This document discusses principles of tooth preparation for restorations. It outlines that the goal of tooth preparation is to mechanically or chemically treat remaining tooth structure to accommodate a restorative material without failure, while minimizing removal of tooth structure. It describes factors like retention, resistance, preservation of tooth structure and margins, and protection of pulpal health as objectives to consider in tooth preparation design. The document also notes that techniques are evolving from traditional cavity preparations to more conservative, minimally invasive approaches.

1. PRINCIPLES OFTOOTH
PREPARATION

2. • Tooth preparation may be definedasthe mechanical treatment ofdental
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 itto accommodate a restorativematerial
without incurring mechanicalorbiological failure.(Marzouk)

3. • The current focus is on conservative tooth preparation that is
noninvasive and that minimally involvesdentin.
• This trend is rational in the light of the reduction of the caries rate
by fluorides, nutritional counseling and oral hygiene programs.
Black’s principles of cavity preparation and Tylman’s principles of
tooth preparation are both presently being modified to
accommodate imaginative approaches i.e., acid etching with
minimum reduction.
• Dentistry is changing from macro tooth preparation to an
environment of molecular chemistry i.e., esthetic bonding. These
techniques are not presently supported by any longitudinal
studies, but are exciting andpromising.

4. Despite these advances,traditional
crownsare still indicated for
majority of patients. The classic
design of the preparation must be
visualized sothat modifications if
required canbeinstituted.
Diagnosis and disciplinedtooth
preparation are essential to
successfulfixed prosthetics.

5. OBJECTIVES OF TOOTH PREPARATION
The objectives of preparation remain clearly defined, but
the methods of securing these goals are constantly being
revised.
1.Reduction of the tooth in miniature to provide
retention.
2.Preservation of healthy tooth structure to
secure resistance form.
3.Provision for acceptable finish line.
4.Performing pragmatic axial tooth reduction to
encourage favorable tissue responses from
artificial crown contour.

6. BIOLOGIC
Conservation oftooth
structure
Avoidance of over
contouring
Supra gingival margins
Harmonious occlusion
Protection against tooth
fracture
MECHANICAL
Retention form
Resistanceform
ESTHETIC
Minimum displayof
metal
Maximum thickness of
porcelain
Porcelainocclusal
surfaces
Subgingival margins
PRINCIPLES OFTOOTHPREPARATION
(Accordingto Rosenstiel)
Optimalrestoration

7. Damage to adjacent teeth isprevented by positioning the
diamondsoa thin lipof enamel isretained between the bur
and the adjacenttooth.
Note that the orientation of the diamondparallelsthe longaxis.
Theenamel shouldbe maintained mesial to the path of the diamondasthe
reductionprogresses.

8. Soft tissues
Damage to the soft tissues of the tongue and cheeks
can be prevented by careful retraction with an aspirator
tip, mouth mirror , or flanged saliva ejector.
Great care is needed to protect the tongue
when the lingual surfaces of mandibular molars are
being prepared.

9. Pulp
Great care also is needed to prevent pulpal injuries
during fixed prosthodontic procedures,
Especially complete crown preparation.
Pulpal degeneration that occurs many years after tooth preparation has
been documented.
Extreme temperatures, chemical irritation, or microorganisms can cause an
irreversible pulpitis particularly when they occur on
freshly sectioned dentinal tubules.

10. • Temperature
• Chemicalaction
• Bacterialaction

11. PRINCIPLESOFTOOTHPREPARATION
According to Shillingburg, the design of a
preparation for a cast restorations and the execution of
that designare governedby five principles:
1.Preservation of toothstructure
2.Retention andresistance
3.Structural durability
4.Marginal integrity
5.Preservation of theperiodontium.

12. Preservation ofTooth Structure
• In addition to replacing lost tooth structure, a restoration must
preserve remaining toothstructure.
• Intact surfaces of tooth structure that can be maintained while
producing astrong, retentive restoration should be saved if patient
acceptance and retention requirements will permit it.
• Preservation of tooth structure in some casesmay
require that limited amounts of sound tooth structure be
removed to prevent subsequent uncontrolled lossof larger
quantities of toothstructure.

