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Cavity Preparation and Inlay Casting*
578
The Journal of the American D ental Association
used by all dentists. W h y not let the public know these things by advertising? T h e newspaper representative seemed amazed to hear of the relation of health to a bridge or set of teeth beyond its masticatory function. T h e degree of doctor of dental surgery authorizes one to be a teacher, a teacher in a “holy sanctuary” known as a dental office. A dentist is a teacher of hygiene, the preservation of health and the pre vention of decay, as well as of methods adequate to reestablish normal function or as nearly so as conditions w ill permit. M aterials are a secondary consideration, and if they are not so considered, the dentist is prostituting his position, and competition becomes certain. Dentists are not competitors, but a real competi tion, created by the dentist himself, may arise in the future from the encroach ment of outside craftsmen. T h e age is fraught w ith greatness and equal dangers. I t is well to reflect on
the old saying that “history repeats it self.” W e do not desire to return to conditions outgrown. An evil day is set aside only by the heeding of wisdom. Professional lore is to be found with those members of our profession who are rendering an honest service created by teaching and doing. T h e dentist, physician and others of the recognized profession have nothing to sell. T h eir services are in demand because of their reputation. T h e perma nent practice demanding substantial fees has been built upon a firm foundation. A professional reputation is created be cause of the ability to educate, to advise and to do. A beginner’s reputation rests in the fact that he has been graduated and granted a license to practice, that he conducts himself properly, which in cludes introducing himself ethically, gives instruction in prevention, advises hon estly and renders an adequate service.
C A V IT Y P R E P A R A T IO N A N D IN L A Y C A ST IN G * By EMORY O. WEST, D.D.S., Des Moines, Iowa N considering cavity preparation for the cast gold inlay, the same principles w ill be followed as those set forth by the late G. V. Black. I t is quite obvious th at those attem pting cavity preparation should have a thorough knowledge of the structural elements of the enamel and dentin and of the anatomy of the pulp. H aphazard cavity preparation is per
I
*Read before the Section on Operative Den tistry, Materia Medica and Therapeutics at the Seventy-Third Annual Session of the Ameri can Dental Association, Memphis, Tenn., Oct. 21, 1931.
Jour. A . D. A ., April, 1982
haps the commonest cause of failure of the restoration, no m atter w hat filling material is used. In the preparation of cavities for in lays, the same fundam ental principles are followed as those of general cavity prep aration. In this brief review of cavity preparation, Class I I cavities will be considered. In studying the outline form, we should have a mental picture of the area of tooth surface to be included within the enamel margins of the finished cavity. T h e cutting of these lines is the first
IV est— Cavity Preparation and Inlay Casting thing to be considered and accomplished. In this procedure, the cavity m argin is extended to an area close to the angle, buccal and lingual. T h is is usually an immune area, or an area less liable to decay, which will be cleansed by the ex cursion of food during mastication. I t is very seldom necessary to extend the cav ity beyond the axial angles unless the tooth is in malposition. Resistance form is the shape given to a cavity which will best afford a seat for the filling to w ithstand the stress of mas tication. I t should be at right angles to the force of stress exerted, or, usually, at right angles w ith the long axis of the tooth. W ith inlay cavity preparation, good resistance form constitutes a large part of the retention form. T h is is in the form of a flat seat or floor. In the preparation of cavities for bridge abut ments, there is a greater need for better resistance form. T h e retention form is the form given to the cavity to prevent the displacement of the filling by the tipping forces that may be brought against it. F o r inlay cavity preparation, it is not permissible to have the slight undercuts that we have in general cavity preparation. T h e re tention, which is usually in the form of a dovetail, should extend into the devel opmental grooves tow ard the buccal and lingual margins. Some operators choose to use small pins as an added means of retention. T h e convenience form is accomplished by having the cavity walls slightly di vergent, in a degree to safely remove the wax pattern w ithout distortion or break ing. T h is is a most im portant step, as any slight undercut may cause the w ax pat tern to be slightly distorted in removal, and hence the inlay w ill not fit the cavity. Lack of serious consideration regarding the convenience form of cavity prepara
579
tion for inlays is no doubt responsible for the failure of many restorations to fit. A t this stage of cavity preparation, there will be little or no carious dentin remaining. I f a small amount of carious dentin remains in the cavity, it should be removed, and the undercuts should be filled w ith cement so that the cavity w ill shed a wax pattern w ithout distortion. If the undercuts are large, usually a reconditioning of the cavity is necessary. T h e finish of the enamel w all is closely associated w ith the convenience form. T h e cutting of the enamel in carrying out the convenience form usually leaves a strong enamel, and there is little left to do except the beveling of the cavosurface angle. T h e purpose of the bevel ing of the cavosurface angle is to cut away any loose ends of enamel rods that might afterw ard fall away and leave an imperfect margin. T h e beveling is ef fected w ith a chisel, enamel hatchet and gingival margin trimmer. Tapered stones are very effective in accomplishing the beveling of the occlusal portion. T h e buccal and lingual walls of the proximal portion are now disked w ith true run ning cuttlefish disks. T h e reason for doing this is that it will produce a very smooth cavity w all. T h e cavity should now have a definite clear-cut bevel throughout the entirety so that when the wax pattern is taken, it will show a well defined bevel in the wax. In the removal of a number of inlays, both from the mouth and from extracted teeth, it is surprising to see the great number in which no attem pt was made to effect a bevel of the cavosurface of the gingival wall. T h is alone foretells fail ure of the restoration in mouths of aver age hygienic condition. F or those who wish to familiarize themselves w ith the principles of cavity preparation, it would be well to prepare cavities in a large num
