Tensile

bond

strength

F. Hadavi, DMD, MS,a E. R. Ambrose, DDSd University

of Saskatchewan,

of repaired

J. H. Hey, College

DDS,

of Dentistry,

MD,b

amalgam

D. Czech,

Saskatoon,

DMD,c

Saskatchewan,

and

Canada

This study evaluated the tensile strength of repaired high-copper amalgams and analyzed the different treatments of the amalgam interface prior to repair. One hundred specimens were divided into 10 groups: group 1 was left intact and was considered as the control group. In groups 2 through 8, the specimens were sectioned into halves after 10 days and were reconstructed with new amalgam. Groups 9 and 10 were condensed with time intervals of 15 minutes and all specimens were subjected to tensile loads in a Universal Testing Machine. The tensile strengths at the junction between old and new amalgam ranged between 50% to 79% of those of the control group and verified that the same type of amalgam and uncontaminated interfaces had higher strengths. The results also suggested that if an amalgam repair is anticipated, additional retention is critical to the longevity of the restoration.(J PROSTHET DENT 1992;670313-7.)

D

entists frequently must consider replacing older amalgamrestorations becauseof fracture, secondary caries, or lost interproximal contact. The question ariseswhether select, well-retained sections of extensive restorations can be satisfactorily repaired or whether the entire restoration shouldbe removed without exception and replaced. The repair of a functional, intact section of an existing restoration has been accepted as a practical alternative procedure but requires sound judgment. However, to ensuresatisfactory results, a definitive technique shouldbe established. Various studies on the strength of repaired amalgamreported different results.‘-10Several investigators have studied the bond of repaired amalgamand most have reported that the bond strength of repaired amalgamwashalf or less than that of an intact specimen.lm6 However, Jorgensenand Saito7contended that the bond strength of repaired amalgamswasalmostidentical to that of the intact restorations. There are few studies regarding the tensile strength of repaired amalgam.s,g Kirks reported in 1962that the tensile strength of repaired conventional amalgam ranged from 23% to 98% of that of the controls. Consani et a1.g more recently investigated the tensile strength of repaired amalgam and concluded that the tensile strength of repaired specimenswas from 14% to 33% of the control group. This investigation determined the tensile strength of repaired high-copper amalgam,and evaluated the influence

aAssociate Professor, Department of Restorative and Prosthetic Dentistry. hClinical Instructor, Department of Restorative and Prosthetic Dentistry. CFormer Clinical Fellow. dProfessor, Department of Restorative and Prosthetic Dentistry.

10/l/32694 THE

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Fig.

1. Mold for making specimens.

of different treatments to repaired amalgamsurfaceson the tensile strength. METHOD

AND

MATERIAL

In this study two types of amalgamalloyswere examined: (1) precapsulated Amalcap nongamma- (Vivadent, Schaan, Liechtenstein) and (2) Amalcap F (Vivadent), a conventional alloy. (This conventional alloy wasonly used in experimental group 7.) The amalgamalloys were triturated in a Silamat (Vivadent) amalgamatorfollowing the manufacturer’s instructions. A two-piece Plexiglas mold (2 mm thick) connected by mortices was assembledin a supporting frame to avoid displacement(Fig. 1); it wassimilar to a mold usedby Rodriquez and Dicksonll and one used by Consaniet a1.g After trituration, amalgamwas hand-condensedwith a condenserhaving a smooth, round surface and a diameter of 1.5 mm, using a condensationpressureof 10 to 50 N per work surface, a pressuresimilar to a clinical force.i2 The mold was filled with one increment of amalgamat a time with pressureintervals of approximately 1 secondon each superimposedsurface. The test sampleswere condensed 313

HADAVI

Table

I.

Treatment

strategies

ET AL

for the 10 groups of

specimens Group No.

Treatment

1 Control group: 2 Fig.

2. Sample

in aligning

grips. 3

for an identical time by the same dentist while the mold was overfilled and excess amalgam was removed with a sharp hand instrument. A Universal testing machine (Frank Corp., Mainz, Germany), with a loading speed of 1 mm per minute and an accuracy of 0.1 N, was used to determine the tensile strength of the specimens. A special holder was designed to align the specimens when the testing loads were applied axially (Fig. 2). A total of 100 specimens were made and divided into 10 test groups. In the first eight groups, the specimens were left in the mold at room temperature for 1 hour and were then carefully removed. After 10 days of storage at room temperature, the specimens in groups 2 to 8 were fractured into halves. Before reconstruction with the new amalgam, the fractured surfaces of samples were subjected to different treatments to simulate clinical conditions. The samples in groups 9 and 10 were condensed in two stages with time intervals of 15 minutes to evaluate delayed condensation and saliva contamination. The specimens were classified according to treatments (Table I). The test samples were reinserted into the mold and new amalgam was condensed in the same manner. All repaired specimens were removed from the mold after 1 hour and were stored at room temperature for 10 days before the tests. The results were computed with the Student t test and the data were evaluated for significance at the 5% or 1% level.