13. This is the rationale for the removal of 1to 1.5mm of occlusal
tooth structure when preparing atooth for anMOD onlay.
The metal on the occlusal surface can protect against dramatic
failures,
such as fracture of tooth structure, as well as the less obvious
failures that may be occasioned by the flexing of tooth structure

14. Conservation of tooth structure by usingpartial-coverage restorations. In this
case,they are usedasfixeddental prosthetic abutmentsto replace congenitally
missinglateralincisors.

15. Retention andResistance
• No cements that are compatible with living tooth
structure and the biologic environment of the oral cavity
possessadequate adhesive properties to hold a
restoration in place solely through
adhesion.(Shillingburg)
• The geometric configuration of the tooth preparation
must place the cement in compression to provide the
necessary retention andresistance.

16. 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.
Retention and resistance are interrelated and often inseparable
qualities.
The essential element of retention is two opposing vertical
surfaces in the same preparation.

17. Taper
The axial walls of the preparation must taper slightly to permit the
restoration to seat; ie,
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 can be used to
describe the respective relationships between the two opposing walls of
a preparation.
The relationship of one wall of a preparation to the long
axis of that preparation is the inclination of that wall.

18. • A tapered diamond or bur will impart an inclination of 2 to 3 degrees to any
surface it cuts if the shank of the instrument is held parallel to the intended
path of insertion of the preparation.
• Two opposing surfaces, each with a 3-degree inclination, would give the
preparation a 6-degree taper.

19. Prothero (1923) indicated that convergence of peripheral surfaces should
range from20-50.
Jorgensen (1955) found that retention decreases as the taper increases. A
range of 2-6.50 has been considered to be optimal. He tested the retention of
crowns at various angles by applying a tensile force on cemented crown.
Maximum strength wasobtained at 50.
Mack (1980) estimated that a minimum taper of 120 is necessary just to
ensure the absence of undercuts. The tendency to over taper preparations
must be avoidedin order to achievemaximum retention.
Ebnashi (1969) to minimize stress in the cement interface between the
preparation and restoration, a taper of 2.5 to 6.50 has been suggested
optimum, but there is only a slight increase in stress astaper is increased from
0to 150.However at 200 stressconcentration wasfound to increasesharply.

20. • Consciously attempting to cut a taper can easily result in an
overtapered and nonretentive preparation.
• A taper or total convergence of 16 degrees has been proposed
as being achievable clinically while still affording adequate
retention.
• This is probably an acceptable overall target. It can be as low as
10 degrees on preparations on anterior teeth and as high as 22
degrees on molars.

21. Freedom of Displacement
Maximum retention isachievedwhenthere isonly onepath.
Afull veneer preparation with long, parallel axial walls and grooves would
produce such retention.
On the opposite extreme, ashort, overtapered preparation would be
without retention becausethe restoration could be removed along an
infinite number ofpaths.
Limiting the freedom of displacement from torquing or twisting forces in a
horizontal plane increasesthe resistance of arestoration.
A groove whose walls meet the axial wall at an oblique angle does not
provide the necessaryresistance.V-shaped grooves produce roughly one-
half asmuch resistance to lingual displacement asdo grooves with a
definite lingualwall.

23. Surface area :
Greaterthe areaof the cement film bound to the preparationand to the
internal detail of the casting, greater the retention.
The total surface areaof the preparationis influenced by the sizeof the tooth,
the extent of the coverage by the restoration, and internal features such as
grooves andboxes

24. Area under shear:
 Most important for retention is the
area of cement that will experience
shearing rather than tensile stress.
 For the shear strength of the cement
to be utilized, the preparation must
have the opposing walls nearly
parallel with each other.
 The direction in which a restoration
can be removed must be limited to
one path. A severely over tapered
preparation has many paths along
which tensile force could remove a
crown.

