Dental Prosthodontic Notes

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    DENT2030 Block Notes:

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    Important Radiographic Findings:

    o Caries

    o Endodontic lesions

    o Assessment of bone loss and periodontal disease

    o Retained roots

    o

    Impacted teeth

    Aesthetic Limitations:

    o Tooth loss generally followed by recession of bone and soft tissues

    o Clasp positioning

    o Diastemas and recession leading to metal show-through

    o Maxillary gingival display on animation

    Functional Limitations:

    o Large anterior or posterior cantilever length leading to rotation about the

    abutment teeth and trauma

    o Over-eruption

    o

    Tilting of abutment teeth

    o Loss of vertical dimension

    o Mobile teeth

    Studies Regarding RPDs:

    o Studies conducted to determine effect of RPDs on oral structures particularly

    the periodontium and remaining teeth. Results not unanimous

    o Earlier studies report increased occurrence of caries and periodontal disease

    which were extensive

    o Others found mild periodontal injuries or practically no caries progression or

    periodontal disease progression after RPD insertiono RPDs promote plaque formation on abutment teeth and teeth in contact with

    them

    Evidence of Increased Caries:

    o Direct relationship between wearing RPDs and prevalence of root caries is

    strong

    o Wearing of removable partial dentures predisposes to high salivary levels of

    streptococcus mutans and yeasts

    Impact of RPDs on Periodontal Patient:

    o Impact of RPD wearing on periodontal health reported to be unfavourable

    o

    Gingivitis more commonly found in glass I Kennedy than class II caseso More frequently occurring in mandibular arch than maxillary

    o Periodontal health of teeth can be maintained if principles of RPD design are

    followed (rigid major connectors, simple design and proper base adaptation)

    o Improper RPD design may lead to changes in tooth mobility and increased

    probing depth due to increase in plaque bacteria accumulation

    o Prior to RPD treatment, periodontal screening in terms of oral hygiene, plaque

    and gingival inflammation presence, attachment loss, remaining osseous

    support and mobility should be conducted followed by a definitive periodontal

    treatment to eliminate periodontal disease, trigger defects that hinder plaque

    control and create better cleaning environmento Strategic extractions of periodontally weakened teeth should be performed

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    o Potential Further Modifications:

    Multiple rests to ensure adequate vertical support remains when

    abutment teeth are lost

    Open lattice base strategically placed in maxillary major connector (to

    facilitate replacement of future loss of teeth)

    Location of finish lines modified to provide a smoother resin-to-metal

    transition when posterior teeth are lost

    Wire direct retainers that provide more physiologically acceptable

    clasping of compromised teeth

    10 year re-evaluation of 74 patients who had worn RPDs during that

    time showed only 36% of dentures were free of hygiene related

    problems. 36% had calculus on acrylic surfaces and 14% had calculus

    on metal surfaces

    However, longitudinal studies indicate that wearing of RPDs is not

    associated with any deterioration of the periodontal status provided

    good oral hygiene is maintained

    High prevalence of plaque, gingivitis and gingival recession, especially

    in dento-gingival surfaces in close proximity (within 3mm) to the

    dentures

    Increased need for regular oral hygiene reinforcement, scaling and

    prophylaxis among RPD wearers

    Low caries incidence in the study, root caries found to be associated

    with contact with the RPDs (P

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    o Advantages of Chrome:

    Can be polished to much thinner proportions without losing strength

    Designed to distribute forces to abutment teeth

    Rigid and well adapted to teeth and tissues

    Rigid and allows soft tissues to take on masticatory force

    Can weld on additions

    o Disadvantages of Chrome:

    Requires accurate master impression

    Requires additional appointment for metal try-in

    Additions are difficult and cannot be done chair-side

    Requires precise planning

    Inaccurate impressions may require complete remake

    Additional lab cost for framework

    o Chrome vs AcrylicDentist Perceptions:

    Dentist experience with chrome and acrylic

    Dentists less experienced at providing cobalt-chromium based RPDs

    were generally less willing to provide this type of prosthesis

    Cobalt-chromium based RPDs seen as harder to get right first time and

    more expensive to get wrong

    o Chrome vs AcrylicPatient Perceptions:

    Patients more motivated to wear their RPD if it filled upper anterior

    gap as they felt gap would be obvious to others

    RPDs also perceived as helping to support facial shape and for smiling

    Aesthetic problems such as pink plastic on acrylic resin based dentures

    being wrong colour or metal on cobalt-chromium based denture being

    visible when they smiled

    Lack of sensation of food in the mouth, inconvenience of having to

    apply denture fixative regularly and rinsing dentures after eating and

    negative patient based factors

    Other negative impacts on physical function were instability of denture

    in mouth (flipping), problems with speech and feeling of having

    something in the mouth which could include gagging

    In conclusion, patient education and continued maintenance essential to success of

    RPDs and preservation of remaining structures

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    Principles of Removable Partial Denture Design Diagnostic Cast:

    Objectives:

    o Systematic protocol for designing cobalt-chromium partial dentures

    o Understand components of CoChr partial dentures

    History:o Physical condition

    o Psychological condition

    o Frequency of dental examinations

    o Previous dental treatment (especially RPDs)

    Diagnostic Casts:

    o Path of Insertion and RemovalMust be different to path of dislodgement by

    sticky foods which is perpendicular to the occlusal plane

    o Rest PreparationNormally placed adjacent to the edentulous space, ideally

    placed on sound tooth structure

    o

    Embrasure ClearanceArticulated casts will allow evaluation of clearance ofminor connector of suprabulge clasp over occlusal surface

    o Cuspal InterferenceArticulated casts will allow space evaluation for metal

    framework and denture teeth/base

    o Excessive Contours Require ReductionTilted/rotated and natural teeth may

    have contours which do not allow for broad guide planes and allow for food

    trap. Over contoured teeth may not allow for sufficient undercut for the

    retentive clasp and may not promote or enable adequate oral hygiene practice

    o Acute Angles Must be RoundedMetal framework cannot pass over acute

    angles and this may create space to allow plaque accumulation. Use polishing

    discs/burs to round line angles for better adaptationo Retentive and Non-Retentive Areas of Abutment TeethAbutment teeth

    (adjacent to edentulous space) will often be clasped for direct retention.

