PRINCIPLES OF TOOTH PREPARATION
DEPT.OF CONS & ENDO
KVG DC ,SULLIA
•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)
Despite these advances, traditional crowns are still indicated for majority of patients. The classic design of the preparation must be visualized so that modifications if required can be instituted. Diagnosis and disciplined tooth preparation are essential to successful fixed prosthetics.
OBJECTIVES OF TOOTH PREPARATION
Conservation of tooth
Avoidance of over contouring
Supra gingival margins
Protection against tooth
Minimum display of metal
Maximum thickness of
Porcelain occlusal surfaces
PRINCIPLES OF TOOTH PREPARATION
(According to Rosenstiel)
Damage to adjacent teeth is prevented by positioning the
diamond so a thin lip of enamel is retained between the bur
and the adjacent tooth.
Note that the orientation of the diamond parallels the long axis.
The enamel should be maintained mesial to the path of the diamond as the reduction progresses.
Damage to the soft tissues of the tongue and cheeks
can be prevented by careful retraction with an aspirator
tip, mouth mirror , or flanged salivaejector.
Great care is needed to protect the tongue
when the lingual surfaces of mandibular molars are
Great care also is needed to prevent pulpal injuries
during fixed prosthodonticprocedures,
Especiallycomplete crown preparation.
Pulpal degenerationthat occurs many years after tooth preparation has
Extreme temperatures, chemical irritation, or microorganisms can cause an irreversible pulpitis particularly when they occur on
freshly sectioned dentinal tubules.
• Chemical action
• Bacterial action
Preservation of Tooth Structure
• In addition to replacing lost tooth structure, a restoration must
preserve remaining tooth structure.
• Intact surfaces of tooth structure that can be maintained while
producing a strong, retentive restoration should be saved if patient
acceptance and retention requirements will permit it.
• Preservation of tooth structure in some cases may
require that limited amounts of sound tooth structure be
removed to prevent subsequent uncontrolled loss of larger
quantities of tooth structure.
Conservation of tooth structure by using partial-coverage restorations. In this
case, they are used as fixed dental prosthetic abutments to replace congenitally
missing lateral incisors.
Retention and Resistance
•No cements that are compatible with living tooth structure and the biologic environment of the oral cavity possess adequate 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 and resistance.
Retentionprevents removal of the restoration along the
path of insertion or long axis of the tooth preparation.
Resistanceprevents 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.
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
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 inclinationof that wall.
•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.
•Consciously attempting to cut a taper can easilyresult in an overtaperedand nonretentivepreparation.
•Ataper or total convergence of 16 degrees has been proposedas being achievable clinically while still affordingadequate retention.
•This is probably an acceptableoverall target. It can be as low as 10 degrees on preparationson anterior teeth and as high as 22 degrees onmolars.
Freedom of Displacement
Maximum retention is achieved when there is only one path.
A full veneer preparation with long, parallel axial walls and grooves would produce such retention .
On the opposite extreme, a short, overtaperedpreparation would be without retention because the restoration could be removed along an infinite number of paths.
Limiting the freedom of displacement from torquingor twisting forces in a horizontal plane increases the resistance of a restoration.
A groove whose walls meet the axial wall at an oblique angle does not provide the necessary resistance. V-shaped grooves produce roughly one- half as much resistance to lingual displacement as do grooves with a definite lingual wall.
Surface area :
Greater the area of the cement film bound to the preparation and to the internal detail of the casting, greater the retention.
The total surface area of the preparation is influenced by the size of the tooth, the extent of the coverage by the restoration, and internal features such as grooves and boxes
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.
Types of preparation :
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).
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
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
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
Leverage And Resistance:
Leverage, is the predominant factor in the dislodgement of the cemented restoration, occur when line of action of force passes outside the supporting tooth structure.
Length of preparation has strong influence on its resistance
Shortening a preparation will produce a proportionally greater diminution of the resisting area
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 incisalportion of the axial wall will resist displacement
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
•The path of insertion is an imaginary line along which the
•restoration will be placed onto or removed from the
•It is determined mentally by the dentist
•before the preparation is begun, and all features of the
•preparation are cut to coincide with that line.
Path of insertion
If the center of the occlusal surface of a preparation
is viewed with one eye from a distance of approximately
30 cm (12 inches), it is possible to sigh! down the
axial walls of a preparation with a minimum taper
For a preparation to be surveyed in the mouth, where
direct vision is rarely possible, a mouth mirror is used.
It is held at an angle approximately 1/2 inch above the preparation, and the image is viewed with one eye.
If fixed partial denture abutment preparations are
being evaluated for a common path of insertion, a firm
finger rest is established and the mirror is maneuvered
until one 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
One of the most important features for providing adequate
bulk of metal and strength to the restoration is OcclusalClearance.
For gold alloys, there should be 1.5 mm of clearance on the functional cusps (lingual of maxillary molars and premolars and buccalof mandibular molars and premolars)
•Metal-ceramic crowns will require 1.5 to 2.0 mm on
functional cusps that will be veneered with porcelain and
•1.0 to 1.5 mm on nonfunctional cusps to receive ceramic coverage.
•There should be 2.0 mm of clearance on preparations for all- ceramic crowns.
•Malposedteeth may have occlusal surfaces that are not parallel with the occlusaltable. Therefore, it may not be necessary to reduce the occlusal surface by 1.0 mm to achieve 1.0 mm of clearance.
