5. Features of carious cavities preparation of II, III, IV classes by Black.

PRINCIPLES OF CAVITY PREPARATION

Basic principles of cavity preparation were developed by Dr. G.V. Black in the early 1900s and are uniquely applied to each class of caries and type of restorative material. Today, the application of his principles has been modified due to the introduction of new dental restorative materials that were not available in his day. A dentist still needs to consider each principle when preparing a tooth for a conservative operative restoration.

A. ESTABLISH AN OUTLINE FORM

The outline form of a preparation is the external shape of the preparation where prepared tooth meets unprepared tooth. It is developed by removing the least amount of tooth structure possible, yet adhering to the following principles:

1. EXTEND THE PREPARATION TO SOUND ENAMEL

The dentist enlarges the preparation outline so that it extends to enamel that has no signs of active decay. Also, when the dentist ends the preparation on enamel margins, the enamel must be able to withstand the forces required when placing the restoration and the forces applied during tooth function. In many cases, this involves extending the preparation to enamel that is supported by, or resting on, sound dentin that is not undermined by the spread of caries within the dentin. Since enamel is brittle, if it is not sufficiently supported by sound dentin and/or bonding techniques, the unsupported, brittle enamel rods may fracture, leaving a gap between the tooth and the restorative material.

2. EXTEND THE PREPARATION FOR PREVENTION

The dentist evaluates the need to enlarge the preparation within enamel beyond the specific area of decay in order to include adjacent tooth structure felt to be prone to the development of future decay. For example, when treating a carious pit and fissure lesion, it may be advisable to include adjacent deep pits and fissures thought to be caries prone, even though they have not yet become carious. Similarly, when developing the cavity preparation for smooth surface carious lesions, the outline of the preparation may be extended to include adjacent smooth surface areas likely to become carious. The dentist must determine whether or not to extend the outline based on a risk assessment of that patient. Over the past 35 years, there has been a tremendous increase in the use of fluoride (in community water, toothpaste, rinses, and topical applications applied periodically in the dental office), as well as improved efforts by dental professionals to educate the population in prevention techniques. Therefore, the need for preventive extension on smooth surface lesions must be weighed against the possibility that excellent hygiene and fluoride could stop or even reverse the decay process, especially if the decay has not progressed too far.

The degree of extension should be based on factors such as the age of the patient (younger enamel is more susceptible to caries than mature enamel), the person’s rate of caries activity, personal oral hygiene, and dietary habits. For example, extension for prevention for a tooth preparation on a younger patient with multiple areas of active decay, poor oral hygiene, and frequent intake of high-sugar snacks and sugar-containing carbonated beverages who is unwilling or unable to change is more appropriate than it would be in an older patient with a lower caries rate, better eating habits, and good or improving oral hygiene.

3. PROVIDE ADEQUATE ACCESS

A restoration outline must be large enough for the dentist to ensure that all carious tooth structure has been removed and that instruments required to insert the filling material will fit. A small, narrow initial cut through the enamel might not permit the dentist to confirm the removal of all caries that may have spread laterally at the DEJ. Further, even when the removal of all caries can be verified visually or by probing, the initial preparation might be too small to place the restoration without voids.

4. PROVIDE RESISTANCE FORM

The dentist must design a preparation to ensure room for an adequate thickness of restorative material for strength, and sufficient remaining solid tooth structure to withstand or resist occlusal forces. This is known as resistance form. If the preparation depth is inadequate for the material of choice to withstand occlusal forces, the restoration could break. If the remaining tooth structure is too thin or undermined, it could fracture.

B. PROVIDE RETENTION FORM

Retention form is the design of a preparation that prevents the restoration from falling out. The methods for providing retention differ depending on the restorative material and on the location of the carious lesion. Retention for amalgam restorations is provided by internal retentive features, such as retentive grooves, and by the convergence of some preparation walls. Retention for composite resin restorations is provided by acid etching the enamel to produce microscopic irregularities (minute undercuts) on the surface. Then, a first layer of flowable resin (bonding agent) can flow into the irregularities forming retentive resin tags that, when hardened, mechanically lock into the microscopic retentive features of the etched enamel (Fig. 10-8). Layers of the stronger composite resin can subsequently be chemically bonded to the initial flowable resin layer to complete the restoration. When using newer adhesive agents, additional retention is gained by chemical bonds formed between tooth and resin.

