Tooth preparation for class II amalgam restorations and its modifications

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INTRODUCTION

Amalgam although a brittle material, has been adequately serving as a restorative material since 175 years, understanding of its properties and judgment of the preparation design are the vital factors controlling its use.

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Class II Restorations.

It is defined as a cavity that is present on the proximal surfaces of premolars and molars.( G.V .BLACK )

DEFINITION

4INDICATIONS for using amalgam as restorative material

Moderate-to-large restorations.

Restorations that are not in highly esthetic areas of the mouth.

Restorations that cannot be well isolated.

Restorations that have heavy occlusal loads.

Temporary and caries control restorations.

Foundations.

Restorations that extends onto root surface.

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Line angles and point angles

6 point angles11 line angles

Tooth preparation walls- Facial (proximal and occlusal portion)Lingual (proximal and occlusal portion)GingivalPulpal AxialDistal / Mesial

6INITIAL CLINICAL PROCEDURES

Marking the occlusal contacts with articulating paper . These contacts serve as guide in tooth preparation and restoration.

Recontouring of any opposing “plunging cusp" or other pointed cusp to reduce the risk of fracture of the new restoration or the cusp from occlusal forces.

For amalgam , the placement of rubber dam is generally recommended.

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Burs used in class Ⅱtooth preparation

Width 0.8 mm & Length 3 mm

8INITIAL TOOTH PREPARATION OUTLINE FORM

Tooth preparation for caries on proximal surface should have occlusal outline such that it allow access to proximal lesion, remove demineralized enamel and dentin, and remove enamel not supported by sound dentin.

If an occlusal carious lesion is present, it should be treated with a separated occlusal restoration.

If the proximal and occlusal lesions are in close proximity so that there is minimal or no sound tooth structure separating the two preparations, they should be joined.

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Mesio-occlusal class II preparation Occlusal outline form:

Enter the pit nearest the involved proximal

surface with a punch cut using a No. 245

bur.

Viewed from the proximal and lingual

aspects, the long axis of bur should be

parallel to the long axis of tooth crown

during cutting.

Bur position for entry;proximal view

10• Proper depth of the initial entry cut

is 1.5-2mm.The pulpal depth is usually 0.1-0.2 mm in to the dentin.

While maintaining the same depth and orientation ,bur is moved to extend the outline to include the central fissure and opposite pit ,if necessary.

Completed occlusal step

Occlusal view

11 Include all the carious occlusal pits and fissures in such a manner that sharp angles in the marginal outline are avoided.

The isthmus width should be as narrow as possible, ideally should be 1/4th of the intercuspal distance

Enameloplasty should be used where indicated to conserve tooth structure.

Enameloplasty .done by , Fine-grit diamond stone in position to remove fault. C, Smooth surface after enameloplasty. D, Cavosurface angle should not exceed 100 degrees, and marginal-amalgam angle should not be less than 80 degrees.

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Distal wall can be made occlusaly divergent by tilting the bur ,if extension of distal margin would reduce the dentinal support of marginal ridge

Maintaining the bur parallel to long axis of the tooth, creates facial lingual and distal walls occlusaly convergent. Dovetail prevents the proximal displacement of the restoration. It is not required in outline form of single step proximal box ,unless a fissure is radiating from the occlusal pit .

Incline bur distally to make proper occlusal divergence of distal wall

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Before extending into the involved proximal marginal ridge, the final location of the facial and lingual walls of the proximal box relative to the contact area should be visualized ,to prevent overextension of the occlusal outline form (i.e., occlusal step) where it joins the proximal outline form (i.e., proximal box).

Visualization of final location of proximo-occlusal margin

14While maintaining the established pulpal depth

and with the bur parallel to the long axis of the

tooth crown, the preparation is extended mesially, stopping approximately 0.8 mm short of cutting through the marginal ridge into the contact area.

