Acid etches bridges and its scope/certified fixed orthodontic courses by Indian dental academy


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Acid etches bridges and its scope/certified fixed orthodontic courses by Indian dental academy

  2. 2. CONTENTS 1. Introduction 2. History 3. Review of literature 4. Design concepts in acid- etched resin bonded bridges. 5. Advantages and disadvantages 6. Indications and contraindications 7. Selection of abutments 8. Preparation of abutment 9. Anterior teeth Posterior teeth Impression procedure 10. Laboratory procedures 11. The Enamel Etch and Resin Bond 12. The Acid for Etching Enamel 13. Alloy etching and bonding 14. Etching of Various Alloys 15. Alloys used for acid etched bridges 16. Acids used for etching 2
  3. 3. 17. Electrolytic etching 18. Bonding the restoration - Acid etching - Resin cements 19. Longevity and scope 20. Summary and conclusion 21. References 3
  4. 4. INTRODUCTION The development of a technique for the fabrication of fixed partial dentures, involving little or no preparation of abutment teeth, has been perhaps the most important single advance in the history of prosthetic dentistry. Although not suitable for many cases where abutment teeth are badly broken down, the resin-bonded fixed prosthesis provides a means for tooth replacement that has both short-term and long-term benefits. The 1970s saw the expansion of the acid etch technique into unexplored areas of clinical dentistry. The 1980s have seen further developments and improvements in the technique of bonding fixed partial dentures. The techniques explored in the 1970s by various investigators, leading up to the present status, where alloy frameworks are electrolytically etched prior to bonding to etched enamel with unfilled and filled resin. Undoubtedly, major changes in the present technique will be made in the years to come. An understanding of all the techniques previously used is important, particularly since these techniques are still viable alternatives to be considered when planning the treatment of certain cases requiring temporary care. Resin –retained FPD have had variable popularity since the technique of splinting lower anteriors with perforated metal casting was described by Rochette in 1973. His alternative to conventional metal –ceramic FPD’s this kind of prosthesis gained popularity which required minimal tooth removal of the abutment teeth that are intact and caries free. The primary goal of resin – retained FPD is replacement of missing teeth and maximum conservation of tooth structure. The development of acid etching of enamel to improve the retention of resin, first described by Buonocore in 1955, has proven to be a means of attaching fixed partial dentures to teeth by less destructive means. Ibsen first described the attachment of an acrylic resin pontic to an unprepared tooth using a composite bonding resin. Others since that time have utilized the technique, but it is probably best suited for use as a long-term provisional restoration or intermediate replacement of a missing tooth. The advent of electrolytic etching of metal to provide micromechanical retention for metal adhesion to enamel has led to the broad application of such bridges. HISTORY- DEVELOPMENT OF RBP Two distinct branches in the development of bonded fixed partial dentures can be traced. The first method utilizes materials readily available in most dental offices, and no 4
  5. 5. laboratory involvement is required. This is the technique using an acrylic resin denture tooth, a composite resin crown, or the extracted tooth as a pontic. The second method utilizes the laboratory manufacture of a cast metal framework with a porcelain or acrylic resin pontic. Both methods have their own indications and contraindications and both branches of the technique offer alternatives for dental practitioners willing to venture into new and fertile areas. 1. Bonded pontics Earliest resin – retained prosthesis were extracted natural teeth or acrylic teeth used as pontics bonded to proximal and lingual surfaces of abutment teeth with composite resins. The use of an acrylic resin denture tooth as a pontic was first published in 1973, although Ibsen in 1974 and Buonocore in 1975. Both Ibsen and Buonocore also described the use of the extracted tooth as a pontic. Buonocore's technique involved etching the adjacent enamel with 50% phosphoric acid (buffered with 7% zinc oxide) for 60 seconds prior to applying the ultra-violet light-curing unfilled and filled resins. In a 1978 publication, Jordan described 86 cases of single and multiple-unit fixed partial dentures followed for up to three years. The only major differences in Jordan's technique are that autopolymerising resins were used instead of ultra-violet light-curing materials, and Class III preparations were cut into the pontic crowns, rather than a lingual retentive groove. Simonsen in addition to describing cases using denture teeth and natural teeth as pontics also described the use of composite resin pontics. A thorough description of the technique with case reports was provided by Simonsen in 1978 and 1980. In a 1979 clinical trial, Hallonsten reported on 49 cases of up to four years duration. A success rate of approximately 50% was reported. Simonsen and Davila and Gwinnett have also described use of the natural tooth as a pontic. Sweeney and co-workers tested similar single-unit pontic attachments against hose where a wire was incorporated within the composite resin for, one would expect increased strength. Lambert and co-workers showed that when testing acrylic resin maxillary lateral incisors bonded with a ultra-violet polymerizing resin. The pontics and pontic-to-tooth bonds withstood a force sufficient to suggest that the application of this technique to the clinical practice of dentistry is warranted. 5
  6. 6. Disadvantages:  Composite resins were brittle  They required supporting wire or stainless steel mesh framework.  Their use is limited to short anterior spans.  Limited lifetime with debonding of resin and subsequent fracture. Such restorations were used as short term replacements. 2. Cast perforated resin – retained FPD’s( Mechanical retention) In 1973 Rochette introduced the concept of bonding metal to teeth using flared perforations of metal casting to provide mechanical retention. He used this for periodontal splinting but also included pontics in his design. Howe and Denehy recognized that metal framework improved retention and began using FPD’s with cast perforated metal retainers bonded to abutment teeth and metal – ceramic pontics to replace anterior teeth. Their design included, extending the metal framework onto the lingual surfaces with little or no tooth reduction. Patient selection limited its use in mandibular teeth with open occlusal relationship. The restorations were luted with heavily filled composites. This concept was expanded to the replacement of posterior teeth by Livaditis. Perforated retainers were used to increase resistance and retention. The castings were extended interproximally into the edentulous areas and onto the occlusal surfaces with a defined occlusogingival path of insertion by tooth modification which involved lowering of proximal and lingual height of contour of enamel on abutment teeth. Survival rate upon recall was up to 13 years. However they had following limitations:  Weakening of metal retainer by perforations with subsequent wear of the resin.  Limited adhesion of metal provided by perforations. 3. Etched cast resin – retained FPD’s ( Micromechanical retention MARYLAND BRIDGE) Based on the work of Tanaka et al on pitting corrosion for retaining acrylic resin facings and metal studies of Dunn and Reisbick, Thompson and Livaditis at University of Maryland developed a technique for electrolytic etching of Ni- Cr and Cr- Co alloys. Etched cast retainers have definite advantages over cast perforated restorations Retention is improved because resin to etched metal bond can be substantially stronger than resin to etched enamel. The retainers can be thinner and still resist flexing. 6
  7. 7. The oral surface of the cast retainers is highly polished and resists plaque accumulation Their use led to first generation of resin cements with low film thickness which permitted micromechanical bonding into the undercuts in metal casting created by etching while providing adequate strength and allowing complete seating of cast retainers. 4. Macroscopic Mechanical retention resin – retained FPD’s (VIRGINIA BRIDGE) As a result of concerns about etching base metal and desire to use alternative alloys several methods were developed to provide visible macroscopic mechanical undercuts on inner surface of FPD retainers. The first was developed at Commonwealth University School of Dentistry known as Virginia Bridge. It involves the lost salt crystal technique. On the cast the abutments wee coated with a model spray and lubricant is then applied. Within outlines of the retainers specially sized crystals (150 – 250 Mm) are sprinkled over the surface in a uniform monolayer leaving 0.5mm border without crystals at the periphery of pattern. Disadvantages  Adequate thickness of casting must be there to allow for undercut thickness.  No long term results reported  Permits use of any metal – ceramic alloy. Advantages  Good bond strength to enamel. An alternative technique was use of a cast mesh pattern on internal surface of retainers given by Taleghani. Mesh made of Nylon was adapted to lingual and proximal surfaces of abutments. The mesh was then covered with wax or resin done carefully to prevent occluding mesh or wax or resin. It was invested and casted. Disadvantage  Technique sensitive  Results in thick lingual casting. 5. Chemical Bonding Resin- Retained FPD’s ( ADHESION BRIDGES) While etched castings were used for retention of resin – retained FPDs in 1980 and early 1990 are extensive research was underway in Japan to develop adhesive systems for direct bonding of metal. 7
  8. 8. Maryland Bridge With etched inner surface for retention Rochette Bridge With metal perforations for retention First Date of Publication Contribution Author 1955 Bonding to enamel Buonocore 1962 Development of composite resin Bowen 1973 First bonded cast alloy framework Rochette 1976 Electrolytically etched dental alloy Dunn and Reisbick 1981 The etched metal bridge developed McLaughlin 1982 Advanced etching techniques McLaughlin REVIEW OF LITERATURE Williams V.D, Dremon D.G et al (1982) studied the effect of retainer design on retention of filled resins to acid - etched FPD’s. They determined which retainer design had the best retention capacity to enamel.  Rochette used 6 holes with sharp spatula in wax pattern 0.8mm in diameter.  Howe and Denehy used 1-1.5mm, 0.5mm in diameter.  Kuhlke and Drenon used 0.33- 0.5 thickness with holes drilled with No.1/2 round bur.  Newman bonded orthodontic brackets using fine mesh screen. They found that all retainer design had enough retention for anterior forces of occlusion. Most retainer failure occurred at retainer - composite resin interface. 8
  9. 9. Love L.D and Britman J.B. (1985) evaluated a new method for etching base metal alloys and the bond strength of ARC to etched alloys. Here the castings made of Ni- Cr alloys were air abraded and then immersed in etchant solution containing 50% nitric solution, 25% hydrochloric acid, and 25% methanol. The control group was etched with 10% sulphuric acid. However there was no significant difference in bond strength. The only advantage was the etchant solution was easy to use. Hill G.L., Zidan O., Gonez M. (1986) evaluated the tensile bond strength of three alloys Ni-Cr, non Be Ni- Cr, and Co- Cr alloys after etching with a lab etching instrument. They were divided into two groups i.e. over etched and under etched. It was concluded that under etching had a negative effect than over etching. Over etching on non Be-Ni-Cr alloys did not reduce the bond strength. Livaditis G.J. (1986) described a method to chemically etching non – precious alloys to create micromechanical retention. ASSURE – Etch a chemically etching solution was heated to 70ºC and applied to surface of metal disc for different periods of time (30, 45, 60, 75 and90 mins). It was observed under SEM. This method used was simple to use and inexpensive. It gave suitable bond strength when etched for 60 minutes. Tanaka T. et al (1986) studied the effect of oxidation of alloys on the bond strength of resin cements. Here two types of alloys Ni- Cr and Co – Cr alloys were used. Two samples were air brushed with 50μm alumina powder whereas others oxidized with an oxidizing agent. A solution of 0.3% sulfuric acid and 1% potassium per manganate forms an oxide layer thus increasing the adherence of resin cements. It was shown that Ni- Cr alloys developed superior bond strength after sandblasting and oxidation but an equivalent strength was obtained with Co –Cr alloys by merely sandblasting and ultrasonic washing. Teleghani Leinfelder K.T. (1987) compared the use of mesh pattern and conventional etching technique on the bond strength. Duralingual mesh pattern were attached to one side of the disc. They were divided into three groups:  Chemically etching with cast screen mesh on surface.  Etching without cast screen mesh an surface  Cast screen mesh on un-etched alloys. 9
