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Bonding of amalgam to composite: Tensile strength and morphology study
Dent Mater 10:83-87, March, 1994
Bonding of amalgam to composite: Tensile strength and morphologystudy Alvaro E. Garcia-Barbero, Javier Garcia-Barbero, Juan A. Lopez-Calvo Department of Conservative Dentistry, Complutense University School of Dentistry, Madrid, SPAIN
ABSTRACT Objectives. The present study was based on the premise that it may be possible to produce useful dental restorations by bonding freshly triturated amalgam to a cured composite restoration (Group 1 specimens), or by bonding uncured composite to hardened amalgam (Group 2 specimens). Methods. To determine the validity of this premise, a phosphonate adhesive resin cement was used to produce simulated, layered dental restorations for each test group. Results. The mean tensile bond strength of 24 hour-old Group 1 specimens (6.74 MPa + 1.63 MPa) was almost twice that of 24 hour-old Group 2 specimens. Cohesive failure of the amalgam-substrate layer was a prominent feature of the fracture pattern of Group 1 specimens. On the other hand, rupture of all Group 2 specimens occurred mainly along the adhesiveamalgam interface. Findings from SEM examination of the layers of amalgam, adhesive cement, and resin composite of intact Group 1 specimens suggested that inclusions of adhesive resin cement may be the cause of the persistent cohesive failure of the amalgam layer. Significance. It may be possible to improve the strength of bonded couples made from freshly triturated amalgam and cured resin composite by minimizing the thickness of the adhesive layer. INTRODUCTION Despite the growing use of posterior composite restoratives, dental amalgam remains the most widely used material for the restoration of posterior teeth (Greener, 1979). The continuing popularity of this remarkable material is attributable to its manipulative features, longevity in the abusive oral environment (Letzel and Vrijhoef, 1984), and overall cost-effectiveness. However, the fact that amalgam does not adhere to tooth structure is thought to be one of the causes of marginal fractures and marginal leakage. Without adhesion, retention of amalgam restorations must rely upon cavity preparation with undercuts and retentive grooves placed in sound tooth tissue to prevent dislodgment of the restoration. The presence of these retentive undercuts and grooves increases the tooth's susceptibility to fracture (Larson et al., 1981; Blaser et al., 1983). The efficacy ofthe using an adhesive resin cement for bonding
amalgam to tooth structure has been demonstrated by Varga et al. (1986), Shimizu et al. (1987), and Staninec and Holt (1988). Furthermore, Staninec (1989) has shown that under simulated occlusal loads, the use of an adhesive resin liner in box-form preparations provides more resistance to the displacement of amalgam restorations than either proximal grooves or dovetails. In fact, it was demonstrated that bonded amalgam restorations inhibit initial marginal microleakage, reduce preparation size, and provide intracoronal support of weakened cusps to improve fracture resistance (Lacy and Staninec, 1990). The present work focuses on two potential clinical applications for bonded amalgam restorations. Both applications are based upon the use ofan adhesive resin cement to bond dental amalgam to a resin composite restorative material. Application No. 1: Bonding amalgam to a cured resin composite. When bonded in situ, the composite would serve to strengthen remaining tooth structure and to facilitate stress transfer during loading. On the other hand, the amalgam overlay would offer an acceptable level of wear resistance, especially in occlusal and interproximal areas. Also, in cavities with gingival margins placed apically to the cemento-enameljunction,freshlytriturated amalgam could be bonded to a cured resin composite to minimize leakage at the restoration-cementumjuncture. Application No. 2: Bonding a resin composite to set amalgam. Selection of this technique would be appropriate when a highly esthetic result is required. The objectives of this study were to: 1) measure the tensile strength of bonded amalgam-resin composite couples; 2) characterize the patterns of failure affiliated with bond rupture; and 3) examine and document the structural features of simulated, layered dental restorations made from freshly triturated amalgam, adhesive resin cement, and previously cured resin composite.
