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A double casting technique to minimize distortion when constructing fixed partial dentures on implants
A double casting technique to minimize distortion when constructing fixed partial dentures on implants Henry W. K. Luk, MPhil, PhD,a Edmond H. N. Pow, BDS, MDS,b Anne S. McMillan, BDS, PhD,c and C. F. Huid The University of Hong Kong, Hong Kong; Prince Philip Dental Hospital, Hong Kong, Republic of China The casting of extensive implant superstructures that have a good passive fit may be technically demanding. A simple, 2-stage casting technique is presented that avoids common problems associated with the casting of large superstructures. The fabrication of a 7-unit metal-ceramic screw-retained fixed partial denture supported by 5 implants is used to illustrate this technique. (J Prosthet Dent 2004;91:93-6.)
T
he importance of a good passive fit of implant superstructures is acknowledged.1,2 Unlike natural teeth, implants are rigidly fixed in bone because of the absence of a periodontal membrane. An imperfectly fitting framework stresses implant components and the surrounding bone. Studies have shown that stresses introduced at the time of prosthesis placement may remain after several years of functioning.3 Clinically, it may be technically difficult to construct a superstructure with a passive fit, especially when the casting is large and extensive casting shrinkage is substantial. Soldering is the conventional method of managing the problem, but it is a technically sensitive process that often results in problems such as porosity.4 To overcome this, methods including modified casting techniques, spark-erosion, laser welding, and cementation techniques have been advocated.5-8 However, these techniques usually require specific technology, equipment, materials, and skills. Thoupos et al9 introduced a method which connected implant framework segments by casting the connector joints instead of soldering. The method is less technique sensitive and more accurate than conventional soldering.10,11 However, a potential problem with the “cast to” technique is air entrapment within the joint that may compromise joint strength.12 In addition, for metal ceramic prostheses, bubbles may be created inside the ceramic material during vacuum firing. To overcome these problems, a new double casting technique is described. A 7-unit metal-ceramic fixed partial denture supported by 5 implants to replace teeth from the maxillary right second premolar to the left lateral incisor is used as an example to illustrate the method. a
Tutor, Dental Technology, Faculty of Dentistry, The University of Hong Kong. b Lecturer, Oral Rehabilitation, Faculty of Dentistry, The University of Hong Kong. c Professor, Oral Rehabilitation, Faculty of Dentistry, The University of Hong Kong. d Dental Technician, Prince Philip Dental Hospital. JANUARY 2004
TECHNIQUE 1. Make an abutment level impression (President; Colte`ne AG, Altsta¨tten, Switzerland) and make a laboratory cast that incorporates abutment analogues (Branemark system; Nobel Biocare AB, Go¨teborg, Sweden). 2. Wax the trial prosthesis to its final dimension and requirement using modeling wax (Dentsply, Surrey, England) and denture teeth (Dentsply Trubyte; Dentsply International, York, Pa), and assess clinically for fit and esthetics (Fig. 1). 3. Convert the trial prosthesis to an investment wax pattern (Plastodent G; Degussa Dental, Hanau, Germany) and make a silicone (Rapid; Colte`ne AG) index to record the shape of the first pattern. 4. Reduce the labial, buccal and occlusal wax pattern to create appropriate space for ceramic material (VITA OMEGA; Zahnfabrik H Rauter GmbH & Co KG, Bad Sackingen, Germany) (Fig. 2). Make a second silicone index to record the shape of the reduced pattern. 5. Further reduce the pattern in all directions to a skeleton pattern (Fig. 3) and strengthen with autopolymerizing resin (Pattern resin; GC, Tokyo, Japan). Invest (Cera-Fina; Whip Mix, Louisville, Ky) and cast using a high-noble alloy (Esteticor Cosmor H; Cendres & Metaux SA, Bienne, Switzerland) (Fig. 4). 6. Place the skeleton casting on the definitive cast and assess the fit. In this situation, the fit was considered imperfect on 2 abutment analogues. Therefore, section the casting and rejoin the segments using the “cast to” technique9 (Figs. 5 and 6). 7. Seat the skeleton casting accurately onto the implant analogues and airborne-particle abrade with alumina (50-m particles), taking care to avoid abrading the machined fitting surface. Clean the casting ultrasonically in distilled water. 8. Add inlay wax (Plastodent G; Degussa Dental) to the skeleton casting with the help of the second THE JOURNAL OF PROSTHETIC DENTISTRY 93
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LUK ET AL
Fig. 1. Trial prosthesis in wax.
