Calcium Products in Dentistry*

916

T he Journal of the American D ental Association

cooling curve from w hich the fusing range can be obtained, this ratio indicates the probability of liquation. T h e hardness determ inations are v al­ uable as a general indication of the m e­ chanical properties and should be made on some indentation m achine rath e r than w ith a sclerom eter or scleroscope, so th a t com parisons w ith other d ata can be made. T h e stress-strain diagram should be avail­ able to perm it calculations of elasticity and elongation, as w ell as to give indica­ tions of toughness and resiliency. I f the mechanical properties are determ ined in both the quenched and the heat-treated condition, the additional d ata are valu­ able. T h e m ethod of heat treatm en t should be indicated, as w ell as a descrip­

C A L C IU M

tion of the m ethod of p reparing the test specimens, including th eir dimensions. W ith such inform ation, the dentist w ould soon learn to distinguish the proper alloy fo r any type of restoration from the list subm itted by the m anufacturers. A l­ low ance should be m ade for the peculiar­ ities of his own ideas of design, combined w ith a consideration of the individual eccentricities of the patient. T h e operator w ould n o t be forced to rely on incomplete inform ation supplied by representatives of gold m an u factu rers, dental technicians or other dentists. T h e d en tist should be qualified to make the final decisions reg ard in g the choice of all m aterials th a t are to used in the m outh.

PR O D U C T S IN

D E N T IS T R Y *

By W. S. RICE, D.D.S., Chicago, 111. O appreciate fully the im portance of plaster of P aris in dentistry, it is only necessary to realize th a t from the taking of an impression to the pro­ duction of all b u t the sim plest type of single tooth restoration, plaster in some form or com bination m ust be employed in practically every step of any technical procedure. Because of their properties of setting to form a solid mass on the addition of w ater, th e low dimensional change on setting and com paratively low changes w ith fluctuation in tem perature,

T

* T h e p ap ers of D rs. Rice and Shell w ere presented as a sym posium on “C ast B ridge A ttachm ents.” *R ead before the Section on P a rtia l D en ­ tu r e ; C row n a n d B ridge a t the M id w in ter C linic of the C hicago D ental Society, Feb. 3, 1931. Jour. A . D . A ., M ay, 1931

together w ith faith fu l reproduction in fine detail of any surface against w hich they m ay be poured, plaster products occupy a unique position am ong the m a­ terials used by the profession. A vast am ount of valuable tim e and scientific research has been devoted to the study of other m aterials for th eir application to den tal lab o rato ry procedure, but, for m any purposes, no satisfactory substitute fo r plaster has been found. Since th e profession is so entirely de­ pendent on these m aterials for the ac­ curacy of its results in restorative w ork, m astery of m anipulative procedures for p laster products and a thorough know l­ edge of th e conditions governing their behavior are essential to th e highest, or even m erely satisfactory, attain m en t in

Rice— Calcium Products in D entistry the a rt of restorative dentistry. O f no less im portance is the responsibility of m anufacturers of dental products to m aintain the highest standards of quality, un iform ity and perform ance in these m a­ terials, as w ell as to supply com plete physical property d ata and detailed in­ structions for obtaining best results. T h e r e are several reasons th a t this one m aterial serves so m any useful purposes in dentistry and th a t it is such a valuable m aterial in the dental la b o rato ry : (1 ) the property of m ixing w ith w ate r to produce a semifluid m oldable mass w hich can be flowed or shaped into any desired fo rm ; ( 2 ) the property of setting to a rigid cast of the body being rep ro d u ce d ; (3 ) the setting tim e, w hich can be regu­ lated to any technical requirem ent regardless of tim e factors involved; (4 ) a setting expansion w hich can be regu­ lated according to the purposes for w hich it is em ployed; ( 5 ) a crushing strength, w hich, th ro u g h recent developm ents, can be regulated from a few hun d red pounds to as high as 10,000 pounds per square inch ; ( 6 ) a low therm al expansion, w hich shows no appreciable change in dim en­ sions w ith ordinary fluctuations in tem ­ p eratu re up to 212 F ., the change being of the order of 0.0009 per cent per degree F ah renheit, and (7 ) resistance to heat and steam in casting, soldering and v u l­ canizing processes. A n intensive educational cam paign is being carried on by m anufacturers of plaster products, the object being to bring to the profession a m ore com prehensive know ledge of these products and their properties. Such know ledge w ill enable the user to develop definite technics of m anipulation and th e re b y be a s su re d of definite results. A high percentage of com plaints of reportedly defective m a­ terials are due, not to faults inherent in the product itself, b u t to im proper stor­