  10. 10. Ag- Pd alloys were etched with NaF and sodium nitrate Base metal alloys were etched with 10% sulfuric acid. It was concluded that cast mesh surface serve as an alternative to chemical or electrolytic etching. The bond strength of base metal alloys with mesh screen was 42.4lbs, etching alone was 34.2lbs, etching with mesh 37lbs. this procedure was advantageous for gold and palladium alloys which have no etchants available. Layer B. Nicholls J.I. and Townsend (1988) used a mixture of 10% sulfuric acid and methanol followed by 18% HCl cleaning. Re G.J. et al (1988) compared three methods of etching metals for resin bonded bridges. Ni- Cr alloys were air abraded with 50μm alumina MET- ETCH Gel, Electrolytic etching with 10% sulfuric acid for 10 mins at 300mA and Silicoating were evaluated as surface treatments. It was observed that silicoating with electrolytic etching gave superior results. MET- Etch Gel produced lowest shear strength. Kern M, Thompson V.P (1995) studied bonding to glass infiltrated alumina ceramicadhesive methods and their durability. Inceram glass infiltrated alumina ceramic has 3-4 times greater flexural strength than other ceramic or glass materials. Therefore can be used as a core material for RBFPD’s. Etching silica based feldspatic or glass ceramics with hydrofluoric acid or ammonium bifluoride creates sufficient resin bond – enhanced with a silane coating of etched ceramics. Inceram can be etched with boiling sulfuric acid. Long term durable bond to Inceram was achieved with combination of tribochemical silica coating and conventional BIS- GMA composite resin or combination of sandblasting and composite resin modified with phosphate monomer. Shakal M.H, Pfeiffer P et al (1997) studied the effect of tooth preparation design on bond strength of resin – bonded prosthesis. A pilot study Here molar teeth were prepared in conventional form or with occlusal coverage and opposing grooves and retainers were silicoated and bonded to etched enamel by bisglycerol methacrylate adhesive. Bond strength was determined after water storage and thermocycling. They concluded that: 10
  11. 11.  Metal surface area and design of tooth preparation had a substantial effect on strength and durability of adhesive bonding joint.  Occlusal coverage improved resistance and retention form of adhesive FPD’s  Placement grooves or increasing bonding surface area improved both strength and durability of bonding joints.  Combination of 180deg opposing grooves, channels with lingual wraparound and occlusal coverage remarkably improved the strength and durability of bonding joints. DESIGN CONCEPTS The guidelines for optimum design of resin-retained FPD’s have been empirically derived. The underlying principle for these restorations has always been that it is necessary to cover as much enamel surface as possible, as long as occlusion, esthetics, or periodontal health are not compromised General Principles of Design The ideal design for an etched cast resin-bonded restoration involves creation of a distinct path of insertion for the restoration by a sequence of modifications to the enamel contours of the abutment teeth. When seated, the restoration should not be displaced or rocked in any direction by occlusal forces. Displacement is prevented by alloy engaging tooth structure. Stated in other terms, the framework mechanically engages each of the abutment teeth. The bonding of the alloy to the tooth structure allows the casting to be supported by the tooth structure. Bonding also prevents displacement back along the path of insertion. Because displacement of the casting in all directions other than back along the path of insertion is prevented by alloy engaging tooth structure, the framework design limits the stresses placed on the luting agent and the bond. Consequently, a well-designed framework and the resin bond between the etched alloy and tooth structure are synergistic. We shall see that this is even carried to the point that the occlusal rest in the posterior and the cingulum notch, sometimes used in the anterior, are designed to allow the casting to mechanically engage the tooth and prevent displacement of the framework in all directions other than incisally or occlusally. Posterior design principles Framework design and the necessary modifications to abutment teeth are most easily described for posterior restorations. An idealized diagram for a three-unit, fixed partial prosthesis and the modifications of the abutment teeth are shown below. 11
  12. 12. The following design elements should be included in any etched cast retainer: 1. A distinct path of insertion must be created in an occlusogingival direction. This is accomplished by parallel modification of first proximal, and then lingual, surfaces of the abutment teeth. The height of contour is lowered to approximately 1 mm from the gingival margin where possible, provided that such modification will not penetrate the enamel. Thus, in some proximal areas the height of contour may only be lowered sufficiently to provide occlusogingival width for the connector - generally a minimum of 2 mm as a result of the concave area from the coronal narrowing in a gingival direction. 2. Proximal resistance form must be created. The alloy framework must extend buccally beyond the distobuccal and mesiobuccal line angles of the respective abutments. Thus, the framework cannot be displaced from the buccal toward the lingual. This is another key element in creating a distinct path of insertion. If aesthetics are compromised by the buccal extent of the alloy, then Judicious modification of the buccal enamel allows the buccoproximal line angle to be moved lingually. The alloy only needs to extend just buccal to this line angle to establish the resistance form and it is easily hidden with proper contour of the buccal porcelain. This proximal enamel modification is distinctly different from that used for a removable partial denture where guiding planes are the norm. Bonded retainers require the modified enamel to retain the approximate original buccal to lingual curve of the proximal surface when viewed from the occlusal. This proximal resistance form can also be created by the use of proximal grooves or boxes when the buccal extent of the preparation might compromise aesthetics by narrowing excessively the mesiodistal width of the tooth (e.g., mesial of maxillary first premolar, where the tooth, when viewed from the buccal, narrows dramatically). Proximal grooves must be prepared judiciously to avoid penetration of enamel. 3. Proximal "wrap-around" should be obtained. The alloy framework should be extended to engage the tooth structure at 180° or more of its circumference when viewed from the occlusal. The framework should not extend to the point where it can compromise the occlusal embrasure form between the abutment and an adjacent tooth. The proximal wrap-around allows the casting to mechanically engage the abutment tooth. This is ideal but is not always clinically possible. Correct design of the occlusal rest can reduce the need for 180° wrap-around. 4. Maximum bonding area is utilized on a given abutment without compromising gingival health or aesthetics. This is obtained with the proximal and lingual modifications, 12
  13. 13. which lower the height of contour. The bonding area can be increased by extending the framework toward the occlusal above the modified enamel, providing it does not interfere with the occlusion. The better the mechanical retention of the framework, the less critical is the necessity to maximize the bonding area. Indeed, in anterior restorations with good proximal wrap-around, the restoration is routinely kept 1.5-2.0 mm short of the incisal edge to prevent graying of the translucent incisal edge. 5. Some form of occlusal rest is required on each abutment of a posterior resin- bonded restoration. The rest should be small but well defined and not a broad spoon shape similar to classic removable partial denture occlusal rests. Usually a number 5 or 6 round bur is employed and the rest created is 1.5-2.0 mm in buccolingual direction, 1.5-2.0 mm in the mesio-distal direction, and 1 mm deep. It is important that the occlusal rest follow the contour of the tooth structure going from the marginal ridge toward the fossa. Thus, relative to the occlusal plane, the rest should become deeper as it approaches the central fossa of the abutment tooth. Utilizing this rest design, removal of the casting from the tooth requires that it be lifted. Alternatively, if the tooth is to migrate out of the framework (due to lack of 180° proximal wrap-around) it must first be displaced gingivally relative to the casting. Thus, the occlusal rest design mechanically retains the tooth in the casting by functioning as a shallow pin. The location of the rest is not critical and it can be placed anywhere along the marginal ridge to remove it from an area of occlusal contact. When a distinct cusp of Carabelli is present, the cusp can be modified to function as a rest. Note, however, that the alloy framework is carried up to, and just over, the height of contour of the marginal ridge and thus functions as a broad additional occlusal rest. 6. Create knife-edge gingival margins on posterior abutment teeth. Enamel is removed only to the extent that a knife-edge supragingival margin results. Thus, the gingival contour of the restoration should duplicate the enamel removed during preparation. These fine margins are aided by the 0.3 mm minimum thickness commonly employed for the lingual portion of the retainer. There is no attempt made to create a chamfer margin at the gingival; this only removes enamel unnecessarily. 13
  14. 14. A definite path of insertion and 180º wrap-around design Note the minimal preparation of molar and mesial rest added to incorporate the mesial occlusal amalgam filling Anterior Design Principles The anterior design depends upon the same general principles for retention as does the posterior retainer. The modifications made to the enamel of abutment teeth are much more subtle than those used in the posterior region. Only minor recontouring of enamel is necessary. The retention is attained by preparing a distinct path of insertion. This path of insertion is the result of three factors. First the surfaces of abutment teeth are modified proximal to the edentulous space so that the casting can be fabricated with proximal wrap-around. This is accomplished in conjunction with lowering the height of contour on the abutment teeth to provide sufficient depth (1.5-2.0 mm) for the connector. When viewed from an inciso-gingival direction, the modification is small but distinct. The wrap-around is facilitated by the divergent orientation of the proximal surfaces of anterior teeth caused by the curvature of the dental arch. Modification interproximally has as its objective recontouring to move the proximo-labial line angle toward the lingual. Alternatively, the proximolingual line angle can be sharpened and, as a result of the divergence in the labial direction between the proximal surfaces, a distinct path of insertion is created. The actual preparation involves the intersection of two gently curving preparations. The intersection line is the path of insertion. The cast framework need only extend labially a 14