    Suitable undercuts are evaluated during surveying

    o Creation of UndercutsTerminal tip of clasp must end in an undercut to allow

    for direct retention. If suitable undercuts are not available these must be

    created

    o Presentation of Treatment Plan to PatientPatients desires should be

    considered. Aesthetics, palatal coverage, previous successful designs and

    alternative treatment plans should all be considered

    o

    Fabrication of Individual TraysRemaining dentition checked for clasping(e.g. periodontal issues, caries and endodontic lesions or heavily restored

    teeth)

    o Assist in Preliminary Design of RPDWorking model is used for surveying

    and design of final casting. Should be drawn to accurate dimensions and coded

    to allow communication between dentist and dental technician. The borders of

    the major connectors should be indicated and are the responsibility of the

    clinician

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    Biomechanical Considerations:

    o Tooth more able to tolerate vertical forces than it can oblique or horizontal

    forces

    o Force ApplicationThe lower the better

    o The retention arm and clasp arm should contact tooth at the same time

    o

    There are differentials in tissue resiliency

    o Movement of the denture base means a torqueing force on the abutment tooth

    Occlusal Plane:o Make occlusal plane of the maxillary arch coincident with that of the

    mandibular arch (less overlap)

    Tilt:

    o Try and minimise undesirable undercuts and maintain desirable undercuts

    o Posterior TiltGenerally for replacement of anterior teeth

    o Horizontal TiltGenerally best orientation. Equalise undesirable undercuts or

    adjust the tooth

    Design Sequence:

    o After examination of the occlusal plane and establishing the most

    advantageous orientation (tilt) of the cast, the RPD design can be formulatedo This is done using a sequence bearing in mind the factors elucidated in the

    treatment planning

    o Following Sequence is Used:

    RestsControls prosthesis in relation to the teeth

    Major ConnectorsRigid components that unites various parts of the

    denture

    Minor ConnectorsComponent that unites other parts with major

    connector

    Retention ArmsFlexible attachment that contacts abutment tooth

    below the survey line Reciprocation ArmsRigid attachment that contacts abutment tooth and opposes the

    retention arm

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    Principles of Removable Partial Denture Design Surveying and Blocking:

    Necessity of Surveying:

    o To determine path of insertion (maximise desirable undercuts and minimise

    undesirable undercuts)

    o

    To determine position of desirable undercuts for retentiono To determine position of undesirable undercuts which need to be blocked out

    Undercut Areas:o Areas below point of contact of analysing rod and tooth or tissue surface

    Non-Undercut Areas:

    o

    Areas above point of contact of analysing rod and tooth and tissue surface

    Analysing Rod:

    o Used in a preliminary study of the model to locate tooth and tissue undercuts

    Carbon Marker:

    o

    Replaces analysing rod after desired tilt has been obtained and marks thesurvey lines in the abutments

    Undercut Gauge:

    o Used as a guide in arriving at a correct tilt of the model and in measuring the

    exact amount of retention needed for a clasp

    o 0.01 Most cast claps

    o 0.02 Wrought stainless steel clasps at least 8mm long and longer cast clasps

    o 0.03 Wrought gold clasps

    o You can be at the mercy of your technician

    Support:

    o

    The mechanical function which prevents tissue-ward movement of a partialdenture supplied primarily by well-designed occlusal rests on all abutments

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    Bracing:

    o The mechanical function which prevents lateral shifting of a partial denture

    supplied chiefly by rigid portions of the clasps

    Retention:

    o The mechanical function which prevents dislodgement of a partial denture,

    mainly derived from the flexible clasp tips

    Reciprocation:

    o Counteracts the force of the retentive clasp arm and prevents movement of the

    abutment tooth. Should stay above the height of contour for the entire length

    of the clasp. The major connector can also serve as a reciprocator

    Reinforcing Sheath:

    o Used solely to strengthen the carbon marker and reduce the amount of

    breakage

    Survey Line:

    o

    Mark around the tooth indicating its greatest circumference in any oneposition relative to the vertical

    Undercut Areas:

    o Always below survey line and accommodate the flexible retentive positions of

    clasp arms

    Non-Undercut Areas:

    o Always above the survey line accommodating occlusal rests and ridge portions

    of clasps

    Direct Retainers:

    o Two teeth with survey lines at the same height but the clasp arm will need to

    be placed much further below the line on the right in order to engage the samedegree of undercut

    o Also known as clasps, can be suprabulge or infrabulge

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    The Four Survey Lines:

    Blocking Out:

    o Block out any undercut areas that will be crossed by rigid parts of the denture

