Production of accurately fitting full-arch framelwo:rks: Part I-Clinical procedures GE Goll, DDS, Seat,tle 15.asir
implant
MSDa
The production of a full-arch metal casting that seats passively on multiple implant fixtures is technically a considerable undertaking. Despite the most meticulous attention to detail, this procedure may be impossible to obtain every time with the resources presently available. This article describes possible errors and makes recommendations as to how the clinical steps can, through careful clinical procedures, be controlled. The question of whether to cast in one piece or in multiple sections that are soldered is discussed. (J PROSTHET DENT 1991;66:377-84.)
0
sseointegrated implant frameworks for full and partial fixed part.-[al dentures should fit passively on the fixtures. Many authors have mentioned the importance of olbtaining this passive fit. 1-6 As yet no research has been cited that discusses the danger in not having a passive fit. Some articlesi-‘” ,. havt> noted the possibility that stresses induced in tine fixture. components, prosthetic reconstruction, and the surrounding bone could lead to the loss of osseointegration. Personal experience has shown that perhaps 25’( of metal frameworks for full-arch restorations do not fit accurately despite the most meticulous attention to det,ail (Fig. 1). A few auth(:rs Is, l6 have also demonstrated and measured &is discrepancy anc: some have described techniques for either correcting discrepancies by soldering the
Presented at the Pacific t :oast Society of Prosthodontists meeting. Napa Valley. (‘alif. %linical Associate E’rofrssor. University of Washington, School ot Dentistry, !icattle. W,ish. 10/l/29231
framework or have recommended that the framework be constructed in sectionsl”, Ii and then soldered after indexing intraorally or on the master cast,. This arricle defines and discusses some of the parameters involved in obtaining a one-piece, full-arch metal casting that accurately fits Nobelpharma (Nobelpharma. Nobel Industries, Gothenburg, Swedeni osseointegrated implants.
REVIEW
OF LITERATURE
The literature is replet,e with articles discussing the accuracy of dental crown margins. Certain authors have studied the ability to produce one-piece castings for multiple-unit fixed partial dentures.‘s-” The conclusion may be drawn that multiple-unit castings are technique sensitive and that some distortion is inevitable. As a casting increases in length so do the inaccuracies.‘” Eden et al.,23 in t,heir experiment. tried to st,andardize the mold conditions for producing single-unit porcelain fused-to-metal crown and fixed partial denture castings. They addressed the following parameters: casting ring size. asbestos liner, position
Fig. 1. A, C’smpleted fixed partial denture demonstrates close intimate contact between abutment and gold cylinder. B, Casting demonstrates improper seating of cylinders on their abutments when tightened with one guide pin.
THE
JOURNAL
OF PROSTHETIC
DRNTISTRY
GOLL
Fig. 2. With center screw wrench and abutment forceps, center screw is checked for tightness to ensure proper seating of abutment each time. This step is essential before impression making, or when checking accuracy of framework.
it may be impossible to obtain an accurately fitting framework with the methods and materials presently available.24-27 Siirila et a1.26claimed that the metal framework shrinks on contraction and recommended that the gold cylinders be joined to the prosthesis with acrylic resin. Parelz4 also described a technique whereby a cast metal bar joined the first and last cylinder, thus obviating the necessity of providing an accurate and complex casting. He recommended joining the middle cylinders to the prosthesis with autopolymerizing acrylic resin. An implant framework is usually horseshoe-shaped and its weight is, on average, approximately 20.5 gm with the casting button. This large mass of metal when molten will be propelled at high speed during casting. As a result it will hit the gold cylinders with a high force and could dislodge them since they are only weakly supported by investment at their exposed ends and through their hollow central core. Controlling the expansion and contraction of metal and investment are essential steps often arrived at by trial and error. The casting ring is necessarily large for a large implant framework, (usually 3 inches in diameter) and the volume of metal is correspondingly great. Alternatives to the standard technique of casting a fullarch superstructure have been described.26v2812g Henry30 described a two-part prosthesis that takes care of poor implant fixture alignment. Stridz5 proposed that on the basis of his photogrammetric technique a large framework could be specially cut from a single monolithic piece of metal, a solution that he admits may be impractical.
MATERIAL
AND METHODS
The following suggested solutions will improve the accuracy of the casting and reduce some of the possible errors.
Impression
Fig. 3. If center screw wrench does not fit properly over hexagonal head of center screw, the center screw must be replaced.
