Fracture resistance by pins coated with
of composite and amalgam new adhesive resins
cores retained
Anthony H. L. T,jan, DrDent, DDS,” James R. Dunn, DDS,b and Ben E. Grant, DMDC Loma Linda University School of Dentistry, Department of Restorative Dentistry, Loma Linda, Calif. This study determined the effects of coating pins with either Panavia EX or with 4-META (Cover-Up) materials on the fracture resistance of pin-retained amalgam and composite cores. Gold-plated stainless steel (TMS) and titanium (Filpin) self-threading pins were used. Findings of this study corroborated the findings of several other studies that the use of pins reduces the fracture resistance of restorations. However, coating the pins with adhesion promoters such as Panavia EX and 4-META materials has been found to be effective in improving the fracture resistance. Cross-preference was observed between TMS and Filpin pins; that is, Panavia material coating was more effective with TMS pins, while 4-META was more effective with Filpin pins. (J PROSTHET DENT 1992;67:752-60.)
P.
ms have simplified the clinician’s task in the restoration of teeth with extensive coronal destruction and in the construction of cores. Self-threading pins have gained wide usage in recognition of their excellent retentive properties.‘, 2 Nonetheless, their use in embrittled endodontitally treated teeth has been somewhat controversial, because they tend to cause dentinal cracking or crazing.3, 4 Several other drawbacks associated with the use of pins have been noted in the literature,5, 6 including a decrease in the crushing, transverse, and tensile strength of amalgam.7-11Further, the breaking load of specimens with pins has been reported to be 15% to 50% of the breaking load of the amalgam, with tbe crack initiating at the pin. In contrast, in the samples without pins, the crack began at the loading site.12,l3 However, when pure silver pins were used, cracks were initiated away from the pin-a phenomenon that can be attributed to the bonding capabilities of silver with amalgam.14 Unfortunately, commercially available pins do not chemically bond to restorative materials, and the retention of pins has relied mainly upon surface irregularities or serrations. This lack of a specific bond between pin and restorative material, in conjunction with the technical difficulty of attaining an intimate adaptation of the material to the pin, is presumed to weaken the restoration. Also, an interfacial gap may be formed
that consequently
functions
as a
flaw (that concentrates stresses), thus undermining the integrity of the restoration.
aProfessor and Director of Biomaterials bAssociate Professor. eProfessor. 10/l/35665
752
Research.
1. Ivorine teeth used in this study (left to right): with four TMS pins; with six TMS pins; and with six tin-plated TMS pins.
Fig.
Dhuru et a1.15reported that stresses formed in amalgam restorations because of the presence of pins can be minimized if a metallurgic bond between the pin and the amalgam exists. Therefore any treatment capable of producing chemical bonding between pin and restorative material would be expected to diminish the weakening effect of pins. A new adhesive resin luting agent, Panavia EX (Kuraray Co. Ltd., Osaka, Japan), has been developed and is claimed to bond chemically with composite resins, nonprecious dental alloys, and tin-plated gold alloy.16 Similarly, Matsumara and Nakabayashl ‘17 introduced another adhesive monomer, 4-META (4-methacryloxyethyl trimellitate anhydride), which acts as an adhesion promoter between composite and metal. A product containing 4-META is now available commercially as a brush-on liquid adhesive JUNE1992
VOLUME67
NUMBER6
CORE FRACTURE
RESISTANCE
Fig. 2. Diagram depicts locations of pins in three pin planes: LL, Lateral lingual plane; MP, midplane; LF, lateral facial plane. Each plane contains two pins.
Fig. 4. Schematic setup for fracture testing; load was applied by means of two small cylinders, which yielded biaxial compressive loading.
Fig. 3. Chrome/cobalt split mold used to produce cores of similar size and configuration. (Cover-Up II, Parke11Biomaterials Division, Farmingdale, N.Y.) that the manufacturer states will chemically bond composite resin to both precious and nonprecious alloys and to amalgams. This study measured the fracture resistance of amalgam and composite cores retained by pins coated with Panavia EX or Cover-Up II (containing 4-META) materials. Two types of self-threading pins were included in this study: gold-plated stainless steel pins (TMS, Whaledent International, New York, N.Y.) and titanium pins (Filpin, Vivadent, Tonawanda, N.Y.). MATERIAL
AND
METHODS
To eliminate variability caused by differences in size and physical properties associated with natural teeth, composite resin tooth analogs were used in this study. A master preparation was made from a composite resin molar analog that was then replicated by the manufacturer (Ivorine tooth, Viade Corp., Camarillo, Calif.) (Fig. 1). THE
JOURNAL
OF PROSTHETIC
DENTISTRY
A total of 160 tooth preparation analogs (which hereafter will be referred to as tooth/teeth) were made and were randomly assigned to 16 test groups of 10 each. Eight groups were then allocated to each restorative material tested-that is, amalgam and composite resin. For each core material tested, both types of self-threading pins were used (TMS, 0.027 in or 0.675 mm diameter; and Filpin, 0.76mm diameter) They were subjected to the following treatment: group 1: four TMS pins with no treatment; group 2: six TMS pins with no treatment; group 3: six TMS pins coated with Panavia EX material; group 4: six TMS pins, tin-plated, then coated with Panavia EX material; group 5: six TMS pins coated with 4-META (Cover-Up II); group 6: six Filpin pins with no treatment; group 7: six Filpin pins coated with Panavia EX material; group 8: six Filpin pins coated with 4-META (Cover-Up II). Pin placement Four 2 mm deep pinholes were drilled parallel to the long axis of the tooth with a slow-speed handpiece at approximately 1.5 mm inside the perimeter and near each axial line angle of the teeth using a twist drill supplied in the pin systems. The TMS pins were manually installed in the prepared pinholes with a hand wrench furnished by the manufacturer. The Filpin pins were placed with a slowspeed handpiece. For the six pin-retained cores, two additional pins were installed in the midplane (MP), thereby positioning them exactly beneath the central groove of the core (Fig. 2). The pins were then shortened so they would be completely embedded in the restorative material. 753
TJAN,
DUNN,
AND
GRANT
90 80 70 60 2 s
50 40 30 20
ia DISPLACEMENT Fig. 5. Typical load-displacement curve shows two peaks, with first peak indicating breaking load of the core, and second peak indicating breaking load of the tooth substrate.
