Cavity Preparation
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The aim of cavity preparation is to remove all carious, defective and compromised tooth structure and prepare the tooth to receive a restoration that restores maximum form, function and esthetics. Generally there is an emphasis on conservative preparations. It is wise to keep margins supragingival, and preparations as far away from the pulp as possible. Minimizing bucco-lingual widths and rounding internal line angles will avoid weakening the tooth and stress concentration that can both lead to fracture.
GENERAL PRINCIPLES
Cavity preparations should be dictated by tooth anatomy, tooth position in the arch, the carious lesion, and the physical properties of the dental filling material. Gingival margins should remain on enamel where possible, and kept supragingival if possible. Weakened or unsupported tooth structure should be removed.
The initial depth into the tooth will be around 0.5mm into the dentin. Preparations need to allow enough access to clean and restore the tooth adequately. The margins are placed where they can be finished and cleaned by the patient (e.g. breaking interproximal contacts). The outline will be determined largely by the damage being repaired, the extension made until the walls rest on sound tooth structure. Unsupported or compromised tooth structure is removed. Cuspal strength needs to be preserved if possible. Wide bucco-lingual extensions weaken cuspal integrity. Too wide and cuspal coverage should be considered.
Marginal ridge strength needs to be considered. If there are two preparations with only 0.5mm or less between them, they should be connected. Avoid placing margins where there is direct cuspal contact or excursive wear. All caries is generally removed, though if lesions extend close to pulp an indirect pulp capping procedure is suggested. Minimal cuspal reduction for the working/functional cusp is 2.5-3mm for amalgam and composite, 2mm for metal-ceramic, 1.5mm for cast gold restorations. Minimal cuspal reduction for the non-working/guiding cusp is 2mm for amalgam and composite, 1.5mm for metal-ceramic, and 1mm for cast gold restorations. For metal-ceramic (PFM) restorations occlusal reduction is recommended 1.5-2mm, 1.5mm facial reduction. A bevel or chamfer margin is prepared if there is a metal collar. A butt joint (shoulder) for a porcelain margin.
Class II inlays will have a beveled metal margin of around 40 degrees, compared to the 90 degree of Class II amalgam preparations.
Primary resistance form features prevent failure from forces along the long axis of the tooth (occlusal forces). Occlusal contacts, amount of tooth structure available and the type of restorative material affects the primary resistance form. Features that would primary resistance:
- Preserve cuspal strength.
- Cap cusps if compromised.
- Flat pulpal and gingival floors perpendicular to the occlusal forces.
- Adequate thickness of material (especially in the isthmus of a class II amalgam).
- Box shape.
- Preserve marginal ridges.
- Remove weakened tooth structure.
- Rounded internal line angles.
Primary retention form features prevent dislodgement of the material, mainly guided by the height, shape and form of the wall preparations (convergent walls, occlusal dovetail etc.). Locks and grooves can be considered primary retention form features. These should be rounded to avoid stress concentration and establish good adaptation. Viscous restorative materials will not readily flow into sharp corners. Convenience form features are alterations made to improve access and visibility.
Secondary resistance and retention forms are mechanical features added once the primary preparation has been completed and clinical judgment suggests it is needed. They include:
- Retentive locks, grooves, coves (mainly for metallic restorations).
- Pins, slots, steps, amalgam pins (for amalgam).
- Groove extensions.
- Skirts (primarily for cast restorations).
- Beveled enamel margins (primarily for composite restorations).
The general sequence followed:
- Obtain initial depth and outline form.
- Establish resistance and retention forms.
- Consider the convenience form.
- Caries removal.
- Finish enamel walls.
- Cavity debridement, final preparation, assessment.
If there are two cavities next to each other, it’s a good idea to prepare the larger lesion first, but repair the smaller lesion first. Care should be taken to make sure that a new restoration is not in hyperocclusion. Radiographic signs of trauma due to a high contact may include:
- Hypercementosis.
- Root resorption.
- Alterations in the PDL space.
- Alterations in lamina dura.
Care must be taken when preparing the mandibular first premolar. The bur should be tilted lingually to prevent compromising the facial pulp horn and to maintain the support of the cusps. The pulpal floor should be parallel to the occlusal plane. One of the most difficult areas to adapt the matrix to is the mesial of the maxillary first premolar due to its developmental depression, the concavity in the cervical third of the crown.
AMALGAM RESTORATIONS
Preparations for amalgam tend to be less conservative than composite preparations. All prepared walls and rounded internal transitional line angles should be on solid dentin or enamel. The preparation is extended to include any carious and compromised tooth structure, and should allow easy restoration. The minimum depth and width required:
- 1/5 width of occlusal table between cusps.
- 0.5mm into dentin.
- 1.5-2mm overall thickness minimum.
The pulpal floor should be flat, sitting parallel to the occlusal surface, with rounded internal angles. It is appropriate to include suspicious fissures if it is believed they may become carious in the future. The preparation is started 0.2-0.5mm inside the DEJ. Amalgam has limited edge strength. A thin unsupported margin risks fracture. An inverted cone carbide bur (No. 245) with its rounded corners provides the desired wall shape and texture.
Bevels of the external margins are generally not used except for perhaps the gingival-cavosurface margin which may be beveled to remove unsupported enamel. This bevel is not needed if it only rests on dentine and cementum. A 90 degree cavosurface angle is optimal. There should be a minimum of 2mm dentin between the amalgam and the pulp. A CaOH liner is used in the case of direct and indirect pulp caps. A sealer is used if no bonding agent is used.
For a Class I prep the mesial and distal walls should diverge occlusally to provide support for the marginal ridges (mesial and distal divergence is also incorporated in direct gold and gold inlay preparations). The buccal and lingual walls should converge to the occlusal surface. If, after preparation, the width of the marginal ridge is less than 1.6mm you should consider turning the prep into a Class II.
For a Class II prep the proximal contacts are cleared so the margins between the filling and tooth can be cleaned. The proximal walls converge towards the occlusal surface. The gingival floor should be parallel with the enamel rods. A wedge is used during the preparation and treatment to depress the gingiva apically, to separate the teeth, to adapt the matrix system to the tooth, and for moisture control. The occlusal isthmus of a Class II dental amalgam restoration is more resistant to fracture if there is adequate depth and the axiopulpal line angle is rounded. Retention grooves may be placed in the axiobuccal and axiolingual line angles. They can extend up to the height of the axial wall (not to cavosurface margin).
For a Class III prep unsupported enamel can be left for esthetic purposes. Caries is usually approached from the palatal/lingual. Composite is generally preferred for Class III and certainly for Class IV preparations that are in the esthetic zone, though the board might still expect you to say a distal prep on an upper canine should receive an amalgam restoration.
For a Class V prep the outline is dictated by the extent of caries, though the occlusal wall is usually longer than the cervical wall (warped trapezoidal or kidney shape). Undercuts are placed occlusally/incisally and gingivally, not axially. Care must be taken during finishing not to ditch the root surface or damage the soft tissue which could lead to recession. A Class V lesion is a great spot for a glass ionomer/resin modified glass ionomer restoration because of good adhesion, snap set, anticariogenic, compatibility with composite (sandwich technique), good esthetics, and adequate physical properties for an area that’s not physically demanding.
The final polish should not be done until after the amalgam has fully set, at least 24 hours after placement. Finishing reduces marginal discrepancies, reduces marginal breakdown, prevents tarnishing, and “improves” the appearance of the restoration. Dry polishing results in heat generation and should be avoided to prevent mercury vapor release or pulpal damage.
RETENTION PINS
Pins are used to retain the restorative material, but doesn’t improve the strength of the restorative material. In fact the use of pins decrease the strength of amalgam restorations. The larger the pin diameter, the stronger the retention, so the largest pin possible is selected in each case. One pin is used per missing axial line angle, placed in dentin only. Care must be taken not to place the pin too close to enamel (at least 0.5mm from DEJ, maybe 1mm to be safe) or too close to the pulp. Optimal depth is 2mm. The pin hole should be parallel to the nearest external surface. They are contraindicated in young teeth with large pulp chambers or in teeth with irreversible pulpitis. Possible disadvantages of pin placements include:
- May not be possible if there is poor access or visualization.
- Pulpal irritation.
- Pulpal exposure.
- External perforation.
- Tooth fracture (creates stresses that cause crazing and eventually fracture).
- Path for microleakage.
If the pin is placed and results in a pulp exposure, it should be treated by introducing a direct pulp capping procedure. The most frequently used pin system is a self threading pin system (e.g TMS), where the hole size is slightly smaller than the pin diameter. These are termed friction-locked pins. The self limiting drill prepares a hole to the optimal 2mm, and the self shearing pin guards against over torquing. The elasticity in the dentin locks the pin in place. The four sizes of pins in the TMS system are called regular, minim, minikin, and minuta. They come in titanium or stainless steel plated with gold. You can get cemented pins that are sealed in holes larger than their diameter. Advancements in modern adhesive dentistry have nearly eliminated the need for pins.
COMPOSITE
Using a bonded restorative system makes use of the following advantages:
- Less microleakage, marginal staining and recurrent caries.
- Less pulpal sensitivity, especially in the cervical area.
- More conservative tooth preparation.
- Improved retention, and reinforcement of remaining tooth structure.
The outline form for composite preparations will be similar to amalgam, including all carious and compromised tooth structure, with proximal walls beyond in the contact where they can be finished and cleaned. But a more conservative approach is used for suspicious fissures. Instead of including these in the preparation, they can be sealed and monitored.
The depth is completely dictated by what you are removing (not uniform) and is also more conservative than amalgam. For example, a Class V restoration would be more rounded if receiving composite. The cavosurface margins are beveled (more than 90 degrees, obtuse cavosurface angle) which exposes more enamel rods to improve bonding. When additional retention or improved esthetics is needed, the bevel can be extended. A diamond bur is often used leaving a rough surface to maximize bonding, but internal line angles are kept rounded.
Generally composite restorations rely far less on primary or secondary retention forms since it physically adheres to the walls of the prep (unlike amalgam which has no inherent retentive properties). This again allows preparations to remain conservative. Bases are not needed but can be used. Liners are used in the case of direct or indirect pulp capping. And an additional sealer is unnecessary because of the bonding system involved in placement. Moisture control is of utmost importance, contamination will severely affect the longevity of the restoration.
Preventive Resin Restorations (PRR) are indicated when the pits and fissure lesions are small and discrete and confined only to the enamel, or when it has reached the dentin without pulpal involvement. Caries removal may not reach the DEJ or remain confined to the superficial dentin. The tooth is restored with Glass ionomer or resin.
Ceramic and metal crowns, veneers, inlays and onlays also greatly benefit from adhesive bonding. It is easier to make repairs with bonded restorative materials.
ISOLATION
The goal of isolation is moisture control, retraction, tissue and airway protection, and improving visualization. Rubber dam is still considered the gold standard isolation technique. Cotton roll “isolation” is very common in real world dentistry, combined with high volume suction to provide adequate isolation and moisture control. Absorbent cheek protectors are a useful adjunct. Isolite is a popular system providing isolation, suction, illumination and soft tissue retraction in one unit.
MATRIX PLACEMENT
The mesial surface of the maxillary 1st premolar is one of the most challenging areas to adapt a matrix band to, due to its depression/developmental groove. The small circumference of the matrix bands faces the gingiva, the larger faces the occlusal table. The slot opening in the Tofflemire matrix points to the gingiva to allow for easy removal. There are many matrix systems available, but they all need to fulfill the following criteria:
- Restore anatomical contours and contact areas.
- Contain the restorative material.
- Be easy to apply and remove.
- Extend below the gingival margin to prevent flash.
- Extend above the marginal ridge height to allow for overfilling.
- Resist deformation during treatment.
- Ability to burnish to establish firm contact with the neighboring teeth.