of pattern, investment type, powder-liquid ratio, spatulation, setting time before casting, burnout sequence, removal of casting ring from oven, casting machine, melt soak time, and melting temperature of the oven. Their findings showed Ni-Cr castings to be consistently undersized and precious metal castings to be oversized. Clearly a large number of parameters must be considered during any dental casting procedure. Because of the complexity of the process, it is not surprising that failures may arise. Most authorities agree that implant framework castings must be accurate. One article17 mentions the increased need for accuracy in the maxillary reconstruction because of the anatomy and type of bone. It has been suggested that
technique
Various approaches L 28to the impression techniques using plaster, hydrocolloid, irreversible hydrocolloid, and elastomeric impression materials have been described. Attention to detail is vital to obtain an exact replica of the oral configuration. It is essential that the superior surface of each abutment is spotlessly clean. The center screw should be checked for tightness every single time an impression is being made or the framework accuracy is being checked to ensure the abutment is secure (Fig. 2). On occasion the wrench does not fit over the center screw’s hexagonal head (Fig. 3) or the center screw does not seat fully. The fit of the gold cylinders must be checked against the brass analogues (Fig. 4) and on the abutment at the first clinical appointment so that any defective components can be changed by the oral surgeon. The use of an abutment cover cap (Implant Innovations, W. Palm Beach, Fla.) helps to maintain a clean surface on the abutment between appointments (Fig. 5). It is preferable to use only machined components instead of castable plastic counterparts because they generally
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:Fig. 4. Passive, accurate seating of brass analogues is checked against ,gold cylinder before clinical appointment identifies any faulty components.
Fig. 5. Cover C,E~PS secured with hexagonal-head screwdiriver maintain cleanliness of abutment head between clinical appointment -.
provide a better fit. Each of the following impression techniques uses a custom open-top tray. Transfer copings. The smooth tapered type of coping is designed for use with hydrocolloid, and the square type is used either with plaster or elastomeric materials1 Humphries’” tested splinting the square copings with dental floss and covering the floss with autopolymerizing resin (Duralay, Reliance Mfg. Co., Worth, Ill.) (Fig. 6). No statistical difference was observed in comparing the accuracy of impressions using smooth, square, or square copings splinted in this fashion. An elastomeric impression material such as Impregum F (Premier Dental Products Co., Norristown, PA.) has i;ufficient body to prevent movement
-*Humphries.
THE
JOURNAL
R.M. :f’erscbnal communication.
OF PROSTHETIC
DENTISTRY
Portage, Mich. 198fl.
Fig. 6. Square transfer copings have been attached to abutments in this maxillary partially edentulous patient. Ribbon floss has been wrapped around transfer copings and bridged with Duralay resin.
Fig. 7. Open-topped autopolymerizing acrylic resin impression tray permits access to superior surface of guide pins. These may he loosened when the impression material has set, permitting removal of tray ,lnd transfer copings in toti;
of the transfer copings on removal from the mouth. The tray is loaded with the material and t,he remainder is injected around the copings before the tray is seated. The open-top design permits loosening of the guide pins so that the entire assembly can be removed as a single unit (Fig. 7). Brass analogues are attached to the transfer copings and the impression is poured in a conventional manner. Gold cylinders. In spaces where access or vertical height is limited. the gold cylinder itself can be used as a t,ransfer coping. A variation of this approach is that of joining the gold cylinders with Duralay resin2. l1 thereby forming a acrylic resin “bridge,” thr fit of which can be tested directly in vivo before rhr Lransfet- impression is made (Fig. h. rl). A furt,her variation 01’this jrchnique is t,he
379
Fig. 8. A, Duralay acrylic resin is painted on top of utility wax, bridging gap between adjacent gold cylinders, which have been secured to abutments with guide pins. B, More rigid connection is desirable for longer spans. Steel burs are luted to Duralay resin bridge with more resin. C, Impression plaster is placed over Duralay resin bridge, ensuring completely rigid structure.
use of used friction-grip burs secured with Duralay resin to bridge and strengthen the span (Fig. 8, B). In addition, the use of impression plaster on top of the acrylic resin index will further strengthen the structure into a rigid mass (Fig. 8, 0
Verification
of fit of plastic
framework
fabrication seem to be either Duralay or Relate (Parke11 Products, Farmingdale N.Y.) resins. Another alternative to an acrylic resin fixed partial denture is the use of a stout wire attached to the transfer copings with Zapit (Dental Ventures of America, Anaheim Hills, Calif.)llr 31or composite resin. This rigid assembly is easily tried onto the abutments. Another recommendation is the use of new guide pins with each patient. Experience has shown that the new flat guide pins (DCA 094; Nobelpharma, Nobel Industries) are more easily distorted during laboratory procedures than the tapered guide pins (DCA 027; Nobelpharma, Nobel Industries). Hence, the use of new guide pins will limit the possibility of distortion. The larger 4 mm gold cylinders are preferable because the acrylic resin fixed partial denture can be made thicker and hence sturdier to prevent springing and bowing of the framework.
Rasmussen2 and Henry l1 have described techniques for verifying the accuracy of the master cast by using a verification jig made of Duralay resin (Fig. 9). It is essential that the plastic facsimile of the fixed partial denture be tested intraorally to verify an accurate fit. If this framework is made directly intraorally,2, I1 verification is straightforward because the framework is loosened, removed, and replaced. If the plastic framework is made indirectly on the master cast, it should be thickened adequately to prevent possible flexure. Curing of acrylic resin should be in a pressure pot. It has been recommended that the resin be processed the night before and left bolted down overnight.* Materials of choice for the resin framework
Alternatives not fit
*White G. Academyof Osseointegrationmeeting, Dallas, Texas, 1989.
When the master cast and the intraoral configuration do not coincide the choices are as follows. 1. Remake the impression by using any of the described alternatives.