580
The Journal of the American D ental Association
ber of extracted teeth, plaster teeth and porcelain teeth. In the principles of cavity preparation, it is the neglect of details that very.often is the direct cause of undesired results when the inlay is placed in the cavity. T here has been much said about the con traction of gold as an outstanding cause of the poor fit of inlays, but it is a known fact that all the schematic procedures of the casting process w ill not compensate for poor cavity preparation. Proper cav ity preparation cannot be too strongly emphasized. Viewing another aspect of inlay fail ures, especially bridge abutments, we are prone to pass hasty judgm ent in our own minds as to the type of restoration to be used. In the mouth extremely susceptible to caries or one negligent of mouth hy giene, we have seen large inlays and three-quarter crowns a failure, not be cause of the restoration itself, but because of the unclean condition and the occur rence of further decay. In such cases in the posterior part of the mouth, is it not better judgm ent to place a well-fitting banded crown, or, better still, to make a full cast crown for the restoration? In securing a wax pattern, it is nec essary to keep in mind the characteristics of w ax: expansion on heating, contrac tion on cooling and elasticity while warm. From a properly prepared cavity, we m ust have a well-adapted w ax pattern or the restoration w ill be a failure. In all compound approximal, three-quarter crown, full crown and large Class I I cavities, it is difficult to control the flow of wax unless a m atrix is used. T h e following systematic procedure may be used in securing the wax pattern. 1. Fit the matrix around the tooth so that it slides on and off easily. Trim the occlusal end of the matrix so that the patient may close in centric relation without striking the matrix. 2. Heat the wax in an open flame, being
careful not to melt the surface. M old it be tween the thumb and finger and work it into the desired shape. Fill the matrix, leaving the more firm portion of w ax in the occlusal end of the matrix to serve as a plunger. 3. Place the matrix on the tooth and press it gingivally w ith heavy thumb pressure. H ave patient close in centric relation. Ask the patient to open, placing a small piece of rubber dam on the wax, and to close again. Maintain this pressure for one or two minutes. 4. Chill the w ax w ith cold water or air and carve the occlusal portion without removing the matrix. A s long as the matrix is in place, the operator has complete control of the wax. 5. Release the matrix and finish the buccal and lingual margins with w ax carvers. A large portion of the surplus w ax at the gingi val is removed w ith w ax carvers. 6. Remove any excess w ax at the gingival margin with fine cuttlefish strips and silk taf feta tape. T he tape w ill burnish the margins so that a very smooth pattern can be had. 7. Mark the position of the contact point on the marginal ridge of the w ax pattern so that it can be added after the pattern is removed.
In placing the sprue, it is better not to insert a heated pin in the wax pattern, but to add a small amount of wax at the desired point and attach the pin into it. By pressing the heated pin into the wax pattern, we invite another hazard of dis tortion. I t is best to place the sprue at a point somewhere near the center of the pattern, thereby giving the gold a more equal flow when casting. W ax patterns should not be allowed to remain any length of time before being invested. By placing the investment upon the wax pattern, any changes that otherwise might take place will be eliminated. T h e sprue pin is now made fast to the base, the pattern is washed w ith soap or a solution of “gold dust” washing pow der, washed again w ith clear w ater (room tem perature) and thoroughly dried. Proper proportions of investment and w ater are placed in a bowl and me chanical spatulation or hand spatulation is applied w ith the bowl upon a vibrator. T his will force the minute air pockets
W est— Cavity Preparation and Inlay Casting from the mix before it is applied to the wax pattern. In applying investment to a wax pattern, it is best to push a mass of investment ahead of the brush so th at the junction of the investment and the wax can be seen at all times. T h is w ill lessen the hazards of trapping air bubbles. If a heavy dense mix is used for the core, it is necessary to vibrate the investment into the core. In producing a core w ith a thin mix of investment, the am ount of expansion obtained is not so great as that with a dense core. A thick dense mix of investment for the core to cover all cav ity surfaces, and a less dense mix to cover the remaining surfaces of the pattern, to gether w ith w ax expansion and therm al expansion of the investment, w ill give the greatest expanded mold for casting in our present casting procedure. In the process of eliminating the wax, the time required w ill depend on the size of the w ax pattern and the tempera ture of the furnace. Occlusal, mesioclusodistal and three-quarter crown types of patterns should be burned out at a tem perature of from 600 to 800 F., for tw enty minutes. T h e full crown type is allowed to remain in the heat for twentyfive minutes. M ore therm al expansion of the investment may be obtained if the mold temperature is carried to 1200 F. T h e higher tem perature is usually neces sary in dimensional casting. I t is well to use an investment w ith a low plaster content, as this w ill tolerate a high degree of heat, and the high heat gains more expansion of the refractory material. M ost low plaster content in vestments expand gradually in propor tion to the degree of heat applied up to the maximum expansion. T h e mold should be placed in the cast ing machine immediately on removal from the furnace and the casting made. If it is allowed to cool before the casting
581
is made, the therm al expansion of the investment is lost. Therefore, castings should be made at the highest tempera ture obtained in the furnace. IN LA Y BRIDGE A T T A C H M E N T S
