2-17. AMALGAM RESTORATIONS WITH CEMENT BASE
a. General. Preparing and restoring teeth with silver amalgam is the dental treatment procedure performed most frequently. The properties of silver amalgam are determined largely by how it is handled. Optimum properties can be achieved only through careful attention to details in its preparation and manipulation. Since the dental assistant normally is responsible for the measurement, preparation, and initial handling of the material, the manner in which he carries out these procedures can determine the success of the restoration. In addition, the manner in which he assists the dentist in the performance of other steps of the restorative operation influences the quality of the results.
b. Instrument and Material Setup. The instruments and materials should not be introduced into the working area until just before they are needed. Instruments used for preceding steps and no longer needed should be removed from the working area. A typical setup for a Class II amalgam restoration includes the instruments and equipment shown in figure 2-29. Class I restorations normally will not require the matrix material.
c. Preparing and Placement of Cement Base.
(1) General. It is common practice to insert a low thermal conducting cement material as a base in deep portions of cavities underlying metallic restorations to insulate the pulp against thermal and electrical shock and pressures induced during placement of the restorations. Zinc phosphate cement is the material most used for this purpose.
(2) Assisting procedures. When the dentist is finishing the cavity preparation, he will direct the dental assistant to prepare the base. The dental assistant manipulates the material in accordance with the manufacturer's directions and the general techniques discussed in a previous subcourse. A thick mix is normally used for this purpose. When the dental assistant has completed the mix to the desired consistency, it is presented to the dentist. Usually, the mix is scraped from the spatula onto the center of one edge of the glass slab. Some dentists may want the glass slab placed on the bracket table, while others may want it held by the specialist while the dentist takes the amount of material required. Some excess powder should remain for the dentist's use in placing the base. Upon completion of the placement of the base, the dental assistant should remove the slab and spatula from the operative area. If zinc phosphate is used as a base, we will of course need a glass slab (which should have a "Z" or "ZP" etched on the side) with a matched set of zinc phosphate powder and liquid. There are several other base materials, which may be used and most of these will be mixed on paper or parchment mixing pads. We will also need a stainless steel spatula in mixing all of the base materials. To carry and pack the cement into the preparation requires the use of one of several cement pluggers available to the dentist. One of the more commonly used pluggers is the Ladmore plugger number 3, which has teardrop shaped working ends on both ends of the instrument. The Woodson pluggers number 1, number 2, and number 3 may also be used. Both nibs of the Woodson number 1 are flat, one in the same plane of the shank and the other at a slight angle to the shank. The Woodson number 2 has a cylinder-shaped plugger on one end and a paddle-shaped nib on the other. A Woodson number 3 has nibs similar to the Woodson number 2 except that the cylinder-shaped nib is larger. Many dentists will prefer to use a much smaller base instrument we call the "Dycal instrument." This instrument is furnished by certain manufacturers with their base materials. It is much shorter and smaller than the others we have discussed and has a beaded end to the nib.
Figure 2-29. Instruments and materials for Class II amalgam restorations.
d. Placement of a Metal Matrix Band.
(1) Proper placement of amalgam requires condensation by applying strong pressure to the surface of the amalgam with amalgam pluggers. In cavities involving more than one tooth surface, it is usually necessary to surround the tooth with a matrix band to help confine the restorative material to the cavity preparation during condensation. The matrix band also helps determine proper contours and the location and form of proximal contacts in the restored tooth. A number of types of matrix retainers are available. Many are designed to be used with matrix bands premade in assorted sizes and configurations. Matrix bands may also be tailored individually from matrix band material and tied or otherwise held in place on the tooth. When the matrix is in place on the tooth, a wedge or wedges are often used to hold the band tightly against the neck of the tooth on proximal surfaces involved in the cavity preparation (see figure 2-30). The wedge keeps the amalgam from squeezing out under the matrix band and into the interproximal space or spaces. Some dentists use red or green stick impression compound to add further support and rigidity to the matrix.
Figure 2-30. Wooden wedge.
(2) The assistant, with the dentist, will prepare materials designed to replace the missing walls of a cavity preparation and to shape or protect the restoration. Placing a matrix for an amalgam restoration will allow the dentist to insert the restorative material without exceeding the limits of the normal tooth structure. We will discuss only the most common type, the Tofflemire matrix retainer and band. This comes in two parts: A small stainless steel band which encircles the tooth is one part, and the other part is the instrument which holds the band in place called the Tofflemire retainer. You will note that the number 1 Universal band is doubled over when it is placed in the retainer. In assembling and placing the matrix, there are three essential rules to follow. First, the small opening of the band must point toward the cervical portion or root of the tooth. Secondly, the slotted side of the retainer always points toward the root of the tooth to facilitate the removal of the retainer after the restorative material has partially set or crystallized. Finally, the retainer is usually positioned on the facial side of the tooth being worked on (in the vestibule).