  15. 15. minimal amount past this line of intersection. Consequently, when the casting is seated along this incisogingival direction it cannot be displaced toward the lingual. The relationship of the wrap-around of the framework to the porcelain can be seen in figure below. The alloy is not visible because the wrap-around does not extend too far toward the labial. The porcelain has some depth, and the composite resin used for bonding fills in the interproximal, masking the presence of the alloy. The subtlety of the tooth modification is difficult to illustrate. On many teeth, simply by lowering the height of contour and retaining the original proximal contour (as seen inciso-gingivally), this wrap- around can be achieved. The proximal wrap-around allows the abutments to reciprocate one with the other for retention of the restoration. Another retentive factor involves extension of the framework over the marginal ridge not involved with the edentulous space as well as over the marginal ridge lingual to the connector. This extension further defines the path of insertion and allows the framework to mechanically engage the abutments. The third and final factor in anterior framework design is the modification of each abutment to create a distinct cingulum notch. This can be considered as a cingulum rest specifically prepared so that the rest becomes deeper gingivally as the preparation extends towards the labial (in cross section, a V shape). This notch functions in a role similar to that of the occlusal rest design suggested previously. The cingulum notch mechanically retains the tooth into the framework and reciprocates with the proximal wrap-around to prevent the tooth from moving out of the framework. The point might be well taken that any one of the above factors might by itself be sufficient to protect the bonding. We try; however, to routinely use all of these retentive components even though clinical experience may, in the future, allow us to eliminate one or more of the factors. 15
  16. 16. ADVANTAGES 1. Minimal Tooth Preparation: Little tooth structure has to be removed for this technique, making it more conservative and less likely to create problems in unblemished abutment teeth. 2. Minimal potential for pulpal trauma: Conservativeness of tooth preparation. 3. No Anesthetic Needed. An anesthetic is not required because most of the preparation will be done in enamel. 4. Supragingival Margins. Although supragingival margins can be used with conventional retainers, they are mandatory for the resin-bonded fixed partial denture. Only when the retainer gingival margins were less then 0.5 mm from the gingival crest was there correlation with gingival response. 5. Easy impression making: due to supragingival margins 6. Provisional not usually required: However judicious placement of composite resin is important to maintain occlusal clearances after final impression and until final restoration is bonded. 7. Reduced chair time: due to conservativeness of tooth preparation 8. Unaltered casts without removable dies 9. Reduced Cost. This is probably not as significant as was first thought when little or no preparation was involved with the technique. However, with the increased use of preparation features, more of the dentist's time and skill are required, and the cost differential between a conventional prosthesis and a resin-bonded fixed partial denture has become less. 10. Rebonding Possible. Resin-bonded fixed partial dentures can be rebonded if the "wings" or axial extensions are not sprung or bent when the restoration debonds. Commonly, one retainer does become loose before the other. If this goes undetected for any significant period of time, caries can develop on the abutment under the retainer and a new conventional fixed partial denture may have to be constructed. However, if the debonding is detected early enough, the retainer that remained attached must be removed without damage to the tooth or to the restoration. This can be quite difficult if the tooth has been well-prepared. Removal of resin-bonded restorations by the use of specially designed ultrasonic scaler tips has been described. The KJS, a straight chisel, is used to develop a fracture in resin along the incisal edge, and the KJC, a curved chisel, is used at the gingival margin. 16
  17. 17. DISADVANTAGES 1. Irreversible. The resin-bonded fixed partial denture, as it is frequently used today, requires the removal of enough tooth structure that it should be considered irreversible. Whether this is really a disadvantage or not is debatable, but the point is raised simply to remind the reader that it is necessary to do some preparation of the tooth. 2. Uncertain Longevity. The resin-bonded fixed partial denture is not new and totally untested, but there is still some concern about the longevity of this type of prosthesis. The results of 27 studies on resin-bonded FPD longevity by Marinello et al, the success rate dropped from 95% after 3 months to 91 % at 6 months, 81.5% at 1 year, and 73% at 18 months. 3. Enamel modification is required: Extensive enamel modifications are required with retentive design to the proximal and lingual surfaces of the abutment teeth. If the restoration is removed, composite resin bonding could restore the enamel contours, but transition to a more traditional prosthesis is likely. Enamel is limited in thickness, which requires precision in design and preparation with attention to detail. Enamel lingual surfaces of anterior teeth are almost always thinner than 0.9mm. 4. No Space Correction. Although some porcelain can be added to the metal retainer on the adjacent abutment teeth, there are definite limitations on what can be done if the edentulous space is significantly wider than the mesiodistal width of the tooth that would normally occupy the space. 5. Good Alignment of abutment is required: It is impossible to correct alignment problems with this restoration, inasmuch as nothing is done to the facial, proximal, and incisal areas of the abutment teeth. Good alignment of abutment teeth is required because the prosthesis' path of insertion is limited by potential penetration of the enamel thickness. However, some posterior teeth, which are mesially or mesiolingually tilted, can be onlayed with a bonded retainer. Thus patient selection in limited. 6. Plaque accumulation can occur: as designs are outside the dimensions of the tooth and bulky contours may be intolerable to some patients. 7. Difficult Temporization. A provisional fixed partial denture cannot be fabricated with this type of restoration. If a missing tooth is to be replaced while the fixed 17
  18. 18. partial denture is being made, it must be accomplished with a muco-adhesion temporary removable partial denture. 8. Esthetics is compromised on posterior teeth: Posterior resin-retained FFD design requires the extension or the metal framework onto the occlusal surface of posterior teeth. These occlusal rests and occasional onlaying of cusps are visible, which might be objectionable to some patients. 9. Graying out of teeth: that are thin labio-lingually due to alloy visibility 10. Dependence on laboratory: for competent treatment of cast metals and selective waxing to avert over-contouring and equipment requirement. 11. Patients expectation of esthetics is high: but routine results are fair to good not outstanding. INDICATIONS In the treatment plan for any fixed prosthesis, the patient's individual needs must be properly identified. The presence of any existing disease, its etiology, and how it relates to the treatment prognosis must be assessed. Periodontal and general denial health must be reestablished, and the proposed abutment teeth should not exhibit mobility; however, periodontal splinting of teeth with a resin-retained FPD has been successful with strict provision of mechanical retention of each tooth within the alloy framework. 1. Replacement of missing anterior teeth in children and adolescents: Conventional fixed prosthodontic techniques are generally contraindicated in young patients because of management problems, inadequate plaque control, the large size of the pulps, and the fact that children, routinely participate in sports. Anteriorly, one or two teeth with mesial and distal abutments can generally be replaced with a resin-bonded FPD. Depending on the circumstances, a greater number of teeth can be involved. 2. Caries-free Abutment Teeth/ Unrestored abutments: If the edentulous span is not too long, the resin-bonded fixed partial denture allows tooth replacement with minimal destruction of tooth structure on undamaged abutment teeth. Sound teeth or those with minimal restorations are suitable as an abutment with a resinbonded retainer. When bonding to anterior teeth, the presence of Class III restorations is not a contraindication to a bonded retainer. However, large 18
  19. 19. multiple restorations or a Class IV restoration would limit the bonding and the abutment's mechanical integrity. In the posterior region, existing Class II lesions proximal to the edentulous space can be incorporated in retainer design. 3. Mandibular Incisor Replacements. The acid-etched resin-bonded fixed partial denture is the restoration of choice for replacing one or two missing mandibular incisors when the abutment teeth are unblemished. 4. Maxillary Incisor Replacements. Maxillary incisors can be replaced if they are in an open-bite, end-to-end, or moderate overbite situation. 5. Periodontal Splints. The splinting of periodontally involved teeth comprised the first published report of the use of a resin-bonded prosthesis by Rochette, and other authors described the use of resin-bonded, perforated and etched-metal splints for long-term usage. However, abutment mobility has been cited as one of the causes of failure by Barrack, and the study by Marinello et al indicated that the failure rate for splints was 13% greater than that for fixed partial dentures' If a resin-bonded prosthesis is to be used as a splint, careful attention must be paid to resistance features on the abutment preparations. In the previously cited study by Marinello et al, the use of grooves on abutments for splints improved the chances for success by nearly 15%. 6. Stabilizing dentition after orthodontics 7. Prolonged placement of interim prosthesis: to augment surgical procedures i.e. cranio-facial anomalies. 8. Single Posterior Tooth Replacements. While replacement of multiple teeth can be done with this type of prosthesis, it becomes a higher-risk procedure. Resinbonded fixed partial dentures of more than three units have a 10% higher failure rate than those that are only three units in length Resin-bonded FPDs of greater than three-unit length should be used only if there is some mitigating treatmentplanning consideration, such as opposing a removable partial denture, which would result in less occlusal stress. Fixed partial dentures with more than two retainers have a failure rate 2.5 times that of resin-bonded FPDs with only two retainers. 9. Significant clinical crown length: should be present maximize retention and resistance. 19
  20. 20. 10. Innovative applications: New and innovative applications include bonding an attachment for RPD to an abutment tooth. The other use maybe bonding the laminate veneer alloy- ceramic facing over prepared tetracycline stained tooth CONTRAINDICATIONS Because of the apparent advantages of resin-retained restorations, they have often been used in inappropriate circumstances, leading to failures that reduced patient (and dentist) confidence in the technique. Fortunately, these failures were usually correctable by more conventional methods. If any of the following contraindications exist in a particular clinical situation, an alternative approach to treatment should be selected. 1. Parafunctional habits: Patients with parafunctional habits should be approached cautiously when the use of resin-retained FPD’s is considered, because the resistance, to displacement of these retainers is lower than in conventional FPD’s. They should be used judiciously where above-average lateral forces are likely to be applied (e.g., in a patient with parafunctional habits or in a patient who requires an anterior tooth replacement in the presence of an unstable or nonexistent posterior occlusion). In these instances, all means of mechanical retention of the framework (grooves, occlusal rests, interproximal extension of metal should be used. The patient should be informed about the possibility of debonding. Periodontal splints can also be fabricated, but they require strict attention to mechanical retention. 2. Long edentulous span: should be avoided because they place excessive force on the metal retention mechanism; with repeated loading, fatigue of the bonding interfaces or even the metal is possible. 3. Extensive Caries. Because the resin-bonded fixed partial denture covers relatively little surface area and relies on bonding to enamel for its retention, the presence of caries of any size will require the use of a more conventional prosthesis. Surgical crown lengthening may therefore be necessary as a way to increase the bondable surface area and because subgingival margins must be avoided. 4. Restored or damaged abutments: Extensively restored or damaged teeth are unsuitable as abutments. One sound abutment and one extensively restored abutment can be incorporated into a combination FPD. 20