Core fabrication To standardize the size and shape of the cores, a cast metal split mold made from chrome/cobalt alloy was used (Fig. 3). The completed core had an occlusal surface with a facial and lingual cuspal slope of 40 degrees, with a maximum height of 4 mm at the facial and lingual cusps. The minimum height at the groove measured 3 mm. The tooth preparation was first fitted inside the mold, held firmly in a vise, then packed with the designated core material. A chemically activated composite resin (Concise, batch No. 890328, 3M, St. Paul, Minn.) and an encapsulated high-copper hybrid amalgam (Dispersalloy, regular set, batch No. 75847, Johnson &Johnson Dental Care Co., New Brunswick, N.J.) were used in this study. The composite resin, mixed according to the manufacturer’s instructions, was placed in the mold and packed. The amalgam was triturated mechanically in a Wig-L-Bug LP 60 instrument (Crescent Dental Mfg. Co., Lyons, Ill.) and manually condensed into the mold by one investigator. Before constructing the cores for the experimental groups, the pins were coated evenly with either 4-META containing primer (Cover Up II primer liquid, batch No. 0588, Parke11Biomaterials Division, Farmingdale, N.Y.) or an adhesive resin cement (Panavia EX, batch No. 64118, Kuraray, Co., Ltd., Osaka, Japan), mixed, and manipulated according to the manufacturer’s instructions. In addition, the TMS pins in group 4 were also tin-plated using an 754
Fig. 6. Vertical fracture through midplane represented by first peak in the load-displacement curve, followed by second peak representing fracture of the tooth substrate through lateral pin plane, the most frequent and typical failure mode (failure mode 1). A, Lateral view of fractured composite specimen; B, frontal view of fractured amalgam specimen.
electrodeposition apparatus (Kura Ace mini, Kuraray Co., Ltd.). Composite resin cores were placed, in which the Panavia resin coating was allowed to set first by covering with Oxyguard gel (Kuraray Co. Ltd.). They were then washed and dried, and following an application of an enamel bonding resin (Concise, batch No. 881104,3M), were packed with a composite resin core material. By contrast, a freshly triturated amalgam was hand-condensed against the pins while the Panavia paste was still wet. This technique was suggested to promote a good bond with the amalgam.ls Before testing, the cores were stored in 100% humidity at 37’ C for 7 days in an incubator (Model 1520, VWR, San Francisco, Calif.). The fracture resistance was then determined using an Instron testing machine (model 1125, Instron Corp., Canton, Mass.) in a biaxial compressive mode at a constant crosshead speed of 0.1 cm/min until fracture occurred (Fig. 4). Two peaks were generally identified on the load-displacement curve, with the first peak indicating a fracture of the core and the second a fracture JUNE
1992
VOLUME
67
NUMBER
6
CORE
FRACTURE
RESISTANCE
. AMALGAM
q
6-FILPIN
T
0
+TMS
NO TX
PANlSn
PAN
4-M
NO TX
PAN
4-M
7. Bar graph depicts mean failure load and standard deviation of amalgam cores. No TX, No treatment; PAN/Sri, tin-plated pins coated with Panavia cement; PAN, coated with Panavia cement; 4-M, coated with 4-META. Fig.
160
60
CTMS
NO TX
PANlSn
PAN
4-M
NO TX
PAN
4-M
8. Bar graph depicts mean failure load and standard deviation of composite resin core. Abbreviations as in legend to Fig. 7.
Fig.
through the tooth substrate (Figs. 5 and 6). The magnitude of the load at first peak was recorded as the fracture strength of the core. The type of fracture or failure mode was either visually or microscopically identified and categorized. The fracture resistance data were analyzed using a three-way analysis of variance (ANOVA), with core material, pin type, and pin treatment representing the independent variables. Duncan’s multiple range test was used for multiple comparisons. The four-pin and six-pin retained cores were compared using two-sample t tests. The failure THE
JOURNAL
OF PROSTHETIC
DENTISTRY
modes, which were nominally categorized and coded numerically, were analyzed by chi square test to determine whether differences in the relative distribution existed resulting from pin treatment. A statistical software package (SPSS/PC+, SPSS, Inc., Chicago, Ill.) was used for the analyses.
RESULTS Table I presents the range, mean, standard deviations, and coefficient of variance of fracture load of amalgam and composite cores. Figs. 7 and 8 illustrate graphically the 755
TJAN,
Fig. 9. Some amalgam remained attached to pins, indicating better wetting and mechanical bond of amalgam to pins.
Fig. 11. Mercury plated TMS pin.
in amalgam
DUNN,
appeared
AND
to attack
GRANT
gold-
pins in
Fig. 12. Fractured specimen of composite resin core retained by Panavia cement-coated TMS pins. Adhesive resin remained attached to pin.
mean and standard deviation of the fracture load for composite resin and amalgam cores. Comparison of four-pin and six-pin retained cores using two-sample t tests demonstrated a highly significant difference at p < 0.0001 (t = 8.02 and t = 5.01 for amalgam, and composite resin, respectively; df = l&3), indicating clearly that pins had weakened the cores. A three-way ANOVA used to analyze the data of the six-pin cores revealed a highly significant
difference between core materials and between pin pretreatments at p < 0.0001. No significant differences were observed between types of pins. However, highly significant interactions were observed between pin type and core material (p < 0.004), and between pin type and pin treatment (p < 0.001) (Table II). The fracture load for amalgam cores retained by untreated titanium pins (Filpin) was higher than that for cores retained by gold-plated stainless
Fig. 10. No composite resin attached fractured composite resin specimen.
756
to exposed
JUNE
1992
VOLUME
67
NUMBER
6
CORE FRACTURE
Table
RESISTANCE
I. Failure load of cores (in kilograms) I-pin
Amalgam Range Mean SD cv
(5)
Composite Range Mean SD cv
(9;)
TMS
B-pin
TMS
B-pin
No TX
No TX
PanlSn
Pan
4-M
66-108
28-58
32-80
52-114 71.5
55-87 66.9
41-80 58.3
54-84 66.3
56-133 81.3
12.60 15
9.76 22
16.81 28
17.82 25
11.26 17
18.72 33
9.68 15
21.09 26
102-188 149.8
72-130 98.4
78- 160 113.8
115-240 162.7
98-200 129.9
72-134 95.1
SO-135 115.6
go-170 131.5
26.62 18
18.60 19
22.37 20
39.80 24
28.98 22
22.08 23
27.91 24
24.82 19
cores
Duncan’s multiple range tests at the 95% confidence level indicated that all three treatments applied to TMS pins significantly improved the fracture resistance of amalgam cores (Table I). For Filpin pins, however, only 4-META enhanced the fracture resistance of amalgam cores. No difference was found between Filpin pins treated with Panavia EX cement and the control with respect to amalgam cores.
resin
cores
Results of this study indicated significantly higher fracture resistance of composite resin cores than amalgam cores under this biaxial compressive loading test condition at p < 0.0001. Duncan’s multiple range tests at the 95% confidence level indicated that TMS pins treated with Panavia EX cement yielded the highest fracture resistance, followed by 4-META-treated TMS pins in composite resin cores. However, no significant difference was found between Panavia cement applied on tin-plated TMS pins and the control specimens. For Filpin pins, 4-META produced the highest fracture resistance at p < 0.05. No significant difference was found between Panavia EX cement-treated Filpin pins and the control specimens. The fractographic analysis identified the following six types of failure mode: (1) fracture through the smallest midplane exposing one or two pins; (2) fracture occurring just outside the midplane, thus not exposing any pin; (3) fracture through the midplane and propagated to another pin in the lateral plane; (4) cuspal fracture without exposing any pin; (5) cuspal fracture with a partial exposure of THE
JOURNAL
4-M
59.3
steel pins (TMS). However, no difference in fracture load for composite cores using either type of pin was observed. While TMS pins performed better with Panavia EX cement pretreatment, Filpin pins were better when treated with 4-META.