2RCl
if the plastic
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framework
1991
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Fig. 11. Fit of sandblasted casting on master cast is evalIFig. 9. Duralay acrvlic resin verification jig constructed on master ,-a&s is tried intreorally to verify accuracy ot master cast.
Fig. 10. Sectioned Duralay acrylic resin bridge is rein dexed intraorally with new ac,rylic resin. New brass analogues are attached to gold cylinders and assembly is placed in patty of fresh ‘die -t,one.
2. Section the plastic framework and index the new relationship with the same autopolymerizing resin. The malpositioned brass analogue is sectioned from the master cast by use of a 4-inch disk (E.C. Moore Co., Inc., Dearborn, Mich.) and .a new brass analogue is secured to the master cast with dental die stone. 3. After the plastic, framework is sectioned and reindexed, secure new analogues to all of the transfer copings and pour a n,ew master cast or a simple stone patty (Fig. 10). This cast serves as an accurate model to verify the fit of the metal framework. The original anatomically correct cast is the one used for articulator msuntings and tooth-positioning.
Casting
uated by use of one guide pin in least fav~orable position. typically terminal abutment. Presence cut’ openings between remaining gold cvlinder and bras5 analogues are looked hr.
Fig. 12. View of gold cylinder fit. surface under magnification demonstrates imperfections on smooth surface (arrou’j due to metal fiash occurring during casting when wax residue has not been adequately remo\.ed
t.he advantages of casting the framework in one piece”, il. :Z or casting in sections and soldering.‘“* ‘, Others have recommended casting in one piece and soldering if the framework does not fit passively.‘, ii’. a2 White* recommended casting in one piece because the work hardening properties of the metal are maintained whereas after soldering two or three pieces of a framework, the properties are altered and the soldered framework cannot be heat treated. Another obvious ramification is that, overall, less laboratory time is involved in casting a framework in one piece.
procedure
The actual casting and soldering techniques will be discussed in a sribsequen- article. Reference has been made to THE
JOURNAL
OF PJJOSTHETIC
DENTISTRY
381
GOLL
Fig. 13. Maxillary fixed partial denture is inspected intraorally for accuracy by use of single guide pin. Observation of openings between any remaining gold cylinders and abutments implies inaccurate casting.
sandblasted and the fit is verified on the master cast by use of one gold screw or new guide pin in what is believed to be the least favorable position. Most often this position is the terminal fixture on one side (Fig. 11). The use of a binocular microscope is encouraged to inspect the inferior surface of each gold cylinder for imperfections of the castings, which should be removed carefully using a small round high speed bur (Fig. 12). The framework is then tried in the mouth according to the protocol described previously, by use of a gold screw in a terminal abutment (Fig. 13). FitChecker (GC Dental Industrial Corp., Tokyo, Japan) material or pressure-indicating paste (Mizzy Inc., Cherry Hill, N.J.) can aid in checking the contact distribution if any discrepancy exists (Fig. 14). Questionable surfaces can then be judiciously adjusted. If the framework does not fit as expected, it should be sectioned with a fine separating disk. The parts are indexed by use of Duralay resin. A new working cast is then produced by either attaching new brass analogues and pouring a new cast, or the malpositioned brass analogue is sectioned from the original master cast and replaced with a new one using die stone (Fig. 15).11The framework can then be soldered and the fit verified on the master cast.
DISCUSSION
Fig. 14. Fit-Checker material is placed on gold cylinders before casting is seated intraorally. Areas of inadequate contact will be demonstrated by presence of material on cylinder surface.
Metal
selection
Dentists select alloys of various types; however, a type IV dental alloy that has a yield strength high enough to withstand the occlusal forces imposed on it should be used. Because of its size and cost, substitutes for gold alloys have been sought. Silver-palladium alloys are popular in the United States because they have the required strength233*5,l6 and are less expensive. Before investment of the framework, the fit surface of the gold cylinder should be cleaned with wax solvent to remove all wax or acrylic resin residues. This procedure reduces the risk of metal flash flowing over the cylinder surface. No wax should extend beyond the perimeter of the fit surface of the gold cylinder margin.32 When recovered from its investment, the framework is 382
The system dentists use to produce an accurate casting has many variables, each of which must be appropriately considered, to provide a final result that will be clinically acceptable. Klineberg and Murray15 have shown that there may be an opening up to 20 to 30 Mm between the cylinder and the abutment. It is evident that careful control of the metal and of the expansion and contraction of the investment are critical steps. Avoidance of detail to these measures could easily result in distortion of the framework dimensions. Contraction of such a large mass of metal on cooling is conceivably too great a force for the investment to counteract. The resultant change in dimensions not only linearly but in all directions causes a rotation of the gold cylinders about their long axes. Although soldering is usually a relatively simple task, in the case of a large framework, it is a formidable task because the contraction of a metal framework of this size can cause the investment to crack and distort. Moreover, soldering multiple pieces of the framework may not be possible when the investment is not strong enough to withstand the enormous forces produced during the cooling of the metal. Perhaps with present techniques it is not likely that a casting of this size can be produced accurately every time.24 The laboratory must receive an accurate master cast on which it can make an accurate casting. Another consideration is the flexure of the mandible itself.33 It is conceivable that with varied openings of the mandible, the precise angulations of the fixtures and hence the transfer copings may change. Such a theory warrants further research. Inadvert-
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1991
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15. A, Inadequately fitting framework has been sectioned and reindexed intraorally with Duralay acrylic resin. New brass analogue has been attached to gold cylinder. Oflending brass analogue was sectioned from master cast. R, Fresh die st,one is poureti into var ant reces,s. securing analogue to cast.
