- Email: [email protected]
Denture bases processed from a fluid resin
Denture
bases
processed
from
a fluid
resin
Walter 1. Shepard, lieutenant Colonel, DC, USA Walter Reed Army Medical Center, Washington, D. C.
1 n meeting the requirements for a dental resin as outlined by Skinner and Phillips,l no available resin has been proved to be superior to the polymethyl methacrylates. When used as a denture base, room-temperature polymerizing* resins have been shown to exhibit a greater degree of accuracy than that exhibited by the heat-polymerizing resins. 3 The conventional compression molding method of denture base processing requires considerable attention to time-consuming details in achieving accurate results. * On the other hand, expensive equipment and special processing does not make for greater accuracy than that obtained with conventional compression molding.5 Setting’ expansion of gypsum investment, as used in compression molding methods, has been shown to change the relative positions of teeth to cast during investing.O Heat-curing resins are somewhat stiffer and stronger than room-temperature polymerizing resins. However, the room-temperature polymerizing resins have sufficient strength and stiffness to withstand the forces to which dentures are subjected without distorting in use.’ This information, as well as laboratory and clinical observation, reveals a need for a method of denture base processing that is accurate, requires no complex special equipment, eliminates time-consuming details, and reduces processing time. The substitution of an accurate inexpensive material for the gypsum investment and the use of a room-temperature polymerizing resin would seem a direction in which to proceed. This article will describe the use of such a material. With the introduction of the pourable type of room-temperature polymerizing resins suitable for use in a hydrocolloid mold, the United States Army Regional Dental Activity at the Walter Reed Army Medical Center undertook a project to Read before the American
Prosthodontic
Society in Chicago.
This material has been reviewed by the Office of the Surgeon General, Department of the Army, and there is no objection to its presentation and/or publication. This review does not imply any endorsement of the opinions advanced or any recommendation of such products as may be named. is used in preference to the term “cold*The term “room-temperature polymerizing” curing,” “ self-curing,” or “autopolymerizing.“”
561
562
J. PI-OS.Dent. June, 1968
Shepard
Fig. 1. The flask disassembled. The bakelite inner ring (A) has a large opening (0) to permit the cutting of the sprue holes. The outer ring (R) is split and held with a rubber band. The metal base (D) and the upper lid (L) are held in position with flexible metal clamps (C). The reservoir (R) is tapered to fit snugly into the lid.
determine whether or not a technique could be developed which would result in an accurate denture base with a minimum of equipment, materials, and expenditure of time. In the light of previous work in this area” and fluid resin techniques for making duplicate dentures, a number of different materials and approaches were evaluated. As a result, a flask was designed (now available from a commercial source+) which would contain the hydrocolloid investment, yet permit adequate access to the denture base for the introduction of the resin material. Following this development, it appeared that our technique resulted in a denture that not only fit the cast more accurately than did compression molded dentures, but had fewer processing changes in its occlusal relationships. Clinical and experimental evidence seemed to support these conclusions. During the past two years, this activity has processed more than 11,000 dentures of all types by the following method.
OUTLINE OF THE METHOD The denture, either placed in a double-ring cating material. (Some be described.) Following its wax-up, is removed
complete or partial, is waxed-up in the usual manner and flask which is then filled with reversible hydrocolloid duplipartial dentures require a modification in waxing that will chilling and gelation of the hydrocolloid, the cast, with in toto. The artificial teeth are removed from the wax-up,
*W. 0. Orsinger:
Personal communication,
tVernon
Company,
Benshoff
Albany,
N. 1’.
Jan. 28, 1965.
Denture
bases processed
from
resin
563
Fig. 2. The wax pattern for an upper denture is in the proper position in the flask. The posterior end of the cast faces the large opening in the inner ring.
cleaned of wax, and replaced in the mold. The cast is cleaned and coated with a separating medium. Sprues and vents are prepared, the cast is replaced in the mold, and the flask is reassembled. A pour-able resin is mixed, and poured into the mold. The flask then is placed into a processing unit under air pressure for 35 minutes, The cured denture is recovered and finished in the usual manner. Materials and armamentarium. The necessary equipment and materials include: ( 1) A pourable type of acrylic resin,* (2) A double-ring flask (Fig. 1)) (3) a reversible hydrocolloid,t (4) standard Nos. 4 and 5 laboratory cork borers, (5) a resin-processing unit, or pressure pot, capable of holding at least 15 psi of air pressure, and (6) an alginate separating medium diluted with one part water. Description of resin used. “The polymer is a very fine, high molecular weight, methymethacrylate. This fine particle size permits it to pass into and through relatively narrow spaces, while the high molecular weight restricts the initial solubility of the material so that the viscosity of a liquid phase of the mixture does not increase rapidly during the first few minutes after mixing. The powder has, however, been given a treatment which causes it to fragment into many smaller irregular particles after a few minutes exposure to monomer. This fragmentation solves the problem of retaining a low viscosity early in the pouring process while still getting a sufficiently rapid reaction of the monomer and polymer that the material can be entirely softened to a gel before the curing reaction starts. “This thorough softening has important effects on both the color of the finished denture, and on the occurrence of internal porosity. Almost all polymer particles contain a dispersion of very small pores which contain air. If the resin is completely softened before the polymerization reaction starts, the applied pressure is sufficient to cause the small bubbles to dissolve in the gel. If the resin is not softened the small pores will remain and scatter light, making the resin appear more “Pronto II: Vernon Benshoff Company, Albany, N. Y. +Perflex: Howmet Corp., Chicago, Ill.