    (which is every part of the denture framework except the retentive claspterminals)

    o In addition, block out areas not involved for convenience, ledges on which

    clasp patterns are placed, relief beneath connectors to avoid tissue

    impingement and relief to provide attachment of the denture base to the

    framework (thin layer over the palate for instance)

    Process of Fabrication:

    o Following surveying and blocking out, the cast should be invested

    o A wax pattern of the denture should be fabricated on the invested cast

    o The cast should be sprued and is then ready to invest

    o

    Following this it is cast with ingotso After casting it is polished

    Components of a Removable Partial Denture:

    o Rests:

    o Major Connector:

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    o Minor Connector:

    o RetentionDesirable Undercuts:

    o Reciprocation:

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    Principles of Removable Partial Design Major Connectors and Rests:

    Kennedy Classification:

    o Allows for communication between clinicians and laboratory

    o Different design principles based on classification

    o

    Class IBilateral distal extension:

    o Class IIUnilateral distal extension:

    o Class IIITooth bound saddle area:

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    Palatal Bar:

    Limited to short class III applications

    Should not be placed anterior to 2ndpremolar as this may feel

    uncomfortable to the patient due to bulk

    Should be avoided due to bulk

    Narrow A-P width means there is little vertical support from

    the palate

    A-P Palatal Strap:

    Each strap should be 8mm wide

    When anterior teeth are not replaced the anterior strap should

    be as far posterior as possible for comfort

    Open area should be a minimum of 20x15mm

    Indicated in cases of palatal tori, loss of anterior or posterior

    teeth

    Rigid and can be made quite thin Due to less contact with palate there is less support from the

    palate and more from the teeth

    Contraindicated with poor periodontal support

    A-P Palatal Bar:

    Anterior bar resembles palatal strap, posterior bar resembles

    palatal bar and is bulky

    Increased rigidity due to its shape (encirclement)

    Same indications as A-P strap

    May be uncomfortable due to tongue and phonetics due to bulk

    Horseshoe:

    Extends onto soft tissues 6-8mm thick

    Borders should be at junction of horizontal and vertical portion

    of the plate

    Rigidity can be increased by extending borders onto horizontal

    portion of the plate

    Used primarily when anterior teeth are missing

    Due to tendency to flex, not a good choice for distal extension

    dentures or when cross-arch stabilisation is required

    Due to limited resistance to flexing may have concentration offorces that result in damage to abutment teeth

    Full Palatal Coverage:

    Ultimate rigidity/support/greatest amount of tissue coverage

    Posterior border should have bead line but cant act as a post-

    dam

    Intimate contact with soft tissues increases retention through

    adhesion and cohesion

    Indicated in periodontally compromised teeth and short

    alveolar ridge height

    Enhances thermal conductivity Can observe papillary hyperplasia with continuous wear

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    Should not substantially alter the natural contour of the lingual surface

    of the mandibular alveolus or the palatal vault

    Cover no more tissue than is absolutely necessary

    Contributes to support of the prosthesis

    Needs support from other elements of the framework to minimise

    rotation in function

    o Other Major Connectors:

    Cingulum bar

    Sub-lingual bar

    Labial bar

    Support:

    o Mucosal Support:

    Tissue borne denture

    Tissue borne denture needs to cover large area of mucosa

    o Tooth Support:

    Support against occlusal loading obtained almost solely from standing

    teeth, usually adjacent to saddle area are tooth borne dentures

    Denture gaining a lot of its support from standing teeth will cover less

    mucosa than a denture obtaining support only from mucosa

    Rests:

    o Any unit of a denture that rests on a tooth surface that has been properly

    prepared to receive them to provide vertical support

    o Prepared surface on abutment tooth is the rest seat

    o Purpose of Rests:

    Maintain components in their prepared positions

    Maintain established occlusal relationships

    Prevent impingement of soft tissue

    Directs and distributes occlusal loads to abutment teeth

    o In Cases with no Rests:

    Cannot maintain components in prepared position

    Cannot maintain established occlusal relationships

    Impinges on soft tissues

    Does not direct and distribute occlusal load to abutment teeth

    o Form of Occlusal Rest and Rest Seat:

    Rounded triangular shape

    Length/width is 2.5mm (if they are smaller they are too weak)

    Marginal ridge needs to be lowered by up to 1.5mm

    Floor should be concave and apical to marginal ridge

    Rest:Minor connector angle should be less than 90o

    Preparation of occlusal rest seats should always follow proximalpreparation (guide planes), never precede it

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    o Interproximal Occlusal Rest Seats:

    Adjacent rests used to avoid interproximal wedging

    Preparation may extend further to lingual and buccal

    o

    Cingulum Rests on Anterior Teeth: Canines preferred over incisors

    Rarely satisfactory on lower incisors due to insufficient enamel

    Preparation of Cingulum Rests:

    Slightly rounded V

    Floor needs to be perpendicular to long axis of the tooth

    Place in a cast restoration

    Cingulum rests function as indirect retention

    o Incisal Rest:

    Used mainly as auxiliary rests or as indirect retainers

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    Direct and Indirect Retention for Denture Bases:

    Clasp Design and Indirect Retainers:o

    Design clasps and indirect retainers to minimise tooth coverageo Excessive coverage will increase plaque retentiono