25. Types of preparation :
Different types of preparations have different retentive values
and these correspond fairly closely to the surface area of the axial
walls, provided other factors (e.g. taper) are kept constant. Thus the
retention of full veneer crowns are almost double that of partial
coverage restorations.
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). However other authors
have reported that, where the addition of grooves or boxes limits the
path of withdrawal, retention is increased.

26.  For the grooves to be effective, the
lingual wall of the groove must be
distinct and perpendicular to adjoining
axial wall
 The walls of a groove that meet the
axial wall at an oblique angle do not
provide necessary resistance (A). The
walls of a groove must be
perpendicular to rotating forces to
resist displacement (B).

27.  Length of the preparation is important
factor in retention
 Longer preparation has a greater
retention than does a short
preparation, due to greater surface
area.
 The preparation with longer walls
interferes with the tipping
displacement of the restoration better
than the short preparation. Because
of greater surface area preparation
with larger diameter will have greater
retention than with narrow
preparation of same length

28. Surface roughness:
 The adhesion of the dental
cements primarily depends on
microscopic irregularities and the
recesses on the surfaces being
joined the prepared tooth
surface should not be highly
polished

29. Resistance:
Resistance prevents dislodgement of restoration by forces
directed in the apical or oblique direction
Resistance to sliding or tipping must be designed into the
preparation
LeverageAndResistance:
Leverage,is the predominant factor in the dislodgement of
the cemented restoration, occur when line of action of
force passesoutside the supporting tooth structure.
Length of preparation hasstrong influence on its resistance
Shortening apreparation will produce aproportionally
greater diminution of the resisting area

30.  A short restoration on a short
preparation is less likely to fail
through tipping than is a long
restoration on same preparation as
the force on it acts through a longer
lever arm.
 A preparation on a tooth with a
smaller diameter resists pivoting
movements better than a preparation
of equal length on a tooth of larger
diameter because smaller teeth will
have a short rotational radius for the
arc of displacement and the incisal
portion of the axial wall will resist
displacement

31.  When relatively long crown must be
made on a short preparation
additional resistance form, in form of
pin retained core must be created.
 Resistance and Tooth Width
 A wider preparation has greater
retention than narrower one of equal
height. Under some circumstances a
crown on the narrow tooth can have
greater resistance to tipping than one
on wider tooth. This is because crown
on narrower tooth has a shorter
radius of rotation resulting in larger
resisting area
 Resistance of a preparation on a
short, wide tooth can be enhanced by
addition of grooves

32. • Thepath ofinsertion is an imaginary linealong whichthe
• restorationwillbeplacedontoorremovedfromthe
• preparation.
• It is determined mentally by the dentist
• before the preparation is begun, andall features of the
• preparation arecut to coincidewith that line.
Path ofinsertion

33. If the center of the occlusal surfaceof apreparation
is viewed with one eyefrom adistance of approximately
30cm (12inches), it is possible to sigh! down the
axial walls of apreparation with aminimum taper

34. For apreparation to be surveyedin the mouth, where
direct vision is rarely possible, amouth mirror is used.
It is held at anangle approximately 1/2inch above the preparation, and
the image is viewed with one eye.
If fixed partial denture abutment preparations are
being evaluated for acommon path of insertion, afirm
finger rest is established and the mirror is maneuvered
untilone preparation is centered.
Then, pivoting on the finger rest, the mirror is moved, without
changing its angulation, until it is centered over the second preparation

35. StructuralDurability
• A restoration must contain a bulk of material that is adequate to
withstand the forces of occlusion.
• This bulk must be confined to the space created by the tooth
preparation.
• Only in this way can the occlusion on the restoration be harmonious
and the axial contours normal, preventing periodontal problems
around therestoration
• There are three preparation features that contribute to the durability
of the restoration that is (1)Occlusal reduction (2)axial reduction (
3)provision for reinforcingstruts

36. Occlusal Reduction
One of the most important features for providing adequate
bulk of metal and strength to the restoration is Occlusal Clearance.
For gold alloys, there should be 1.5 mm of clearance on the functional
cusps (lingual of maxillary molars and premolars and buccal of
mandibular molars and premolars)

