plane, or a retentive undercut. If
tooth reduction is too great then endodontic treatment should be considered.
If the strategically positioned teeth need to be retained as abutment teeth for
partial denture and required to be corrected extensively then intentional
endodontics with crowns are considered.
Extraction-It should be the goal of a designer of removable partial dentures to
retain as many of the remaining teeth as possible. However, at times retaining
certain teeth can greatly complicate or even compromise the success of the
treatment. For example, if orthodontic treatment cannot be accomplished to
realign severely malposed molars or premolars, extraction must be considered.
When teeth interfere with the placement of the major connector and no other
solution (such as crowning the tooth) feasible, extraction must be planned.
Surgical repositioning- Surgical repositioning of one or both jaws or ofsegments of one or both jaws can be performed to correct malrelationship of
teeth. Various forms of mandibulectomies, usually to correct gross prognathic
jaw relationships, have been performed. Maxillary segmental osteotomy is done
to superiorly repositioning posterior segments of maxillae. This is one of the
most effective methods of regaining interarch space lost due to downward
migration of the teeth and tuberosity
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6) CONSERVATIVE/ENDODONTIC PREPARATION
Fillings
Onlays
Endodontic treatment with crown/coping
Fillings: When fillings are required in abutment or other teeth, only gold or
amalgam are suitable materials lo come into contact with partial dentures as
these materials have the necessary strengths to form a foundation for occlusal
rests. Amalgam fillings or gold inlays are used to restore lost contours of the
teeth.
Onlays: The occlusal surfaces of worn teeth can be restored by onlays. The
occlusal surface of a tooth to be covered by an onlay should be free of pits and
fissures and if present, should be removed by an enameloplasty. If onlay rests
are placed than they should be constructed short of occlusal contact with
retentive beads present on the metal surface, for tooth-colored acrylic resin to be
processed over it. This is done to prevent the metal coming in contact with
natural teeth which if otherwise would cause rapid wear of the opposing enamelsurface.
Endodontic with crown/coping: Some of the strategic important teeth present
in the arch, like an anterior tooth present in a long anterior edentulous span,
should be retained and used as abutment for the partial denture. But most of the
time these strategic important tooth/teeth are over erupted are have lost some of
their periodontal support which is needed to serve as an abutment. In such casesendodontic therapy followed by cementation crown will allow such tooth/teeth
to serve as normal abutments. Porcelain jacket crowns should generally be
avoided in partially edentulous mouths as they make very poor abutments
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7) CORRECTION OF MALALIGNMENT
Teeth that are malposed, facially or lingually are more difficult to correct
There are definite, limitation to the repositioning of these malposed teeth.
Orthodontic correction of these malposed teeth is the first line of treatment.
Enameloplasty and crowns are also treatment choices. Surgical intervention is
planned only if all other measures fail to reposition these malposed teeth.
II) Prosthetic Mouth Preparation
It is done to modify the existing structures to further enhance the placement of
prosthesis.
It mainly involves reshaping of teeth
The steps involved are:
Developing guiding planes Changing height of contour Modifying retentive undercut. Abutment preparation using cast crowns Rest seat preparation
Tooth surfaces often need to be reshaped to accomplish specific purposes.
This changing of tooth contour may be accomplished in the enamel, on the
surface of an existing restoration, or by placing a new restoration.
Enameloplasty
Conservatism must be the rule when tooth preparation is to be accomplished on
enamel surfaces for a removable partial denture. Sufficient tooth reduction must
be accomplished to ensure adequate space or proper contour, but never at the
expense of overcutting the tooth
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Enameloplasty to Develop Guiding Planes
Guiding planes are those surfaces on the teeth, of sufficient area and parallel
relationship to each other, so that they may serve to determine positively the
direction of appliance movement (Applegate 1954)
Two or more vertical parallel surfaces of abutment teeth, so shaped to direct
prosthesis during placement and removal.(McCracken 12th
edn)
Two or more vertically parallel surfaces of abutment teeth, so orientated as to
direct the path of placement of removable partial dentures.( GPT 8th )
Functions of guiding planes
1. To provide one path of placement and removal2. To ensure planned and intended action of the retentive and bracing
components of the partial denture
3. To eliminate detrimental strain to the abutment teeth and the componentsof the framework in placing and removing the prosthesis
4. To eliminate gross food traps between the abutment teeth and the denturebase
5. To provide retentive characteristics against dislodgement of the denturewhen the dislodging force is other than parallel to the path of removal
6. To provide bracing characteristics against horizontal rotation of thedenture
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Guiding Planes On Abutment Teeth Adjacent to Tooth Supported
Segments
A cylindrical diamond point is generally the instrument to make the preparation.