  21. 21. 5. Compromised enamel: on abutment teeth as a result of hypoplasias, demineralization, or congenital, problems (e.g. amelogenesis imperfecta or dentinogenesis imperfecta) will adversely affect resin bond strength. 6. Significant pontic width discrepancy: The labiolingual thickness of anterior abutment teeth and translucency of the enamel should be assessed to determine whether the shade of the abutment teeth will be changed (a consequence of reduction of tooth translucency by the metal retainer. Graying of the abutments can be eliminated with opaque resins and by limiting the incisal extent of the metal on the lingual surface. Translucent resins cause optical coupling of the metal to the tooth; appreciable graying of the enamel will result. A trial insertion of the metal with water between the metal and tooth provides a preview of the possible graying. Similarly, when custom-staining the pontic of resin-retained FPD, a trial resin (which will not polymerize) should be used to visualize the final shade of the abutment teeth with the metal backing. 7. Nickel Sensitivity. Since most resin-bonded fixed partial dentures are etched nickel-chromium restorations, nickel sensitivity in a patient requires that another alloy be used or that another type of prosthesis be employed. Tin-plating and laboratory-applied bonding systems allow the use of noble alloys. However with the reduced elastic modulus of most noble alloys, the metal thickness should be increased approximately 30% to 50% to make the stiffness of noble metal framework equal to that of base metal. This is an important factor in treatment planning and can influence the amount of occlusal clearance required for the metal (which is critical in patients with deep vertical overlap. 8. Deep Vertical Overbite. So much enamel must be removed from the lingual surface of a maxillary incisor in this occlusal relationship that retention would be drastically reduced because of the poor bonding strength afforded by the exposed dentin. 9. Incisors with thin faciolingual dimension. Lingual Enamel Thickness of Maxillary Anterior Teeth (m millimeters) Millimeters from cementoenamel junction Central incisor 0.3 0.5 0.6 0.7 0.7 0.7 Lateral incisor 0.4 0.5 0.5 0.6 0.7 0.7 Canine 0.2 0.4 0.6 0.7 0.9 0.9 21
  22. 22. SELECTION OF ABUTMENT The dentist should determine the restoration design before beginning tooth preparation. This may include surveying the abutment teeth and making trial modifications on the cast. Abutment teeth that are caries free and periodontally sound with adequate thickness of enamel forms ideal abutment. eg. missing upper lateral incisor with central incisor and canine as abutments When designing the anterior prosthesis, use largest possible surface area of enamel that will not compromise the esthetics of the abutment teeth. The retentive retainers (wings) should extend one tooth (mesial and distal if a single tooth is replaced. If two teeth are replaced, double abutment on either side can be considered, but only if periodontal support of the abutments is compromised. With proper mechanically retentive designs, two maxillary incisors pontics can be retained by two lateral incisors, unless the laterals have short clinical crowns or a deep vertical overlap is present. If a combination of tooth replacement and splinting is cover more used, the framework may teeth. Cantilevering pontics with resin-retained FPD’s is also possible. Replacement of lateral incisors where cantilevers from either the central incisor or canine are possible. The retainer design is critical and requires adequate mechanical engagement of the abutment tooth. RBFPD with missing upper left central incisor ABUTMENT PREPARATION The early use of acid-etched resin-bonded fixed partial dentures was accomplished with no preparation of the abutment teeth. Although some authors advocate little or no tooth preparation for this type of prosthesis, emphasizing its, reversibility preparation features are used by many to enhance the resistance of resin-bonded fixed partial dentures 22
  23. 23. Whether anterior or posterior teeth are prepared, common principles dictate tooth preparation design.  Nearly parallel walls i.e. 6 degree taper  A specific path of insertion  Sufficient occlusal clearance  Maximum coverage of virginal enamel  Vertical stops A distinct path of insertion must exist, proximal undercuts must be removed to provide "planes of metal" on the lingual and proximal surfaces, with a slight extension onto the facial surface to achieve a faciolingual lock. The preparation should encompass at least 180 degrees of the tooth to enhance the resistance of the retainer. Occlusal rest seats and proximal groove/slots must provide resistance form, and a definite and distinct margin gingival margin should be established wherever possible about 1mm supragingivally. Occlusal clearance is needed on very few teeth that are prepared as abutments for acid-etched resin-bonded fixed partial dentures. Specifically, 0.5 mm is needed on maxillary incisors, where preparation is done on the lingual surface of the teeth. . Because of the limited thickness of enamel near the cemento-enamel junction, this type of restoration cannot be used on patients with a severe Class II vertical overlap. Vertical stops are placed on all the preparations. This will consist of two or three flat countersinks on the lingual surface of an incisor, a cingulum rest on a canine or an occlusal rest seat on a premolar or molar. Wilkes found rests to be the dominant feature in a preparation, contributing to both resistance and rigidity. The occlusal rest directs the applied force from the pontic to the abutments On anterior teeth, the procedure is similar in many ways to the lingual reduction needed for a pinledge, preparation, but the amount of reduction is less because the enamel must not be penetrated, if necessary, the opposing teeth can be recontoured to increase interocclusal clearance. There must be sufficient, enamel area for successful bonding, and the metal retainers must encompass enough tooth structure and have sufficient resistance form to prevent the individual abutment tooth from being displaced in any direction out of the framework. Bur selection depends on operator preference. Gingival margins and circumferential preparation are easily accomplished with a chamfer or round tipped diamond. Occlusal rest seats and cingulum notches can be prepared with a diamond or 23
  24. 24. carbide inverted cone bur. The other critical retentive features (e.g., slots, grooves) can be made with tapered fissure bur. Anterior tooth preparation and framework design Steps involved are:  Clearing the occlusion  Creating a path of insertion  Cingulum notch or rest The occlusion is assessed to ensure at least 0.5 mm of interocclusal clearance for the metal retainer in the intercuspal position and throughout the lateral and protrusive excursive pathways. If inadequate clearance exists, selective enameloplasty is performed. Occasionally, additional clearance can be obtained by reducing the opposing teeth. However, this is contraindicated if there is wear or attrition on the incisal edges. Modification of the proximal surfaces is necessary to create a parallel path of insertion. This also depends on the presence or absence of proximal restoration. Another modification is a slight alteration of the marginal ridge to allow the casting to wrap over the marginal ridges of each abutment tooth. This modification results in a slight chamfer which acts as a guide to locate the gingival margin and at the same time creates a smooth transition from the casting to enamel. The gingival margin should be designed so that slight supragingival chamfer exists that delineates the gingival extension of the preparation. Any undercut enamel is removed at this time. The chamfer finish line should extend incisally through the distal marginal ridge area so that mesial, lingual, and distal “planes" are created. Abutments should have parallel proximal surfaces whenever possible. The finish line on the proximal surface, adjacent to the edentulous space, should be placed as far facially as practical. A 0.5-mm-deep slot, prepared with a tapered carbide bur, should be placed slightly lingual to labial termination of the proximal reduction. Great care paralleling the proximal grooves is required. A paralleling instrument is very helpful during this procedure and has demonstrated its importance in highly successful clinical studies. This device is also helpful when many teeth are to be incorporated in the framework (e.g., an extensive periodontal splint). The restoration must extend labially past the proximal contact point. To optimize esthetics, the proximal wrap in the anterior region may be achieved in part with the porcelain pontic. 24
  25. 25. A V- shaped notch can be prepared with an inverted cone bur in lingual approach. The preparation must be small but distinct. Judicious preparation is necessary and penetration into enamel in avoided. Preparation of mandibular anterior teeth is similar to that for the maxillary incisors. Lingual enamel thickness is 11% to 50%less than for maxillary teeth and tooth preparation must therefore be more conservative. Combinations of periodontal splinting and tooth replacement are commonly used in mandibular anterior region. This presents a challenge for providing mechanical retention for the abutment and splinted teeth. Posterior tooth preparation and framework design The basic framework for the posterior resin-retained FPD consists of three major components:  the occlusal rest (for resistance to gingival displacement),  the retentive surface (for resistance to occlusal displacement), and  the proximal wrap and proximal slots (for resistance to torqumg forces) A spoon-shaped occlusal rest seat, similar to that described for a removable partial denture (RPD) is placed in the proximal marginal ridge area of the abutments adjacent to the edentulous space. An additional rest seat may be placed on the opposite side of the tooth. The rest is an important retention feature for resistance to both occlusal and lateral forces and should be designed to function as a shallow "pin." To resist occlusal displacement, the restoration is designed to maximize the bonding area without unnecessarily compromising periodontal health or esthetics. Proximal and lingual axial surfaces are reduced to lower their height of contour to approximately I mm from the crest of the free gingiva. The proximal surfaces are prepared so that parallelism results without undercuts. In the interproximal area, no gingival chamfer is necessary to prevent penetration of the enamel, resulting in a knifeedge margin. Occlusally, the framework should be extended high on the cuspal slope, well beyond the actual area of enamel recontouring (provided it does not interfere with the occlusion). Resistance to lingual displacement is more easily managed in the posterior region of the mouth. A single path of insertion should exist. The alloy framework should be designed to engage at least 180 degrees of tooth structure when viewed from the occlusal. This proximal wrap enables the restoration to resist lateral loading by engaging the underlying tooth structure and is assisted in this regard by grooves in the proximal 25