MATERIALSANDMETHODS Two groups (n=25 each) for measurement oftensile bond strength were produced from a single composition, high-copper, spherical dental amalgam (Amalcap Plus Non Gamma 2, Vivadent, Schaan, Liechtenstein) and a hybrid, organo-inorganic resin composite restorative material (Herculite XR, Kerr Mfg. Co., Romulus, MI, USA). Specimens belonging to Group 1 were made by bonding Dental Materials/March 1994 83
freshly triturated amalgam to cured composite. Conversely, specimens belonging to Group 2 were fabricated by bonding uncured resin composite to hardened dental amalgam. In both test groups, bonding of the two dissimilar materials (freshly triturated amalgam and cured composite, or uncured composite and hardened amalgam) was achieved with the use of an adhesive phosphonate cement of the Bis-GMA type (Panavia EX, Kuraray Co. Ltd., Osaka, Japan). To determine the effectiveness of the resin adhesive, 5 control specimens were added to each test group. These specimens were made without the use of an adhesive. Each bond strength specimen was formed and ultimately tested in a split, two-piece stainless steel device (Fig. 1). Each piece was a cylinder with a truncated, conical cavity. Upon assembly of the two pieces (Part A and Part B), the mold cavity assumed the shape of an hourglass. The conical shape of both pieces allowed application of tension to the specimen until fracture and the easy recovery of both halves. The following procedure was used to produce Group 1 specimens. Part A was inverted and placed on a cover glass. This step allowed the composite to be inserted through the large diameter orifice of Part A and condensed against a flat surface. The inverted, truncated conical cavity of Part A was filled incrementally. The uncured thickness ofall incremental additions was less than 2 mm. Each increment was condensed and photopolymerized for 40 s before placement of another. Upon completion of the filling process, Part A was returned to its normal position (Fig. 2). A mylar strip with a 5 mm hole was placed on the composite's bonding surface to prevent the adhesive cement from contacting the walls of the mold. Then Part B of the mold was coupled to Part A. The adhesive was mixed according to the manufacturer's instructions and applied to the exposed substrate-surface with a soft brush. The amalgam was triturated in an amalgamator (Silimat Type 3, Vivadent, Schaan, Liechtenstein) for 6 s and hand condensed against the adhesive until Part B was filled. Group 2 specimens were prepared in the same manner, but the sequence of mold-filling was reversed. Accordingly, the amalgam component of each specimen was formed and aged for 24 h in Part A. After coupling Part B to Part A, the adhesive resin cement was applied to the amalgam, and Part B was filled with composite. The circumferential joint between mold Parts A and B was covered with a polyethylene gel to minimize oxygen inhibition of the adhesive's polymerization. After 6 min, the gel was removed with water, and the mold was stored for 7 d in tap water at ambient temperature. With the use of an alignment apparatus, the forming devices for bond strength specimens were positioned for measuring the tensile strength of the amalgam-to-composite bond and the composite-to-amalgam bond. The crosshead speed of the universal testing machine (Hounsfield HTE, Croydon, England, UK) was 1 mm/min. Crosshead displacement and load were recorded continuously. Mean bond strength values of the two groups were compared by Student's t-test. The two substrate-surfaces of each ruptured bond strength specimen were examined by light microscopy to determine the mode and site ofbond failure. SEM specimens that simulated the layered dental restorations were formed in molds made from thin-walled copper tubing. The materials, methods, and conditions specified for production of Group 1 bond strength specimens 84 Alvaro et aL/Bonding to amalgam composite
B
B
8mm
12mm
,oI 19rnm
A
A
Fig. 1. Schematic (longitudinal section) of the split, 2-piece device in which bond strength specimens were formed and tested.
Firstsubstrate
(~)
A ~ e s i v e resin
J.
,I. A
®
(~
~Sec.ondsub,rate
B A
A
Fig. 2. Sequential steps in assembly and filling of the 2-piece mold.
