Wax Patterns for Gold Inlays

Wax Patterns for Gold Inlays

WAX PATTERNS FOR GOLD INLAYS B y E u g e n e B o d m e r ,* D .D .S., M .S.D ., Chicago, 111. H E R E is no single step in the proc­ ess o f making a...

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WAX PATTERNS FOR GOLD INLAYS B y E u g e n e B o d m e r ,* D .D .S., M .S.D ., Chicago, 111.

H E R E is no single step in the proc­ ess o f making a gold inlay that does not require the utmost care and precision. Each step must be considered a link in a continuous chain, the success of which depends on the success or strength of each individual link. Although this paper will deal entirely with wax patterns, full cognizance is given to the importance o f the other steps in the proc­ ess o f making a past gold inlay. There are three principal methods of making wax patterns; namely, the direct, semi-indirect, or impression, method, and the indirect. In this paper, consideration of the direct method will include all technics in which w ax is directly adapted to the cavity and completely trimmed in the mouth. T h e semi-indirect method will include those technics in which the wax pattern is made on a casting investment model, developed from a copper band impression in inlay wax. A ny technic in which the wax pattern is developed on a stone or metallic die will be considered as indirect. T he question most frequently asked in relation to the methods o f preparing wax patterns is, W hich o f the three technics will give the most satisfactory results? The answer to this question m ay be sim­ plified by two statements o f fact. First of all, there is no technic that will com­ pensate for poor cavity preparation. Secondly, the only model spared all in­ accuracies due to the physical changes

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Read before the Section on Operative Den­ tistry at the Seventy-Sixth Annual Midwinter M eeting of the Chicago Dental Society, Feb­ ruary 14, 1940. * Assistant professor of operative dentistry, Northwestern University Dental School.

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of the materials used and the errors of the operator himself is the tooth with the properly prepared cavity. As the di­ rect method is the only technic utilizing the tooth as the actual model for the wax pattern, it must be considered the method o f choice. It is true that we are able to produce accurately fitting inlays by the other methods, but we must concede the fact that the chances o f error in the other methods are multiplied owing to the greater number o f steps involved in the procedures. Occasionally, we are confronted by mouth conditions or certain types of cavities or restorations that make it im ­ possible to obtain accurate, well-adapted and carved direct patterns. In such cases, the skilful employment of one or the other technic m ay be the best solu­ tion. There are two types of one-surface restorations. These are the occlusal and the buccogingival or linguogingival third. In the case of the occlusal restoration, the difficulties most frequently encoun­ tered are (1) the adjustment o f the inlay to occlusion and (2) removal of the in­ lay from the cavity without injury to the margins after the fit has been ascertained. Both of these difficulties m ay be avoided by using the direct technic in making the wax pattern. It is very simple to obtain correct occlusion in the w ax pattern. Interference is much more easily elim­ inated in wax than in gold. By attach­ ing the sprue wire in «the center of the occlusal surface, we obtain an excellent means of removing the pattern from the cavity. A fter the casting has been made, the sprue serves as a handle and makes it possible to try in and remove the inlay 120

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from the cavity without the slightest danger o f injuring the margins. Just be­ fore cementation, the sprue m ay be cut off and the point of its attachment to the inlay reduced so that there will be no interference in occlusion. Though we need not be concerned with the contact and occlusion in gin­ gival inlay restorations, they are no less simple than other types because of the in­ creased problem of their retention. In the preparation of these cavities, we must frequently use a minimum depth owing to the proximity of the pulp. Retention o f the inlay must therefore depend on sharp angles and almost parallel walls in the prepared cavity, and, last but not least, a well-adapted wax pattern which takes full advantage o f this retention. Inlays o f this type made from patterns lacking accurate adaptation are doomed to certain failure. As we need not be concerned with contact and occlusion, the most accurate technic for m aking wax patterns o f this type is the semi-indirect, or impression, method. O nce an accurate impression in w ax is obtained and poured in casting investment, there is no chance o f distor­ tion during the subsequent process of developing the w ax pattern. This is not true in the case of either the direct or the indirect technic. In both, distortion or loss of adaptation m ay occur during the process o f carving, owing to the lack of inherent retention in ' the prepared cavity, and because the smallness of these cavities often prevents the operator from supporting the w ax in case a displacing force must be applied in the process of carving. Inlays resulting from such pat­ terns fit loosely into the cavity and de­ pend almost entirely on cement for re­ tention. Two-surface cavities m ay be classified as to the direction o f withdrawal of the w ax pattern. Using this manner of classification, we find that there are two types, those prepared so that the pattern can be withdrawn in a labial or lingual