  26. 26. just lingual, to the buccal line angle. Distal to the edentulous space, the retainer resistance is augmented by a groove at the lingual proximal line angle. Moving a properly designed resin-bonded FPD in any direction except parallel to its path of insertion should not be possible, nor should it be possible to displace any tooth to the buccal from the framework. In general, preparation differs between maxillary and mandibular molar teeth only on the lingual surfaces. The lingual wall of the mandibular tooth maybe prepared in a single plane. The lingual surface of the maxillary molars requires a two-plane reduction due to occlusal function and the taper of these functional cusps in the occlusal two thirds. However, the mandibular lingual retainer may be carried over the lingual cusps to augment resistance and retention form on short clinical crowns of mesially and lingually inclined molars (this may require a two- plane modification). A wide range of extensions of the casting onto the occlusal surfaces of posterior teeth is possible. They include shoeing of cusps, encircling, of cusps, and extensions of metal through the central fossa from mesial to distal with the lingual cusps exposed. The clinician is limited only by imagination, available enamel, occlusion, and the display of metal tolerated by the patient. Occasionally, a combination restoration can be used. This type of FPD includes a resin-bonded retainer on one of the abutment teeth and a conventional cast restoration on the other. As previously noted, this type of FPD has been very successful in clinical studies. Periodontal splinting is the most demanding of the restoration designs; splints and splint-FPD combinations require care in designing adequate mechanical retention. IMPRESSION PROCEDURE An accurate impression is necessary as marginal fit is as critical for a resin-retained restoration as for a conventional FPD. Bond strengths are reduced with thick resin layers. A custom tray can be fabricated to minimize the distortion of using stock trays. Elastomeric impressions are superior and reversible hydrocolloid is less desirable for final replica. Provide temporary occlusal stops. Significant supraocclusion of the abutment teeth can occur rapidly, particularly in younger patient or patients with reduced periodontal support. This can be avoided on anterior teeth by placing a small amount of composite resin on the opposing lower teeth. This is rarely needed for posterior teeth unless significant onlaying of the abutment is planned (in which case small composite resin 26
  27. 27. stops can be bonded to the enamel). The resin is removed just before placement of the resin-retained FPD. LABORATORY PROCEDURES Framework Fabrication Framework design for the resin-bonded fixed partial denture is important. It cannot be divorced from the preparation since the extensions for the retainers will be determined by the preparation. There must be an adequate thickness of the metal in the finished restoration so that it will be immune to distortion and/or dislodgment. Caputo et al found through the use of photoelastic stress analysis that stresses could be lowered significantly by thickening the wraparound arms of the retainer to 0.6mm and including proximal extensions." Failure to reduce stress in the arms of the retainer will be translated into fatigue failure of the underlying resin bonding material. Framework can be fabricated using resin copings or pattern waxes.  Wax the framework. The approximate thickness of the wax pattern should be no less than 0.4mm and preferably closure to 0.6mm. Carve and flush the wax on the lingual surface. There should be no voids. Use a spoon excavator or a discoid carver to cut 1mm deep grooves in the facial, lingual and gingival surfaces of the pontic. The grooves on the incisal edge should be 1.5mm deep. These grooves will ensure adequate thickness of porcelain when added to the metal framework.  Cast in nickel- chromium alloy or any other metal ceramic alloy  Different alloys require different surface preparation or tin-plating; the dentist should use an alloy that has been well tested with the adhesive composite resin of choice (see below).  Build up the pontic in porcelain, fire it, and contour it. 27
  28. 28.  Evaluate the restoration clinically; when the fit is satisfactory, characterize and glaze it. Opaque resins are necessary to prevent metal from graying the abutment teeth. Depending on the opacity of the resin and the teeth, the value of the abutment may be increased.  Try-in for anterior teeth should involve a try-in paste for proper characterization of the pontic. Any remaining try-in paste will be burned out during the glaze firing.  After this is completed, the restoration can be polished. Regular finishing compound is suitable.  Clean the fitting surface with a particle abrasion unit, using aluminum oxide (50Mm at a minimum of 40 psi [0.3 MPa] pressure); rinse thoroughly with water and dry. If the restoration is tried-in again, particle abrasion should be repeated just before bonding. THE ENAMEL ETCH AND RESIN BOND The development of the acid etch technique has had a profound effect on many phases of clinical dentistry. The expanded adaptation of the acid etch technique into the bonding of etched alloy frameworks to etched enamel has significantly and irrevocably changed the treatment choices available to practitioners in the area of fixed partial dentures. The late Michael Buonocore's pioneering work in the field started in 1955 with the publication of his paper entitled “A Simple Method of Increasing the Adhesion of Acrylic Filling Materials to Enamel Surfaces.'" Recognizing that one of the major shortcomings of the resin filling materials was their lack of adhesion to dentin and enamel, Buonocore embarked on the development of "bonding", as the acid etch technique is popularly referred to Buonocore's choice of the use of phosphoric acid to etch enamel was not an accident. For years, this acid had been used in industry to obtain better adhesion of paint and resin to metal. The strength of acid used in industry (85%) was also used by Buonocore in his initial work. Buonocore showed that acrylic resin can be attached to human enamel in vivo simply by etching the enamel surface for 30 seconds with 85% phosphoric acid. The increased bond strength of acrylic resin to etched enamel compared to unetched enamel was attributed by Buonocore to several factors: 1. A large increase in surface area of enamel available for interaction with resin as a result of the etching process. 28
  29. 29. 2. The exposure of the organic framework of enamel, which then serves as a framework for adhesion. 3. The removal of inert enamel surface structure, exposing a fresh reactive surface 4. The presence on the enamel of a strongly absorbed layer of highly polar phosphate groups derived from the acid THE ACID FOR ETCHING ENAMEL Several investigators have conducted in vitro studies into various acids and their effect on both bovine and human enamel. Brauer and Termini, Laswell and co-workers Lee and co-workers, and Silverstone have all contributed to the scientific literature in this field. Silverstone's 1974 data were critical in the choice, by several manufacturers, of the acid and the strength of acid to recommend with composite resin systems. Silverstone tested 20%, 30%, 40%, 50%, 60%, and 70% phosphoric acid, 50% phosphoric acid buffered with 7% zinc oxide by weight, 5% and 50% citric acid, 10% polyacrylic acid, and 5% and 50% neutral and acid solutions of EDTA for exposure times varying from one to five minutes. Silverstone observed two types of attack of the enamel by phosphoric acid. The first was a loss of surface contour, and the second was the creation of a porous subsurface region. It is this porous subsurface region that is the key to the retention of resins. Silverstone found the greatest retention of resin with phosphoric acid in the range of 20-50%. By further selecting the acid that combined the least amount of surface contour loss with the greatest depth of subsurface porous region, Silverstone concluded that a 30% solution of phosphoric acid was the most effective etching agent. This most significant study resulted in the majority of the resin manufacturers choosing to supply phosphoric acid in the strength range of 30-40% for the acid etching of enamel. This does not mean that 50% phosphoric acid will not give a clinically acceptable etch. Silverstone's work, however, shows that the degree of surface etching decreases with increasing concentration of acid. Only about 5 microns of surface enamel are lost with a three-minute etch of 60% phosphoric acid, whereas a three-minute etch with 20% phosphoric acid produces a 40 micron loss of surface enamel. The same effects are seen with the subsurface histologic changes in tissue – the weaker the acid, the deeper the changes. However, weak acids do not produce such a consistent etching pattern and it is not desirable to lose large amounts of surface enamel each time etching takes place. A 30% phosphoric acid solution produces a 10 micron loss of surface contour and 29
  30. 30. a 20 micron depth of histologic change. The reason for a weaker acid affecting the surface enamel more than a stronger acid may be related to the degree of ionization of the acid. The weaker the acid, the greater is the degree of ionization leading to greater diffusion into the tissue. Silverstone has also suggested that the formation of soluble or insoluble precipitates with the various strengths of acids can affect the dissolution rate of enamel. In a 1975 report, Silverstone and co- workers defined three basic etching patterns of human enamel produced after exposure to phosphoric acid. In the type1 etching pattern, the enamel prism cores are preferentially removed, leaving the prism peripheries standing. The enamel surface thus appears cratered under the scanning electron microscope (SEM). The type 2 etching pattern is the reverse of type 1. The prism cores are left relatively intact and the peripheries removed. The SEM allows us to view such a surface, which appears much like a tightly packed forest of trees as viewed from above. The Type 3 etching pattern was defined as one where small areas of types 1 and 2 could be seen intermixed with areas that could not be related to prism morphology. Any one or all three of these patterns can be seen on a single sample of etched enamel. Clinically, etched enamel takes on a frosty white appearance. If this is not seen after etching for 60 seconds, an extended etch time may be required. In general, if teeth are carefully cleaned prior to etching, and the phosphoric acid is continually and carefully reapplied during the course of the etch time, it is seldom necessary to increase the etch time. Type 1 etching pattern Type 3 etching pattern Type 2 etching pattern Resin tags 30
  31. 31. ALLOY ETCHING AND BONDING The electrolytic etching of non-precious casting alloys in order to create a microretentive surface for resin bonding (Figure Ill-1) was a natural progression from earlier work on perforated retainers at the University of Maryland. Based upon the work of Rochette in France and Howe and Denehy of the University of Iowa, the perforated retainer was introduced at the University of Maryland by Kuhlke. Impressed with the possibilities of the technique, Livaditis in consultation with others instituted a study to evaluate the perforated retainer for replacement of missing posterior teeth with the restoration in normal occlusion. Indeed, these posterior restorations, although of limited number, are continuing to function without failure for up to four years. Denehy has recently reported on 250 anterior restorations of up to seven years duration with excellent results. The following limitations of the perforated retainer were noted: 1. The resin retention into the perforations is the limiting factor in strength of the system. That is, failure can occur through the resin projections into the perforations leaving the resin retained on the abutment teeth. This has been observed in several anterior failures and confirmed in in-vitro tests by Eshleman and others: 2. The composite resin used for bonding is exposed at the perforations and can wear, causing loss of mechanical retention. This is of particular concern with the low film thickness composite resin developed for this technique (Comspan, L. D. Caulk Company, Milford, Delaware), which has a filler loading of only 65% as compared to 75-85% for conventional composite resins. 3. The perforations weaken the alloy framework. This necessitates making the lingual arms thicker in cross section for stiffness and fatigue resistance. In two earlier anterior cases lacking some of the later design criteria, fatigue fractures of the alloy propagating through the perforations were observed. (This problem is somewhat eliminated with proper design employing a distinct path of insertion with the casting engaging tooth structure in all displacement directions Initial alloy etching The appearance in September of 1979 of an elegant article by Tanaka and others, utilizing pitting corrosion of a non- precious alloy for mechanically retaining acrylic facings, caused us to immediately attempt to apply these techniques to nonprecious alloys, with the above- mentioned limitations in mind. Pitting corrosion proved to be difficult and highly variable for the nonprecious (Ni-Cr) alloy selected 31
