This chapter emphasizes procedural organization for tooth preparation and associated nomenclature, including the historical classification of caries lesions. In the past, most restorative treatment was for caries, and the term cavity was used to describe a caries lesion that had progressed to the point that part of the tooth structure had been destroyed. The tooth was cavitated (a breach in the surface integrity of the tooth) and was referred to as a cavity. Likewise, when the affected tooth was treated, the cutting or preparation of the remaining tooth structure (to receive a restorative material) was referred to as cavity preparation. Currently, many indications for treatment are not related to carious destruction, and the preparation of the tooth no longer is referred to as cavity preparation, but as tooth preparation.
Tooth preparation is the mechanical alteration of a defective, injured, or diseased tooth such that placement of restorative material re-establishes normal form and function, including esthetic corrections, where indicated. This textbook covers such preparations, with the exception of preparation for either a three quarter crown or full crown.
Much of the scientific foundation of tooth preparation techniques was presented by Black.1 Modifications of Black’s principles of tooth preparation have resulted from the influence of Bronner, Markley, J. Sturdevant, Sockwell, and C. Sturdevant; from improvements in restorative materials, instruments, and techniques; and from the increased knowledge and application of preventive measures for caries.2–6
In the past, most tooth preparations were precise procedures, usually resulting in uniform depths, particular wall forms, and specific marginal configurations. Such precise preparations are still required for amalgam, cast metal, and ceramic restorations and may be considered conventional preparations. Conventional preparations require specific wall forms, depths, and marginal forms because of the properties of the restorative material. The use of adhesive restorations, primarily composites and glass ionomers, has allowed a reduced degree of precision of tooth preparations. Many composite restorations may require only the removal of the defect (caries, fracture, or defective restorative material) and friable tooth structure for tooth preparation, without specific uniform depths, wall designs, retentive features or marginal forms. This simplification of procedures results in a modified preparation and is possible because of the physical properties of the composite material and the strong bond obtained between the composite and the tooth structure (Table 5-1).
|Outline form||Include defect||Same|
|May extend to break proximal contact||Same|
|Include adjacent suspicious area||No|
|Seal these areas|
|Pulpal depth||Uniform 1.5 mm||Remove defect; not usually uniform|
|Axial depth||Uniform 0.2-0.5 mm inside DEJ||Remove defect; not usually uniform|
|Cavosurface margin||Create 90-degree amalgam margin||≥90 degrees|
|Bevels||None (except gingival)||Large preparation, esthetics, and seal|
|Texture of prepared walls||Smoother||Rough|
|Cutting instrument||Burs||Burs or diamonds|
|Primary retention form||Convergence occlusally||None (roughness/bonding)|
|Secondary retention form||Grooves, slots, pins, (bonding)||Bonding; grooves for very large or root-surface preparation|
|Resistance form||Horizontal floors, rounded angles, box-shaped (floors perpendicular to occlusal forces)||Same for large preparations; no special form for small- to moderate-size preparations|
|Base indications||Provide 2 mm between pulp and amalgam||Not needed|
|Liner indications||Ca(OH)2, for pulp exposures or near exposures RMGI in deep preparations||Same (also may use RMGI liner on root-surface extensions)|
|Desensitizer||Dentin desensitizer (5% glutaraldehyde + 35% HEMA) when not bonding||Sealed by bonding system used|
Much of this chapter presents information about the conventional tooth preparations because of the specificity required. The fundamental concepts relating to conventional and modified tooth preparation are the same: (1) all unsupported enamel tooth structure is normally removed; (2) the fault, defect, or caries is removed; (3) the remaining tooth structure is left as strong as possible; (4) the underlying pulpal tissue is protected; and (5) the restorative material is retained in a strong, esthetic (whenever possible), and functional manner. Conventional preparations achieve these concepts by specific, exact forms and shapes. Modified preparations are usually smaller and have more variable and less complex forms and shapes.
Teeth need restorative intervention for various reasons. Dental caries is an infectious disease, and prevention often requires prophylactic restorative procedures (see Chapter 2). Caries progression may cause destruction of tooth structure which requires repair. Another common need is the replacement or repair of restorations with serious defects such as improper proximal contact, gingival excess of restorative material, defective (open) margins, or poor esthetics. Restorations also are indicated to restore proper form and function to fractured teeth. Such teeth present with minor to major amounts of missing tooth structure or with an incomplete fracture (“greenstick fracture”), resulting in a tooth that has compromised function and often also associated pain or sensitivity. A tooth may require a restoration simply to restore form or function that is absent as a result of congenital malformation or improper position. Restorations also are required for teeth simply as part of fulfilling other restorative needs. When replacing a missing tooth with a fixed or removable partial denture, the teeth adjacent to the space may require some type of restorative procedure to allow for optimal placement and function of the prosthesis. Careful diagnosis and development of a comprehensive treatment plan must be accomplished before the restoration of individual teeth is pursued to ensure appropriate restorative intervention.