Fig.
!. Check the fit of gold cylinders and transfer copings onto the abutment ‘cylinders at the first clinical appointment following the second stage surgery to ensure that all of the components fit together properly. 3. Cover the abutments with ,zover caps to prevent debris from accumulating on the abutment surface. 4. Double check the accuracy of the master cast by using an acrylic resin fixed partial denture facsimile before proceeding to the casting of the framework. Construct the acrylic resin fixed partial denture the night before it is needed and lea\e it bolted down overnight. 5. IJse new guide pin+ for the laboratory stages and when checking the fit of she cast metal framework intraorally. Test t,he tit of the ( asting by%using only one screw at one end of the frameuork. 6. Clean the fitting surface of each gold cylinder with wax solvent before investing. ‘i. Cast the metal framework n one piece if possible. 8. Cover the cylinder:, with stainless steel protector caps or duplicate brass annlogues to protect the surface when polishing the framework. 9. For full-arch castiiigs use 4 mm gold cylinders because the framework will have more bulk and sturdiness. The acrylic resin try-in i’ramework will similarly be less likely to distort.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
SUMMARY Each step in the fabrication of a one-piece metal implant framework casting is of critical importance in producing an accurately fitting framework. The various stages of this procedure have been described and discussed in this part of the articles. Alternative techniques to limit inaccuracies have been suggested. A list of recommendations have been included
GOLL
15. Klineberg IJ, Murray GM. Design of superstructures for osseointegrated fixtures. Swed Dent J 1985;28(Suppl):63-9. 16. Leung N, Zarb G, Watson P. Non gold alloy systems for prosthetic frameworks [Abstract]. J Dent Res 1982;62:324. 17. Lundquist S, Carlsson GE. Maxillary fixed prostheses on osseointegrated dental implants. J PROSTHET DENT 1983;50:262-70. 18. Fusayama T, Wakumoto S, Hosoda H. Accuracy of fixed partial dentures made by various soldering techniques and one-piece casting. J PROSTHET DENT 1964;14:334-42. 19. Bruce RW. Evaluation of multiple unit castings for fixed partial dentures. J PROSTHET DENT 1964;14:939-43. 20. Bruce RW. Clinical applications of multiple unit castings for fixed prostheses. J PROSTHET DENT 1967;18:359-64. 21. Huling JS, Clark RE. Comparative distortion in three-unit fixed prostheses joined by laser welding, conventional soldering or casting in one piece. J Dent Res 1977;56:128-34. 22. Hinman RW, Tesk JA, Parry EE, Eden GT. Improving the casting accuracy of fixed partial dentures. J PROSTHET DENT 1985;53: 466-71.
23. Eden GT, Franklin OM, Powell JM, Ohta Y, Dickson G. Fit of porcelain fused-to-metal crown and bridge castings. J Dent Res 1979;58: 2360-S. 24. Pare1 SM. Modified casting techniques for osseointegrated fixed prosthesis fabrication: A preliminary report. Int J Oral Maxillofac Imp 1989;4:33-40.
384
25. Strid KG. On the application of photogrammetry to the fitting of jawbone-anchored bridges. Swed Dent J 1985;28(Suppl):93-105. 26. Siirila HS, Nordberg L, Olkarinen VJ. Technique for converting an existing complete denture to a tissue-integrated prosthesis. J PROSTHET DENT 1988;59:463-7.
27. Chiche GJ, Weaver C, Pinault A, Elliot R. Auxiliary substructure for screw-retained prostheses. Int J Prosthodont 1989;2:407-12. 28. Hobo S, Ichida E, Garcia LT. Osseointegration and occlusal rehabilitation. Chicago: Quintessence Pub1 Co Inc, 1989;176. 29. Pare1 SM. Implants and overdentures: the osseointegrated approach with conventional and compromised applications. Int J Oral Maxillofac Imp 1986;1:93-9. 30. Henry PJ. Two-piece cast superstructure for mandibular osseointegrated bridgework. Quintessence of Dent Tech 1987;11:321-7. 31. Knudson RC, Williams EO, Kemple KP. Implant transfer coping verification jig. J PROSTHET DENT 1989;61:601-2. 32. Loos LG. A fixed prosthodontic technique for mandibular osseointegrated titanium implants. J PROSTHET DENT 1986;55:232-42. 33. Brunski JB. Biomaterials and biomechanics in dental implant design. Int J Oral Maxillofac Imp 1988;3:85-97. Reprint requests to: DR. GEORGE E. GOLL 737 MEDICAL-DENTAL BLDG. SEATTLE, WA 98101
SEPTEMBER
1991
VOLUME
66
NUMBER
3
implant
MSDa
The production of a full-arch metal casting that seats passively on multiple implant fixtures is technically a considerable undertaking. Despite the most meticulous attention to detail, this procedure may be impossible to obtain every time with the resources presently available. This article describes possible errors and makes recommendations as to how the clinical steps can, through careful clinical procedures, be controlled. The question of whether to cast in one piece or in multiple sections that are soldered is discussed. (J PROSTHET DENT 1991;66:377-84.)