T h e use of inlays as bridge abutments in the posterior part of the mouth, as a standard, is problematic and open to dis cussion. In mouths of average hygienic condition, the inlay bridge attachment of proper construction has prophylactic ad vantages over the gold shell crown. In some instances, the inlay attachment may loosen on one end of the bridge, and the patient be unaw are of this fact until caries has made considerable progress. I t is then necessary to make another abut ment and, many times, add another tooth to the bridge. If the best results are to be obtained from bridge abutments, they must be inspected often. I have had the unpleasant, yet educational, experience of having some inlay bridge abutments of the mesioclusodistal type become loosened. T h e three-quarter crown of fers much more success, yet some of these lose anchorage on short teeth. T h e loose abutments have been replaced with full cast crowns which seem to hold very firmly. M y experience w ith full cast crowns as bridge abutments has been very satisfactory. W hen we see crowns th at have been giving good service for from twenty to twenty-five years, we sometimes wonder whether the large inlay restoration can give similar service. In conclusion, the following principles must be incorporated in any inlay pro cedure for the operator to get consistent results: 1. T here must be proper cavity prep aration or defeat must be acknowledged in the first step. 2. A well-adapted wax pattern is nec essary, with definite margins.
582
The Journal of the American D ental Association
it would be assumed from Dr. W est’s paper that all inlay waxes have the same amount of thermal expansion when subjected to the same changes in temperature. I wish to call your attention to the extensive experiments on these waxes made by the United States Bureau of Standards. T he charts shown by them in the January, 1931 issue of T h e J o u r n a l indi cate that almost every inlay w ax has a d if ferent expansion curve. T he Bureau advo cates the printing on each package of w ax or investment of the characteristics of that certain 608 Equitable Building. brand of material. W ithout definite knowl DISCUSSION edge of the expansion produced by each degree H. D. Coy, Hamburg, Iow a: T h e three of temperature, it is difficult to determine how requisites for success in the making of a cast much w ill be necessary in compensating for ing are: (1) a w ell prepared and w ell finished the shrinkage of the gold. Standardization of cavity; (2) a properly fashioned w ax pattern these waxes and investments and the printing which fits the cavity, and (3) a casting which of the thermal expansion curve, with the other exactly reproduces the w ax pattern. Failure physical data on each package of material, to give due consideration to any one of these w ill do much toward sim plifying the casting principles w ill inevitably result in the failure process. T h e shrinkage of gold has been of the finished restoration. Conservation of proved, by extensive experiments, to be con tooth structure and the protection of the pulp stant at 1.25 per cent by volume of the gold. should be given first consideration in the prep By expanding the mold or pattern or both to, aration of any cavity. T h e careful operator compensate for this shrinkage, we are able to w ill have little difficulty in preparing cavities cast to dimension. T his w ill not be difficult of the Black type without endangering the life to accomplish if the dentist is able to secure of the pulp. T hey are boxlike, w ith a flat his favorite brand of w ax and investment with gingival seat and definite angles. I f the same the thermal expansion curve accurately de painstaking care were required in the making termined and printed on the package by the of an inlay restoration as that employed by manufacturer. the gold foil operator in his work, the inlay J. J. Trams, A nn Arbor, Mich.: If more would more nearly approach the ideal already men were able to make perfect w ax patterns reached by the foil filling. A w ax pattern in the mouth, a more excellent service to our may be made of a very poorly prepared cavity. patients would result, but like many other Failure in this first requisite dooms many inlay operative procedures, this one has its lim ita restorations almost before they are begun. tions. T he remote portion of the mouth, the W ith the vast number of scientific facts now interference of the restless tongue, thick cheeks available concerning the physical characteris and ever-present saliva, the difficult access tics of the materials used in dental casting, w e and poor light render direct pattern making find it comparatively easy to cast to any de next to impossible. Deep cervical margins w ill sired dimension. There would seem to be little extend below any point that can be reached to justify argument in favor of any type of with strips, and finishing these margins with cavity preparation which does not include the instruments by the sense of touch alone in a principles of a flat seat and a definite angle. highly sensitive area where hemorrhage is Dr. W est states that most investments of high sure to result leaves too much to the imagina silica and low plaster content reach their max tion. A good amalgam model, in the hands, imum expansion at about 1200 F. Rather than correctly articulated, with good light and none to carry the mold to a temperature of only of the sources of interference present, gives 800 F., as recommended, it would seem to be the operator great advantages. I therefore beg a better procedure to expand it to the max to amend what Dr. W est has said concerning imum, thereby relying less on the expansion inlay cavity preparation by suggesting a modi of the frail and plastic w ax pattern. As fication of the accepted cavity form to facili regards the use of w ax expansion as an aid tate the withdrawal of a modeling compound in overcoming the shrinkage of gold from impression without distortion. T h is modifica the molten state to the point of crystallization,
3. A conscientious application of the investment to the wax pattern is required. 4. Elimination and casting m ust be a systematic procedure. In a word, possibly the greatest point in any inlay technic is system. H ap hazard methods afford little in return. In any line of endeavor, system tends to success and few are successful w ithout it.
