Reusable metal molds for ocular parts of orbital prostheses

Reusable metal molds for ocular parts of orbital prostheses David

M. Casey, D.D.S.*

State University

of New York at Buffalo, School of Dentistry, Buffalo, N.Y.

C

ustom-fitted methyl methacrylate resin ocular prostheseswere developed at the U.S. Naval Dental School in 1944 becauseof a scarcity of imported stock glass ocular prostheses. ’ Since that time acrylic resin has become the standard material for use in ocular prostheses,whether custom made or manufactured. Several articles have appeared in the prosthodontic literature that describevarious methods of fabrication of custom ocular prostheses.“’ Superior results for ocular prosthesesare obtained with the modified impression method.8 This method virtually eliminates the presence of irritating dead spacesin the defect and is far superior to attempting to adjust and modify the shapeof a manufactured ocular prosthesisto fit the defect. Ocular prostheseshave not usually been custom made for incorporation into an orbital prosthesis becausethe marginal shape is not critical and extra time is involved in custom molding. There are several advantagesto developing a simplified method for custom molding ocular portions of orbital prostheses.First, estheticscould be customized. Second,a relatively large supply of manufactured eyes would not have to be kept in stock. Third, the scleral curvature of the prosthesiscould be decreased.5Frequently when the inner and outer canthi of an orbital prosthesis are carved to the proper dimensions, the curvature of the globe is found to be too severe. The scleral surface of the ocular part becomestoo deep in relation to the canthi becausethe curvature is too great (Fig. 1). The purposeof this article is to describethe fabrication of a reusable metal mold that simplifies the fabrication of customocular portions of orbital prosthe-

Presented at the American Academy of Maxillofacial Prosthetics, San Diego, Calif. *Clinical Assistant Professor, Departments of Removable Prosthodontics and Otolaryngology; Consultant, Department of Dentistry and Maxillofacial Prosthetics, Roswell Park Memorial Institute, Buffalo. N.Y.

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Fig. 1. Curvature of manufactured ocular prostheses is often too great and appearstoo deep when carving inner and outer canthi.

Fig. 2. Custom tooth mold used by Payne? ses.The use of thesemetal maids puts custom fabrication within the realm of practicality. The molds are used to form ocular prosthesesby a method that is most often usedby ocuhuiststoday. The method useshand-painted iris disks (Robert B. Scott Ocular&s of Florida Inc., Tampa, Fla.) attached to preformed cornea1buttons (Robert B. Scott Ocularists of Florida Inc.) and embeddedin scleral acrylic resin. The sclera is pulled back, characterized, and awered with a layer of clear corneal acrylic r&n. The method has been describedby Cain,’ Cox,” and Parr.‘”

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Fig. 3. Layer of baseplate wax is adapted over sto ck ocular prosthesis, and wax that overlies iris is removed.

MOLD

Fig. 4. Wax shell is separated from stock ocular prosthesis.

CONSTRUCTION

Metal molds have been used for a number of years for custom molding denture teeth to improve esthetics. Payne” simplified mold construction by using autopolymerizing master teeth and low-fusing Cerrotru metal (Ontario Metal Supply, Inc., Rochester, N.Y.) for the molds. The method of mold construction for ocular prostheses presented here is adapted from the technique described by Payne” for custom tooth molds (Fig. 2).

Master

ocular prosthesis

The master ocular prosthesis is used as the pattern to form the mold and is made of acrylic resin. The following steps describe its fabrication. 1. The anterior surfaces of a right and left manufactured ocular prosthesis are covered with a layer of softened baseplate wax. The wax that extends beyond the outer edge of the prosthesis is removed. 2. A circle of the wax that overlies the iris is cut out. Its diameter is slightly less than the diameter of the iris on the stock eye (Fig. 3). 3. The wax shell is chilled and carefully separated from the stock eye (Fig. 4). 4. A cornea1 button with a black iris disk cemented to it for contrast is placed from the back into the opening in the wax shell. The peg on the iris-corneal piece should be positioned exactly in the center of the opening. The wax shell around the iris opening should

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Fig. 5. Cornea1 button is added to wax shell. be carefully softened with an alcohol torch to allow the iris-cornea1 button to be positioned properly anteriorly (Fig. 5). 5. The corneal button is sealed to the wax shell and waxed to the proper contour. Another layer or two of baseplate wax is softened and used carefully to form the posterior surface of the wax-up to be similar to the form of the stock prosthesis. 6. After the wax-up is complete, it is immersed in 140” F water to soften it. When soft, the cornea1 curvature is reduced carefully by stretching the wax in

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Fig. 6. Inner and outer canthi decrease curvature of prosthesis.

are stretched

to Fig. 9. Open mold after boiling out.

Fig. 7. Diagram shows improved contour of prosthesis for incorporation into orbital prosthesis.