Generally, the objectives of tooth preparation are to (1) remove all defects and provide necessary protection to the pulp, (2) extend the restoration as conservatively as possible, (3) form the tooth preparation so that under the forces of mastication, the tooth or the restoration (or both) will not fracture and the restoration will not be displaced, and (4) allow for the esthetic and functional placement of a restorative material.
A careful examination must be performed to determine an accurate diagnosis and to render subsequent appropriate treatment. An assessment of pulpal and periodontal status influences the potential treatment of the tooth.
Likewise, an assessment of the occlusal relationships must be made. Such knowledge often affects the design of tooth preparation and the choice of restorative material. For instance, a preparation may require further extension of the outline form to avoid heavy occlusal contact on a marginal interface between the tooth and the restoration.
The relationship of a specific restorative procedure to other treatment planned for the patient also must be considered. For example, if a tooth is planned to be an abutment for a fixed or removable partial denture, the design of the restoration may need to be altered to accommodate optimal success of the prosthesis.
Proper tooth preparation is accomplished through systematic procedures based on specific physical and mechanical principles. A prerequisite for understanding tooth preparation is knowledge of the anatomy of each tooth and its related parts. A mental image of the individual tooth being prepared must be visualized. The direction of the enamel rods, the thickness of enamel and dentin, the size and position of the pulp, the relationship of the tooth to its supporting tissues, and other factors all must be considered to facilitate appropriate tooth preparation.
Patient factors play an important role in determining the appropriate restorative treatment rendered. The patient’s esthetic concerns, economic status, medical condition, and age should be taken into consideration when selecting the various restorative materials to be used in a given procedure. Older adults who have physical or medical complications may require special positioning for restorative treatment and shorter, less stressful appointments. Because many older adults have new or replacement restorative needs that are completely or partially on the root surfaces, the treatment of many of these areas is more complex.
It is imperative that the level of caries risk be assessed for all patients prior to the initiation of restorative treatment. Patients at high risk for dental caries may require an initial treatment plan designed to limit disease progression (i.e., control caries) until caries risk factors are reduced or eliminated. This initial treatment plan, usually termed caries control treatment plan, may be followed by more definitive treatment once the patient’s risk for caries has been reduced. In the design of the definitive treatment plan, the patient’s ongoing risk of caries is taken into consideration. More conservative, less expensive definitive restorative procedures may be indicated until the patient develops oral conditions consistent with low caries risk.
The primary objective of operative dentistry is to repair the damage from dental caries or trauma while preserving the vitality of the pulp. Pulp tolerance to insult is usually favorable; however, the pulp should not be subjected to unnecessary abuse from poor or careless operative procedures. When less tooth structure is removed, the potential for damage to the pulp is lower.
Every effort should be made to create restorations that are as conservative as possible. Small tooth preparations result in restorations that have less effect on intra-arch and inter-arch relationships and esthetics. Also, it follows that the smaller the tooth preparation is, the stronger will be the remaining unprepared tooth structure.
The choice of restorative material affects the tooth preparation and is made by considering many factors. The patient’s input into the decision is important. Economic and esthetic considerations are primarily patient decisions. The ability to isolate the operating area and the extent of the lesion or defect are factors that the operator must consider in presenting material options to the patient. Table 5-1 compares factors related to restorative choices when choosing between amalgam and composite materials.
An amalgam restoration requires a specific tooth preparation form that ensures (1) retention of the material within the tooth and (2) strength of the material in terms of bulk thickness and marginal edge strength. An indirect cast-metal restoration also requires a specific tooth preparation form that provides (1) draw to provide seating of the rigid restoration, (2) a beveled cavosurface configuration to provide optimal fit, and (3) retention of the casting by virtue of the degrees of parallelism of the prepared walls.
