Principles of tooth preparations

Principles of Tooth PreparationsDeepak Kumar Gupta

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• The design of a preparation for a cast restoration and the execution of that design are governed by five principles:

– Preservation of tooth structure

– Retention and resistance

– Structural durability

– Marginal integrity

– Preservation of the periodontium


According to Rosenteil, 3rd edition


Preservation of tooth structure


Preservation of tooth structure

• Use of partial-coverage rather than complete coverage restorations

• Preparation of teeth with the minimum practical convergence angle (taper) between axial walls

• Preparation of the occlusal surface following the anatomic planes to give uniform thickness.

• Preparation of the axial surfaces so tooth structure is removed evenly; if necessary, teeth should be orthodontically repositioned

• Selection of a conservative margin compatible with the other principles of tooth preparation

• Avoidance of unnecessary apical extension of the preparation


Use of partial-coverage rather than complete coverage restorations : ¾ OR

7/8 crown


Preparation of teeth with the minimum practical convergence angle (taper) between axial walls


Preparation of the occlusal surface following the anatomic planes to give

uniform thickness.

A flat occlusalpreparation will result in either (1) insufficient clearance or (2) an excessive amount of reduction.


Preparation of the axial surfaces so tooth structure is removed evenly; if necessary, teeth should be

orthodontically repositioned

• The path of withdrawal should coincide with the long axis of the tooth, which for a mandibularpremolar is typically inclined 9 degrees lingually.


Preparing the tooth perpendicular to the occlusal plane is a commonly seen error and results in additional tooth reduction

Tooth structure is conserved by uprighting a tilted FPD abutment.


A shoulder margin (2) is less conservative

than a chamfer (1)


Apical extension of the preparation can necessitate additional tooth reduction

Preparations for periodontally involved teeth may necessitate considerable reduction if the margins are to be placed subgingivally for esthetic reasons


Retention and Resistance


Retention and Resistance• 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.

• No tooth bio-compatible cement possess adequate adhesive properties to hold a restoration in place solely through adhesion -geometric configuration of the tooth preparation must place the cement in compression


Retention

1. Magnitude of the dislodging forces

2. Geometry of the tooth preparationa) Cylindrical preparation - Taper

b) Surface area

c) Types of preparation

3. Roughness of the fitting surface of the restoration

4. Materials being cemented

5. Film thickness of the luting agent


Magnitude of dislodging force

• Type of food – highly sticky food will exert greater dislodging force.

• surface area

• texture of the restoration


Geometry of the tooth preparation• Most of the fixed prosthesis is

dependent on geometric form rather than cementation for retention– Grains of cement prevents two

surfaces from sliding – shear stress

– although they do not prevent one surface from being lifted from another – tensile stress

– Cement is effective only if the restoration has a single path of withdrawl• the tooth is shaped to restrain

the free movement of the restoration

sliding


a. Tensile stress b. Shear stress

c. Both compressive

and shear stress

d. compressive stress


A preparation is cylindrical if the two horizontal cross

sections of the prepared axial tooth surface (1 and 2) are

coincident. A, This complete crown is cylindrical and therefore

retentive. B, A partial crown will be

retentive if its sections are coincident and perpendicular

movement is prevented by grooves.

C, This preparation iscylindrical (1 and 2 coincide) but not retentive, because itcan move perpendicularly to

the axis of the cylinder

Tooth preparation should be cylindrical as much as possible


Maximum retention is obtained if a tooth preparation has parallel walls.

A, Cross sections 1 and 2 do not coincide, and the preparation thus has little retention.B, Under these circumstances, very little friction develops between the cement and the axial walls, and the cement is subjected to tensile stress.C, A retentive near-parallel preparation with frictional resistance. The cement is placed under shearstress


Taper• Near parallel axial wall is impossible to prepare

clinically, so a slight taper is acceptable– Smaller taper - limited path of withdrawal– Attempt to make parallel wall may lead to formation

of undercut - defined as a divergence between opposing axial walls, or wall segments, in a cervical-occlusal direction

– Excess taper may lead to loss of retention– Acceptable taper 2- 6.50, clinically achievable – 160

mean– teeth are readily prepared with a rotary instrument of

the desired taper held at a constant angulation– deliberately tilting a rotary cutting instrument to

create a taper - lead to over preparationfacebook.com/notesdental

Retention decrease with increasing taper angle – Jorgensen


Retention can be increased by limiting possible path of insertion and removal

– making grooves


If axial wall is unveneered then the retention can be increased by placing

grooves(a), boxes(b) or pin slots ©


In case of ¾ crown the lingual wall of groove is made perpendicular to the path of displacement, in case of (b) its V-shaped which will acts as inclined plane favoring displacement