Composite
Pan
43.9
No TX, No treatment; Pan, Pan&a coating; Sn, tin plating; 4-M, 4-META coating; efficient of variation. Horizontal lines connect means that are not significantly different at p < 0.05.
Amalgam
No TX
Filpin
OF PROSTHETIC
DENTISTRY
Z’MS, gold-plated
stainless
steel; Filpin,
titanium
self-threading;
CV, co-
a pin; and (6) fracture through a lateral pin plane, either lingual or facial pin plane (Fig. 2). Type 1 failure mode was the most typical and occurred most frequently amounting to 59.3%) and was followed by type 2, which consisted of 15% (Fig. 6). Table III presents the relative frequency distribution of the failure modes. Contingency tables were constructed from the data of the failure mode of each combination of core material and pin type and tests for statistical significance were made by chi square analysis at the 0.05 level of significance to determine whether the pattern of failure modes was related to pin pretreatments. For this statistical analysis the failure modes were classified into two more relevant categories: (1) fracture through the midplane or central groove-initiated and (2) fracture outside the midplane, which may be attributed to better bonding capability of the pretreated pins. Therefore, types 1 and 3 failure modes were combined to form one category, since both failed at the midplane, and the remainder of types of failure modes formed another category. Results indicated that only the combination of amalgam and TMS pins was significantly different at p < 0.0012 (x2 = 15.833, DF = 3). Amalgam cores retained by Panavia cement-coated TMS pins yielded significantly more fracture outside the midplane.
DISCUSSION The fracture strength of a core as a foundation for a final restoration is very important. The presence of discontinuity or flaws such as cracks, pores, gaps, or inclusions in the core can disrupt an otherwise uniform pattern of stress distribution by producing stress concentrations within the vicinity of the discontinuity, which could weaken the cores. Lack of adhesion between the pin and the restorative material has been implicated in the decreased strength of pin-retained restorations. Polymerization contraction of 757
TJAN,
Table
II.
of variation
ss
effects
SS, Sum of squares;
99187.500 529.200 22926.350 14633.133
MS
GRANT
30660.762
1 1
99187.500 529.200
2 5
.11463.175 2932.627 4368.133 1518.975 3628.525 606.858 606.858
4368.133
1 2 2 2 2
7257.050
1213.717 1213.717 138519.900
11
53803.800
108
192323.700
119
Significance ofF
F
4
3037.950
61.545
0.000
199.098
12592.718 498.183 1616.166
0.000
1.062 23.010 5.887 8.168 3.049 7.284 1.218 1.218
0.305 0.000 0.000 0.004 0.052 0.300 0.300
25.277
0.000
0.001
MS, mean squares.
the restorative material or mismatch of the coefficients of thermal expansion between the pin and restorative material may create an interfacial gap-a discontinuity-that could act as a stress raiser. In addition, these factors permit microleakage that degrades or corrodes both the pin and restorative materials. The incorporation of an excessive number of pins or of a pin longer than necessary for retention to the amalgam or composite resin may also complicate and compromise the proper condensation or packing of the restorative materials. Voids or reduced density may thereby be created. The rationale of using a coupling agent to treat the pins was to promote adhesion between the pins and the restorative material, thereby enhancing the strength of the cores by eliminating the interfacial gap or discontinuity. This agent would act as an interface between the core material and the pin. Because of the claims that a strong bond can be achieved by 4-META and Panavia EX cement to tooth structures, composite resin, and several dental alloys,i6?I791g-21they were selected as coupling agents to improve the bond between the pin and the core material. The presence of an aromatic acid anhydride functional group in the adhesive 4-META monomer is responsible for adhesion of the resin to nonprecious alloys.17 Atsuata et a1.22 reported that 4-META also increases bonding between the polymeric matrix and the filler particles in composite resin and enhances the transverse strength. Panavia EX is a chemically activated, BIS-GMA-based resin cement for use in final cementation of crowns and fixed partial dentures. The addition of phosphate monomer (MlOP or lo-methacryloyloxydecyl dihydrogen phosphate) in the formulation has contributed to the adhesive properties to various dental as mentioned
previously.16
Comparable tensile strengths of pin-retained amalgam and composite resin restorations have been reported.23 In this study, composite resin cores almost doubled the frac-
758
DF
122643.050
Core material Pin type Treatment Two-way interactions Core material/Pin type Core material/Treatment Pin type/Treatment Three-way interactions Core material/Pin type/ Treatment Explained Residual Total
materials,
AND
Summary of three-way ANOVA
Source Main
DUNN,
ture resistance
of amalgam
cores. A possible
explanation
for this difference could be that the composite resin used in this experiment showed some ductile behavior and did not fail in a pure brittle fashion. The plastic deformation occurring at the tip of the propagating crack associated with this type of composite material could have consumed substantially more energy causing failure. Findings of this
study seemed to suggest that the fracture toughness value (KIc) of the composite resin was higher than that of the amalgam. This value may vary for different types and brands of materials. Lloyd and Adamson reported higher (KIc) values of posterior composite resins than of amalgams. Nonetheless, microscopic examination of the fractured specimens showed that some amalgam remained attached to the exposed pins, indicating better wetting and mechanical bond of amalgam to untreated pins than to composite resin, which exhibited clean pins (Figs. 9 and 10). In addition, it was noted that the free mercury in the amalgam could dissolve the thin-layered gold plating of TMS pins (Fig. 11). However, in composite resin cores retained by Panavia-coated TMS pins, the adhesive resin remained attached to the pin, indicating a better bonding (Fig. 12). Obviously, different values of fracture strength may be expected by using different types of amalgams. Findings of this study have shown that pin pretreatment with the two adhesive promoters used in this experiment was effective in substantially improving the fracture resistance of the cores. From an engineering standpoint, failure analysis is extremely essential, because it is the only means of isolating the failure-causing problem. This analysis helps determine whether failure occurred as a result of a design fault or because of material limitation or deficiency. Until this can be determined, effort cannot be efficiently directed toward finding
a solution.
The path of a crack or fracture as it propagates through a specimen provides substantial information about the JUNE
1992
VOLUME
67
NUMBER
6
CORE
FRACTURE
Table
RESISTANCE
Relative frequency distribution
III.
of failure modes of six-pin retained amalgam and composite cores Failure
Core
material
Pin treatment
Pin type
Amalgam
No.