Adhesive composite restorations do not typically require preparations as precise as those for amalgam and cast-metal restorations. Unlike amalgam, adhesively bonded composite does not exhibit low edge strength and micromechanically “bonds” to the tooth structure. These features allow a reduction in the complexity of the tooth preparation. Other adhesive restorations may require more precise tooth preparations. Ceramic inlay or onlay restorations require specific preparation depths, wall designs, and cavosurface marginal configurations that allow for sufficient strength to resist fracture.
Nomenclature refers to a set of terms used in communication among individuals in the same profession, which enables them to understand one another better. This section details terminology related to tooth defects and preparations.
Dental caries is an infectious microbiologic disease that results in localized dissolution and destruction of the calcified tissues of teeth. Caries is episodic, with alternating phases of demineralization and remineralization, and these processes may occur simultaneously in the same lesion.
Primary caries is the original caries lesion of the tooth. The etiology, morphology, control, and prevention of caries are presented in Chapter 2. Variations of this pathologic condition are associated with certain areas of teeth and fundamentally influence tooth preparation. Three morphologic types of primary caries are evident in clinical observation: (1) lesions originating in enamel pits and fissures, (2) lesions originating on enamel smooth surfaces, or (3) lesions originating on root surfaces. Also described in the following sections are backward caries, forward caries, and residual caries. Of these, the terms backward caries and forward caries are rarely used.
Complete coalescence of the enamel developmental lobes results in enamel surface areas termed grooves and fossae. Usually, these areas are not susceptible to caries because they are cleansed by the rubbing of food during mastication. Caries may develop in a groove or fossa, however, in areas of no masticatory action in neglected mouths. Imperfect coalescence of the developmental enamel lobes will result in enamel surface pits and fissures. When such areas are exposed to oral conditions conducive to demineralization, caries may develop (Fig. 5-1, A). The caries forms a small area of penetration in the enamel at the bottom of a pit or fissure and does not spread laterally to a great extent until the dentinoenamel junction (DEJ) is reached. Dentin caries initially spreads laterally along the DEJ and begins to penetrate the dentin toward the pulp via the dentinal tubules. This lateral and pulpal progression results in unsupported enamel. In diagrammatic terms, pit-and-fissure caries may be represented as two cones, base to base, with the apex of the enamel cone at the point of origin and the apex of the dentin cone directed toward the pulp. As caries progresses in these areas, sometimes little evidence is clinically noticeable until the forces of mastication fracture the increasing amount of unsupported enamel.
Smooth-surface caries does not begin in an enamel defect but, rather, in a smooth area of the enamel surface that is habitually unclean and is continually, or usually, covered by plaque (see Figs. 5-1, B and C). It is emphasized in Chapter 2 that plaque is necessary for caries and that additional oral conditions also must be present for caries to ensue. The enamel disintegration in smooth-surface caries also may be pictured as a cone, but with its base on the enamel surface and the apex at, or directed toward, the DEJ. The caries again spreads at this junction in the same manner as in pit-and-fissure caries. The apex of the cone of caries in the enamel contacts the base of the cone of caries in the dentin.
Residual caries is caries that remains in a completed tooth preparation, whether by operator intention or by accident. Such caries is not acceptable if it is present at the DEJ or on the prepared enamel tooth wall (Fig. 5-3). It may be acceptable, however, when it exists as affected dentin, especially near the pulp (see the section Affected and Infected Dentin).
Root-surface caries may occur on the tooth root that has been exposed to the oral environment and habitually covered with plaque (Fig. 5-4). Additional oral conditions (discussed in Chapter 2) conducive to caries development also must be present and often are prevalent in older patients. Root caries is usually more rapid than other forms of caries and should be detected and treated early. Root caries is becoming more prevalent because a greater number of older individuals are retaining more of their teeth and experiencing gingival recession, both of which increase the likelihood of root caries development.
Secondary caries occurs at the junction of a restoration and the tooth and may progress under the restoration. It is often termed recurrent caries. This condition usually indicates that microleakage is present, along with other conditions conducive to caries development (Fig. 5-5).
Incipient caries is the first evidence of caries activity in enamel. On smooth-surface enamel, the lesion appears opaque white when air-dried and seems to disappear when wet. This lesion of demineralized enamel has not extended to the DEJ, and the enamel surface is fairly hard, intact, and smooth to the touch. The lesion can be remineralized if immediate corrective measures alter the oral environment, including plaque removal and control. This lesion may be characterized as reversible. A remineralized lesion usually is either opaque white or a shade of brown-to-black from extrinsic coloration, has a hard surface, and appears the same whether wet or dry.
Chronic caries is slow, or it may be arrested after several active phases. The slow rate results from periods when demineralized tooth structure is almost remineralized (the disease is episodic over time because of changes in the oral environment). The condition may be found in only a few locations in a mouth, and the lesion is discolored and fairly hard. The slow rate of caries allows time for extrinsic pigmentation. An arrested enamel lesion is brown-to-black in color and hard and as a result of fluoride may be more caries resistant than contiguous, unaffected enamel. An arrested, dentinal lesion typically is “open” (allowing debridement from toothbrushing), dark, and hard, and this dentin is termed sclerotic or eburnated dentin.
Chapter 1 presented information on the development of the enamel surface of the tooth. Anatomic depressions mark the location of the union of developmental enamel lobes. Where such union is complete, this “landmark” is only slightly involuted, smooth, hard, shallow, accessible to cleansing, and termed groove. Where such union is incomplete, the landmark is sharply involuted to form a narrow, inaccessible canal of varying depths in the enamel and is termed fissure. The distinction made between a groove and a fissure also applies to an enamel surface fossa, which is nondefective enamel lobe union, and a pit, which is defective. A fissure (or pit) may be a trap for plaque and other oral elements that together can produce caries, unless the surface enamel of the fissure or pit walls is fluoride rich.
Black noted that in tooth preparations for smooth-surface caries, the restoration should be extended to areas that are normally self-cleansing to prevent recurrence of caries.1 This principle was known as extension for prevention and was broadened to include the extension necessary to remove remaining enamel defects such as pits and fissures. The practice of extension for the prevention on smooth surfaces virtually has been eliminated, however, because of the relative caries immunity provided by preventive measures such as fluoride application, improved oral hygiene, and a proper diet. This change has fostered a more conservative philosophy defining the factors that dictate extension on smooth surfaces to be (1) the extent of caries or injury and (2) the restorative material to be used. Likewise, extension for prevention to include the full length of enamel fissures has been reduced by treatments that conserve tooth structure. Tooth structure conservation ultimately leads to restored teeth that are stronger and more resistant to fracture. Such treatments are enameloplasty, application of pit-and-fissure sealant, and preventive resin or conservative composite restoration.9
Enameloplasty is the removal of a shallow developmental fissure or pit in enamel to create a smooth, saucer-shaped surface that is self-cleansing or easily cleaned. This prophylactic procedure can be applied not only to fissures and pits and deep supplemental grooves but also to some shallow, smooth-surface enamel defects (see Initial Tooth Preparation Stage later in the chapter).
Prophylactic odontotomy is presented only as a historical concept.10 The procedure involves minimal preparation and amalgam filling of the developmental, structural imperfections of enamel, such as pits and fissures, to prevent caries originating in these sites. Prophylactic odontotomy is no longer advocated as a preventive measure.
Fusayama reported that carious dentin consists of two distinct layers—an outer layer and an inner layer.11 This textbook refers to the outer layer as infected dentin and the inner layer as affected dentin. In tooth preparation, it is desirable that only infected dentin be removed, leaving affected dentin, which may be remineralized in a vital tooth after the completion of restorative treatment. This principle for the removal of dentinal caries is supported by the observation by Fusayama et al. that the softening front of the lesion always precedes the discoloration front, which always precedes the bacterial front.12
Infected dentin has bacteria present, and collagen is irreversibly denatured. It is not remineralizable and must be removed. Affected dentin has no bacteria, and the collagen matrix is intact, is remineralizable, and should be preserved. To clinically distinguish these two layers, the operator traditionally observes the degree of discoloration (extrinsic staining) and tests the area for hardness by the feel of an explorer tine or a slowly revolving bur. Some difficulties occur with this approach because (1) the discoloration may be slight and gradually changeable in acute (rapid) caries, and (2) the hardness (softness) felt by the hand through an instrument may be an inexact guide. To differentiate between remineralizable and non-remineralizable dentin, staining carious dentin was proposed by Fusayama.11 Caries-detecting dyes are not specific for infected dentin and will stain the slightly demineralized protein matrix of affected dentin as well as normal DEJ.13 Caries-detecting dyes should be used with caution and only as an adjunct to clinical evaluation.
