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COMPARING THE RESISTANCE OF DENTIN BONDING AGENTS AND PINS IN AMALGAM RESTORATIONS
COMPARING THE RESISTANCE OF DENTIN BONDING AGENTS ANED PINS INN AMALGAM RESTORATIONS TERENCE A. IMBERY, D.D.S.; JOHN
hen challenged to restore a severely compromised tooth with a complex amalgam restoration, a clinician must provide optimal forms of retention and resistance to displacement. The use of retentive pins is well documented to provide retention and resistance for amalgam restorations. Markley' first described the use of a cemented stainless steel pin. Goldstein2 subsequently reported the friction lock pin technique, and Going3 later suggested a third method in which a self-threading pin is placed into a smaller diameter dentinal channel. Several studies have demonstrated favorable results using self-threaded pin systems.4`7 Even though clinical success and relative ease of placement have made self-threaded pin systems very popular,8 they are not without disadvantages. Use of these systems can result in pulpal inflammation,9 pulpal perforation and periodontal ligament perforation,10'11 crazing of dentin'2 and a decrease in the amalgam's compressive and transverse strength.'3"4 BONDING AMALGAM TO
.TEETH The increased emphasis on the use of dentin bonding agents has led to proposals of bonding
0.
BURGESS, D.D.S., M.S.; RICHARD C. BATZER, D.D.S.
This in vitro study involving 84
caries-free molar specimens examined the resistance of com-
plex amalgam restorations retained by two dentin bonding
agents-Amalgambond and Amalgambond Plus-four regular
TMS pins, six regular TMS pins and four pins in conjunction with
Amalgambond. Analysis indicated that the Amalgambond restorations were significantly
weaker than the other types.
Amalgambond Plus restorations were significantly stronger than the Amalgambond restorations
but not different from the re-
maining groups. The authors concluded that complex amalgam restorations should be retained with a combination of
adhesive and mechanical retention.
amalgam to tooth structure.15-'7 One material capable of this is Amalgambond (Parkell). Developed by Nakabayashi and colleagues,'8 Amalgambond is a self-cured dentin bonding agent capable of bonding amalgam
and composite resin to dentin, enamel and alloys. Shear bond strengths of amalgam bonded to dentin with Amalgambond reportedly range from 3.119 to 4.7 megaPascals.20 Amalgambond provided as much resistance (1,834 Newtons) in complex amalgam restorations as four amalgapins (1,597 N) and greater resistance than four regular TMS pins (1,259 N). Other studies have reported greater cuspal fracture resistance in wide mesio-occlusaldistal amalgam restorations when Amalgambond is compared to a copal varnish.22'23 With the inclusion of a HighPerformance Additive (HPA) in the original formula, Amalgambond became Amalgambond Plus (Parkell). The HPA is a polymethyl methacrylate powder that may be added to the mixture of the Base (Parkell) and Catalyst (Parkell) for added retention and resistance. The shear bond strength of amalgam to dentin with Amalgambond Plus reportedly ranges from 8.724 to 15.3 MPa. 25 Burgess, Alvarez and Summitt26 compared resistance form of complex amalgam restorations with Amalgambond Plus and four minim pins; they reported a higher bond strength with Amalgambond Plus (1,361 N) than with four minim pins JADA, Vol. 126, June 1995 753
BESEARCHI
Figure 1. Specimen mounted in acrylic resin.
(1,028 N). The greatest resistance was achieved when Amalgambond Plus was used with four minim pins (2,360 N). We undertook a study to compare the fracture resistance of complex amalgam restorations retained by a variety of agents:
Amalgambond, Amalgambond Plus, four regular TMS pins, six regular TMS pins and four regular TMS pins used in combination with Amalgambond. We also examined the effect of water storage on Amalgambond and Amalgambond Plus to determine if the bond would undergo hydrolysis. MATERIALS AND METHODS
Materials. We collected 84 unerupted maxillary molars that had been extracted and were free of caries and defects. 754 JADA, Vol. 126, June 1995
Figure 2. Specimen after occlusal reduction.
These were cleaned of debris and disinfected using a 1:10 solution of bleach and water. After disinfection, all teeth were stored in deionized water before preparation. We notched the roots of each tooth for retention and embedded each one separately in an orthodontic acrylic resin base (L.D. Caulk Co.) to a level 2 millimeters apical to the cemento-enamel junction (Figure 1). We carefully reduced the occlusal surface of each tooth with a conventional model trimmer (Handler Mfg. Co.), using water to provide a flat dentinal surface 2 mm above the CEJ (Figure 2). Methods. The 84 teeth were randomly distributed into seven groups of 12 teeth (Table 1). Groups A and B were retained by Amalgambond. A copper band matrix (Myco Industries,
Inc.) was adapted to the prepared tooth and supported with impression compound (Kerr Manufacturing Co.). We carefully applied cavity varnish to the internal surface of the copper band to prevent bonding of amalgam to the matrix. Amalgambond was placed according to the following protocol as directed by the manufacturer. The dentinal surface was treated for 10 seconds with Amalgambond Activator (Parkell) and rinsed with an air/water spray for 30 seconds, then the Adhesive Agent (Parkell) was applied for 30 seconds. Excess Adhesive Agent was removed with a dry brush. Two drops of Base and one drop of Catalyst (both chilled as directed by the manufacturer) were mixed together and applied to the prepared tooth surface. Dispersalloy (L.D.
RBESEARCH Caulk) was triturated in a Varimix II amalgamator (L.D. Caulk) for 15 seconds at the medium setting. The amalgam was mechanically condensed (Condensaire, Teledyne Densco) while the Amalgambond was still wet. Ten minutes after condensation, we placed the specimens in a 130 F water bath (Teledyne Hanau) to soften the impression compound. We removed the matrices and adjusted the specimens using hand instrumentation and a high-speed handpiece (Midwest) to produce an amalgam restoration 4 mm high with a 1 mm bevel at the occlusal surface (Figure 3). All specimens were stored in deionized water at 68 F, Group A for three months and Group B for six months. Groups C and D were retained by Amalgambond Plus. We used the same protocol for the placement of Amalgambond Plus as described for Groups A and B except that three drops of Base was mixed with one drop of Catalyst and one level scoop (0.05 grams) of HPA. All specimens were stored in deionized water at 68 F, Group C for three months and Group D for six months. In Group E, amalgams were retained with four regular TMS pins placed at the line angles 1 mm from the dento-enamel junction. We used a self-limiting twist drill (Coltene/Whaledent) to prepare the pin channels to a depth of 2 mm. We inserted single shear pins manually and reduced them to a length of 2 mm using a highspeed handpiece with water spray. Group F received six regular TMS pins as described for Group E, except that two line angles received two pins and the other two line angles re-
TABLE I
ceived one pin. Copper band matrices were adapted and supported with impression compound. Two applications of cavity varnish were applied to the prepared tooth surfaces prior to the mechanical condensation of Dispersalloy. The specimens were adjusted as described for Groups A through D and stored in deionized water at 68 F for six months. The specimens in Group G received four regular TMS pins and Amalgambond. The same protocol was adhered to for the placement of the pins and Amalgambond as described for Groups A and E. After the amalgam restorations were completed, they were stored in deionized water at 68 F for six months. Before being tested, all specimens were thermocycled in 6 C to 60 C water for 1,250 cycles with a dwell time of 30 seconds. Specimens were placed in a fixture attached to an Instron (Model 1125, Instron) so that the load was applied 45 degrees to the long axis of the tooth at a constant crosshead speed of 1 mm/minute (Figure 4). The load required for restoration failure was recorded in Newtons. The parametric data were analyzed with a two-way analysis of vari-
ance (ANOVA). A post-hoc Scheffe analysis was used to assess significant intergroup dif-
ferences. RESULTS
Water storage did not significantly affect the resistance of restorations retained by Amalgambond or Amalgambond Plus. Therefore, we combined data for Groups A and B, and C and D. The mean force and standard deviation required to fracture the samples are shown in Table 2. The Scheffe post-hoc analysis indicated that the Amalgambond restorations (Groups A and B) were significantly weaker than those in the other groups. Amalgambond Plus restorations (those in Groups C and D) were significantly stronger than the Amalgambond restorations but not different from those in the remaining groups. DISCUSSION
The resistance provided by Amalgambond Plus (1,386 N) is similar to that reported in an earlier investigation (1,361 N).26 The bond strength of Amalgambond (802 N) is lower than the 1,834 N reported by Imbery and
colleagues.21 JADA, Vol. 126, June 1995 755
R~~0ESEABCH
Figure 3. Specimen with completed amalgam restoration.
It is possible that earlier removal of the matrices 10 minutes after amalgam condensation rather than 24 hours after condensation, as in the study by Imbery and colleagues21-may result in the lower fracture resistance. Four regular TMS pins (1,591 N) provided similar resistance (1,259 N) as that reported by Imbery and others," but less than the 1,980 N reported by Buikema and others.4 Furthermore, the lower resistance provided by six regular TMS pins (1,234 N) is not in accordance with the 2,210 N reported by Davis and others27 or the 2,520 N reported by Buikema and others.4 The lower value may have resulted from differences in protocol among studies or may be the result of recording early failure as slippage or 756 JADA, Vol. 126, June 1995
Figure 4. Specimen mounted in a block on a 45-degree inclined plane with an oblique load applied by a universal testing machine.
bending of pins. We reported the first alteration in the stressstrain curve as failure, which gives a lower value than fracturing the amalgam off the tooth. It is reported that earlier failure occurs at 75 to 85 percent of the peak load.28 Increasing the number of regular TMS pins from four to six did not result in increased resistance or retention; rather, it caused earlier failure and more tooth fractures. A pin's retention of amalgam is totally mechanical, relying mainly on the surface serrations of the pin. The lack of intimate contact between the amalgam and the pin results in flaws, cracks and gaps.29 Furthermore, thermocycling produces a mismatch of thermal expansion between pins and
amalgam. These incongruities disrupt the uniform stress distribution, which ultimately weakens the restoration and decreases the resistance and retention. Stresses in amalgam restorations caused by the presence of pins can be minimized by a metallurgic bond between the pin and amalgam.30 Amalgambond acts as a coupling agent to improve the resistance and retention by bonding amalgam to both tooth and pins. We found that the addition of Amalgambond to four regular TMS pins increased the fracture resistance from 1,591 N to 1,858 N. This finding is supported by an investigation in which the addition of Amalgambond Plus to four minim pins increased the fracture resistance from 1,098 N with four minim pins
RESEARCHI alone to 2,360 N.26 Thus, restorations requiring maximum retention and resistance should combine adhesive and mechanical retention. Bond strength. Amalgambond bonds to tooth structure through a process called "hybridization." A hybrid layer is formed by the diffusion and subsequent polymerization of monomers into the pretreated dentinal substrate. The acid-resistant hybrid layer is reportedly 5 microns thick and composed of the adhesive resin, collagen and hydroxyapatite.31 The bonding of amalgam to Amalgambond is believed to be primarily mechanical.32 The Amalgambond must be wet (unpolymerized) and of sufficient thickness to be incorporated into the freshly condensed amalgam. Amalgambond is an unfilled resin; the addition of HPA (polymethyl methacrylate) transforms the bonding agent into a filled resin. Higher bond strengths between amalgam and more filled and viscous resins have been attributed to greater entrapment between the filler and amalgam particles.32 The bond between amalgam and Amalgambond depends on the particle shape of the alloy. A recent study indicated that amalgam alloys containing a greater proportion of spherical particles to lathe-cut particles have greater bond strengths to Amalgambond than alloys with a higher percentage of lathe-cut particles.33 Visual examination of the debonded specimens revealed that failures occurred between the resin and the amalgam. This demonstrates that the dentin-resin bond through the hybrid layer is more durable than the amalgam-resin bond.
TABLE 2
Effects of water storage. Water storage of up to six months did not significantly affect the specimens' fracture resistance of the specimens- a finding we believe to be the result of a stable hybrid layer. This is supported by another study25 that demonstrated no adverse effect of six months' water storage on the shear bond strength of amalgam to Amalgambond Plus. This is not to suggest that the hybrid layer is stable for longer durations. The main determining factor of bond durability appears to be the development of a well-organized resin interdiffusion zone into the pre-conditioned dentin.34 The pros and cons of a bonded amalgam restoration. Pros. The bonded amalgam restoration has several potential advantages, including reduced microleakage, decreased postoperative sensitivity and
reinforcement of remaining weakened tooth structure. Numerous studies have reported reduced microleakage at both the enamel and cementum margins of amalgam restorations lined with Amalgambond compared to restorations lined with a copal varnish.35-38 The observed reduction in microleak-
age is a result of the polymerization of 4-methyacryloxyethyl trimallitate anhydride (4META/MMA) in the dentinal substrate. Thus, the volumetric dimensional change occurring with polymerization is toward the dentinal substrate's surface. Reduced microleakage is believed to be responsible for the reported reduction in post-operative sensitivity of amalgam restorations lined with Amalgambond.39 Additionally, there is a direct correlation between biocompatibility, pulpal health and microleakage.40'1 Another advantage of Amalgambond includes strengthening of cusps. When Amalgambond is used in wide MOD amalgam restorations instead of copal varnish, the fracture resistance of the cusps increased from 57 kilograms for copal varnish to 98 kg.22 Similarly, Christensen and others23 reported an increased resistance to cusp fracture between copal varnish and Amalgambond from 2,085 N to 2,635 N. These in vitro studies support the use of Amalgambond to improve fracture resistance of teeth that have been restored with a wide MOD amalgam restoration. However, teeth with more conJADA, Vol. 126, June 1995 757
RDESEARCH
_I
ff1
Dr. imbery is a lioutenant colonel, U.S. Air Force DC, 1st
Medical Group, Langley Air Force
Base, Va. Address reprint requests to Dr. Imbery at 105 Chestnut Court, Yorktown, Va. 23692.
Dr. Burgess Is division head, Biomaterials, Department of Restorative Dentistry, The University of Texas Health Science Center at San Antonio. 4
servative MOD cavity preparations are not Dr. Batzer is a weakened and major, U.S. Air therefore have
Force DC, Wilford
need of
Lackland Air Force Base, Texas.
no
Amalgam-
Hall Medical Center,
bond.22,42 During amalgam condensation, both Amalgambond and Amalgambond Plus are visibly incorporated into the restoration. Charlton and others43 reported the absence of adverse effects on the compressive strength and creep of Tytin amalgam (Kerr) when Amalgambond is incorporated into the alloy. Similarly, Naguib and Millstein44 reported that Amalgambond did not affect the compressive or tensile strength of Dispersalloy. However, the effect of Amalgambond Plus with polymethyl methacrylate powder on an amalgam's physical properties are not yet known. Cons. There are disadvantages of using resins to bond amalgam to tooth structure. The application of Amalgambond and Amalgambond Plus is a multi-step procedure requiring great concentration on the clinician's part. The clinician also will require a learning period to acquire the skills necessary to execute this 758 JADA, Vol. 126, June 1995
procedure successfully. The components are heat-sensitive and require refrigeration until immediately before use. Matrices must be rigidly reinforced to prevent movement of the restoration during condensation. Furthermore, removal of the matrix requires care, and excessive resin beyond the cavosurface margins must be removed to prevent periodontal complications. Amalgambond and Amalgambond Plus are not radiopaque and therefore cannot be identified on radiographs. Study limitations. The results of this in vitro study should be interpreted with caution. In vitro studies produce a model that can only speculate on clinical performance. Dentin is a dynamic substrate with different histologic, morphologic and compositional variations. The caries-free teeth in this study are not a true representation of dentin that is seen clinically. Therefore, clinical trials remain the only conclusive and predictive performance of dental materials.45'46 CONCLUSION
We found that restorations retained by Amalgambond were significantly weaker than those retained by Amalgambond Plus, four or six regular TMS pins and four regular pins in conjunction with Amalgambond. Amalgambond Plus provided as much resistance as four or six regular TMS pins. The greatest resistance was achieved when pins and Amalgambond were combined. Water storage did not significantly affect the resistance of restorations retained
by Amalgambond or Amalgambond Plus. . The American Dental Association has no
commercial interest in the products mentioned. The opinions expressed or implied are strictly those of the authors and do not necessarily reflect the opinions or policies of the American Dental Association or its subsidiaries, the United States Air Force Dental Corps, the Department of Defense or other departments of the United States government.
The authors acknowledge the support of Parkell and L.D. Caulk in supplying the materials for this study. This article is based on a paper presented at the meeting of the International Association for Dental Research, Seattle, 1994. The authors gratefully acknowledge the assistance of the Audio Visual Services at Kunsan Air Base, Republic of Korea, with the graphics and illustrations. 1. Markley MR. Pin reinforcement and retention of amalgam foundations and restorations. JADA 1958;56:675-9. 2. Goldstein PM. Retention pins are friction-locked without the use of cement. JADA 1966;73:1,103-6. 3. Going RE. Pin retained amalgam. JADA 1966;73:619-24. 4. Buikema DJ, Mayhew RB, Voss JE, Bales DJ. Pins and their relation to cavity resistance form for amalgam. Quintessence Int 1985;16:187-90. 5. Robbins JW, Burgess JO, Summitt JB. Retention and resistance features for complex amalgam restorations. JADA 1989;118:437-42. 6. Lambert RL, Robinson FB, Lindemuth JS. Coronal reinforcement with cross-splinted pin-amalgam restorations. J Prosthet Dent 1985;54:346-9. 7. Burgess JO. Horizontal pins: a study in tooth reinforcement. J Prosthet Dent 1985;53:317-22. 8. Munk MB, Brokaw WC. Pins and intracoronal retentive features for multisurface amalgam restorations. Gen Dent 1989;37:320-3. 9. Felton DA, Webb EL, Kanoy BE, Cox CF. Pulpal response to threaded pin and retentive slot techniques: a pilot investigation. J Prosthet Dent 1991;56:597-602. 10. Seng GF, Rupell OL, Nance GI, Pompura JP. Placement of retentive amalgam inserts in tooth structure for supplemental retention. Gen Dent 1980;28:62-6. 11. Schurchard A, Reed OM. Pulpal response to pin placement. J Prosthet Dent 1973;29:292-300. 12. Webb EL, Straker WF, Phillips CL. Tooth crazing associated with threaded pins: a three dimensional model. J Prosthet Dent 1989;61:624-7. 13. Welk DA, Dilts WE. Influence of pins on the compressive and transverse strength of dental amalgam and retention of pins in amalgam. JADA 1969;78:101-4. 14. Going RE, Moffa JP, Nostrant GW, Johnson BE. The strength of dental amalgam as influenced by pins. JADA 1968;77:1,331-4. 15. Staninec M, Holt M. Bonding of amalgam to tooth structure: tensile adhesion and microleakage. J Prosthet Dent 1988;59:397-402. 16. Staninec M. Retention of amalgam restorations: undercuts versus bonding. Quintessence Int 1989;20:347-51. 17. Eakle WS, Staninec M, Lacey AM. Effect of bonded amalgam on the fracture resistance of teeth. J Prosthet Dent 1992;68:257-60.
.RESEARCH 18. Nakabayashi N, Masuhara E, Mochida E, Ohmori I. Development of adhesive pit and fissure sealants using MMA resin initiated by tri-n-butyl borane derivative. J Biomed Mater Res 1978;12:149-65. 19. Pashley EL, Comer RW, Parry EE, Pashley DH. Amalgam buildups: shear bond strength and dentin sealing properties. Oper Dent 1991;16:82-9. 20. Tjan AHL, Tan DE, Berry FA. Shear bond strength of amalgam to dentin pretreated with Optibond (Abstract no. 958). J Dent Res 1994;73(Special Issue):221. 21. Imbery TA, Hilton TJ, Reagan SE, Caldon WP. Fracture resistance of amalgams retained by Amalgambond, pins and amalgapins (Abstract no. 227). J Dent Res 1993;72(Special Issue):132. 22. Roth L, Boyer D. Fracture resistance of teeth with bonded amalgams (Abstract no. 278). J Dent Res 1991;70(Special Issue):300. 23. Christensen GJ, Hunsaker KJ, Bangerter V, Christensen R. Influence of Amalgambond on molar cusp fracture resistance (Abstract no. 279). J Dent Res 1991;70(Special Issue):300. 24. Lo CS, Millstein PL, Nathanson D. In vitro shear strength of pin retained vs. resin bonded amalgam (Abstract 2,285). J Dent Res 1994;73(Special Issue):387. 25. Hasawaga T, Retief DH. Shear bond strengths of two commercially available dentin-amalgam bonding systems (Abstract no. 957). J Dent Res 1994;73(Special Issue):221. 26. Burgess JO, Alvarez AN, Summitt JB. Fracture resistance of complex amalgams (Abstract no. 228). J Dent Res 1993;72(Special Issue):132. 27. Davis SP, Summitt JB, Mayhew RB, Hawley RJ. Self-threading pins and amalga-
pins compared in resistance form for complex amalgam restorations. Oper Dent 1983;8:8893. 28. Outhwaite WC, Garman TA, Pashley DH. Pin vs. slot retention in extensive amalgam restorations. J Prosthet Dent 1979;41:396-400. 29. Ijan AHL, Dunn JR, Grant BE. Fracture resistance of composite and amalgam cores retained by pins coated with new adhesive resins. J Prosthet Dent 1992;67:752-60. 30. Dhuru VB, Mclachlan K, Kassloff Z. A photoelastic study of stress concentration produced by retentive pins in amalgam restorations. J Dent Res 1979;58:1,060-4. 31. Nakabayashi N. Adhesive bonding with 4-META. Oper Dent 1992;17(Supplement 5):125-30. 32. Ruzickova M, Stanined M, Marshall GW. SEM analysis of resin-amalgam adhesion after debonding (Abstract no. 2,289). J Dent Res 1994;73 (Special Issue):388. 33. Morrill F, Galburt R, Kugel G, Zive M, Habib C. Comparison of different shaped particles of amalgam alloy bonded to composite and dentin with 4-methacryloxyethyl trimellitate anhydride (Abstract no. 959). J Dent Res 1994;73(Special Issue):221. 34. Reeve JB, Hunter KE, Conn LJ, Duke ES. Phosphoric acid concentration and time on the resin interdiffusion zone (Abstract no. 976). J Dent Res 1994;73(Special Issue):223. 35. Charlton DG, Moore BK, Swartz ML. In vitro evaluation of the use of resin liners to reduce microleakage and improve retention of amalgam restorations. Oper Dent 1992;17:112-9. 36. Paraussis D, Kakaboura A, Doukoudakis S. Microleakage of class V restorations using
dentin adhesive systems (Abstract no. 443). J Dent Res 1992;71(Special Issue):571. 37. Ben-Amar A, Urstein M, Serebro L, Lieberman R. The effect of new sealants around class V amalgam restorations (Abstract no. 42). J Dent Res 1990;69(Special Issue):1,036. 38. Edgren BN, Denehy GE. Microleakage of amalgam restorations using Amalgambond and Copalite. Am J Dent 1992;5:296-308. 39. Masaka N. Restoring the severely compromised molar through adhesive bonding of amalgam to dentin. Compend Contin Educ Dent 1991;12:90-8. 40. Cox CF, Keall CL, Keall HJ, Ostro E, Bergenholtz G. Biocompatibility of surfacesealed dentinal materials against exposed pulps. J Prosthet Dent 1987;57:1-8. 41. Cox CF. Biocompatibility of dental materials in the absence ofbacterial infection. Oper Dent 1987;12:146-52. 42. Santos AC, Meiers JC. Fracture resistance of premolars with MOD amalgam restorations lined with Amalgambond. Oper Dent 1994;19:2-6. 43. Charlton DG, Murchison DF, Moore KB. Incorporation of adhesive liners in amalgam: effect on compressive strength and creep. Am J Dent 1991;4:184-8. 44. Naguib G, Millstein PL. Effects of resin adhesives on mechanical properties of set amalgam (Abstract no. 2,288). J Dent Res 1994;73(Special Issue):388. 45. Duke ES, Lindemuth JS. Polymeric adhesion to dentin: contrasting substrates. Am J Dent 1990;3:264-70. 46. Duke ES, Lindemuth JS. Variability of clinical dentin substrates. Am J Dent 1991;4:241-6.
hen challenged to restore a severely compromised tooth with a complex amalgam restoration, a clinician must provide optimal forms of retention and resistance to displacement. The use of retentive pins is well documented to provide retention and resistance for amalgam restorations. Markley' first described the use of a cemented stainless steel pin. Goldstein2 subsequently reported the friction lock pin technique, and Going3 later suggested a third method in which a self-threading pin is placed into a smaller diameter dentinal channel. Several studies have demonstrated favorable results using self-threaded pin systems.4`7 Even though clinical success and relative ease of placement have made self-threaded pin systems very popular,8 they are not without disadvantages. Use of these systems can result in pulpal inflammation,9 pulpal perforation and periodontal ligament perforation,10'11 crazing of dentin'2 and a decrease in the amalgam's compressive and transverse strength.'3"4 BONDING AMALGAM TO
.TEETH The increased emphasis on the use of dentin bonding agents has led to proposals of bonding
0.
BURGESS, D.D.S., M.S.; RICHARD C. BATZER, D.D.S.
This in vitro study involving 84
caries-free molar specimens examined the resistance of com-
plex amalgam restorations retained by two dentin bonding
agents-Amalgambond and Amalgambond Plus-four regular
TMS pins, six regular TMS pins and four pins in conjunction with
Amalgambond. Analysis indicated that the Amalgambond restorations were significantly
weaker than the other types.
Amalgambond Plus restorations were significantly stronger than the Amalgambond restorations
but not different from the re-
maining groups. The authors concluded that complex amalgam restorations should be retained with a combination of
adhesive and mechanical retention.
amalgam to tooth structure.15-'7 One material capable of this is Amalgambond (Parkell). Developed by Nakabayashi and colleagues,'8 Amalgambond is a self-cured dentin bonding agent capable of bonding amalgam
and composite resin to dentin, enamel and alloys. Shear bond strengths of amalgam bonded to dentin with Amalgambond reportedly range from 3.119 to 4.7 megaPascals.20 Amalgambond provided as much resistance (1,834 Newtons) in complex amalgam restorations as four amalgapins (1,597 N) and greater resistance than four regular TMS pins (1,259 N). Other studies have reported greater cuspal fracture resistance in wide mesio-occlusaldistal amalgam restorations when Amalgambond is compared to a copal varnish.22'23 With the inclusion of a HighPerformance Additive (HPA) in the original formula, Amalgambond became Amalgambond Plus (Parkell). The HPA is a polymethyl methacrylate powder that may be added to the mixture of the Base (Parkell) and Catalyst (Parkell) for added retention and resistance. The shear bond strength of amalgam to dentin with Amalgambond Plus reportedly ranges from 8.724 to 15.3 MPa. 25 Burgess, Alvarez and Summitt26 compared resistance form of complex amalgam restorations with Amalgambond Plus and four minim pins; they reported a higher bond strength with Amalgambond Plus (1,361 N) than with four minim pins JADA, Vol. 126, June 1995 753
BESEARCHI
Figure 1. Specimen mounted in acrylic resin.
(1,028 N). The greatest resistance was achieved when Amalgambond Plus was used with four minim pins (2,360 N). We undertook a study to compare the fracture resistance of complex amalgam restorations retained by a variety of agents:
Amalgambond, Amalgambond Plus, four regular TMS pins, six regular TMS pins and four regular TMS pins used in combination with Amalgambond. We also examined the effect of water storage on Amalgambond and Amalgambond Plus to determine if the bond would undergo hydrolysis. MATERIALS AND METHODS
Materials. We collected 84 unerupted maxillary molars that had been extracted and were free of caries and defects. 754 JADA, Vol. 126, June 1995
Figure 2. Specimen after occlusal reduction.
These were cleaned of debris and disinfected using a 1:10 solution of bleach and water. After disinfection, all teeth were stored in deionized water before preparation. We notched the roots of each tooth for retention and embedded each one separately in an orthodontic acrylic resin base (L.D. Caulk Co.) to a level 2 millimeters apical to the cemento-enamel junction (Figure 1). We carefully reduced the occlusal surface of each tooth with a conventional model trimmer (Handler Mfg. Co.), using water to provide a flat dentinal surface 2 mm above the CEJ (Figure 2). Methods. The 84 teeth were randomly distributed into seven groups of 12 teeth (Table 1). Groups A and B were retained by Amalgambond. A copper band matrix (Myco Industries,
Inc.) was adapted to the prepared tooth and supported with impression compound (Kerr Manufacturing Co.). We carefully applied cavity varnish to the internal surface of the copper band to prevent bonding of amalgam to the matrix. Amalgambond was placed according to the following protocol as directed by the manufacturer. The dentinal surface was treated for 10 seconds with Amalgambond Activator (Parkell) and rinsed with an air/water spray for 30 seconds, then the Adhesive Agent (Parkell) was applied for 30 seconds. Excess Adhesive Agent was removed with a dry brush. Two drops of Base and one drop of Catalyst (both chilled as directed by the manufacturer) were mixed together and applied to the prepared tooth surface. Dispersalloy (L.D.
RBESEARCH Caulk) was triturated in a Varimix II amalgamator (L.D. Caulk) for 15 seconds at the medium setting. The amalgam was mechanically condensed (Condensaire, Teledyne Densco) while the Amalgambond was still wet. Ten minutes after condensation, we placed the specimens in a 130 F water bath (Teledyne Hanau) to soften the impression compound. We removed the matrices and adjusted the specimens using hand instrumentation and a high-speed handpiece (Midwest) to produce an amalgam restoration 4 mm high with a 1 mm bevel at the occlusal surface (Figure 3). All specimens were stored in deionized water at 68 F, Group A for three months and Group B for six months. Groups C and D were retained by Amalgambond Plus. We used the same protocol for the placement of Amalgambond Plus as described for Groups A and B except that three drops of Base was mixed with one drop of Catalyst and one level scoop (0.05 grams) of HPA. All specimens were stored in deionized water at 68 F, Group C for three months and Group D for six months. In Group E, amalgams were retained with four regular TMS pins placed at the line angles 1 mm from the dento-enamel junction. We used a self-limiting twist drill (Coltene/Whaledent) to prepare the pin channels to a depth of 2 mm. We inserted single shear pins manually and reduced them to a length of 2 mm using a highspeed handpiece with water spray. Group F received six regular TMS pins as described for Group E, except that two line angles received two pins and the other two line angles re-
TABLE I
ceived one pin. Copper band matrices were adapted and supported with impression compound. Two applications of cavity varnish were applied to the prepared tooth surfaces prior to the mechanical condensation of Dispersalloy. The specimens were adjusted as described for Groups A through D and stored in deionized water at 68 F for six months. The specimens in Group G received four regular TMS pins and Amalgambond. The same protocol was adhered to for the placement of the pins and Amalgambond as described for Groups A and E. After the amalgam restorations were completed, they were stored in deionized water at 68 F for six months. Before being tested, all specimens were thermocycled in 6 C to 60 C water for 1,250 cycles with a dwell time of 30 seconds. Specimens were placed in a fixture attached to an Instron (Model 1125, Instron) so that the load was applied 45 degrees to the long axis of the tooth at a constant crosshead speed of 1 mm/minute (Figure 4). The load required for restoration failure was recorded in Newtons. The parametric data were analyzed with a two-way analysis of vari-
ance (ANOVA). A post-hoc Scheffe analysis was used to assess significant intergroup dif-
ferences. RESULTS
Water storage did not significantly affect the resistance of restorations retained by Amalgambond or Amalgambond Plus. Therefore, we combined data for Groups A and B, and C and D. The mean force and standard deviation required to fracture the samples are shown in Table 2. The Scheffe post-hoc analysis indicated that the Amalgambond restorations (Groups A and B) were significantly weaker than those in the other groups. Amalgambond Plus restorations (those in Groups C and D) were significantly stronger than the Amalgambond restorations but not different from those in the remaining groups. DISCUSSION
The resistance provided by Amalgambond Plus (1,386 N) is similar to that reported in an earlier investigation (1,361 N).26 The bond strength of Amalgambond (802 N) is lower than the 1,834 N reported by Imbery and
colleagues.21 JADA, Vol. 126, June 1995 755
R~~0ESEABCH
Figure 3. Specimen with completed amalgam restoration.
It is possible that earlier removal of the matrices 10 minutes after amalgam condensation rather than 24 hours after condensation, as in the study by Imbery and colleagues21-may result in the lower fracture resistance. Four regular TMS pins (1,591 N) provided similar resistance (1,259 N) as that reported by Imbery and others," but less than the 1,980 N reported by Buikema and others.4 Furthermore, the lower resistance provided by six regular TMS pins (1,234 N) is not in accordance with the 2,210 N reported by Davis and others27 or the 2,520 N reported by Buikema and others.4 The lower value may have resulted from differences in protocol among studies or may be the result of recording early failure as slippage or 756 JADA, Vol. 126, June 1995
Figure 4. Specimen mounted in a block on a 45-degree inclined plane with an oblique load applied by a universal testing machine.
bending of pins. We reported the first alteration in the stressstrain curve as failure, which gives a lower value than fracturing the amalgam off the tooth. It is reported that earlier failure occurs at 75 to 85 percent of the peak load.28 Increasing the number of regular TMS pins from four to six did not result in increased resistance or retention; rather, it caused earlier failure and more tooth fractures. A pin's retention of amalgam is totally mechanical, relying mainly on the surface serrations of the pin. The lack of intimate contact between the amalgam and the pin results in flaws, cracks and gaps.29 Furthermore, thermocycling produces a mismatch of thermal expansion between pins and
amalgam. These incongruities disrupt the uniform stress distribution, which ultimately weakens the restoration and decreases the resistance and retention. Stresses in amalgam restorations caused by the presence of pins can be minimized by a metallurgic bond between the pin and amalgam.30 Amalgambond acts as a coupling agent to improve the resistance and retention by bonding amalgam to both tooth and pins. We found that the addition of Amalgambond to four regular TMS pins increased the fracture resistance from 1,591 N to 1,858 N. This finding is supported by an investigation in which the addition of Amalgambond Plus to four minim pins increased the fracture resistance from 1,098 N with four minim pins
RESEARCHI alone to 2,360 N.26 Thus, restorations requiring maximum retention and resistance should combine adhesive and mechanical retention. Bond strength. Amalgambond bonds to tooth structure through a process called "hybridization." A hybrid layer is formed by the diffusion and subsequent polymerization of monomers into the pretreated dentinal substrate. The acid-resistant hybrid layer is reportedly 5 microns thick and composed of the adhesive resin, collagen and hydroxyapatite.31 The bonding of amalgam to Amalgambond is believed to be primarily mechanical.32 The Amalgambond must be wet (unpolymerized) and of sufficient thickness to be incorporated into the freshly condensed amalgam. Amalgambond is an unfilled resin; the addition of HPA (polymethyl methacrylate) transforms the bonding agent into a filled resin. Higher bond strengths between amalgam and more filled and viscous resins have been attributed to greater entrapment between the filler and amalgam particles.32 The bond between amalgam and Amalgambond depends on the particle shape of the alloy. A recent study indicated that amalgam alloys containing a greater proportion of spherical particles to lathe-cut particles have greater bond strengths to Amalgambond than alloys with a higher percentage of lathe-cut particles.33 Visual examination of the debonded specimens revealed that failures occurred between the resin and the amalgam. This demonstrates that the dentin-resin bond through the hybrid layer is more durable than the amalgam-resin bond.
TABLE 2
Effects of water storage. Water storage of up to six months did not significantly affect the specimens' fracture resistance of the specimens- a finding we believe to be the result of a stable hybrid layer. This is supported by another study25 that demonstrated no adverse effect of six months' water storage on the shear bond strength of amalgam to Amalgambond Plus. This is not to suggest that the hybrid layer is stable for longer durations. The main determining factor of bond durability appears to be the development of a well-organized resin interdiffusion zone into the pre-conditioned dentin.34 The pros and cons of a bonded amalgam restoration. Pros. The bonded amalgam restoration has several potential advantages, including reduced microleakage, decreased postoperative sensitivity and
reinforcement of remaining weakened tooth structure. Numerous studies have reported reduced microleakage at both the enamel and cementum margins of amalgam restorations lined with Amalgambond compared to restorations lined with a copal varnish.35-38 The observed reduction in microleak-
age is a result of the polymerization of 4-methyacryloxyethyl trimallitate anhydride (4META/MMA) in the dentinal substrate. Thus, the volumetric dimensional change occurring with polymerization is toward the dentinal substrate's surface. Reduced microleakage is believed to be responsible for the reported reduction in post-operative sensitivity of amalgam restorations lined with Amalgambond.39 Additionally, there is a direct correlation between biocompatibility, pulpal health and microleakage.40'1 Another advantage of Amalgambond includes strengthening of cusps. When Amalgambond is used in wide MOD amalgam restorations instead of copal varnish, the fracture resistance of the cusps increased from 57 kilograms for copal varnish to 98 kg.22 Similarly, Christensen and others23 reported an increased resistance to cusp fracture between copal varnish and Amalgambond from 2,085 N to 2,635 N. These in vitro studies support the use of Amalgambond to improve fracture resistance of teeth that have been restored with a wide MOD amalgam restoration. However, teeth with more conJADA, Vol. 126, June 1995 757
RDESEARCH
_I
ff1
Dr. imbery is a lioutenant colonel, U.S. Air Force DC, 1st
Medical Group, Langley Air Force
Base, Va. Address reprint requests to Dr. Imbery at 105 Chestnut Court, Yorktown, Va. 23692.
Dr. Burgess Is division head, Biomaterials, Department of Restorative Dentistry, The University of Texas Health Science Center at San Antonio. 4
servative MOD cavity preparations are not Dr. Batzer is a weakened and major, U.S. Air therefore have
Force DC, Wilford
need of
Lackland Air Force Base, Texas.
no
Amalgam-
Hall Medical Center,
bond.22,42 During amalgam condensation, both Amalgambond and Amalgambond Plus are visibly incorporated into the restoration. Charlton and others43 reported the absence of adverse effects on the compressive strength and creep of Tytin amalgam (Kerr) when Amalgambond is incorporated into the alloy. Similarly, Naguib and Millstein44 reported that Amalgambond did not affect the compressive or tensile strength of Dispersalloy. However, the effect of Amalgambond Plus with polymethyl methacrylate powder on an amalgam's physical properties are not yet known. Cons. There are disadvantages of using resins to bond amalgam to tooth structure. The application of Amalgambond and Amalgambond Plus is a multi-step procedure requiring great concentration on the clinician's part. The clinician also will require a learning period to acquire the skills necessary to execute this 758 JADA, Vol. 126, June 1995
procedure successfully. The components are heat-sensitive and require refrigeration until immediately before use. Matrices must be rigidly reinforced to prevent movement of the restoration during condensation. Furthermore, removal of the matrix requires care, and excessive resin beyond the cavosurface margins must be removed to prevent periodontal complications. Amalgambond and Amalgambond Plus are not radiopaque and therefore cannot be identified on radiographs. Study limitations. The results of this in vitro study should be interpreted with caution. In vitro studies produce a model that can only speculate on clinical performance. Dentin is a dynamic substrate with different histologic, morphologic and compositional variations. The caries-free teeth in this study are not a true representation of dentin that is seen clinically. Therefore, clinical trials remain the only conclusive and predictive performance of dental materials.45'46 CONCLUSION
We found that restorations retained by Amalgambond were significantly weaker than those retained by Amalgambond Plus, four or six regular TMS pins and four regular pins in conjunction with Amalgambond. Amalgambond Plus provided as much resistance as four or six regular TMS pins. The greatest resistance was achieved when pins and Amalgambond were combined. Water storage did not significantly affect the resistance of restorations retained
by Amalgambond or Amalgambond Plus. . The American Dental Association has no
commercial interest in the products mentioned. The opinions expressed or implied are strictly those of the authors and do not necessarily reflect the opinions or policies of the American Dental Association or its subsidiaries, the United States Air Force Dental Corps, the Department of Defense or other departments of the United States government.
The authors acknowledge the support of Parkell and L.D. Caulk in supplying the materials for this study. This article is based on a paper presented at the meeting of the International Association for Dental Research, Seattle, 1994. The authors gratefully acknowledge the assistance of the Audio Visual Services at Kunsan Air Base, Republic of Korea, with the graphics and illustrations. 1. Markley MR. Pin reinforcement and retention of amalgam foundations and restorations. JADA 1958;56:675-9. 2. Goldstein PM. Retention pins are friction-locked without the use of cement. JADA 1966;73:1,103-6. 3. Going RE. Pin retained amalgam. JADA 1966;73:619-24. 4. Buikema DJ, Mayhew RB, Voss JE, Bales DJ. Pins and their relation to cavity resistance form for amalgam. Quintessence Int 1985;16:187-90. 5. Robbins JW, Burgess JO, Summitt JB. Retention and resistance features for complex amalgam restorations. JADA 1989;118:437-42. 6. Lambert RL, Robinson FB, Lindemuth JS. Coronal reinforcement with cross-splinted pin-amalgam restorations. J Prosthet Dent 1985;54:346-9. 7. Burgess JO. Horizontal pins: a study in tooth reinforcement. J Prosthet Dent 1985;53:317-22. 8. Munk MB, Brokaw WC. Pins and intracoronal retentive features for multisurface amalgam restorations. Gen Dent 1989;37:320-3. 9. Felton DA, Webb EL, Kanoy BE, Cox CF. Pulpal response to threaded pin and retentive slot techniques: a pilot investigation. J Prosthet Dent 1991;56:597-602. 10. Seng GF, Rupell OL, Nance GI, Pompura JP. Placement of retentive amalgam inserts in tooth structure for supplemental retention. Gen Dent 1980;28:62-6. 11. Schurchard A, Reed OM. Pulpal response to pin placement. J Prosthet Dent 1973;29:292-300. 12. Webb EL, Straker WF, Phillips CL. Tooth crazing associated with threaded pins: a three dimensional model. J Prosthet Dent 1989;61:624-7. 13. Welk DA, Dilts WE. Influence of pins on the compressive and transverse strength of dental amalgam and retention of pins in amalgam. JADA 1969;78:101-4. 14. Going RE, Moffa JP, Nostrant GW, Johnson BE. The strength of dental amalgam as influenced by pins. JADA 1968;77:1,331-4. 15. Staninec M, Holt M. Bonding of amalgam to tooth structure: tensile adhesion and microleakage. J Prosthet Dent 1988;59:397-402. 16. Staninec M. Retention of amalgam restorations: undercuts versus bonding. Quintessence Int 1989;20:347-51. 17. Eakle WS, Staninec M, Lacey AM. Effect of bonded amalgam on the fracture resistance of teeth. J Prosthet Dent 1992;68:257-60.
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