    Gap between partial denture clasp and posterior tooth is a minimum of 6mm Support:

    o Resists tissue ward movement of prosthesis

    o Distributes masticatory load

    o Prevents damage to periodontal structures

    Bracing:o Resistance to horizontal movement of prosthesiso Horizontal forces are generated during function by occlusal contacts and by

    the oral mucosa surrounding the denture. These forces tend to displace the

    denture in both antero-posterior and lateral directionso

    Bracing occurs only when the denture is fully seatedo Posterior movement of the distal extension saddle is prevented by the

    coverage of the retro-molar pad and the minor connectoro The distal end saddle can undergo rotation in the horizontal plane. If a single

    abutment tooth is rigidly clasped, it can undergo considerable forces

    Retention:

    o Resistance to dislodgement in an occlusal direction

    o Flexibility of metal determines amount of undercut

    Shape, bulk and stiffness of metal determine flexibility

    o

    Retentive areas similar on opposite sides of the archo Retention should be the minimum necessary to resist reasonable dislodging

    forces

    Reciprocation:

    o Opposes the force exerted by the clasp arm terminal during seating and

    unseating of the prosthesis

    o Prevents tooth movement from over adjustment of clasps

    o The reciprocal arm does not engage an undercut

    Encirclement >180o:

    o The clasp assembly must engage more than 180 degrees to prevent tooth

    movemento Prevents clasp from slipping off the tooth

    o Minimum three-point contact is necessary

    Passivity:

    o When a clasp is in place it should not grip the tooth

    Factors Affecting Resiliency of a Clasp:

    o Cross-sectional size

    o Length

    o Taper

    o Kind of metals (cast metal not as resilient as a wrought metal)

    o

    Proper heat treatment will increase resiliency

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    Additional Factors in Proper Clasp Design:

    o Position of survey line and undercut depth

    o Occlusal rest must be thick enough

    o Rests should never be placed on inclined surfaces

    o When anterior teeth are replaced, lingual or Incisal rests should be placed in

    prepared rest seats

    o Clasp arms that cross a groove on a tooth should follow the groove contour to

    maintain a uniform thickness

    Types of Clasp:

    o SuprabulgePull type:

    Approaches from occlusal aspect of the survey line

    o InfrabulgePush (trip) type:

    Approaches from the gingival aspect of the survey line

    o SuprabulgeCircumferential (Akers):

    Clasp with arms that originate at the minor connector, usually near the

    occlusal rest and approach the undercut from an occlusal direction

    o SuprabulgeReverse C:

    Modification of circumferential Useful if undercut near embrasure where clasp arm originates

    o SuprabulgeDev Van:

    Clasp arm originates in the saddle and follows the line of the acrylicsaddle above the undercut and then turns

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    o SuprabulgeBack action:

    Engages 0.010 inches in the undercut on the distal if the buccal

    undercut is small (0.010 inches or less)

    o SuprabulgeRing:

    Encircles nearly all the tooth

    Engages 0.020 inches in the undercut in bilateral tooth borne partial

    dentures

    Frequently used with tilted molars

    o

    InfrabulgeBar or roach: The approach arm of a bar clasp must never impinge on soft tissue

    The approach arm must never designed over a deep soft tissue

    undercut

    o Infrabulge RPI Clasp:

    Important with a distal extension base

    R Mesial rest

    P Proximal plate

    I I bar clasp arm

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    Indirect Retainers:

    o A part of a removable partial denture which assists the direct retainers in

    preventing displacement of an extension base by functioning through lever

    action on the opposite side of fulcrum line

    o In extension base partial dentures, dislodging forces occur which tend to lift

    the bases and cause a displacement

    o Imaginary lines drawn between distal rests will indicate fulcrum lines

    o Indirect retainers are effective in proportion to their support and distance from

    the fulcrum line, the further the better

    o Types of Indirect Retainers:

    Occlusal rest

    Incisal rest

    Cingulum rest

    Others (lingual plate is poor and should not be considered as an

    indirect retainer)

    o

    Also important for reline procedures of extension base partial dentures

    Denture Bases:

    o Part of the RPD that rests on the oral mucosa to which the artificial teeth are

    attached

    o Consideration for Denture Bases:

    Support is the primary concern

    Cover the greatest area possible within the confines of the musculature

    The base should be able to be modified or relined easily and

    economically

    o Types:

    Plastic Resin Base:

    Extension base RPDs

    Long edentulous span

    Relining anticipated

    Strong junction between base and major connector

    Tissue stops

    Tight mesh not desirable

    Metal Base:

    Tooth supported RPDs with short spans

    Inadequate occlusal space for a resin base Dont over extend

    Base can be thinner than framework for resin denture base

    Avoid sharp margins in finishing

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    Fundamentals of Occlusion:

    Occlusion:

    o Static relationship between incising and masticating surfaces of the maxillary

    and mandibular teeth or tooth analogues

    Facial Types:o Mesofacial:

    Most symmetrical, balanced pattern

    o Dolicofacial:

    Increased lower face height

    Increased mandibular angle

    o Brachyfacial:

    Reduced lower face height

    Reduced mandibular plane angle

    Angles Classification:

    o

    Measuring parametero Based on first molars

    o Class I, II and III

    o Fails to address horizontal (over jet) and vertical (overbite) dimensions of

    malocclusion

    o Fails to address underlying skeletal discrepancy

    Ideal Occlusion:

    o A standard against which patient occlusion can be compared and evaluated

    o Treatment plans can then aim to improve the occlusal scheme

    Natural (Organic) Occlusion:

    o

    Bilateral posterior centric contact

    o Anterior guidance

    o Mutually protective scheme of occlusion

    o Ideally occlusion may be studied in be unrestored complete dentition

    o Canine Guidance:

    Form of mutually protected articulation in which vertical and

    horizontal overlap of the canine teeth disengage the posterior teeth in

    the excursive movements of the mandible

    o Posterior occlude in centric relation only

    o Incisors are only teeth contacting in protrusion

    o

    Canines are only teeth contacting in lateral excursion

    Lateral Movements Types:

    o Group function

    o Partial group function

    o Canine guidance (posterior disclusion)

    Group Function:

    o Multiple contact relations between maxillary and mandibular teeth in lateral

    movements on the working side whereby simultaneous contact of several teeth

    acts as a group to distribute occlusal forces

    o Whenever the arch relationship does not allow anterior guidance to do its job

    of discluding the non-working side. Case by case choice

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    o Indications:

    Class III with all mandibular anterior teeth outside of maxillary

    anterior teeth

    Some end to end bites

    Anterior open bite

    Periodontally/traumatically compromised canines

    Partial Group Function:

    o Allows some of the posterior teeth to share the load in excursions while others

    contact only in MIP

    Occlusion Disorders:

    o Parafunction results in bruxism/clenching

    o Missing teeth result in over eruption and tipping

    o Loss of anterior guidance

    o Malocclusions

    o Organic occlusion is highest mechanical arrangement given to teeth

    Signs of Instability:

    o Hypermobility of one or more of the teeth

    o Excessive wear (interferences)

    o Migration of one or more teeth

    Horizontal shifting

    Intrusion

    Supraeruption

    Types of Contacts:

    o Tripodization:

    Tripod contacts difficult to accomplish but can be done as long as theanterior teeth are capable of discluding the posterior teeth in all

    excursions

    Should not be used when posterior teeth are involved in group function

    Errors in centric relation result in a loss of tripodization

    o Cusp-tip-to-Fossa:

    Easiest occlusion to equilibrate, offers stability and flexibility to

    choose any distribution of lateral forces

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    Guidelines:

    o When occlusal reconstruction is considered, the prosthetic replacement of the

    articulating surfaces is the last phase of treatment

    o Prior to prosthodontic therapy all active disease should be controlled, hygiene

    should be optimized and periodontic, endodontic and orthodontic treatment

    should be completed

    o A comfortable, functional and physiologically appropriate position of the

    condyles in the glenoid fossa must be achieved

    o Braced, centric condylar position with an interposed disk in centric closure

    o Occlusal forces directed along long axis of the teeth

    o Working side dental contacts disarticulating teeth on the balancing side during

    lateral excursion

    o Anterior teeth disarticulating the posterior teeth during protrusive excursions

    o Posterior teeth contact heavier than anterior teeth in centric or habitual

    closure

    Occlusal Vertical Dimension:

    o Recording of the OVD should always precede the bite record

    o Bite record in MIP or CR should be taken at the correct vertical dimension.

    Failure to do so results in occlusal interferences and excursive interferences

    o Consideration of facial shape and extent of vertical dimension increase should

    be matched

    o Measuring OVD:

    Physiological rest positionFreeway space (FWS)

    To achieve rest position, ask patient to swallow then relax lips, ask

    patient to pronounce M

    Should be examined with patient sitting upright and reference points

    marked on the nose and chin in the midline of the face. Repeat

    measurements till consistency is achieved

    Can differ with muscle tonicity, time of day (e.g. fatigue) and tongue

    posture

    Underlying disease (TMJ, skeletal, neuromuscular, mental can alter

    OVD)

    Variations in Freeway Space:

    o Class I Skeletal Cases3mm (normal)

    o Class II CasesCan be up to 4-5mm

    o Class III CasesDown to 1-2mm

    o Look for:

    Face fallen in (excessive FWS)

    Small barely perceptible movement of mandible when swallowing

    (correct)

    If no movement or stained lips there is insufficient FWS

    Periodontal Ligament:

    o Responds to occlusal overload by allowing drifting of teeth, bone apposition

    or loss. Highly sensitised shock absorber

    o

    In bruxers 500-600N of force can be generated during night time parafunctiono In dentate patient the PDL will compress 125m during mastication

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    Differences Between Natural and Denture Occlusion:

    o Natural Dentition:

    Retained in PDL

    Units move independently

    Malocclusion effects not immediate

    Non-vertical forces affect only teeth involved and usually well

    tolerated

    Incising doesnt affectposteriors

    Bilateral balance is rare

    Tactile sensitivity

    o Denture Dentition:

    Mobile bases on mucosa

    Teeth move as a unit

    Malocclusion affects entire base immediately

    Non-vertical forces affect all teeth and are traumatic

    Incising affects all teeth attached to base

    Bilateral balance is often desired for base stability

    Decreased tactile sense

    Bilateral Balanced Occlusion:

    o Stable, simultaneous contact of opposing upper and lower teeth in centric

    relation position with a smooth bilateral gliding contact to any eccentric

    position within the normal range of mandibular function, developed to lessen

    or limit tipping or rotation of the denture bases in relation to supporting

    structures

    Partial Dentate Patient:

    o

    In the partially dentate patient one must decide whether to use CR or MIP

    when restoring the mouth if the two dont coincide

    Centric Relation or MIP:

    o Decision based on multiple factors

    o Number of teeth occluding

    o Position of teeth

    o Presence or absence of joint dysfunction symptoms

    o Extent of tooth alteration requirements to achieve centric relation should

    occlusal interferences be present

    o

    Sufficient restorative space for denture components or whether the OVD needsre-evaluation and increasing at which point CR is the only restorative position

    Occlusal Contact Relationships for RPDs:

    o Occlusion of tooth support partial dentures may be arranged similar to

    occlusion in normal dentition as long as there is anterior guidance

    o Balanced occlusion in eccentric positions should be formulated when the

    partial denture is opposed by a maxillary complete denture

    o This promotes the stability of the complete denture

    o Simultaneous working side and balancing side contact should be obtained for

    the distal extension denture

    o

    Only working side contacts need to be formulated for either the maxillary ormandibular unilateral distal extension partial denture

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    Distal Extension Removable Partial Dentures:

    Distal Extension Problem:

    o The two tissues (tooth and mucosa) that support a distal extension removable

    partial denture differ markedly in their visco-elastic response to loading

    o

    Mucosa displaces far more readily than tooth due to the resilience of toothpermitted by the PDL compared to the mucosa.