Fig. 10. Acrylic resin master blank is embedded in stone in section of IL-inch diameter automobile radiator hose. Fig. 8. Wax-up is half flasked in drag of fixed partial denture flask. the regions of the medial and lateral canthi (Fig. 6). This procedure, described by Couillard and Schaaf,5 provides a cornea1 surface that can accommodate the wider intercanthi distance often needed in carving orbital prostheses (Fig. 7). If a flatter eye is needed, the stock eyes can be bent after the application of heat. The eye is heated in a glass bead box, used by opticians for frame adjustment, at a temperature of 375°F. The eye is heated until it is

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just pliable to bend and quickly placed on a flat surface. The cornea is pressed to reduce the curvature and held for a few seconds until it cool~.‘~ 7. The wax-up is chilled, and the anterior surface is carefully polished. The wax-up is flasked in a bubblefree mix of die stone in a fixed partial denture flask (Fig. 8). 8. After boiling out the wax, the gypsum mold is coated with a tinfoil substitute, packed with autopolymerizing acrylic resin, and placed in a press to cure. The iris-cornea1 button is retained in the upper half of the flask by its peg (Fig. 9).

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Fig. 13. Completed top half of mold separated from stone base.

Fig. 11. Keying of stone.

Fig. 14. Pouring top half of mold.

Fig. 12. Cerrotru mold.

metal used to pour top half of

9. After the resin is cured, the master acrylic resin ocular prosthesis is removed from the gypsum mold, with caution taken not to break the corneal peg. The blank is carefully trimmed and polished and is ready for use as a pattern in the preparation of the metal molds.

Metal

mold

Materials for the metal mold include two sections of 2-inch diameter automobile radiator hose in lengths of % and 1% inch. Cerrotru metal, composed of 58% bismuth and 42% tin, is used for the molds. It has a melting point of 281 o F, which is low enough to be cast

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Fig. 15. Completed mold. against acrylic resin without melting it and high enough to allow it to be placed into boiling water for processing without melting. The technique is as follows. 1. The %-inch section of hose is filled with vacuumspatulated stone. The concave back of the master blank

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Ei& 16. Patterns for obturation of peg holes sprued in preparation for casting.

Fig. 17. Completed iris-cornea1 button is inserted into top half of mold

is filled with stone and set into the hose, with care taken not to sink it below the height of contour (Fig. 10). 2. After the stone has set, it’s removed from the hose, and three wedge-shaped keys are cut into the stone: one on one side and two on the opposite side close together. This separation of keys allows for easy orientation and closure of the die during the molding procedure (Fig. 11). 3. The stone that contains the acrylic resin blank is placed in an oven at 250” F overnight to dehydrate the stone. 4. After the stone is dehydrated, the stone-blank assembly is placed into the bottom of the li%-inch piece of hose. Immediately, Cerrotru metal, which has been melted in a plumber’s ladle over a gas flame, is poured into the ring to the top of the cornea1 peg but not over it (Fig. 12). The metal must not be overheated. An unmelted piece of metal will float in the molten metal because of slight expansion of the metal when it hardens (decreased density). The molten metal will be at its casting temperature (281” F) when the edges of the floating piece stop melting or begin to grow. This is the ideal pouring temperature and is most critical when the second half is poured against the first half; otherwise fusion will occur between the two pieces. 5. After the metal cools, it is removed from the ring and separated from the stone base. The acrylic resin blank will be retained in the metal half by the cornea1 peg (Fig. 13). It can be removed by gently tapping out

the peg with a laboratory handpiece mandrel. If the stone was properly dehydrated and the acrylic resin blank was not contaminated by skin oils, a smooth surface should be evident in the metal. 6. The hottom half of the mold can now be poured. The acrylic resin blank is carefully reseated in the upper half of the Cerrotru mold. The assembly is placed in the bottom of the 1W-inch hose (Fig. 14) and warmed to 150” F in an oven to eliminate moisture condensation from the air. The Cerrotru metal is again melted as described in step No. 4 and poured against the top half of the mold to create the bottom half of the mold. After it is cool, the completed mold is removed from the radiator hose ring, and the acrylic resin blank is removed (Fig 15). 7. When the outer clear corneal layer is being processed, the positioning hole from the cornea1 button peg in the top half of the mold must be closed. A plastic sprue of the correct diameter is inserted into the mold to the proper length, which is level with the comeal surface of the mold. The plastic sprue is attached to a sprued wax-up ifor a chrome alloy removable partial denture frame and cast in a chrome alloy (Fig. 16). The cast pin is cut to length, and used as a filler in the peg hole during the final step of the molding procedure. The right and left ocular molds are complete and ready for use in the fabrication of the ocular part of any orbital prosthesis. After it is processed, the ocular prosthesis can be ground in the same way that a

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Fig. 18. Scleral blank with embedded iris-cornea1 button ready for trimming and pull-back of sclera.