RESULTS The results are summarized in Table II. The tensile strength in the various groups ranged from 50 % to 79 % of that of the control group, but the tensile strength of group 8 was not measured because the test samples fractured during removal from the mold. The tensile strength of all groups was significantly lower than the tensile strength of the control group (p < 0.05). Graphic representations are shown of the tensile strength required to fracture repaired amalgam (Figs. 3 and 4).

DISCUSSION Most studies on the durability of repaired amalgam evaluated the flexural or bond strength of the repaired amalgam. However, tensile properties are critical parameters for the strength of an amalgam restoration.ll Mahler13 concluded after analysis of the stresses in a distocclusal amalgam restoration that failures at the isthmus were

314

4

5

6

7

Intact test samples bur: Fractured surface roughened using a slow-speed diamond bur No. 123-014 (Phingst & Co., Inc., South Plainfield, N.J.) Abrasion with carbide bur: Fractured surface abraded using a high-speed carbide bur No. 170 (Phingst & Co., Inc.) Notches: Nine retentive notches without undercuts were prepared in abraded surface similar to group 3, described by CowanlO using an inverted cone bur No. 33 ‘/L Slope: On the fractured surface an inclination of 45 degrees convergent from top to bottom was made with a high-speed carbide bur No. 170 (Horico) Step: On the fractured surface, a step with depth of 1.4 mm and i width of 1.4 mm was made using a slow-speed diamond bur No. 123-014 (Horico)

Abrasion

with

diamond

Amalcap nongammaversus Amalcap F (conventional amalgam): The fractured surface of conventional amalgam

was treated as in group 2, before the repair with nongammaamalgam 8 Copalite contamination: On the abraded surface of group 3 a thin layer of Copalite (Cooley & Cooley Ltd., Houston, Texas) was applied before the repair 9 Aging: One half of the mold was filled with amalgam and after 15 minutes the other half was completed 10 Saliva contamination: One half of the mold was filled with amalgam; after 15 minutes the surface of the aged specimen was moistened with saliva and dried for a short period with a cotton pellet before the repair

caused by tensile stresses. Although masticatory forces on an extensive restoration are a complex combination of different stresses, a fracture in amalgam commonly occurs at the weakest point, such as an area of impurity, porosity, and at macrocracks or microcracks.14 Jorgensen15 reported a tensile strength of 51 N/mm2 for Amalcap nongammaalloys without describing the test method. Vrijhoef et a1.16 reported an average tensile strength of 37 N/mm2 using the diametral tensile test method. Vrijhoef et al. l6 also stated that during mastication tensile stresses in the isthmus area can lead to gross fracture, but they believed that diametral tensile strengths higher than 35 N/mm2 were probably sufficient to avoid bulk fractures. In this investigation, the tensile strength of the repaired test samples did not reach this value, but the values for groups 2 and 3 were in the same range. Asgar et a1.17 demonstrated that when one uses a diametral test method to determine the tensile strength, amalgam has a tendency to flow under load, resulting in a gradual change from a line contact to an area contact. They concluded that the greater the creep of the material, the higher the diametral tensile strength. Walker and Reese2 and Terkla et al.’ discovered that

MARCH

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AMALGAM

45 40 35 30 25 N/mm*

20

15 10 5 12

3 Fig.

Table

II.

Results of tensile strength

3. Tensile

4 5 Groups strength

6

7

8

9

IO

of samples.

testing

Groups*

values (N/mm’) S.D. Mean

1

2

42.23

33.36

0.51

5%

100

*Ten specimens per group. tAl1 samples contaminated $Tensile strength of group ITensile strength of group lpensile strength of group TTensile strength of group

2.4 79

3

5

30.71

26.39

29.72

27.30

26.21

1.45

1.68

2.25

1.42

1.51

73

62

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70

65

7/l

62

9

w

31.1 1.86 74

loll

-

16.92 0.92 40

with Copalite were fractured during removal from the mold. 4 was significantly lower than tensile strength of group 2 (p < 0.05). 6 was significantly lower than tensile strength of group 2 @ < 0.05). 7 was significantly lower than tensile strength of group 2 (p < 0.05). 10 was highly significantly lower than tensile strength of group 9 (p < 0.01).