A gentle, light sweeping stroke from the buccal line angle to the lingual line
angle should be used. The flat surface created should ideally be 2 to 4mm in
occluso-gingival height The reduction must not be a straight slice across the
tooth surface; rather it should follow the curvature of the surface so that nearly
uniform amounts of enamel are removed
Guiding Planes on Abutment Teeth Adjacent to Distal Extension
Edentulous Spaces
The tooth preparation on the proximal surface ofabutment teeth adjacent to
distal extension edentulous spaces is accomplished in thesame manner with a
cylindrical diamond stone held parallel to the path of insertion.
A guiding plane prepared adjacent to a distal extension space should be slightly
shorter than a guiding plane prepared adjacent to a tooth supported segment.Typically, a guiding plane adjacent to a distal extension space is 1.5 to 2.0 mm
in height. The reduced height results in decreased contact with the associated
minor connector (ie, proximal plate) and permits greater freedom of movement
for the associated removable partial denture as a result; potentially destructive
torquing forces are minimized
Guiding Planes On Lingual Surfaces Of Abutment TeethMandibular posterior teeth are usually inclined lingually with a resultant high
lingual survey line. Minor recontouring can frequently improve the position of
the survey line to allow placement of the reciprocal clasp arm in its proper
position
The purpose of providing guiding planes on lingual surfaces of teeth is to
provide maximum resistances to lateral stresses. The more teeth involved in
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guiding plane preparation, the less will be the stress transmitted to each
individual tooth.
The occluso-gingival height of the preparation is 2 to 4 mm. The plane ideally
should be located in the middle third of the clinical crown of the tooth. Special
care must be shown to avoid changing the contour of the gingival third of the
tooth because damage to the marginal gingiva through the improper shunting of
food may occur if the normal morphology of the gingival third of the crown is
lost
Guiding Planes on Anterior Abutment Teeth
Guiding planes on anterior teeth adjacent to edentulous spaces provide the
parallelism needed to ensure stabilization, minimize wedging action between
the teeth, decrease undesirable space between the denture and the abutment
tooth, and increase retention through frictional resistance.
Another important purpose of these guiding planes is to reestablish the normal
width of an edentulous space. If one or more anterior teeth are lost,adjacentteeth tend to drift or tip into these spaces.Both actions result in reduced space
and make esthetic replacement of the missing teeth much more difficult.
Tipping is relatively common and often results in a large undercut apical to the
height of contour
If the tooth is not recontoured, this undercut will appear as an unsightly space
between the tipped tooth and the removable partial denture.Such a space detracts from the esthetic value of the removable partial denture
and acts as a food trap.
Recontouring should be performed to minimize the effects of tipping and to
improve the esthetic and functional results of the removable partial denture
service. This recontouring should be performed with the proposed path of
insertion in mind, and the resultant guiding planes should be parallel to the
planned path of insertion.
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Enameloplasty to Change Height of Contour
The height of contour is changed most frequently to provide better positions for
clasp arms or for lingual plating.
Ideally the retentive clasp arm should be located no higher than the junction of
the gingival and the middle thirds. This position not only enhances the esthetic
quality of the clasp, but also places clasp nearer the tooths centre of rotation
The amount of correction that can be accomplished by recontouring the enamel
surface is limited by the thickness of the enamel. Care has to be taken not to
penetrate the enamel and expose dentin
The height of contour is best lowered by using tapered diamond stones.