The effect of etching enamel. A. Magnified

view of a nonetched enamel surface (3260 times bigger).

B. Magnified view of an etched enamel surface (3600 times

bigger) after application of 50% phosphoric acid. This etched

surface allows the resin bonding agent of the composite systems

to flow into the irregular microscopic undercuts, thus affording

mechanical retention for the material.

C. REMOVE CARIES AND TREAT THE PULP

All principles of the cavity preparation described up to this point assume that caries has spread just beyond the DEJ into dentin. The dentist usually prepares the outline form and retention for a cavity preparation to a depth just beyond the DEJ with a high-speed dental handpiece using carbide or diamond burs that cut quickly, minimizing the potentially damaging heat by use of an effective water coolant spray. When removing carious lesions that have progressed deeper into dentin, the dentist uses slowly rotating round burs in slow-speed handpieces, or hand instruments. The slow-speed handpiece, or hand instruments, permit the dentist to differentiate between the softer carious dentin and the harder healthy or non-carious dentin.

When caries extends close to the pulp, it may be advisable to protect the vital tissues of the tooth (odontoblasts, blood vessels, and nerves within the pulp) with dental liners and cement bases prior to placing the final restoration (Fig. 10-9). Various dental materials have been developed for this purpose. When used in the appropriate combination and in the correct order, they can prevent bacterial penetration, provide thermal insulation, sedate the pulp, or stimulate the production of secondary dentin.

D. FINISH THE PREPARATION WALLS

This step involves using a handpiece with appropriate burs or hand instruments (chisel type) designed to smoothly plane the walls while removing unsound enamel (i.e., enamel that is crazed or cracked, or not supported by sound dentin).

E. CLEAN THE PREPARATION

Prior to the restoration of any cavity preparation, the operator must remove tooth debris, hemorrhage, saliva, and any excess cement base. In this way, the restorative material will contact only sound, clean tooth structure.

F. FINAL EVALUATION OF THE PREPARATION

Finally, it is critical to evaluate the finished preparation to ensure that all of the principles of cavity preparation have been addressed.

Dental Caries - is a complex pathological process that occurs after tooth eruption, with subsequent demineralization and softening of dental hard tissues, thus leading to cavitation.

Black’s classification is as follows:

1. Class I: caries affecting pits and fissures; commonly used to refer to caries affecting the occlusal surfaces of premolars and molars.

2. Class II: caries affecting the proximal(contact) surfaces of posterior teeth(molars and premolars).

3. Class III: caries affecting the proximal surfaces of anterior teeth(incisors, canines).

4. Class IV: caries affecting the proximal surfaces of anterior teeth and also including the incisal angle and cutting edge.

5. Class V: caries affecting the cervical surfaces.

6. In some American issues we can find the additional VI class .Class VI – caries-resistant zones of teeth - cusps and equator of the tooth.

Preparation is aimed at dissection of pathologically altered dental hard tissues, in order to stop further progression of the caries process and the creation of necessary conditions for reliable fixing of filling material, restore anatomical form and function of the tooth.

There are several principles of cavities preparation :

The principle of "extension for prevention" (Black)- preventive extension of the cavity boundaries, is aimed to dissect caries- unstable areas (pits and fissures) to the so-called immune zones that are relatively rare is affected by dental caries (cusps, smooth convex surface-equator).

The principle of "biological suitability" - the dissection of tooth tissue is sparing, preparation is finished within the visibly healthy tissue. Thus, the basic principle that should be guided while preparing the cavity : full dissection of pathologically altered tissue and sparing preparation of unaffected by caries process tissues of enamel and dentin.

However, regardless of cavity location, there are common stages of dental hard tissues preparation, which are come to:

- Anaesthetizing

- Disclosure(opening and enlargement) of cavity ( is conducted by using round-shaped, fissures burs, burs that is chosen, should have the size of the working end not bigger than the entrance aperture of this cavity)

-

- Necrectomy

- Formation the cavity for fillings (is done with fissures, inverted-cone and cone-shaped burs)

- Smoothing the edges of enamel

Necrectomy - is a removal of the necrotic dental hard tissues from carious cavity. There are total and partial necrectomy. Total -is complete removal of necrotic dentin from the walls and bottom of the cavity. Partial - is complete removal of necrotic dentin from walls and partly from the bottom of the cavity. Partial necrectomy is allowed in the case of deep dental caries, when the bottom of the cavity is very thin and there is a danger of the pulp horn disclosure. In this case is permitted to leave on the bottom of the cavity a dense pigmented dentin, and in the course of acute deep caries – is allowed to leave a small layer of softened dentin with the next remineralization influence on it. Necrectomy is done with the help of round-shaped burs and the excavator.

Elements of the carious cavity: bottom (surface that is turned to the pulp chamber), walls, corners, edges. There are terms such as main and additional cavity.

The main cavity is prepared in the place of pathological focus, additional cavity is created within the healthy tissues, for better fixation of the filling material.

Features of cavity formation, mainly, depend on the localization of the pathological process and the group of teeth.

However, there are general rules for the preparation of cavities, namely they are:

- transition the bottom of the cavity (the surface which is turned to a pulp) to the side wall should be at right angle

- transition of one wall to another should be at an angle –the form of the cavity-box- shaped form (except the V class)

- enamel edges should be straight and smooth

- bottom of the cavity should be flat or somewhat remind the form of the occlusal surface of the tooth

Dissection of tooth tissues for filling with composites materials is slightly different from the traditional preparation by Black. This is because the traditional preparation is used for mechanical fixing of fillings in the carious cavity. Composite materials have the ability to chemically bind to tissues, so there is no need to prepare a wall at right angles. However, you must create enamel bevel at an angle of 45, around the edge of the cavity to increase the adhesion and to mask the line of transition "enamel-composite material".

Peculiarities of Black’s preparation

1. According to Black’s principles of preparation, a cavity of the I class should be: with straight walls at right angle to the bottom, a form of the cavity should be cylindrical, box-shaped, rhombic, X-like;

2. According to Black’s principles of preparation, a cavity of the II class should be: if there is no neighbouring tooth and the carious cavity is placed below the equator, it is formed on the proximal surface; when an access is complicated, a cavity is extended to the occlusal surface and an additional cavity is formed there, additional cavity occupies the 1/3 – 1/4 length of the occlusal surface. Peculiarities of a carious cavity disclosure of the II class according to Black principles of preparation: an access is gained from unaffected occlusal surface.

3. According to Black’s preparation, a cavity of the III class should be: in a shape of triangle; if teeth are stand tightly one to another it is extended to the lingual / palatal surface, and an additional cavity is formed there. Disclosure of a carious cavity of the III class according to Black preparation is done in such a way: an access is gained from a lingual/palatal surface, in some rare cases, from a labial surface.

4. According to Black’s principles of preparation, a cavity of the IV class should be prepared in such way: an additional cavity is formed either in the area of incisal edge (when it is wide) or on the palatal (lingual) surface within the limits of dentin.

5. According to Black’s preparation, a cavity of the V class should be: an oval shape, walls and the bottom should be at the right angle, bottom is convex, because of pulp proximity at cervical area, thus preventing pulp exposure.

Treatment of posterior proximal lesions – II class cavities

Caries on the proximal surfaces of posterior teeth occurs because plaque can collect cervical to the contact area, resulting in a stagnation area (or plaque trap). The diagnosis of proximal caries requires careful clinical examination of the marginal ridges; this area may appear darker or more opaque than surrounding tooth tissue.

Bitewing radiographs are essential for the diagnosis and assessment of posterior proximal lesions. If the lesion is confined to enamel, as assessed radiographically, then it may be possible to arrest, or even reverse, the progress of the caries. Appropriate dietary advice and interdental cleaning instruction should be given and fluoride, either as an operator-applied varnish or in toothpaste, should be used.

If the lesion has cavitated and spread into dentine then operative intervention will normally be necessary to restore the surface integrity of the tooth. Access to caries on the posterior proximal surfaces may be gained in a number of ways:

■ Through the marginal ridge from the occlusal aspect. The most common technique to gain access to the caries is through the marginal ridge from the occlusal surface of the tooth and this technique will be described in detail.

■ From the occlusal surface (tunnel preparation), preserving the marginal ridge. The tunnel preparation is difficult to execute unless there is a pre-existing occlusal restoration which is removed and access to the proximal caries can be gained from the occlusal cavity, without removing the marginal ridge. This technique is not suitable if there is extensive proximal caries as the marginal ridge will collapse. The main difference between this method is that during preparation contact point is preserved, access to the carious cavity is created from the occlusal surface like a tunnel. This method helps to preserve the most of unmodified tissue. This preparation is recommended to be carried out with turbine handpieces, round-shaped burs with the simultaneous cooling of water.

■ From the buccal (or lingual) aspect. This technique is only suitable where there is no risk of marginal ridge collapse and in situations where resin composite can be used as the restorative material.

■ Directly, if the adjacent tooth is absent.

Technique for posterior proximal restorations through the marginal ridge

■ Local analgesia is usually required.

■ Check occlusion and mark occlusal stops with articulating paper.

■ Ensure effective isolation.

■ Protect adjacent teeth: some operators like to place a matrix band on the adjacent tooth to prevent damage of this tooth during preparation of the box component of the cavity. This is no guarantee that the tooth will not be damaged and care should always be taken in the preparation of proximal cavities to protect the adjacent tooth.

■ Gaining access: access is gained through the marginal ridge using a pear-shaped diamond or tungsten carbide bur in a high-speed handpiece. Start slightly away from the marginal ridge and direct the bur downwards and towards the contact area. The bur should drop down into the caries. Try to leave a thin wall of proximal enamel to protect the adjacent tooth. This can be removed subsequently with gingival margin trimmers. This creates a shape described as a box but it should not be square: it should have round internal line angles and should be wider cervically than occlusally. If there is also occlusal caries then the cavity should be extended into the occlusal fissure (Figs 13.11 and 13.12). If there is no occlusal caries then the cavity does not need to extend into the fissure (Fig. 13.13).

■ Caries removal: caries should be removed with a round stainless steel or tungsten carbide bur in the slow-speed handpiece. Remove the caries from the enamel dentine junction first before moving to the axial wall (and pulpal floor if the cavity has been extended into the occlusal fissure). An excavator may also be used to remove soft dentine caries. This should result in a cavity that clears the contact area cervically and is wider cervically than occlusally.

■ Retentive features: additional retentive features are only necessary if amalgam is to be used as the restorative material. If the cavity has extended into the

occlusal fissure then this will act as a key or dovetail to retain the amalgam and prevent its displacement. If there is no occlusal key and amalgam is to be used, then small grooves should be cut at the junctions between the axial wall and the buccal and lingual walls.

■ Lining: if the cavity is suitably deep to require lining then this should be placed on the pulpal floor and on the axial wall (Fig. 13.14).

■ Matrix band: a matrix band is placed to help retain the restorative material during placement, to give shape to the proximal surface of the restoration and to allow close adaptation of the restorative material to the cavity. The band should be closely adapted to the cervical margin and should be burnished against the adjacent tooth to help formation of a good contact. There are many types of matrix bands and holders, but commonly used ones are:

Ø Siqveland: this system uses a straight band and the holder and band are removed from the tooth simultaneously. This can sometimes result in removal of part of the newly packed amalgam.

Ø Tofflemire: this system has the advantage that the holder is removed before the band and this may prevent removal of the restoration with the band.

Ø Circumferential: a number of systems exist that have no retainer/holder. The band is tightened by a spring mechanism.

Ø Ivory: this has a holder which engages into a selection of holes in a metal band. The metal band replaces only one proximal wall and therefore cannot be used for cavities involving both proximal walls.

■ Wedge: the next stage is to place a wedge at the cervical margin of the band, normally from the buccal aspect. The wedge has several functions:

Ø It separates the teeth slightly so that when the matrix band is removed there is no space between the adjacent teeth and a tight contact is formed. Wooden wedges swell slightly by absorbing moisture in the mouth so are preferable to plastic wedges.

Ø It prevents excess material at the cervical area of the cavity forming a ledge.

Ø It shapes the band at the cervical margin of the tooth.

Ø It can help retain the band in place.

■ Material placement (amalgam): once the amalgam has been mixed, it starts to set so the operator must work quickly to pack and carve the restoration. The amalgam is transferred in increments from the amalgam carrier to the deepest area of cavity – usually the base of the box. It is condensed first with the wider end of the amalgam condenser and then with the narrower end. It is important to condense the amalgam well to adapt the material to the cavity walls and to reduce porosity. Place the next increment, condense and continue until the cavity is over-filled. The cavity is over-filled to allow removal of the weak, mercury-rich (γ2) layer that is at the surface of a well-condensed amalgam. Run a straight probe around the inside surface of the matrix band to remove gross excess of amalgam and to start to shape the marginal ridge. Carefully remove the wedge, matrix retainer and band. Check the cervical margin for excess amalgam with a straight probe and remove any excess, either with the probe or an amalgam carving instrument, such as a ½ Hollenbach. Use an instrument designed for carving as it will cut through the amalgam, rather than smearing it (as would be the result if a flat plastic were used).

Using the tooth as a guide, rest the blade of the carver against the tooth and carve through the amalgam to recreate the cuspal shapes of the tooth. Check that the marginal ridge is a similar height to that of the adjacent tooth. Check the occlusion by asking the patient to close gently on the restoration. Listen for the sound of the teeth coming together and any impact on the amalgam. Look for any high spots and adjust. Should the amalgam fracture at this stage, it is better to remove the partially set material and start again, rather than try to add to the fractured amalgam.

■ Material placement (resin composite): dental adhesive should be applied to all the surfaces and margins of the cavity. The first increment of restorative material may be placed either at the base of the box or to form the proximal wall. Light cure for the recommended time, then place the next increment, ensuring that this increment only touches either the buccal or lingual wall but not both (Fig. 13.7). Light cure and continue with incremental packing and curing. Carefully shape the marginal ridge by running a straight probe round the inside of the matrix band and finally recreate the cusp shapes to give the correct occlusal contour. Remove the wedge, matrix holder and band and check cervically for excess material. Check the occlusion by asking the patient and by the use of articulating paper. Shape and polish as required.

Treatment of anterior proximal lesions – III class cavities

Caries occurs on the anterior proximal surfaces owing to the accumulation of plaque gingival to the contact area (Fig. 13.16). Detection of these lesions is by direct vision or by transillumination: reflected light in the mouth mirror.

The technique for treatment of these lesions is as follows:

■ Gaining access: access to the lesion should be from the palatal or lingual aspect if at all possible, as this will allow preservation of the labial enamel. A small round diamond in the high-speed handpiece is used to drop into the caries.

■ Removal of caries: a round bur in the slow-speed handpiece is used to remove the caries, trying to preserve the labial enamel. Additional preparation to create a retentive cavity will probably not be necessary as the shape of the carious lesion will result in an undercut cavity. With adhesive restorations, an undercut cavity is unnecessary and amalgam restorations are contraindicated in anterior proximal cavities primarily because of their poor appearance.

■ Lining: a lining should be placed as required. Beware that calcium hydroxide lining materials are opaque and can look unsightly through thin labial enamel.