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direction, and those prepared to be w ith­ drawn in an occlusal or incisal direction. There is little choice as to the wax pat­ tern technic for the cavities in the first group. These patterns must be made by the direct technic as it is impossible to get enough of an impression of the tooth with the prepared cavity by the other methods. T h e best technic for making wax pat­ terns for two-surface cavities in which the direction o f withdrawal is in an oc­ clusal direction is the direct. In making a pattern by this technic, we have the surrounding structures to guide us in establishing the correct anatomic form, occlusion and proxim al contour. The margins too can be carefully examined. The only conditions which would contraindicate the use of the direct technic are ( i ) an overabundance or a thick, ropey condition of the saliva; (2) an over-active or large tongue; (3) ab­ sence o f the proxim al tooth, making the retention o f the w ax in the cavity diffi­ cult during the process of carving, and (4) very deep extension gingivally of the proximal portion o f the cavity. In order to more successfully cope with these con­ ditions, a technic should be employed which will cut dow n as much as possible the actual work in the mouth. Either the semi-indirect o r indirect technic will serve the purpose. A word of warning in relation to cavity preparation is appropriate at this time. A cavity prepared to meet the re­ quirements of convenience form for a direct w ax pattern will not necessarily meet the same requirements in case the pattern is to be m ade by one of the band impression methods. Should the tooth in question be quite bell-crowned, it will be noted that the proxim al walls must be made more divergent, to escape the undercuts created by the curvature of these walls. T h e latter condition is not a factor when the direct technic is used, as all o f the excess wax is removed from these curved m argins before any attempt

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is made to remove the w ax impression from the cavity. T h e indications for the various meth­ ods o f m aking wax patterns for threesurface restorations are, quite naturally, similar to those for the two-surface in­ lay. O f all the different types o f pat­ terns that can be m ade by the direct method, the one which most operators avoid is the three-surface pattern, be­ cause they are not certain that good adaptation can be obtained and they feel that too m uch chair time is consumed. This attitude can be changed by adop­ tion o f and adherence to a good direct technic. T h e few minutes more o f chair time that this technic m ay require will be recovered at the final appointment. Inlays m ade by the direct method re­ quire m uch less time fo r the adjustment o f occlusion and contact. The only contraindications for the direct m ethod are as fo llo w s: ( i ) a new bite relation in the process o f being established; (2) lack o f proximal teeth; (3) unfavorable mouth conditions, such as a ropey condition or overabundance of the saliva, and overactivity o f the tongue, and (4) inaccessibility, as in second and third m olar cases. In all o f these cases, one of the band impression technics is preferable. M y choice is the indirect method. A direct w ax pattern technic should be selected w h ich is simple and does not re­ quire elaborate equipment. It should be based upon sound physical principles. A technic which meets these requirements has been developed by R . H. Volland. There are three important points to be considered in m aking a w ax pattern by the direct m ethod; namely, adapting, tempering and contouring the wax. The first step in making the pattern is the preparation o f the wax, which must be so shaped that it will reach the most in­ accessible portion of the cavity first, in order to assure that the w ax will be properly adapted to that portion o f the cavity. F or the simple occlusal, proximo-

