Parallel bonded cast attachments for removable partial dentures Michael D. Murray, BDS, PhD*
Key words: Acid-etch bonding, removable partial denture, semi-precision attachment. Abstract The original design of a semi-precision extra-coronal attachment for partial dentures is illustrated, and its fabrication in a cast non-precious bondable alloy is described. A step-by-step method is also described of bonding multiple attachments in parallel for the retention and support of removable partial dentures. Clinical cases are illustrated. The results of trials indicate that the method is clinically viable and, in selected cases, appears to offer distinct advantages over existing retention systems. (Received for publication February 1991. Revised September 1991. Accepted September 1991.)
Introduction and review of the literature A removable partial denture should restore both appearance and function, while contributing to the preservation of the remaining dentition and its supporting structures. In most instances requests for partial dentures are made by patients whose prime concern is the restoration of their appearance, without betraying the presence of a denture, which is particularly difficult to achieve with a clasped removable partial denture restoring upper anterior teeth.' In that situation and despite attempts at concealment through a wide variety of design modifications, the clasps are commonly visible, and their display mars the appearance.' T o add to their
*Department of Prosthetic Dentistry, Prince Philip Dental Hospital, Hong Kong. Australian Dental Journal 1992;37(6):419-26.
aesthetic shortcomings it has also been demonstrated that clasps may fit imprecisely and in addition are likely to be inefficient as direct retainer^.^ They are, however, simple to construct and their continued widespread use, despite their deficiencies, indicates that no ideal substitute has yet been found. Several alternative methods of retention have been described. A rotational or dual path of insertion, for instance, may be e m p l ~ y e dbut ~ - ~is feasible only in Kennedy Class IV cases and only when the premaxilla is undercut relative to the occlusal plane. The denture may then be inserted parallel to the anterior undercut and rotated so that its posteriorly placed clasps may engage undercuts on molar teeth, and the undercut premaxilla affords anterior retention. Sectional dentures have also been de~cribed,'~~ the parts being separately inserted to engage divergent undercuts and then locked together, but the fabrication of the framework in interlocking parts is technically demanding and the system is not widely used. Clasps as direct retainers may be substituted by locking devices in the form of anchors or bolts9-" or by precision attachments.12Both, in theory offer aesthetic and mechanical advantages over clasps but both also have real disadvantages. Locking devices are difficult to place with accuracy and consequently are imprecise in their function. They are permeable to oral fluids, difficult to clean, mechanically fragile and none has achieved popular acceptance in practice. Existing precision attachment systems are expensive, wear and break easily, are difficult to replace, clinically demanding, and technique-sensitive.They involve the crowning of abutment teeth with all 419
Mouth pr& extended
Id
1
Fig. la.-Key and Keyway components. Fig. 1b.- Analysing rod modified with keyway component. Fig. 1c.-Tooth preparation showing the proximal flat area with small dimple. Fig. Id.-Preparation extended to form an occlusal rest seat.
its attendant disadvantages, principally the considerable destruction of tooth tissue with inevitable pulpal compromise. Despite all this, precision attachments do offer efficient retention and support, and do so inconspicuously. An alternative method of attaching them may be etching and bonding rather than by incorporation in a crowned abutment, particularly in view of the widespread and successhl employment of bonding techniques to retain fixed b r i d g e ~ o r k , ' ~and ~ ' ~the development of improved adhesive agents, notably those based on methacrylate sy~tems.'~If multiple bonded attachments could be placed in parallel this would offer a retention system that would combine the aesthetic advantages and mechanical efficiency of attachments with the simplicity and nondestructiveness of bonding. Lingual rest seats were attached to anterior teeth by Wong, Nicholls and Smitha6using stainless steel foil-mesh pads. They evaluated the stresses trans420
mitted to abutment teeth by masticatory forces and concluded that the bonded attachment could withstand such functional forces. Lyon" described the fabrication and bonding of an ideal cast rest seat and guide plane and suggested that a resin-bonded casting would be the restoration of choice when there was adequate sound enamel present. Similar techniques were employed by Leupold and Faraone,'* to modify abutment teeth by the provision of (a) vertical stops (b) cingulum rests (c) occlusal rests (d) improved guide plane and (e) improved tooth contour. Thirty-one castings in all were placed for 20 patients. Bond strengths were found to be sufficient to support removable partial dentures over periods of 26 to 42 months. Saunder~,'~ however, in a review of the techniques of resin-bonded bridges stated that electrolytic etching had proved to be extremely techniquesensitive, and quoted several corroborative reports. This difficulty was circumvented by the developAustralian Dental Journal 1992;37:6.