1
Cumulative % No TX Pan 4-M Cumulative % Combined %
10 10 10 10 40 10 10 10 30 70
70 10 50 80 52.5 60 90 80 76.7 62.9
20 50 40 10 30
No TX PanlSn Pall 4-M Cumulative %
10 10 10 10 40
No TX Pan/Sri Pan
TMS
4-M
Filpin
Composite
TMS
Filpin
No TX Pall 4-M
Cumulative % Combined % Total % Abbreviations
10 10 10 30 70 140
JOURNAL
3 10 10 10 7.5 30
10 -
-
3.3 18.6
10 8.6
50 70 60 20 50
10 -
10 -
60 70 60 63.3 55.7 59.3
10 -
20 20
20 10 10 15
13.3 7.1 7.9
30 10
(“lo) 4
6
6
30 7.5 10 -
-
10 -
2.5 20 6.7 2.9
-
20 20 40 40 30
10 -
-
10 10
-
-
6.7 20 11.4
20 6.7 5.7 3.6
3.3 5.7
10 -
2.5
2.5
1.4 3.6
10 5
as in Table I.
stress distribution at the time of failure. Examination of the fractured specimens has suggested that most failures followed a typical pattern. With this bicuspal core design (with facial and lingual slopes), in conjunction with a biaxial-compressive loading that basically generated a wedging force, it was anticipated that most failures would occur primarily at the midplane, beginning at the tip of the V-shaped center groove (because of the higher level of stress in the notch region). The failure travelled vertically down to the tooth substrate (perpendicular to the tooth surface) because of its smaller cross-sectional surface area (approximately 25.9 mm2; the cross-sectional surface area of the lateral pin planes was 31.5 mm2). The use of this design configuration allowed measurement and comparison of the effect of pins on fracture resistance of cores. For example, the effect of pins was evaluated by comparing the fracture strength of four-pin cores, in which no pins were placed in the midplane, versus the six-pin cores, where two additional pins were positioned in the midplane. A low fracture strength would be expected if no adhesive bond existed between the pins and the core material. On the other hand, a high fracture strength was expected when the coupling agents tested were capable of achieving an adhesive bond between the pins and the core material. The highest bonding strength was indicated when, upon stressing, the fracture occurred in the body of the core material or within the coupling agent and not at the interface. A typical fracture pattern was a vertical one through the THE
2
mode
OF PROSTHETIC
DENTISTRY
midplane that was represented by the first peak on the load-displacement curve. Further application of load caused fracture of the tooth substrate through the highly prestressed pinholes in the lateral plane, identified by a second peak that required an additional load of approximately 20% to 30% (Fig. 5). Because of the difference in the resiliency and elasticity of tooth dentin, perhaps the magnitude of stress concentration in the vicinity of the pinholes may be at variance with the analog teeth used in this study. Many investigators have reported that a selfthreading pin causes crazing in dentin and weakening of the tooth.3, 25,26 Since the study was intended to measure the fracture resistance of the cores, only the values represented by the first peak were presented. A higher load was also recorded when the fracture occurred outside the midplane (e.g., at the lateral lingual or lateral facial plane) or when the pin treatment was considered effective (Fig. 2). The presence of flaws, pores, and inclusions, which could not be avoided in condensing amalgam or packing composite resin, contributed to much of the variation in the results. The failure modes did not correlate well with the fracture load. Corrosion by water and mismatch of the coefficients of thermal expansion between metal pins and composite resin may create an interfacial flaw that could reduce the fracture resistance of the cores. A longer water immersion time than 7 days is probably needed to evaluate the effect of water corrosion. Since the core is totally encased in a metal 759
TJAN,
crown when the final restoration of the tooth is completed, the strength difference between composite resin and amalgam might not be of clinical significance. However, cores with high fracture strength clearly are desirable when allceramic crowns are indicated for the final restorations. Although this investigation was conducted on cores, the information obtained from this study is valid and useful and must be considered in the construction of any restoration using pins for retention., A greater improvement of fracture strengths was achieved by applying Panavia EX cement directly to gold-plated TMS pins rather than to tin-plated TMS pins. The reason for this contradictory result was not fully understood. It could be speculated that the electrodeposition apparatus used in this study was not effective for this purpose, and that the tin deposit was too thick or was not bonded properly to the gold-plated ‘TMS pins. In addition, excess tin particles may be deposited in the serration’s crevices (valleys), which reduces their depth and subsequently the mechanical interlocking of the restorative materials. To avoid problems related to tin plating, unplated stainless steel TMS pins should be acquired and used to evaluate the effect of Panavia cement coating on fracture strength. The findings of this study have indicated that Panavia cement coating was not effective in improving the fracture resistance of cores retained by titanium pins. This result may or may not be extrapolated to other nonprecious metal alloys. Variation in bond strength of Panavia EX cement to different types of amalgam has been reported in the literature.27 SUMMARY
AND
CONCLUSIONS
This study determined the relationship between pin treatment with Panavia EX cement or with Cover-Up II material (4-META) and the fracture resistance of pin-retained amalgam and composite resin cores. Two types of self-threading pins were used in this study-TMS goldplated stainless steel and Filpin titanium pins. Conclusions from the study were: 1. The use of pins significantly decreased the fracture resistance of both amalgam and composite resin cores. 2. All three treatments applied to TMS pins improved the fracture resistance of amalgam cores. 3. Only Cover-Up (4META) material improved the fracture resistance of amalgam cores retained by Filpin pins. 4. The fracture resistance of a composite core was substantially higher than that of an amalgam core. 5. TMS pins treated with Panavia EX cement yielded the highest fracture resistance in composite resin cores, followed by pins pretreated with Cover-Up II material. 6. Panavia EX cement pretreatment on tin-plated TMS pins failed to improve the fracture resistance of composite resin cores. 7. For Filpin pins, only Cover-Up material treatment improved the fracture resistance of composite resin cores. 760
DUNN,
AND
GRANT
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I-META
acrylic
resin denture
PP, Watanabe LG. Bond strength of base to cobalt chromium alloy. J PROS-
THET DENT 1988;60:570-6.
22. Atsuata M, Abel1 AK, Turner DT, Nakabayashi N, Takeyama M. A new coupling agent for composite materials: 4-methacryloxyethyl trimethylic anhydride. J Biomed Mater Res 1982;16:619-28. 23. Fujimoto J, Norman RD, Dykema RW, Phillips RW. A comparison of pin-retained amalgam and composite resin cores. J PROSTHET DENT 1978;39:512-9.