37. • Metal-ceramic crowns will require 1.5to 2.0 mm on
functional cuspsthat will be veneeredwith porcelain and
• 1.0to 1.5mm on nonfunctional cuspsto receive ceramic
coverage.
• There should be 2.0 mm of clearance on preparations for all-
ceramic crowns.
• Malposed teeth may have occlusal surfaces that are not
parallel with the occlusal table. Therefore, it may not be
necessaryto reducethe occlusalsurface by 1.0mm to achieve
1.0 mm ofclearance.

38. Thebasic inclined plane pattern of the occlusal
surface
should be duplicated to produce adequate
clearance
without overshortening thepreparation

39. Functional CuspBevel
An integral part of the occlusal reduction isthe functional
cuspbevel .Awide bevel on the lingual
inclines of the maxillary lingual cuspsand the buccal
inclinesof mandibular buccalcuspsprovidesspacefor
anadequate bulk of metal in anareaof heavyocclusal
contact.
If awide bevel is not placedon the functional cusp,
several problems mayoccur

40. Toprevent athincasting
when there is no functional cuspbevel, anattempt may
be made to wax the crown io optimal thickness in this
area.An overcontoured restoration will result and a
deflective occlusal contact is likely to occur unless the
opposing tooth isreduced

41. Axialreduction:
Secondprerequisite for structural durability is axialreduction.
When it is sufficient, restoration walls have satisfactory
thicknesseswith out over contouring
Reinforcingstruts:the features that serveto provide spacefor the
metal that will improve the durability and the rigidity of the
restoration:Offset the occlusalshoulder, the isthmus, the proximal
grooves, and the box. Isthmus connects the boxes, and the offset
ties the groovestogether to enhancethe reinforcing “truss effect”.

42. MarginalIntegrity
Therestoration cansurvive in the biological environment
of the oral cavity only if the margins are closely adapted
to the cavosurface finish line of the preparation. The
configuration
of the preparation finish line dictates the shape
and bulk of restorative material in the margin of the
restoration It alsocanaffect both marginal adaptation
and the degree of seating of the restoration

43. FINISH LINEREQUIREMENTS
Definition:
Thepoint at which apreparation terminates on the
tooth is called the finish line. It is also defined asthe
peripheral extension of atooth preparation (GPT).

44. Therearethree requirements for successfulrestoration margins.
Fundamentals of toothpreparation:Shillingburg,Jacobi,Bracket

45. Functions :
The finish line serves the following functions :
•During visual evaluation of the tooth preparation, it is
a measure of the amount of tooth structure already
removed. It also delineates the extent of the cut in an
apical direction. The more distinct it is, the better it
serves these purposes.
•The finish line is one of the features that can be used
to evaluate the accuracy of the impression made for
indirect procedures.
•In the die, a distinct finish line helps to evaluate the
quality of the die and helps in accurate die trimming.
•The correct marginal adaptation of the wax pattern
depends on an obvious finish line.
•The evaluation of the restoration is also aided by a
proper finish line.
•At cementation, a sharp finish line aids in
determining whether the restoration is fully seated.

46. • Historicallya bevelwas introduced tocompensateforthecasting shrinkage
ofalloysused tofabricatecrowns.
• Metal margins should be ideally acute incross section rather than right-
angled to facilitate acloser fit.
• D is the distance by which the crown fails to seat
• dis the shortest distance between the tooth structure and the restoration
• If the inner angle of the metal margin forms anangle m ,of lessthan 90
degrees with the path of insertion ,asdoes abevel or achamfer ,d will be
smaller thanD.
• Theshortest distance from the casting margin to tooth structure ,d, can
be stated asafunction of D and the sine of the angle m or the cosine of
the angle p,which is the angle between the surface of the bevel and the
path ofinsertion.

48. d = D sinm
or
d = D cosp
Asthe angle m isreduced its sinevalue alsoreducesandsodoes
the value of d…thusreducing the marginal discrepancy.