Larger diameter offers more resistance than narrower crown


Longer axial wall may provide more resistance as compared to shorter wall


Surface Area

• Provided the restoration has a limited path of withdrawal, retention will depend on surface area

• length of this path - long axial walls > short axial walls

• molar crowns > premolar crowns of similar taper

• Surfaces where the crown is essentially being pulledaway from rather than sliding along the tooth, such as the occlusal surface, do not add much to total retention.


Types of preparation

• Retention of a complete crown is about double that of partial-coverage restorations

• grooves or boxes in preparation with limited path of withdrawal does not markedly affect its retention – surface area is not increased significantly

• But where addition of a groove limits the paths of withdrawal, retention is increased


Roughness of the Surfaces Being Cemented

• Retentive failure doesn’t occur at cement-tooth surface – so tooth surface shouldn’t be roughened

– difficulty of impression making and waxing.

• Fitting surface – roughened to increase retention

– Air abraded with alumina (50µm)


Materials Being Cemented

• Conflicting finding for this factor

• more reactive the alloy is, the more adhesion

• base metal alloys are better retained than less reactive high-gold content metals


Type of Luting Agent

• decision regarding which agent to use is also based on other factors

• adhesive resin cements are the most retentive


Film Thickness of the Luting Agent

Conflicting evidence about the effect of increased thickness of the cement film on retention of a restoration



Resistance form

The resistance depends on

• Magnitude and direction of the dislodging forces

• Geometry of the tooth preparation

• Physical properties of the luting agent


Magnitude and Direction of the dislodging forces

• Important factor in FPD design for habits like Bruxism

• In a normal occlusion, biting force is distributed over all the teeth; most of it is axially directed.

– Incisor - 43.3kg,

– Bicuspid - 99.11kg

– first molar - 120.66kg


Geometry of the tooth preparation

When the line of force is passing through the margin, then there is no lifting force produced. B) When the line of force is outside the margin then it will produce torque that will tend to rotate or tip the crown at the point of contact of margin


F: Fulcrum point of centre of rotationP1: Tangent point which is perpendicular

from the fulcrum point to opposite wall of preparation

At P1 arc of rotation is tangent to surface of preparation.

There is component of compression on P2 and occlussal to P1 which becomes higher

above itBelow P1 there is point of compression.

F


Decreasing the length of preparation cause lesser resistance to the tipping

force


Shorter crown with short preparation will have good resistance as compared longer crown and shorter preparation. In such case

extra retentive feature like pin retained crown may be given


The length of primary lever arm is the shortest distance from line of force to the fulcrum point. Shorter will be the lever arm lesser

will the tipping force


Because of Smaller diameter, the tangent line of this preparation falls too low on the opposite wall, thus creating much resistance.

B. Height and taper are the same but because of larger diameter the area of resistance is less


Weak resistance of short preparation can be enhansed by addition of

vertical groove


Resisting are decreases as the taper increases


The inclination of the walls of a preparation will depend on height /width ratio.

A) height/width ratio 1:1 can have a inclination of 150 but still can have resitance

B) Height/width ratio 1:2, must have inclination no greater tha 7


Rotation around vertical axis

• When ecentric horizontal force, a moment of torque occur around a vertical as well as horizontal axis.

• It can be encountered by placing grooves or wings


A) partial veneer crown which has no grooves offers less resitance to vertical rotation.

B) Grooves –resist by blocking

arc of rotation


A) Axial symmetry of full veneer

crown may allow rotation of crown.B) Wings (vertical

planes) perpendicular to

the arc of rotation may provide

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Path of Insertion

• The ideal path should be parallel with long axis for full veneer crown, but for ¾ crown it prooves to be unesthetic –more display metal


Preffered path of insertion for an anterior preparation parallels the incisal 2/3 of the labial surface.

This minimizes the display of metal and allows the groove to be longer and more retentive


Path of insertion for tipped tooth

• A) full veneer crown on posterior tooth in normal allignment parallels the long axis

• B) Tilted crown requires different

• C) if its parallel, the crown will be prevented from seating of those parts of adjacent teeth which protrude into the path of insertion.

• D) Correct path for tipped tooth is perpendicular to the occlussal plane.