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clusal, proximolingual or gingival third cavity preparations, the best form for the w ax is that of a cone or wedge. In order to obtain this form, a sufficiently large piece o f w ax should be slowly warmed by rotation through the flame o f a Bun­ sen burner. Care should be exercised not to melt or overheat the wax, to avoid loss of certain desirable and necessary properties. For the mesioclusodistal, or mesiolinguodistal cavity, the best plan is to heat the w ax in the previously described manner, and then mold it to the shape o f a fan rather than that o f a cone or wedge. B y cutting a notch in the center o f this wax, the form o f a fork will be acquired, enabling the operator to better adapt the w ax to the two gingival walls simultaneously. It is highly inadvisable to bend a straight piece o f w ax into a U shape with the intention o f using this form in securing adaptation in a threesurface cavity, the reason being that too m any strains are set up in the wax that cannot later be relieved. T h e second consideration in the prep­ aration o f the w ax should be met during the above-described process o f manipu­ lation. A n attempt should be made to develop a hard inner cone o f wax, cov­ ered by a warmer and softer outer layer. T h e reason for this procedure is two­ fold : 1. T h e hard inner cone will act as a plunger, carrying the softer w ax on the surface into the fine lines o f the cavity. 2. T h e w ax comprising the inner cone will form the greatest bulk in the finished pattern. This wax, being just slightly warmed, will undergo less temperature change as it cools to mouth temperature and subsequently to room temperature. This treatment of the w ax will insure a pattern whose adaptation to the walls of the cavity will not be lost by shrinkage of the wax, due to its cooling through a wider range o f temperature. W hen the w ax has been properly pre­ pared, it is forced into the cavity and driven to place with great pressure. The

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patient should then be instructed to close in centric position and m aintain the pres­ sure sufficiently long to insure that the w ax has congealed and is no longer flow­ ing. This will eliminate much o f the occlusal excess o f w ax and establish maximum cusp height. T h e w ax should next be molded into the embrasures with the fingers, after which the patient is instructed to close again. T h e wax, which b y this time has cooled to mouth temperature, should be irrigated with water the temperature of which approximates i2 0 ° F . This re­ warming o f the w ax results in dissipation o f the molecular strain and also causes some degree o f expansion, later followed b y some contraction. However, only a slight amount o f m anipulation is neces­ sary to correct the adaptation. T he strains developed during this secondary adaptation are so small that their effect on the w ax pattern is negligible. T h e carving of the direct pattern is a simple procedure. First o f all, the oc­ clusal contour must be developed. This is done with a blunt right-angled bur­ nisher the working face o f which is 1.5 mm. in diameter. This instrument is carried along the inclined planes of the tooth structure, projecting the contour of the tooth into the wax. Refinements in the occlusal carving m ay be made with a beaver-tail burnisher or other suitable instrument. In the case o f lingual dovetail or mesiolinguodistal patterns, the lingual surface and embrasures should be com­ pleted first for the reason that the excess w ax forced through and molded over the labial surface locks the w ax firmly in the cavity and permits forces to be applied in a lingual direction, which would nor­ m ally dislodge the pattern. A fter the lingual portion has been completed, the w ax m ay be stabilized in the cavity, while the labial portion is being trimmed, by labial pressure on a ball o f cotton held against the lingual surface o f the pattern. A fter the occlusal surface has been