49. Anangle of 30to 45degreesisconsideredoptimal
• Angles above 50degrees will not reducethe value of d.
• Angles below 45degrees will result in too thin acasting.

50. TYPES OF FINISH LINES
Chamfer finish line :
The preferred finish line for the veneer metal
restorations is the chamfer. This finish line has been
shown experimentally to exhibit the least stress, so that
the cement underlying it will less likelihood of failure. It
can be cut with the tip of a round end diamond, while the
axial reduction is being done with the side of that
instrument. However, a torpedo diamond is less likely to
produce a butt joint. The margin of the cast restoration
that fits against it combines an acute edge with a nearby
bulk of metal.

51. Heavy chamfer finish line :
A heavy chamfer is used to provide a
90-degree cavosurface angle with a large radius
rounded internal angle. It is created with a
round end tapered diamond. In the hands of an
unskilled operator, this instrument can create an
undesirable fragile lip of enamel at the
cavosurface. The heavy chamfer provides better
support for a ceramic crown than does a
conventional chamfer, but it is as good as a
shoulder. A bevel can be added to the heavy
chamfer for use with a metal ceramic
restoration.

52. Shoulder :
The shoulder has long been the finish
line of choice for the all-ceramic crowns. The
wide ledge provides resistance to occlusal
forces and minimizes stresses that might lead to
fracture of the porcelain. It produces the space
for healthy restoration contours and maximum
esthetics. However, it does require destruction
of more tooth structure than any other finish
line, the sharp 90-degree internal line angle
associated with the classic variety of this finish
line concentrates stress in the tooth and is
conductive to coronal fracture. The shoulder
generally is not used as a finish line for cast
metal restorations.

53. Shoulder with bevel :
The shoulder with a bevel is a used as a
finish line in a variety of situations. It is utilized
as the gingival finish line on the proximal box
of inlays and onlays, and for the occlusal
shoulder of onlays and mandibular three quarter
crowns. This design can also be used for the
facial finish line of metal ceramic restorations
where gingival esthetics is not critical. It can be
used in those situations where a shoulder is
already present, either because of destruction by
caries or the presence of previous restorations.
It is also a good finish line for preparations with
extremely short walls, since it facilities axial
walls that are nearly parallel.

54. Knife-edge :
The knife-edge margin provides for an
acute margin of metal. But its use can create
problems. Unless it is carefully prepared, the
axial reduction may fade out instead of
terminating in a definite finish line. The thin
margin of the restoration that fits this finish line
may be difficult to accurately wax and cast. It is
also more susceptible to distortion in the mouth
when the casing is subject to occlusal forces.

55. Featheredge :
A featheredge finish line is unacceptable
because it is not sufficiently distinct and results
in so little cervical tooth reduction that the
restoration must be over contoured to possess
adequate rigidity. Also, since a feather edge is
more difficult to see visually, occlusocervical
undulations and irregularities in the finish line
are more likely to be present, making it much
more difficult to fabricate a restoration that fits
accurately.

56. Instrumentation
The advent of hand piece capable of speedsin excessof 100,000 rpm made possible
efficient cutting with smallerinstruments,whichmade more sophisticatedpreparations
practical.
With high speedinstruments the problemof over heating the tooth duringpreparation
iscritical.
Cuttingdry cancausethree timesmore dentinalburning andthermal changesleading
to pulpalinflammationandnecrosisthan with adequateair water spray.
Brownet` al calculatedthe temperatureof highspeeddry cutting to be 118 degree
celcius.
The seriousnessof whichcanbeunderstood fromZach`scontentions that even an
increaseof 20degreeFahrenheit will leadto pulpaldeath in 60%of the teeth.
Dry cutting of non-vitalteeth alsoshouldbe avoidedasit canleadto micro fracturesin
the enamel.

