New concepts in the preparation of teeth for full coverage

XEW

CONCEPTS

IX THl;, FOR FULI,

PREPARATION COVERAGE

012 TbXTIE

HE CHIEF PROBLEJI in the preparation of teeth for full coverage is the removal of tooth enamel. In addition to being endowed with great hardness, the enamel rods are arranged with classic engineering skill. They arr generally situated at right angles to the outer surface and in this fashion are capable of offering maximum resistance to masticatory stresses and abrasion. ‘IX gnarled arrangement of the rods at the points of greatest wear and strain, namely, the cusp eminences, is further evidence of functional design. According to conventional methods. abrasive wheel stones are commonly en)ployed for grinding away tooth enamel. These stones usually consist of a narron cylindrical member with a flat peripheral edge and opposite flat sides. They are mounted on a mandrel for attachment to a source of rotary power. The stone is applied directly against the outer enamel surface and pressure is applied in in longitudinal or pulpal direction, in order to grind away the enamel by a direct abrading process. Tnasmuch as the pressure is exerted in the longitudinal direction of the enamel rods, the grinding process is exceedingly slow and tedious, I-~~auti~ of the great compressive strength and altrading resistance of the enamel rod structure. This results in the generation of csccssire heat \vith its associated problems of proper heat dissipation, discomfiture tr) the patient, and the possibility oi pulpal injury. In addition, the long grinding process results in the fatigue of tht patient, as well as of the dentist. The (SC of diamond surfaced stones, in cotI. junction with more efficient peripheral cutting sl’eeds, lighter Imssure, anti water spravs have helped to reduce some c,f these difficulties.

T

A

XEW

INSTRUMISNT

In order to improve upon conventional methods of removing portion,s of enamel from a tooth, and accordingly considerably reduce the time required in preparation, obviate the danger of pulp injury, and relieve both patient and dentist from some of the strain inherent in the procedure, a new tool and technique were devised. The mechanical origin of this technique is shown in the following machine shop procedure. Fig. 1 shows a cylinder of cast iron being machined in a lathe. The outer crust A, is tough and abrasive, making it difficult to remove by a surface cutting action. Tnstead, the cutting tool I3 is angulated so that it cuts into the relatively soft metal underneath the crust, undermining and removing it with ease. Received

for

publication

Nov.

12, 1954.

Volume 5 .Numlxr s

I’REPARATION

OF TEETH

FOR FULL

393

COVERAGE

The biomechanical application of this principle was described by G. V. Black.’ Fig. 2 shows the use of an inverted cone bur during cavity preparation. The enamel rods are undermined because of the angulation of the cutting blades and are therefore removed with greater ease. In the accomplishment of this principle in tooth preparation, a method is employed, whereby during a considerable portion of the process, the direct longitudinal pressure usually applied to the tooth is reduced or eliminated, the cutting instrument providing for the undermining of the enamel rods by a lateral undercutting action. By this method, the enamel rods are deprived of their respective base supporting areas, and are thereby caused to flake off very rapidly. Fig. 1. 1

I

Fig. l.-The Fig. 2.-The

Fig. 2. hard outer crust of a casting is removed by an undermining process. inverted cone bur is used to undermine the enamel rods during cavity prepara-

tion.

In order to greatly reduce the manual effort required to maintain the stone at a level for the desired depth of cut, an inclined,cutting surface is provided during the lateral movement of the tool, the operative pressure during such movement at all times having a longitudinal or vertical component in the direction of the depth of cut. In other words, there is always present a longitudinal resultant force which helps to maintain the tool in its proper position with respect to the depth of the cut. This reduces the need to employ pressure against the tooth, thereby reducing the amount of heat generated in the grinding process. The inclined cutting or grinding surface is effective in creating a larger cutting area per revolution of the stone. This increases cutting efficiency for equal depths of cut. It Fig. 3 shows a cross-sectional and end view of the cutting instrument. is a permanently mounted, disc-like diamond wheel stone having a circumferential peripheral wall and two opposite depressed sides.* The peripheral wall is used *U. S. Patent