917

age, incorrect m ethods of m ixing, con­ tam ination w ith oth er m aterials and sim ilar conditions beyond the co n tro l of the m an u factu rer. I t is surprising to note how flagrantly the few simple rules fo r the successful h an dling of these products are violated, an d how m uch resentm ent is engendered against a perfectly inno­ cent m aterial as a resu lt of such abuses. A greater degree of accuracy and perfection is constantly being dem anded of the technician. T h erefo re, it is essen­ tial th a t all types of d en tal plaster products conform to the most rigid speci­ fications. D en ta l plasters to m eet these specifications m ust be prepared from the purest, especially selected gypsum rock. E very step in the m a n u factu rin g process m ust be carefully controlled in order to assure uniform ity of quality. E very batch of the final p roduct should be carefully tested fo r conform ity w ith specifications before it is perm itted to leave the factory. H erm etically sealed steel containers are now being employed, as a fu rth e r precau­ tion against deterioration of th e product u n til it reaches the consum er. T h e only rem aining requirem ent for satisfactory results, th a t of proper m a n ip u la tio n ,. is up to the individual user. T h e exacting lim its for settin g time, s e ttin g e x p a n s io n a n d d i m e n s i o n a l changes in laboratory procedures have necessitated the developm ent of highly refined m aterials w hich require g reater care in th eir m anipulations th an ju st o rdinary plaster. T h e form ulas are quite com plicated, changing m ore o r less fre ­ quently to keep pace w ith technical ad ­ vancem ents. T h e m ixing of one m aterial w ith another, or even th e m ixing of d if­ ferent batches of th e same m aterial, may cause disastrous results. P laster of P aris ( C a S 0 4. ^ H 20 ) , o r calcium sulphate hem ihydrate as it is know n technically, is m ade by heating

918

The Journal of the American D ental Association

gypsum u nder carefully controlled tem ­ p era tu re conditions u n til one and onehalf molecules of w ate r have been driven off. H e a tin g or calcining above 350 C. removes the last h alf molecule of w ater, a soluble anhydrite w hich is ra th e r u n ­ stable and has a g reat affinity for w ate r resulting. I t rapidly absorbs m oisture from th e atm osphere and is thereby con­ verted into the hem ihydrate form . O r ­ d in ary plaster, w hen fresh, usually con­ tains a sm all percentage of this soluble anhydrite, w hich reverts to hem ihydrate a fte r exposure to the air for a few days. D u rin g the period of this conversion, or aging, the setting tim e of the products may vary by several m inutes. I t was previously necessary to age plaster by long storage, but m odern m ethods of pre-aging render it m ore stable and less liable to change in physical properties. S etting tim e usually lengthens d uring the aging process, the extent depending on m odifying m aterials used in com­ pounding. Chem ically, set plaster is n eu tral, be­ ing neither alkaline nor acid. P laster of P aris contains, theoretically, 6.2 per cent chem ically com bined w ater. O n long ex­ posure under conditions of high hum idity, the w ate r content may increase by as much as from 1 to 2 per cent. T h is w ate r does not chem ically combine w ith the plaster to produce set plaster, but is absorbed. P laster should be stored in a clean, dry place, and w ell covered to prevent contam ination and deterioration. If stored under dam p conditions, plaster rapidly takes up m oisture, loses strength and soon becomes unfit for use. T h e properties of plaster of P aris vary w idely. Its setting tim e, setting expan­ sion and crushing strength cover a w ide range. T h ese properties are dependent largely on the source of m aterial and on the m ethod and accuracy of control in

the calcination process. I t is necessary, therefore, if best results are to be ob­ tained, th a t care be observed in storage, cleanliness and m anipulative procedures. R ecent developm ents in the field of calcium research have b ro u g h t to lig h t a new m ethod of calcining gypsum w hich produces a m a terial possessing m any of the characteristics of o rdinary plaster, but w ith several advantages over plaster in physical properties. I t is m ore stable than plaster and m uch stro n g er and has a higher resistance to heat and vulcan iz­ ing conditions. P H Y S IC A L P R O P E R T IE S