W est— Cavity Preparation and. Inlay Casting tion in cavity form would consist merely in taking a slice from the mesial and distal convexity around which the impression ma terial wraps itself and, when chilled, cannot be withdrawn without distortion or fracture. To be sure that I am not misunderstood, allow me to state clearly that this modification applies only when the individual operator feels that he can render a better service by employ ing the indirect method of operating. Dr. W est has described an almost universally ap proved cavity form with flaring buccal and lingual w alls which lends itself perfectly to the direct method of operating and offers com pensation for the shrinkage of the cast. Where the direct method of operating can be em ployed advantageously, this cavity form can not be improved on from the standpoint of esthetics or strength, but it is not adapted to indirect work. A s dentists, let us always re member that w e are rendering primarily a health service; and w hile the perfect sealing of a cavity is very important, the most essen tial factor in these operations lies in givin g the best possible protection to the vital pulp. Thousands of pages of literature in our dental magazines and hundreds of hours of discussion on the floors of our dental meetings have been devoted to the question of the pulpless tooth. Negligence on the part of the laity and opera tive mistakes on the part of the operative dentist are in the main responsible for the condition of these dead, dying or degenerating pulps. W e cannot be held accountable for the negligence of the laity, for most intelligent people are learning something of the dire re sults of foci of infection in the mouth. Neither can w e evade the responsibility for operative mistakes that lead eventually to the death of the pulp. It therefore seems to me that any consideration of cavity preparation for the gold inlay is not complete without some ref erence to the protection of the pulp. If a frac tion of the time that has been devoted to the study of the pulpless tooth problem and the treatment and filling of some of these root canals had been spent in earlier operations on these teeth, in conscientious cavity prepara tion, disinfection, the sealing of open tubules in freshly cut dentinal surfaces and the placing of nonconductors to protect the pulp from oftrepeated thermal shock, the number of dying and degenerating pulps would be reduced
583
tremendously. W hen the generally accepted principles given to us by Dr. Black were worked out, they took into consideration the materials with which the restorations were to be made. T hat is axiomatic. Those materials were gold foil and amalgam. Since then, the casting technic has been evolved and greatly improved, so that materials many times stronger are now in common use; which ren ders deeply cut cavities, for retention only, no longer necessary or defendable. During those days when gold foil and amalgam were used to fill teeth, brick and mortar were used to construct buildings that rarely were higher than five or six stories. Now , there seems to be no limit to the height of buildings. T he fundamental principles of architecture have not changed, but the form and details have been modified to conform with the materials in use. T h e fundamental principles of cavity preparation need not be changed in order that we may take advantage of the greater strength of filling materials by using thinner walls in our inlays and thus protecting the pulp from thermal shock. It is not necessary that we eleminate a definite cervical floor, but it should be narrower mesiodistally and be cut with greater precision. T he resistance form on the occlusal surface can even be widened if it seems necessary, but let us be careful about encroaching too closely on the horns of the pulps or the recessional lines because of their peculiar histologic structure. Nothing has has been said about investments, their phys ical properties, manipulation or behavior under heat. Nothing has been said about alloys, their adaptability or behavior in the mouth or heat treatment. Nothing has been said about waxes, their physical properties or lim its in expansion. In fact, nothing has been said about dimensional casting, although my interest in these topics has been greatly stim ulated by my work w ith the Bureau of Stand ards Research Committee during the past year. I appeal to you for more conservative cutting in our cavity preparation, finer de tails in line angles and point angles, elimina tion of frictional heat, proper sealing of dentinal tubules, conscientious removal of de cay, disinfection of cavity w alls and the employment of cement as a nonconductor in thermal change in all deep cavities.
