3 22
Australian Dental Journal, August, 1976 Volume 21, No. 4
A simulated clinical appraisal of base materials for amalgam restorations* Roland W. Bryant, M.D.S.(Syd.) Lecturer, Department of Operative Dentistry, University o f Sydney AND
George Wing, M.D.S., D.D.Sc.(Syd.), F.A.C.D., F.R.A.C.D.S. Associate Professor, Department of Operative Dentistry, University of Sydney
ABSTRACT-Eight materials were placed as bases on the pulpal surfaces of cavities prepared in extracted teeth. Seven minutes after mixing of the material commenced, amalgam was condensed on to the lined cavity. The restored teeth were sectioned subsequently for microscopic examination. The results suggested that all uniformly mixed, carefully placed base forming materials that have reached the state of “clinical set” are able to withstand loads applied during amalgam condensation irrespective of their strength. (Received for publication February, 1975)
lntroduction
In the light of current knowledge of the strength properties of materials commonly used as bases for amalgam restorations, it is necessary to assess whether or not materials at different strength levels possess adequate strength to withstand the condensation forces applied during the placement of amalgam. Present knowledgels 2,394 indicates that accelerated zinc oxide-eugenol materials and
*
This research was supported by the University of Sydney Research Grant.
~
1 Messing J. J.-A polystyrene-fortified zinc oxide-eugenol c e m e k Brit. D . J., 110-3, 95-100 (Feb.) 1961. 2Colcman. J. M . . and Kirk, E. E. J.-An assessment of a modified zinc oxide-eugenol cement. Brit. D. J., 118:11, 482-487 (June) 1965. 3 Chong. W. F.. Swartz. M. L., and Phillips. R. W.Displacement of cement bases by amalgam condensaction. J.A.D.A.. 74:l. 97-102 (Jan.) 1967. 4 Bryant. R. W. and Wing, G.-The rate of development of strength in base forming materials for dental amalgam. Austral. D. J . , in press.
quick-setting calcium hydroxide cements possess significantly lower strength values, a t the time of clinical importance, than those exhibited by zinc phosphate cements. The aim of this investigation was to determine whether materials possessing significantly lower compressive and tensile strengths than zinc phosphate cements were sufficiently strong to withstand the packing pressures during amalgam condensation. Experimental Method To investigate the possibility of either fracture or displacement of a base forming material occurring during amalgam condensation, cavities were prepared in extracted teeth and restored with amalgam after the placement of different base forming materials. Following sectioning, the restored teeth were examined microscopically. Class I, Class I1 and Class V cavities were prepared in extracted teeth to a depth considered
323
Australian Dental Journal, August, 1976 TABLE 1
Base forming materials investigated and proportions used for clinical consistency Category
Material
Clinical consistency gm powder/0.5 ml liquid
Manufacturer and/or Distributor
I
S.S. White Zinc Cement
2.30
IIA
Unmodified zinc oxideeugenol
2.50
IIB
Zinc oxide (1% zinc acetate)-eugenol Kalzinol Caulk ZOE B & T Opotow EBA Cement
2.25
Non commercial product
2.70 1.65
A.D. International Ltd., London L. D. Caulk Co., Milford, Del., Toronto, Ont.
2.50
-tow
S. S. White Dental Mfg. CO. (G.B.) Ltd,
Improved
London
Dental Mfg. Corp., Brooklyn, N.Y.
.
111 Dycal
Hydrex
Equal lengths of base and catalyst
L. D. Caulk Co., Milford, Del., U.S.A.
Equal lengths of base and catalyst
Kerr Mfg. Co., Detroit, Michigan
greater than ideal, both on the axial and pulpal walls. These cavities were then restored. Base forming material was placed on both the axial and pulpal walls to provide a cavity of approximately ideal depth into which amalgam was to be condensed. Eight base forming materials were tested and included materials from the three categories, zinc phosphate, zinc oxide-eugenol and calcium hydroxide cements (Table 1). With the exception of unmodified zinc oxide-eugenol, each material has been found'-5 to reach a state of clinical set at a time which would suggest its suitability for use as a base under an amalgam restoration. Excess cement was removed and a mechanical matrix applied. A load of approximately three kilograms was used to condense amalgam into the cavity using a 1 5mm condenser. Condensation of amalgam was commenced seven minutes after mixing of the base forming material and was carried out in a manner similar to that which would be used in a clinical situation. After completion of the packing of the restoration, the tooth was placed in distilled water for a period of at least one week to ensure setting of both the base forming material and the amalgam. Before sectioning for microscopic examination, the restored teeth were mounted in cavities prepared in lucite rod, using small amounts of autopolymerising resin. The teeth were positioned in such a way as t o allow investigation of the significant areas of the base and amalgam. The
teeth were positioned, so that after sectioning, the axial and/or pulpal walls of the cavities could be observed. Because of the considerable differences in the hardness of the materials under investigation, namely, tooth structure, base forming material, and amalgam, it was necessary to use a sectioning technique which would cut each of the materials at approximately the same rate without a tendency for flow or smearing to occur. The method employed was the metallographic polishing technique developed by Samuelse. 7. Silicon carbide abrasive papers in three grades, 220, 400 and 600, were used with a constant stream of running water. Following the silicon carbide papers, use was made of an alumina-wax lap using light hand pressure to produce a flat specimen. Finally the specimens were ground on a slowly revolving cloth-covered wheel, charged in turn with two grades of diamond paste. The diamond pastes used were in the grades of 4-8am and 0-lam. After the completion of specimen preparation, microscopic examination was carried out at magnifications ranging from IOX to SOX, using reflected light on a Zeiss Neophot I1 Metallographic microscope*. Photographic records at magnifications of 12.5X were made and these were examined for evidence
*
Carl Zeiss Jena, East Germany.