564
J. Pms. Dent. June, 1968
Shefard
opaque and lighter in color. Those pores which lie at the borderline between the softened and unsoftened material may be exposed to pressure for too short a time to allow them to dissolve, and they can then serve as nuclei of bubbles into which monomer vapor will evaporate as the temperature rises, and thus grow into holes of substantial size.“* Cast and wax-up (wax pattern). The base of the cast should have a slight taper, with the convergence toward its occlusal surface. This permits easier withdrawal from the gelled hydrocolloid mold. There should be an adequate shelf beyond the border roll of the (land area) of approximately 3 mm. extending upper cast. The wax-up should represent exactly the desired form of the finished denture. It is important that the teeth be cleaned of all wax residue prior to investing. If the teeth are not well cleaned, the resin will flash onto the tooth surface when the resin is poured, and this may result in tooth movement. Even if tooth movement does not occur, careless waxing results in more difficult finishing requirements. Partial dentures should have severe undercut areas blocked out with plaster of Paris to prevent tearing of the hydrocolloid during the removal and reinsertion of the cast. The labial region and the part beneath the lingual bar are the regions most commonly requiring a plaster block-out. The use of modeling clay is not recommended, because it is easily distorted and may prevent the proper reseating of the cast in the hydrocolloid. When an anterior denture base on a partial denture is to be processed, a roll of utility wax is used to connect the facial aspect of the anterior denture base wax-up with the posterior denture base wax-up or with the posterior surface of the cast. This will permit a connection to be made with the vent. Following wax-up, the cast should be soaked for about 5 minutes in room-temperature slurry water to displace the air from the cast. This helps to prevent bubble formation in the hydrocolloid mold. Before the hydrocolloid is poured, the cast is centered in the flask and positioned so that the large openin g of the inner rin
Personal communication,
Nov. IO, 1965.
Denture
Volume 19 Number 6
Fig.
3. The flask is filled with a reversible
bases
from
processed
resin
565
hydrocolloid.
are used to cut sprue holes in the hydrocolloid. A No. 4 cork borer is the vent, and the No. 5 is used to make the sprue hole. Note the location
Fig. 4. Cork borers used for making of these holes.
after
they have been separated
dry tooth denture,
surfaces
from
the incisal edge and buccal
and the lingual
the wax base. The
prior to their being separated
distinguish
the first and second bicuspids
additional
mark
marked, marked
on the labial
of both
sides is marked.
from one another. surface
the central
of the central
and lateral
after they are separated
from the wax base. The markings
dewaxing,
but they are readily, removed
wax pattern
is warmed
slightly
knife blade.
The wax base is removed,
and a household
detergent.
with hot water,
While
to
will help
incisor incisor
an
on the side on the un-
incisors from one another
teeth
during
This
In the lower arch,
and a mark is placed on the lingual side of the central side. This will distinguish
are applied
cusp of the upper teeth of one side are marked,
cusp of the first bicuspid is made
markings
from the wax base. For an upper
will usually stay on the
during
final polishing.
and the teeth are removed
and the cast is cleaned
The
with a
with a wax solvent
the cast is still warm, a thin coating
of separat-
566
Shepard
J. Pros. Dent. June, 1968
ing medium is painted on the cast and blown dry with air. Do not allow the separating medium to build up on the cast to a thick layer, because the fluid resin tdtl not displace the excess separating medium when it is poured. The denture base will be inaccurate where the separating medium is too thick. The cast is put aside to cool at this time. The teeth arc cleaned with a household detergent and clean boiling water only. A small amount of ammonia added to the detergent will help cut the wax residue. A tea strainer serves well to hold the teeth during dewaxing procedures. Wax solvents are not used in cleaning the teeth because they leave an oily residue which prevents good bonding with the base. Some resin teeth are made of a highly cross-linked plastic, and they do not seem to bond well with cross-linked resin base materials. The ridge lap of these resin teeth should be lightly ground with an arbor band prior to set-up in order to roughen the glazed
Fig. 5. The marked te eth are replaced in the mold after being thorou ighly c:Iraned. ‘I‘he marks hJP to get the teeth in the correct sockets in the mold.
Fig. 6. The cast is being replaced in the mold. It has been treated wi .th a separating medium. Note the location of sprue hole (S) and the vents (V).
Denture
yul~&l~ u
bases processed
from
resin
567
surface. This seems to result in a better bond between the teeth and the denture base resin. Spruing the mold. A No. 5 cork borer is used to cut an ingate sprue for filling the mold. For lower dentures, the cork borer should be positioned so that it will cut through to the denture base at the lingual side of the lower anterior teeth. This .will permit the fluid resin to begin filling the mold at the deepest anterior part of the denture base. The sprues for complete upper dentures should be located so they enter the mid-portion of the posterior palatal seal area. The No. 4 cork borer is used to cut the vents (Fig. 4). The vents should always be located so they will permit the resin to flow out of the most distal part of the denture base. With the flask standing on end, the vents will engage the highest part of the wax-up. If the vents are not positioned at the most distal aspect of the mold, air is likely to be trapped within the mold. This would result in voids in the base. Spruing and venting are not difficult once the principles are understood. Spruing for partial dentures will vary with each denture, but the same basic principles apply. The main sprue should permit entry of the resin at the deepest anterior base surface accessible, and the vent should permit exit of the resin at the most distal aspect of the base. spruing, the mold must be inspected Repositioning teeth and base. Following carefully for any loose pieces of hydrocolloid. These must be removed or they may become incorporated into the resin and result in a defect in the denture base. A light air blast is usually sufficient to remove any loose pieces that might be present.