0
sseointegrated implant frameworks for full and partial fixed part.-[al dentures should fit passively on the fixtures. Many authors have mentioned the importance of olbtaining this passive fit. 1-6 As yet no research has been cited that discusses the danger in not having a passive fit. Some articlesi-‘” ,. havt> noted the possibility that stresses induced in tine fixture. components, prosthetic reconstruction, and the surrounding bone could lead to the loss of osseointegration. Personal experience has shown that perhaps 25’( of metal frameworks for full-arch restorations do not fit accurately despite the most meticulous attention to det,ail (Fig. 1). A few auth(:rs Is, l6 have also demonstrated and measured &is discrepancy anc: some have described techniques for either correcting discrepancies by soldering the
Presented at the Pacific t :oast Society of Prosthodontists meeting. Napa Valley. (‘alif. %linical Associate E’rofrssor. University of Washington, School ot Dentistry, !icattle. W,ish. 10/l/29231
framework or have recommended that the framework be constructed in sectionsl”, Ii and then soldered after indexing intraorally or on the master cast,. This arricle defines and discusses some of the parameters involved in obtaining a one-piece, full-arch metal casting that accurately fits Nobelpharma (Nobelpharma. Nobel Industries, Gothenburg, Swedeni osseointegrated implants.
REVIEW
OF LITERATURE
The literature is replet,e with articles discussing the accuracy of dental crown margins. Certain authors have studied the ability to produce one-piece castings for multiple-unit fixed partial dentures.‘s-” The conclusion may be drawn that multiple-unit castings are technique sensitive and that some distortion is inevitable. As a casting increases in length so do the inaccuracies.‘” Eden et al.,23 in t,heir experiment. tried to st,andardize the mold conditions for producing single-unit porcelain fused-to-metal crown and fixed partial denture castings. They addressed the following parameters: casting ring size. asbestos liner, position
Fig. 1. A, C’smpleted fixed partial denture demonstrates close intimate contact between abutment and gold cylinder. B, Casting demonstrates improper seating of cylinders on their abutments when tightened with one guide pin.
THE
JOURNAL
OF PROSTHETIC
DRNTISTRY
GOLL
Fig. 2. With center screw wrench and abutment forceps, center screw is checked for tightness to ensure proper seating of abutment each time. This step is essential before impression making, or when checking accuracy of framework.
it may be impossible to obtain an accurately fitting framework with the methods and materials presently available.24-27 Siirila et a1.26claimed that the metal framework shrinks on contraction and recommended that the gold cylinders be joined to the prosthesis with acrylic resin. Parelz4 also described a technique whereby a cast metal bar joined the first and last cylinder, thus obviating the necessity of providing an accurate and complex casting. He recommended joining the middle cylinders to the prosthesis with autopolymerizing acrylic resin. An implant framework is usually horseshoe-shaped and its weight is, on average, approximately 20.5 gm with the casting button. This large mass of metal when molten will be propelled at high speed during casting. As a result it will hit the gold cylinders with a high force and could dislodge them since they are only weakly supported by investment at their exposed ends and through their hollow central core. Controlling the expansion and contraction of metal and investment are essential steps often arrived at by trial and error. The casting ring is necessarily large for a large implant framework, (usually 3 inches in diameter) and the volume of metal is correspondingly great. Alternatives to the standard technique of casting a fullarch superstructure have been described.26v2812g Henry30 described a two-part prosthesis that takes care of poor implant fixture alignment. Stridz5 proposed that on the basis of his photogrammetric technique a large framework could be specially cut from a single monolithic piece of metal, a solution that he admits may be impractical.
MATERIAL
AND METHODS
The following suggested solutions will improve the accuracy of the casting and reduce some of the possible errors.
Impression
Fig. 3. If center screw wrench does not fit properly over hexagonal head of center screw, the center screw must be replaced.