The Journal of the American D ental Association
used by all dentists. W h y not let the public know these things by advertising? T h e newspaper representative seemed amazed to hear of the relation of health to a bridge or set of teeth beyond its masticatory function. T h e degree of doctor of dental surgery authorizes one to be a teacher, a teacher in a “holy sanctuary” known as a dental office. A dentist is a teacher of hygiene, the preservation of health and the pre vention of decay, as well as of methods adequate to reestablish normal function or as nearly so as conditions w ill permit. M aterials are a secondary consideration, and if they are not so considered, the dentist is prostituting his position, and competition becomes certain. Dentists are not competitors, but a real competi tion, created by the dentist himself, may arise in the future from the encroach ment of outside craftsmen. T h e age is fraught w ith greatness and equal dangers. I t is well to reflect on
the old saying that “history repeats it self.” W e do not desire to return to conditions outgrown. An evil day is set aside only by the heeding of wisdom. Professional lore is to be found with those members of our profession who are rendering an honest service created by teaching and doing. T h e dentist, physician and others of the recognized profession have nothing to sell. T h eir services are in demand because of their reputation. T h e perma nent practice demanding substantial fees has been built upon a firm foundation. A professional reputation is created be cause of the ability to educate, to advise and to do. A beginner’s reputation rests in the fact that he has been graduated and granted a license to practice, that he conducts himself properly, which in cludes introducing himself ethically, gives instruction in prevention, advises hon estly and renders an adequate service.
C A V IT Y P R E P A R A T IO N A N D IN L A Y C A ST IN G * By EMORY O. WEST, D.D.S., Des Moines, Iowa N considering cavity preparation for the cast gold inlay, the same principles w ill be followed as those set forth by the late G. V. Black. I t is quite obvious th at those attem pting cavity preparation should have a thorough knowledge of the structural elements of the enamel and dentin and of the anatomy of the pulp. H aphazard cavity preparation is per
I
*Read before the Section on Operative Den tistry, Materia Medica and Therapeutics at the Seventy-Third Annual Session of the Ameri can Dental Association, Memphis, Tenn., Oct. 21, 1931.
Jour. A . D. A ., April, 1982
haps the commonest cause of failure of the restoration, no m atter w hat filling material is used. In the preparation of cavities for in lays, the same fundam ental principles are followed as those of general cavity prep aration. In this brief review of cavity preparation, Class I I cavities will be considered. In studying the outline form, we should have a mental picture of the area of tooth surface to be included within the enamel margins of the finished cavity. T h e cutting of these lines is the first
IV est— Cavity Preparation and Inlay Casting thing to be considered and accomplished. In this procedure, the cavity m argin is extended to an area close to the angle, buccal and lingual. T h is is usually an immune area, or an area less liable to decay, which will be cleansed by the ex cursion of food during mastication. I t is very seldom necessary to extend the cav ity beyond the axial angles unless the tooth is in malposition. Resistance form is the shape given to a cavity which will best afford a seat for the filling to w ithstand the stress of mas tication. I t should be at right angles to the force of stress exerted, or, usually, at right angles w ith the long axis of the tooth. W ith inlay cavity preparation, good resistance form constitutes a large part of the retention form. T h is is in the form of a flat seat or floor. In the preparation of cavities for bridge abut ments, there is a greater need for better resistance form. T h e retention form is the form given to the cavity to prevent the displacement of the filling by the tipping forces that may be brought against it. F o r inlay cavity preparation, it is not permissible to have the slight undercuts that we have in general cavity preparation. T h e re tention, which is usually in the form of a dovetail, should extend into the devel opmental grooves tow ard the buccal and lingual margins. Some operators choose to use small pins as an added means of retention. T h e convenience form is accomplished by having the cavity walls slightly di vergent, in a degree to safely remove the wax pattern w ithout distortion or break ing. T h is is a most im portant step, as any slight undercut may cause the w ax pat tern to be slightly distorted in removal, and hence the inlay w ill not fit the cavity. Lack of serious consideration regarding the convenience form of cavity prepara
579
tion for inlays is no doubt responsible for the failure of many restorations to fit. A t this stage of cavity preparation, there will be little or no carious dentin remaining. I f a small amount of carious dentin remains in the cavity, it should be removed, and the undercuts should be filled w ith cement so that the cavity w ill shed a wax pattern w ithout distortion. If the undercuts are large, usually a reconditioning of the cavity is necessary. T h e finish of the enamel w all is closely associated w ith the convenience form. T h e cutting of the enamel in carrying out the convenience form usually leaves a strong enamel, and there is little left to do except the beveling of the cavosurface angle. T h e purpose of the bevel ing of the cavosurface angle is to cut away any loose ends of enamel rods that might afterw ard fall away and leave an imperfect margin. T h e beveling is ef fected w ith a chisel, enamel hatchet and gingival margin trimmer. Tapered stones are very effective in accomplishing the beveling of the occlusal portion. T h e buccal and lingual walls of the proximal portion are now disked w ith true run ning cuttlefish disks. T h e reason for doing this is that it will produce a very smooth cavity w all. T h e cavity should now have a definite clear-cut bevel throughout the entirety so that when the wax pattern is taken, it will show a well defined bevel in the wax. In the removal of a number of inlays, both from the mouth and from extracted teeth, it is surprising to see the great number in which no attem pt was made to effect a bevel of the cavosurface of the gingival wall. T h is alone foretells fail ure of the restoration in mouths of aver age hygienic condition. F or those who wish to familiarize themselves w ith the principles of cavity preparation, it would be well to prepare cavities in a large num
580
The Journal of the American D ental Association
ber of extracted teeth, plaster teeth and porcelain teeth. In the principles of cavity preparation, it is the neglect of details that very.often is the direct cause of undesired results when the inlay is placed in the cavity. T here has been much said about the con traction of gold as an outstanding cause of the poor fit of inlays, but it is a known fact that all the schematic procedures of the casting process w ill not compensate for poor cavity preparation. Proper cav ity preparation cannot be too strongly emphasized. Viewing another aspect of inlay fail ures, especially bridge abutments, we are prone to pass hasty judgm ent in our own minds as to the type of restoration to be used. In the mouth extremely susceptible to caries or one negligent of mouth hy giene, we have seen large inlays and three-quarter crowns a failure, not be cause of the restoration itself, but because of the unclean condition and the occur rence of further decay. In such cases in the posterior part of the mouth, is it not better judgm ent to place a well-fitting banded crown, or, better still, to make a full cast crown for the restoration? In securing a wax pattern, it is nec essary to keep in mind the characteristics of w ax: expansion on heating, contrac tion on cooling and elasticity while warm. From a properly prepared cavity, we m ust have a well-adapted w ax pattern or the restoration w ill be a failure. In all compound approximal, three-quarter crown, full crown and large Class I I cavities, it is difficult to control the flow of wax unless a m atrix is used. T h e following systematic procedure may be used in securing the wax pattern. 1. Fit the matrix around the tooth so that it slides on and off easily. Trim the occlusal end of the matrix so that the patient may close in centric relation without striking the matrix. 2. Heat the wax in an open flame, being
careful not to melt the surface. M old it be tween the thumb and finger and work it into the desired shape. Fill the matrix, leaving the more firm portion of w ax in the occlusal end of the matrix to serve as a plunger. 3. Place the matrix on the tooth and press it gingivally w ith heavy thumb pressure. H ave patient close in centric relation. Ask the patient to open, placing a small piece of rubber dam on the wax, and to close again. Maintain this pressure for one or two minutes. 4. Chill the w ax w ith cold water or air and carve the occlusal portion without removing the matrix. A s long as the matrix is in place, the operator has complete control of the wax. 5. Release the matrix and finish the buccal and lingual margins with w ax carvers. A large portion of the surplus w ax at the gingi val is removed w ith w ax carvers. 6. Remove any excess w ax at the gingival margin with fine cuttlefish strips and silk taf feta tape. T he tape w ill burnish the margins so that a very smooth pattern can be had. 7. Mark the position of the contact point on the marginal ridge of the w ax pattern so that it can be added after the pattern is removed.
In placing the sprue, it is better not to insert a heated pin in the wax pattern, but to add a small amount of wax at the desired point and attach the pin into it. By pressing the heated pin into the wax pattern, we invite another hazard of dis tortion. I t is best to place the sprue at a point somewhere near the center of the pattern, thereby giving the gold a more equal flow when casting. W ax patterns should not be allowed to remain any length of time before being invested. By placing the investment upon the wax pattern, any changes that otherwise might take place will be eliminated. T h e sprue pin is now made fast to the base, the pattern is washed w ith soap or a solution of “gold dust” washing pow der, washed again w ith clear w ater (room tem perature) and thoroughly dried. Proper proportions of investment and w ater are placed in a bowl and me chanical spatulation or hand spatulation is applied w ith the bowl upon a vibrator. T his will force the minute air pockets
W est— Cavity Preparation and Inlay Casting from the mix before it is applied to the wax pattern. In applying investment to a wax pattern, it is best to push a mass of investment ahead of the brush so th at the junction of the investment and the wax can be seen at all times. T h is w ill lessen the hazards of trapping air bubbles. If a heavy dense mix is used for the core, it is necessary to vibrate the investment into the core. In producing a core w ith a thin mix of investment, the am ount of expansion obtained is not so great as that with a dense core. A thick dense mix of investment for the core to cover all cav ity surfaces, and a less dense mix to cover the remaining surfaces of the pattern, to gether w ith w ax expansion and therm al expansion of the investment, w ill give the greatest expanded mold for casting in our present casting procedure. In the process of eliminating the wax, the time required w ill depend on the size of the w ax pattern and the tempera ture of the furnace. Occlusal, mesioclusodistal and three-quarter crown types of patterns should be burned out at a tem perature of from 600 to 800 F., for tw enty minutes. T h e full crown type is allowed to remain in the heat for twentyfive minutes. M ore therm al expansion of the investment may be obtained if the mold temperature is carried to 1200 F. T h e higher tem perature is usually neces sary in dimensional casting. I t is well to use an investment w ith a low plaster content, as this w ill tolerate a high degree of heat, and the high heat gains more expansion of the refractory material. M ost low plaster content in vestments expand gradually in propor tion to the degree of heat applied up to the maximum expansion. T h e mold should be placed in the cast ing machine immediately on removal from the furnace and the casting made. If it is allowed to cool before the casting
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is made, the therm al expansion of the investment is lost. Therefore, castings should be made at the highest tempera ture obtained in the furnace. IN LA Y BRIDGE A T T A C H M E N T S