Removing remaining undermined enamel

Breaking proximal contact

Isolating proximal enamel

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The occlusal step in this region is made slightly wider faciolingually than in the Class I preparation because additional width is necessary for the proximal box.

This extension includes part of the mesial marginal ridge, it also exposes the marginal ridge DEJ. The location of the DEJ is an important guide in the development of the proximal preparation.

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Direction of mesiofacial and mesiolingual walls. A, Failure caused by weak enamel margin. B, Failure caused by weak amalgam margin. C, Proper direction to proximal walls results in full-length enamel rods and 90-degree amalgam at preparation margin

17PROXIMAL OUTLINE FORM

Again we visualizes the desired final location of the facial and lingual walls of the proximal box relative to the contact area.

The objectives for extension of proximal margins are to: Include all caries, faults, or existing restorative material.Create 90-degree cavosurface margins (i.e., butt joint margins).Establish (ideally) not more than 0.5 mm clearance with the

adjacent proximal surface facially, lingually, and gingivally.

18 The initial procedure in preparation of outline form is the isolation of the proximal enamel by giving the proximal ditch cut.

Allow the end of the bur to cut a ditch gingivally along the exposed DEJ, two thirds at the expense of dentin and one third at the expense of enamel. The 0.8-mm diameter bur end will cut approximately 0.5 to 0.6 mm into dentin and 0.2 to 0.3 mm into enamel.

The ditch is extended gingivally just beyond the caries or the proximal contact, whichever is greater.

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When extension places the gingival margin in cementum, the initial pulpal depth of the axiogingival line angle should be 0.7 to 0.8 mm (the diameter of the tip end of the No. 245 bur is 0.8 mm). The bur may shave the side of the wedge that is protecting the rubber dam and underlying gingiva.

20 Ideally the extension of facial and lingual margins of proximal

box should be such that ,it provides clearance of 0.2-0.3 mm from the adjacent tooth.

but this clearance greater than 0.5 mm is excessive unless indicated to include caries, undermined

enamel, or existing restorative material.

Ideally the gingival margin should

clear the adjacent tooth by only

0.5mm ,which may be measured

with the side of explorer.

21Preparation of Axial wall- It is an internal prepared wall that is parallel to the long axis

of the tooth & It is always placed in dentin to obtain:

It is always placed in the dentin to obtain:-• Resistance & elasticity of dentin• Bulk of the restoration.• Placement of retentive locks.

Length of the axial wall :- 0.4-0.6 mm from the axio-pulpal line angle to the axio-gingival line angle.

the axial wall should be straight or convex but it should never be concave.

It should follow the contour of proximal surface .

22 PREPRATION OF GINGIVAL SEAT:-

It is an external cavity wall that is perpendicular to the long axis of the tooth.

It is extended beyond the contact area or up to the proximal lesion whichever is more. This also help in providing a contact clearance gingivally.

It is made flat so that the masticatory forces are distributed equally.

The width of gingival seat:- 0.6-0.8mm(premolars) 0.8-1.0mm(molars) It consists of 2/3rd of dentin and 1/3rd of enamel.

23GINGIVAL DIVERGENCE OF FACIAL AND LINGUAL WAALS OF PROXIMAL BOX

The proximal ditch cut may be diverged gingivally to ensure that the faciolingual dimension at the gingival is greater than at the occlusal.

This gingival divergence :- Increases the retention form .- Provides desirable extension of the facial and lingual

proximal margins to include defective tooth structure or old restorative material at the gingival level.

- Conserve the marginal ridge strength.- Provides 90-degree amalgam at the

margins on this ridge .

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In completing the proximal extensions, next make two cuts, one starting at the facial limit of the proximal ditch and the other starting at the lingual limit, extending toward and perpendicular to the proximal surface (until the bur is nearly through the enamel at contact level).

The side of the bur may emerge slightly through the surface at the level of the gingival floor.

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a matrix band may be used around the adjacent tooth to prevent marring its proximal surface.

The isolated enamel, if still in place, may be fractured out with a spoon excavator or by additional movement of the bur.

