principle of tooth preparation
TRANSCRIPT
05/03/2023
PRINCIPLES OF TOOTH PREPARATION
Dr Apurva Thampi
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WHAT IS TOOTH PREPARATION???
Tooth preparation may be defined as the mechanical treatment of dental disease or injury to hard tissue that restores a tooth to the original form
Tylman
The mechanical preparation or the chemical treatment of the remaining tooth structure, which enables it to accommodate a restorative material without incurring mechanical or biological failure.
Marzouk
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NEED FOR TOOTH PREPARATION
• Teeth - no regenerative ability restoration• Teeth require preparation to receive restoration• Conservative tooth preparation
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BROAD OBJECTIVES
Optimal tooth preparation
According to Rosensteil,
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GUIDELINES OF TOOTH PREPARATIONTotal occlusal convergence
Occlusocervical/incisocervical dimensionRatio of OC and FL dimensionCircumferential form of the prepared toothReduction uniformityReduction depthsFinish line locationLine angle form
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PRINCIPLES OF TOOTH PREPARATION
Preservation of tooth structure
Retention &
ResistanceStructural durability
Marginal integrity
Preservation of the
periodontium
According to Shillingburg,
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PRESERVATION OF TOOTH STRUCTURE
Preservation of Adjacent teeth Soft tissue pulp
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PRESERVATION OF ADJACENT TEETH
• Metal matrix band around the tooth – can be perforated
• Thin layer of proximal enamel retained – protect adjacent tooth structure
• Thin tapered diamond – “fin” of enamel• Undesirable angulation to be avoided
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PRESERVATION OF SOFT TISSUE
• Careful retraction with mouth mirror or suction tip
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PRESERVATION OF PULP
Temperature Chemical action Bacterial action• Friction • Materials – on
freshly cut dentin• All carious dentin
– removed • Water spray
should be used• Cavity varnish or
dentin bonding agents - prevents
• Bacteria gain access - microleakage
• Retention features – at low speeds
• Cleaning and degreasing agents – pulpal irritants
• Zinc phosphate cement - antibacterial
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10 – 19years
20-2930-3950-59
20-2910 – 19years
30-39
50-59
10 – 19years
20-2930-3950-59
Maxillary central incisors with metal ceramic crown
preparation
Maxillary lateral incisors with metal ceramic crown
preparation
Maxillary canine with metal ceramic crown
preparation
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PULPAL TEMPERATURE RISE IN RELATION TO
TOOTH CONTACT
Grp I : air turbine ; water cooledGrp II : air turbine ; dryGrp III : low speed ; water cooledGrp IV : low speed ; dry
According to Zach and Cohen –• Rise of 5.5 o C - 15% necrosis• Rise of 11.1o C - 60% necrosis• Rise of 16.6o C - 100% necrosis
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CONSERVATION OF TOOTH STRUCTURE
• Extensive amount of reduction – every dentinal tubule exposed – communicated directly with the dental pulp
• Any damage to the odontoblastic process would adversely affect the cell nucleus at the dentin-pulp interface
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Partial coverage. Minimum convergence angle. Anatomic reduction
Axial surface reduction. Apical extension. Conservative margin
Thickness of remaining dentin inversely proportional to pulpal response
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CONSIDERATIONS AFFECTING FUTURE DENTAL HEALTH
• Insufficient axial reduction
• Inadequate occlusal reduction
• Poor choice of margin location
• Over contoured restoration – periodontal disease
• Poor form – occlusal dysfunction
• Chipping of enamel or crown fracture
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RETENTION AND RESISTANCE• Taper• Freedom of displacement• Length• Substitution of internal features• Path of insertion
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necessarily true.
Retention – A +BResistance – B+C+D
A B
C D
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RETENTION FORM
Retention prevents removal of the restoration along the path of insertion
or long axis of the tooth preparation.
Resistance prevents dislodgment of the
restoration by forces directed in an apical or oblique
direction and prevents any movement of the restoration
under occlusal forces.
RESISTANCE FORM19
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TAPER
• The axial walls of the preparation must taper slightly • two opposing external walls must gradually
converge or• two opposing internal surfaces of tooth
structure must diverge occlusally. • The terms angle of convergence and angle of
divergence - relationships between the two opposing walls of a preparation.