of pattern, investment type, powder-liquid ratio, spatulation, setting time before casting, burnout sequence, removal of casting ring from oven, casting machine, melt soak time, and melting temperature of the oven. Their findings showed Ni-Cr castings to be consistently undersized and precious metal castings to be oversized. Clearly a large number of parameters must be considered during any dental casting procedure. Because of the complexity of the process, it is not surprising that failures may arise. Most authorities agree that implant framework castings must be accurate. One article17 mentions the increased need for accuracy in the maxillary reconstruction because of the anatomy and type of bone. It has been suggested that
technique
Various approaches L 28to the impression techniques using plaster, hydrocolloid, irreversible hydrocolloid, and elastomeric impression materials have been described. Attention to detail is vital to obtain an exact replica of the oral configuration. It is essential that the superior surface of each abutment is spotlessly clean. The center screw should be checked for tightness every single time an impression is being made or the framework accuracy is being checked to ensure the abutment is secure (Fig. 2). On occasion the wrench does not fit over the center screw’s hexagonal head (Fig. 3) or the center screw does not seat fully. The fit of the gold cylinders must be checked against the brass analogues (Fig. 4) and on the abutment at the first clinical appointment so that any defective components can be changed by the oral surgeon. The use of an abutment cover cap (Implant Innovations, W. Palm Beach, Fla.) helps to maintain a clean surface on the abutment between appointments (Fig. 5). It is preferable to use only machined components instead of castable plastic counterparts because they generally
SEPTEMBER
1991
VOLUME
66
NUMBER
3
FULL-ARCH
IMPLANT
I RA.ME:WORKS:
PART
1
:Fig. 4. Passive, accurate seating of brass analogues is checked against ,gold cylinder before clinical appointment identifies any faulty components.
Fig. 5. Cover C,E~PS secured with hexagonal-head screwdiriver maintain cleanliness of abutment head between clinical appointment -.
provide a better fit. Each of the following impression techniques uses a custom open-top tray. Transfer copings. The smooth tapered type of coping is designed for use with hydrocolloid, and the square type is used either with plaster or elastomeric materials1 Humphries’” tested splinting the square copings with dental floss and covering the floss with autopolymerizing resin (Duralay, Reliance Mfg. Co., Worth, Ill.) (Fig. 6). No statistical difference was observed in comparing the accuracy of impressions using smooth, square, or square copings splinted in this fashion. An elastomeric impression material such as Impregum F (Premier Dental Products Co., Norristown, PA.) has i;ufficient body to prevent movement
-*Humphries.
THE
JOURNAL
R.M. :f’erscbnal communication.
OF PROSTHETIC
DENTISTRY
Portage, Mich. 198fl.
Fig. 6. Square transfer copings have been attached to abutments in this maxillary partially edentulous patient. Ribbon floss has been wrapped around transfer copings and bridged with Duralay resin.
Fig. 7. Open-topped autopolymerizing acrylic resin impression tray permits access to superior surface of guide pins. These may he loosened when the impression material has set, permitting removal of tray ,lnd transfer copings in toti;
of the transfer copings on removal from the mouth. The tray is loaded with the material and t,he remainder is injected around the copings before the tray is seated. The open-top design permits loosening of the guide pins so that the entire assembly can be removed as a single unit (Fig. 7). Brass analogues are attached to the transfer copings and the impression is poured in a conventional manner. Gold cylinders. In spaces where access or vertical height is limited. the gold cylinder itself can be used as a t,ransfer coping. A variation of this approach is that of joining the gold cylinders with Duralay resin2. l1 thereby forming a acrylic resin “bridge,” thr fit of which can be tested directly in vivo before rhr Lransfet- impression is made (Fig. h. rl). A furt,her variation 01’this jrchnique is t,he
379
Fig. 8. A, Duralay acrylic resin is painted on top of utility wax, bridging gap between adjacent gold cylinders, which have been secured to abutments with guide pins. B, More rigid connection is desirable for longer spans. Steel burs are luted to Duralay resin bridge with more resin. C, Impression plaster is placed over Duralay resin bridge, ensuring completely rigid structure.
use of used friction-grip burs secured with Duralay resin to bridge and strengthen the span (Fig. 8, B). In addition, the use of impression plaster on top of the acrylic resin index will further strengthen the structure into a rigid mass (Fig. 8, 0
Verification
of fit of plastic
framework
fabrication seem to be either Duralay or Relate (Parke11 Products, Farmingdale N.Y.) resins. Another alternative to an acrylic resin fixed partial denture is the use of a stout wire attached to the transfer copings with Zapit (Dental Ventures of America, Anaheim Hills, Calif.)llr 31or composite resin. This rigid assembly is easily tried onto the abutments. Another recommendation is the use of new guide pins with each patient. Experience has shown that the new flat guide pins (DCA 094; Nobelpharma, Nobel Industries) are more easily distorted during laboratory procedures than the tapered guide pins (DCA 027; Nobelpharma, Nobel Industries). Hence, the use of new guide pins will limit the possibility of distortion. The larger 4 mm gold cylinders are preferable because the acrylic resin fixed partial denture can be made thicker and hence sturdier to prevent springing and bowing of the framework.
Rasmussen2 and Henry l1 have described techniques for verifying the accuracy of the master cast by using a verification jig made of Duralay resin (Fig. 9). It is essential that the plastic facsimile of the fixed partial denture be tested intraorally to verify an accurate fit. If this framework is made directly intraorally,2, I1 verification is straightforward because the framework is loosened, removed, and replaced. If the plastic framework is made indirectly on the master cast, it should be thickened adequately to prevent possible flexure. Curing of acrylic resin should be in a pressure pot. It has been recommended that the resin be processed the night before and left bolted down overnight.* Materials of choice for the resin framework
Alternatives not fit
*White G. Academyof Osseointegrationmeeting, Dallas, Texas, 1989.
When the master cast and the intraoral configuration do not coincide the choices are as follows. 1. Remake the impression by using any of the described alternatives.