Proximal enamel plate is broken by hand instrument

Margins palned with GMT or hatchet

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To protect the gingiva and the rubber dam when extending the gingival wall gingivally, a wooden wedge should already be in place in the gingival embrasure to depress the soft tissue and rubber dam.

Rounded toothpick wedge Triangular wedge

Indicated in deepGingival extension

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With the enamel hatchet, the bin-angle chisel, or both, cleave away any remaining undermined proximal enamel, establishing the proper direction to the mesiolingual and mesiofacial walls. .

Proximal margins having cavosurface angles of 90 degrees are indicated.

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PRIMARY RESISTANCE FORM

1. Pulpal & gingival walls being flat and perpendicular to the forces directed to the long axis of the tooth.

2. Restrict the extension of the walls to allow strong cusps & ridges to remain with sufficient dentin support.

3. Restricting the occlusal outline form to areas receiving minimal occlusal contact.

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4.Reverse curve optimizing the strength of both the amalgam and tooth structure at the junction of the occlusal step and proximal box.

5. Rounding off the internal line angles.

6.Providing enough thickness of the restorative material.

30REVERSE CURVEWhen the direction of the mesiofacial wall is parallel to the

direction of the enamel rods, thus creating a ‘s’ shaped curve in the proximal outline.

It is created to:

Provide butt joint in the preparation margin.To relieve the contactTo place the proximal margins in a self cleansing area.

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PRIMARY RETENTION FORM Occlusal convergence of facial & lingual

walls

Dovetail design of occlusal step(when present).

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FINAL TOOTH PREPARATION Removal of any remaining defective enamel and infected carious dentin

Accomplished in the same manner as in the Class I preparation.

Infected carious dentin is removed with a slowly revolving round bur of appropriate size or a discoid-type spoon excavator or both.

. A, Infected carious dentin extending beyond ideal pulpal wall position. B, Incorrect lowering of pulpal wall to include infected carious dentin. C, Correct extension facially and lingually beyond infectedcarious dentin.

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The presence of infected carious dentin on a portion of either the pulpal wall (floor) or axial wall does not indicate deepening the entire wall.

. Infected carious dentin on axial wall does not call for preparing axial wall toward pulp as shown by dotted lines. It should be removed with round bur

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A partial extension of a facial or lingual wall is permissible if:

The entire wall is not weakened

The extension remains accessible and visible

Sufficient gingival seats remain to support the restoration

A butt joint fit at the amalgam and enamel margin (90-degree amalgam angle and 90-degree cavosurface angle) is possible.

35PULP PROTECTION

It is desirable to have approx 2mm bulk beneath the pulp and metallic restoration which may be Dentin, Liner or a Base.

Liner/Base- Mechanical, Chemical, Thermal Pulp protection and act as a barrier that protects the dentin.

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Remaining Dentin Thickness 0.5-1 mm -Zinc Phosphate Cement, Glass Ionomer Cement base is used followed by calcium hydroxide liner.

The base insulates the pulp from thermal changes, bonds to the dentin, releases fluoride, and is strong enough to resist the forces of condensation.

37SECONDARY RESISTANCE AND RETENTION FORMS

Using the gingival margin trimmer to bevel or round the axiopulpal line angle.

Proximal retention locks: Placed on axiofacial and axiolingual line angles. Terminate at axiopulpal point angle. Prepared with No. 169L bur or No. ¼ bur There are four characteristics or determinants of proximal

locks: (1) position, (2) translation, (3) depth, and (4) occlusogingival orientation

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Proximal retention locksA, Position of No. 169L bur and pulpally. B, Lingual lock..C, Completed locks. D, Locks preparedwith No. '/, bur. E, Completed locks

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Position refers to the axiofacial and axiolingual line angles of initial tooth preparation It is important to note that the retention locks should be placed 0.2 mm inside the DEJ, regardless of the depth of the axial walls and axial line angles.