The basic inclined plane pattern of the occlusal surface
should be duplicated to produce adequate clearance
without overshorteningthe preparation
Functional Cusp Bevel
An integral part of the occlusal reduction is the functional
cusp bevel . A wide bevel on the lingual
inclines of the maxillary lingual cusps and the buccal
inclines of mandibular buccalcusps provides space for
an adequate bulk of metal in an area of heavy occlusal
If a wide bevel is not placed on the functional cusp,
several problems may occur
To prevent a thin casting
when there is no functional cusp bevel, an attempt may
be made to wax the crown iooptimal thickness in this
area. An overcontouredrestoration will result and a
deflective occlusal contact is likely to occur unless the
opposing tooth is reduced
Second prerequisite for structural durability is axial reduction. When it is sufficient, restoration walls have satisfactory thicknesses with out over contouring
Reinforcing struts: the features that serve to provide space for the metal that will improve the durability and the rigidity of the restoration: Offset the occlusal shoulder, the isthmus, the proximal grooves, and the box. Isthmus connects the boxes, and the offset ties the grooves together to enhance the reinforcing “truss effect”.
The restoration can survive in the biological environment
of the oral cavity only if the margins are closely adapted
to the cavosurfacefinish 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 also can affect both marginal adaptation
and the degree of seating of the restoration
FINISH LINE REQUIREMENTS
The point at which a preparation terminates on the tooth is called the finish line. It is also defined as the peripheral extension of a tooth preparation (GPT).
There are three requirements for successful restoration margins.
Fundamentals of tooth preparation:Shillingburg,Jacobi,Bracket
•Historically a bevel was introduced to compensate for the casting shrinkage of alloys used to fabricate crowns.
•Metal margins should be ideally acute in cross section rather than right- angled to facilitate a closer fit.
•Dis 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 an angle m ,of less than 90 degrees with the path of insertion ,as does a bevel or a chamfer ,dwill be smaller than D.
•The shortest distance from the casting margin to tooth structure ,d, can be stated as a function of Dand the sine of the angle m or the cosine of the angle p,whichis the angle between the surface of the bevel and the path of insertion.
d = D sin m
d = D cosp
As the angle m is reduced its sine value also reduces and so does the value of d…thus reducing the marginal discrepancy.
An angle of 30 to 45 degrees is considered optimal
•Angles above 50 degrees will not reduce the value of d.
•Angles below 45 degrees will result in too thin a casting.
TYPES OF FINISH LINES
The advent of hand piece capable of speeds in excess of 100,000 rpm made possible efficient cutting with smaller instruments,whichmade more sophisticated preparations practical.
With high speed instruments the problem of over heating the tooth during preparation is critical.
Cutting dry can cause three times more dentinal burning and thermal changes leading to pulpal inflammation and necrosis than with adequate air water spray.
Brown et` al calculated the temperature of high speed dry cutting to be 118 degree celcius.
The seriousness of which can be understood from Zach`s contentions that even an increase of 20 degree Fahrenheit will lead to pulpal death in 60% of the teeth.
Dry cutting of non-vital teeth also should be avoided as it can lead to micro fractures in the enamel.
There are basically three main rotary instruments used in tooth preparation.
2.Tungsten carbide bur
Numerous small ,irregularly placed sharp diamond chips are electroplated with a nickel or chromium bonding medium to steel instrument blanks whose head is machined to the desired final 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 chips and the binding material.
They are best suited for making precise preparation features and smooth surfaces on enamel and dentin.
They can also 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 heat and vacuum.
These are then cut into desired lengths and attached to steel rods using soldering or welding.
Most burs have six and occasionally eight blades.
Finishing burs will have 12 blades.
The finer the finish more the number of blades.
The angle at which the face of the blade meets a line extending from the cutting edge to the bur axis is known as the rake angle.
The more positive the rake angle.
The twist deillis made up of steel
It cuts only at its tip as it is pushed into the tooth in the direction of the long axis of the instrument
It has deep twin heliocalflutes that wind around the shaft in a tight spiral,helpingto remove chips from the hole.
Used to make small,uniformdiameter,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 a small cement space.
The working portion of this type of drill should be 3 –5 mm long.
a shallow pilot hole is made with no.1/2 round bur on a narrowhorizontalledge to ensure that the hole will be drilled precisely in its intended position.
0.6 mm twist drill,pinholes for parallel pins for cast restorations
0.5 mm Kodexdrill used for creating pinholes for minim threaded pins,whichretain amalgam and compositecores.
Diamond burs cut through enamel more efficiently than carbide burs but they leave micro scratceson the surface reducing the finish of the preparation.
Leading to rough cavosurfaceand marginal preparation with diamond burs.
Hence for preparation of grooves ,box forms , isthmus etc
Here both diamond and carbide burs are used of the same length and diameter which are configured
The restorative dentist should develop skill in determining the esthetic expectations of the patient. Patients prefer their dental restorations to look as natural as possible. However, care must be taken that the esthetic considerations are not preserved at the expense of the patient’s long term oral health or functional efficiency.
Whenever possible, accomplishment of an esthetically acceptable result without the use of metal-ceramic crowns is preferred, not only because tooth structure is conserved but also because no restorative material can approach the appearance of intact tooth enamel.