24. Lloyd CH, Adamson M. The development of fracture toughness and fracture strength in posterior restorative materials. Dent Mater 1987;3:225-31. 25. Markley MR. Pin-retained and reinforced restorations and foundations. Dent Clin North Am 1967;11:229-44. 26. Pameijer CH, Stallard RE. Effect of self-threading pins. J Am Dent Assot 1972;85:895-9. 27. Rueggeberg FA, Caughman WF, Gao F, Kovarik RE. Bond strength of Panavia EX to dental amalgam. Int J Prosthodont 1989;2:371-5. Reprint requests to: DR. ANTHONY H. L. TJAN LOMA LINDA UNIVERSITY SCHOOL OF DENTISTRY DEPARTMENT OF RESTORATIVE DENTISTRY LOMA LIYDA, CA 92350
JUNE
1992
VOLUME
67
NUMBER
6
of composite and amalgam new adhesive resins
cores retained
Anthony H. L. T,jan, DrDent, DDS,” James R. Dunn, DDS,b and Ben E. Grant, DMDC Loma Linda University School of Dentistry, Department of Restorative Dentistry, Loma Linda, Calif. This study determined the effects of coating pins with either Panavia EX or with 4-META (Cover-Up) materials on the fracture resistance of pin-retained amalgam and composite cores. Gold-plated stainless steel (TMS) and titanium (Filpin) self-threading pins were used. Findings of this study corroborated the findings of several other studies that the use of pins reduces the fracture resistance of restorations. However, coating the pins with adhesion promoters such as Panavia EX and 4-META materials has been found to be effective in improving the fracture resistance. Cross-preference was observed between TMS and Filpin pins; that is, Panavia material coating was more effective with TMS pins, while 4-META was more effective with Filpin pins. (J PROSTHET DENT 1992;67:752-60.)
P.
ms have simplified the clinician’s task in the restoration of teeth with extensive coronal destruction and in the construction of cores. Self-threading pins have gained wide usage in recognition of their excellent retentive properties.‘, 2 Nonetheless, their use in embrittled endodontitally treated teeth has been somewhat controversial, because they tend to cause dentinal cracking or crazing.3, 4 Several other drawbacks associated with the use of pins have been noted in the literature,5, 6 including a decrease in the crushing, transverse, and tensile strength of amalgam.7-11Further, the breaking load of specimens with pins has been reported to be 15% to 50% of the breaking load of the amalgam, with tbe crack initiating at the pin. In contrast, in the samples without pins, the crack began at the loading site.12,l3 However, when pure silver pins were used, cracks were initiated away from the pin-a phenomenon that can be attributed to the bonding capabilities of silver with amalgam.14 Unfortunately, commercially available pins do not chemically bond to restorative materials, and the retention of pins has relied mainly upon surface irregularities or serrations. This lack of a specific bond between pin and restorative material, in conjunction with the technical difficulty of attaining an intimate adaptation of the material to the pin, is presumed to weaken the restoration. Also, an interfacial gap may be formed
that consequently
functions
as a
flaw (that concentrates stresses), thus undermining the integrity of the restoration.
aProfessor and Director of Biomaterials bAssociate Professor. eProfessor. 10/l/35665
752
Research.
1. Ivorine teeth used in this study (left to right): with four TMS pins; with six TMS pins; and with six tin-plated TMS pins.
Fig.
Dhuru et a1.15reported that stresses formed in amalgam restorations because of the presence of pins can be minimized if a metallurgic bond between the pin and the amalgam exists. Therefore any treatment capable of producing chemical bonding between pin and restorative material would be expected to diminish the weakening effect of pins. A new adhesive resin luting agent, Panavia EX (Kuraray Co. Ltd., Osaka, Japan), has been developed and is claimed to bond chemically with composite resins, nonprecious dental alloys, and tin-plated gold alloy.16 Similarly, Matsumara and Nakabayashl ‘17 introduced another adhesive monomer, 4-META (4-methacryloxyethyl trimellitate anhydride), which acts as an adhesion promoter between composite and metal. A product containing 4-META is now available commercially as a brush-on liquid adhesive JUNE1992
VOLUME67
NUMBER6
CORE FRACTURE
RESISTANCE
Fig. 2. Diagram depicts locations of pins in three pin planes: LL, Lateral lingual plane; MP, midplane; LF, lateral facial plane. Each plane contains two pins.
Fig. 4. Schematic setup for fracture testing; load was applied by means of two small cylinders, which yielded biaxial compressive loading.
Fig. 3. Chrome/cobalt split mold used to produce cores of similar size and configuration. (Cover-Up II, Parke11Biomaterials Division, Farmingdale, N.Y.) that the manufacturer states will chemically bond composite resin to both precious and nonprecious alloys and to amalgams. This study measured the fracture resistance of amalgam and composite cores retained by pins coated with Panavia EX or Cover-Up II (containing 4-META) materials. Two types of self-threading pins were included in this study: gold-plated stainless steel pins (TMS, Whaledent International, New York, N.Y.) and titanium pins (Filpin, Vivadent, Tonawanda, N.Y.). MATERIAL
AND
METHODS
To eliminate variability caused by differences in size and physical properties associated with natural teeth, composite resin tooth analogs were used in this study. A master preparation was made from a composite resin molar analog that was then replicated by the manufacturer (Ivorine tooth, Viade Corp., Camarillo, Calif.) (Fig. 1). THE
JOURNAL
OF PROSTHETIC
DENTISTRY
A total of 160 tooth preparation analogs (which hereafter will be referred to as tooth/teeth) were made and were randomly assigned to 16 test groups of 10 each. Eight groups were then allocated to each restorative material tested-that is, amalgam and composite resin. For each core material tested, both types of self-threading pins were used (TMS, 0.027 in or 0.675 mm diameter; and Filpin, 0.76mm diameter) They were subjected to the following treatment: group 1: four TMS pins with no treatment; group 2: six TMS pins with no treatment; group 3: six TMS pins coated with Panavia EX material; group 4: six TMS pins, tin-plated, then coated with Panavia EX material; group 5: six TMS pins coated with 4-META (Cover-Up II); group 6: six Filpin pins with no treatment; group 7: six Filpin pins coated with Panavia EX material; group 8: six Filpin pins coated with 4-META (Cover-Up II). Pin placement Four 2 mm deep pinholes were drilled parallel to the long axis of the tooth with a slow-speed handpiece at approximately 1.5 mm inside the perimeter and near each axial line angle of the teeth using a twist drill supplied in the pin systems. The TMS pins were manually installed in the prepared pinholes with a hand wrench furnished by the manufacturer. The Filpin pins were placed with a slowspeed handpiece. For the six pin-retained cores, two additional pins were installed in the midplane (MP), thereby positioning them exactly beneath the central groove of the core (Fig. 2). The pins were then shortened so they would be completely embedded in the restorative material. 753
TJAN,
DUNN,
AND
GRANT
90 80 70 60 2 s
50 40 30 20
ia DISPLACEMENT Fig. 5. Typical load-displacement curve shows two peaks, with first peak indicating breaking load of the core, and second peak indicating breaking load of the tooth substrate.