    Necessity of Removable Partial Denture:

    o Resultant forces from muscles of mastication pass through the distal of the 35

    and 45, may not be necessary to replace teeth distal to these to maintain

    equilibrium

    o If the maxillary posterior teeth are present it is necessary to replace

    mandibular posteriors to prevent over eruption

    Aims of RPD Fabrication:

    o Evenly distribute the loading forces (mastication and parafunction) between

    the teeth and soft tissues covering the edentulous spano Minimise torqueing forces to the teeth

    o Various Design Factors can be Employed to Minimise Torqueing Forces:

    Indirect RetentionMinimize the rotation of the denture base around

    the posterior abutments on application of a lifting force

    Indirect RetainersEffective in proportion to their support and

    distance from fulcrum line

    Cover the Greatest Possible Denture Bearing AreaDecreasing

    denture bearing area by half increases displacement of denture 4 times

    Cover no more tissue than is absolutely necessary

    Major connectors contribute to prosthesis support Reduce Load AppliedTo distal extension by decreasing size of

    occlusal table (use fewer and narrower teeth)

    RestsIf placed on mesial of posterior abutment, more mucosal

    support will be utilised and in a better direction. Forces will tend to tilt

    the abutment towards the mesial

    Consider Mandibular Ridge Shape:

    round

    Undercut

    Flat

    Sharp

    Fibrous

    Consider Surrounding Muscles:

    Buccinator

    Mylohyoid

    Hyoglossus

    Styloglossus

    Palatoglossus

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    Proximal Plates:

    o Kratchovil Mesial Rest, Long Proximal Plate:

    Fulcrum occurs at base of guide plane under load

    Causes rest to be lifted out of seat

    No torque applied to tooth

    o

    Krol Mesial Rest, Short Proximal Plate:

    Fulcrum point at mesial rest

    Torqueing forces are minimised

    May still result in fulcrum at guide plane

    o McGivney Mesial Rest, 1mm Proximal Plate:

    Similar principles to Krol

    Direct Retention:

    o Use I-bar on maximum point of curvature on abutment to allow clasp to

    disengage the undercut and minimise torqueing of the tooth

    o Several have stated that good adaptation of the base to the tissues outweighs

    importance of clasp design

    Anterior Abutment of Kennedy Class II RPD:

    o As distal extension moves down under loading, the anterior abutment, if

    clasped, will tend to be extruded

    o Keep retentive element to the distal as much as possible

    Impression Technique:

    o No single material that can record both anatomic form of teeth and tissues in

    the dental arch at the same time

    o Dental soft tissues do not compress evenly

    o

    Selective Tissue Placement Impression Method: Soft tissues that cover basal seat areas may be placed, displaced or

    recorded in their resting or anatomic form

    e.g. Palatal tissues in vicinity of vibrating line can be slightly displaced

    to develop posterior palatal seal for maxillary complete denture and

    remain in a healthy state but will become inflamed if they are overly

    displaced

    Oral tissues that are overly displaced or distorted attempt to regain

    their anatomic form

    Maximise soft tissue support while utilizing the teeth to their

    supporting advantageo Altered Cast TechniqueAllows for different compressibility of different

    parts of the mucosa and ensures that an even pressure is applied everywhere

    when force is applied

    Metal casting is made in usual manner

    Acrylic base then added to saddle area of metal casting

    Impression of saddle area taken under light pressure

    Model that casting was made on is cut so that DE saddle area is

    removed, casting with impression is then repositioned

    New distal extension saddle poured in stone

    This process accommodates for disparity between resilience of toothpermitted by PDL and that of the mucosa

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    Altered Cast Technique:

    Supporting structures of edentulous ridge and remaining teeth are utilized to best

    advantage by altering the edentulous part of the cast upon which a distal extension

    RPD is fabricated

    Employs principles of impressions for complete dentures to fabrication of tissuesurfaces of extension RPDs, refined edentulous tissue impression made after metal

    casting is used to alter edentulous areas of master cast

    The resultant cast reproduces supporting tissues in a form that provides the correct

    denture base extension and favourable physiologic support when denture is in its

    fully seated position

    Rationale:

    o Tissues of edentulous ridge are displaced and distorted when impression made

    with stock tray, therefor extension RPD can move excessively if it has been

    fabricated on a cast made from such an impression, even if it was done with a

    specially made tray

    o Alginate material in a stock tray is satisfactory for making impression for cast

    on which metal framework is fabricated (PVS more likely used now)

    Procedure:

    o Metal framework is fabricated on a cast made from an impression obtained in

    a dimensionally accurate and stable material (formerly irreversible

    hydrocolloid)

    o Solution of rouge and chloroform is painted on surfaces that contact teeth and

    framework is placed in mouth where it is moved as it would in function

    (disclosing) indicating metal contacts which may interfere with placement of

    framework and should be relieved

    o After cast has been fitted in mouth, a layer of base plate wax is placed over

    edentulous regions to the master cast. The denture base retention meshworks

    of the casting are warmed over a flame and the framework is seated on the

    master cast

    o Wax that flows over the denture base retentive part of the casting is removed

    and autopolymerising acrylic resin is adapted over the edentulous ridge to

    form a tray attached to the frame work

    o Metal framework with the resin tray attached is tried in mouth and tray is

    shortened to eliminate interference with tongue and cheek movements

    o

    Low fusing model plastic placed on borders of resin tray and softened in

    controlled water bath. Modelling borders are moulded as dictated by

    movement of tongue and cheek. Modelling plastic (impression compound) that

    flows inside tray is trimmed away without reducing border side and escape

    holes for material are drilled in acrylic resin as well

    o ZOE paste placed on tissue side of resin tray and carried to patients mouth

    o Metal casting is seated on teeth and firm pressure applied to parts of the metal

    that contact the teeth to be certain there is no framework movement

    o Pressure is maintained on metal until impression material is set, no finger

    pressure on resin covering edentulous ridge

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    Material Properties:

    Consider:

    o How strong it is

    o What force will deform/break it

    o

    Thermal expansion/contractiono Reproduction detail of teeth and soft tissues

    o Setting shrinkage/expansion

    o Degree of wear/pattern of wear

    o Biocompatibility

    Stress:

    o When a force acts on a body to produce a deformation a resistance is

    developed equal in intensity and opposite in direction

    o Stress = Force/Area

    o Calculated in MPa

    o

    e.g. Stress on an RPD claspo Types of Stress:

    TensionForce away (pulling force)

    CompressionForce directed towards a body

    Shear StressResistance of one body sliding against another,

    substance must be immediately adjacent to the interface

    Strain:

    o Change in length per unit length of a body subjected to stress

    o Described as a percentage change or absolute value

    Stress/Strain Graph:

    o

    Proportional Limit:

    Linear part of the graph

    Area below designated the elastic portion of material

    If force is removed the material returns to its original state (no

    permanent deformation)

    Slope of the line gives modulus of elasticity (relative stiffness/rigidity

    of the material)

    o Ultimate Compressive/Tensile Strength - Point at which material will fail

    o Plastic Region - Area below the graph at which material will start to

    permanently deform

    o

    Yield Strength - Point at which material begins to exhibit plastic behaviour

    Wear and Hardness:

    o Ability of material to resist abrasion or wear

    o Hardness is ability to resist indentation

    o TestsBarcol, Brinell, Rockwell, Shore, Vickers, Knoop

    o Wear tests use determined force to penetrate surface of material and measure

    resultant depth and width

    o 4 Types of WearAdhesive, corrosive (chemically initiated), surface fatigue

    (loose particles leading to subsurface cracks) and abrasive wear (soft surface

    in contact with harder surface)

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    Malleability and Ductility:

    o Pertains to metallic properties

    o MalleabilityAbility to be hammered or rolled into a sheet

    o DuctilityAbility to be drawn into a wire

    o BrittlenessOpposite of ductility. Amalgam, ceramics and composites are

    brittle at oral temperatures (5-55oC) and sustain little or no plastic strain before

    fracture (fracture close to proportional limit)

    Other Properties:

    o Viscosity

    o Tear strength

    o Biocompatibility

    o Co-efficient of thermal expansion

    Gypsum:

    o Naturally obtained from gypsum rock

    o Mainly calcium sulphate dihydrate

    o

    Dihydrate is material with two parts water to one part compound

    o One part calcium to two parts water

    o Plaster of Paris can become gypsum and vice versa

    o CaSO4+ 0.5 H2OCaSO4+ 2H2O

    o During manufacturing process gypsum is converted to Plaster of Paris and

    artificial stone via a process called calcining

    o Gypsum is first ground to fine powder of particle size (Plaster of Paris derived

    when gypsum subjected to heat in open vat, artificial stone produced when

    gypsum processed by steam heat under pressure)

    o In both products, the reaction converts calcium sulphate hemihydrate by the

    removal of 75% of the water molecules

    o Type 1:

    Impression plaster

    4-8 MPa compressive strength

    0-0.15% expansion

    Impressions only

    o Type 2:

    Model plaster

    9MPa compressive strength

    0-0.3% expansion

    Articulation

    o Type 3:

    Dental stone

    20MPa compressive strength

    0-0.2% expansion

    Edentulous casts, denture investment

    o Type 4:

    Dental Stone

    35MPa compressive strength

    0-0.15% expansion (low) Crown and bridge dyes

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    o Type 5:

    Dental stone

    35MPa compressive strength

    0.16-0.3% expansion (high)