Fig. 7. The mold is filled
through
the sprue hole until
the resin overflows
through
the vents.
568
Shepard
.I. Pros. Dent. June, 1968
If properly waxed, the average denture will present no problem in the repositioning of the teeth in the mold. The teeth will fit into place readily and not shift while the resin is being poured. Cotton pliers are used to replace the teeth in the mold (Fig. 5). After the teeth are repositioned, the attachment of the teeth to the mold is tested by inverting the flask and shakin g slightly. If no teeth fall from the mold, good tooth engagement is assumed. If a tooth has been ground extensively to accommodate occlusal or denture space requirements, a small amount of petroleum jelly is applied to the occlusal and facial surface of the tooth to retain it in position. The petroleum jelly must not be allowed to contact any part of the tooth that will be joined to the base. The cast should be cool before it is placed back into the mold, because a warm cast will accelerate the polymerization of the fluid resin and result in a lack of density of the denture base. If the cast is cleaned and coated first, it can be cooling while other steps in the preparation of the mold are carried out. Care should be exercised in replacing partial denture casts so that the artificial teeth are not moved by the elements of the metal framework. When the cast is repositioned in the mold (Fig. 61, the flask is reassembled and placed on end with the sprue holes accessible for filling the mold with resin. Mixing and pouring the rp.riz. The acrylic resin monomer should be stored under refrigeration, if possible. If the monomer is kept cool, the resin will have a longer .working time. The polymer and monomer must be accurately measured in cubic centimeters at the ratio of 30:12. This amount is sufficient for the average complete denture. The amounts used for partial dentures will vary. There will be increased shrinkage of the base during polymerization if too much monomer is used. Approximately 0.5 C.C. of the monomer wiil remain in the graduate after the monomer has been poured into the mixing beaker. This should be compensated for at the time the monomer is measured. (To avoid a build-up of set monomer in the graduate over a period of time, which will render the graduate inaccurate, the graduate should be inverted following each use to permit it to drain. j The monomer is poured into a glass beaker, and the polymer is added to it. The resin is stirred rapidly for 10 seconds with a metal spatula, swirled in the beaker for several seconds and then poured into the main sprue hole. The pouring should be completed within one minute. The resin tends to become too viscous for accurate results aftfter this time. A maximum pouring of three dentures at one time is recommended. A slight rocking of the flask from side to side during pouring will help displace any air that might be trapped within the mold and will prevent voids. The mold cannot be considered to be filled until the resin overflows through the vents (Fig. 7). Following pouring, the flask is allowed to set on the bench for 3 minutes before being placed in the processing unit. Curing. The flask is positioned in the processing? unit so that the vents and sprue holes are not directly under the. air intake valve. If they are positioned directly under the intake valve, the ini&l blast of air may displace some of the Auid resin in the mold and redult in a*void in the denture base. The flasks were designed so that two molds can be placed in a processing unit at the same time. About one inch of water at room temperatgre is placed in the processing unit, and the lid and air outlet valve axe closed; At, least 15 psi, of air pressu?e is turned into the pro-
VOlUrnP Number
19 6
Denture
bases ~rocessrd
from
resin
569
Fig. 8. The denture and cast are easily pushed out of the hydrocolloid mold.
Fig. 9. The processed dentures. Note the position of the sprues and vents.
Fig. 10. The casts are reseated in the mounting plaster on the articulator after the sprues are removed. Note the pin position, tooth contacts, and the detail of the wax pattern copied in the denture base with a clean sharp separation of the dentures from the hydrocolloid molds. cessing unit and maintained for 35 minutes. Air pressures of over 15 psi neither improve the density of the denture .base nor shorten the curing time, but air pressures under 10 psi result in less dense baseqg and may lead to pinpoint porosity. After 35 minutes, the air is evacuated from the processing unit, and the flasks are removed and disassembled. The dentures can be ejected from the hydrocolloid mold with finger pressure (Fig. 8). The sprues are removed with large carborun(Fig. 10). The dum discs (Fig. 9). Th en the denture is returned to the articulator hydrocolloid taken from the flask and denture is washed clean and stored for reuse.
570
Shepard
J. Pros. Dent.
June, 1968
Fig. 11. These test denture bases have been allowed to dry for,.one week after processing. The bases were removed from their casts and finished and polished through the pumice stage. Then they were replaced on their original casts, and the distal ends were ground flat on a model trimmer. A heat-cured base is less accurate even though it was processed according to the recommended procedures and meets A.D.A. Specification No. 12 (left). A denture base processed by the technique described in this article for fluid resin processing is shown on the right. After soaking these test bases for several days in water, both denture bases showed some improvement in fit.
Fig. 12. A processed, bilateral distal extension lower partial denture showing the locations of the sprues and vents. The space beneath the lingual bar and the cast in front have been blocked out with plaster.
Fig. 13. The location of the sprues and vents for a lower partial denture with three separate denture base sections. The sprue hole leading to the anterior segment is poured first, then the main sprues leading to the posterior bases are filled. A roll of utiiity wax is placed in the buccal fold region during the wax-up to connect anterior and posterior base segments.