T h e physical properties of plaster p ro d ­ ucts w hich are capable of m easurem ent are settin g tim e, setting expansion, th e r­ m al expansion, vulcanizing expansion and crushing stren g th . S ettin g T im e .— T h e first property en­ countered in w o rking these m aterials is the setting tim e. F o r p ractical purposes, this reaction is divided into tw o stages, initial set an d final set. T h e in itial set denotes the tim e in w hich the m ate­ rial is still w orkable, and the final set indicates the period w hich m ust elapse before the m aterial is h ard enough for use. In certain products, such as im pres­ sion plaster and investm ents fo r w ax expansion technics, the tim e of setting is very im portant. A stable setting tim e in these products is the m ost difficult prop­ erty to control. By the addition of accel­ erators or retarders, any settin g tim e may be secured, b ut establishing a definite set­ tin g tim e a t the factory does n ot neces­ sarily m ean th a t this w ill n o t change, from aging, exposure to m oisture ' or varying clim atic conditions. F o r this reason, a know ledge of the m ethods of changing setting tim e is very im portant. T h e first facto r in the setting tim e is the am ount of w a te r used in

Rice— Calcium Products in D entistry m ixing. A good ru le reg ard in g varying percentages of w a te r is “increasing the percentage of w ate r increases the setting time, b u t decreases setting expansion and crushing stre n g th .” T h e re are num erous m aterials w hich may be used to acceler­ ate or re ta rd the setting of calcium prod­ ucts. Each m a n u fa ctu re r has his own retarders and m ethods of regulating the setting tim e. I t frequently happens th a t modifiers used in one m aterial m ay be incom patible w ith those used in another. In m ost instances, acceleration is best obtained by the use of a potassium sul­ phate solution. T h is chem ical not only gives the norm al acceleration but w ill fu rth e r act to reduce setting expansion. T h e use of excessive quantities of this m aterial may result in retardation. A n o th e r im p o r ta n t f a c to r in th e m anipulative procedures w hich may be m ade use of to regulate the setting tim e is spatulation. T w o kinds of spatulation are generally employed, hand and m e­ chanical. H a n d spatulation for th irty seconds w ill result in a longer setting tim e by several m inutes th an w ill m e­ chanical spatulation. T h e ratio is approx­ im ately 2 0 :8 . T h a t is, a m aterial w hich has a setting tim e of tw enty m inutes w hen hand spatulated for th irty seconds w ill set in eight m inutes w hen m echani­ cally spatulated for th irty seconds at 1,200 revolutions per m inute. T h e tim e of spatulation has also an im portant bearing on setting. A m aterial w hich has a setting tim e of tw enty-nine m inutes w hen spatulated for th irty seconds may be accelerated to a tw elve-m inute setting tim e by spatulation for 150 seconds. C on­ versely, the setting tim e may be len g th ­ ened by reducing the period of spatulation. S ettin g E xpansion.— S etting expansion is recorded as the percentage of linear length change in plaster products d uring the process of setting. (V olum e change

919

is three tim es linear len g th change.) T h is indicates th e percentage of increase or decrease w hich may be expected in the size of impressions or m odels made from them . A s a resu lt of this change, the appliance constructed over a model of these m aterials w ill be a like percentage la rg e r o r sm aller than the object to be duplicated o r th an the m outh for w hich it is constructed. By the addition of certain ingredients to impression plasters, it has been possible to reduce the settin g change to the very sm all m agnitude of from 0.02 to 0.04 per cent. W ith a n eu tra l (no expansion or con tractio n ) impression plaster, no w arpage w ould occur as a result of the confining action of a tray, and, from our present inform ation, it w o u ld seem th at this type of plaster should be employed. If an impression plaster w ith a norm al percentage of expansion is used, the p al­ atal portion of the impression w ill be raised, a model w ith the e rro r of an ex­ cessively deep v au lt resulting. T h is may contribute to undue pressure in the palate. T h e effect of setting expansion on the w ax p attern in the casting process has been reported in a previous paper, b u t is repeated here because of its intim ate con­ nection w ith this paper. T h e setting ex­ pansion of investm ents has been measured w ith plaster unconfined, and the assump­ tion has been th a t the benefits of this expansion are obtained in the investing process. T h is change is to be expected in an invested mass w hich is free to expand in all directions. In a practical case, the invested mass is confined in a rigid, practically nonex­ panding brass ring. In such a case as this, w h a t w ould be the effect of the settin g expansion? W o u ld there be no expansion, or w ould the expansion press inw ard on the w ax p attern ? In o rd er to determ ine this point qualitatively, a tapered brass