  32. 32. (Biobond C&B, Dentsply International Inc., York, Pennsylvania). Review of the literature established that Dunn and Reisbick had previously used electrolytic techniques to etch a cobalt- chromium implant alloy (Surgical Vitalhum, Howmedica Inc., Chicago, Illinois) to provide mechanical retention for a ceramic coating. Following their example, work was initiated in the winter of 1979-1980 at the University of Maryland to determine the etching conditions for the above Ni-Cr alloy. The test configuration was that employed by Tanaka and involved a low voltage DC power supply connected to a stainless steel cathode (—) and a disc of the alloy mounted on a conductive anode (+). All areas of the anode except the face of the test disk were masked with sticky wax. Both electrodes were immersed in a stirred bath of acid, and current was passed through the bath for a specific period of time. Following etching, a debris layer was present on the alloy and this was removed by immersion in 18% hydrochloric acid in an ultrasonic bath for 10-15 minutes. The details of the testing procedures are covered elsewhere. Nitric acid was selected for the etchant as a result of the similarity between the composition and microstructure of the Ni-Cr alloy investigated by Dunn and Reisbick and this Ni-Cr alloy. This choice was fortuitous and results were almost immediate, but trial and error involving acid concentration, etching current, and etching time were necessary to refine the process. Nitric acid at a concentration of 0.5 N and a current density (mill amperes per square centimeter of it resulted in surfaces exhibiting a great area to be etched) of 250 ma/cm2 for five minutes deal of three-dimensional relief. This is most easily seen in the scanning electron microscope (SEM). Immediately after successfully etching this alloy, clinical evaluations of etched casting resin-bonded restorations were begun with the first fixed prosthesis placed in March 1980. Investigations of the etching conditions for other alloys were begun in the spring of 1980, and a great deal of effort was expanded in trying to etch a representative alloy (Rexillium III, Jeneric Industries, Wallingford, Connecticut) of the Ni-Cr- Be class to obtain sufficient three-dimensional relief. The 10% sulfuric acid etching of Ni-Cr-Be alloys was finalized in December of 1980. Here the slight flecking of the surface due to relief of individual grains is apparent. This is characteristic of Ni-Cr-Be alloys for porcelain bonding. Rexillium III alloy has been the alloy with which the authors have the most experience, but it should be pointed out that other alloys have since been shown to etch equally as well and have excellent resin-to-alloy bond strengths. The Rexillium III alloy can be thought of as representative of the class of Ni-Cr-Be alloys for porcelain bonding. The choice of alloy for this technique should also be concerned with 32
  33. 33. other characteristics of the alloy such as corrosion resistance, yield strength, and case of casting or polishing. The electrolytic etching is most conveniently evaluated using a light micro- scope at 40-80 magnifications to inspect for surface relief and etching patterns. (It is helpful to use lighting incident at a shallow angle to the alloy surface to highlight the surface relief.) The etching pattern is most easily observed with a stereomicroscope and a characteristic pattern showing the junction of an unetched control area with 10% sulfuric acid etched area is seen In Figure Ill-5. Higher magnification photomicrographs lose quality due to prolonged exposure times. Viewed with the SEM, the junction between the unetched and etched areas becomes more apparent. Here it can be appreciated that the amount of surface relief created depends upon selective removal of one or more of the phases present. Consequently, this technique is limited to alloys that solidify with a multiphasic structure - that is, nonprecious alloys. There are at this time no known precious or semi-precious alloys suitable for this technique. Particular efforts have been made to grain-refine the high noble metal content alloys for porcelain bonding in order to prevent sag at porcelain firing temperatures, which precludes a retentive surface being formed during etching. The selective removal of the alloy phases present between the dendritic arms in the Ni-Cr-Be alloy can be evaluated. The phases removed are the lamellar interdentritic eutectic high in Beryllium according to Baran. Even the intra-dentritic gamma prime phase ascribed to be NI; AI in com- position is attacked to some degree by sulfuric acid. Any factor which changes the solidification structure of the alloy will affect the etching process. This can be seen in recast Ni-Cr-Be alloy where the interlamellar eutectic phase is hardly present due likely to loss of some constituents during the reheating of alloy. ETCHING OF VARIOUS ALLOYS Subsequent studies in our laboratories have established etching conditions to a number of alloys. These alloys can be generally classified as Ni-Cr-Be, Ni-Cr and Co-Or although within any one class there are wide variations of other metallic constituents within the alloys. 33
  34. 34. Conclusions 1. Ni-Cr-Be alloys as a class yield micromechanically retentive surfaces when electrolytically etched in sulfuric acid at a concentration of 10%. There are, however, individual variations. 2. Micromechanical retention in Ni-Cr- Be alloys is achieved by massive removal of the lamellar interdendritic phases and to a lesser degree by removal of the intradendritic gamma- prime phase. 3. Ni-Cr and Co-Or alloys generally etch in nitric acid. Porcelain firing causes oxide layer formation that binds nitric acid etching. 4. The Ni-Cr and Co-Cr alloys have microstructures in which the secondary phases are less broadly distributed than those for Ni-Cr-Be alloys and etching conditions must be tailored to each alloy composition. 5. The presence of a retentive surface on a candidate alloy should be confirmed with the scanning electron microscope and bond studies con- ducted to determine the efficacy of the etching conditions. In order to investigate the clinical situation, lingual retainer castings have been fabricated in a Ni-Cr-Be alloy, electrolytically etched and bonded to phosphoric acid etched tooth structure with a low film thickness composite resin. Such a casting was embedded in methyl methacrylate. When this cross-section is examined under the SEM an appreciation is gained of the relative size difference between the degree of relief on the alloy surface as compared to the etched enamel. The small particle size in the composite resin is easily appreciated to be less than 5 microns (the interspace between the tops of the alloy and the etched enamel is 8-12 microns in this selected area). 34
  35. 35. 35
  36. 36. ALLOYS USED FOR ACID ETCHED BRIDGES Ni-Cr-Be Alloys  RexiBium III  Pacific 5B  Vera Bond  Litecast B  Bak-On N.P.  Unitbond  Ticonium 100 Ni-Cr Alloys  Biobond C&B  NP2 (Howmedica, Chicago. IL.) Co-Cr Alloys  Neobond II Special (Neolloy Posen, IL.)  Vitallium (Howmedica, Chicago. IL.) Silver-Palladium  Albabond (Heraeus, Queen Village,NY.) High Palladium  Spirit (Jensen Industries, New Haven, CT.) ACIDS USED FOR ETCHING SULFURIC ACID Sulfuric acid is easily obtainable in a 98% concentration. It is available in both reagent grade and technical grade. Some researchers have found that use of the technical grades of sulfuric acid has resulted in a poorly etched surface with red and brown discolorations. For this reason reagent grade sulfuric acid should always be used. To achieve an approximately 10°/o solution from a 98% solution of sulfuric acid it is only necessary to dilute the reagent grade sulfuric acid 9:1 by volume with water. In order to do this properly it is important that you be aware of one detail. Concentrated sulfuric acid is a very strong dessicant. In the presence of compounds containing water it has the ability to withdraw water from the compound very rapidly. When mixed with actual water, this reaction takes place so rapidly that much heat is given off. If water is added to concentrated sulfuric acid, the reaction takes place so rapidly that it causes boiling. Therefore, the correct technique for diluting sulfuric acid is to add the sulfuric acid to cold water, rather than to add the water to the acid. To create a 10% solution of sulfuric acid follow these steps. 1. Fill a 1-liter polypropylene bottle about two thirds full of cold water. 36
  37. 37. 2. Measure out 102 m of 98% reagent grade sulfuric acid and slowly add this to the cold water. Remember, it is important that the acid be added to the water rather than the other way around. 3. Fill the bottle with additional cold water until the total contents equal 1 liter. This will result in an approximately 10% solution of sulfuric acid. NITRIC ACID Nitric acid is available in reagent grade at a 70% concentration. The steps for diluting this to 0.5 N solution are as follows; 1. Fill a polypropylene bottle with I liter of cold water. 2. To this, add 30 ml of 70% nitric acid. The resultant solution is 0.5 N nitric acid. To mix the 10% solution of nitric acid that is used for the cleaning phase of some high palladium alloys, the directions would be as follows: 1. Into an unbreakable bottle pour 85.7 ml cold water 2. Add to this 14.3 ml of 70% nitric acid. The resultant solution is 10% nitric acid. HYDROCHLORIC ACID Hydrochloric acid is a solution of hydrogen chloride gas dissolved in water. It is available in various grades, and readily available solutions range in concentration from 32% to 36%. One important thing to realize about hydrochloric acid is that the hydrogen chloride gas is constantly being released from the solution. Therefore, it is important that when the solution is not actually being used, it should always be in a closed container. The final concentration of the hydrochloric acid in the cleaning phase of the twostep technique is not critical. Success can be expected when using solutions in the range of 16% to 18%. Thus, it is easiest simply to take the concentrated hydrochloric acid and dilute it 1:1 with cold water. As with all gases in solution, the gas is more soluble in a cold liquid then in a hot liquid. Therefore, when mixing and handling the hydrochloric acid, it is usually best to use cold solutions. 37
  38. 38. MIXTURES OF ELECTROLYTES With the introduction of the one-step etching technique, it has become increasingly common to use more exotic and sophisticated electrolytes than those used in the single component system. By combining various electrolytes, we have been able to increase the efficiency of the entire etching process. The first such combination to be put to use in dentistry was the combination used in the one-step etch. ONE-STEP SOLUTION This patented solution is a mixture with a resultant concentration of 10% sulfuric acid and 18% hydrochloric acid. It is mixed in the following manner: 1. Make a 20% solution of sulfuric acid. This is accomplished by adding 98% reagent grade sulfuric acid to chilled water in a 2:8 ratio. Refrigerate the resulting mixture. 2. In a 1-liter polypropylene bottle pour 500 ml of 36% hydrochloric acid. Refrigerate this liquid. 3. When both liquids have chilled sufficiently, slowly add the 20% sulfuric acid solution to the 36% hydrochloric acid solution. The resulting solution is a 10% sulfuric acid and 18% hydrochloric acid mixture. When mixing the one-step solution, exercise great care. Because hydrogen chloride gas, which is extremely caustic, is liberated from the hydrochloric acid in great quantities when the solution becomes warm and because the dilution of sulfuric acid by water solutions results in the liberation of heat, this procedure should take place only under a vent. Even with a vent, it is best to chill the solutions adequately before mixing, in order to ensure that the final concentrations will be that which is anticipated. That having been said, the process is quite simple. The solution that has been developed for etching, Spirit alloy," requires a solution of 33.25% methanol and 23.4% hydrochloric acid. This solution is mixed as follows: 1. Into an unbreakable bottle pour 35 ml of anhydrous methanol. 2. Add 65 ml of cold 36% hydrochloric acid. It is important to observe carefully the following order when mixing these chemicals. If the methanol were to be added to the concentrated hydrochloric acid, it could react in various ways to produce aldehydes, fatty acids, explosive nitrogen compounds. Once the hydrochloric acid concentration has been reduced below 35% 38