Volume 19 Number 6
Denture
bases processed
from
resin
571
SUMMARY We have made about 14,000 dentures of all types using a fluid resin and a hydrocolloid mold. Technicians’ working time, and the over-all, elapsed processing time have been greatly reduced from that required by other methods. The working time per denture that has been saved amounts to 40 or 50 minutes. The finishing and remounting of dentures is simplified due to the clean and sharp separation of the cured dentures from the mold. The bases are more accurately adapted to the casts than were heat-cured resins. This was observed when test specimens of two A.D.A.-certified heat-curing resins were compared with the fluid resin specimens, both in a dry state and after six months in water (Fig. 11). The elimination of the gypsum investment and the use of the split flask seem to have reduced the amount of processing changes in the occlusion that had been consistently noted in our compression-molded dentures. Loss of vertical height of the occlusion of the dentures has been noted occasionally, but this has rarely exceeded 1 mm. when measured at the incisal pin of a Hanau articulator. About 10 per cent of the dentures have had voids as a result of incorrect sprue positioning or of pouring a too viscous resin. Most voids have been readily repaired with a small amount of resin and placed back into the processing unit with warm water for 5 minutes under 15 psi of air pressure. Repairs to the base are practically undetectable, even under magnification. This is probably due to the elimination of gypsum contamination and to the use of the original denture base material for repair. Pinpoint porosity will result if the processing unit loses air pressure, if inadequate air pressure is used initially, or if polymerization begins before the denture is put into the processing unit. All materials have handled well, even in room temperatures in excess of 90’ F., but polymerization is accelerated and the working time is shortened. Some partial dentures require rather complicated spruing procedures, but once the principles are learned, they presented no serious problem (Figs. 12 and 13). The movement of the distal extension metal framework due to packing pressures, such as is frequently observed when compression-molded techniques are used, appears to have been eliminated. Some tooth shifting has been noted where there has been extensive grinding on the teeth. With excessive grinding, the hydrocolloid does not adequately grip the tooth, and this permits tooth movement during pouring. Careful attention to detail must be followed, particularly during the wax-up. If a wax residue is left on the teeth prior to investing, a flash of resin over the tooth surface will invariably result. The average technician can be trained to perform the necessary processing procedures with a few days’ observation and practical experience. The procedure lends itself well to small or large laboratory operations and is well suited for use in a dental office. The equipment required is minimal and requires a small amount of storage space. The over-all cleanliness of the laboratory area is greatly improved due to the elimination of the gypsum investing materials. Our results with other fluid-type resins have been generally satisfactory, but our best results have been obtained with the resin described. This is probably due to the incorporation of a greater amount of polymer (2’/2 parts) in proportion to monomer (1 part) than the other resins usually contain (2 parts polymer to 1 part monomer). The glycerine or glycol type of hydrocolloid has given better results than those
572
Shepard
thinned
J.
June,
with water.
It seems to be tougher,
ing life. It does not have a tendency tacky
surface
PI<& Drnt.
which
seems to maintain
cut cleaner,
to dehydrate
and have a longer work-
as readily,
teeth in position
1968
better
and has a slightly during the pouring
and curing procedures. CONCLUSION A technique scribed.
After
significant
do not detract
certain
over
bases from fluid acrylic
resin has been de-
it is felt that the technique
compression-molding
problem
significantly
and technologic
denture
period of evaluation,
improvements
of development, material
for processing a two-year
methods,
areas exist which have been mentioned,
from the value of the procedure. research
in this area of denture
presents
In its present
state
but these
The need for further base processing
is indi-
cated. The author wishes to thank Colonel Duke and George C. Gresham for their cessing procedure.
William invaluable
T. Fisher and Sergeants First assistance in the development
Class Jeff J. of this pro-
References 1. Skinner, 2. 3. 4. 5. 6. 7. 8. 9.
E. W., and
Phillips,
R. W.: The
Science
of Dental
Materials,
ed. 5, Philadelphia,
1963, W. B. Saunders Company, p. 136. Roydhouse, R. H.: Materials in Dentistry; A Discussion for the Users of Dental Materials, Chicago, 1962, Year Book Medical Publishers, Inc., p. 88. Anthony, D. H., and Peyton, F. A.: Dimensional Accuracy of Various Denture-Base Materials, J. PROS. DENT. 1’2: 67-81, 1962. Rudd, K. D.: Processing Complete Dentures Without Tooth Movement, D. Clin. North America, pp. 675-691, Nov., 1964. Woelfel, J. B., Paffenbarger, G. C., and Sweeney, W. T.: Dimensional Changes Occurring in Dentures During Processing, J. A. D. A. 61: 413-430, 1960. Grant, A. A.: Effect of Investment Procedure on Tooth Movement, J. PROS. DENT. 12: 1053-1058, 1962. American Dental Association: Guide to Dental Materials, ed. 2, Chicago, 1964, American Dental Association, p. 61. Applegate, 0. C.: Essentials of Removable Partial Denture Prosthesis, ed. 3, Philadelphia, 1965, W. B. Saunders Smith, D. E., Lord, polymerizing Acrylic 103-l 15, 1967.
Company, p. 353. J. L., and Bolender, C. L.: Complete Resin Processed in Water Under Air
USA REGIONAL DEKTAL ACTIVITY WALTER REED ARMY MEDICAL CENTER WASHINGTON, D. C. 20012
Denture Pressure,
Relines With AutoJ. PROS. DENT. 18:
bases
processed
from
a fluid
resin
Walter 1. Shepard, lieutenant Colonel, DC, USA Walter Reed Army Medical Center, Washington, D. C.