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Translation refers to the direction of movement of the axis of the bur i.e. parallel to DEJ.

Depth refers to the extent of translation (i.e., 0.5 mm at gingival floor level).

Occlusogingival orientation refers to the tilt of the No. 169L bur, which dictates the occlusal height of the lock, given a constant depth.

41Retention grooves:Placed on axiofacial and axiolingual line angles.

Extend from the gingival floor to the occlusal surface

Prepared with No. ¼ round bur with head diameter of 0.5mm or No. 1/8 bur with head diameter of 0.4mm

In preparation with deep proximal box grooves should be in the proximal walls just inside the DEJ and not in the corners of the box

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Circumferential slots:

Prepared with No. 33 ½ inverted cone bur0.5 to 1 mm deep inside DEJ0.6 mm deep

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Pins: A)Cemented B)Friction locked C) Self threaded

Horizontal Vertical As a rule one pin per missing axial angle should be used . Pin hole should be positioned no closer than 0.5 to 1 mm to the DEJ or 1 - 1.5 mm to external surface of the tooth.

Pinhole should be parallel to adjacent external surface of tooth.

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Amalgapins:Depth of 1.5 to 2mm is adequateDiameter 0.8 to 1mmPrepared with No.1156, No.1157, No.330 or No.56 burs

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Amalgam bonding agents:Should have auto-polymerizing property as amalgam does not allow light transmission

Ex: All bond-2, Amalgam bond plus, etc.

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PROCEDURE FOR FINISHING EXTERNAL WALLS:

1. There should be no unsupported enamel and marginal irregularities present.

2. There should be a butt joint relation between the tooth & amalgam

3. Cavosurface bevel [20°] at the gingival margin can be given by G.M.T , to remove the unsupported enamel rods.

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• When the gingival margin is positioned gingival to the cementoenamel junction (CEJ) on the tooth root, the bevel is not indicated.

48Final procedures: cleaning, inspecting, desensitizing,and bonding

First step includes removing all chips and loose debris and then drying the preparation(not dessicating) and check for any infected dentin and any unsound enamel margins.

Cleaning is to free the preparation of visible debris with warm water from syringe and then to remove moisture with a few light surges of air syringe.

Varnishes should be applied on all the preparation walls to prevent the microleakege.

GLUMA desensitizer can also be used to reduce the postoperative sensitivity.

Disinfection of the preparation walls can be done by silver nitrate, phenol,ethyl alchol (short duration).

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RESTORATIVE TECHNIQUES

MATRICES Are used in class ii restorations.It is defined as “a properly shaped piece of metal or non

metal which supports the restoration during its insertion and its setting.”

ESSENTIAL QUALITIES:-1. Rigidity2. Help to re establish proper anatomic contour.3. Help to re establish proper proximal contact.4. To prevent over hang of the restoration and to maintain the

health of the interdental gingiva.5. Easily removable.6. Should neither adhere to ,nor react with the restorative

material.

Many types of matrices are available:- *Universal matrix *auto matrix *Precontoured bands *compound supported

matrix

50MATRICES

UNIVERSAL MATRIXAUTO MATRIX

PRE CONTOURED MATRICES

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VARIATIONS OF ONE PROXIMAL SURFACE TOOTH PREPARATION

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SIMPLE BOX PREPARATIONA proximal box that is prepared without occlusal step.

When restoring a small, cavitated, proximal lesion in a tooth with neither occlusal fissures nor a previously inserted occlusal restoration, a proximal box preparation without an occlusal step has been recommended

Indicated in narrow proximal contacts allowing minimum facial and lingual extension.

Facial & lingual proximal walls converge occlusally.

Proximal retention locks-0.5mm at gingival point angles tapering to 0.3mm at occlusal surface are prepared

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53 The proximal preparation begins with the creation of a slot, cut with a

small bur in the center (mesiodistally) of the crest of the marginal ridge and occlusal to the caries lesion.

The slot is deepened gingivally until the bur “falls” into the soft carious dentin

The preparation is widened facially and lingually to eliminate all demineralized tooth structure at the DEJ and to remove enamel that is not supported by sound dentin.

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SLOT PREPARATION FOR ROOT CARIES When caries is present in the proximal aspect without involving the marginal ridge.

Indicated in cases of gingival recession having root surface caries gingival to the proximal aspect.

When the adjacent tooth is missing. Preparation for Class II restorations with facial or lingual access are sometimes referred to as Key Hole Preparations.

The tooth preparation is usually approached from the facial and has the form of a slot.

-

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Outline form extension to sound tooth structure is at a limited depth axially (i.e., 0.75 to 1 mm at the gingival aspect [if no enamel is present], increasing to 1 to 1.25 mm at the occlusal wall [if margin in enamel]

The remaining infected carious dentin (if any) will be removed during final tooth preparation.

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• Prepare retention grooves with a No. 1/4, bur into the occlusoaxial and gingivoaxial line angles, 0.2 mm inside the DEJ or 0.3 to 0.5 mm inside the cemental cavosurface margin .

• The depth of these grooves is one half the diameter of the bur head (i.e., 0.25 mm), and the bur is directed to bisect the angle formed by the junction of occlusal (or gingival) and axial walls.

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Mandibular 1st premolar The support of the small lingual cusp may be

conserved by preparing the occlusal step more at the expense of tooth structure facial to the central groove than lingual.

The lingual cusp may need to be reduced for capping if the lingual margin of the occlusal step extends more than two thirds the distance from the central fissure to the cuspal eminence

58SLOPING PULPAL FLOOR.

This preparation is done in the mandibular 1st premolar because of a short lingual cusp and also due to the lingual inclination of the tooth.

This preparation coincides with the occlusal morphology of the tooth.

For a preparation that will not cross the transverse ridge, prepare the proximal box before the occlusal portion to prevent removing the tooth structure that will form the isthmus between the occlusal dovetail and the proximal box.

The bur axis is parallel to the tooth crown which is tilted slightly lingually

The pulpal wall is facially inclined

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MAXILLARY MOLAR

Extension into the enamel oblique ridge is avoided whenever possible to maintain the cross-splinting strength it provides to the tooth

61It may require extending through the oblique ridge to unite the proximal preparations with the occlusal step.

Cutting through the oblique ridge is indicated only if: (1) the ridge is undermined with caries, (2) it is crossed by a deep fissure, or(3) occlusal portions of the separate MO and DO outline forms leave less than 0.5 mm of tooth structure between them.

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Maxillary 1st premolar

A Class II preparation involving the mesial surface requires special attention because the mesiofacial embrasure is esthetically prominent.

The occlusogingival preparation of the facial wall of the mesial box should be parallel to the long axis of the tooth instead of converging occlusally to minimize an unaesthetic display of amalgam in the faciogingival corner of the restoration.

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The facial extension of the mesiofacial proximal wall should be minimal and margins may be placed lingual to the facial contact.

64ROTATED TEETH. Tooth preparation for rotated teeth follows the same principles as for normally aligned teeth.

When the tooth is rotated 90 degrees and the "proximal“ lesion is on the facial or lingual surface or orthodontic correction is declined or ruled out, the preparation may require an isthmus that includes the cuspal eminence.

If the lesion is small, consideration should be given to the slot preparation.

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ADJOINING RESTORATION.• It is permissible to repair or replace a defective

portion of an existing amalgam restoration if the remaining portion of the original restoration retains adequate resistance and retention form.

• Where the two restorations adjoin, care should be taken that the outline of the second restoration does not weaken the amalgam margin of the first.

• It includes replacing a defective portion of an

existing restoration , while remaining portion retains adequate resistance and retention form.

• The intersecting margins of the two restorations should be at right angles as much as possible.

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UNUSUAL OUTLINE FORM.