Mesially shifted tooth due long standing carious lesion of adjacent teeth

B) Vertical path iof insertion will not permit seating of crown without removal of excessive tooth structure from adjacent tooth

C) Path of insertion can be inclined slight mesially, removing the small of coronal enamel from botadajcent tooth.

Incase of severely collapsed tooth, where more than 50% of enamel needs to be reduced, it needs to orthodontoicallyuprighted.


Evaluating tooth preparation

Binocular veiw should never be employed to evaluate a preparation for correct taper.With both eye open a preparation that is undercut appears to have acceptable taper


When viewed from a distance from 30 cm, all axial preparation with ideal taper and angle oiconvergence can be seen


A mirror is used to evaluate a preparation where direct vision is not possible,

An unobstructed view of entire finish line barely outside the circumference of the occlusal surface indicates correct taper


Evaluation of matching bridge abutment paths of insertion

Firstly centering the image of one preparation in the mirror

Secondly move the mirror without tilting to adjacent abutment until it is centred.

If there is need of change of angulation of mirror to see the finish line of adjacent teeth, then the path of insertion of two preparation donot match


Structural durability


• Casting should be rigid enough to withstand occlussal load.

• Must have adequate bulk

• Three preparation for the structural durability of the crown preparation

– Oclussal reduction

– Axial reduction

– Provision for reinforcing struts.


Oclussion reduction• Sufficient tooth reduction from occlusal aspect

must be done to allow crown to be thick enough to prevent wearing through and distortion.

• Occlusal reduction should be as uniform as possible following the cuspal planes of teeth– Sufficient occlusal clearance is combined with

preservation of as much tooth structure as possible– anatomically prepared occlusal surface - rigidity to the

crown because of the "corrugated effect""6 of the planes.

Crown material Functional cusp Non-functional cusp

Gold crown 1.5 mm 1 mm

Veneer 2.0 1.5 mmfacebook.com/notesdental

• A) correct occlusal plane reduction parallel the major planes of the tooth

• B) flat single plane reduction results in insufficient thickness of restoration over the grooves and fossae.

• C) single plane reduction deep enough – unnecessary loss of dentin over the pulp horns and excessive shortening of axial walls with loss of retention


There is enough occlusal

clearance for the

adequate bulk of

crown on the mesial

cusp, so the cusp should

left unprepared


Functional Cusp bevel

• As part of occlusalreduction, wide bevel should be placed on the functional cusp of posterior teeth

• Rounding helps bearing high stress in this area

• Mandible : facial cusps paralleling the inclination of the cusp plane it opposes

• Maxilla: lingual cuspfacebook.com/notesdental

a) The functional cusp Bevel in a blane paralleling that of opposing cusp allows for adequate bulk without undue sacrifice of tooth structure.

B) Functional cusp bevel is omitted – restoration too thin in the stress bearing aresC) absence of functional cusp bevel – overbulk the crown

d) leading to superocclusion of restoration

Functional cusp Bevel


Teeth in crossbite: the

functional cusp bevel is placed on the buccal

cusps of maxillary teeth and the lingual

cusp in mandibular

teeth


Axial reduction

• If restorations are made with normal contours over preparations with inadequate axial reduction, they will have thin walls that will be subject to distortion.

• To compensate this technician overcontour thaxial surface of tooth – inflammation of gingiva


Adequate axial reduction creates space for a strong bulk of metal within the normal contour of tooth

b) Inadequate axial reduction can result in a crown with thin, weak walls.

C) Overcontouring to counter this deficiency will lead to plaque depositing factor resulting in gingival

inflamation


Provision for reinforcing struts

• Offset : ties the grooves together in maxilla

• the occlusal shoulder: ties the groove together in mandible

• the isthmus : connects the boxes

• the proximal groove,

• the box



Marginal Integrity


• There are three requirement for successful margin– Must fit as closely as possible against the finish

line of the preparation – minimize the width of exposed cement

– Sufficient strength to withstand the force of mastication

– Whenever possible, they should be located in areas where the dentist can finish and inspect them and the patient can clean them


• Metal margin should be acute in cross section– d = D sine m– d = D cos pd – shortest

distance from the cast margin to the tooth structure

D – distance by which crown fails to seat

m – angle of margin of restoration

p – surface of bevel and path of insertion


As the angle m is reduced sine

becomes smaller and so does d.

