Class III and V Amalgam Restorations
Lee W. Boushell, Theodore M. Roberson and Aldridge D. Wilder, Jr.
This chapter presents information about Class III and V amalgam restorations. Class III restorations are indicated for defects located on the proximal surface of anterior teeth that do not affect the incisal edge. Part of the facial or the lingual surfaces also may be involved in Class III restorations. Class V restorations are indicated to restore defects on the facial or lingual cervical one third of any tooth.
The Class III amalgam restoration is rarely used. Its use has been supplanted by tooth-colored restorations (primarily composite), which have become increasingly wear-resistant and color-stable. Because indications exist for Class III amalgam restorations, however, practitioners should be familiar with this restorative technique.
The Class V amalgam restoration can be especially technique-sensitive because of location, extent of caries, and limited access and visibility. Cervical caries usually develops because of the chronic presence of acidogenic plaque located in the non–self-cleansing area just beneath the coronal height of contour. Patients with gingival recession are predisposed to cervical caries because dentin is more susceptible to demineralization than enamel. Patients with a reduced salivary flow caused by certain medical conditions (e.g., Sjögren’s syndrome), medications, or head and neck radiation therapy also also are predisposed to cervical caries. These patients usually have less saliva to buffer the acids produced by oral bacteria. Patients with gingival recession that has exposed the root surface have a predisposition to root caries because dentin is more susceptible to demineralization than enamel. Class V restorations may be used to treat both cervical and root caries lesions.
Incipient, smooth-surface enamel caries appears as a chalky white line just occlusal or incisal to the crest of the marginal gingiva (usually on the facial surface) (Fig. 15-1). These areas often are overlooked in the oral examination, unless teeth are free of debris, isolated with cotton rolls, and dried gently with the air syringe. When incipient cervical caries has not decalcified the enamel sufficiently to result in cavitation (i.e., a break in the continuity of the surface), the lesion may be remineralized by appropriate techniques, including patient motivation toward proper diet and hygiene. Occasionally, an enamel surface that is only slightly cavitated may be treated successfully by smoothing with sandpaper disks, polishing, and treating with a fluoride varnish or a dentin adhesive in an attempt to prevent further caries that may require treatment. This prophylactic, preventive treatment cannot be instituted if caries has progressed to decalcify and soften enamel to an appreciable depth. In this instance, a Class V tooth preparation and restoration is indicated, particularly if caries has penetrated to the dentinoenamel junction (DEJ) (Fig. 15-2, A). When numerous cervical lesions are present (see Fig. 15-2, B), a relatively high caries index is obvious. In addition to the restorative treatment, the patient should be instructed and encouraged to implement an aggressive prevention program to avoid recurrent decay.
Pertinent Material Qualities and Properties
Material qualities and properties important for Class III and V amalgam restorations are strength, longevity, ease of use, and past success. See Chapter 13 for a discussion of the pertinent material qualities and properties of amalgam.
Few indications exist for a Class III amalgam restoration. It is generally reserved for the distal surface of maxillary and mandibular canines if (1) the preparation is extensive with only minimal facial involvement, (2) the gingival margin primarily involves cementum, or (3) moisture control is difficult. For esthetic reasons, amalgam rarely is indicated for the proximal surfaces of incisors and the mesial surface of canines.
Class V amalgam restorations may be used anywhere in the mouth. As with Class III amalgam restorations, they generally are reserved for non-esthetic areas, for areas where access and visibility are limited and where moisture control is difficult, and for areas that are significantly deep gingivally. Because of limited access and visibility, many Class V restorations are difficult and present special problems during the preparation and restorative procedures.
One measure of clinical success of cervical amalgam restorations is the length of time the restoration serves without failing (Fig. 15-3). Properly placed Class V amalgams have the potential to be clinically acceptable for many years. Some cervical amalgam restorations show evidence of failure, however, even after a short period. Inattention to tooth preparation principles, improper manipulation of the restorative material, and moisture contamination contribute to early failure. Extended service depends on the operator’s care in following accepted treatment techniques and proper care by the patient.
Amalgam may be used on partial denture abutment teeth because amalgam resists wear as clasps move over the restoration. Contours prepared in the restoration to retentive areas for the clasp tips may be achieved relatively easily and maintained when an amalgam restoration is used. Occasionally, amalgam is preferred when the caries lesion extends gingivally enough that a mucoperiosteal flap must be reflected for adequate access and visibility (Fig. 15-4). Proper surgical procedures must be followed, including sterile technique, careful soft tissue management, and complete debridement of the surgical and operative site before closure.
Class III and V amalgam restorations usually are contraindicated in esthetically important areas because many patients object to metal restorations that are visible (Fig. 15-5). Generally, Class V amalgams placed on the facial surface of mandibular canines, premolars, and molars are not readily visible. Amalgams placed on maxillary premolars and first molars may be visible. The patient’s esthetic demands should be considered when planning treatment.
Amalgam restorations are stronger than other Class III and V direct restorations. In addition, they are generally easier to place and may be less expensive to the patient. Because of its metallic color, amalgam is easily distinguished from the surrounding tooth structure. Amalgam restorations are usually easier to finish and polish without damage to the adjacent surfaces.
The primary disadvantage of Class III and V amalgam restorations is that they are metallic and unesthetic. In addition, the preparation for an amalgam restoration typically requires 90-degree cavosurface margins and specific axial depths that allow incorporation of secondary retentive features. These features result in a less conservative preparation than that required for most esthetic restorative materials.