Core fabrication To standardize the size and shape of the cores, a cast metal split mold made from chrome/cobalt alloy was used (Fig. 3). The completed core had an occlusal surface with a facial and lingual cuspal slope of 40 degrees, with a maximum height of 4 mm at the facial and lingual cusps. The minimum height at the groove measured 3 mm. The tooth preparation was first fitted inside the mold, held firmly in a vise, then packed with the designated core material. A chemically activated composite resin (Concise, batch No. 890328, 3M, St. Paul, Minn.) and an encapsulated high-copper hybrid amalgam (Dispersalloy, regular set, batch No. 75847, Johnson &Johnson Dental Care Co., New Brunswick, N.J.) were used in this study. The composite resin, mixed according to the manufacturer’s instructions, was placed in the mold and packed. The amalgam was triturated mechanically in a Wig-L-Bug LP 60 instrument (Crescent Dental Mfg. Co., Lyons, Ill.) and manually condensed into the mold by one investigator. Before constructing the cores for the experimental groups, the pins were coated evenly with either 4-META containing primer (Cover Up II primer liquid, batch No. 0588, Parke11Biomaterials Division, Farmingdale, N.Y.) or an adhesive resin cement (Panavia EX, batch No. 64118, Kuraray, Co., Ltd., Osaka, Japan), mixed, and manipulated according to the manufacturer’s instructions. In addition, the TMS pins in group 4 were also tin-plated using an 754
Fig. 6. Vertical fracture through midplane represented by first peak in the load-displacement curve, followed by second peak representing fracture of the tooth substrate through lateral pin plane, the most frequent and typical failure mode (failure mode 1). A, Lateral view of fractured composite specimen; B, frontal view of fractured amalgam specimen.
electrodeposition apparatus (Kura Ace mini, Kuraray Co., Ltd.). Composite resin cores were placed, in which the Panavia resin coating was allowed to set first by covering with Oxyguard gel (Kuraray Co. Ltd.). They were then washed and dried, and following an application of an enamel bonding resin (Concise, batch No. 881104,3M), were packed with a composite resin core material. By contrast, a freshly triturated amalgam was hand-condensed against the pins while the Panavia paste was still wet. This technique was suggested to promote a good bond with the amalgam.ls Before testing, the cores were stored in 100% humidity at 37’ C for 7 days in an incubator (Model 1520, VWR, San Francisco, Calif.). The fracture resistance was then determined using an Instron testing machine (model 1125, Instron Corp., Canton, Mass.) in a biaxial compressive mode at a constant crosshead speed of 0.1 cm/min until fracture occurred (Fig. 4). Two peaks were generally identified on the load-displacement curve, with the first peak indicating a fracture of the core and the second a fracture JUNE
1992
VOLUME
67
NUMBER
6
CORE
FRACTURE
RESISTANCE
. AMALGAM
q
6-FILPIN
T
0
+TMS
NO TX
PANlSn
PAN
4-M
NO TX
PAN
4-M
7. Bar graph depicts mean failure load and standard deviation of amalgam cores. No TX, No treatment; PAN/Sri, tin-plated pins coated with Panavia cement; PAN, coated with Panavia cement; 4-M, coated with 4-META. Fig.
160
60
CTMS
NO TX
PANlSn
PAN
4-M
NO TX
PAN
4-M
8. Bar graph depicts mean failure load and standard deviation of composite resin core. Abbreviations as in legend to Fig. 7.
Fig.
through the tooth substrate (Figs. 5 and 6). The magnitude of the load at first peak was recorded as the fracture strength of the core. The type of fracture or failure mode was either visually or microscopically identified and categorized. The fracture resistance data were analyzed using a three-way analysis of variance (ANOVA), with core material, pin type, and pin treatment representing the independent variables. Duncan’s multiple range test was used for multiple comparisons. The four-pin and six-pin retained cores were compared using two-sample t tests. The failure THE
JOURNAL
OF PROSTHETIC
DENTISTRY
modes, which were nominally categorized and coded numerically, were analyzed by chi square test to determine whether differences in the relative distribution existed resulting from pin treatment. A statistical software package (SPSS/PC+, SPSS, Inc., Chicago, Ill.) was used for the analyses.
RESULTS Table I presents the range, mean, standard deviations, and coefficient of variance of fracture load of amalgam and composite cores. Figs. 7 and 8 illustrate graphically the 755
TJAN,
Fig. 9. Some amalgam remained attached to pins, indicating better wetting and mechanical bond of amalgam to pins.
Fig. 11. Mercury plated TMS pin.
in amalgam
DUNN,
appeared
AND
to attack
GRANT
gold-
pins in
Fig. 12. Fractured specimen of composite resin core retained by Panavia cement-coated TMS pins. Adhesive resin remained attached to pin.
mean and standard deviation of the fracture load for composite resin and amalgam cores. Comparison of four-pin and six-pin retained cores using two-sample t tests demonstrated a highly significant difference at p < 0.0001 (t = 8.02 and t = 5.01 for amalgam, and composite resin, respectively; df = l&3), indicating clearly that pins had weakened the cores. A three-way ANOVA used to analyze the data of the six-pin cores revealed a highly significant
difference between core materials and between pin pretreatments at p < 0.0001. No significant differences were observed between types of pins. However, highly significant interactions were observed between pin type and core material (p < 0.004), and between pin type and pin treatment (p < 0.001) (Table II). The fracture load for amalgam cores retained by untreated titanium pins (Filpin) was higher than that for cores retained by gold-plated stainless
Fig. 10. No composite resin attached fractured composite resin specimen.
756
to exposed
JUNE
1992
VOLUME
67
NUMBER
6
CORE FRACTURE
Table
RESISTANCE
I. Failure load of cores (in kilograms) I-pin
Amalgam Range Mean SD cv
(5)
Composite Range Mean SD cv
(9;)
TMS
B-pin
TMS
B-pin
No TX
No TX
PanlSn
Pan
4-M
66-108
28-58
32-80
52-114 71.5
55-87 66.9
41-80 58.3
54-84 66.3
56-133 81.3
12.60 15
9.76 22
16.81 28
17.82 25
11.26 17
18.72 33
9.68 15
21.09 26
102-188 149.8
72-130 98.4
78- 160 113.8
115-240 162.7
98-200 129.9
72-134 95.1
SO-135 115.6
go-170 131.5
26.62 18
18.60 19
22.37 20
39.80 24
28.98 22
22.08 23
27.91 24
24.82 19
cores
Duncan’s multiple range tests at the 95% confidence level indicated that all three treatments applied to TMS pins significantly improved the fracture resistance of amalgam cores (Table I). For Filpin pins, however, only 4-META enhanced the fracture resistance of amalgam cores. No difference was found between Filpin pins treated with Panavia EX cement and the control with respect to amalgam cores.