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Nearly parallel sides gives maximum retention
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TAPER
• Initially – Ward – 5% - 20% taper (3-12 degrees)• Recently – 3-5 ; 6 ; 10-14 degrees• Several studies suggest – 16 degrees clinical
taper (TC)• Range – 10 degrees on anteriors and 22
degrees on molars
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Relationship between taper and retention – as taper increases, retention decreases
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ACCORDING TO CHARLES GOODACRE…
Posterior teeth were prepared with greater taper than anterior (mandibular molars)
FL surfaces had greater convergence than MD surfaces
FPD abutments were prepared with greater taper
Monocular vision created greater taper than binocular vision
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FREEDOM OF DISPLACEMENT
• Limiting path of insertion• Limiting torqueing forces in a horizontal plane –
grooves with definate lingual walls
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Walls of the groove must be perpendicular to the rotating forces
Buccal and lingual walls will not resist displacement if they meet the pulpal wall at
oblique angles
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LENGTH
• Longer preparations – more surface area – more retention
• Short walls – decrease taper – increase resistance
• Preparation on smaller tooth – smaller arc of displacement
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Preparattion of a tooth with smaller diameter resists displacement better than in
a larger tooth with same heightResistance of a short preparation can be
improved by adding grooves
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SUBSTITUTION OF INTERNAL FEATURES
• Minimal taper may always not be possible
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Adding grooves or boxes to a preparation with a limited path of withdrawal does not markedly affect its retention because the surface area is not increased significantly
(Rosenstiel)
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A flat gingival seat and adequate axial depth will aid in resisting forces by increasing surface area
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PATH OF INSERTION
Path of insertion is an imaginary line along which the restoration will be placed into or removed from the preparation
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PATH OF INSERTION
• Considered FL and MD
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STRUCTURAL DURABILITY• Occlusal reduction• Functional cusp bevel• Axial reduction
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OCCLUSAL REDUCTION
Preparation Occlusal reduction
Gold crown 1.5 mmMetal ceramic crown
1.5 – 2mm
All ceramic crown 2 mm
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OCCLUSAL REDUCTION
• Occlusal anatomy must be maintained• Flat occlusal anatomy – does not allow good
functional morphology
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FUNCTIONAL CUSP BEVEL
• Wide bevel (on functional cusps)• Lingual cusps of maxillary teeth• Buccal cusps of maxillary teeth
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• If no bevel – excessive axial inclination – destroy excessive tooth structure – lessens retention
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AXIAL REDUCTION
• Space for adequate restoration material• Inadequate axial reduction – over contoured
restorations – disastrous periodontal effects
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“TRUSS EFFECT”
To maximise space – • Offset• Occlusal shoulder• Isthmus• Proximal groove
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MARGINAL INTEGRITY• Finish line
configuration• Bevels
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FINISH LINE CONFIGURATION
• Fit as closely to the finish line to minimise exposed cement
• Sufficient strength to withstand the forces of mastication
• Should be located where the dentist can inspect them and the patient can clean them
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FUNCTIONS OF FINISH LINE
Measure of tooth structure that has been removed
Used to measure the accuracy of an impression
Helps to evaluate a die and trim accurately
Proper fabrication of a wax pattern
Evaluation of a restorationHelps in determining if the restoration is seated
completely
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TYPES OF FINISH LINE
• Least stress• Round end diamond• Torpedo – less likely
to produce a butt joint CHAMFER MARGIN
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TYPES OF FINISH LINES
• 90 degree cavo-surface angle
• Rounded internal line angle
• Better support than Chamfer
• Bevel can be added to provide better support
HEAVY CHAMFER
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TYPES OF FINISH LINES
• Healthy restoration contours and maximum esthetics
• Minimises stress that may lead to fracture of porcelain
• Sharp line angle – stress concentration – coronal fracture
SHOULDER
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TYPES OF FINISH LINES
• Essentially same as the shoulder
• Internal line angle rounded
• Cavo surface margin – 90 degrees
• Stress concentration less than in classic shoulder RADIAL SHOULDER
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TYPES OF FINISH LINES
• Permits an acute margin of metal
• Thin margin – difficult to wax up
• Susceptible to distortion when subjected to occlusal forces
• May result in over contoured restoration to compensate for bulk
KNIFE EDGE
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TO BEVEL OR NOT TO BEVEL….
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TO BEVEL…..