Fig. 2.-Key pattern positioned and on the model and held with wax. Fig. 3.-Copings and key attachment cast and replaced on a model.
ment of chemically active resins which exhibited good bond strength both to tooth and to sandblasted non-precious alloy." Atta, Smith and Browntg compared three of the resins then available and reported high bond strengths with the sand-blasted surfaces of a beryllium free nickelchromium alloy. Of the three cements compared, Panavia Ex,? a modified phosphate ester of dimethacrylate, gave highest values for tensile and shear bond strength. A method of duplicating and bonding prefabric~
~
TKuraray Co., Osaka, Japan. Australian Dental Journal 1992;37:6.
ated attachments based on previous work with lingual rest and guide plane bondingl6-I9 was described by Marinello and S ~ h a r e r . ' ~ In* their ~~ system, however, when more than one attachment was required, parallelism could not be achieved and then only the ring and ball type could be used. Such attachments without guiding planes do not give the positive retention and lateral stability that is associated with a single path of withdrawal. The purposes of this study were (1) to investigate the clinical feasibility of bonding multiple extracoronal attachments in parallel and (2) to demonstrate the technique employed in so utilizing these attachments for removable partial dentures. 421
Fig. 4.-Coping plus key attachments on a duplicate model with undercuts blocked out. Fig. 5.-Denture framework waxed up on a refractory cast.
Materials and methods The attachments employed in this study were external, key and keyway, semi-precision design, sectioned truncated cone in shape. The prototype keyway component (Fig. la) was machined in stainless steel in an engineering workshop$ to the author’s specification. The attachment had an overall length of 4 mm and tapered at 14 O from a base width of 2.5 mm to apical width of 1.5 mm externally. The internal diameter at the base was 1.5 mm, and at the apex 0.5 mm. The wall thickness was 0.5 mm. A key pattern was formed in Duralays to the prototype keyway component and invested in Bellavest T 1 according to the manufacturer’s instructions. It was burnt out at 1000°C, then induction cast in nickel chromium alloy of tensile
SHKU Engineering Workshop, Hong Kong. §Reliance Dental Manufacturing Co., Worth, ILL, USA. 1)BEGO Bremer Goldschlagerei Wilh Herbst Gmb H & Co., 422
strength 524 MPa, and yield strength 276 MPa (Super 3).1 The casting was sand-blasted with 50 pm aluminium oxide particle at 0.62 MPa to ensure removal of all investment material. This key casting was used as a master die. Several keyway counterparts were individually formed to it in Duralay and were cast in Super 3 alloy by the method described above. Their sprues were cut off at approximately 3 mm and were indented to act as retention tags for a denture. To each keyway in turn a key pattern was individually fabricated in Duralay. A paralleling rod was constructed by welding a keyway component to the tip of a standard analysing rod in a linear extension of the rod (Fig. lb). This was fitted to a standard surveyor.** Potentially suitable patients on the prosthetic waiting list of the Prince Philip Dental Hospital, H.K. were examined. Those selected for treatment required removable bounded-saddle (Kennedy C1 111)partial dentures. In most instances replacement of anterior as well as posterior teeth was required (Kennedy C1 I11 modification I or 11). The patients chosen had good oral hygiene and abutment teeth suitable for bonding. Study casts were made and designs were outlined for removable partial dentures, some to be retained solely by attachments, others by a combination of attachment and conventional clasping. In this technique the anatomy of the buccal surfaces of those abutment teeth receiving attachments was irrelevant, their lingual and proximal surfaces having sole influence on the design. Proximal flat surfaces of 3-4 mm’, lingual guide planes, and rest seats were prepared as planned on the abutment teeth. A small locating dimple was also cut in the prepared surface with a round No. 5 bur (Fig. Icyd). This dimple, combined with the occlusal rest, assisted in the precise location of the coping both on the cast and in the mouth. Working impressions were made, cast in stone, and duplicated. The stone casts were mounted on a semiadjustable articulator, artificial teeth were arranged in wax for aesthetic evaluation. The design was finalized.