Enameloplasty to Modify Retentive Undercuts
It is used to increase a less than adequate retentive undercut only if the
oral hygiene of the patient is good & caries index is low. But this should not be
substituted for adequate design procedures.For the procedure to be successful, the buccal and lingual surfaces should be
nearly vertical. If surface to receive undercut is sloped, indentation has to be
excessively deep. If opposing surface is sloped, the reciprocal clasp arm cannot
prevent retentive clasp tip from dislodging. Retentive undercut should be in the
form of a gentle depression. Create slight concavity (0.010 inch deep, 4mm
MD, 2mm OG), parallel to gingival margin without encroaching it. A round endtapered diamond held parallel to gingival margin is used to create a gentle
depression.
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Abutment preparation using Inlays Onlays and Crowns
If the remaining teeth do not possess usable natural contours and enamel
surfaces cannot be corrected to produce them, cast restorations must be planned.
Guiding planes, height of contour and retentive undercuts can be placed in the
wax patterns for the cast restorations. Also many abutment teeth will require
restorations for more routine reasons such as caries, endodontic therapy etc.
Shaping the Wax Pattern
The die of the tooth preparation in the cast of the remainder arch is
analyzed on the surveyor. Working cast is mounted at the same tilt as the
diagnostic cast. Once correct tilt is established substitute analyzing rod with
wax knife and carve guiding plane by shaving the wax. Pattern must be hand
carved to place height of contour at the junction of gingival and middle third for
retentive clasp. Refining can be done in cast restoration.
Occlusion Rest Seat Preparation
Rest -rigid extension of a partial removable dental prosthesis that contacts the
occlusal surface of a tooth or restoration, the occlusal surface of which may
have been prepared to receive it
Rest seat-the prepared recess in a tooth or restoration created to receive the
occlusal, incisal, cingulum, or lingual restFunctions-
Direct forces of mastication parallel to long axis.
Prevent gingival displacement of denture.
Maintain the clasp in proper position.
Function as indirect retainer in distal extension partial denture.
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Occlusal Rest Seat in Enamel
Form
Triangular in outline with base at marginal ridge and apex pointing
towards the centre of the tooth. Apex of the triangle should be rounded as
should all external margins of preparation
Should follow outline of mesial or distal fossa.
Minimum 0.5mm at thinnest point, 1-1.5mm at marginal ridge.
Extension
1/3rd to 1/2 of mesiodistal diameter.
1/2 of the distance between buccal and lingual cusp tips.
Floor
Inclined towards the centre.
Spoon shaped.
Enclosed angle with the proximal surface less than 90.
Preparation
Round diamond stone is used approximating no.4 round carbide bur for
preparation. Create an outline using small round diamond stone. The island of
enamel within the outline can then be removed with the same bur. Deepest
portion of the rest seat is towards the center of the tooth. Verify preparation byred beading wax. Polishing of preparation is done using carborundum
impregnated rubber point in low speed hand piece
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Occlusal Rest Seat in New Gold Restoration
It should always be placed in wax patterns. Sufficient occlusal clearance
must be given to permit proper dimensions of rest seat. A depression can be
added to the preparation to accommodate rest seat. Rest seat in wax pattern is
prepared by using no.4 round steel bur.
In Existing Gold Restoration
Patient must be warned of the possibility of the need to replace the
restoration. If restoration has marginal integrity and occlusal harmony, attempt
can be made to contour a rest seat in it.
Occlusal Rest Seat Preparation In Amalgam Restorations
An occlusal rest preparation in a multi surface amalgam restoration is less
desirable than that in either sound enamel or a gold restoration. Amalgam alloy
tends to flow when placed under constant pressure. Care must be taken not toweaken the proximal portion of the amalgam restoration at the isthmus during
the preparation. This may result in fracture during function.
Rest Seat Preparation For Embrasure Clasp
This preparation extends over the occlusal embrasure of two approximating
posterior teeth, from the mesial fossa of one tooth to the distal fossa of other.Insufficient tooth removal will generally lead to occlusal interferences between
the metal of the clasp and the opposing cusps. Relieving the metal to gain
occlusal freedom ultimately leads to breakage of the clasp during function.