  39. 39. however, this danger does not exist. It is therefore the mixing and not the using that is particularly tricky. METHANOL-SULFURIC ACID SOLUTION The addition of a small percentage of alcohol to most etching solutions seems to facilitate the etching procedure. Although this is a well-known fact in industry, it is still poorly understood. It has been hypothesized that the alcohol facilitates the etching process at the surface, perhaps by lowering the viscosity in the debris layer in the immediate areas of the etching. One such mixture commonly used in dentistry for etching dental alloys is a mixture that is approximately 9% sulfuric acid and 10% methanol. This solution should be mixed in the following manner: 1. Fill a 1-liter polypropylene bottle about two thirds full with cold water. 2. Measure out 92 ml of 98% reagent grade sulfuric acid and slowly add this to the cold water (Note: it is important that the acid be added to the water rather than the other way around). 3. Add 100 ml methanol to this solution. 4. Fill the bottle with additional cold water until it equals 1liter in volume. Alternatively, a 10% sulfuric acid can be made, and this then diluted with a 9:1 mixture of sulfuric acid and methanol. HYDROCHLORIC ACID- METHANOL- SULFURIC ACID SOLUTION In order to create a retentive etch on one silver palladium alloy, Albabond- 60, it was necessary to resort to an extremely corrosive solution. After extensive testing, the specific solution that proved to combine the greatest effectiveness with the highest degree of safety was a solution of 16.2% hydrochloric acid, 10% anhydrous methanol, and 9% sulfuric acid by volume. The simplest way to mix this etchant is first to prepare a batch of one- step etchant. Then dilute it by adding a 9:1 mixture of one-step etchant and methanol. As with all the previous recipes, the order of mixing is of extreme importance. As already mentioned, hydrochloric acid is nothing more than hydrogen chloride gas dissolved in water. For this reason it is advisable to refrigerate the acid before diluting it, because gasses are more soluble in cold liquids. 39
  40. 40. When concentrated sulfuric acid is diluted, a great deal of heat is liberated. This, then, is both the reason for mixing small quantities at a time, and for adding the sulfuric acid to the water, instead of the reverse. If the water were to be added to 98% sulfuric acid, the solution would probably begin to boil and splatter. Even after diluting the sulfuric acid, it is best to refrigerate the liquid before going on to the next step. If the solutions are not quite cool when they are combined, voluminous amounts of extremely caustic HCI gas can be released. Once the solution has cooled down again, this problem is diminished. It is also important to delay adding methanol until the final step, because the combination of methanol with concentrated sulfuric acid may form dimethylene sulfate, an odorless, tasteless compound that may be fatal if absorbed in sufficient quantities into the skin or respiratory tract. Even gas masks do not offer adequate protection. If methanol were to be added to concentrated hydrochloric acid, it could react in various ways to produce aldehydes, fatty acids, or explosive nitrogen compounds. Once the hydrochloric acid content has been reduced below 35%, however, this danger does not exist. It is therefore the mixing and not the using of this electrolyte that is particularly tricky. ELECTROLYTIC ETCHING TECHNIQUE It must be borne in mind that electrolytic etching of alloys in order to create threedimensional relief for micromechanical bonding of resins represents an entirely new technology. There has never been any basic research published in this area. Tanaka and his co-workers' pointed the way with their pitting corrosion studies and based part of their concept on the previous work of Dunn and Reisbick. The metallurgical literature has been concerned with slight amounts of etching relief on polished surfaces in order to visualize metal and alloy micro- structure. At the other extreme, there has been extensive research on electro-polishing, which occurs at higher current densities, and this is well covered in the classic text of Teggart. In the early investigation of electrolytic etching for resin bonding, it was assumed that factors critical in electropolishing were also critical in this technique. As we shall see later in this chapter, this is not the case, and we are learning daily about this new technology. There are a number of research projects being conducted at the University of Maryland and at other universities to better understand the etching process. 40
  41. 41. Consequently, there are likely to be changes in the following technique as the dental profession gains experience in this new area. Laboratory procedures in etching 1. Complete the restoration The restoration is completed prior to etching. All adjustments, characterization and staining, glazing, and final polishing should have been finished. Adjustments and polishing following etching can lead to contamination of the etched surface. Cleaning in an ultrasonic bath with a soap solution and then thorough rinsing could remove this contamination. However, the etched surface is quite friable and easily scratched with a sharp instrument. Repeated try-ins with the inner surface of the casting etched are definitely not recommended. The best procedure is to have all aspects of the restoration completed prior to etching. 2. Mounting the restoration The restoration is initially tacked to an electrode with a brittle sticky wax. Holding the restoration and electrode in contact while tacking them with sticky wax is difficult. It is best accomplished with the use of a "soldering assistant" for splints or for restorations with pontics by placing the buccal or facial surface of the pontic into a mound of modeling clay on the bench top, being careful not to allow the clay to contact the "wings" of the retainer. This frees a hand for the mounting procedure. The electrode can be any conductive metal. This is awkward since making metalto-metal contact at more than two points on the restoration is difficult due to the stiffness of the rod. It is recommended that a less stiff electrode material such as no. 12 or no. 14-gauge copper wire be used. This is easily adjusted to make contact at two points on a three-unit bridge and multiple contacts on more extensive cases. Multiple contacts on extensive cases do not appear to be critical in the procedure, but they are routine. The electrode to which the restoration is attached (the positive electrode) is to be masked with a sticky wax or insulation and should never be in contact with the etching solution. Thus it can be any conductive metal. One manufacturer of etching equipment (Time Dental Laboratories, Baltimore, Maryland) routinely uses 0.036 inch diameter stainless steel orthodontic wire as the electrode material. If any portion of the electrode other than the restoration is in contact with the etching solution, it will give false current 41
  42. 42. reading. The mounting electrode is also bent to allow the maximum surface area for etching to be at right angles to the long axis of the electrode. 3. Assuring electrical contact A conductive paint (Silversol, Hanau Teledyne Inc., Buffalo, New York) is then applied with a brush at the contact points between the mounting electrode and the restoration. This assures a broad electrical contact between these curved surfaces. It also prevents the shrinkage of the sticky wax from opening the point contact. Any conductive paint may be used. Do not allow the conductive paint to extend to a margin or it will become the main current path and inhibit etching. 4. Masking the restoration All areas of the restoration not to be etched are then masked with sticky wax, taking special care that the wax is brought just to the margins. Any sharp margins that are exposed will be preferentially etched due to the higher local current density. This results in ragged margins. Remember that thorough masking of all areas (even porcelain) is necessary in order to prevent loss of polish. (Note that the electrode must be completely insulated also.) This is a time-consuming procedure, and several alternative masking paints are being evaluated which are essentially rubberized paints (e.g. Plating Mask, Yarter-Tech Inc., Denver, Colorado). These masks are applied with a brush. After etching, they can be peeled off the restoration or dissolved solvent. Attaching to electrode Metal immersed in electrolyte solution 5. Cleaning areas to be etched The surfaces of the restoration to be etched are routinely cleaned by air abrading with 50 micron alumina, and rinsed with tap water. The margins are checked and sticky wax reapplied to areas inadvertently exposed. 42
  43. 43. 6. Determining the etching current The total area on the restoration to be etched is estimated by comparing it to a one square centimeter standard. An oblong piece of paper 5 mm x 20 mm is very convenient. It is necessary to estimate the area to be etched in order to determine the total amount of current to be passed through the etching solution. For instance, if the current density called for in the etching formula is 300 ma/cm2 and the area to be etched is estimated as 0.75 cm", then the current through the etching bath should be 0.75 x 300 = 225 milliamperes. It is better to underestimate the area to be etched (lower current) than to overestimate (higher current) and perhaps get into the electropolishing domain of the potentiostatic anode current density curve (Figure Vll-10). This will be covered later. The suggested etching current densities have been selected to allow some latitude in area estimation and still be assured of an adequate etch. 7. Electrode arrangement Attach the electrode with the mounted restoration to the positive (anode) lead of a low voltage DC power supply. The remaining electrode (cathode) is then attached to the negative (-) lead of the power supply. The cathode must be stainless steel. The cathode is bent at right angles at the end of the shaft to allow a 1.5-2.0 cm length of the cathode to point toward the anode. The spatial relationship of the anode to the cathode has not been found to influence the etching process to any extent. This is quite the opposite of what is found in pitting corrosion' and electropolishing. The electrode "clamp assembly" is manipulated to position the cathode so that the end of the cathode is directed at the maximum surface area of the restoration. Inter-electrode distance is usually 1.5-2.0 cm. Since the resistance of the etching solution changes with the length of the current path through the solution, increasing the inter-electrode distance increases the voltage drop across the electrodes. Alternatively, if a constant voltage is maintained, the current through the solution will decrease as the inter-electrode distance is increased. 8. The etching process Submerge the electrodes in the etching solution. Solutions are covered in the next section. Following the manufacturer’s instructions for the utilization of the power supply: 43