    Accounts for casting shrinkage in base metals vs noble metal alloys

    Plaster vs Stone:

    o Chemically plaster and artificial stone are identical, however plaster particles

    are rough, irregular and porous while stone particles are prismatic, more

    regular in size and dense

    o When plaster or stone is mixed with water, a hard substance is formed and the

    process described above is reversed so that the hemihydrate converts to

    dihydrate

    o In the setting reaction, crystals of gypsum intermesh and become entangled

    with one another giving the set material its strength and rigidity

    o Reaction also released heat at a rate of 3900 cal/gm mole

    Storage of Plaster and Stone:

    o Keep containers tightly closed otherwise humidity above 70% causes partial

    conversion of hemihydrate to dihydrate which greatly increases speed of

    setting reaction CaSO4.0.5H2OCaSO4.2H2O

    Setting Expansion:

    o Setting reaction of calcined calcium sulphate hemihydrate reaction with water

    to form a hard mass of calcium sulphate dihydrate is associated with an

    expansion of 0.3-0.6%

    o When this occurs within the confines of an impression tray it will lead to a

    significant reduction in accuracy

    o

    Mixing the plaster with anti-expansion solution (containing 4% potassium

    sulphate and 0.4% borax) will reduce this

    o The potassium sulphate reduces expansion to 0.05% but this also accelerates

    setting reaction and borax is added as a retarded which gives more time to

    pour up the impression

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    Process of Removable Partial Denture Construction Start to Finish:

    Clinical Steps:

    o 1Consultation/impression

    o 2Survey and formulate treatment plan

    o

    3Pre-prosthetic treatment (extractions, caries control, survey crowns andoral hygiene)

    o 4Mouth preps, secondary impressions and border moulding if distal end

    saddle

    o 5Metal framework try-in

    o 6Bite registration (MIP, CR) and determine tooth shade and mould

    o 7Wax try-in

    o 8Insertion, home care instructions

    o 9Review

    Clinical Consultation/Examination:

    o

    General Questionnaire: Medical history and personality assessment

    Diet history

    Dental history

    Reason for tooth loss

    History of existing and previous RPDs

    Evaluate existing partial denture

    Intra-oral examination (oral hygiene, caries susceptibility, patients

    responsibility in home care and obstacles such as arthritis, dry mouth

    and systemic conditions)

    o

    Treatment Planning: Periodontal management

    Surgical modifications as needed

    Restorations/endodontics

    Fixed prosthodontic treatment

    Removable prosthodontic treatment

    o Important Radiographic Findings:

    Caries

    Endodontic lesions

    Assessment of bone loss and periodontal disease

    Retained roots Impacted teeth

    o Components:

    Major Connector:

    Functions:

    o Be RigidFlexible major connectors may cause

    damage to soft and hard tissue as well as allowing for

    forces to be contained on individual teeth instead of

    distributed

    o Protect Associated Soft Tissue6mm away from

    gingival margin in maxilla and 3mm away from margin

    in mandible

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    o Provide Means of Placement of One or More Denture

    Bases

    o Promote Patient ComfortMajor connector must not

    end on the anterior border of the rugae in the palate. It

    should end at the posterior slow so the thickness is not

    discernable by the tongue

    Maxillary Major Connectors:

    o Palatal bar

    o Palatal strap

    o A-P Palatal bar

    o Horseshoe

    o A-P Palatal strap

    o Full palate

    Mandibular Major Connectors:

    o Lingual plate

    o

    Lingual bar

    o Double lingual (Kennedy) bar

    o Labial bar

    o Swing-lock

    Minor Connector

    Rests

    Dire Retainers/Clasps

    Indirect Retainers

    Denture Bases Associated with Denture Teeth

    Survey and Treatment Plan:

    o

    Step 1Study Casts:

    Good preliminary casts

    No bubbles/drags

    Clear gingival margins

    Extending to hamular notch and retromolar pad region

    Floor of mouth captured and tongue space cleared

    o Step 2Check Occlusion:

    Hand articulate casts

    Use pencil to mark occlusal overlap which is the incisal limit of

    framework extension

    o Step 3Check Tilt and Tripod

    o Step 4Survey Undercuts (Tooth and Soft Tissue)

    o Design:

    Colours:

    RedRest seats/modifications

    BlueSaddle areas

    BrownMetal framework

    BlackWiring instructions on the cast

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    o Components:

    Major connector

    Minor connector

    Rests

    Direct retainers/clasps

    Indirect retainers

    Denture bases associated with denture teeth

    o For Maxilla:

    Draw framework design in brown

    All lines should be rounded

    Plating crosses midline perpendicularly

    Proximal plate should extend to contact point

    If large diastemas use Cummer fingers

    Avoid horse-shoe major connectors where possible

    Design to accommodate for future tooth loss

    Check proximal planes are as parallel as possible or mark for

    modification

    o For Mandible:

    Measure gingival margin to floor of mouth

    Remove all calculus prior to secondary impression

    Check all proximal planes are as parallel as possible

    Distal End Saddle Rules:

    If both premolars remain then 35 distal rest and 34 mesial rest

    If only one premolar remains then 34 mesial rest and circular

    cast clasp or wrought wire combination clasp

    Consider RPI and RPA where appropriate

    Occlusal forces on the distal extension base cause rotation

    about the mesial rest. The retentive terminus disengages into

    the mesial undercut minimising torque at the abutment

    Mouth Preps/Impression and Shade Guides:

    o Purpose of Mouth Prep:

    To parallel guide planes

    To provide rest seats for which rest components of framework will seat

    To create favourable undercuts for direct retainers

    To recontour tipped or over-erupted teeth which interfere with occlusalplane

    o Border Moulding:

    Tooth borne partial dentures do not require border moulding

    Distal end partial dentures should be fabricated with broad base

    extension principles and thus should extend as far as possible to the

    functional depth of the sulcus

    o Lab Prescription:

    Master model in high strength stone

    Construct CoChr partial framework as per design indicated on study

    model, mandibular cast enclosed for occlusal examination Return framework for metal try-in, no wax bases

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