roughening of the surface improved the bond strength. Groups 2 and 3 recorded the highest tensile strength compared with the control group, and this confirmed their findings. The increased strength of the repair associated with samples roughened from a bur enhanced the mechanical retentions. The diminished strength of all the repaired samples was an indication for special retention when one is repairing amalgam restorations. Samples in group 4 surprisingly displayed almost the lowest tensile strength compared with the first six groups. The fractures were observed along the margins and pervading the retentive notches. This was probably a result of the difficulty of condensing the fresh amalgam into the notched surfaces. Gordon et a1.5 believed that an elevated flexural strength of the repaired amalgam was related to an increased surface area, and that this enhanced the mechanical retention. In this study the increased surface area (group 5) did not significantly improve the tensile strength (70 % of the control group), and this result was not significantly different from that in group 3 (p > 0.05). Terkla et al.’ hypothesized that the lower strength of a

THE

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4$

PROSTHETIC

DENTISTRY

repaired amalgam was related to the lack of crystalline growth by the old amalgam. They found that the bond strength of conventional amalgam repaired at 15 minutes is approximately 50% stronger than that of conventional amalgam repaired at 7 days. Scott and Grisiu@ reported similar results after testing the bond strength of spherical alloy. Brown et a1.3 advised repair as soon as possible to ensure a greater flexural strength. However, with tensile strength testing in the present study there was no significant difference between repaired amalgam whether 15 minutes or 10 days had elapsed, and these findings were supported by Walker and Reese2 and by Jorgensen and Saito.7 However, it was difficult to condense fresh amalgam against the 15-minute set amalgam. After 15 minutes the amalgam was not completely hardened, therefore less condensing pressure was used to prevent damage to the repaired surface, and it is uncertain how this influenced the results. Cowan1o reported in a clinical study that there was no evidence of corrosion when nongammahigh-copper amalgam was bonded to a conventional alloy containing gamma-2. It is possible to repair conventional amalgam with

315

HADAVI

ET AL

100 90 -80 -. 70 1. 60 -1

x

50 9. 40 -= 30 -. 20 -10 -* 0-12

3

4

5

6

7

8

9

lo

Groups Fig. 4. Percentages of tensile strength.

high-copper amalgam (group 7), but the tensile strength of the repaired specimen was significantly less than that of a repair with the same high-copper alloy (group 2) (p < 0.05). There was a significant difference between samples contaminated with saliva and uncontaminated samples after 15 minutes. The tensile strength decreased by 50% after contamination, and this finding agreed with Consani et a1.g Jorgensen and Saito7 also reported a decreased bond strength of approximately 67 % if the repaired surface was contaminated with saliva. Copalite (Cooley & Cooley Ltd., Houston, Texas) application caused a complete failure between the bond of two amalgams. All the contaminated repaired specimens fractured during removal from the mold, and contamination of any repaired surface was detrimental to the tensile strength and should thus be avoided. Investigators1,6, lo have suggested that wetting the old amalgam with mercury increased the bond strength. Cowanlo proposed a clinical technique of wetting the surface of the old amalgam with a mixture of mercury-rich amalgam, but did not evaluate the physical properties of the amalgam. These procedures do not encourage appropriate clinical hygiene for mercury, while they may adversely affect the physical properties of the amalgam. Therefore this procedure was not considered in the present investigation. SUMMARY To determine the strength of repaired amalgam a tensile strength test was developed, and the effects of several factors on the repaired amalgam were investigated. The following conclusions were drawn: 1. The mean tensile strength for intact Amalcap nongamma-2 amalgam was 42.23 N/mm.2

316

2. Fractures in the repaired amalgams always occurred at the junction between old and new amalgam. 3. The tensile strength at the junction between old and new amalgam ranged between 50% to 79% of that of the control group. 4. Microretention and macroretention in the fractured surface had only a negligible influence on the tensile strength. 5. Time was not an appreciable factor affecting the repaired amalgam. 6. The adhesion of nongamma- amalgam to conventional amalgam was significantly less. 7. Saliva contamination decreased the adhesion by approximately 50%. 8. Copalite completely prevented adhesion to the new amalgam. CONCLUSIONS The repaired amalgam exhibited a reduced tensile strength when compared with intact restorations. The tensile strength of repaired amalgam was greater when the same type of amalgam was used and the interfaces were uncontaminated. When an amalgam repair is anticipated, precise mechanical retention must be prepared in the tooth and in the remaining amalgam restoration to complement the union between new and old amalgam alloys. Where the amalgam repair is in functional occlusion, the additional retention is critical to the longevity of the restoration. REFERENCES 1. Terkla LG, Mahler DB, Mitchem JC. Bond strength gam. J PROSTHET DENT 1961;11:942-7. 2. Walker AC, Reese SB. Bond strength of amalgam high-copper amalgam. Oper Dent 1982;8:99-102.