2RCl
if the plastic
SEPTEMBER
framework
1991
VOLUME
86
does
NUMBER
3
FULL-ARCH
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J RAMEWORKS:
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Fig. 11. Fit of sandblasted casting on master cast is evalIFig. 9. Duralay acrvlic resin verification jig constructed on master ,-a&s is tried intreorally to verify accuracy ot master cast.
Fig. 10. Sectioned Duralay acrylic resin bridge is rein dexed intraorally with new ac,rylic resin. New brass analogues are attached to gold cylinders and assembly is placed in patty of fresh ‘die -t,one.
2. Section the plastic framework and index the new relationship with the same autopolymerizing resin. The malpositioned brass analogue is sectioned from the master cast by use of a 4-inch disk (E.C. Moore Co., Inc., Dearborn, Mich.) and .a new brass analogue is secured to the master cast with dental die stone. 3. After the plastic, framework is sectioned and reindexed, secure new analogues to all of the transfer copings and pour a n,ew master cast or a simple stone patty (Fig. 10). This cast serves as an accurate model to verify the fit of the metal framework. The original anatomically correct cast is the one used for articulator msuntings and tooth-positioning.
Casting
uated by use of one guide pin in least fav~orable position. typically terminal abutment. Presence cut’ openings between remaining gold cvlinder and bras5 analogues are looked hr.
Fig. 12. View of gold cylinder fit. surface under magnification demonstrates imperfections on smooth surface (arrou’j due to metal fiash occurring during casting when wax residue has not been adequately remo\.ed
t.he advantages of casting the framework in one piece”, il. :Z or casting in sections and soldering.‘“* ‘, Others have recommended casting in one piece and soldering if the framework does not fit passively.‘, ii’. a2 White* recommended casting in one piece because the work hardening properties of the metal are maintained whereas after soldering two or three pieces of a framework, the properties are altered and the soldered framework cannot be heat treated. Another obvious ramification is that, overall, less laboratory time is involved in casting a framework in one piece.
procedure
The actual casting and soldering techniques will be discussed in a sribsequen- article. Reference has been made to THE
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Fig. 13. Maxillary fixed partial denture is inspected intraorally for accuracy by use of single guide pin. Observation of openings between any remaining gold cylinders and abutments implies inaccurate casting.
sandblasted and the fit is verified on the master cast by use of one gold screw or new guide pin in what is believed to be the least favorable position. Most often this position is the terminal fixture on one side (Fig. 11). The use of a binocular microscope is encouraged to inspect the inferior surface of each gold cylinder for imperfections of the castings, which should be removed carefully using a small round high speed bur (Fig. 12). The framework is then tried in the mouth according to the protocol described previously, by use of a gold screw in a terminal abutment (Fig. 13). FitChecker (GC Dental Industrial Corp., Tokyo, Japan) material or pressure-indicating paste (Mizzy Inc., Cherry Hill, N.J.) can aid in checking the contact distribution if any discrepancy exists (Fig. 14). Questionable surfaces can then be judiciously adjusted. If the framework does not fit as expected, it should be sectioned with a fine separating disk. The parts are indexed by use of Duralay resin. A new working cast is then produced by either attaching new brass analogues and pouring a new cast, or the malpositioned brass analogue is sectioned from the original master cast and replaced with a new one using die stone (Fig. 15).11The framework can then be soldered and the fit verified on the master cast.
DISCUSSION
Fig. 14. Fit-Checker material is placed on gold cylinders before casting is seated intraorally. Areas of inadequate contact will be demonstrated by presence of material on cylinder surface.
Metal
selection
Dentists select alloys of various types; however, a type IV dental alloy that has a yield strength high enough to withstand the occlusal forces imposed on it should be used. Because of its size and cost, substitutes for gold alloys have been sought. Silver-palladium alloys are popular in the United States because they have the required strength233*5,l6 and are less expensive. Before investment of the framework, the fit surface of the gold cylinder should be cleaned with wax solvent to remove all wax or acrylic resin residues. This procedure reduces the risk of metal flash flowing over the cylinder surface. No wax should extend beyond the perimeter of the fit surface of the gold cylinder margin.32 When recovered from its investment, the framework is 382
The system dentists use to produce an accurate casting has many variables, each of which must be appropriately considered, to provide a final result that will be clinically acceptable. Klineberg and Murray15 have shown that there may be an opening up to 20 to 30 Mm between the cylinder and the abutment. It is evident that careful control of the metal and of the expansion and contraction of the investment are critical steps. Avoidance of detail to these measures could easily result in distortion of the framework dimensions. Contraction of such a large mass of metal on cooling is conceivably too great a force for the investment to counteract. The resultant change in dimensions not only linearly but in all directions causes a rotation of the gold cylinders about their long axes. Although soldering is usually a relatively simple task, in the case of a large framework, it is a formidable task because the contraction of a metal framework of this size can cause the investment to crack and distort. Moreover, soldering multiple pieces of the framework may not be possible when the investment is not strong enough to withstand the enormous forces produced during the cooling of the metal. Perhaps with present techniques it is not likely that a casting of this size can be produced accurately every time.24 The laboratory must receive an accurate master cast on which it can make an accurate casting. Another consideration is the flexure of the mandible itself.33 It is conceivable that with varied openings of the mandible, the precise angulations of the fixtures and hence the transfer copings may change. Such a theory warrants further research. Inadvert-
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15. A, Inadequately fitting framework has been sectioned and reindexed intraorally with Duralay acrylic resin. New brass analogue has been attached to gold cylinder. Oflending brass analogue was sectioned from master cast. R, Fresh die st,one is poureti into var ant reces,s. securing analogue to cast.
