2-16. CAVITY PREPARATION
a. General. Cavity preparation is largely a mechanical procedure in which hand excavating instruments and motor driven burs, disks, and stones are used to remove caries and debris and to cut and shape tooth structure. Speeds used with rotating cutting instruments vary from the range of 4,000 to 10,000 revolutions per minute (rpm) to "high" speeds of 500,000 rpm or more. Instruments and techniques vary with the speeds, and the dental assistant should be familiar with these differences and be prepared to adapt readily to assist in the use of any technique.
b. Instrument and Material Setup. Specific instruments used in cavity preparation will be determined by the dentist's preference and by the location of the cavity and the type of restorative material to be used. Instruments and materials used in preparing the cavity for an amalgam restoration include the contra-angle hand piece and appropriate hand instruments and burs, stones, and disks.
(1) Resistance form, retention form, and convenience form. The dentist will begin each procedure by selecting one of the burs we have discussed, and then outlining the cavity preparation. This establishes the general outline that the preparation will take, or how it will appear on the surface of a tooth. The bur used to accomplish this procedure will depend on the dentist's preference. The burs that might be used are the inverted cone, straight fissure, or tapered fissure burs. After outlining the cavity preparation, the dentist will want to obtain the resistance and retention forms. Though resistance and retention are not the same, both are accomplished at the same time. Resistance form is a form whereby the tooth and the restoration are able to resist the normal forces of mastication regardless of loss of tooth structure caused by caries and cavity preparation. Retention form is a form that tends to prevent dislodgment of the restoration and is characterized by lock, dovetails, and undercuts. This step may also be accomplished with burs: a small round bur, inverted cone, or tapered fissure bur. Convenience form will be accomplished at this time to obtain access for removing the remaining decay and to facilitate placement of the restorative material.
(2) High speed procedures. Use of high speed cutting instruments reduces cutting time, increases operator control of the instrument (no tendency to bind, wedge, or hop), and lessens patient apprehension and operator strain and fatigue.
(a) Control of heat. The main clinical problem in restorative dentistry is the heat generated during the cutting of the tooth. Temperatures high enough to damage the pulp can be developed at any speed, but, because elevated temperatures develop more rapidly at higher speeds, greater care and attention must be given to heat control. Operating at speeds up to 4,000 rpm and applying moderate force may be done without generating too much heat, but at greater speeds, coolants must be used. Streams of water, air, or mixture of air and water are used as coolants. Air is effective only in the lower ranges of high speeds. For higher speeds, a spray or stream of water must be used. In addition to lowering the temperature in the field of operation, the coolant carries away debris produced by the cutting operations of the bur, and helps prevent clogging of the bur's cutting surface. Instruments so cleaned cut more rapidly and last longer. A number of devices have been developed for delivering the coolant to the cutting site by the hand piece. These may be attached to or built into the hand piece. The dentist may prefer not to use such a device or one may not be available. In this case, the dental assistant must apply the coolant with a hand-held water syringe.
(b) High-speed hand pieces. Hand pieces designed to operate at speeds up to 8,000 rpm will not operate satisfactorily at higher speeds. Increased speed, therefore, has required greater precision in manufacture and replacement of sleeve-type bearings with ball, roller, or needle bearings. Care of the hand piece, which was always important, has become even more critical.
(c) High velocity evacuation. Since high-speed hand pieces require a continual flow of water for cooling during cavity preparation, there is a need for more efficient removal of water from the oral cavity than is possible with the saliva ejector. One method employs a suction apparatus, which uses a vacuum pump to remove large volumes of saliva, water, and debris through a tube and mouthpiece of large diameter. Thus, high velocity suction can be achieved with little suction pressure. A number of mouthpieces have been designed for different operations in different locations of the mouth. The mouthpiece of this instrument functions best when handled by the chair assistant. Advantages of this system are: It relieves tension of the patient by keeping fluids out of his throat and relieving him of the need to use the cuspidor, it helps to keep the operating field dry and the mirror clean, and it saves time by eliminating rinsing.
(3) Removal of decay. The next major step that the dental office will perform is the removal of the bulk of decay. Here he will be taking out the carious dentin and other debris in the cavity itself. The dentist might choose to use burs in this step of the procedure. Rather than using burs, the dentist might use hand instruments, excavators being the instruments used to accomplish this procedure. The dentist can choose from several pairs of excavators. He might choose the large pair of Black's excavators number 63 and number 64 or the smaller pair of Black's number 65 and number 66. These are paired instruments, the face of which has parallel sides and a concave surface. The other excavators that might be used are the smaller pair of Darby-Perry number 5 and number 6 or the larger pair number 21 and number 22. The face of the Darby-Perry excavator is shaped much like a rounded spoon. The shape of the working end of all these excavators makes them ideal for scooping out decayed dentin and debris.
(4) Finishing. In the next step, the dentist will finish the cavity preparation by cleaving unsupported enamel walls and, where needed, establish bevels on the proximal surface at the cervical seat(s) of the pre- pared cavity. The dentist might employ hand instruments. He could use the large pair of mono-beveled hatchets number 51 and number 52, or the small pair number 53 and number 54. These are called "mono-beveled" because there is only a single bevel on the blade of the instrument. They are also paired instruments (if the dentist used one, he will probably use the other). The other hatchet that might be used is the bi-beveled hatchet number 17. Looking at the cutting edge, you will notice that it is sharpened on both sides, thus creating a bevel (angle) on both sides of the blade. The dentist might also choose to use any of the chisels available to him. The bin-angled chisels have a sharp bend in the shank, and the blade itself is wider than the shank. Bin-angled chisels are number 81 and number 83, either 1, which may be placed out in the setup. The straight chisels that might be used are numbers 84, 85, and 86. Chisel number 84 is the largest and number 86 the smallest of the set. Only one of these is required for the setup. Some chisels are put out as a pair. These are the Wedelstaedt numbers 41 and 42. They have a graceful curve in the shank down to the cutting edge rather than a distinct bend or angle, as did another set of chisels we have already discussed. The cutting edge is on opposite sides of the blade, indicative of numbers 41 and 42.
(5) Trimming. Sometimes when finishing the cavity preparation, the dentist will find it necessary to bevel the enamel at the cervical seat. Gingival margin trimmers are used to accomplish this. These instruments are also used in pairs. There are a pair of mesial gingival margin trimmers and a pair of distal gingival margin trimmers. They are so designated because they can be used on only one of the two proximal surfaces. The mesial gingival margin trimmers have the sharp point of the cutting edge on the convex side of the blade. The distal gingival margin trimmers have the cutting edge on the concave side of the blade. Both the mesial and distal trimmers are made in large and small sizes.