Positioning the attachment A duplicate cast was aligned on the surveyor table in the chosen path of insertion. A Duralay key
Bremen, Germany. 1Dental Alloy Products Inc., Compton, CA, USA. **Nesor Model F. Australian Dental Journal 1992;37:6.
**'
3 II
Clearance between framework 6 tooth
Dont ure framework
/
6 Laboratory
7 Fig. 6.-Relationship between key attachment and cast denture framework in the mouth. Fig. 7.-Denture framework and key attachment in position on a duplicate model and joined together with cold-curing acrylic resin.
pattern was inserted in the keyway attachment on the modified analysing rod and held there with a spot of petroleum jelly. It was conveyed to the abutment tooth on the cast and attached with blue inlay wax on the proximal prepared area, its upper and lower extremities well clear of the gingival margin and of the occlusal table respectively. The surveyor rod was then withdrawn, leaving the key pattern in place (Fig. 2). This procedure was repeated in turn for all other attachments to be used in the arch, thus ensuring that all were set in parallel. The copings and occlusal rests were waxed up for each attachment pattern in blue inlay wax approximately 0.3 mm thick, extending over as much of the lingual enamel surface as possible, again without impinging on the occlusal or gingival margins. All margins of the pattern were bevelled to a knife edge. The combined Duralay and wax patterns were removed, Australian Dental Journal 1992;37:6
invested and cast in nickel chromium alloy. The cast key components were accurately placed in position on a duplicate model with the assistance of the modified analysing rod (Fig. 3). Undercuts were blocked out and a refractory cast was made (Fig. 4). On this the denture framework was formed in wax to the designated pattern, ensuring that there was clearance between the framework and the key component (Fig. 5). The denture framework was cast in cobalt-chromium alloy.
Trial insertions Both the framework and the key component were tried in the mouth at the same appointment, ensuring that the denture framework could be inserted and removed without interference (Fig. 6). In the laboratory the key components were refitted on a duplicate model, using the modified 423
Fig. 10.-View of a cemented key component and coping.
Fig. 8.-Denture teeth waxed up for final try-in. Fig. 9.-Key attachments positioned in their keyway counterparts in the processed denture ready for conveying to the mouth.
surveying rod to ensure parallelism. They were secured in position with modelling wax. The denture framework was also fitted to the duplicate model. The keyway attachments were placed in position on their key counterparts, and were permanently attached to the denture framework with auto-polymerizing acrylic resin (Fig. 7). The chosen teeth were set up in wax on the denture framework (Fig. 8), in accordance with the previous aesthetic try-in. In the clinic, each coping bearing the key attachment was inserted in the appropriate keyway component on the denture framework, held in position with petroleum jelly and conveyed to the mouth (Fig. 9). A final check was made of the fit of all components and of the appearance of the prosthesis in the mouth. In the laboratory the denture was processed. The copings were ultrasonically cleaned?? and kept in a sealed plastic bag pending their return to the clinic. ttFSl200. Decon Ultrasonics Ltd, Hove, UK. 424
Final fitting of the attachments and delivery of the denture At the delivery appointment one of the key attachments was placed in its own keyway attachment in the finished denture. The tooth surface was acidetched in the appropriate area, isolated with cottonwool rolls and with an individual rubber-dam apron. Panavia Ex was applied to the tissue-fitting surface of the coping. The denture was inserted, conveying the coping to the abutment tooth. There it was painted over with an oxygen-inhibiting gelt in accordance with manufacturer’s instructions and held firmly against the tooth while fully seated within the keyway attachment and until the cement had set. Then the denture was removed leaving the bonded key attachment in place (Fig. 10). This procedure was repeated for each attachment individually to ensure that the source of any individual discrepancy in alignment was immediately apparent, could be accurately identified and easily rectified, thus avoiding any difficulty in removing the framework from the mouth when the cement had set. (This method of using the denture framework as a transfer coping ensures parallelism between attachments and obviates any minor discrepancies in positioning that might occur during cementation. In some instances it may be feasible to insert more than one attachment at a time but if there is convergence between the abutments it will not be possible to do so.) When all attachments had been placed the denture was re-inserted and final adjustments were made. The patient was instructed in the handling and care of the appliance and allowed to practise its insertion and removal before leaving the clinic. Australian Dental Journal 1992;37:6.