1 n meeting the requirements for a dental resin as outlined by Skinner and Phillips,l no available resin has been proved to be superior to the polymethyl methacrylates. When used as a denture base, room-temperature polymerizing* resins have been shown to exhibit a greater degree of accuracy than that exhibited by the heat-polymerizing resins. 3 The conventional compression molding method of denture base processing requires considerable attention to time-consuming details in achieving accurate results. * On the other hand, expensive equipment and special processing does not make for greater accuracy than that obtained with conventional compression molding.5 Setting’ expansion of gypsum investment, as used in compression molding methods, has been shown to change the relative positions of teeth to cast during investing.O Heat-curing resins are somewhat stiffer and stronger than room-temperature polymerizing resins. However, the room-temperature polymerizing resins have sufficient strength and stiffness to withstand the forces to which dentures are subjected without distorting in use.’ This information, as well as laboratory and clinical observation, reveals a need for a method of denture base processing that is accurate, requires no complex special equipment, eliminates time-consuming details, and reduces processing time. The substitution of an accurate inexpensive material for the gypsum investment and the use of a room-temperature polymerizing resin would seem a direction in which to proceed. This article will describe the use of such a material. With the introduction of the pourable type of room-temperature polymerizing resins suitable for use in a hydrocolloid mold, the United States Army Regional Dental Activity at the Walter Reed Army Medical Center undertook a project to Read before the American
Prosthodontic
Society in Chicago.
This material has been reviewed by the Office of the Surgeon General, Department of the Army, and there is no objection to its presentation and/or publication. This review does not imply any endorsement of the opinions advanced or any recommendation of such products as may be named. is used in preference to the term “cold*The term “room-temperature polymerizing” curing,” “ self-curing,” or “autopolymerizing.“”
561
562
J. PI-OS.Dent. June, 1968
Shepard
Fig. 1. The flask disassembled. The bakelite inner ring (A) has a large opening (0) to permit the cutting of the sprue holes. The outer ring (R) is split and held with a rubber band. The metal base (D) and the upper lid (L) are held in position with flexible metal clamps (C). The reservoir (R) is tapered to fit snugly into the lid.
determine whether or not a technique could be developed which would result in an accurate denture base with a minimum of equipment, materials, and expenditure of time. In the light of previous work in this area” and fluid resin techniques for making duplicate dentures, a number of different materials and approaches were evaluated. As a result, a flask was designed (now available from a commercial source+) which would contain the hydrocolloid investment, yet permit adequate access to the denture base for the introduction of the resin material. Following this development, it appeared that our technique resulted in a denture that not only fit the cast more accurately than did compression molded dentures, but had fewer processing changes in its occlusal relationships. Clinical and experimental evidence seemed to support these conclusions. During the past two years, this activity has processed more than 11,000 dentures of all types by the following method.
OUTLINE OF THE METHOD The denture, either placed in a double-ring cating material. (Some be described.) Following its wax-up, is removed
complete or partial, is waxed-up in the usual manner and flask which is then filled with reversible hydrocolloid duplipartial dentures require a modification in waxing that will chilling and gelation of the hydrocolloid, the cast, with in toto. The artificial teeth are removed from the wax-up,
*W. 0. Orsinger:
Personal communication,
tVernon
Company,
Benshoff
Albany,
N. 1’.
Jan. 28, 1965.
Denture
bases processed
from
resin
563
Fig. 2. The wax pattern for an upper denture is in the proper position in the flask. The posterior end of the cast faces the large opening in the inner ring.
cleaned of wax, and replaced in the mold. The cast is cleaned and coated with a separating medium. Sprues and vents are prepared, the cast is replaced in the mold, and the flask is reassembled. A pour-able resin is mixed, and poured into the mold. The flask then is placed into a processing unit under air pressure for 35 minutes, The cured denture is recovered and finished in the usual manner. Materials and armamentarium. The necessary equipment and materials include: ( 1) A pourable type of acrylic resin,* (2) A double-ring flask (Fig. 1)) (3) a reversible hydrocolloid,t (4) standard Nos. 4 and 5 laboratory cork borers, (5) a resin-processing unit, or pressure pot, capable of holding at least 15 psi of air pressure, and (6) an alginate separating medium diluted with one part water. Description of resin used. “The polymer is a very fine, high molecular weight, methymethacrylate. This fine particle size permits it to pass into and through relatively narrow spaces, while the high molecular weight restricts the initial solubility of the material so that the viscosity of a liquid phase of the mixture does not increase rapidly during the first few minutes after mixing. The powder has, however, been given a treatment which causes it to fragment into many smaller irregular particles after a few minutes exposure to monomer. This fragmentation solves the problem of retaining a low viscosity early in the pouring process while still getting a sufficiently rapid reaction of the monomer and polymer that the material can be entirely softened to a gel before the curing reaction starts. “This thorough softening has important effects on both the color of the finished denture, and on the occurrence of internal porosity. Almost all polymer particles contain a dispersion of very small pores which contain air. If the resin is completely softened before the polymerization reaction starts, the applied pressure is sufficient to cause the small bubbles to dissolve in the gel. If the resin is not softened the small pores will remain and scatter light, making the resin appear more “Pronto II: Vernon Benshoff Company, Albany, N. Y. +Perflex: Howmet Corp., Chicago, Ill.