No. 2$X35,738

Canadian

Patent

No. 505087

Foreign

Patents

Pending.

to create an initial cut into the dentinc, and the lateral undercutting or striI)pi~~~ of the enamel is accomplished hy the inclined side cvall~ of the instrument. Fig. 4. is a graphic representation 11i the interpla?- of forces created against the side cutting walls of the instrument while in use. Triangle ;2II<. F’ is a I;~teral co111 ponent of force, at right angles to X117, 1; is thta vertical component of force which

Fig. -1. Fig. 3.-A Fig. 4.-The

cross-sectional interplay

and end view of The mw of forces against

wtting

the side cutting

instrument. wall

of the instrumrnt.

automatically helps to maintain the depth oi cut. The valuc~ of 1’ can be expressed hy the equation I: zz 1;’ Sin N By experimentation, it was found that a 1.2J degree taper of the cutting walk creates a correct value for F. Triangle DGE shows the relationship of the cutting wall DE to the depth of cut, represented by the dotted line IX;. Since DI< is the hypotenuse of the triangle DGE, it is longer than DG, thereby providing a larger cutting surface for a given depth of cut. REMOVAL

OF ENAMEL

Following the creation of an initial step into the dentine, occlusal or incisal reduction can be accomplished by a lateral stripping action (Fig. .5) , Buccal or labial enamel reduction is similarly accomplished, using the dento-enamel junction as a depth guide (Fig. 6). Lingual enamel reduction using the same method is shown in Fig. 7. Wide areas of enamel which cannot be removed with :I

l’olume 5 Xumber 3

PREPARATION

OF TEETH

FOR FULL

COVISRAC,E

B.

A

Fig. 5.--4,

Fig.

G-A,

Fig.

7-A

Occlusal enamel reduction.

Buccal enamel reduction.

and

B,

Lingual

B,

B,

Incisal

enamel redurtion.

Labial enamel reduction.

enamel reduction.

396 single stripping movement, require two or mote sweeps of the wheel. .j‘l)<. amoutlt of tooth structure removed can alwavs be judged accuratelv_ 1~ - obsrrvm~ the depth of the step created by the 1vhec4. This helps to maintain l)roper ilrr spective, thus avoiding the possibility (ri overcutting. Where rherr i:, lintiteci operating space due to interference of the tongue or cheek wall, a smaller wht%el can be used. The wheel can also be tilted in order to accommodate the dccreasrl operating space, and the stripping action accomplished in a diagonal fashion. The major part of the enamel has now been removed by a very efficient lateral stripping action. This also scrvcs to reduce the enamel area of the proximal burfaces by the amount of reduction of the three aforementioned surfaces. The proxi-~ ma1 cuts are now accomplished I)y mc;ul,~ oi permancntl~ mounted cliamontl disc*-.

.A

Fig. 8.---A and B, A perforated incisor tooth.

II

diamond

disc being used for proximal

cuts of a molar

and

Fig. 8 shows the use of a pi inch perforated disc, which is edge cutting and safe sided on one side, being used on the proximal surfaces of a molar and incisor tooth respectively. The cut is started at the dento-enamel junction, and the extent and amount of tooth structure removed will determine the presence or absence of a shoulder. The disc perforations allow for visibility of the tooth surface being prepared and access for the water spray. This is necessary in spite of the fact that the perforations also tend to increase vibration. In order to provide proper cutting angulation for the distal surfaces of posterior Fig. 9 shows how the cut is started at thr teeth, a cup-shaped disc is provided.

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PREPARATION

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COVERAGE

In the case of the cup-shaped disc, as well as the flat dento-enamel junction. disc which is similarly used on anterior teeth (Fig. lo), a safe side protects the adjacent tooth. In order to avoid overcutting, the proximal tooth surfaces can be engaged directly by the cutting face of the disc whenever access permits. Caution should be exercised if disc saws are used, since they do not have a safe side to prevent injury to adjacent teeth. In order to allow for a better edge cutting action and prevent binding at the contact area, the discs have been made slightly thicker. This also helps to prevent accidental bending of the disc during use, and minimizes the injury to soft tissues in case of accidental contact.