Clinical Technique for Class III Amalgam Restorations
After appropriate review of the patient records (including medical history), treatment plan, and radiographs, the gingival extension of the preparation should be anticipated. Anesthesia is usually necessary when a vital tooth is to be restored. Pre-wedging in the gingival embrasure of the proximal site to be restored provides (1) better protection of soft tissue and the rubber dam, (2) better access because of the slight separation of teeth, and (3) better re-establishment of the proximal contact. The use of a rubber dam is generally recommended; however, cotton roll isolation is acceptable if moisture can be adequately controlled.
A lingual access preparation on the distal surface of the maxillary canine is described here because the use of amalgam in that location is more likely. For esthetic reasons, use of amalgam is best suited for caries that can be accessed from the lingual rather from the facial. A facial approach for a mandibular canine may be indicated, however, if the lesion is more facial than lingual. The mandibular restoration is often not visible at conversational distance (Fig. 15-6).
The outline form of the Class III amalgam preparation may include only the proximal surface. A lingual dovetail may be indicated if one existed previously or if additional retention is needed for a larger restoration.
Initial Tooth Preparation
Bur size selection depends on the anticipated size of the lesion. Bur options may include a No. 2 (or smaller) round bur or No. 330 bur. The bur is positioned so that the entry cut penetrates into the caries lesion, which is usually apical to (and slightly into) the contact area. Ideally, the bur is positioned so that its long axis is perpendicular to the lingual surface of the tooth, but directed at a mesial angle as close to the adjacent tooth as possible. (The bur position may be described as perpendicular to the distolingual line angle of the tooth.) This position conserves the marginal ridge enamel (Fig. 15-7, A through C). Penetration through enamel positions the bur so that additional cutting isolates the proximal enamel affected by caries and removes some or all of the infected dentin. In addition, penetration should be at a limited initial axial depth (i.e., 0.5–0.6 mm) inside the DEJ (see Fig. 15-7, C and D) or at a 0.75-0.8-mm axial depth when the gingival margin is on the root surface (in cementum) (Fig. 15-8). This 0.75-mm axial depth on the root surface allows a 0.25-mm distance (the diameter of the No. bur is 0.5 mm) between the retention groove (which is placed later) and the gingival cavosurface margin. Infected dentin that is deeper than this limited initial axial depth is removed later during final tooth preparation.
For a small lesion, the facial margin is extended 0.2-0.3 mm into the facial embrasure (if necessary), with a curved outline from the incisal to the gingival margin (resulting in a less visible margin). The lingual outline blends with the incisal and gingival margins in a smooth curve, creating a preparation with little or no lingual wall. The cavosurface angle should be 90 degrees at all margins. The facial, incisal, and gingival walls should meet the axial wall at approximately right angles (although the lingual wall meets the axial wall at an obtuse angle or may be continuous with the axial wall) (Fig. 15-9). If a large round bur is used, the internal angles are more rounded. The axial wall should be uniformly deep into dentin and follow the faciolingual contour of the external tooth surface (Fig. 15-10). The initial axial wall depth may be in sound dentin (i.e., shallow lesion), in infected dentin (i.e., moderate to deep lesion), or in existing restorative material, if a restoration is being replaced.
Incisal extension to remove carious tooth structure may eliminate the proximal contact (Fig. 15-11). It is important to conserve as much of the distoincisal tooth structure as possible to reduce the risk for subsequent fracture. When possible, it is best to leave the incisal margin in contact with the adjacent tooth.
When preparing a gingival wall that is near the level of the rubber dam or apical to it, it is beneficial to place a wedge in the gingival embrasure earlier to depress and protect soft tissue and the rubber dam. As the bur is preparing the gingival wall, it may lightly shave the wedge. A triangular (i.e., anatomic) wedge, rather than a round wedge, is used for a deep gingival margin.
The initial tooth preparation is completed by using a No. round bur to accentuate the axial line angles (Fig. 15-12, A and B), particularly the axiogingival angle. This facilitates the subsequent placement of retention grooves and leaves the internal line angles slightly rounded. Rounded internal preparation angles permit more complete condensation of the amalgam. The No. round bur also may be used to smooth any roughened, undermined enamel produced at the gingival and facial cavosurface margins (see Fig. 15-12, C). The incisal margin of the minimally extended preparation is often not accessible to the larger round bur without marring the adjacent tooth (see Fig. 15-12, D). Further finishing of the incisal margin is presented later. At this point, the initial tooth preparation is completed.
Final Tooth Preparation
Final tooth preparation involves removing any remaining infected dentin; protecting the pulp; developing secondary resistance and retention forms; finishing external walls; and cleaning, inspecting, and desensitizing or bonding. Any remaining infected carious dentin on the axial wall is removed by using a slowly revolving round bur (No. 2 or No. 4), appropriate spoon excavators, or both. (See Chapter 5 for the indications and technique for placing a liner.)
For the Class III amalgam restoration, resistance form against post-restorative fracture is provided by (1) cavosurface and amalgam margins of 90 degrees, (2) enamel walls supported by sound dentin, (3) sufficient bulk of amalgam (minimal 1-mm thickness), and (4) no sharp preparation internal angles. The box-like preparation form provides primary retention form. Secondary retention form is provided by a gingival groove, an incisal cove, and sometimes a lingual dovetail.
The gingival retention groove is prepared by placing a No. round bur (rotating at low speed) in the axio-facio-gingival point angle. It is positioned in the dentin to maintain 0.2 mm of dentin between the groove and the DEJ. The rotating bur is moved lingually along the axiogingival line angle, with the angle of cutting generally bisecting the angle between the gingival and axial walls. Ideally, the direction of the gingival groove is slightly more gingival than axial (and the direction of an incisal [i.e., occlusal] groove would be slightly more incisal [i.e., occlusal] than axial) (Fig. 15-13; see also Fig. 15-8).
Alternatively, if less retention form is needed, two gingival coves may be used, as opposed to a continuous groove. One each may be placed in the axio-gingivo-facial and a/>