  44. 44. a) Turn the power on and adjust the current (milliamperes) to the level calculated based on the current density necessary for the particular alloy. Begin to time the etching process. b) Check that the current level is maintained (it may fluctuate initially). The current need only be maintained to within ± 20 ma for the average three- unit retainer and it is less critical for larger cases. c) The restoration should begin to turn dark and go to a black color within the first 30 seconds. Bubbles should form on the cathode and a yellow colored solution should form around the restoration. If a large number of bubbles form on the restoration and it does not turn black, the electrodes are reversed. Occasionally, a few bubbles may form on the restoration, these can usually be displaced by tapping on the clamp assembly and they should not reform. d) At the end of the required etching time, turn the unit off and remove the electrode on which the restoration is mounted, using caution to avoid skin contact with the acid. Rinse with tap water and then observe the uniform black debris layer present on the etched surface. Note that stirring of the etching bath is not mentioned. Del Castillo and Thompson have recently investigated this and determined that there is no significant difference in resin-to-alloy bond strength with stirring, with no stirring, or with ultrasonic agitation of the solution during etching, 9. Cleaning the restoration The restoration, still attached to the electrode, is placed in a closed container with an 18% hydrochloric acid solution. Approximately 150 ml of fresh solution is necessary for a three-unit retainer. The acid should not contact the exposed upper portion of the electrode where it connected to the electrode clamp assembly. Place the closed container in an ultrasonic cleaner for 10 minutes. When the ultrasonic cleaner is first turned on, the debris layer will fall away from the etched surface as if black ink were being released from the surface. The cleaning is continued for approximately 10-15 minutes or until a uniform gray surface appears. The electrode is removed carefully from the acid, rinsed, and inspected. Ideally, the surface should appear a uniform gray. This can vary widely. In general, Ni-Cr alloys give brighter, more uniformly metallic surfaces. On the other hand, Ni-Cr-Be alloys tend to be a darker gray, which is due likely in part to optical effects created by the much finer etched structure. In addition, Ni-Cr-Be alloys have been found 44
  45. 45. to have areas of brown or gray-black those are not cleaned away in the 18% HCl solution. These discolored areas appear to be properly etched when viewed with the SEM, and no debris layer remains. They may be due to chemical contaminants in the etching solution. Bond strengths appear to be unaffected by this discoloration. The use of 18% HCI solution can be questioned, as this is not substantiated in the literature. Whether this is the optimal concentration for cleaning these various alloys has not been determined. There is much research to be done in this area. 10. Checking the etch It is necessary to check the alloy surface for characteristic etching patterns prior to its separation from the electrode. Without a great deal of experience with a given alloy, it will be impossible to determine the presence of a properly etched alloy surface with the naked eye. It is strongly suggested that the etch be verified by observation of the alloy surface at a minimum magnification of 60x. This is most conveniently and accurately done with a stereomicroscope. A compound microscope can be used, but the depth of field is so limited that verification of the etching pattern on the curved surfaces of the retainers is difficult. To aid in visualization of the three- dimensional relief on the surface, it is recommended that the light source be directed at a very shallow angle to the surface to be observed. There are a number of stereomicroscopes on the market, and it is felt that this is a wise purchase for a dental laboratory, as the microscope has many other uses. If the etching pattern of the given alloy cannot be observed or is limited to a few areas, the retainer should be returned to the etching solution for an additional 60-90 seconds of etching and then cleaned again in HCI solution. Once the etching pattern can be observed over approximately 90% or more of the surface, etching should be stopped. The majority of cases will routinely etch completely unless there is a gross underestimation of surface area with, therefore, a very low current density. Contaminants in the etching solution or on the alloy surface can also cause problems. Any additional etching does not create more three-dimensional relief of the surface but instead strips alloy from the surface. If this is continued, the etching can, in time, perforate the retainer in thin areas. 45
  46. 46. 11. Separating restoration and electrode Removal of the restoration from the electrode is best achieved by chilling the sticky wax in cold water and then breaking the restoration away from the electrode under water. The sticky wax can be chipped away under water. This allows the wax to float away and prevents it from becoming embedded in the etched alloy surface. If this is not done, wax invariably gets into the etched surface and can only conveniently be removed by steam cleaning. Once all the wax is removed and the restoration is dried, it should be handled carefully in order to avoid contamination. The etched casting should not be placed back on the master cast. Rather, the restoration should be placed in a hard paper or plastic envelope. Packing directly in cotton or soft fibrous materials is to be avoided as fibers become entangled in the etched surface and nearly impossible to remove. The restoration is now ready for bonding. Etching in process Etched metal Etching apparatus A variety of products have been manufactured to be self-contained power supplies and timers for electrolytic etching of castings. It is helpful to bear .in mind that the etching can be satisfactorily completed using a six-volt storage battery, a rheostat, and a current meter. The next step up would be to purchase a low voltage DC power supply with continuously variable current control between 100 mA to 1.5 mA and an accurate current meter. Beyond this there are package systems with integral timers, stirrers, and even integrated circuit controls with preprogrammed functions. Some systems monitor only current, while some display both current and voltage. Others monitor one or the other. Monitoring of voltage is not critical to the procedure, but it does help to confirm that estimated area has not been grossly (two or three times) overestimated. The following is helpful to remember: when in doubt monitor current, not voltage. The reason for this will be covered in the section on factors in the etching process. 46
  47. 47. Etching Solutions The various etching solutions and conditions for selected alloys are given in Tables below. The depth of etching pattern has been verified in the scanning electron microscope. In making the etching solutions it is important that reagent grade acids be used. It has been found, particularly for sulfuric acid etching, that the several varieties of technical grade acid give very poorly etched surfaces with brown and red-brown discolorations- The restorations and test disks, when subsequently etched in reagent grade acid solutions, gave excellent results. The hydrochloric acid cleaning solution, which is consumed in considerable quantity, can be made with technical grade acid. Handling and storage of these solutions require some care. Contact with skin and clothing is to be avoided, and closed unbreakable containers should be used as much as possible. Observe good technique when diluting concentrated acid and always add acid to water, rather than water to acid. Factors in the etching process In order to further investigate this previously described method of creating a micromechanically retentive surface by electrolytic etching of non-precious alloys, studies were initiated to elucidate the laboratory variables that influence the etching process. In order to better understand this process and compare it to the well-known phenomenon of electropolishing, curves of current density vs. voltage were determined for one representative alloy (Rexillium III, Jeneric Industries, Wallingford, Connecticut). Below 2.7 volts and current densities less than approximately 600 ma/cm", etching occurs. As the voltage is increased, the current density falls and electropolishing is observed at the edge of the sample. At higher voltages, the current rises again and uniform electropolishing occurs. Note that over a narrow range of voltage (2.1-2.7 volts) the current density changes from 50 ma/cm" to over 550 ma/cm. Based on this curve, the etching current density of 300 ma/cm" was selected for this alloy. This allows for a fair variation in the estimation of area to be etched while the current density is still within the etching region. If only voltage is monitored, then as little as 0.2v change can alter the current density by a factor of two, and either greatly prolong the etching time or, at higher current densities, strip excessive amounts of alloy from the framework for a given 47
  48. 48. three-minute etching time. This also explains why the current drops substantially when voltage is kept constant and inter-electrode distance is increased (increasing the resistance). A definite relationship between current density and etching time over the etching portion of this curve has been verified by Del Castillo and Thompson. This is shown In Table below, where the control etching conditions of 300 ma/cm2 for three minutes, etching at 150 ma/cm 2 for six minutes and 600 ma/cm2 for one and one-half minutes give equivalent bond strength values. At this higher current density, if area is misjudged and over-estimated, it is easy to begin to electropolish the restoration. Should the concentration of the sulfuric acid solution be low, then the current density vs. voltage curve is shifted to higher voltages. The resulting surface etched at 300 mA/cm2 for three minutes has very poor surface relief and cannot be expected to give good bond strength values. 1. Not stirring the etching bath gives higher bond strength values (not significant) than control etching with stirring or ultrasonic agitation. Thus, ultrasonic agitation does not increase bond strength. 2. Recast alloy gives significantly lower bond strength values and has a highly changed etched morphology. 3. Orientation of the test disks relative to the cathode had little effect on bond strength values, whether facing the cathode or facing in an opposite direction. Ultrasonic agitation of the etching solution results in over etched dendritic arms of the specimens. The significance of this is not clearly understood. 48
  49. 49. Etching apparatus 49
  50. 50. BONDING THE RESTORATION Resin Cements The first resin-bonded restorations described by Rochette, which were splints, were held in place by a unfilled resin, poly(methyl methacrylate)(Sevriton), attached to etched enamel, based on the work of Laswell et al. While a whole generation of resinbonded fixed partial dentures would bear the title of Rochette Bridge, they made use only of the perforated retainers described by Rochette, ignoring the silane coupling with which he augmented resin attachment to the metal framework. Unfilled/filled composite resins (Adaptic/Adaptic Bonding Agent, J&J Dental Products Co, East Windsor, NJ, and Concise Composite and Enamel Bond System, 3M Co, St Paul, MN'447) were used with perforated retainers. Then a modified unfilled/filled composite resin with a thin film thickness specifically intended for luting resin-bonded fixed partial dentures was released, closely following the development of electrolytic etching. The next step was "chemically active (adhesive) resin cements: 4META, or 4-methacryloxyethyl trimellitate anhydride (C&B Metabond, Parkeli Corp, Farmingdale, NY) .It was developed with a unique tri-n-butylborane catalyst system that is added to the liquid before combining with the powder. On base metal alloys Super bond has the highest initial bond strengths of any adhesive resin system. Unfortunately there is some concern about the hydrolytic stability of these bonds over time which depends on the alloy‘s Ni-Cr ratio. Its advantages include:  Lower elastic modulus  Higher fracture toughness when compared to BIS-GMA based resin cements which could result in less brittleness and better clinical results with less well adapted castings. This system has shown poor clinical results with bonding high gold alloy retainers to abutment teeth. However alloy primers are being developed to provide more stable bond to noble alloy surfaces. Super Bond was followed by introduction of BIS-GMA based composite resin luting cement that is modified with adhesion promoter MDP, or 10-methacryloytoxydecyl dihydrogen phosphate (Panavia EX, Kuraray Co, Osaka, Japan). These cements rely on adhesion to the metal and not on microretention in the surface of the metal for bond strength. Etching was no longer necessary. Panavia has excellent bonds to air abraded Ni-Cr and Co- Cr alloys. 50