Fundamentals of Tooth Preparation and Pulp Protection

Lee W. Boushell, Theodore M. Roberson and Ricardo Walter

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.

Definition of 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).

Table 5-1

Tooth Preparation: Amalgam versus Composite

	Amalgam	Composite
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

Ca(OH)₂, calcium hydroxide; HEMA, 2-hydroxyethyl methacrylate; RMGI, resin-modified glass ionomer.

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.

Need for Restorations

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.

Objectives of Tooth Preparation

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.

Factors Affecting Tooth Preparation

General Factors

Nomenclature

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.

Caries Terminology

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.

Location of Caries

Caries can be described according to location, extent, and rate.⁷

Primary Caries

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.

Caries of Pit-and-Fissure Origin

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.

Fig. 5-1 Graphic example of cones of caries in pit and fissure of tooth (A) and on the facial (B) and proximal (C) surfaces when caries has penetrated approximately same depth into dentin. Note the differences in loss of enamel on the external surfaces. Sectional view (D) of initial stage of conventional (amalgam) tooth preparations for lesions in A and B shows cavosurface angle (cs), axial wall (a), pulpal wall (floor) (p), enamel wall (e), dentinal wall (d), margin (m), and DEJ (j). Note, in the upper exploded view, that the cavosurface angle (cs) can be visualized by imaginary projections of the preparation wall (w′) and of the unprepared surface (*us′*) contiguous with the margin, forming angle *cs′*. Angles cs and *cs′* are equal because opposite angles formed at the intersection of two straight lines are equal. Likewise, minimal restorative material angle rm is equal to angle *rm′*.

Caries of Enamel Smooth-Surface Origin

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.

Backward Caries

When the spread of caries along the DEJ exceeds the caries in the contiguous enamel, caries extends into this enamel from the junction and is termed backward caries (Fig. 5-2).

Forward Caries

Forward caries is said to be present wherever the caries cone in enamel is larger or at least the same size as that in dentin (see Fig. 5-1, A).⁸

Residual Caries

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

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 (Recurrent) Caries

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).

Extent of Caries

Incipient Caries (Reversible)

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.

Cavitated Caries (IRReversible)

In cavitated caries, the enamel surface is broken (not intact), and usually the lesion has advanced into dentin. Usually, remineralization is not possible, and treatment that includes tooth preparation and restoration is indicated.

Rate (Speed) of Caries

Acute (Rampant) Caries

Acute caries, often termed rampant caries, refers to disease that rapidly damages the tooth. It is usually in the form of numerous soft, light-colored lesions in a mouth and is infectious. Less time for extrinsic pigmentation explains the lighter coloration.

Chronic (Slow) or Arrested Caries

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.

Grooves and Fissures; Fossae and Pits

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.

Extension for Prevention

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

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

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.

Affected and Infected Dentin

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

Fig. 5-6 Comparison of acute and chronic caries regarding closeness, hardness, and depth factors of the softening, discoloration, and bacterial invasion fronts.

Non-carious Tooth Defects Terminology

Abrasion

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

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.

Attrition

Attrition is the mechanical wear of the incisal or occlusal surface as a result of functional or parafunctional movements of the mandible (tooth-to-tooth contacts). Attrition also includes proximal surface wear at the contact area because of physiologic tooth movement.

Abfraction

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

Fractures

Fractures are among the more difficult and challenging defects of teeth, in both diagnosis and treatment.

Incomplete Fracture Not Directly Involving Vital Pulp

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.

Complete Fracture Not Involving Vital Pulp

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.

Fracture Involving Vital Pulp

Fracture involving vital pulp always results in pulpal infection and severe pain. If the tooth is restorable, immediate root canal therapy is indicated; otherwise the tooth must be extracted.

Non-hereditary Enamel Hypoplasia

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.

Line Angle

A line angle is the junction of two planar surfaces of different orientation along a line (Figs. 5-8 and 5-9). An internal line angle is the line angle whose apex points into the tooth. The external line angle is the line angle whose apex points away from the tooth.