Fig. 20. Grinding

back sclera.

Fig. 21. Characterizing Pig. 19. Close-up of cast peg in place in top half of mold in preparation for addition of cornea1 layer. manufactured ocular prosthesis is adjusted to any desirable size to fit an orbital defect.

MAKING CUSTOM OCULAR FOR ORBITAL PROSTHESES

PORTIONS

This technique allows the dentist the freedom of artistry that is one of the main advantages of the custom prosthesis, combined with the ease of use of prefabricated reusable molds. The technique is unique only in the use of the prefabricated metal molds. The method has been described by Cox,9 Paar,” and Cain.’ Only the basic steps of the procedure will be outlined, because the main purpose of this article is to describe construction of the metal molds. 1. With dry artists pigments thinned with a 1O:l clear acrylic resin monomer-polymer mixture and pure

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sclera.

monomer, paint the iris disk to match the iris of the natural eye of the patient. The disk is cured in an oven at 40” C. 2. The painted disk is sealed to a cornea1 button with the monomer-polymer mixture and cured at 40“ C for % hour. 3. The peg of the iris-cornea1 button is inserted into the hole in the top half of the appropriate mold (right or left) and seated fully (Fig. 17). 4. A mix of the appropriate shade of scleral acrylic resin is packed in the mold, placed in the press, and heat cured. 5. The mold is opened and the prosthesis is removed (Fig. 18). A precast chrome alloy sprue is placed into the peg hole in the upper part of the mold (Fig. 19). 6. The acrylic resin peg is ground off the cornea1 button, and the sclera is ground off the cornea to leave only a slight amount remaining over the outer limbus

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custommolded with practicality. The metal usedin the construction of the mold8 has a life span of approximately 100 moldings before distortions limit its further usefulness.” At that point the mold must be repoured. If a more permanent mold is desired,a cobalt-chromefaced mold can be made according to the method outlined by Casey et a1.13

1. 2. 3.

Fig. 22. Trial

packing clear corneal layer.

area of the iris (Fig. 20). The entire scleral surface is reduced 1 to 2 mm and polished. The prosthesis is ready for scleral characterization and addition of the cornea4layer. 7. The sclera is characterized by a wash of appropriately colored pigmentsto match the patient’s natural eye. Matching vasculature in the form of red cotton fibers is added(Fig. 21), coveredwith a layer or two of the monomer-polymer mixture, and cured at 40” C for 10 minutes. 8. The prosthesis is completed by addition of the final corneal layer. Clear heat-cured acrylic resin is trial packed with cellophane between the acrylic resin and the painted sclera (Fig. 22). When the proper amount is in the mold, the cellophane is removed for final closure of the mold, and processing is completed. 9. The finished prosthesisis retrieved, trimmed, and polished. CONCLUSION With the useof one right and one left metal ocular mold, ocular portions of orbital prosthesescan be

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5.

6. 7. 8. 9.

10. 11. 12. 13.

Bartlett, S. O., and Moore, D. J.: Ocular prosthesis: A physiologic system. J PROSTHET DENT 29:450, 1973. Welden, N.: Ocular prosthesis. J PRCXSTHCT DENT 6~272, 1956. Brown, K. E.: Fabrication of an ocular prosthesis. J PROWHET DENT 24:225, 1970. Moore, D. J., Ostrowski, J. S., and King, 1,. M.: A quasiintegrated custom ocular prosthesis. J PROSTHFT DENT 32:439, 1974. Couillard, P., and Schaaf, N. G.: Fabrication of the ocular portion of an orbital prosthesis. J PROSTHEI’ DENT 35:478, 1976. Benson, P.: The fitting and fabrication of a custom resin artificial eye. .J PUOSTHET DENT 38~352, 1977. Cain, J. R.: Custom ocular prosthetics. J PRUFTHET DENT 48~690, 1982. Allen, L., and Webster, H. E.: Modified impression method of artificial eye fitting. Am J Opthalmol 6El89, 1969. Cox, Jr., C. W.: Ocular and orbital acrylic prostheses. Presented at the American Academy of Maxillafacial Prosthetics, New Orleans, 1979. Paar, G.: Ocular prosthesis. Presented at the American Academy of Maxillofacial Prosthetics, St. Louis, 1981. Payne, S. H.: ~Construction of custom denture teeth. Dent Clin North Am l&333, 1975. American Optical Corporation: Monoplex Bulletin No. 46. Southbridge, Mass., 1964, p 5. Casey, D. M., Lauciello, F. R.. Crother, D., Payne, S. H., and Wiegel, H.: Fabrication of cobalt-chromium dies for custom tooth molding. Quintessence lnt Dent Digest 9:29, 1980.

Re#mnt request\ to: DR. DAVID M. CASE! 2 CONCORD DR. ORCH,\RLI PARK, NY 14217

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