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Principles of tooth preparation


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simplified contemporary fixed prostho

simplified contemporary fixed prostho

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  • 1. PRINCIPLES OF TOOTH PREPARATION 1. biological considerations 2. mechanical considerations 3. esthetic considerations 1. Biological considerations A) prevention of damage during tooth preparation of : a. adjacent teeth - use metal matrix band - use thin, tapered diamond - common clinical error – tipping diamond away from adjacent proximal surface b. soft tissue - eg ; tongue, cheek, oral mucosa, peeriodontal ligament, gingiva - use ; aspirator tip, mouth mirror, flanged saliva ejector, tongue depressor, rubber dent - great care needed to protect tongue ; especially when ling surface of mand molars are prepared - for pulp ; prevent pulpal injuries ; causes - temperature, chemical irritation, Mos (Causes of Injury) 1. temperature - friction from heat generated - rotatory instrument - excessive pressure - type, shape, condition of cutting instrument - how to prevent ; a. use water spray - to remove debris b. make shoulder & chamfer finish line c. slow speed handpiece d. avoid high pressure 2. chemical action - bases, restorative resins, solvents, luting agents - these cause pulpal damage - how to prevent ; a. use cavity varnish b. use dentin bonding agent adv : effective barrier disadv : effect retention 3. bacterial action - from microleakage - how to prevent ; a. zn phosphat cement b. chlorhexidine gluconate disinfecting solutions (consepsis) c. remove all carious dentin before placement of restoration d. not recommended ! - indirect pulp capping
  • 2. B) conservation of tooth structure why we have to conserve ; - thickness of dentin inversely proportional to pulpal response - damage to odontoblastic processes affect cell nucleus at dentin-pulp interface how to conserve ; 1. use partial-coverage crown 2. min convergence angle between axial walls (taper) 3. preparation of occlusal surface - reduction follow anatomical planes - give uniform thickness 4. preparation of axial surfaces - there's enough thickness of residual tooth struc surrounding pulpal tissue - teeth should orthodontically repositioned 5. select margin geometry - must be conservative - compatible with tooth preparation 6. avoid unnecessary apical extension C) consideration affecting future dental health axial reduction – indications : 1. to avoid plaque control 2. avoid pdl disease 3. to avoid poor form & subsequent occlusal dysfunction 4. avoid chipping of enamel/cusp fracture 5. avoid gingival inflammation – how : 1. tooth preparation must provide sufficient space for development of good axial contours - enable jucntion between restoration * tooth - to make smooth & free preparation from ledges 2. crown should duplicate contours of original tooth - except for malaliged/malformed tooth 3. slightly undercontoured flat restoration is better - when error is made - to keep plaque free 4. increase proximal contour on anterior crown - to maintain interproximal papilla 5. sufficient tooth structure must be removed margin placement – whenever possible should be SUPRAGINGIVAL! – advantages of supragingival 1. easily finished w/o associated with soft tissue trauma 2. kept plaque free 3. impressions are more easily made 4. restoration can be easily evaluated at the time of placement & at recall appointment 5. situated on hard enamel – should use subgingival margin placement if ; 1. no indication of dental caries, cervical erosion 2. proximal contact area extends to gingival crest 3. additional retention/resistance is needed 4. margin of PFM crown is to be hidden behind labiogingival crest
  • 3. 5. indication of modification of axial contour 6. short crown – min height 3.5mm margin adaptation 1. consider the junction between restoration & tooth - potential site for recurrent caries 2. increase adaptation will decrease caries & decrease pdl disease 3. smooth & even margin 4. preparing smooth margin cannot be overemphasized margin geometry – guidelines should be considered ; 1. to ease of preparation w/o overextension or undermined enamel 2. ease of identification in impression/die 3. distinct boundary for easy finishing of wax pattern 4. sufficient bulk of material - enable wax patterm to be handles w/o distortion - give restoration strength - give esthetics when porcelain is used 5. conservation of tooth geometry 6. refer page 220 (table 7-2) occlusial reductions 1. need considerable reduction to compensate supraeruption of abutment teeth 2. if too much reduction will shorten axial wall and reduce the retension & resistance preventing tooth structure 1. cuspal fracture occur from bruxism 2. tooth prepartion is designed to min destructive stresses - complete cast better than onlay & inlay 2. Mechanical considerations aim – to prevent dislodgement or distortion or fracture of the restoration during service 3 categories : a. retention form b. resistance form c. prevent deformation of restoration A) Retention form def : resistance that prevents restoration from becoming dislodged by such forces parallel to path of placement (vertical direction) 5 factors : 1. magnitude of dislodging forces def : forces that tend to remove a cemented restoration along its p.o.p : disclose forces > seating force causes : pulling with floss under connectors : eat sticky food factors affecting : stickiness of food : surface area of restoration : surface texture of restoration
  • 4. 2. geometry of tooth preparation - cement is effective if restoration has single p.o.p - the best shape is cylindrical - if overtapered : cemented restoration not constrained by preparation : restoration has multiple paths of withdrawal - factors affecting 1. taper (decrease taper, increase retention) - def : convergence of 2 opposite external walls of tooth preparation - theory : parallel walls – max retention - mechanically : 6 degrees – optimum for max retention - too small degree – undercuts - too large degree – lack of retention - recommended to use grooves to decrease displacement 2. surface area (increase s.a, increase retention) - depends on length of this p.o.p - occlusal surface not include - tall crown > retentive than short crown - molar crown > retentive than premolar crown 3. stress concentration (decrease s.c, inccrease retention) - lead to cohesive & adhesive failure - causes : sharp occlusoaxial line angle : stress not uniform & concentrated - should make roundation of all line angles 4. type of preparation - complete crown > retentive than partial crown 3. roughness of surfaces being cemented - increase roughness, increase retention - use : air abrasion, tin plating, acid etching 4. materials - increase the reactivity of alloy, increase adhesion, increase retention - base metal alloy (high gold content) > retentive than less reactive metal - complete resin > retentive than amalgam 5. film thickness - 25mm 6. type of luting agent - adhesive resin cement (most retentive) B) Resistance form def : features that enhances the stability of a restoration & resist horizontal/lateral dislodgement 3 factors : 1. magitude & direction of dislodging forces - mastication – cause horizontal/oblique/lateral dislodgement - lateral force tend to displace restoration by rotation around gingiva margin - individual biting force – 4340N (443kg)
  • 5. - complete cast crown can withstand more than 13500N (1400kg) 2. geometry of tooth preparation - increase taper/increase diameter/decrease height lead to decrease resistace how to increase resistance : - acceptable degree of convergence (taper) : 5-22 degress - min wall height : 3.5-4mm - make pyramidal preparation - make grooves/boxes - pinholes - complete crown - u-shaped grooved/flared boxes 3. physical properties of luting agent - highest compressive strength : Glass Ionomer - highest MOE : Zinc Phosphate C) Preventing deformation 1. alloy selection - type I & II gold alloy : intracoronal cast restorations - type III & IV gold alloy : crown - metal-ceramic alloys with high noble metal content = type IV - nickel-chromium alloys (harder) 2. adequate tooth reduction - to get sufficient bulk of tooth to withsatnd occlusal forces - make func cusp (1.5mm) : mand – buccal : max – palatal/lingual - make non func cusp (1mm) : mand – palatal/lingual : max – buccal – for malaligned/overerupted/supraerupted tooth (reduction > 1.5mm) 3. margin design - prevent distortion by : a. design outline form b. avoid occlusal contact c. keep preparation margin 1-1.5mm away from occlusal contact areas d. maintain adequate dentin thickness e. make grooves/ledged/ant pinledge retainers