Fig. 1la-d.-Attachment is an aesthetic retainer in an awkward clinical situation.
Conclusions Results to date (over a two-year period) indicate that the technique is viable and endorse previous reports that the bond strength between attachment and abutment is adequate to sustain the stresses to which it will be subjected in the retention and
support of a removable partial denture. There are several advantages over traditional removable partial denture techniques. The greatest gain compared with clasp retention is in appearance (Fig. 1la-d) and there is also an obvious improvement in retentiveness and in functional efficiency. When compared with crown-retained attachments there is the great advantage of minimal tooth reduction and of accessible supragingival margins which can be easily cleaned. In addition, the attachment is simple in design, the keyway component is unobtrusively contained in the fabric of the denture (Fig. 12), the technique does not demand exceptional clinical skills, it is not time-consuming, and any part of the combined prosthesis can be easily repaired or replaced without requiring major reconstruction of other components. It is, therefore, cost-effective and appears to offer many advantages over alternative systems to both patient and operator.
References Fig. 12.-Keyway component fits neatly in the denture. Australian Dental Journal 1992;37:6.
1. Zarb GA, Mackay JF. Cosmetics and removable partial dentures - the Class IV partially edentulous patient. J Prosthet Dent 1981;46:360-8. 425
2. Murray MD. Aesthetic considerations in removable partial denture design. Dent Annual 1989;201-10. 3. Murray MD, Dyson JE. A study of the clinical fit of cast cobalt-chromium clasps. J Dent 1988;16:135-9. 4. King GE, Barco MJ, Olson RJ. Inconspicuousretention for removable partial dentures. J Prosthet Dent 1978;39:505-7. 5. Firtell DN, Jacobson JE. Removable partial dentures with rotational paths of insertion; Problem analysis. J Prosthet Dent 1983;50:8-15. 6. Chow JW, Clark RKF, Clarke D, Ho GFC. A rotational path of insertion for Kennedy IV removable partial dentures. Br Dent J 1988;164:180-3. 7. L'Estrange PR, Pullen-Warner E. Sectional dentures. In: Preiskel HW. Precision attachments in dentistry. London: Kimpton, 1979:267-86. 8. Walter JD. Partial denture technique. Br Dent J. 1980;148~257-60. 9. Handlers M, Lenchner BA, Weissman B. A retaining device for partial dentures. J Prosthet Dent 1957;7:483-8. 10. Wright SM. Use of spring-loaded attachments for retention of removable partial dentures. J Prosthet Dent 1984;51: 605-10. 11. Murray MD. An aesthetic retention device for removable partial dentures. Proc Excerpta Media Asia Pacific Congress 1984;37:46. 12. Preiskel HW. Precision attachments in dentistry. London: Kimpton, 1979. 13. Saunders WP. Resin bonded bridgework: A review. J Dent 1989;17~255-65.
426
14. Marinello CP, Scharer P, Meyenberg K. Resin-bonded etched castings with extracoronal attachmentsfor removable partial denture. J Prosthet Dent 1991;68:52-5. 15. Flood AM, Brockhurst P, Harcourt JK. The bond strengths of various adhesives used for Maryland Bridges. Aust Dent J 1989;34:449-53. 16. Wong R, Nicholls JI, Smith DE. Evaluation of prefabricated lingual rest seats for removable partial dentures. J Prosthet Dent 1982;48:521-6. 17. Lyon HE. Resin-bonded etched-metal rest seats. J Prosthet Dent 1985;53:366-8. 18. Leupold RJ, Faraone KL. Etched castings as an adjunct to mouth preparation for removable partial dentures. J Prosthet Dent 1985;53:655-8. 19. Atta MO, Smith BGN, Brown D. Bond strengths of three chemical adhesive cements adhered to a nickel-chromium alloy for direct bonded retainer. J Prosthet Dent 1990;63:137-43. 20. Marinello CP, Scharer P. Resin-bonded etched cast extracoronal attachments for removable partial dentures: Clinical experiences. Int J Periodontics Restorative Dent 1987;2:37-49.