(3) However, when you are assigned to a clinic, you may find that these rules are not always followed or applied as they are described here. Your dentist might desire to use the retainer in the mouth proper, for example, rather than in the vestibule, but the majority of dentists will use it according to rules that we have given you. You may also encounter other matrix retainers. You may find that the dentist you are assisting will not always use the number 1 Universal band. There are bands for molars (larger bands) and for bicuspids (smaller bands). In any case, you must learn how to assemble the retainer and band. You must also know how to adjust the size of the band by turning the adjusting screw, which is the inner screw on the retainer, as well as assembling the set and tightening the band in place. This is done with the securing screw, which is the outer screw on the retainer. In some instances, the restorative material may actually be placed before applying the matrix.
e. Preparing Amalgam. The dental assistant then prepares the amalgam in accordance with the manufacturer's instructions and general techniques discussed in a previous lesson. The manner in which the assistant handles the dental amalgam will have much to do with the success of the resulting restoration. Correct trituration time and avoidance of moisture contamination are important steps in producing the material with the best properties.
f. Filling the Cavity. In an amalgam restoration, the amalgam will be prepared in an amalgamator, and the dentist can place the amalgam into the preparation with the amalgam carrier. This instrument has a barrel type nib that is used to pick up and transfer the amalgam from the squeezed cloth to the mouth.
CAUTION: Remember never to touch the amalgam with your hands.
Once it is placed in the restoration, the amalgam must be condensed. This is done with amalgam pluggers. The two pluggers that will be used are the Black's number 1 and number 2. They both have round-shaped plugger ends with the end of the number 2 somewhat larger than the number 1. The alternate pluggers take different shapes on the working ends. The dental assistant loads the amalgam carrier with the amalgam and places it in the dentist's operating hand in the position in which he will carry it to the tooth. The dental assistant then picks up the first amalgam plugger in his right hand. When the dentist is ready for the plugger, the dental assistant takes the amalgam carrier from him and replaces it with the plugger. While the dentist is condensing the first segment of amalgam, the dental assistant loads the carrier with the next segment. In this way, the cavity is filled segment by segment, the dental assistant alternately passing the loaded amalgam carrier and appropriate instruments to the hand of the dentist. When the cavity is sufficiently filled, the dental assistant selects the appropriate carving instruments and hands them one at a time to the dentist as needed. The exchange of instruments between the hands of the assistant and operator is done to keep the dentist's hands close to the patient's mouth and to foster rapid and smooth accomplishment of the operative procedure. When the matrix material has been removed from the mouth, the dental assistant should also be prepared to rinse the patient's mouth as necessary during and after final carving procedures.
g. Finishing and Polishing Amalgam Restorations
(1) Amalgam covers. Finally, the dentist is ready to finish the restoration. In this step, he will complete the procedure so that patient is able to function normally. In placing the amalgam, the dentist will overfill the preparation during the condensation phase to obtain sufficient bulk of material to carve the amalgam down to the original contour of the tooth. He will use amalgam carvers to contour the partially crystallized amalgam. He may employ the Hollenback number 3, the cleoid number 92, and the discoid number 89. The Hollenback number 3 has two grain-shaped blades. The cleoid carver has only one blade, a concave pointed oval that resembles a claw. In fact, you will find that in the Medical Supply Catalog this instrument is listed as a claw excavator. The discoid amalgam carver has a single blade that is round or disc-shaped (from which its name comes, discoid).
(2) Articulating paper. The final item is articulating paper. It closely resembles blue carbon paper and is issued in a roll or package form. Desired lengths are cut from the roll or selected from the package, and the patient is instructed to bite down normally with the strip of articulating paper over the occlusal surfaces of the teeth. Any high spots in the patient's occlusion will be detected by noting blue spots on the teeth. The dentist may have a patient return for final finishing and polishing of the amalgam restoration 24 or more hours after its insertion. This is done to eliminate small excesses of amalgam that may have been left overhanging cavity margins, to refine occlusal contours of the restoration, and to develop a smooth, shiny surface that will resist contamination and formation of surface oxides.
(3) Materials. Finishing and polishing is done with fine polishing disks and strips, green stones, finishing burs, chalk, and other fine polishing agents. An old contra-angle hand piece is operated at slow speed along with "wet" solutions (mixtures) of polishing materials to minimize heat production. Heat produced during the polishing could weaken the surface of the amalgam. Rubber polishing cups and polishing brushes are commonly used to apply the polishing materials.
NOTE: Figure 2-31 outlines steps of procedure for the dental assistant and the dentist, when preparing amalgam restorations with a cement base.