resin
cores
Results of this study indicated significantly higher fracture resistance of composite resin cores than amalgam cores under this biaxial compressive loading test condition at p < 0.0001. Duncan’s multiple range tests at the 95% confidence level indicated that TMS pins treated with Panavia EX cement yielded the highest fracture resistance, followed by 4-META-treated TMS pins in composite resin cores. However, no significant difference was found between Panavia cement applied on tin-plated TMS pins and the control specimens. For Filpin pins, 4-META produced the highest fracture resistance at p < 0.05. No significant difference was found between Panavia EX cement-treated Filpin pins and the control specimens. The fractographic analysis identified the following six types of failure mode: (1) fracture through the smallest midplane exposing one or two pins; (2) fracture occurring just outside the midplane, thus not exposing any pin; (3) fracture through the midplane and propagated to another pin in the lateral plane; (4) cuspal fracture without exposing any pin; (5) cuspal fracture with a partial exposure of THE
JOURNAL
4-M
59.3
steel pins (TMS). However, no difference in fracture load for composite cores using either type of pin was observed. While TMS pins performed better with Panavia EX cement pretreatment, Filpin pins were better when treated with 4-META.
Composite
Pan
43.9
No TX, No treatment; Pan, Pan&a coating; Sn, tin plating; 4-M, 4-META coating; efficient of variation. Horizontal lines connect means that are not significantly different at p < 0.05.
Amalgam
No TX
Filpin
OF PROSTHETIC
DENTISTRY
Z’MS, gold-plated
stainless
steel; Filpin,
titanium
self-threading;
CV, co-
a pin; and (6) fracture through a lateral pin plane, either lingual or facial pin plane (Fig. 2). Type 1 failure mode was the most typical and occurred most frequently amounting to 59.3%) and was followed by type 2, which consisted of 15% (Fig. 6). Table III presents the relative frequency distribution of the failure modes. Contingency tables were constructed from the data of the failure mode of each combination of core material and pin type and tests for statistical significance were made by chi square analysis at the 0.05 level of significance to determine whether the pattern of failure modes was related to pin pretreatments. For this statistical analysis the failure modes were classified into two more relevant categories: (1) fracture through the midplane or central groove-initiated and (2) fracture outside the midplane, which may be attributed to better bonding capability of the pretreated pins. Therefore, types 1 and 3 failure modes were combined to form one category, since both failed at the midplane, and the remainder of types of failure modes formed another category. Results indicated that only the combination of amalgam and TMS pins was significantly different at p < 0.0012 (x2 = 15.833, DF = 3). Amalgam cores retained by Panavia cement-coated TMS pins yielded significantly more fracture outside the midplane.
DISCUSSION The fracture strength of a core as a foundation for a final restoration is very important. The presence of discontinuity or flaws such as cracks, pores, gaps, or inclusions in the core can disrupt an otherwise uniform pattern of stress distribution by producing stress concentrations within the vicinity of the discontinuity, which could weaken the cores. Lack of adhesion between the pin and the restorative material has been implicated in the decreased strength of pin-retained restorations. Polymerization contraction of 757
TJAN,
Table
II.
of variation
ss
effects
SS, Sum of squares;
99187.500 529.200 22926.350 14633.133
MS
GRANT
30660.762
1 1
99187.500 529.200
2 5
.11463.175 2932.627 4368.133 1518.975 3628.525 606.858 606.858
4368.133
1 2 2 2 2
7257.050
1213.717 1213.717 138519.900
11
53803.800
108
192323.700
119
Significance ofF
F
4
3037.950
61.545
0.000
199.098
12592.718 498.183 1616.166
0.000
1.062 23.010 5.887 8.168 3.049 7.284 1.218 1.218
0.305 0.000 0.000 0.004 0.052 0.300 0.300
25.277
0.000
0.001
MS, mean squares.
the restorative material or mismatch of the coefficients of thermal expansion between the pin and restorative material may create an interfacial gap-a discontinuity-that could act as a stress raiser. In addition, these factors permit microleakage that degrades or corrodes both the pin and restorative materials. The incorporation of an excessive number of pins or of a pin longer than necessary for retention to the amalgam or composite resin may also complicate and compromise the proper condensation or packing of the restorative materials. Voids or reduced density may thereby be created. The rationale of using a coupling agent to treat the pins was to promote adhesion between the pins and the restorative material, thereby enhancing the strength of the cores by eliminating the interfacial gap or discontinuity. This agent would act as an interface between the core material and the pin. Because of the claims that a strong bond can be achieved by 4-META and Panavia EX cement to tooth structures, composite resin, and several dental alloys,i6?I791g-21they were selected as coupling agents to improve the bond between the pin and the core material. The presence of an aromatic acid anhydride functional group in the adhesive 4-META monomer is responsible for adhesion of the resin to nonprecious alloys.17 Atsuata et a1.22 reported that 4-META also increases bonding between the polymeric matrix and the filler particles in composite resin and enhances the transverse strength. Panavia EX is a chemically activated, BIS-GMA-based resin cement for use in final cementation of crowns and fixed partial dentures. The addition of phosphate monomer (MlOP or lo-methacryloyloxydecyl dihydrogen phosphate) in the formulation has contributed to the adhesive properties to various dental as mentioned
previously.16
Comparable tensile strengths of pin-retained amalgam and composite resin restorations have been reported.23 In this study, composite resin cores almost doubled the frac-
758
DF
122643.050
Core material Pin type Treatment Two-way interactions Core material/Pin type Core material/Treatment Pin type/Treatment Three-way interactions Core material/Pin type/ Treatment Explained Residual Total
materials,
AND
Summary of three-way ANOVA
Source Main
DUNN,
ture resistance
of amalgam
cores. A possible
explanation
for this difference could be that the composite resin used in this experiment showed some ductile behavior and did not fail in a pure brittle fashion. The plastic deformation occurring at the tip of the propagating crack associated with this type of composite material could have consumed substantially more energy causing failure. Findings of this
study seemed to suggest that the fracture toughness value (KIc) of the composite resin was higher than that of the amalgam. This value may vary for different types and brands of materials. Lloyd and Adamson reported higher (KIc) values of posterior composite resins than of amalgams. Nonetheless, microscopic examination of the fractured specimens showed that some amalgam remained attached to the exposed pins, indicating better wetting and mechanical bond of amalgam to untreated pins than to composite resin, which exhibited clean pins (Figs. 9 and 10). In addition, it was noted that the free mercury in the amalgam could dissolve the thin-layered gold plating of TMS pins (Fig. 11). However, in composite resin cores retained by Panavia-coated TMS pins, the adhesive resin remained attached to the pin, indicating a better bonding (Fig. 12). Obviously, different values of fracture strength may be expected by using different types of amalgams. Findings of this study have shown that pin pretreatment with the two adhesive promoters used in this experiment was effective in substantially improving the fracture resistance of the cores. From an engineering standpoint, failure analysis is extremely essential, because it is the only means of isolating the failure-causing problem. This analysis helps determine whether failure occurred as a result of a design fault or because of material limitation or deficiency. Until this can be determined, effort cannot be efficiently directed toward finding
a solution.