Address for correspondence: 2 Sylvan Green, Grantstown Park, Dunmore Road, Waterford, Ireland.
Australian Dental Journal 1992;37:6.
Key words: Acid-etch bonding, removable partial denture, semi-precision attachment. Abstract The original design of a semi-precision extra-coronal attachment for partial dentures is illustrated, and its fabrication in a cast non-precious bondable alloy is described. A step-by-step method is also described of bonding multiple attachments in parallel for the retention and support of removable partial dentures. Clinical cases are illustrated. The results of trials indicate that the method is clinically viable and, in selected cases, appears to offer distinct advantages over existing retention systems. (Received for publication February 1991. Revised September 1991. Accepted September 1991.)
Introduction and review of the literature A removable partial denture should restore both appearance and function, while contributing to the preservation of the remaining dentition and its supporting structures. In most instances requests for partial dentures are made by patients whose prime concern is the restoration of their appearance, without betraying the presence of a denture, which is particularly difficult to achieve with a clasped removable partial denture restoring upper anterior teeth.' In that situation and despite attempts at concealment through a wide variety of design modifications, the clasps are commonly visible, and their display mars the appearance.' T o add to their
*Department of Prosthetic Dentistry, Prince Philip Dental Hospital, Hong Kong. Australian Dental Journal 1992;37(6):419-26.
aesthetic shortcomings it has also been demonstrated that clasps may fit imprecisely and in addition are likely to be inefficient as direct retainer^.^ They are, however, simple to construct and their continued widespread use, despite their deficiencies, indicates that no ideal substitute has yet been found. Several alternative methods of retention have been described. A rotational or dual path of insertion, for instance, may be e m p l ~ y e dbut ~ - ~is feasible only in Kennedy Class IV cases and only when the premaxilla is undercut relative to the occlusal plane. The denture may then be inserted parallel to the anterior undercut and rotated so that its posteriorly placed clasps may engage undercuts on molar teeth, and the undercut premaxilla affords anterior retention. Sectional dentures have also been de~cribed,'~~ the parts being separately inserted to engage divergent undercuts and then locked together, but the fabrication of the framework in interlocking parts is technically demanding and the system is not widely used. Clasps as direct retainers may be substituted by locking devices in the form of anchors or bolts9-" or by precision attachments.12Both, in theory offer aesthetic and mechanical advantages over clasps but both also have real disadvantages. Locking devices are difficult to place with accuracy and consequently are imprecise in their function. They are permeable to oral fluids, difficult to clean, mechanically fragile and none has achieved popular acceptance in practice. Existing precision attachment systems are expensive, wear and break easily, are difficult to replace, clinically demanding, and technique-sensitive.They involve the crowning of abutment teeth with all 419
Mouth pr& extended
Id
1
Fig. la.-Key and Keyway components. Fig. 1b.- Analysing rod modified with keyway component. Fig. 1c.-Tooth preparation showing the proximal flat area with small dimple. Fig. Id.-Preparation extended to form an occlusal rest seat.