920

T h e Journal of the American D ental Association

plug w as invested inside a brass ring. W h e n the investm ent had reached its in itial set, the plug w as removed, and the investm ent allow ed to come to its final set. A fter this state w as reached, w ith the nonexpanding ring, the hole w as sm aller th a n th e plug, and the plug w ould go only p a rt w ay into it. T h e experim ent w as repeated w ith a slotted ring capable of free expansion. In this case, the plug, w hen placed in position in the set plaster, extended through the bot­ tom of the investm ent. T h u s, it w as dem onstrated th a t, in the case of a non­ expanding brass ring, the expansion of the mass of investm ent took place in such a w ay as to close the hole in the center of the investm ent. T h e investing process as it is frequently carried out in a nonexpanding ring w ith a uniform investing mass w as show n to result in compression and elongation of the w alls of the w ax pattern, w ith an appreciable bulging at the center of the core w hen the thin m ix is used. T h is bulge appeared to be negligible w ith the thick mix. I t w as thus dem onstrated th a t w hereas, w ith the use of a thin m ix throughout, the benefits of the settin g expansion at the gingival m argin are lost and the crow n is slightly larg er m idw ay between the gingival and occlusal surfaces than at the gingival surface, the practice of using a thick m ix in the core and a t the m argins of a crow n p a tte rn and a th in m ix o u t­ side affords the benefits of p a rt of the setting expansion of the investm ent. V ulcanizing E xpansion.— V ulcanizing expansion relates to the change w hich takes place in models and investing m ate­ rial as a result of exposure to vulcanizing conditions. T h is varies w ith the different types of m aterials being used. In the case of plaster free to expand, the change am ounts to from 2 to 4 per cent, caused

by the calcination of th e plaster and coincident absorption of w ater. W h ite stones are little b etter in this respect, show ing a length change of from 2.5 to 3 per ccnt. V u lcan izin g stones w hich have been com pounded to correct exces­ sive change in v ulcanization show expan­ sions ran g in g from 0.10 to 0.17 per cent. In the investigation of vulcanizing change, tw o m ethods w ere employed. T h e first consisted of the construction of test specimens, approxim ately one-half inch square by six inches long, w hich w ere exposed on end in the vulcanizer to 320 degrees F ah ren h eit for three hours. T h e values above recorded w ere obtained from these specimens. D eterm in atio n of the relative effect of protecting m aterials by flasks w as a t­ tem pted. T o date, definite values have not been obtained, b ut indications are th a t tw o changes in the model occur. F irst, there is a w arpage of the model, resulting in increasing the v au lt depth produced by setting expansion of the m a­ terial used for investing the model in the flask. T h is, in some instances, am ounted to as m uch as 0.5 per cent. A fte r these m easurem ents w ere made, the flasked case was packed w ith rubber and ru n through the vulcanizer. U pon rem oval from the vulcanizer, accurate m easurem ents w ere again m ade and a fu rth e r increase in the v au lt depth w as noted. T h e re w as also a slight increase in the distance between tuberosities. Both of these changes, oper­ ating to increase the v au lt depth of models, together w ith the increase in this dimension produced by confining a highsetting impression m aterial, may partially account for misfits w here th e den tu re rides in th e palate. T h is leads us to the conclusion th at, for flasking purposes, a m aterial having a m in im u m s e ttin g a n d v u lc a n iz in g change should be used or, as has been

Rice— Calcium Products in D entistry recommended in the casting process, some provision, such as lining the flasks, or the use of a flask w hich w ill perm it expansion w ith o u t distortion, should be given con­ sideration. A n o th er factor in the vulcan­ izing process w hich may contribute to distortion is th e fact th a t a flask w hen heated to vulcanizing tem p eratu re ex­ pands m ore th a n the flasking m aterials, w hich makes room fo r expansion of the m aterial, or distortion, as the case may be. T h e rm a l E xpansion.— T h e rm a l expan­ sion is recorded as the percentage of linear length change in dental m aterials, especially investm ents, caused by changes in tem perature. M o st calcium products expand from room tem p eratu re to about 250 F . Above this tem perature, the elim i­ nation of free w a te r and w a te r of crystal­ lization occurs. T h is loss of w ate r is accompanied by a contraction, the m agni­ tude of w hich depends on the type of m aterial and, to a certain extent, its plaster content. P lasters and stones show m arked con­ traction w hen heated above 250 to 300 F ., this contraction often am ounting to a net shrinkage of from 1.5 to 2 per cent. Shrinkage is still fu rth e r increased w hen the specimen again cools to room tem per­ ature. L ow heat (high plaster co n ten t) invest­ m ents show therm al expansion character­ istics very sim ilar to those of plaster. Recently, there has appeared in the literatu re reference to a new m aterial for com pounding dental investm ents, know n as cristoballite. T h is m aterial has been selected because of its high therm al ex­ pansion a t com paratively low tem pera­ tures. A th erm al expansion of as much as 1.25 per cent a t 1,000 F . m ay be obtained in a properly com pounded in­ vestm ent. A lim ited am ount of experi­ ence w ith this m aterial w ould indicate th a t there is a possibility of overexpansion

921

instead of the underexpansion earlier ex­ perienced w ith other m aterials. In present high heat investm ents, w e are able to obtain as m uch as 0.4 per cent setting expansion as w ell as 0.8 per cent expan­ sion on h eating to a casting tem pera­ ture, from 1,200 to 1,400 F . T h ese tw o expansions com bined w ill effectively neu tralize the casting shrinkage of gold in th e casting process. T h e use of this type of investm ent also makes possible th e casting of inlays of ac­ curate dimensions by th e old technic of investing the p attern , settin g it aside u n til the pain tin g investm ent had set and then com pleting the investm ent w ith a second mix, provided a casting tem p eratu re of red heat (1 ,3 0 0 F .) is used. In p artial and fu ll den tu re casting w ith this new type of investm ent, it has been necessary to issue cautions against the use of too high b u rn o u t tem peratures, because of the danger of overexpansion. T h is danger is p articu larly p revalent in technics em ­ ploying m aster models. In using calcium products, there are certain precautions th a t th e dentist and the technician m ust observe if they wish to obtain the best results. 1. T h e products should be stored in a dry place. 2. C ontainers m ust be kept closed w hen n ot in use to prevent undue ex­ posure to air. E xposure for a long tim e w ill resu lt in deterioration, w ith conse­ quent changes in physical properties. 3. If w a te r is accidentally splashed or spilled in dry plaster, all w et m aterial and surro u n d in g plaster should be re­ moved im m ediately and discarded. 4. Such m aterials as investm ents, stones and sim ilar products composed of m ix­ tu res of plaster and other ingredients should be rem ixed d ry in th e container before used. W h ile they are thoroughly mixed w hen packaged, th ere is a tendency

4

922

T he Journal of the A merican D ental Association

to segregation d u rin g shipm ent. Also, the top layer of dry m a terial m ay be affected by long storage. 5. C areful control of the plaster-w ater ratio for any given plaster p ro d u ct assures uniform ity of set, of strength and of ex­ pansion. 6. In mixing, the plaster is sifted into w ater, allow ed to soak for a few seconds and spatulated thoroughly. M echanical spatulation, hand spatulation on a vi­ b rato r and the ord in ary m ethod of hand spatulation are effective, and are recom­ m ended in the order nam ed. 7. Im pressions m ust be thoroughly soaked to prevent soft surfaces on models

caused by absorption of w ater from the new ly mixed m aterial. 8. A lacquer type separator should be employed. Soap and other soluble sepa­ rato rs do n o t produce the best surfaces on models and m ay affect the properties of m odel m aterials. 9. T h e use of a m echanical vib rato r in pouring plaster m aterials is recommended to assure smooth, bubble-free surfaces. ?0. D ifferen t plaster m aterials should not be m ixed, as one p roduct may contain ingredients w hich w ill destroy the setting properties of the other. F o rm u las are occasionally changed for im provem ent of products, an d m ixing different batches of the m aterial m ay cause trouble.

W H A T F A C T O R S D E T E R M IN E T H E E A R L Y ST A G E P A R A D E N T O S I S (P Y O R R H E A )? *

OF

By H E R M A N N BECKS, M .D., D.D.S., San Francisco, Calif. L I N I C A L and laboratory experi­ ence of the last few years has seemed to confirm the supposition of dental practitioners th a t a constitutional or in­ tern al factor is involved in m any of the cases of so-called pyorrhea (parad en tosis). T h e re has been little scientific foundation fo r this theory up to the present tim e, and w e are only very slow ly gaining an insight into the etiology and pathogenesis of this disease.

C

C linical observation does not advance us very rapidly, and a deeper know ledge of the w hole organism is necessary in order to disclose the relationship between *R ead before the Section on Biologic Sciences a n d R esearch a t the M id w in ter Clinic of the C hicago D ental Society, Feb. 3, 1931. Jour. A . D . A ., M a y , 1931

m anifestations of p arad en tal lesions and the organism on the one side, an d any constitutional anom alies on th e other. M y discussion w ill be based upon some recent publications in this field by the

Arbe'itsgemeinschaft fur Paradentoseforschung ( P aradentosis Research G ro u p ) and some of those w ho are follow ing their lead, as w ell as upon my ow n obser­ vations. I w as a m em ber of this group before I came to A m erica, and, since th a t tim e, a reciprocal relationship has been established w ith my laboratory in San Francisco, w hich form s the only con­ necting link w ith this w ork in the U n ited States. O u r group is follow ing the lead of all of th e other laboratories of th e E u ro ­ pean P aradentosis Research G ro u p and w orks in close collaboration w ith intern-