T h e use of inlays as bridge abutments in the posterior part of the mouth, as a standard, is problematic and open to dis cussion. In mouths of average hygienic condition, the inlay bridge attachment of proper construction has prophylactic ad vantages over the gold shell crown. In some instances, the inlay attachment may loosen on one end of the bridge, and the patient be unaw are of this fact until caries has made considerable progress. I t is then necessary to make another abut ment and, many times, add another tooth to the bridge. If the best results are to be obtained from bridge abutments, they must be inspected often. I have had the unpleasant, yet educational, experience of having some inlay bridge abutments of the mesioclusodistal type become loosened. T h e three-quarter crown of fers much more success, yet some of these lose anchorage on short teeth. T h e loose abutments have been replaced with full cast crowns which seem to hold very firmly. M y experience w ith full cast crowns as bridge abutments has been very satisfactory. W hen we see crowns th at have been giving good service for from twenty to twenty-five years, we sometimes wonder whether the large inlay restoration can give similar service. In conclusion, the following principles must be incorporated in any inlay pro cedure for the operator to get consistent results: 1. T here must be proper cavity prep aration or defeat must be acknowledged in the first step. 2. A well-adapted wax pattern is nec essary, with definite margins.
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The Journal of the American D ental Association
it would be assumed from Dr. W est’s paper that all inlay waxes have the same amount of thermal expansion when subjected to the same changes in temperature. I wish to call your attention to the extensive experiments on these waxes made by the United States Bureau of Standards. T he charts shown by them in the January, 1931 issue of T h e J o u r n a l indi cate that almost every inlay w ax has a d if ferent expansion curve. T he Bureau advo cates the printing on each package of w ax or investment of the characteristics of that certain 608 Equitable Building. brand of material. W ithout definite knowl DISCUSSION edge of the expansion produced by each degree H. D. Coy, Hamburg, Iow a: T h e three of temperature, it is difficult to determine how requisites for success in the making of a cast much w ill be necessary in compensating for ing are: (1) a w ell prepared and w ell finished the shrinkage of the gold. Standardization of cavity; (2) a properly fashioned w ax pattern these waxes and investments and the printing which fits the cavity, and (3) a casting which of the thermal expansion curve, with the other exactly reproduces the w ax pattern. Failure physical data on each package of material, to give due consideration to any one of these w ill do much toward sim plifying the casting principles w ill inevitably result in the failure process. T h e shrinkage of gold has been of the finished restoration. Conservation of proved, by extensive experiments, to be con tooth structure and the protection of the pulp stant at 1.25 per cent by volume of the gold. should be given first consideration in the prep By expanding the mold or pattern or both to, aration of any cavity. T h e careful operator compensate for this shrinkage, we are able to w ill have little difficulty in preparing cavities cast to dimension. T his w ill not be difficult of the Black type without endangering the life to accomplish if the dentist is able to secure of the pulp. T hey are boxlike, w ith a flat his favorite brand of w ax and investment with gingival seat and definite angles. I f the same the thermal expansion curve accurately de painstaking care were required in the making termined and printed on the package by the of an inlay restoration as that employed by manufacturer. the gold foil operator in his work, the inlay J. J. Trams, A nn Arbor, Mich.: If more would more nearly approach the ideal already men were able to make perfect w ax patterns reached by the foil filling. A w ax pattern in the mouth, a more excellent service to our may be made of a very poorly prepared cavity. patients would result, but like many other Failure in this first requisite dooms many inlay operative procedures, this one has its lim ita restorations almost before they are begun. tions. T he remote portion of the mouth, the W ith the vast number of scientific facts now interference of the restless tongue, thick cheeks available concerning the physical characteris and ever-present saliva, the difficult access tics of the materials used in dental casting, w e and poor light render direct pattern making find it comparatively easy to cast to any de next to impossible. Deep cervical margins w ill sired dimension. There would seem to be little extend below any point that can be reached to justify argument in favor of any type of with strips, and finishing these margins with cavity preparation which does not include the instruments by the sense of touch alone in a principles of a flat seat and a definite angle. highly sensitive area where hemorrhage is Dr. W est states that most investments of high sure to result leaves too much to the imagina silica and low plaster content reach their max tion. A good amalgam model, in the hands, imum expansion at about 1200 F. Rather than correctly articulated, with good light and none to carry the mold to a temperature of only of the sources of interference present, gives 800 F., as recommended, it would seem to be the operator great advantages. I therefore beg a better procedure to expand it to the max to amend what Dr. W est has said concerning imum, thereby relying less on the expansion inlay cavity preparation by suggesting a modi of the frail and plastic w ax pattern. As fication of the accepted cavity form to facili regards the use of w ax expansion as an aid tate the withdrawal of a modeling compound in overcoming the shrinkage of gold from impression without distortion. T h is modifica the molten state to the point of crystallization,
3. A conscientious application of the investment to the wax pattern is required. 4. Elimination and casting m ust be a systematic procedure. In a word, possibly the greatest point in any inlay technic is system. H ap hazard methods afford little in return. In any line of endeavor, system tends to success and few are successful w ithout it.