In chronic caries, infected dentin usually is discolored, and because the bacterial front is close to the discoloration front, it is advisable, in caries removal, to remove all discolored dentin unless judged to be within 0.5 mm of the pulp (Fig. 5-6). Because the discoloration is slight in acute caries, and the bacterial front is well behind the discoloration front, some discolored dentin may be left, although any “clinically remarkable” discoloration should be removed.12
Abrasion is abnormal tooth surface loss resulting from direct forces of friction between teeth and external objects or from frictional forces between contacting teeth components in the presence of an abrasive medium.8 Abrasion may occur from (1) improper brushing techniques, (2) habits such as holding a pipe stem between teeth, (3) tobacco chewing, or (4) vigorous use of toothpicks between adjacent teeth. Toothbrush abrasion is the most common example and is usually seen as a sharp, V-shaped notch in the gingival portion of the facial aspect of a tooth.
Erosion is the wear or loss of tooth surface by chemico-mechanical action. Regurgitation of stomach acid can cause this condition on the lingual surfaces of maxillary teeth (particularly anterior teeth). Other examples are the dissolution of the facial aspects of anterior teeth because of habitual sucking on lemons or the loss of tooth surface from ingestion of acidic beverages.
It has been proposed that the predominant causative factor of some cervical, wedge-shaped defects is a strong eccentric occlusal force (frequently manifested as an associated wear facet) resulting in microfractures or abfractures. Such microfractures occur as the cervical area of the tooth flexes under such loads. This defect is termed idiopathic erosion or abfraction.14
An incomplete fracture not directly involving vital pulp is often termed a “greenstick” fracture. This phenomenon is caused by excessive cyclic loading (or traumatic injury) from occlusal contact with resultant fracture development. The fracture begins in enamel, but becomes painful following propagation into dentin. This condition is very sensitive, and yet the patient may only be able to tell which side of the mouth is affected rather than the specific tooth. It is, therefore, sometimes challenging to diagnose and treat.
This represents complete separation of a fragment of the tooth structure in such a way that the pulp is not involved. Usually, pain is not associated with this condition, unless the gingival border of the fractured segment is still held by periodontal tissue. Restorative treatment (sometimes along with periodontal treatment) is indicated.
Non-hereditary enamel hypoplasia occurs when ameloblasts are injured during enamel formation, resulting in defective enamel (diminished form, calcification, or both). It usually is seen on anterior teeth and the first molars in the form of opaque white or light brown areas with smooth, intact, hard surface or as pitted or grooved enamel, which is usually hard and discolored and caused by fluorosis or high fever. The reader should consult a textbook on oral pathology for additional information.
For brevity in records and communication, the description of a tooth preparation is abbreviated by using the first letter, capitalized, of each tooth surface involved. Examples are as follows: (1) An occlusal tooth preparation is an “O”; (2) a preparation involving the mesial and occlusal surfaces is an “MO”; and (3) a preparation involving the mesial, occlusal, and distal surfaces is an “MOD”.
The floor (or seat) is the prepared wall that is reasonably horizontal and perpendicular to the occlusal forces that are directed occlusogingivally (generally parallel to the long axis of the tooth). Examples are pulpal and gingival floors. Such floors may be purposefully prepared to provide stabilizing seats for the restoration, distributing the stresses in the tooth structure rather than concentrating them. This preparation feature increases the resistance form of the restored tooth against post-restorative fracture.
Although the junction of two or more prepared surfaces is referred to as angle, the junction is almost always “softened” so as to present a slightly rounded configuration. Despite this rounding, these junctions are still referred to as angles for descriptive and communicative purposes.
The cavosurface angle is the angle of tooth structure formed by the junction of a prepared wall and the external surface of the tooth. The actual junction is referred to as cavosurface margin. The cavosurface angle may differ with the location on the tooth, the direction of the enamel rods on the prepared wall, or the type of restorative material to be used. In Figure 5-1, D, the cavosurface angle (cs) is determined by projecting the prepared wall in an imaginary line (w′) and the unprepared enamel surface in an imaginary line (us′) and noting the angle (cs′) opposite to the cavosurface angle (cs). For better visualization, these imaginary projections can be formed by using two periodontal probes, one lying on the unprepared surface and the other on the prepared external tooth wall (Fig. 5-10).
When discussing or writing a term denoting a combination of two or more surfaces, the –al ending of the prefix word is changed to an –o. The angle formed by the lingual and incisal surfaces of an anterior tooth would be termed linguoincisal line angle. The tooth preparation involving the mesial and occlusal surfaces is termed mesio-occlusal preparation, or MO preparation. The preparation involving the mesial, occlusal, and distal surfaces is a mesio-occluso-distal tooth pr/>