E.-The use of diamond abrasives for a universal system of metallographic polishing. J. Inst. Metals, 81. 471478, 1952-1953. 7 Samucls. L. E.-Metallographic polishing by mechanical methods. Melbourne and London, Pitman and Sons. 1st ed.. 1967.
6 Samucls. L.
6
Bryant, R. W.. and Wing. G.-TI!@ effects of manipulative variables on base forming materials for amalgam restorations. Austral. D. J.. in press,
324
Australian Dental Journal, August, 1976
of fracture or displacement of the base forming material. Results Zinc phosphate cements A typical situation of amalgam packed over zinc phosphate cement is shown in Fig. I*. In general there was no evidence of displacement of the base
Fig. I.-Amalgam
with base of S.S. White Zinc Cement Improved.
Fig. 3.-Amalgam
with base of set, unaccelerated zinc oxide-eugenol.
this specimen because of the very obvious displacement that was occurring. In contrast with the unset material, unaccelerated zinc oxide-eugenol, when allowed to set fully (one week) before condensation of the amalgam, possessed adequate strength to resist displacement (Fig. 3).
Fig. 2.--Unsatisfactory restoration placed with base of unset, unaccelerated zinc oxide-eugenol.
Fig. 4.-Amalgam
with base of zinc oxide (1 per cent zinc acetate)-eugenol.
forming material, although at times very slight fractures of the base were detected as shown in this illustration on the axial wall. Almost without exception, this type of base forming material appeared to remain relatively intact during amalgam condensation. Unmodified zinc oxide-eugenol cement Gross displacement of the base material resulted when an attempt was made to pack amalgam on to unset, unaccelerated, zinc oxide-eugenol as seen in Fig. 2. The cement was grossly incorporated within the amalgam and also extruded from the cavity at the margins. In two places, seen in Fig. 2, it was observed that amalgam had penetrated the lining and was in direct contact with the dentine in spite of the fact that only a very light condensation load had been used for ______
*
~~
AU illustrations show restorations placed teeth. Original magnification X 12.5.
in extracted
Fig. 5.-Amalgam
with base of Knlzinol.
Modified zinc oxide-eugenol materials Figures 4 and 5 show amalgam condensed onto two typical zinc oxide-eugenol materials. There was no evidence of fracture or displacement of the base. The occasional presence of large voids in the base (Fig. 4) apparently did not weaken the material sufficiently to enable condensation
3 25
Australian Dental Journal, August. 1976 forces to crush the unsupported material. The commercial zinc oxide-eugenol material, Kalzinol, shown in Fig. 5 allowed good condensation of the amalgam with no evidence of fracture or displacement of the base. Opotow EBA Cement also appeared to allow good condensation of amalgam as seen in Fig. 6.
over base forming materials which are weak in compressive strength and contain air bubbles Some evidence of fracture and crushing has been observed in such situations. Because, in general, this is localized to the area of the void, it may not have an effect on the satisfactory performance of the material as a base.
--
~~
Fig. 6.-Amalgam
with base of Opotor EBA Cement.
Fig. 7.-Amalgam
with base of Hydrex.
c
Fig. E.-Amalgam with base of a thinly applied, relatively weak calcium hydroxide material with internal voids.
Fig. 9.-Amzlgzm with base of a calcium hydroxide material. Note evidence of crushing.