W est— Cavity Preparation and. Inlay Casting tion in cavity form would consist merely in taking a slice from the mesial and distal convexity around which the impression ma terial wraps itself and, when chilled, cannot be withdrawn without distortion or fracture. To be sure that I am not misunderstood, allow me to state clearly that this modification applies only when the individual operator feels that he can render a better service by employ ing the indirect method of operating. Dr. W est has described an almost universally ap proved cavity form with flaring buccal and lingual w alls which lends itself perfectly to the direct method of operating and offers com pensation for the shrinkage of the cast. Where the direct method of operating can be em ployed advantageously, this cavity form can not be improved on from the standpoint of esthetics or strength, but it is not adapted to indirect work. A s dentists, let us always re member that w e are rendering primarily a health service; and w hile the perfect sealing of a cavity is very important, the most essen tial factor in these operations lies in givin g the best possible protection to the vital pulp. Thousands of pages of literature in our dental magazines and hundreds of hours of discussion on the floors of our dental meetings have been devoted to the question of the pulpless tooth. Negligence on the part of the laity and opera tive mistakes on the part of the operative dentist are in the main responsible for the condition of these dead, dying or degenerating pulps. W e cannot be held accountable for the negligence of the laity, for most intelligent people are learning something of the dire re sults of foci of infection in the mouth. Neither can w e evade the responsibility for operative mistakes that lead eventually to the death of the pulp. It therefore seems to me that any consideration of cavity preparation for the gold inlay is not complete without some ref erence to the protection of the pulp. If a frac tion of the time that has been devoted to the study of the pulpless tooth problem and the treatment and filling of some of these root canals had been spent in earlier operations on these teeth, in conscientious cavity prepara tion, disinfection, the sealing of open tubules in freshly cut dentinal surfaces and the placing of nonconductors to protect the pulp from oftrepeated thermal shock, the number of dying and degenerating pulps would be reduced
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tremendously. W hen the generally accepted principles given to us by Dr. Black were worked out, they took into consideration the materials with which the restorations were to be made. T hat is axiomatic. Those materials were gold foil and amalgam. Since then, the casting technic has been evolved and greatly improved, so that materials many times stronger are now in common use; which ren ders deeply cut cavities, for retention only, no longer necessary or defendable. During those days when gold foil and amalgam were used to fill teeth, brick and mortar were used to construct buildings that rarely were higher than five or six stories. Now , there seems to be no limit to the height of buildings. T he fundamental principles of architecture have not changed, but the form and details have been modified to conform with the materials in use. T h e fundamental principles of cavity preparation need not be changed in order that we may take advantage of the greater strength of filling materials by using thinner walls in our inlays and thus protecting the pulp from thermal shock. It is not necessary that we eleminate a definite cervical floor, but it should be narrower mesiodistally and be cut with greater precision. T he resistance form on the occlusal surface can even be widened if it seems necessary, but let us be careful about encroaching too closely on the horns of the pulps or the recessional lines because of their peculiar histologic structure. Nothing has has been said about investments, their phys ical properties, manipulation or behavior under heat. Nothing has been said about alloys, their adaptability or behavior in the mouth or heat treatment. Nothing has been said about waxes, their physical properties or lim its in expansion. In fact, nothing has been said about dimensional casting, although my interest in these topics has been greatly stim ulated by my work w ith the Bureau of Stand ards Research Committee during the past year. I appeal to you for more conservative cutting in our cavity preparation, finer de tails in line angles and point angles, elimina tion of frictional heat, proper sealing of dentinal tubules, conscientious removal of de cay, disinfection of cavity w alls and the employment of cement as a nonconductor in thermal change in all deep cavities.