apply also to the production of SEM specimens. The resin composite was cured in a relatively small (diameter = 8 mm, length = 5 mm) tube. Subsequently, this tube was placed within a larger (diameter = 9 mm, length = 10 mm) tube. Tight contact between the two tubes was established by crimping the large tube. Freshly triturated amalgam was placed in the superior half of the large tube and condensed against a thin layer of adhesive cement that covered the bonding surface of the resin composite. Upon removal from storage, 5 bonded amalgamcomposite couples were sectioned longitudinally with the use of a circular wafering blade. Thus, 10 specimens for SEM evaluation were made available. The cut surfaces of these specimens were ground through 240 to 600 grit abrasive papers, polished with 6 mm through 0.25 mm diamond polishing compound, and coated with gold. The specimens' coated surfaces were examined with the use of a scanning electron microscope (JSM 6.400, JEOL Ltd., Tokyo, Japan). RESULTS The apparent mean bond strength values for Group 1 specimens (freshly triturated amalgam bonded to cured composite) and
Group 2 specimens (composite bonded to 24 hourTABLE 1: FAILURE PATTERNS OF BONDED AMALGAM-RESIN COMPOSITE old amalgam) were 6.74 MPa_+ 1.63 MPa and 3.31 COUPLES: BOND FAILURE MODE, SITE, AND FREQUENCY MPa _+ 0.70 MPa, respectively. The difference Frequency between these values, as shown by Student's tFailure Mode Site Group 1' Group 2' test, was significant (p<0.01). The strength of each of the 10 specimens made without using Cohesive failure phosphonateadhesive cement 6 3 adhesive cement was nil. Cohesive failure resin composite substrate 0 0 Data pertaining to the failure patterns ofbonded amalgam-composite specimens are presented Adhesive failure composite-adhesivecement interface 1 0 in the Table. In Group 1, incidents of bond failure involved only the amalgam layer. Seven of these Mixed failure involving resin composite substrate 0 0 failures were cohesive, and three were adhesive. Fifteen specimens exhibited evidence of mixed Cohesive failure amalgamsubstrate 16 (7) 19 (3) failure. In most mixed failures, however, more than two-thirds of the fracture surface was conAdhesive failure amalgam-adhesivecement interface 0 25 (22) fined to the adhesive cement-amalgam interface (10 incidents) or to the amalgam substrate Mixed failure involving amalgam substrate 15 3 (1 incident). Values in parentheses indicate frequency of single mode-single site bond failure In Group 2, rupture of all specimens occurred along the adhesive-amalgam interface. *Freshly triturated amalgam bonded to cured resin composite. Additionally, however, 3 of 25 specimens exhibit+Resin composite bonded to 24 h old amalgam. ed cohesive failure of the adhesive cement that n = 25 involved about one-third of the fracture surface. The structural features of SEM specimens are shown in Figs. 3 to 6. Intimate apposition of amalgam chanical factor (Staninec and Holt, 1988; Temple-Smithsonet al., and adhesive was an attribute of all specimens, but the profiles 1992). of the amalgam-adhesive interfaces were irregular. Although The irregular thickness of the adhesive within each SEM variable from specimen to specimen, the depth of the irregulari- specimen is attributed to the absence of a suitable exit portal for ties ranged from 10 mm to 65 mm. Thickness ofthe adhesive film excess adhesive cement. It would appear that back pressure, i.e., ranged from nil to 80 mm. Multiple, irregularly shaped voids and resistance exerted by the fluid adhesive, countered the pressure infrequent solitary isolates of adhesive resin cement were found exerted at the face of the amalgam plugger-head. Thus, in areas within the body of the amalgam component of most specimens. where back pressure was greater than plugger-head pressure, Typically, the location of such defects was limited to the central the advancing front of the soft, plastic amalgam mass became region of the specimen (Fig. 3). The amalgam component of one indented. Whether the shape or thickness of the adhesive layer specimen, however, exhibited an interrupted, wavy line of multi- affects the strength of the fresh amalgam-adhesive cement bond ple adhesive resin inclusions (Fig. 4) that seemed to run almost remains to be determined. parallel to the amalgam-adhesive interface. The presence of Entrapment of adhesive cement during the condensation of substantial amounts of displaced, "amalgam-locked" adhesive amalgam adversely affects the strength of the hardened amal(Fig. 5) was confined to the peripheral areas of the specimens. A gam substrate. This contention is supported by the relatively few specimens exhibited bulky patches ofadhesive-encasedamal- high incidence of cohesive amalgam failures in Group 1 bond gam (Fig. 6) that seemed to be detached from the main body of strength specimens. Clinically, large ectopic accumulations of amalgam. adhesive cement such as those found in peripheral areas of SEM specimens could be highly troublesome, especially in thin, bondDISCUSSION ed amalgam overlays. The basis