Repair of the embrasure clasp is usually difficult.
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As the preparation passes over the buccal and lingual embrasures it should be
approximately 3.0 to 3.5 mm wide and 1.5t o 2.5 mm deep. All contours of the
preparation must be rounded after the preparation is complete
Rest Seat Preparation on Anterior Teeth
An occlusal rest on a molar or a premolar is preferred over a lingual or an
incisal rest on anterior teeth to provide support for a partial denture. Forces are
better directed down the long axis of the abutment tooth by an occlusal rest than
by a lingual or incisal rest. A canine is preferred over an incisor for support of a
denture. When a canine is not present, multiple rests on incisor teeth are needed
in place of a single rest on a single incisor tooth. A lingual rest is preferred to an
incisal rest.
Lingual Rest Seat preparation In Enamel
A lingual rest seat may be prepared in the enamel surface of an anterior tooth if
the tooth is sound, the patient practices good oral hygiene, and the caries indexis low. The cingulum should also be prominent to present a gradual slope to the
lingual surface rather than a steep vertical slope. This is the principal reason
why mandibular canines are poor candidates for a lingual rest. The lingual
surface of the tooth normally has too great a vertical slope to permit the rest seat
to be prepared without penetrating into dentin. In some instances a lingual rest
can be placed on maxillary central incisors that have prominent cingulum, butmost, often this is a compromise effort unless it is placed in a cast restoration.
The lingual rest can be prepared nearer the center of the tooth, preventing the
tipping action that an incisal rest may produce. Lingual rests are also more
acceptable esthetically and less subject to breakage and distortion. The most
satisfactory lingual rest from the standpoint of support is one that is placed on a
prepared rest seat in a cast restoration. This should be used wherever possible.
A lingual rest on a cast restoration may be used on any anterior tooth, either
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maxillary or mandibular. A lingual rest prepared in a enamel surface should be
used primarily on maxillary canines and on a limited number of maxillary
incisor teeth.
Outline Form -
Half moon shaped forming smooth curve from one marginalridge to other.
Should cross the centre of tooth incisally to cingulum. The rest seat itself is V shaped. The labial incline of lingual surface makes one wall. Other wall starts of cingulum and inclines labio-gingivally
towards the centre of tooth.
Preparation
Preparation of a cingulum rest seat is accomplished using a No. 38 carbide bur
in a high-speed handpiece. The No. 38 bur is an inverted cone with side- and
end cutting surfaces. During the preparation process, the bur is oriented at a
slight angle to the lingual surface of the tooth. The bur is then used to create acrescent-shaped rest seat that begins on one marginal ridge, passes over the
cingulum, and terminates on the opposite marginal ridge .The walls of the rest
seat are relatively smooth and that they do not present any mechanical
undercuts.
The preparation is finished using a green stone in a low-speed handpiece.
Polishing is accomplished using a carborundum impregnated rubber wheel orpoint in a low-speed handpiece
Lingual rest seat preparation in cast restorations
If a cast restoration is to be placed on abutment tooth, the rest seat should be
carved in the wax pattern and not cut in the cast restoration. A definite rest seat
thus developed will direct the forces of occlusion through the long axis of the
abutment tooth.
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Conclusion
The preparation of mouth is fundamental to a successful removal partial
denture. The prime objective of all the mouth preparation procedures is to return
the mouth to optimum health and to eliminate any condition that compromises
the success of the partial denture.
REFERENCES
1. McCrackens removable partial prosthodontics 12th edition 20112. Stewarts: Clinical removable partial prosthodontics,3rd edition 20033. Applegate OC. Essentials of Removable Partial Denture Prosthesis, 3RD .
Philadelphia: Saunders, 1965
4. Osborne & Lammies Partial prosthodontics 5th edition 19865. McCracken .L.W.Mouth preparation for partial dentures , J. Prosthet.
Dent 1956;6,(1) :39-52
6. Glan G.W., Appleby R.C. Mouth preparation for removable partialdentures. J. Prosthet. Dent. 1960;10:124-134.
.