564
J. Pms. Dent. June, 1968
Shefard
opaque and lighter in color. Those pores which lie at the borderline between the softened and unsoftened material may be exposed to pressure for too short a time to allow them to dissolve, and they can then serve as nuclei of bubbles into which monomer vapor will evaporate as the temperature rises, and thus grow into holes of substantial size.“* Cast and wax-up (wax pattern). The base of the cast should have a slight taper, with the convergence toward its occlusal surface. This permits easier withdrawal from the gelled hydrocolloid mold. There should be an adequate shelf beyond the border roll of the (land area) of approximately 3 mm. extending upper cast. The wax-up should represent exactly the desired form of the finished denture. It is important that the teeth be cleaned of all wax residue prior to investing. If the teeth are not well cleaned, the resin will flash onto the tooth surface when the resin is poured, and this may result in tooth movement. Even if tooth movement does not occur, careless waxing results in more difficult finishing requirements. Partial dentures should have severe undercut areas blocked out with plaster of Paris to prevent tearing of the hydrocolloid during the removal and reinsertion of the cast. The labial region and the part beneath the lingual bar are the regions most commonly requiring a plaster block-out. The use of modeling clay is not recommended, because it is easily distorted and may prevent the proper reseating of the cast in the hydrocolloid. When an anterior denture base on a partial denture is to be processed, a roll of utility wax is used to connect the facial aspect of the anterior denture base wax-up with the posterior denture base wax-up or with the posterior surface of the cast. This will permit a connection to be made with the vent. Following wax-up, the cast should be soaked for about 5 minutes in room-temperature slurry water to displace the air from the cast. This helps to prevent bubble formation in the hydrocolloid mold. Before the hydrocolloid is poured, the cast is centered in the flask and positioned so that the large openin g of the inner rin
Personal communication,
Nov. IO, 1965.
Denture
Volume 19 Number 6
Fig.
3. The flask is filled with a reversible
bases
from
processed
resin
565
hydrocolloid.
are used to cut sprue holes in the hydrocolloid. A No. 4 cork borer is the vent, and the No. 5 is used to make the sprue hole. Note the location
Fig. 4. Cork borers used for making of these holes.
after
they have been separated
dry tooth denture,
surfaces
from
the incisal edge and buccal
and the lingual
the wax base. The
prior to their being separated
distinguish
the first and second bicuspids
additional
mark
marked, marked
on the labial
of both
sides is marked.
from one another. surface
the central
of the central
and lateral
after they are separated
from the wax base. The markings
dewaxing,
but they are readily, removed
wax pattern
is warmed
slightly
knife blade.
The wax base is removed,
and a household
detergent.
with hot water,
While
to
will help
incisor incisor
an
on the side on the un-
incisors from one another
teeth
during
This
In the lower arch,
and a mark is placed on the lingual side of the central side. This will distinguish
are applied
cusp of the upper teeth of one side are marked,
cusp of the first bicuspid is made
markings
from the wax base. For an upper
will usually stay on the
during
final polishing.
and the teeth are removed
and the cast is cleaned
The
with a
with a wax solvent
the cast is still warm, a thin coating
of separat-
566
Shepard
J. Pros. Dent. June, 1968
ing medium is painted on the cast and blown dry with air. Do not allow the separating medium to build up on the cast to a thick layer, because the fluid resin tdtl not displace the excess separating medium when it is poured. The denture base will be inaccurate where the separating medium is too thick. The cast is put aside to cool at this time. The teeth arc cleaned with a household detergent and clean boiling water only. A small amount of ammonia added to the detergent will help cut the wax residue. A tea strainer serves well to hold the teeth during dewaxing procedures. Wax solvents are not used in cleaning the teeth because they leave an oily residue which prevents good bonding with the base. Some resin teeth are made of a highly cross-linked plastic, and they do not seem to bond well with cross-linked resin base materials. The ridge lap of these resin teeth should be lightly ground with an arbor band prior to set-up in order to roughen the glazed
Fig. 5. The marked te eth are replaced in the mold after being thorou ighly c:Iraned. ‘I‘he marks hJP to get the teeth in the correct sockets in the mold.
Fig. 6. The cast is being replaced in the mold. It has been treated wi .th a separating medium. Note the location of sprue hole (S) and the vents (V).
Denture
yul~&l~ u
bases processed
from
resin
567
surface. This seems to result in a better bond between the teeth and the denture base resin. Spruing the mold. A No. 5 cork borer is used to cut an ingate sprue for filling the mold. For lower dentures, the cork borer should be positioned so that it will cut through to the denture base at the lingual side of the lower anterior teeth. This .will permit the fluid resin to begin filling the mold at the deepest anterior part of the denture base. The sprues for complete upper dentures should be located so they enter the mid-portion of the posterior palatal seal area. The No. 4 cork borer is used to cut the vents (Fig. 4). The vents should always be located so they will permit the resin to flow out of the most distal part of the denture base. With the flask standing on end, the vents will engage the highest part of the wax-up. If the vents are not positioned at the most distal aspect of the mold, air is likely to be trapped within the mold. This would result in voids in the base. Spruing and venting are not difficult once the principles are understood. Spruing for partial dentures will vary with each denture, but the same basic principles apply. The main sprue should permit entry of the resin at the deepest anterior base surface accessible, and the vent should permit exit of the resin at the most distal aspect of the base. spruing, the mold must be inspected Repositioning teeth and base. Following carefully for any loose pieces of hydrocolloid. These must be removed or they may become incorporated into the resin and result in a defect in the denture base. A light air blast is usually sufficient to remove any loose pieces that might be present.