for selection of the phosphonate adhesive resin The presence of seemingly detached, "adhesive-locked" patchcement used in this study was its demonstrated ability to bond to es of amalgam in some specimens is an artifact of specimen metals (Omura et al., 1984) and its use in other research involving preparation. The patches so depicted merely reflect the threeamalgam (Varga et al., 1986; Staninec and Holt, 1988; Torii et al., dimensional configuration of the amalgam-adhesive cement in1988; Cooley et al., 1989; Rueggeberget al., 1989; Staninec, 1989; terface. Lacy and Staninec, 1990). Furthermore, the stability of the The findings pertaining to the bonding of freshly triturated phosphonate adhesive is documented by its ability to withstand amalgam to a resin composite are encouraging. Nonetheless, long-term aging (Aboush and Jenkins, 1989) and thermocycling further studies are required to enable rational preclinical assess(Thompson et al., 1985; Atta et al., 1990). ment of the potential usefulness of bonded amalgam-resin comA precise explanation cannot be offered for the large variance posite restorations. between the mean bond strength values of Group 1 and Group 2 The present results of bond strength testing imply that the specimens. Nonetheless, one might speculate that the products bonding of amalgam to resin composite cannot be achieved of chemical reactions involving phosphonate adhesive cement without the use of an intermediary cement, i.e., bonding agent. It and freshly triturated amalgam are not the same as those must be mentioned, however, that the adhesive cement, amalresulting from reactions of the phosphonate adhesive and hard- gam alloy, and resin composite identified herein are not necessarened amalgam (Wing, 1966). Alternatively, the intermingling of ily the best available materials for the production of layered adhesive with fresh amalgam may introduce a beneficial me- restorations. Dental Materials~March 1994 85
Fig.3. SEM micrograph of dental amalgam-adhesive cement and resin compositeadhesive cement interfaces (A = dental amalgam; P = phosphonate adhesive resin cement; and C = resin composite),
Fig. 5. SEM micrograph of a peripheral area showing substantial accumulation of adhesive resin cement in the amalgam layer,
Fig. 4. SEM micrograph depicting multiple isolates of adhesive resin cement in amalgam,
Fig. 6. SEM micrograph showing bulky patches of amalgam in adhesive resin cement.
Complutense University School of Dentistry, Madrid, Spain.
For instance, the bondability of the amalgamated product of one specific amalgam alloy powder is known to be greater than that of the amalgamates of seemingly similar amalgam alloy powders (Rueggeberg et al., 1989). This fact alone points to the need to systematically test a broad spectrum of combinations of various dental amalgams, resin composites, and bonding agents. In future studies, care will be exercised to keep the adhesive layer as thin as possible. Hopefully, this effort will minimize the likelihood of displaced adhesive being engulfed by condensed amalgam.
Address corresponding and reprint requests to: Alvaro Enrique Garcia-Barbero Departamento de Odontologia Conservadora Facultad de Odontologia de la Universidad Complutense de Madrid Ciudad Universitaria 28040 Madrid, Spain
ACKNOWLEDGMENT This investigation was supported by the Scientific Research Program of the Department of Conservative Dentistry,
REFERENCES Aboush YEY, Jenkins CBG (1989). Tensile strength of enamelresin-metal joints. J Prosthet Dent 61:688-694.
86
Alvaro et al./Bonding to amalgam composite
Received November 2, 1993 / Accepted January 25, 1994
Atta MO, Smith BGN, Brown D (1990). Bond strengths of three chemical adhesive cements adhered to a nickelchromium alloy for direct bonding retainers. JProsthetDent 63:137-143. Blaser PK, Lund MR, Cochran MA (1983). Effects of designs of Class II preparations on resistance of tooth to fracture. Oper Dent 8:6-10. CooleyRL, McCourt JW, Train TE (1989). Bond strength ofresin to amalgam as affected by surface finish. Quint Int 20:237239. Greener EH (1979). Amalgams: Yesterday, today and tomorrow. Oper Dent 4:24-35. Lacy AM, Staninec MA (1990). La restauracion con amalgama adherida. Quint Int (Spanish edition) 3:214-217. Larson TD, Douglas WH, Geistfeld RE (1981). Effect ofprepared cavities on the strength of teeth. OperDent 6:2-5. Letzel H, Vrijhoef MMA (1984). Long-term influence on marginal fracture of amalgam restorations. J Oral Rehabil 11:89-94. Omura I, Yamauchi J, Harada I, Wada T (1984). Adhesive and mechanical properties of a new dental adhesive. J Dent Res 63:233, Abstr. No. 561. Rueggeberg FA, Caughman WF, Gao F, Kovarik RE (1989). Bond