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completed, the m ajor excess o f wax is removed from the buccal and lingual embrasures, being trimmed out roughly with a semisharp instrument such as the W ard w ax carver No. 2. Care should be taken not to trim the w ax to the enamel margin. Excess w ax should never be cut from an enamel margin with a sharp or semisharp instrument. W ax is elastic and m ay be quite easily sprung out of the cavity as it feeds onto the blade of the cutting instrument. A fter the instrument has passed over the margin, the tension is released and the w ax resumes its nor­ mal position in the cavity, slightly short o f the margin. When the m ajor excess has been trimmed from the embrasures, the final contouring is done with a “ shepherd hook” exploring tine, No. 23, which is used as a burnisher. A gain the instru­ ment is allowed to follow the contour of the remaining tooth structure, projecting that contour into the wax. T h e w ax pattern is now complete with the exception of a slight excess that m ay remain gingivally to the contact point. This is removed by means of number A sewing silk, used as one would pass a ligature between the teeth. T h e first cut made with the thread is gingivally and toward the proximal tooth. Just enough clearance is obtained to free the wax from contact with the adjoining tooth. In making the second cut, the silk thread is carried occlusogingivally to a point gingivally from the desired contact point o f the wax pattern with the proximal tooth. Pressure is then brought against the w ax in the gingival direction. T h e silk thread will act as a lancet, cutting aw ay any gingival excess. T h e final step in the preparation of the w ax pattern is the polishing o f the prox­ imal surface, which is done with a plain linen or silk strip. In polishing the prox­ im al surface, care must be taken that the strip tends to burnish the w ax toward the margin o f the cavity. In other words, in polishing and burnishing toward the

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lingual margin, the strip should be held away from the buccal margin, and vice versa. The occlusal portion o f the pat­ tern m ay be polished w ith dry cotton tightly wrapped about the beaks of cot­ ton pliers. During the foregoing procedure, the chips and excess w ax are washed away with water from a glass, the temperature o f the water being originally 120° F. T h e frequent bathing o f the pattern with this warm water has a tendency to relieve all deep and superficial strains that may have developed in the w ax during its manipulation. T h e water gradually loses its heat until, by the end of the process,

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it probably reaches room temperature. This room-temperature water has a tendency to reduce the mouth tempera­ ture somewhat, so that when the pat­ tern is removed, the change in degrees of temperature is slight. T h e resulting pat­ tern will be one in which all strains of any consequence have been dissipated. T h e pattern m ay be removed from the cavity either with an exploring tine or by attaching the sprue wire in the center of the occlusal surface. It is inadvisable to attach the sprue wire in the mouth if access is difficult or i f the occlusal por­ tion of the pattern is relatively small. 3 11 East Chicago Avenue.

SILVER NITRATE AND ZINC OXIDE IN THE TREATMENT OF CHILDREN’S TEETH By F. W . G r a h a m , J r ., Morris, 111. A S far back as one hundred years ago and prior to the discovery o f ar­ senical paste for the devitalization of teeth, silver nitrate was used for that purpose. As early as 1846, the value of silver nitrate as an aid in controlling dental caries was recognized. In 1917, Percy R. Howe, present director o f the Forsyth Dental Infirmary, o f Boston, in­ troduced a new solution o f silver nitrate, a saturated aqueous solution to which was added strong ammonia water (37 per cent). As a reducing agent, he recommended a 25 per cent solution of formalin. Since 1917, this solution of silver nitrate has become popularly known as “ Howe’s Solution.” Directions for making the solution can be obtained Read before the Section on Children’s Den­ tistry at the Seventy-Sixth Annual Midwinter M eeting of the Chicago Dental Society, Feb­ ruary 13, 1940.

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from an article entitled “Ammoniacal Silver Nitrate.” 1 T he one fact concerning the use of this solution to which I wish to call your attention is that it is an alkaline solu­ tion, differing in this from the ordinary silver nitrate solution. Silver nitrate, in its ordinary state, is an acid salt. When dissolved in water, it contains a certain amount of free nitric acid, which is it­ self an irritant of considerable degree. By alkalizing this solution with am­ monium hydroxide, we produce a deli­ cately balanced solution having practic­ ally no irritating action. T h e chief advantage to be obtained from the use o f Howe’s Solution is its ability to sterilize disintegrated dentinal structure and to neutralize the acid reaction of dental caries. Sterilization results from the action of the metallic silver upon the proteins of the bacteria in the decayed