Fig. 2. Wax pattern reduced to create space for ceramic material.
Fig. 3. Reduced pattern is further reduced to create skeleton pattern for casting.
Fig. 4. Assessment of fit of skeleton casting on definitive cast.
silicone index to recreate the reduced pattern (Fig. 7). Embed and cast the framework using traditional methods13 to yield the definitive casting (Fig. 8). 9. Verify the fit of the definitive casting clinically. 10. Place dental porcelain (VITA OMEGA) on the final casting.
filled any micro-defects that might occur in the first casting. There was a good union between the 2 castings since the same type of metal was used.14 Thus, the final framework provided a strong, junction-free, uniform, and homogenous surface for the veneering materials. There were other technical advantages when using this technique. Because the framework was composed of 2 castings, the time period of the casting alloy in its molten state was shorter than if the framework had been cast in one piece. Thus, the gold cylinders (machine milled components) were less likely to be affected because they were exposed to the high casting temperature for a shorter time. The method described can be applied generally with different implant systems. The method was found to be useful in situations in which significant shrinkage may be encountered, such as with large castings. Some additional laboratory time is required for adding waxing and
DISCUSSION This new method permitted the fabrication of a passive-fitting implant framework by 2 castings, completely encasing the first casting with the second. The first casting served as a rigid base to allow precise fit of the framework. It also reduced the volume of the second casting and hence limited any unwanted effects such as distortion. The second casting that completely encased the first solved the problem of potential weak joint strength in the “cast to” or soldering technique.13 It also 94
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THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 5. Sectioning of skeleton casting.
Fig. 6. Skeleton casting rejoined using “cast to” technique.
Fig. 7. Wax added to corrected skeleton casting to re-form reduced pattern.
Fig. 8. Definitive casting.
casting procedures. However, less clinical time is needed because chairside re-indexing of the framework is avoided. The cost of casting alloy is comparable with the conventional technique since the definitive casting is of the same dimension. To date, 6 frameworks for 4 patients have been fabricated using this technique. The fit of the castings was found to be satisfactory at chairside and no additional adjustment or soldering was required.
SUMMARY A double-casting technique for constructing an implant framework was presented. The framework has satisfactory fit, and the need for chairside sectioning and laboratory soldering procedures was therefore minimized. JANUARY 2004
REFERENCES 1. Wicks RA, deRijk WG, Windeler AS. An evaluation of fit in osseointegrated implant components using torque/turn analysis. J Prosthodont 1994;3:206-12. 2. Kunavisarut C, Lang LA, Stoner BR, Felton DA. Finite element analysis on dental implant-supported prostheses without passive fit. J Prosthodont 2002;11:30-40. 3. Jemt T, Book K. Prosthesis misfit and marginal bone loss in edentulous implant patients. Int J Oral Maxillofac Implants 1996;11:620-5. 4. el-Ebrashi MK, Asgar K, Bigelow WC. Electron microscopy of gold soldered joints. J Dent Res 1968;47:5-11. 5. Parel SM. Modified casting technique for osseointegrated fixed prosthesis fabrication: a preliminary report. Int J Oral Maxillofac Implants 1989;4: 33-40. 6. Ru¨ beling G. New techniques in spark erosion: the solution to an accurately fitting screw-retained implant restoration. Quintessence Int 1999; 30:38-48. 7. Wat PYP, Pow EH, Chow TW. A new prosthodontic technique for fabricating cement-retained implant-supported prostheses. Quintessence Int 2000;31:187-90. 8. Iglesia MA, Moreno J. A method aimed at achieving passive fit in implant prostheses: case report. Int J Prosthodont 2001;14:570-4.
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THE JOURNAL OF PROSTHETIC DENTISTRY
9. Thoupos GA, Zouras CS, Winkler S, Roussos VG. Connecting implant framework Segments. Implant Dent 1995;4:97-9. 10. Zoidis PC, Winkler S, Karellos ND. The effect of soldering, electrowelding, and cast-to procedures on the accuracy of fit of cast implant bars. Implant Dent 1996;5:163-8. 11. Romero GG, Engelmeier R, Powers JM, Canterbury AA. Accuracy of three corrective techniques for implant bar fabrication. J Prosthet Dent 2000; 84:602-7. 12. Rasmussen EJ, Goodkind RJ, Gerberich WW. An investigation of tensile strength of dental solder joints. J Prosthet Dent 1979;41:418-23. 13. Rhoads JE, Rudd KD, Morrow RM. Dental laboratory procedures: fixed partial dentures, vol 2. St. Louis: Mosby; 1985. p. 213-37. 14. Carr AB, Brantley WA. Characterization of noble metal implant cylinders: as-received cylinders and cast interfaces with noble metal alloys. J Prosthet Dent 1996;75:77-85.
Noteworthy Abstracts of the Current Literature
LUK ET AL
Reprint requests to: DR EDMOND H. N. POW ORAL REHABILITATION, FACULTY OF DENTISTRY UNIVERSITY OF HONG KONG 34 HOSPITAL ROAD HONG KONG, REPUBLIC OF CHINA FAX: 852-2858-6114 E-MAIL: [email protected] 0022-3913/$30.00 Copyright © 2004 by The Editorial Council of The Journal of Prosthetic Dentistry.
doi:10.1016/j.prosdent.2003.09.013
Posterior mandibular residual ridge resorption in patients with conventional dentures and implant overdentures Kordatzis K, Wright P, Meijer HJ. Int J Oral Maxillofac Implants 2003;18:447-52.
Purpose. This study investigated the effects of certain systemic and local factors on resorption of the posterior mandibular residual ridge under conventional dentures and overdentures supported by 2 implants. Materials and Methods. Proportional area measurements of the posterior mandible were made on rotational tomograms taken immediately before and 5 years after treatment. The area was bounded by a line joining gonion to the lowest point of the mental foramen, and the crest of the residual ridge and was expressed as a proportion of an area that was not dependent on the ridge. The use of proportions rather than actual measurements minimized errors related to magnification and distortion. Results. The estimated average reduction in height was 1.25 mm in 5 years (1.63 mm for conventional denture groups and 0.69 mm for implant overdenture groups, ie, almost 1 mm less in the overdenture group). Discussion and Conclusion. Female gender was a risk factor for greater resorption. Other factors, such as the number of years a patient had been edentulous, initial height of the mandible, and the number of dentures used, failed to show an association with resorption of the residual posterior mandibular ridge, while the statistically significant effect of age was unlikely to be clinically significant.—Reprinted with permission of Quistessence Publishing.
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VOLUME 91 NUMBER 1
T
he importance of a good passive fit of implant superstructures is acknowledged.1,2 Unlike natural teeth, implants are rigidly fixed in bone because of the absence of a periodontal membrane. An imperfectly fitting framework stresses implant components and the surrounding bone. Studies have shown that stresses introduced at the time of prosthesis placement may remain after several years of functioning.3 Clinically, it may be technically difficult to construct a superstructure with a passive fit, especially when the casting is large and extensive casting shrinkage is substantial. Soldering is the conventional method of managing the problem, but it is a technically sensitive process that often results in problems such as porosity.4 To overcome this, methods including modified casting techniques, spark-erosion, laser welding, and cementation techniques have been advocated.5-8 However, these techniques usually require specific technology, equipment, materials, and skills. Thoupos et al9 introduced a method which connected implant framework segments by casting the connector joints instead of soldering. The method is less technique sensitive and more accurate than conventional soldering.10,11 However, a potential problem with the “cast to” technique is air entrapment within the joint that may compromise joint strength.12 In addition, for metal ceramic prostheses, bubbles may be created inside the ceramic material during vacuum firing. To overcome these problems, a new double casting technique is described. A 7-unit metal-ceramic fixed partial denture supported by 5 implants to replace teeth from the maxillary right second premolar to the left lateral incisor is used as an example to illustrate the method. a
Tutor, Dental Technology, Faculty of Dentistry, The University of Hong Kong. b Lecturer, Oral Rehabilitation, Faculty of Dentistry, The University of Hong Kong. c Professor, Oral Rehabilitation, Faculty of Dentistry, The University of Hong Kong. d Dental Technician, Prince Philip Dental Hospital. JANUARY 2004
TECHNIQUE 1. Make an abutment level impression (President; Colte`ne AG, Altsta¨tten, Switzerland) and make a laboratory cast that incorporates abutment analogues (Branemark system; Nobel Biocare AB, Go¨teborg, Sweden). 2. Wax the trial prosthesis to its final dimension and requirement using modeling wax (Dentsply, Surrey, England) and denture teeth (Dentsply Trubyte; Dentsply International, York, Pa), and assess clinically for fit and esthetics (Fig. 1). 3. Convert the trial prosthesis to an investment wax pattern (Plastodent G; Degussa Dental, Hanau, Germany) and make a silicone (Rapid; Colte`ne AG) index to record the shape of the first pattern. 4. Reduce the labial, buccal and occlusal wax pattern to create appropriate space for ceramic material (VITA OMEGA; Zahnfabrik H Rauter GmbH & Co KG, Bad Sackingen, Germany) (Fig. 2). Make a second silicone index to record the shape of the reduced pattern. 5. Further reduce the pattern in all directions to a skeleton pattern (Fig. 3) and strengthen with autopolymerizing resin (Pattern resin; GC, Tokyo, Japan). Invest (Cera-Fina; Whip Mix, Louisville, Ky) and cast using a high-noble alloy (Esteticor Cosmor H; Cendres & Metaux SA, Bienne, Switzerland) (Fig. 4). 6. Place the skeleton casting on the definitive cast and assess the fit. In this situation, the fit was considered imperfect on 2 abutment analogues. Therefore, section the casting and rejoin the segments using the “cast to” technique9 (Figs. 5 and 6). 7. Seat the skeleton casting accurately onto the implant analogues and airborne-particle abrade with alumina (50-m particles), taking care to avoid abrading the machined fitting surface. Clean the casting ultrasonically in distilled water. 8. Add inlay wax (Plastodent G; Degussa Dental) to the skeleton casting with the help of the second THE JOURNAL OF PROSTHETIC DENTISTRY 93
THE JOURNAL OF PROSTHETIC DENTISTRY
LUK ET AL
Fig. 1. Trial prosthesis in wax.
Fig. 2. Wax pattern reduced to create space for ceramic material.
Fig. 3. Reduced pattern is further reduced to create skeleton pattern for casting.
Fig. 4. Assessment of fit of skeleton casting on definitive cast.
silicone index to recreate the reduced pattern (Fig. 7). Embed and cast the framework using traditional methods13 to yield the definitive casting (Fig. 8). 9. Verify the fit of the definitive casting clinically. 10. Place dental porcelain (VITA OMEGA) on the final casting.
filled any micro-defects that might occur in the first casting. There was a good union between the 2 castings since the same type of metal was used.14 Thus, the final framework provided a strong, junction-free, uniform, and homogenous surface for the veneering materials. There were other technical advantages when using this technique. Because the framework was composed of 2 castings, the time period of the casting alloy in its molten state was shorter than if the framework had been cast in one piece. Thus, the gold cylinders (machine milled components) were less likely to be affected because they were exposed to the high casting temperature for a shorter time. The method described can be applied generally with different implant systems. The method was found to be useful in situations in which significant shrinkage may be encountered, such as with large castings. Some additional laboratory time is required for adding waxing and
DISCUSSION This new method permitted the fabrication of a passive-fitting implant framework by 2 castings, completely encasing the first casting with the second. The first casting served as a rigid base to allow precise fit of the framework. It also reduced the volume of the second casting and hence limited any unwanted effects such as distortion. The second casting that completely encased the first solved the problem of potential weak joint strength in the “cast to” or soldering technique.13 It also 94
VOLUME 91 NUMBER 1
LUK ET AL
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 5. Sectioning of skeleton casting.
Fig. 6. Skeleton casting rejoined using “cast to” technique.
Fig. 7. Wax added to corrected skeleton casting to re-form reduced pattern.
Fig. 8. Definitive casting.
casting procedures. However, less clinical time is needed because chairside re-indexing of the framework is avoided. The cost of casting alloy is comparable with the conventional technique since the definitive casting is of the same dimension. To date, 6 frameworks for 4 patients have been fabricated using this technique. The fit of the castings was found to be satisfactory at chairside and no additional adjustment or soldering was required.
SUMMARY A double-casting technique for constructing an implant framework was presented. The framework has satisfactory fit, and the need for chairside sectioning and laboratory soldering procedures was therefore minimized. JANUARY 2004
REFERENCES 1. Wicks RA, deRijk WG, Windeler AS. An evaluation of fit in osseointegrated implant components using torque/turn analysis. J Prosthodont 1994;3:206-12. 2. Kunavisarut C, Lang LA, Stoner BR, Felton DA. Finite element analysis on dental implant-supported prostheses without passive fit. J Prosthodont 2002;11:30-40. 3. Jemt T, Book K. Prosthesis misfit and marginal bone loss in edentulous implant patients. Int J Oral Maxillofac Implants 1996;11:620-5. 4. el-Ebrashi MK, Asgar K, Bigelow WC. Electron microscopy of gold soldered joints. J Dent Res 1968;47:5-11. 5. Parel SM. Modified casting technique for osseointegrated fixed prosthesis fabrication: a preliminary report. Int J Oral Maxillofac Implants 1989;4: 33-40. 6. Ru¨ beling G. New techniques in spark erosion: the solution to an accurately fitting screw-retained implant restoration. Quintessence Int 1999; 30:38-48. 7. Wat PYP, Pow EH, Chow TW. A new prosthodontic technique for fabricating cement-retained implant-supported prostheses. Quintessence Int 2000;31:187-90. 8. Iglesia MA, Moreno J. A method aimed at achieving passive fit in implant prostheses: case report. Int J Prosthodont 2001;14:570-4.
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THE JOURNAL OF PROSTHETIC DENTISTRY
9. Thoupos GA, Zouras CS, Winkler S, Roussos VG. Connecting implant framework Segments. Implant Dent 1995;4:97-9. 10. Zoidis PC, Winkler S, Karellos ND. The effect of soldering, electrowelding, and cast-to procedures on the accuracy of fit of cast implant bars. Implant Dent 1996;5:163-8. 11. Romero GG, Engelmeier R, Powers JM, Canterbury AA. Accuracy of three corrective techniques for implant bar fabrication. J Prosthet Dent 2000; 84:602-7. 12. Rasmussen EJ, Goodkind RJ, Gerberich WW. An investigation of tensile strength of dental solder joints. J Prosthet Dent 1979;41:418-23. 13. Rhoads JE, Rudd KD, Morrow RM. Dental laboratory procedures: fixed partial dentures, vol 2. St. Louis: Mosby; 1985. p. 213-37. 14. Carr AB, Brantley WA. Characterization of noble metal implant cylinders: as-received cylinders and cast interfaces with noble metal alloys. J Prosthet Dent 1996;75:77-85.
Noteworthy Abstracts of the Current Literature
LUK ET AL
Reprint requests to: DR EDMOND H. N. POW ORAL REHABILITATION, FACULTY OF DENTISTRY UNIVERSITY OF HONG KONG 34 HOSPITAL ROAD HONG KONG, REPUBLIC OF CHINA FAX: 852-2858-6114 E-MAIL: [email protected] 0022-3913/$30.00 Copyright © 2004 by The Editorial Council of The Journal of Prosthetic Dentistry.
doi:10.1016/j.prosdent.2003.09.013
Posterior mandibular residual ridge resorption in patients with conventional dentures and implant overdentures Kordatzis K, Wright P, Meijer HJ. Int J Oral Maxillofac Implants 2003;18:447-52.
Purpose. This study investigated the effects of certain systemic and local factors on resorption of the posterior mandibular residual ridge under conventional dentures and overdentures supported by 2 implants. Materials and Methods. Proportional area measurements of the posterior mandible were made on rotational tomograms taken immediately before and 5 years after treatment. The area was bounded by a line joining gonion to the lowest point of the mental foramen, and the crest of the residual ridge and was expressed as a proportion of an area that was not dependent on the ridge. The use of proportions rather than actual measurements minimized errors related to magnification and distortion. Results. The estimated average reduction in height was 1.25 mm in 5 years (1.63 mm for conventional denture groups and 0.69 mm for implant overdenture groups, ie, almost 1 mm less in the overdenture group). Discussion and Conclusion. Female gender was a risk factor for greater resorption. Other factors, such as the number of years a patient had been edentulous, initial height of the mandible, and the number of dentures used, failed to show an association with resorption of the residual posterior mandibular ridge, while the statistically significant effect of age was unlikely to be clinically significant.—Reprinted with permission of Quistessence Publishing.
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