strength ofPanavia EX to dental amalgam. Int JProsthodont 2:371-375. Shimizu A, Ui T, Kawakami M (1987). Microleakage ofamalgam restoration with adhesive resin cement lining, glass ionomer cement and fluoride treatment. Dent Mater 6:64-69. Staninec M, Holt M (1988). Bonding of amalgam to tooth structure: Tensile adhesion and microleakage tests. JProsthet Dent 59:397-402. Staninec M (1989). Retention of amalgam restorations: Undercuts versus bonding. Quint Int 20:347-351. Temple-Smithson PE, Causton BE, Marshall KF (1992). The adhesive amalgam--fact or fiction? Br Dent J 172:316-319. ThompsonVP, Grolman KM, Liao R (1985). Bonding of adhesive resins to various nonprecious alloys. JDentRes 64:314,Abstr. No. 1258. ToriiY, Staninec M, Kawakami M, Imazato S, Torri M, Tsuchitani Y (1988). Inhibition ofcaries around amalgam restorations by amalgam bonding. JDent Res 67:308, Abstr. No. 1562. Varga J, Matsumura H, Masuhara E (1986). Bonding of amalgam filling to tooth cavity with adhesive resin. Dent Mater 2:158-164. Wing G (1966). Phase identification in dental amalgam. Aust Dent J 11:105-110.
Dental Materials~March 1994 87
Bonding of amalgam to composite: Tensile strength and morphologystudy Alvaro E. Garcia-Barbero, Javier Garcia-Barbero, Juan A. Lopez-Calvo Department of Conservative Dentistry, Complutense University School of Dentistry, Madrid, SPAIN
ABSTRACT Objectives. The present study was based on the premise that it may be possible to produce useful dental restorations by bonding freshly triturated amalgam to a cured composite restoration (Group 1 specimens), or by bonding uncured composite to hardened amalgam (Group 2 specimens). Methods. To determine the validity of this premise, a phosphonate adhesive resin cement was used to produce simulated, layered dental restorations for each test group. Results. The mean tensile bond strength of 24 hour-old Group 1 specimens (6.74 MPa + 1.63 MPa) was almost twice that of 24 hour-old Group 2 specimens. Cohesive failure of the amalgam-substrate layer was a prominent feature of the fracture pattern of Group 1 specimens. On the other hand, rupture of all Group 2 specimens occurred mainly along the adhesiveamalgam interface. Findings from SEM examination of the layers of amalgam, adhesive cement, and resin composite of intact Group 1 specimens suggested that inclusions of adhesive resin cement may be the cause of the persistent cohesive failure of the amalgam layer. Significance. It may be possible to improve the strength of bonded couples made from freshly triturated amalgam and cured resin composite by minimizing the thickness of the adhesive layer. INTRODUCTION Despite the growing use of posterior composite restoratives, dental amalgam remains the most widely used material for the restoration of posterior teeth (Greener, 1979). The continuing popularity of this remarkable material is attributable to its manipulative features, longevity in the abusive oral environment (Letzel and Vrijhoef, 1984), and overall cost-effectiveness. However, the fact that amalgam does not adhere to tooth structure is thought to be one of the causes of marginal fractures and marginal leakage. Without adhesion, retention of amalgam restorations must rely upon cavity preparation with undercuts and retentive grooves placed in sound tooth tissue to prevent dislodgment of the restoration. The presence of these retentive undercuts and grooves increases the tooth's susceptibility to fracture (Larson et al., 1981; Blaser et al., 1983). The efficacy ofthe using an adhesive resin cement for bonding
amalgam to tooth structure has been demonstrated by Varga et al. (1986), Shimizu et al. (1987), and Staninec and Holt (1988). Furthermore, Staninec (1989) has shown that under simulated occlusal loads, the use of an adhesive resin liner in box-form preparations provides more resistance to the displacement of amalgam restorations than either proximal grooves or dovetails. In fact, it was demonstrated that bonded amalgam restorations inhibit initial marginal microleakage, reduce preparation size, and provide intracoronal support of weakened cusps to improve fracture resistance (Lacy and Staninec, 1990). The present work focuses on two potential clinical applications for bonded amalgam restorations. Both applications are based upon the use ofan adhesive resin cement to bond dental amalgam to a resin composite restorative material. Application No. 1: Bonding amalgam to a cured resin composite. When bonded in situ, the composite would serve to strengthen remaining tooth structure and to facilitate stress transfer during loading. On the other hand, the amalgam overlay would offer an acceptable level of wear resistance, especially in occlusal and interproximal areas. Also, in cavities with gingival margins placed apically to the cemento-enameljunction,freshlytriturated amalgam could be bonded to a cured resin composite to minimize leakage at the restoration-cementumjuncture. Application No. 2: Bonding a resin composite to set amalgam. Selection of this technique would be appropriate when a highly esthetic result is required. The objectives of this study were to: 1) measure the tensile strength of bonded amalgam-resin composite couples; 2) characterize the patterns of failure affiliated with bond rupture; and 3) examine and document the structural features of simulated, layered dental restorations made from freshly triturated amalgam, adhesive resin cement, and previously cured resin composite.