The path of a crack or fracture as it propagates through a specimen provides substantial information about the JUNE
1992
VOLUME
67
NUMBER
6
CORE
FRACTURE
Table
RESISTANCE
Relative frequency distribution
III.
of failure modes of six-pin retained amalgam and composite cores Failure
Core
material
Pin treatment
Pin type
Amalgam
No.
1
Cumulative % No TX Pan 4-M Cumulative % Combined %
10 10 10 10 40 10 10 10 30 70
70 10 50 80 52.5 60 90 80 76.7 62.9
20 50 40 10 30
No TX PanlSn Pall 4-M Cumulative %
10 10 10 10 40
No TX Pan/Sri Pan
TMS
4-M
Filpin
Composite
TMS
Filpin
No TX Pall 4-M
Cumulative % Combined % Total % Abbreviations
10 10 10 30 70 140
JOURNAL
3 10 10 10 7.5 30
10 -
-
3.3 18.6
10 8.6
50 70 60 20 50
10 -
10 -
60 70 60 63.3 55.7 59.3
10 -
20 20
20 10 10 15
13.3 7.1 7.9
30 10
(“lo) 4
6
6
30 7.5 10 -
-
10 -
2.5 20 6.7 2.9
-
20 20 40 40 30
10 -
-
10 10
-
-
6.7 20 11.4
20 6.7 5.7 3.6
3.3 5.7
10 -
2.5
2.5
1.4 3.6
10 5
as in Table I.
stress distribution at the time of failure. Examination of the fractured specimens has suggested that most failures followed a typical pattern. With this bicuspal core design (with facial and lingual slopes), in conjunction with a biaxial-compressive loading that basically generated a wedging force, it was anticipated that most failures would occur primarily at the midplane, beginning at the tip of the V-shaped center groove (because of the higher level of stress in the notch region). The failure travelled vertically down to the tooth substrate (perpendicular to the tooth surface) because of its smaller cross-sectional surface area (approximately 25.9 mm2; the cross-sectional surface area of the lateral pin planes was 31.5 mm2). The use of this design configuration allowed measurement and comparison of the effect of pins on fracture resistance of cores. For example, the effect of pins was evaluated by comparing the fracture strength of four-pin cores, in which no pins were placed in the midplane, versus the six-pin cores, where two additional pins were positioned in the midplane. A low fracture strength would be expected if no adhesive bond existed between the pins and the core material. On the other hand, a high fracture strength was expected when the coupling agents tested were capable of achieving an adhesive bond between the pins and the core material. The highest bonding strength was indicated when, upon stressing, the fracture occurred in the body of the core material or within the coupling agent and not at the interface. A typical fracture pattern was a vertical one through the THE
2
mode
OF PROSTHETIC
DENTISTRY
midplane that was represented by the first peak on the load-displacement curve. Further application of load caused fracture of the tooth substrate through the highly prestressed pinholes in the lateral plane, identified by a second peak that required an additional load of approximately 20% to 30% (Fig. 5). Because of the difference in the resiliency and elasticity of tooth dentin, perhaps the magnitude of stress concentration in the vicinity of the pinholes may be at variance with the analog teeth used in this study. Many investigators have reported that a selfthreading pin causes crazing in dentin and weakening of the tooth.3, 25,26 Since the study was intended to measure the fracture resistance of the cores, only the values represented by the first peak were presented. A higher load was also recorded when the fracture occurred outside the midplane (e.g., at the lateral lingual or lateral facial plane) or when the pin treatment was considered effective (Fig. 2). The presence of flaws, pores, and inclusions, which could not be avoided in condensing amalgam or packing composite resin, contributed to much of the variation in the results. The failure modes did not correlate well with the fracture load. Corrosion by water and mismatch of the coefficients of thermal expansion between metal pins and composite resin may create an interfacial flaw that could reduce the fracture resistance of the cores. A longer water immersion time than 7 days is probably needed to evaluate the effect of water corrosion. Since the core is totally encased in a metal 759
TJAN,
crown when the final restoration of the tooth is completed, the strength difference between composite resin and amalgam might not be of clinical significance. However, cores with high fracture strength clearly are desirable when allceramic crowns are indicated for the final restorations. Although this investigation was conducted on cores, the information obtained from this study is valid and useful and must be considered in the construction of any restoration using pins for retention., A greater improvement of fracture strengths was achieved by applying Panavia EX cement directly to gold-plated TMS pins rather than to tin-plated TMS pins. The reason for this contradictory result was not fully understood. It could be speculated that the electrodeposition apparatus used in this study was not effective for this purpose, and that the tin deposit was too thick or was not bonded properly to the gold-plated ‘TMS pins. In addition, excess tin particles may be deposited in the serration’s crevices (valleys), which reduces their depth and subsequently the mechanical interlocking of the restorative materials. To avoid problems related to tin plating, unplated stainless steel TMS pins should be acquired and used to evaluate the effect of Panavia cement coating on fracture strength. The findings of this study have indicated that Panavia cement coating was not effective in improving the fracture resistance of cores retained by titanium pins. This result may or may not be extrapolated to other nonprecious metal alloys. Variation in bond strength of Panavia EX cement to different types of amalgam has been reported in the literature.27 SUMMARY
AND
CONCLUSIONS
This study determined the relationship between pin treatment with Panavia EX cement or with Cover-Up II material (4-META) and the fracture resistance of pin-retained amalgam and composite resin cores. Two types of self-threading pins were used in this study-TMS goldplated stainless steel and Filpin titanium pins. Conclusions from the study were: 1. The use of pins significantly decreased the fracture resistance of both amalgam and composite resin cores. 2. All three treatments applied to TMS pins improved the fracture resistance of amalgam cores. 3. Only Cover-Up (4META) material improved the fracture resistance of amalgam cores retained by Filpin pins. 4. The fracture resistance of a composite core was substantially higher than that of an amalgam core. 5. TMS pins treated with Panavia EX cement yielded the highest fracture resistance in composite resin cores, followed by pins pretreated with Cover-Up II material. 6. Panavia EX cement pretreatment on tin-plated TMS pins failed to improve the fracture resistance of composite resin cores. 7. For Filpin pins, only Cover-Up material treatment improved the fracture resistance of composite resin cores. 760
DUNN,
AND
GRANT
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JUNE
1992
VOLUME
67
NUMBER
6