Fig. 7. The mold is filled
through
the sprue hole until
the resin overflows
through
the vents.
568
Shepard
.I. Pros. Dent. June, 1968
If properly waxed, the average denture will present no problem in the repositioning of the teeth in the mold. The teeth will fit into place readily and not shift while the resin is being poured. Cotton pliers are used to replace the teeth in the mold (Fig. 5). After the teeth are repositioned, the attachment of the teeth to the mold is tested by inverting the flask and shakin g slightly. If no teeth fall from the mold, good tooth engagement is assumed. If a tooth has been ground extensively to accommodate occlusal or denture space requirements, a small amount of petroleum jelly is applied to the occlusal and facial surface of the tooth to retain it in position. The petroleum jelly must not be allowed to contact any part of the tooth that will be joined to the base. The cast should be cool before it is placed back into the mold, because a warm cast will accelerate the polymerization of the fluid resin and result in a lack of density of the denture base. If the cast is cleaned and coated first, it can be cooling while other steps in the preparation of the mold are carried out. Care should be exercised in replacing partial denture casts so that the artificial teeth are not moved by the elements of the metal framework. When the cast is repositioned in the mold (Fig. 61, the flask is reassembled and placed on end with the sprue holes accessible for filling the mold with resin. Mixing and pouring the rp.riz. The acrylic resin monomer should be stored under refrigeration, if possible. If the monomer is kept cool, the resin will have a longer .working time. The polymer and monomer must be accurately measured in cubic centimeters at the ratio of 30:12. This amount is sufficient for the average complete denture. The amounts used for partial dentures will vary. There will be increased shrinkage of the base during polymerization if too much monomer is used. Approximately 0.5 C.C. of the monomer wiil remain in the graduate after the monomer has been poured into the mixing beaker. This should be compensated for at the time the monomer is measured. (To avoid a build-up of set monomer in the graduate over a period of time, which will render the graduate inaccurate, the graduate should be inverted following each use to permit it to drain. j The monomer is poured into a glass beaker, and the polymer is added to it. The resin is stirred rapidly for 10 seconds with a metal spatula, swirled in the beaker for several seconds and then poured into the main sprue hole. The pouring should be completed within one minute. The resin tends to become too viscous for accurate results aftfter this time. A maximum pouring of three dentures at one time is recommended. A slight rocking of the flask from side to side during pouring will help displace any air that might be trapped within the mold and will prevent voids. The mold cannot be considered to be filled until the resin overflows through the vents (Fig. 7). Following pouring, the flask is allowed to set on the bench for 3 minutes before being placed in the processing unit. Curing. The flask is positioned in the processing? unit so that the vents and sprue holes are not directly under the. air intake valve. If they are positioned directly under the intake valve, the ini&l blast of air may displace some of the Auid resin in the mold and redult in a*void in the denture base. The flasks were designed so that two molds can be placed in a processing unit at the same time. About one inch of water at room temperatgre is placed in the processing unit, and the lid and air outlet valve axe closed; At, least 15 psi, of air pressu?e is turned into the pro-
VOlUrnP Number
19 6
Denture
bases ~rocessrd
from
resin
569
Fig. 8. The denture and cast are easily pushed out of the hydrocolloid mold.
Fig. 9. The processed dentures. Note the position of the sprues and vents.
Fig. 10. The casts are reseated in the mounting plaster on the articulator after the sprues are removed. Note the pin position, tooth contacts, and the detail of the wax pattern copied in the denture base with a clean sharp separation of the dentures from the hydrocolloid molds. cessing unit and maintained for 35 minutes. Air pressures of over 15 psi neither improve the density of the denture .base nor shorten the curing time, but air pressures under 10 psi result in less dense baseqg and may lead to pinpoint porosity. After 35 minutes, the air is evacuated from the processing unit, and the flasks are removed and disassembled. The dentures can be ejected from the hydrocolloid mold with finger pressure (Fig. 8). The sprues are removed with large carborun(Fig. 10). The dum discs (Fig. 9). Th en the denture is returned to the articulator hydrocolloid taken from the flask and denture is washed clean and stored for reuse.
570
Shepard
J. Pros. Dent.
June, 1968
Fig. 11. These test denture bases have been allowed to dry for,.one week after processing. The bases were removed from their casts and finished and polished through the pumice stage. Then they were replaced on their original casts, and the distal ends were ground flat on a model trimmer. A heat-cured base is less accurate even though it was processed according to the recommended procedures and meets A.D.A. Specification No. 12 (left). A denture base processed by the technique described in this article for fluid resin processing is shown on the right. After soaking these test bases for several days in water, both denture bases showed some improvement in fit.
Fig. 12. A processed, bilateral distal extension lower partial denture showing the locations of the sprues and vents. The space beneath the lingual bar and the cast in front have been blocked out with plaster.
Fig. 13. The location of the sprues and vents for a lower partial denture with three separate denture base sections. The sprue hole leading to the anterior segment is poured first, then the main sprues leading to the posterior bases are filled. A roll of utiiity wax is placed in the buccal fold region during the wax-up to connect anterior and posterior base segments.