57. There arebasicallythree main rotary instruments usedin tooth preparation.
1. Diamondstone
2. Tungsten carbidebur
3. Twistdrill

58. Diamond stones
Numerous small ,irregularly placed sharp diamond chipsare
electroplated with anickel or chromium bonding medium to
steel instrument blanks whose head is machined to the
desiredfinal shape of the instrument.
They most effective against cutting enamel and porcelain.
An ideal diamond instrument should have diamond stones
evenly placed with intimate contact between the chipsand
the bindingmaterial.

62. Tunsten carbide burs
They are best suited for making precise preparation
features and smooth surfaces on enamel and dentin.
They canalso be used to cut metal.
The metal in the head of the tungsten carbide bur is
formed by sintering tungsten carbide powder and cobalt
powder under heatand vacuum.
Theseare then cut into desired lengths and attached to
steel rods using soldering or welding.
Most burs have sixand occasionally eight blades.
Finishing burs willhave 12blades.
Thefiner the finish more the number of blades.

63. The angle at which the face of the blade meets aline extending from the cutting edge
to the bur axis is known asthe rake angle.
The more positive the rake angle.

64. The twist deill is made up of steel
It cuts only at its tip asit is pushed into the tooth in the
direction of the long axis of the instrument
It hasdeep twin heliocal flutes that wind around the shaft
in atight spiral,helping to remove chips from the hole.
Used to make small,uniform diameter,parallel-sided holes
in dentin to receive retentive pins for restorations.
The drill diameter is slightly larger than the pins that are
incorporated into cast restorations to allow for asmall
cement space.
The working portion of this type of drill should be 3–5
mm long.
ashallow pilot hole is made with no.1/2 round bur on a
narrowhorizontal ledge to ensure that the hole will be
drilled precisely in its intended position.

65. 0.6 mm twist drill,pin holes for parallel pins for cast
restorations
0.5 mm Kodex drill usedfor creating pinholes for minim
threaded pins,which retain amalgam and
compositecores.

66. Dualinstrumentation
Diamond burs cut through enamel more efficiently
than carbide burs but they leave micro scratces on
the surfacereducing the finish of the preparation.
Leading to rough cavosurface and marginal
preparation with diamondburs.
Hence for preparation of grooves ,box forms ,
isthmus etc
Here both diamond and carbide burs areusedof
the same length and diameter which are
configured

67. Clearanceangle
Rakeangle

68. Common errors of tooth preparation :
1.Insufficient occlusal or incisal reduction.
2.Lack of uniform reduction of labial or buccal surfaces compromising esthetics.
3.Minimum axial reduction on the buccal and lingual surface of the posterior teeth,
which increases the incidence of working prematurities. The distinction between
reduction and clearance is crucial.
4.Inappropriate proximal reduction, which prevents having a cleanable embrasure
space.
5.Over reduction of teeth and/or violation of the biologic width.
6.Insufficient gingival reduction to accommodate a definite finish line.
7.Undercuts on the distolingual surface of the preparation and/or lack of parallelism
of the FPD abutments.
8.Failure to contour proximal surfaces of adjacent teeth to allow seating of a
restorations.

69. ESTHETICCONSIDERATIONS
The restorative dentist should develop skill in
determining the esthetic expectations of the patient. Patients
prefer their dental restorations to look asnatural aspossible.
However, caremust be taken that the esthetic considerations are
not preserved at the expense of the patient’s long term oral
health or functionalefficiency.
Whenever possible, accomplishment of an esthetically
acceptable result without the useof metal-ceramic crowns is
preferred, not only because tooth structure is conserved but also
because no restorative material canapproach the appearance of
intact toothenamel.

70. CONCLUSION
The principles of tooth preparation can be categorized into biologic,
mechanical and esthetic considerations. Often those principles conflict, and the
clinician must decide how the restoration should be designed. Each tooth
preparation must be measured by clearly defined criteria, which can be used to
identify and correct problems. It is important to understand the pertinent theories
underlying each step. Successful preparations can be obtained by systematically
following these steps. It is important to critically evaluate each step before
proceeding to the next step to ensure an optimal quality final restoration, which
will serve the patient for a long time.