Fig. 9. Fig. S.-The Fig. IO.-The

distal

surface

proximal

of a molar

surface

Fig. 10. tooth

being prepared

of an incisor

COMPLETION

OF THE

tooth

being

with prepared

a cup shaped disc. with

a flat disc.

PREPARATION

The major part of the tooth preparation has now been completed, and the rest of the cutting is primarily in dentine. For this purpose, there have been provided three tapered cylindrical diamond points in graduated sizes that are both side and end cutting. They are used to straighten axial walls, create shoulders, round line angles, carve occlusal anatomy, and finish tooth surfaces gingival to shoulders. Some of these uses are illustrated in Fig. 11. One of the cutting characteristics of diamond abrasive is the lack of cutting efficiency against soft tissues. It is primarily the angulation and sharpness of the

A

c

E

Fig. cylindrical coverage.

1 1.-A-E, points

Several uses of the tapered in tooth preparation for full

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PREPARATION

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TEETH

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COVERAGE

399

edges of the instrument, rather than the presence of diamond particles, that cause tissue laceration. Thus the presence of diamond abrasive on the ends of the tapered instruments greatly increases their utility, as for example ill the creation of shoulders, without endangering the gingival tissues. CUTTING

SPEEDS

A practical consideration of cutting speed is indicated at this point. The excellent cutting characteristics of the diamond wheel previously described permit its efficient use at relatively slow speeds, although higher speeds increase cutting efficiency and allow lighter cutting pressures to be employed. The use of a water and air spray is indicated in order to prevent overheating of the tooth and clogging of the cutting tool. This is important at all speeds, since the production of heat depends on instrument pressure as well as peripheral speed. The discs have large cutting areas and can operate efficiently at relatively low speeds. This is desirable in order to avoid the dangers inherent in the use of discs, e.g. where tissue damage might result from binding at the contact area, with the resultant loss of instrument control. The possibility of disc slippage or movement by the patient are additional reasons for using low speeds. The peripheral cutting speed of a disc would be represented by the following equation. Diameter of wheel X r X r.p.m. = Cutting Speed In the case of a g inch disc rotating at 1000 r.p.m. the equation would be g inch X r X 1000 = peripheral speed in inches per minute. In order to attain the same peripheral speed with a fine tapered cylindrical point that has a diameter of l/32 inch at its tip, it would be necessary to use 28,000 r.p.m. The concept of such a high speed involves too radical a transition from our present day thinking, and is actually unnecessary in view of the limited amount of cutting performed by this small instrument. The medium size tapered cylindrical points can be considered in similar fashion from the standpoint of quantitative tooth reduction. An efficient cutting speed is required however for the heavy tapered cylindrical point which has a tip of approximately 3/32 inch diameter. This heavy point is used extensively to straighten axial walls, create shoulders, round accessible line angles and carve occlusal anatomy. This involves a reduction of the dentine in bulk, whereas the refining of the shoulders, line angles, and bevels by means of the finer tapered points, requires considerably less cutting. The recommended speed should be approximately 9,500 r.p.m., which would provide the same peripheral speed for the heavy tapered point as 1,000 r.p.m. would provide for the periphery of a g inch disc. Technical improvements are constantly simplifying the problem of speed control at the dental unit. The use of hand instruments to plane tooth shoulders and the refining and smoothing of the preparation is essential in the quest for an even and properly placed finishing line.

400 CONCLUSIONS

A new concept of tooth preparation is made possible by the development ~ri an entirely new type of cutting instrument for the efficient removal of enamel by an undermining process. A step by step technique utilizing a minimum number of instruments greati! simplifies the procedure. The avoidance of excessive or dangerous handpiece speeds has been stressed tty means of a suitable evaluation of peripheral cutting speed. This allows for the safe and efficient use of each instrument, thereby producing less wear and tear on both the patient and the dentist. REFERENCE

1. G. V. Black’s Work on Operative Dentistry, Publishing Co., P. 137. 833

ST. JOHNS PL. BROOKLYN 16, N. Y.

Vol. 3, ed. 7, Chicago, 1936, Medico-Dental