• Diminish marginal inaccuracy
d = D sin mor
d = D cos p• As the angle m is reduced its
sine value also reduces and so does the value of d…thus reducing the marginal discrepancy.
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….NOT TO BEVEL
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PRESERVATION OF THE PERIODONTIUM
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MARGIN PLACEMENT
• Direct effect on ultimate success of restoration• Margins should be as smooth as possible • Placed in area that can be finished well by the dentist
and kept clean by the patient• Placed in enamel whenever possible• Should be supragingival whenever possible
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SUPRA GINGIVAL MARGIN
Click icon to add picture
• Less potential for soft tissue damage
• Easily prepared and finished
• More easily kept clean• Impressions are more
easily made• Restorations easily
evaluated at recall appointments
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SUBGINGIVAL MARGIN:
• Esthetics• Existing caries, cervical erosion, or restorations extend
subgingivally, and crown-lengthening is not indicated• Proximal contact area extends to the gingival crest• Additional retention is needed• Margin of a metal-ceramic crown is to be hidden behind the
labiogingival crest• Root sensitivity cannot be controlled by more conservative
procedures, such as the application of dentin bonding agents
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• Finish line should not be closer than 2mm to the alveolar crest
• Placement in this area – • gingival inflammation• loss of alveolar crest height• pocket formation
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MARGINAL ADAPTATION
• Junction between a cemented restoration and the tooth - potential site for recurrent caries
• Casting- fits within 10 µm• Porcelain margin- 50 µm
• Stepped irregular margin- poor adaptation
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CURRENT CONCEPTS
Tooth preparations for complete crowns: An art form based on scientific principles
Charles Goodacre
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• The total occlusal convergence - between 10 and 20 degrees.
• 3 mm should be the minimal occlusocervical /incisocervical dimension of incisors and premolars prepared within the recommended 10 to 20 degrees of total occlusal convergence.
• The minimal occlusocervical dimension of molars should be 4 mm when prepared with 10 to 20 degrees total occlusal convergence.
• The ratio of the occlusocervical/incisocervical dimension of a prepared tooth to the faciolingual dimension should be at least 0.4 or higher for all teeth.
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• Teeth should be prepared so that the facioproximal and linguoproximal corners are preserved.
• Teeth without natural circumferential morphology after tooth preparation (round teeth) or teeth that lack adequate resistance form should be modified with the creation of grooves/boxes.
• Many molars need auxiliary grooves or boxes to enhance resistance form because of their short occlusocervical dimensions and the unfavorable ratio of the occlusocervical dimensions to the faciolingual dimensions.
• Axial grooves/boxes should be used routinely when mandibular molars are prepared for fixed partial dentures, and they should be located on the proximal surfaces.
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• When tooth conditions and esthetics permit, finish lines should be located supragingivally.
• When subgingival finish lines are required, they should not be extended to the epithelial attachment.
• Chamfer finish lines approximately 0.3 mm deep are well suited for all-metal crowns.
• Both shoulder and chamfer finish lines can be used with all-ceramic crowns if the crowns are bonded to the prepared teeth. Depths greater than 1 mm are not required when a semitranslucent type of allceramic crown is used.
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• Axial and occlusal reductions for all-metal crowns should be at least 0.5 mm deep and 1.0 mm deep, respectively.
• For metal-ceramic crowns, Facial /axial reductions in excess of 1 mm can compromise the remaining tooth structure external to the pulp, whereas 2.0 mm of occlusal reduction is commonly achievable even on a young tooth.
• With all-ceramic crowns, it is not necessary to exceed 1 mm of axial reduction with semitranslucent systems and higher value, lower chroma shades.
• 2 mm incisal/occlusal reduction for allceramic crowns.
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14. Line angles should be rounded on all-ceramic tooth preparations to reduce stress in the definitive restoration.
With crowns that use metal, the primary purpose of line angle rounding is to facilitate pouring impressions and investing wax patterns without trapping air bubbles and to facilitate removing casting modules.
15. Smooth tooth preparation appears to enhance the fit of restorations. Surface roughness generally increases retention with zinc phosphate cement, but its effect with adhesive cements (polycarboxylate, glass ionomer, resin) has not been as definitely determined.
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CONCLUSION
Successful restoration
Accurate diagnosis
Thoughtful Rx planning
Preparation design
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