Usually seen in mandibular third molars having unusual groove pattern.

central fissure segmented by coalesced enamel

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CLASS II AMALGAM RESTORATIONS INVOLVING BOTH PROXIMAL SURFACES

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THE CONVENTIONAL DESIGN OR MESIO OCCLUSO DISTAL CAVITY.

A moderate to large sized lesion involving both the proximal boxes in which it becomes imperative to involve the occlusal box for proper accesibility of proximal lesion.

69OCCLUSAL EXTENSIONS:

Requires extending grooves that are fissured, capping cusps that are undermined, or extending the outline form up the cuspal inclines.

Groove extension occurs at the same initial pulpal floor depth (i.e., 1.5 mm) but follows the DEJ as the groove is extended in a facial or lingual direction.

This extension (and groove extension) will usually require some alteration in the orientation of the bur: a slight lingual tilt when extending in a facial direction, and a slight facial tilt when extending in a lingual direction.

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PROXIMAL EXTENSIONS:

Larger Class II restorations will often require larger proximal box preparations.

Extensive proximal boxes are usually prepared the same as a more conservative proximal box, but may require modifications.

The increased dimensions of a large proximal box usually require the use of retention locks or other secondary retention form features.

If the proximal box is extended onto the root surface, the axial wall depth is no longer dictated by the DEJ.

Axial wall depth of approximately 0.8 mm.

Cusp cappingThe small distal cusp of mandibular first molars, the distolingual cusp of maxillary molars, and the lingual cusp of some mandibular premolars (especially first premolars) may be weakened when normal preparations of surrounding areas of the tooth are included.

This requires capping of these cusps

Cusp capping increases the resistance form of the tooth .

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Cusp reduction for an amalgam restoration should result in a uniform amalgam thickness over the reduced cusp of 1.5 to 2 mm.

The thicker amount is necessary for functional cusps.

To reduce the cusp, orient the No. 245 bur parallel to the cuspal incline and make several depth cuts in the cusp (to a depth of 1.5 or 2 mm).

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Use the bur to reduce the cusp, following the mesiodistal inclines of the cusp. This results in a uniform reduction.

Cusp capping reduces the amount of vertical preparation wall heights and, therefore, increases the need for the use of secondary retention features.

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Maxillary first molar.May require extending through the oblique ridge to unite the

proximal preparations with the occlusal step.

Cutting through the oblique ridge is indicated only if: (1) the ridge is undermined with caries, (2) it is crossed by a deep fissure, or (3) occlusal portions of the separate MO and DO outline forms

leave less than 0.5 mm of tooth structure between them.

The remainder of the outline form is similar to the two-surface outline forms

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Extending the facial or lingual wall of a proximal box to include the entire cusp is done (if necessary) to include weak or carious tooth structure or existing restorative material.

MANDIBULAR FIRST MOLARThe distal cusp on the mandibular first molar may be weakened

when positioning the distofacial wall and margin.

When the distal cusp is small or weakened or both, extension of the distal gingival floor and distofacial wall to include the distal cusp places the margin just mesial to the distofacial groove.

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77 Capping the distal cusp is an alternative

to extending the entire distofacial wall when the occlusal margin crosses the cuspal eminence.

Whenever possible, capping the distal cusp is more desirable than extending the distofacial margin because the remaining portion of the cusp helps in applying the matrix for the development of proper embrasure form. It also conserves tooth structure.

The cusp reduction should result in a butt joint between the tooth structure and the amalgam.

78CONCLUSIONThe successful and intelligent practice of operative dentistry is

based on a thorough knowledge of fundamentals of laws of science.

The wise operator who views each preparation with a discerning eye can render the patient an excellent service!!

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References Theodore M. Roberson, Harald O. Heymann, Edward J. Swift,

Sturdevant’s Art and Science of Operative Dentistry; Fifth edition

James B Summit, J.W. Robbins, Richard S Schwartz, Fundamentals of operative Dentistry a contemporary approach; Second Edition