MATERIALSANDMETHODS Two groups (n=25 each) for measurement oftensile bond strength were produced from a single composition, high-copper, spherical dental amalgam (Amalcap Plus Non Gamma 2, Vivadent, Schaan, Liechtenstein) and a hybrid, organo-inorganic resin composite restorative material (Herculite XR, Kerr Mfg. Co., Romulus, MI, USA). Specimens belonging to Group 1 were made by bonding Dental Materials/March 1994 83
freshly triturated amalgam to cured composite. Conversely, specimens belonging to Group 2 were fabricated by bonding uncured resin composite to hardened dental amalgam. In both test groups, bonding of the two dissimilar materials (freshly triturated amalgam and cured composite, or uncured composite and hardened amalgam) was achieved with the use of an adhesive phosphonate cement of the Bis-GMA type (Panavia EX, Kuraray Co. Ltd., Osaka, Japan). To determine the effectiveness of the resin adhesive, 5 control specimens were added to each test group. These specimens were made without the use of an adhesive. Each bond strength specimen was formed and ultimately tested in a split, two-piece stainless steel device (Fig. 1). Each piece was a cylinder with a truncated, conical cavity. Upon assembly of the two pieces (Part A and Part B), the mold cavity assumed the shape of an hourglass. The conical shape of both pieces allowed application of tension to the specimen until fracture and the easy recovery of both halves. The following procedure was used to produce Group 1 specimens. Part A was inverted and placed on a cover glass. This step allowed the composite to be inserted through the large diameter orifice of Part A and condensed against a flat surface. The inverted, truncated conical cavity of Part A was filled incrementally. The uncured thickness ofall incremental additions was less than 2 mm. Each increment was condensed and photopolymerized for 40 s before placement of another. Upon completion of the filling process, Part A was returned to its normal position (Fig. 2). A mylar strip with a 5 mm hole was placed on the composite's bonding surface to prevent the adhesive cement from contacting the walls of the mold. Then Part B of the mold was coupled to Part A. The adhesive was mixed according to the manufacturer's instructions and applied to the exposed substrate-surface with a soft brush. The amalgam was triturated in an amalgamator (Silimat Type 3, Vivadent, Schaan, Liechtenstein) for 6 s and hand condensed against the adhesive until Part B was filled. Group 2 specimens were prepared in the same manner, but the sequence of mold-filling was reversed. Accordingly, the amalgam component of each specimen was formed and aged for 24 h in Part A. After coupling Part B to Part A, the adhesive resin cement was applied to the amalgam, and Part B was filled with composite. The circumferential joint between mold Parts A and B was covered with a polyethylene gel to minimize oxygen inhibition of the adhesive's polymerization. After 6 min, the gel was removed with water, and the mold was stored for 7 d in tap water at ambient temperature. With the use of an alignment apparatus, the forming devices for bond strength specimens were positioned for measuring the tensile strength of the amalgam-to-composite bond and the composite-to-amalgam bond. The crosshead speed of the universal testing machine (Hounsfield HTE, Croydon, England, UK) was 1 mm/min. Crosshead displacement and load were recorded continuously. Mean bond strength values of the two groups were compared by Student's t-test. The two substrate-surfaces of each ruptured bond strength specimen were examined by light microscopy to determine the mode and site ofbond failure. SEM specimens that simulated the layered dental restorations were formed in molds made from thin-walled copper tubing. The materials, methods, and conditions specified for production of Group 1 bond strength specimens 84 Alvaro et aL/Bonding to amalgam composite
B
B
8mm
12mm
,oI 19rnm
A
A
Fig. 1. Schematic (longitudinal section) of the split, 2-piece device in which bond strength specimens were formed and tested.