Volume 19 Number 6
Denture
bases processed
from
resin
571
SUMMARY We have made about 14,000 dentures of all types using a fluid resin and a hydrocolloid mold. Technicians’ working time, and the over-all, elapsed processing time have been greatly reduced from that required by other methods. The working time per denture that has been saved amounts to 40 or 50 minutes. The finishing and remounting of dentures is simplified due to the clean and sharp separation of the cured dentures from the mold. The bases are more accurately adapted to the casts than were heat-cured resins. This was observed when test specimens of two A.D.A.-certified heat-curing resins were compared with the fluid resin specimens, both in a dry state and after six months in water (Fig. 11). The elimination of the gypsum investment and the use of the split flask seem to have reduced the amount of processing changes in the occlusion that had been consistently noted in our compression-molded dentures. Loss of vertical height of the occlusion of the dentures has been noted occasionally, but this has rarely exceeded 1 mm. when measured at the incisal pin of a Hanau articulator. About 10 per cent of the dentures have had voids as a result of incorrect sprue positioning or of pouring a too viscous resin. Most voids have been readily repaired with a small amount of resin and placed back into the processing unit with warm water for 5 minutes under 15 psi of air pressure. Repairs to the base are practically undetectable, even under magnification. This is probably due to the elimination of gypsum contamination and to the use of the original denture base material for repair. Pinpoint porosity will result if the processing unit loses air pressure, if inadequate air pressure is used initially, or if polymerization begins before the denture is put into the processing unit. All materials have handled well, even in room temperatures in excess of 90’ F., but polymerization is accelerated and the working time is shortened. Some partial dentures require rather complicated spruing procedures, but once the principles are learned, they presented no serious problem (Figs. 12 and 13). The movement of the distal extension metal framework due to packing pressures, such as is frequently observed when compression-molded techniques are used, appears to have been eliminated. Some tooth shifting has been noted where there has been extensive grinding on the teeth. With excessive grinding, the hydrocolloid does not adequately grip the tooth, and this permits tooth movement during pouring. Careful attention to detail must be followed, particularly during the wax-up. If a wax residue is left on the teeth prior to investing, a flash of resin over the tooth surface will invariably result. The average technician can be trained to perform the necessary processing procedures with a few days’ observation and practical experience. The procedure lends itself well to small or large laboratory operations and is well suited for use in a dental office. The equipment required is minimal and requires a small amount of storage space. The over-all cleanliness of the laboratory area is greatly improved due to the elimination of the gypsum investing materials. Our results with other fluid-type resins have been generally satisfactory, but our best results have been obtained with the resin described. This is probably due to the incorporation of a greater amount of polymer (2’/2 parts) in proportion to monomer (1 part) than the other resins usually contain (2 parts polymer to 1 part monomer). The glycerine or glycol type of hydrocolloid has given better results than those
572
Shepard
thinned
J.
June,
with water.
It seems to be tougher,
ing life. It does not have a tendency tacky
surface
PI<& Drnt.
which
seems to maintain
cut cleaner,
to dehydrate
and have a longer work-
as readily,
teeth in position
1968
better
and has a slightly during the pouring
and curing procedures. CONCLUSION A technique scribed.
After
significant
do not detract
certain
over
bases from fluid acrylic
resin has been de-
it is felt that the technique
compression-molding
problem
significantly
and technologic
denture
period of evaluation,
improvements
of development, material
for processing a two-year
methods,
areas exist which have been mentioned,
from the value of the procedure. research
in this area of denture
presents
In its present
state
but these
The need for further base processing
is indi-
cated. The author wishes to thank Colonel Duke and George C. Gresham for their cessing procedure.
William invaluable
T. Fisher and Sergeants First assistance in the development
Class Jeff J. of this pro-
References 1. Skinner, 2. 3. 4. 5. 6. 7. 8. 9.
E. W., and
Phillips,
R. W.: The
Science
of Dental
Materials,
ed. 5, Philadelphia,
1963, W. B. Saunders Company, p. 136. Roydhouse, R. H.: Materials in Dentistry; A Discussion for the Users of Dental Materials, Chicago, 1962, Year Book Medical Publishers, Inc., p. 88. Anthony, D. H., and Peyton, F. A.: Dimensional Accuracy of Various Denture-Base Materials, J. PROS. DENT. 1’2: 67-81, 1962. Rudd, K. D.: Processing Complete Dentures Without Tooth Movement, D. Clin. North America, pp. 675-691, Nov., 1964. Woelfel, J. B., Paffenbarger, G. C., and Sweeney, W. T.: Dimensional Changes Occurring in Dentures During Processing, J. A. D. A. 61: 413-430, 1960. Grant, A. A.: Effect of Investment Procedure on Tooth Movement, J. PROS. DENT. 12: 1053-1058, 1962. American Dental Association: Guide to Dental Materials, ed. 2, Chicago, 1964, American Dental Association, p. 61. Applegate, 0. C.: Essentials of Removable Partial Denture Prosthesis, ed. 3, Philadelphia, 1965, W. B. Saunders Smith, D. E., Lord, polymerizing Acrylic 103-l 15, 1967.
Company, p. 353. J. L., and Bolender, C. L.: Complete Resin Processed in Water Under Air
USA REGIONAL DEKTAL ACTIVITY WALTER REED ARMY MEDICAL CENTER WASHINGTON, D. C. 20012
Denture Pressure,
Relines With AutoJ. PROS. DENT. 18: