NAG’S TECHNIQUE: AN INNOVATIVE INDIRECT RESTORATION IN A SINGLE VISIT
ABSTRACT:
Objective: To restore a tooth using Nag’s technique, which is a new method of fabrication of an indirect restoration in a single visit and has the potential to substitute traditional lost wax technique.
Materials & Methods: Restoration is fabricated by melting the alloy and directly filling the cavity preparation in the cast( eliminates the casting procedure). Alloys usually ball up upon melting leading to improper filling of the cavity. This is basically the reason why metals are cast using lost-wax technique. As an alternative to casting various methods to enable the molten metal to flow into the cavity are mentioned. Au80Sn20 , Au60Ag20Sn20Ir0.005 ( AME Alloy 1)and Ag31Cu8Sn61Ir0.005( AME alloy 2 ) alloys compatible with this technique are discussed with their advantages and disadvantages. The equipments used to melt and contour the alloy are temperature controllable soldering station, gas soldering iron etc. After filling the cavity & contouring it ,the restoration is removed from the cast and subject to finishing and polishing.
Results: A satisfactory restoration could be fabricated with this technique in terms of contour, fit, marginal adaptation, surface smoothness , luster & other properties. Fabrication and placement of restoration is possible in a single visit (30-45 minutes).Resulted restorations with Au80 Sn20 , Au60Ag20Sn20 and Ag31Cu8Sn61 alloys exhibits good properties.
Conclusion: This paper presents an alternative to lost wax technique of fabricating an indirect restoration.. Technique is simple to master & can be done in a single visit with in 30-45 minutes. But extensive research is required to evaluate the biological compatibility and physical properties of the alloys.
INTRODUCTION: Over the years numerous materials and techniques have been developed and tried for restoration of teeth. Basically, these dental restorations can be broadly divided into two types: direct restorations and indirect restorations. In the direct restorations, a tooth preparation is designed with retention features and restored with a pliable material that is capable of hardening in situ (1) .While in a moldable stage the material can be adapted to the tooth structure and shaped to recreate normal anatomical contours. All this is accomplished directly in the intra oral environment .The development or selection of material for direct application may require compromise of mechanical properties or other desired characteristics. Commonly used materials for direct restorations are, Silver amalgam, Composites, Glass ionomer cement, Direct filling gold etc.
The advantages of direct restorations are that it is a single step procedure (but amalgam polishing requires an additional sitting usually 24 hours after the first sitting (1) ) and economical .However the disadvantage is achieving the ideal properties of restorative material directly in the intraoral environment is difficult.
If there is extensive loss of tooth structure, then the restorative material must provide better stress distribution characteristics and be more carefully bonded to remaining tooth structure. In most cases, this requires the use of materials that cannot be made fluid for direct use. These materials must be fabricated into a restoration out side of the mouth and cemented or bonded in place. The procedures involved with this approach are characterized as indirect restorative dentistry(1).Common indirect restorations include inlays, onlays, crowns, bridges, and veneers. This technique of fabricating the restoration outside of the mouth requires the use of dental impressions of the prepared tooth. Usually a dental technician fabricates the indirect restoration from records the dentist has provided of the prepared tooth. The finished restoration is usually bonded permanently. This procedure generally requires two or more visits and temporization (to seal dentinal tubules(2) , protect the pulp and maintain the surrounding dental tissues) till the patient receives the permanent restoration. Common indirect restorative materials are gold alloys, ceramics, non precious alloys, composites etc.The advantages of indirect method are that large restorations can be planned. Superior physical properties of the material and better contacts and contours can be achieved. The disadvantages of indirect technique are that it is a two visit procedure, some times the second visit may take several days after the first visit (because of laboratory processing time).It is also expensive because it requires many costly equipments, more time and labour.
Copy milling or CAD-CAM milling (Computer Aided Design-Computer Aided Manufacturing) of blocks of ceramics or composites can produce indirect inlays and onlays chair side. One well-known CAD-CAM machine is made by Cerec (Siemens, Germany). The machine scans an optical impression of the tooth preparation following caries removal and mills an inlay or onlay out of a block of ceramic or composite in only a few minutes(3). The disadvantages of this technique include the need for significant training and the inaccuracy of the fit of the inlay or onlay. Currently, these machines cost $90,000 to purchase plus $40 for each ceramic blank, which makes it very expensive for patients.
This paper presents Nag’s technique, an innovative method which tries to combine the advantages of both the direct and indirect restorative procedures, minimizing the complexity and time requirement involved in the fabrication of an indirect restoration. In this technique the restoration is fabricated directly on the cast by melting and carving the alloy. The steps of preparation of the wax pattern, investing the pattern, wax elimination, casting the pattern are eliminated.
MATERIALS AND METHODS:
Cavity preparation - the design of cavity preparation is similar to that of an indirect restoration with rounded internal line angles. Bevels are placed, if the alloy chosen is malleable. Make an impression of the prepared cavity with addition silicone impression material.
Pour the cast and prepare the die- The material selected for the preparation of the cast depends upon the melting temperature of the alloy selected. For low fusing alloys dental stone can be used. For high melting alloys refractory cast materials are required. Preparation of the die is done according to standard techniques.
Melting of alloy and filling the preparation: - Molten alloy has very high surface tension. Filling of the cavity with molten alloy causes ball up of molten alloy. Because of the acute contact angle formed, the molten alloy doesn’t fill the preparation completely. This can be overcome by following methods-
1) Liquidus Temperature is the temperature at which metals of an alloy system begin to solidify on cooling or become totally liquid on heating. Above the liquidus temperature the alloy is completely liquid in state. Solidus Temperature is the temperature at which metals of an alloy system become completely solidified on cooling or start to melt on heating. Below the solidus temperature the alloy is completely solid.In eutectic mixtures the solidus and liquidus temperatures are identical, i.e., the mixture melts completely at one temperature, the eutectic point.
The solidus and liquidus do not align or overlap in all cases. If a gap exists between the solidus and liquidus, then within that gap, the substance consists of solid and liquid phases simultaneously and the consistency is like slurry or clay. Using various alloys which will have sufficient gap between their solidus and liquidus temperature. . so when ever these alloys are melted below their liquidus and well above their solidus temperature, the partially melted alloy will have slurry/clay like consistency. This clay type of molten material is easy to carve in the desired shape.
2) Cavity is coated with an adhesive. Thin coating is preferable this can be achieved by blowing air immediately after applying the adhesive. Temperature resistant adhesives are preferable. Some examples are – cyanocrylate(super glue, Fevikwik etc) ,n-butyl cyanocrylate,2 –octyl cyanocrylate ,Polyimide adhesive, TK 200,TK 300 (4), etc. Molten alloy does not ball up due to the adhesive action.
3) Step soldering or step brazing - In this initially high melting temperature alloy is used. Over that slightly lesser melting temperature alloy is used to build the material bulk. Using low melting alloy than the initial alloy it is possible to prevent melting of initially soldered part. This method is suitable for certain molten alloys which have clay type of consistency and workability. Due to good wetting characteristics of alloy it is possible to prevent ball up of the molten alloy.
4) Placing prefabricated/tailor made inlay forms/onlay forms into the preparation and adjusting them with soldering/brazing tool or rotary instrument in such a way that it perfectly fits the preparation.
5) Combination of above methods in which adhesive is used to hold the wire mesh / wires /particles in place molten alloy is prevented from ball up by adhesive action or/and capillary action
Once the cavity is made receptive by above methods, alloy is melted by the following equipment-
1) Temperature controllable soldering station (if the temperature requirement is less than 450 degree centigrade) accurate temperature calibration is possible with microprocessor based electronic soldering stations. Example:- Wellers temperature controllable soldering station. (figure1)
2) Gas soldering iron (if the temperature requirement is less than 580 degree centigrade) butane, propane based gas soldering iron can be used. Accurate temperature calibration is not possible with this equipment. Example :- Portasol Pro II , Portasol Super Pro gas soldering irons. (figure 2)
3) Modified gas torch (brazing)– in which normal gas torch (figure3) is modified and used .The tip of the gas torch is modified as mentioned in figure4 this will assist the carving of the restoration. The tip can be modified using copper or high melting alloy. The modified portion can be fixed by screw to the torch. This is useful to achieve temperatures up to 900 degree centigrade.
If the alloy is in the form of stick, application will become easy. Using the heat source, tip of the alloy stick is heated and the melted alloy is allowed to directly drop and fill the preparation. Then pressure can be applied by condensation tool or a blunt metal instrument /heated tip to achieve proper adaptation to the walls and margins of the preparation. Additional alloy can be added to the fluid alloy in the cavity.
With this type of successive addition of alloy total cavity is filled with alloy. Slight over filling should be done. Preheating the model in an oven or hot air tool (figure 5) increases working time and increases accuracy of the filling. Heat treatment after filling the cavity improves quality of the restoration because it affects the grain size of the alloy. Heat treatment can be done either by oven or hot air tool. Some alloys do not require flux but most alloys require flux to remove oxide layers. Use appropriate flux. Once the filling is complete carving can be done with carving tool or sharp point of the soldering/brazing iron. If the surface is rough this can be smoothed by specially designed tip of soldering/brazing iron in which wire brush is attached to the soldering iron. Remove the finished restoration carefully from the model. If the adhesive is used that can be removed by application of acetone or other solvents. If necessary internal surface can be roughened by chair side sandblasting /acid pickling. Finishing and polishing procedures can be initiated. Finishing can be done with carbide burs. After the completion of polishing with various grades of polishing discs mirror like polished surface can be achieved. In a few situations finishing and polishing can be performed before the removal of restoration from model.
Finished restoration can be luted with dental cements. The whole procedure takes about 30- 45 minutes.
Alloys use full are SEE TABLE 1
Other alloys and their melting temperatures that can be tried with this techniques are 70 Au, 20 In, 10 Sn,(370-445)/60 Au,30 In,10 Sn(505)/70 Au, 20 In, 10 Sn(415)/Au 96.8, Si 3.2(370)/Au 88,Ge12(356)/Au 98,Si2(370-800)/Au90,Sn10(380-850) etc.
The technique has been used to fabricate a restoration on a left maxillary first molar tooth(16). Class one cavity with cuspal involvement is prepared in this typhodont tooth(figure 6). Impression of the preparation taken with rubber base material( figure 7).Quick setting stone is poured and cast prepared( figure 8). Cyanocrylate adhesive applied to the preparation in the cast. Microprocessor based soldering station is used to melt and carve the alloy.. Tip of the soldering iron should be flat in shape to facilitate carving. Au60Ag20Sn20 alloy was selected. Temperature setting adjusted well above the solidus but blow the liquidus temperature of Au60Ag20Sn20 that is above 2800 C but below 475 0C . Due to the clay consistency when the alloy is melted it is easy to apply the alloy and carve it according to the requirement. Occlusal morphology can be carved with sharp point of the soldering tip and slow speed rotary instruments. After finishing and polishing resulted restoration is shown in figure10.
DISCUSSION: The current paper presents Nag’s technique, a new method of fabrication of an indirect restoration. The indirect restorative technique has the advantages of achieving superior contacts, contours and enhanced properties of materials. However indirect restorative technique is commonly a two step procedure, and it involves a chain of procedures from waxing a pattern of the intended final restoration on a die, investing the pattern to create a mold space for casting, casting the restoration. Because of multiple stages of these techniques errors that enter into the procedures at any point tend to be compounded and carried into the next stages. The current method seeks to retain the advantages of indirect restorations and eliminate many of its disadvantages.
The alloys used in the present technique are mostly high noble metal alloys,(alloy classification of the ADA -198A high noble alloy contains more than 40% gold and noble metal content is more than 60%) hence exhibit less corrosion and good physical properties. Most of these alloys are considered as hard solders in electronic industry. Since many years they are into use wherever high properties of alloys are required(5,6,7,8) . Tin, indium, germanium, silicon help in lowering melting temperature of the parent alloy. Lead, antimony etc should not be used because of their biological toxicity nature(9,10). Because of the low melting temperature, one can anticipate lower shrinkage and better adaptation (11) . Abundant number of alloys are compatible with this technique ex:- gold, silver , copper ,aluminum alloys etc.
This method is a single sitting procedure in which restorations will be ready within 30- 45 minutes. Time can be further reduced by use of quick setting stone, microwave drying of casts(12) . Less chair time is required and no provisional restorations are required. So overall treatment cost will be reduced. Technique of fabrication of restoration extraorally is simple to master and trained dental assistant can perform this. Dentist’s precious time is saved.
An amalgam restoration provides many years of service(13,14,15). If cost benefit considerations are a concern, amalgam is still the most convenient restorative material for posterior teeth (16). Posterior amalgam restorations performed better than did posterior composite restorations. The difference in performance was accentuated in large restorations and in those with more than three surfaces involved (17). However amalgam has certain drawbacks like mercury toxicity(18,19,20.) , brittleness, inability to support weakened tooth structure, insufficient initial strength etc. The Nag’s technique and alloys can be a possible amalgam alternative. The alloy satisfies the mechanical property requirements of ADA specification No.5 and comes under category of type III ( hard alloy) see table2. Amalgam tensile strength is in the range of 40-70 MPa but Au80 Sn 20 alloy’s tensile strength is nearly 275 MPa. it has the capacity to support weakened tooth structure and can be used where amalgam is not indicated. Whenever Ag31Cu8Sn61 alloy or other economical alloys are used cost of the final restoration will be same or less than the amalgam. Second sitting for polishing is not required.
This method is better than the cold welding of gold/silver technique which is not suitable for patients with large pulp chambers, periodontally weakened teeth, causes discomfort to the patient, more chair time requires costly material (1).
This technique is an alternative to traditional lost wax technique ( CAD/CAM also an alternative to lost wax technique). But when compared to CAD/ CAM less number of equipments are required. The equipment use full in this technique is not expensive compared to many dental equipments. Development of dedicated equipment for this technique improves the outcome.
Some of the disadvantages with Nag’s technique are that Voids may be present which weakens the restoration. Some of the alloys(silver & tin based ) are more prone for corrosion and oxide formation . controlled atmosphere soldering, nitrogen atmosphere soldering or other flux less soldering techniques can be used to prevent oxide formation. All these alloys are good electrical/ thermal conductors so adequate pulp protection is required. Harm full adhesive fumes and flux fumes may be generated during soldering/brazing . Adequate protection (fume extractor, mouth mask, goggles etc ) and ventilation is required (21,22,23.).
Like any new technique more biological evaluation is required. Extensive research is needed in developing alloys which are economical, low melting temperature, with adequate physical properties.
CONCLUSION: This paper presents an alternative to lost wax technique of fabricating an indirect restoration.. Technique is simple to master & can be done in a single visit with in 30-45 minutes. But extensive research is required to evaluate the biological compatibility and physical properties of the alloys.
DONOT TRY IT ON PATIENTS.
THIS PAPER IS PATENT PENDING UNDER THE PATENT ACT 1970 INDIA S.NO. 2258/CHE/2009 DATED 17/9/2009 AND 1247/CHE/2010 DATED 3/5/10 IN THE NAME OF Dr.K.NAGA SRINIVAS.
REFERENCES
(1).STURDEVANTS ART AND SCIENCE OF OPERATIVE DENTISTRY-Theodore M. Roberson, Herald O.Heymann,Edward J.Swift-5 th edition page 152 & 777 & 924
(2) Magne P. immdediate dentin sealing :A fundamental procedure for indirect bonded restorations. Journal of esthetic and restorative dentistry 2005;17:144-155
(3) Leinfelder KF, Kurdziolek SM. Contemporary CAD/CAM technologies: the evolution of a restorative system.
Practical procedures & Aesthetic dentistry 2004 Apr;16(3):224-6, 228, 231.
(4) O. N. Klenovich and A. M. Vetrova -Filled Cyanoacrylate Adhesive Compositions
ISSN 1811-2382, Journal of polymer science Ser. C, 2007, Vol. 49, No. 1, pp. 50–51.
(5)G. Elger, M. Hutter, H. Oppermann, R. Aschenbrenner, H.Reichl, and E. Jager, “Development of an assembly process and
reliability investigations for flip-chip LEDs using AuSn soldering,” Microsystem Technologies, Volume 7, Numbers 5-6 / January, 2002 Springer, pp. 239-243, 2002.
(6) J.Y. Tsai, C.W. Chang, Y.C. Shieh, Y.C. Hu, and C.R. Kao,
“Controlling the microstructures from the gold-tin reaction,”Journal of Electronic Materials, Volume 34, Number 2 / February, 2005 TMS, pp. 182-187, 2005.
(7) Jeong-Won Yoon, Hyun-Suk Chun, Ja-Myeong Koo and Seung-Boo Jung, Au–Sn flip-chip solder bump for microelectronic
and optoelectronic applications Microsystem Technologies ,Volume 13, Numbers 11-12 / July, 2007: pages 1463-1469
(8) J. Doesburg and D. G. Ivey ; Microstructure and preferred orientation of Au–Sn alloy plated deposits , Materials Science and
Engineering B, Volume 78, Issue 1, 31 October 2000, Pages 44-52(9) Demayo, A., M.C. Taylor, K.W. Taylor, and P.V. Hodson. 1982. Toxic effects of lead and lead compounds on human health, aquatic life, wildlife plants, and livestock. CRC Critical Reviews in Environmental Control 12:257-305.
(10) R D Jones, Survey of antimony workers: mortality 1961-1992. Occupational and Environmental Medicine 1994;51:772-776.
(11) R. Castillo, C. Ercoli, G. Graser, R. Tallents, M. Moss Effect of ring liner and casting ring temperature on the dimension of cast posts.The Journal of Prosthetic Dentistry, Volume 84, Issue 1, Pages 32-37(12)N.Hersek, Tensile strength of type IV dental stones dried in a microwave ovenThe Journal of Prosthetic Dentistry, Volume 87, Issue 5, Pages 499-502 (13)Letzel H, M.A. Van 'T Hof , G.W. Marshall , S.J. Marshall :The influence of the amalgam alloy on the survival of amalgam restorations: a secondary analysis of multiple controlled clinical trials, Journal of Dental Research, Vol. 76, No. 11, 1787-1798 (1997)
(14)bjertness E. Sonju T : Survival analysis of amalgam restorations in long term recall patients, Acta Odontologica Scandinavica, Volume 48, Issue 2 April 1990 , pages 93 - 97
(15) Downer MC, Azli NA, Bedi R, Moles DR, Setchell DJ: How long do routine restorations last?Asystematic review, British Dental Journal. 1999 Dec 25;187(12):632.
(16) J. -F. Roulet Benefits and disadvantages of tooth-coloured alternatives to amalgam References and further reading may be available for this article. To view references and further reading you must purchase this article.
,Journal of Dentistry Volume 25, Issue 6, November 1997, Pages 459-473
(17) Mario Bernardo, DMD, PhD, Henrique Luis, MS, Michael D. Martin, DMD, MSD, MPH, MA, PhD, Brian G. Leroux, MSc, PhD, Tessa Rue, MS, Jorge Leitão, MD and Timothy A. DeRouen, PhD Survival and reasons for failure of amalgam versus composite posterior restorations placed in a randomized clinical trial- Journal of the American Dental Association, Vol 138, No 6, 775-783.
(18) Bauer JG, First HA. The toxicity of mercury in dental amalgam. (1982) Calif. Dent Association . J., 10:47-61
(19) Women in dental surgeries: reproductive hazards in occupational exposure to metallic mercury, International Archives of Occupational and Environmental Health Volume 59, Number 6 / September, 1987 551-557
(20) Eley BM. The future of dental amalgam: a review of the literature. Part 2: Mercury exposure in dental practice. British Dental Journal. 1997 Apr 26;182(8):293-7.
(21) PENGELLY, J. GROVES A. SIMPSON and C. NORTHAGE Workplace Exposure to Rosin-based Solder Flux Fume During Hand Soldering Annals of Occupational Hygiene., Vol. 42, No. 5, pp. 295-302, 1998
(22)P S Burge, G Edge, R Hawkins, V White, A J Taylor Occupational asthma in a factory making flux-cored solder containing colophony.Thorax 1981;36:828-834;
(23)A. E. JOHNSON and R. C. BROWN Measurement of the Performance of Air Cleaners Against the Particulate Element of Rosin-based Solder Flux Fume Annals of Occupational Hygiene., Vol. 42, No. 8, pp. 511-519, 1998
TABLE 1
Alloy compositions
Melting temperature in degree centigrade(Celsius)
Advantages
Disadvantages
Remarks
60% gold
20% silver
20%tin, (Au60Ag20Sn20)
280(solidus)-475(liquidus)
Clay type of consistency when it is melted.
Easy to contour with soldering iron.
Fluxless soldering.
Resistance to corrosion.
good wettability.
good surface hardness.
High cost. Brittle.
Excellent alloy in terms of carving capability.
31% silver,
8% copper,
61%tin, (Ag31Cu8Sn61)
280-4500c
Clay type of consistency when it is melted.
Easy to contour with soldering iron.
Economical.
Consist metals that present in a dental amalgam
Less surface hardness. Borax flux is required. Mild corrosion potential.
Excellent alloy in terms of carving capability.
80% gold
20% tin, (Au80Sn20)
2800c
Easily achievable melting temperature.
Fluxless soldering
. Resistance to corrosion. good wettability.good surface hardness.ultimate tensile strength 275Mpa.
Modulus of elasticity 59.1GPa;elongation at break 2%.Suitable for “step soldering” applications.
High cost. Brittle. Burnishing may be difficult
Even though Versatile alloy best results can be achieved if it used as preform or along with adhesive. High tensile strength when comparing to many ceramics.Along with luting agent it may yield more tensile strength.
TABLE 2-- ABANSI/ADA SPECIFICATION NO.5 FOR DENTAL CASTING ALLOYS.
TYPE
HARDNESS
YIELD STRENGTH
MPa
PERCENTAGE ELONGATION %
TYPE I for low-strength castings _ such as inlays
SOFT
<140>340
10