Firstsubstrate
(~)
A ~ e s i v e resin
J.
,I. A
®
(~
~Sec.ondsub,rate
B A
A
Fig. 2. Sequential steps in assembly and filling of the 2-piece mold.
apply also to the production of SEM specimens. The resin composite was cured in a relatively small (diameter = 8 mm, length = 5 mm) tube. Subsequently, this tube was placed within a larger (diameter = 9 mm, length = 10 mm) tube. Tight contact between the two tubes was established by crimping the large tube. Freshly triturated amalgam was placed in the superior half of the large tube and condensed against a thin layer of adhesive cement that covered the bonding surface of the resin composite. Upon removal from storage, 5 bonded amalgamcomposite couples were sectioned longitudinally with the use of a circular wafering blade. Thus, 10 specimens for SEM evaluation were made available. The cut surfaces of these specimens were ground through 240 to 600 grit abrasive papers, polished with 6 mm through 0.25 mm diamond polishing compound, and coated with gold. The specimens' coated surfaces were examined with the use of a scanning electron microscope (JSM 6.400, JEOL Ltd., Tokyo, Japan). RESULTS The apparent mean bond strength values for Group 1 specimens (freshly triturated amalgam bonded to cured composite) and
Group 2 specimens (composite bonded to 24 hourTABLE 1: FAILURE PATTERNS OF BONDED AMALGAM-RESIN COMPOSITE old amalgam) were 6.74 MPa_+ 1.63 MPa and 3.31 COUPLES: BOND FAILURE MODE, SITE, AND FREQUENCY MPa _+ 0.70 MPa, respectively. The difference Frequency between these values, as shown by Student's tFailure Mode Site Group 1' Group 2' test, was significant (p<0.01). The strength of each of the 10 specimens made without using Cohesive failure phosphonateadhesive cement 6 3 adhesive cement was nil. Cohesive failure resin composite substrate 0 0 Data pertaining to the failure patterns ofbonded amalgam-composite specimens are presented Adhesive failure composite-adhesivecement interface 1 0 in the Table. In Group 1, incidents of bond failure involved only the amalgam layer. Seven of these Mixed failure involving resin composite substrate 0 0 failures were cohesive, and three were adhesive. Fifteen specimens exhibited evidence of mixed Cohesive failure amalgamsubstrate 16 (7) 19 (3) failure. In most mixed failures, however, more than two-thirds of the fracture surface was conAdhesive failure amalgam-adhesivecement interface 0 25 (22) fined to the adhesive cement-amalgam interface (10 incidents) or to the amalgam substrate Mixed failure involving amalgam substrate 15 3 (1 incident). Values in parentheses indicate frequency of single mode-single site bond failure In Group 2, rupture of all specimens occurred along the adhesive-amalgam interface. *Freshly triturated amalgam bonded to cured resin composite. Additionally, however, 3 of 25 specimens exhibit+Resin composite bonded to 24 h old amalgam. ed cohesive failure of the adhesive cement that n = 25 involved about one-third of the fracture surface. The structural features of SEM specimens are shown in Figs. 3 to 6. Intimate apposition of amalgam chanical factor (Staninec and Holt, 1988; Temple-Smithsonet al., and adhesive was an attribute of all specimens, but the profiles 1992). of the amalgam-adhesive interfaces were irregular. Although The irregular thickness of the adhesive within each SEM variable from specimen to specimen, the depth of the irregulari- specimen is attributed to the absence of a suitable exit portal for ties ranged from 10 mm to 65 mm. Thickness ofthe adhesive film excess adhesive cement. It would appear that back pressure, i.e., ranged from nil to 80 mm. Multiple, irregularly shaped voids and resistance exerted by the fluid adhesive, countered the pressure infrequent solitary isolates of adhesive resin cement were found exerted at the face of the amalgam plugger-head. Thus, in areas within the body of the amalgam component of most specimens. where back pressure was greater than plugger-head pressure, Typically, the location of such defects was limited to the central the advancing front of the soft, plastic amalgam mass became region of the specimen (Fig. 3). The amalgam component of one indented. Whether the shape or thickness of the adhesive layer specimen, however, exhibited an interrupted, wavy line of multi- affects the strength of the fresh amalgam-adhesive cement bond ple adhesive resin inclusions (Fig. 4) that seemed to run almost remains to be determined. parallel to the amalgam-adhesive interface. The presence of Entrapment of adhesive cement during the condensation of substantial amounts of displaced, "amalgam-locked" adhesive amalgam adversely affects the strength of the hardened amal(Fig. 5) was confined to the peripheral areas of the specimens. A gam substrate. This contention is supported by the relatively few specimens exhibited bulky patches ofadhesive-encasedamal- high incidence of cohesive amalgam failures in Group 1 bond gam (Fig. 6) that seemed to be detached from the main body of strength specimens. Clinically, large ectopic accumulations of amalgam. adhesive cement such as those found in peripheral areas of SEM specimens could be highly troublesome, especially in thin, bondDISCUSSION ed amalgam overlays. The basis for selection of the phosphonate adhesive resin The presence of seemingly detached, "adhesive-locked" patchcement used in this study was its demonstrated ability to bond to es of amalgam in some specimens is an artifact of specimen metals (Omura et al., 1984) and its use in other research involving preparation. The patches so depicted merely reflect the threeamalgam (Varga et al., 1986; Staninec and Holt, 1988; Torii et al., dimensional configuration of the amalgam-adhesive cement in1988; Cooley et al., 1989; Rueggeberget al., 1989; Staninec, 1989; terface. Lacy and Staninec, 1990). Furthermore, the stability of the The findings pertaining to the bonding of freshly triturated phosphonate adhesive is documented by its ability to withstand amalgam to a resin composite are encouraging. Nonetheless, long-term aging (Aboush and Jenkins, 1989) and thermocycling further studies are required to enable rational preclinical assess(Thompson et al., 1985; Atta et al., 1990). ment of the potential usefulness of bonded amalgam-resin comA precise explanation cannot be offered for the large variance posite restorations. between the mean bond strength values of Group 1 and Group 2 The present results of bond strength testing imply that the specimens. Nonetheless, one might speculate that the products bonding of amalgam to resin composite cannot be achieved of chemical reactions involving phosphonate adhesive cement without the use of an intermediary cement, i.e., bonding agent. It and freshly triturated amalgam are not the same as those must be mentioned, however, that the adhesive cement, amalresulting from reactions of the phosphonate adhesive and hard- gam alloy, and resin composite identified herein are not necessarened amalgam (Wing, 1966). Alternatively, the intermingling of ily the best available materials for the production of layered adhesive with fresh amalgam may introduce a beneficial me- restorations. Dental Materials~March 1994 85
Fig.3. SEM micrograph of dental amalgam-adhesive cement and resin compositeadhesive cement interfaces (A = dental amalgam; P = phosphonate adhesive resin cement; and C = resin composite),
Fig. 5. SEM micrograph of a peripheral area showing substantial accumulation of adhesive resin cement in the amalgam layer,
Fig. 4. SEM micrograph depicting multiple isolates of adhesive resin cement in amalgam,
Fig. 6. SEM micrograph showing bulky patches of amalgam in adhesive resin cement.
Complutense University School of Dentistry, Madrid, Spain.
For instance, the bondability of the amalgamated product of one specific amalgam alloy powder is known to be greater than that of the amalgamates of seemingly similar amalgam alloy powders (Rueggeberg et al., 1989). This fact alone points to the need to systematically test a broad spectrum of combinations of various dental amalgams, resin composites, and bonding agents. In future studies, care will be exercised to keep the adhesive layer as thin as possible. Hopefully, this effort will minimize the likelihood of displaced adhesive being engulfed by condensed amalgam.
Address corresponding and reprint requests to: Alvaro Enrique Garcia-Barbero Departamento de Odontologia Conservadora Facultad de Odontologia de la Universidad Complutense de Madrid Ciudad Universitaria 28040 Madrid, Spain
ACKNOWLEDGMENT This investigation was supported by the Scientific Research Program of the Department of Conservative Dentistry,
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Alvaro et al./Bonding to amalgam composite
Received November 2, 1993 / Accepted January 25, 1994
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