30-45 is optimal – turns weaker if

its made more acute


Bevel or not to bevel

•The film thickness of the cement imposes a limit on the reduction of the perpendicular distance from the margin to the tooth, d. •This distance therefore becomes a constant, and the previous equation is solved for D instead of d:

D = d/sin mD = d/cos p

•As the angle of the margin bevel becomes more acute, its sine becomes smaller, and as the angle of the finish line becomes more obtuse, its cosine becomes smaller, and D becomes larger


Occlusal bevel

• Margins away – 1mm from centric occlusalcontact – distortion of margin and fracture of adjoining enamel.

• Acute angle of metal and obtuse angle of enamel created by bevel – withstand much imoact


Occlusal bevel• A) without bevel –

unsupported enamel

• B) occlusal force deforms the thin overlying metal crown – fracture brittle crown

• C) margin strengthened by simple finishing bevel

• D) esthetic situation allows – contrabevelwill give more strength

• E) inclination of occlusal surface is relatively flat – bevel strength is unecessary


Flares• The vertical finish line for

onlay, inlay and partial veneer crown is finished with a flare.

• Flare is a geometric plane inclined slightly to the path of insertion cutting through contour of teeth

• Bevel follows the contour of tooth.

• Used only on finish line which is perpendicular to the path of insertion.

• Bevel on vertical finish line will produce undercuts –convexity of the tooth


Direction of flare

Planes of two facial flare should intersect slightly facial to path of insertion.

Planes of lingual flare should intersect lingual to path of insertion


Flares are formed by removing equal amount of tooth structure from the walls of box/groove and outer surface.b) Flares cut more at the expense of internal tooth structure will b etoo parallel to the walls of box – resulting in margins that are sufficiently acute and probably underextendedc) Flares cut too much at the expense of outer tooth surface are too flat – thin, weak and possible overextended and undercut gingivally


Finish line configurations•Wide, shallow bevels that are nearly parallel with the outer surface of the tooth should be avoided

• Lead to overcontouring

•not overcontoured - the resultant thin, unsupported wax at the margin potentially will break or distort when the wax pattern is withdrawn from the die and invested.


Chamfer finish line

• veneer metal restorations

• least stress, so the cement underlying it will have less likelihood of failure

• tip of a round-end diamond


Deep chamfer

• Also known as heavy chamfer.• 90-degree cavosurface angle

with a large-radius rounded internal angle

• roundend tapered diamond,• better support for a ceramic

crown not as good as a shoulder• A bevel can be added - metal

restoration.

(a) Deep chamfer on a preparation for an all-ceramic crown.

(b) Ninety degree finish line formation.


Operator inexperience•undesirable fragile lip of enamel at the cavosurface

•friable, unsupported enamel is very easily fractured during or after cementation of the restoration


Classic shoulder• all-ceramic crown

• wide ledge provides resistance to occlusal forces

• minimizes stresses that might lead to fracture

• space for healthy restoration contours and maximum esthetics

• destruction of more tooth structure

• 90-degree internal line angle concentrates stress in the tooth - coronal fracture.

•Not for cast metal restorations.


Radial shoulder• cavosurface angle is 90 degrees• shoulder width is only slightly

lessened by the rounded internal angle

• one-fourth to one-fifth the depth of the axial reduction

• Rounded internal angle –withstand higher stress

1) Initial instrumentation

- coarse, flatend tapered diamond

2) small-radius rounded internal angle - Fine flat-end tapered diamond

3) Finishing - specially modified binangle chisel


shoulder with a bevelfinish line in a variety of situations

gingival finish line on the proximal box of inlays

and onlays and for the occlusal shoulder of onlays and mandibular threequartercrowns

facial finish line

metalceramic restorations where gingival esthetics is not critica

shoulder is already present either because of destruction by caries or the presence of previous restorations.

preparations with extremely short walls

facilitates axial

walls that are nearly parallel

acute

edge of metal at the margin

not be used

routinely for full veneer restorations

axial reduction required to obtain it is unnecessarily destructive of tooth structure


knifeedge

create problems

Unless it is carefully cut, the axial reduction may fade out instead of terminating in a definite

finish line.



Preservation of the Periodontium

•as smooth as possible and are fully exposed to cleansing

•finish line should be placed in an area

•where the margins of the restoration can be finished by dentist and cleaned by patient

•duplicated by the impression - without tearing or deforming the impression

•placed in enamel when it is possible to do so

•Supragingival finish line is prefferred


Health & Medicine

Principles of tooth preparations