its attendant disadvantages, principally the considerable destruction of tooth tissue with inevitable pulpal compromise. Despite all this, precision attachments do offer efficient retention and support, and do so inconspicuously. An alternative method of attaching them may be etching and bonding rather than by incorporation in a crowned abutment, particularly in view of the widespread and successhl employment of bonding techniques to retain fixed b r i d g e ~ o r k , ' ~and ~ ' ~the development of improved adhesive agents, notably those based on methacrylate sy~tems.'~If multiple bonded attachments could be placed in parallel this would offer a retention system that would combine the aesthetic advantages and mechanical efficiency of attachments with the simplicity and nondestructiveness of bonding. Lingual rest seats were attached to anterior teeth by Wong, Nicholls and Smitha6using stainless steel foil-mesh pads. They evaluated the stresses trans420
mitted to abutment teeth by masticatory forces and concluded that the bonded attachment could withstand such functional forces. Lyon" described the fabrication and bonding of an ideal cast rest seat and guide plane and suggested that a resin-bonded casting would be the restoration of choice when there was adequate sound enamel present. Similar techniques were employed by Leupold and Faraone,'* to modify abutment teeth by the provision of (a) vertical stops (b) cingulum rests (c) occlusal rests (d) improved guide plane and (e) improved tooth contour. Thirty-one castings in all were placed for 20 patients. Bond strengths were found to be sufficient to support removable partial dentures over periods of 26 to 42 months. Saunder~,'~ however, in a review of the techniques of resin-bonded bridges stated that electrolytic etching had proved to be extremely techniquesensitive, and quoted several corroborative reports. This difficulty was circumvented by the developAustralian Dental Journal 1992;37:6.
Fig. 2.-Key pattern positioned and on the model and held with wax. Fig. 3.-Copings and key attachment cast and replaced on a model.
ment of chemically active resins which exhibited good bond strength both to tooth and to sandblasted non-precious alloy." Atta, Smith and Browntg compared three of the resins then available and reported high bond strengths with the sand-blasted surfaces of a beryllium free nickelchromium alloy. Of the three cements compared, Panavia Ex,? a modified phosphate ester of dimethacrylate, gave highest values for tensile and shear bond strength. A method of duplicating and bonding prefabric~
~
TKuraray Co., Osaka, Japan. Australian Dental Journal 1992;37:6.
ated attachments based on previous work with lingual rest and guide plane bondingl6-I9 was described by Marinello and S ~ h a r e r . ' ~ In* their ~~ system, however, when more than one attachment was required, parallelism could not be achieved and then only the ring and ball type could be used. Such attachments without guiding planes do not give the positive retention and lateral stability that is associated with a single path of withdrawal. The purposes of this study were (1) to investigate the clinical feasibility of bonding multiple extracoronal attachments in parallel and (2) to demonstrate the technique employed in so utilizing these attachments for removable partial dentures. 421
Fig. 4.-Coping plus key attachments on a duplicate model with undercuts blocked out. Fig. 5.-Denture framework waxed up on a refractory cast.
Materials and methods The attachments employed in this study were external, key and keyway, semi-precision design, sectioned truncated cone in shape. The prototype keyway component (Fig. la) was machined in stainless steel in an engineering workshop$ to the author’s specification. The attachment had an overall length of 4 mm and tapered at 14 O from a base width of 2.5 mm to apical width of 1.5 mm externally. The internal diameter at the base was 1.5 mm, and at the apex 0.5 mm. The wall thickness was 0.5 mm. A key pattern was formed in Duralays to the prototype keyway component and invested in Bellavest T 1 according to the manufacturer’s instructions. It was burnt out at 1000°C, then induction cast in nickel chromium alloy of tensile
SHKU Engineering Workshop, Hong Kong. §Reliance Dental Manufacturing Co., Worth, ILL, USA. 1)BEGO Bremer Goldschlagerei Wilh Herbst Gmb H & Co., 422
strength 524 MPa, and yield strength 276 MPa (Super 3).1 The casting was sand-blasted with 50 pm aluminium oxide particle at 0.62 MPa to ensure removal of all investment material. This key casting was used as a master die. Several keyway counterparts were individually formed to it in Duralay and were cast in Super 3 alloy by the method described above. Their sprues were cut off at approximately 3 mm and were indented to act as retention tags for a denture. To each keyway in turn a key pattern was individually fabricated in Duralay. A paralleling rod was constructed by welding a keyway component to the tip of a standard analysing rod in a linear extension of the rod (Fig. lb). This was fitted to a standard surveyor.** Potentially suitable patients on the prosthetic waiting list of the Prince Philip Dental Hospital, H.K. were examined. Those selected for treatment required removable bounded-saddle (Kennedy C1 111)partial dentures. In most instances replacement of anterior as well as posterior teeth was required (Kennedy C1 I11 modification I or 11). The patients chosen had good oral hygiene and abutment teeth suitable for bonding. Study casts were made and designs were outlined for removable partial dentures, some to be retained solely by attachments, others by a combination of attachment and conventional clasping. In this technique the anatomy of the buccal surfaces of those abutment teeth receiving attachments was irrelevant, their lingual and proximal surfaces having sole influence on the design. Proximal flat surfaces of 3-4 mm’, lingual guide planes, and rest seats were prepared as planned on the abutment teeth. A small locating dimple was also cut in the prepared surface with a round No. 5 bur (Fig. Icyd). This dimple, combined with the occlusal rest, assisted in the precise location of the coping both on the cast and in the mouth. Working impressions were made, cast in stone, and duplicated. The stone casts were mounted on a semiadjustable articulator, artificial teeth were arranged in wax for aesthetic evaluation. The design was finalized.
Positioning the attachment A duplicate cast was aligned on the surveyor table in the chosen path of insertion. A Duralay key
Bremen, Germany. 1Dental Alloy Products Inc., Compton, CA, USA. **Nesor Model F. Australian Dental Journal 1992;37:6.
**'
3 II
Clearance between framework 6 tooth
Dont ure framework
/
6 Laboratory
7 Fig. 6.-Relationship between key attachment and cast denture framework in the mouth. Fig. 7.-Denture framework and key attachment in position on a duplicate model and joined together with cold-curing acrylic resin.
pattern was inserted in the keyway attachment on the modified analysing rod and held there with a spot of petroleum jelly. It was conveyed to the abutment tooth on the cast and attached with blue inlay wax on the proximal prepared area, its upper and lower extremities well clear of the gingival margin and of the occlusal table respectively. The surveyor rod was then withdrawn, leaving the key pattern in place (Fig. 2). This procedure was repeated in turn for all other attachments to be used in the arch, thus ensuring that all were set in parallel. The copings and occlusal rests were waxed up for each attachment pattern in blue inlay wax approximately 0.3 mm thick, extending over as much of the lingual enamel surface as possible, again without impinging on the occlusal or gingival margins. All margins of the pattern were bevelled to a knife edge. The combined Duralay and wax patterns were removed, Australian Dental Journal 1992;37:6
invested and cast in nickel chromium alloy. The cast key components were accurately placed in position on a duplicate model with the assistance of the modified analysing rod (Fig. 3). Undercuts were blocked out and a refractory cast was made (Fig. 4). On this the denture framework was formed in wax to the designated pattern, ensuring that there was clearance between the framework and the key component (Fig. 5). The denture framework was cast in cobalt-chromium alloy.
Trial insertions Both the framework and the key component were tried in the mouth at the same appointment, ensuring that the denture framework could be inserted and removed without interference (Fig. 6). In the laboratory the key components were refitted on a duplicate model, using the modified 423
Fig. 10.-View of a cemented key component and coping.
Fig. 8.-Denture teeth waxed up for final try-in. Fig. 9.-Key attachments positioned in their keyway counterparts in the processed denture ready for conveying to the mouth.
surveying rod to ensure parallelism. They were secured in position with modelling wax. The denture framework was also fitted to the duplicate model. The keyway attachments were placed in position on their key counterparts, and were permanently attached to the denture framework with auto-polymerizing acrylic resin (Fig. 7). The chosen teeth were set up in wax on the denture framework (Fig. 8), in accordance with the previous aesthetic try-in. In the clinic, each coping bearing the key attachment was inserted in the appropriate keyway component on the denture framework, held in position with petroleum jelly and conveyed to the mouth (Fig. 9). A final check was made of the fit of all components and of the appearance of the prosthesis in the mouth. In the laboratory the denture was processed. The copings were ultrasonically cleaned?? and kept in a sealed plastic bag pending their return to the clinic. ttFSl200. Decon Ultrasonics Ltd, Hove, UK. 424
Final fitting of the attachments and delivery of the denture At the delivery appointment one of the key attachments was placed in its own keyway attachment in the finished denture. The tooth surface was acidetched in the appropriate area, isolated with cottonwool rolls and with an individual rubber-dam apron. Panavia Ex was applied to the tissue-fitting surface of the coping. The denture was inserted, conveying the coping to the abutment tooth. There it was painted over with an oxygen-inhibiting gelt in accordance with manufacturer’s instructions and held firmly against the tooth while fully seated within the keyway attachment and until the cement had set. Then the denture was removed leaving the bonded key attachment in place (Fig. 10). This procedure was repeated for each attachment individually to ensure that the source of any individual discrepancy in alignment was immediately apparent, could be accurately identified and easily rectified, thus avoiding any difficulty in removing the framework from the mouth when the cement had set. (This method of using the denture framework as a transfer coping ensures parallelism between attachments and obviates any minor discrepancies in positioning that might occur during cementation. In some instances it may be feasible to insert more than one attachment at a time but if there is convergence between the abutments it will not be possible to do so.) When all attachments had been placed the denture was re-inserted and final adjustments were made. The patient was instructed in the handling and care of the appliance and allowed to practise its insertion and removal before leaving the clinic. Australian Dental Journal 1992;37:6.
Fig. 1la-d.-Attachment is an aesthetic retainer in an awkward clinical situation.
Conclusions Results to date (over a two-year period) indicate that the technique is viable and endorse previous reports that the bond strength between attachment and abutment is adequate to sustain the stresses to which it will be subjected in the retention and
support of a removable partial denture. There are several advantages over traditional removable partial denture techniques. The greatest gain compared with clasp retention is in appearance (Fig. 1la-d) and there is also an obvious improvement in retentiveness and in functional efficiency. When compared with crown-retained attachments there is the great advantage of minimal tooth reduction and of accessible supragingival margins which can be easily cleaned. In addition, the attachment is simple in design, the keyway component is unobtrusively contained in the fabric of the denture (Fig. 12), the technique does not demand exceptional clinical skills, it is not time-consuming, and any part of the combined prosthesis can be easily repaired or replaced without requiring major reconstruction of other components. It is, therefore, cost-effective and appears to offer many advantages over alternative systems to both patient and operator.
References Fig. 12.-Keyway component fits neatly in the denture. Australian Dental Journal 1992;37:6.
1. Zarb GA, Mackay JF. Cosmetics and removable partial dentures - the Class IV partially edentulous patient. J Prosthet Dent 1981;46:360-8. 425
2. Murray MD. Aesthetic considerations in removable partial denture design. Dent Annual 1989;201-10. 3. Murray MD, Dyson JE. A study of the clinical fit of cast cobalt-chromium clasps. J Dent 1988;16:135-9. 4. King GE, Barco MJ, Olson RJ. Inconspicuousretention for removable partial dentures. J Prosthet Dent 1978;39:505-7. 5. Firtell DN, Jacobson JE. Removable partial dentures with rotational paths of insertion; Problem analysis. J Prosthet Dent 1983;50:8-15. 6. Chow JW, Clark RKF, Clarke D, Ho GFC. A rotational path of insertion for Kennedy IV removable partial dentures. Br Dent J 1988;164:180-3. 7. L'Estrange PR, Pullen-Warner E. Sectional dentures. In: Preiskel HW. Precision attachments in dentistry. London: Kimpton, 1979:267-86. 8. Walter JD. Partial denture technique. Br Dent J. 1980;148~257-60. 9. Handlers M, Lenchner BA, Weissman B. A retaining device for partial dentures. J Prosthet Dent 1957;7:483-8. 10. Wright SM. Use of spring-loaded attachments for retention of removable partial dentures. J Prosthet Dent 1984;51: 605-10. 11. Murray MD. An aesthetic retention device for removable partial dentures. Proc Excerpta Media Asia Pacific Congress 1984;37:46. 12. Preiskel HW. Precision attachments in dentistry. London: Kimpton, 1979. 13. Saunders WP. Resin bonded bridgework: A review. J Dent 1989;17~255-65.
426
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Australian Dental Journal 1992;37:6.