■ Matrix: a clear cellulose matrix strip should be placed before use of the dental adhesive to prevent bonding the adjacent teeth together. The strip should be placed so that it is cervical to the gingival margin of the cavity.

■ Dental adhesive: apply the adhesive to the cavity and the cavity margins.

■ Material placement: place the resin composite in the cavity in small increments and light cure. After the final increment has been placed, pull the matrix band tight cervically to prevent formation of a ledge, and light cure.

■ Finishing: check the occlusion as before and finish as required.

Treatment of incisal edge lesions – IV class cavities

Incisal edge lesions are the result of trauma, failure of a proximal restoration or extensive proximal caries.

The technique for treatment of incisal edge lesions is as follows:

■ Access: access to the lesion is not normally difficult; the difficulty is creating good bonding potential. A labial bevel or chamfer will increase the area of tooth tissue for bonding and will improve the appearance of the final restoration as it will allow the composite to merge gradually with the tooth, rather than having a butt joint. Palatally, a small shoulder will increase the strength of the restoration in this area of occlusal loading. The lack of cavity walls has the advantage of reduced stress from polymerisation shrinkage.

■ Lining: in trauma cases, direct or indirect pulp capping with setting calcium hydroxide may be necessary.

■ Composite placement: composite can be built up free-hand or by using a matrix. To achieve optimal appearance, composites of different opacity, such as ‘dentine’, ‘body’, ‘enamel’ and translucent, should be built up in incremental layers. The types of available matrices are:

Ø Custom-made: an impression of the palatal aspect of an intact tooth can be used to aid formation of this aspect of the final restoration. To achieve an intact tooth, a temporary restoration can be placed, or a laboratory wax-up used.

Ø Preformed: the main types used are clear cellulose strips, incisal corners and complete crown forms.

■ Shaping and finishing: the adjacent teeth may be used as a guide to the shape of the final restoration. Care should be taken not to damage the remaining tooth tissue in the polishing of incisal edge restorations when it may be difficult to distinguish between tooth and restoration.

PROGRESSION OF DENTAL CARIES AND ITS LOCALIZATION

Superficial dental caries Dental caries starts to develop in fissures, pits, proximal surfaces: in places of plaque retention, where plaque is undisturbed by toothbrush.	Medium dental caries Once bacteria reach the dentinoenamel junction (DEJ), lateral spread occurs undermining the overlying enamel. Thus causing progression of carious cavity in dentine.
Deep dental caries Caries has reached circumpulpal dentine; thin layer of dentine separates carious cavity from pulp chamber.	Complication of dental caries (pulpitis) – a pulp is involved in the inflammation process due to progression of carious lesion.

Elements of carious cavity

In a carious cavity distinguish walls, corners, bottom, edges. Edges and walls limit the in gate of cavity. The walls of cavity are named depending on the surface of crown to which they fitting. Edges and walls the cavities turned to the threshold of mouth and adjoining to the cheek are named cheeks, turned to the tongue — tongues. The corners of cavity are named on walls generatrices them: cheek-middle and turned to the middle plane — front, turned back — back. Consider the formed surface turned to mash the bottom of cavity, or pulp wall, regardless of localization of carious cavity.

• Cavity wall: Side or surface of a tooth prepared

for restoration.

• Internal wall: Cavity wall that does not extend

to the external tooth surface.

• External wall: Portion of the tooth preparation

that extends to the external tooth surface,

named according to the tooth surface involved:

distal, mesial, facial, lingual, and gingival.

• Axial wall: Internal wall of prepared tooth that

runs parallel to the long axis of the tooth.

• Pulpal wall: Internal wall of prepared tooth that

is perpendicular to the long axis of the tooth;

also known as the pulpal floor.

• Line angle: Angle formed by the junction of two

walls in a cavity preparation (similar to the angle

formed where-two walls of a room meet to form

a corner). To identify a line angle, the names of

the two involved walls are combined.*