Fig.
!. Check the fit of gold cylinders and transfer copings onto the abutment ‘cylinders at the first clinical appointment following the second stage surgery to ensure that all of the components fit together properly. 3. Cover the abutments with ,zover caps to prevent debris from accumulating on the abutment surface. 4. Double check the accuracy of the master cast by using an acrylic resin fixed partial denture facsimile before proceeding to the casting of the framework. Construct the acrylic resin fixed partial denture the night before it is needed and lea\e it bolted down overnight. 5. IJse new guide pin+ for the laboratory stages and when checking the fit of she cast metal framework intraorally. Test t,he tit of the ( asting by%using only one screw at one end of the frameuork. 6. Clean the fitting surface of each gold cylinder with wax solvent before investing. ‘i. Cast the metal framework n one piece if possible. 8. Cover the cylinder:, with stainless steel protector caps or duplicate brass annlogues to protect the surface when polishing the framework. 9. For full-arch castiiigs use 4 mm gold cylinders because the framework will have more bulk and sturdiness. The acrylic resin try-in i’ramework will similarly be less likely to distort.
THE
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SUMMARY Each step in the fabrication of a one-piece metal implant framework casting is of critical importance in producing an accurately fitting framework. The various stages of this procedure have been described and discussed in this part of the articles. Alternative techniques to limit inaccuracies have been suggested. A list of recommendations have been included
GOLL
15. Klineberg IJ, Murray GM. Design of superstructures for osseointegrated fixtures. Swed Dent J 1985;28(Suppl):63-9. 16. Leung N, Zarb G, Watson P. Non gold alloy systems for prosthetic frameworks [Abstract]. J Dent Res 1982;62:324. 17. Lundquist S, Carlsson GE. Maxillary fixed prostheses on osseointegrated dental implants. J PROSTHET DENT 1983;50:262-70. 18. Fusayama T, Wakumoto S, Hosoda H. Accuracy of fixed partial dentures made by various soldering techniques and one-piece casting. J PROSTHET DENT 1964;14:334-42. 19. Bruce RW. Evaluation of multiple unit castings for fixed partial dentures. J PROSTHET DENT 1964;14:939-43. 20. Bruce RW. Clinical applications of multiple unit castings for fixed prostheses. J PROSTHET DENT 1967;18:359-64. 21. Huling JS, Clark RE. Comparative distortion in three-unit fixed prostheses joined by laser welding, conventional soldering or casting in one piece. J Dent Res 1977;56:128-34. 22. Hinman RW, Tesk JA, Parry EE, Eden GT. Improving the casting accuracy of fixed partial dentures. J PROSTHET DENT 1985;53: 466-71.
23. Eden GT, Franklin OM, Powell JM, Ohta Y, Dickson G. Fit of porcelain fused-to-metal crown and bridge castings. J Dent Res 1979;58: 2360-S. 24. Pare1 SM. Modified casting techniques for osseointegrated fixed prosthesis fabrication: A preliminary report. Int J Oral Maxillofac Imp 1989;4:33-40.
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25. Strid KG. On the application of photogrammetry to the fitting of jawbone-anchored bridges. Swed Dent J 1985;28(Suppl):93-105. 26. Siirila HS, Nordberg L, Olkarinen VJ. Technique for converting an existing complete denture to a tissue-integrated prosthesis. J PROSTHET DENT 1988;59:463-7.
27. Chiche GJ, Weaver C, Pinault A, Elliot R. Auxiliary substructure for screw-retained prostheses. Int J Prosthodont 1989;2:407-12. 28. Hobo S, Ichida E, Garcia LT. Osseointegration and occlusal rehabilitation. Chicago: Quintessence Pub1 Co Inc, 1989;176. 29. Pare1 SM. Implants and overdentures: the osseointegrated approach with conventional and compromised applications. Int J Oral Maxillofac Imp 1986;1:93-9. 30. Henry PJ. Two-piece cast superstructure for mandibular osseointegrated bridgework. Quintessence of Dent Tech 1987;11:321-7. 31. Knudson RC, Williams EO, Kemple KP. Implant transfer coping verification jig. J PROSTHET DENT 1989;61:601-2. 32. Loos LG. A fixed prosthodontic technique for mandibular osseointegrated titanium implants. J PROSTHET DENT 1986;55:232-42. 33. Brunski JB. Biomaterials and biomechanics in dental implant design. Int J Oral Maxillofac Imp 1988;3:85-97. Reprint requests to: DR. GEORGE E. GOLL 737 MEDICAL-DENTAL BLDG. SEATTLE, WA 98101
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