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amal-

to amalgam

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NUMBER

in a

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STRENGTH

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AMALGAM

3. Brown KB, Molvar MP, Demarest VA, Hasegawa TK, Heinecke PN. Flexural strength of repaired high-copper amalgam. Oper Dent 1986; 11:131-5. 4. Berge M. Flexural strength of joined and intact amalgam. Acta Odontal Stand 1982;40:313-7. 5. Gordon M, Ben-Amar A, Librus S, Liberman R. Bond strength of mechanically condensed repaired high-copper amalgam. Quintessence Int 1987;18:471-4. 6. Scott GL, Grisius RJ. Bond strength at the interface of new and old spherical amalgam. US Navy Med News Lett 1969;54:34. 7. Jorgensen KD, Saito T. Bond strength of repaired amalgam. Acta Odontal Stand 1968;26:605-15. 8. Kirk EEJ. Amalgam to amalgam bond: a preliminary report. Dent Practioner 1962;12:371-2. 9. Consani S, Ruhnke LA, Stolf WL. Infiltration of a radioactive solution into jointed silver-amalgam. J PROSTHET DENT 1977;37:158-63. 10. Cowan RD. Amalgam repair-a clinical technique. J PROSTHET DENT 1983;49:49-51. 11. Rodriguez MS, Dickson G. Some tensile properties

of amalgam. J Dent

Res 1962;41:840-52.

Tooth contacts morphology Bengt

Ingervall,

Bruno

Stettler,

University

of Bern,

in eccentric DDS,

Odont

DDS, Bern,

Dr

Med

Dr,8

Daniel

12. Jorgensen KD. Amalgame in der Zahnheilkunde. Munchen-Wien: Hanser-Verlag, 1977. 13. Mahler DB. An analysis of stresses in a dental amalgam restoration. J Dent Res 1958;37:516-26. 14. Asgar K, Sutfin L. Brittle fracture of dental amalgam. J Dent Res 1965;44:977-88. 15. Jorgensen KD. Recent developments in alloys for dental amalgams: their properties and proper use. Int Dent J 1976;26:369-77. 16. Vrijhoef MMA, Vermeersch AG, Spanauf AJ. Diametral tensile strength of twenty-three hardened commercial amalgams. J Oral Rehabil 1979;6:153-7. 17. Asgar K, Arfaei AH, Mahler DB. Evaluation of amalgam tensile test methods [Abstract]. J Dent Res 1977;56:77. Reprint

requests to:

DR. FARHAD HADAVI COLLEGE OF DENTISTRY UNIVERSITY OF SASKATCHEWAN SASKATOON S7N OWO CANADA

mandibular Meyer,

DDS,

positions Dr

Med

Dent,b

and facial

and

Dentb

Switzerland

Correlations between facial morphology and tooth contacts in excursive mandibular positions were studied in ‘75 men aged 20 to 33 years. The morphology of the dentition was verified on dental casts and the face was measured by use of roentgen cephalometry. No correlation was observed between facial morphology and the number of tooth contacts in the retruded position; however, wide dental arches and jaws displayed many contacts on protrusion. Numerous contacts on the functional side in group function were noted in individuals with a facial morphology associated with distal occlusion, such as Angle class II, division 1. Wide dental arches were associated with multiple functional-side contacts whereas tooth contacts on the nonfunctional side were related to the inclination of the mandible. A “long-face” morphology was related to contacts on the nonfunctional side. There was a negative correlation between the extent of the overbite (vertical overlap) and the number of tooth contacts on the nonfunctional side. (J PROSTHET DENT 1992;07:317-22.)

D

uring lateral excursions of the mandible or laterotrusion, two types of tooth contact are evident on the functional or working side: group function and canine protection. Group function is defined as contacts between two or more pairs of opposing teeth on the working side,le3 whereas a canine-protected occlusion implies contact only between the canines on the working side.*T5 During laterotrusion, some individuals have contacts between opposing aProfessor and Chairman, bPrivate practice. 1011132710

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of Orthodontics.

DENTISTRY

teeth on the nonfunctional or balancing side. However, the tooth contacts during retrusion and protrusion of the mandible vary widely between young adults.6 Information is sparse on associations between patterns of tooth contact and facial morphology. Madone and Ingerval17 reported a negative correlation between group function and the width of the maxillary dental arch but a positive correlation with the sagittal jaw relation in young adults after orthodontic treatment. Canine protection was positively correlated with the width of the maxillae and molar relation but negatively with the sagittal jaw relation and with the inclination of the mandibular incisors. Group

317

Tensile bond strength of repaired amalgam.

This study evaluated the tensile strength of repaired high-copper amalgams and analyzed the different treatments of the amalgam interface prior to rep...
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