The irregular contours of the base possibly resulted from flow during condensation of the amalgam. It was considered more likely however, that this was attributable to the virtual lack of trimming of the base prior to amalgam condensation. No trimming of the Opotow material was possible in the in v i m studies because of the material’s slow setting rate and “tacky” nature, when tested out of the mouths. Calcium, hydroxide cements Of all the materials examined for the development of strength, calcium hydroxide cements possessed the lowest compressive and tensile strengths‘. It was therefore somewhat surprising to find that Hydrex (Fig. 7) appeared to be one of the base forming materials showing least tendency to be fractured or displaced during the condensation of amalgam. Influence of internal voids Figures 8 and 9 illustrate amalgam condensed
The nature of materials such as Hydrex and Dycal and their method of placement is such that they should be used in thin layers only. If placed in an attempt to build up large deficiencies to provide thermal protection in extremely deep cavities, the material may exhibit layering and the presence of voids and bubbles between successive layers. A summary of the results of this investigation following the placement of amalgam restorations is shown in Table 2. Conclusions and Discussion The strength of a base forming material has been used frequently as an indication of its ability to resist fracture or displacement. Displacement of the base. resulting from amalgam condensation, may: (1) Result in contact between amalgam and dentine, thereby negating the insulating
3 26
Australian Dental Journal, August, 1976 TABLE 2
Assessment of base forming materials for clinical usage based on the microscopic examination ~ e s u ~ tof s microscopic examination
Category of base forming material
Seven ,,,inUtes kg/cm2
600
I. Zinc phosphate IIA. Unmodified zinc oxide-eugenol i. Unset ii. Set
Displacement ot base
Voids in and around base
Overall assessment of the strength Of the base
t
Nil
t
Adequate
Nil
ttt
***
t
Nil
t
Not adequate Adequate
t
tt t tt
~~~~t~~~of base
**
~
IIB. Modified zinc oxide-eugenol i. Non commercial ii. Commercial
315 150-350
Nil
Nil Nil
60
t
Nil
111. Calcium hvdroxide
* Approximate' ** Taka between five and *** Not possible to assess t Mild t t Moderate
ttt
Adequate Adequate Adeauate
48 hours lo set
scvera
effect of the base and giving rise to the possibility of post operative hypersensitivity to thermal stimuli. (ii) Lead to eventual marginal failure of the restoration. The extrusion of material at the margin of the cavity and its subsequent loss by washing out or dissolution may provide a site for recurrent caries. Fracture of the base is unlikely t o affect the restoration. In the absence of large voids and in the presence of a reasonable thickness of base, it is unlikely that forces exerted through the amalgam during condensation are sufficient to crush a cement lining. Unfortunately the majority of base forming materials are unable to be mixed without the incorporation of some air, producing internal voids. Fracture and subsequent crumbling of the material around a large void, may then render the base liable to failure for the reasons given above with reference to displacement of a base. Traditionally, zinc phosphate cements have been widely acknowledged as clinically satisfactory base forming materials, at least from the point of view of their resistance t o fracture and displacement, It is likely that slight fractures which may occur in these materials in the presence of small voids, do not constitute a threat to the satisfactory functioning of the lining. Displacement of the base on the other hand, may seriously reduce the effectiveness of a base forming material. On the basis of this simulated clinical investigation, it can be concluded that clinical consistencies of zinc phosphate cements, unmodified but set
zinc oxide-eugenol cement, modified zinc oxideeugenol cements and calcium hydroxide cements all possess sufficient early strength to be used successfully beneath amalgam restorations. The weakest of the materials tested has been found4 to possess seven minute compressive and tensile strengths of 60 kg/cml and 15 kg/cmz, respectively. That this material has been shown to possess strength adequate to withstand the placement of amalgam is consistent with the findings of Chong, Swartz, and Phillips3 who established that base forming materials require a compressive strength of only 7-12 kg/cm2. Unaccelerated zinc oxide-eugenol cement is of little use in the clinical situation, as a base for the immediate placement of amalgam, when unset. If allowed to set, as occurs when this material is used as a temporary restoration and then subsequently cut back to be left as a base, unaccelerated zinc oxide-eugenol cement is almost certainly an adequate base forming material to withstand the forces of condensation of amalgam. Irrespective of the strength values, the results of this investigation suggest that all base forming materials, which are clinically set at the time of amalgam condensation and have been placed so as to minimise the presence of large voids, possess adequate strength to withstand the condensation pressures during the placement of amalgam. Department of Operative Dentistry, University of Sydney, 2 Chalmers Street, Surry Hills, N.S.W. 2010. Australia.
Australian Dental Journal, August, 1976 Volume 21, No. 4
A simulated clinical appraisal of base materials for amalgam restorations* Roland W. Bryant, M.D.S.(Syd.) Lecturer, Department of Operative Dentistry, University o f Sydney AND
George Wing, M.D.S., D.D.Sc.(Syd.), F.A.C.D., F.R.A.C.D.S. Associate Professor, Department of Operative Dentistry, University of Sydney
ABSTRACT-Eight materials were placed as bases on the pulpal surfaces of cavities prepared in extracted teeth. Seven minutes after mixing of the material commenced, amalgam was condensed on to the lined cavity. The restored teeth were sectioned subsequently for microscopic examination. The results suggested that all uniformly mixed, carefully placed base forming materials that have reached the state of “clinical set” are able to withstand loads applied during amalgam condensation irrespective of their strength. (Received for publication February, 1975)
lntroduction
In the light of current knowledge of the strength properties of materials commonly used as bases for amalgam restorations, it is necessary to assess whether or not materials at different strength levels possess adequate strength to withstand the condensation forces applied during the placement of amalgam. Present knowledgels 2,394 indicates that accelerated zinc oxide-eugenol materials and
*
This research was supported by the University of Sydney Research Grant.
~
1 Messing J. J.-A polystyrene-fortified zinc oxide-eugenol c e m e k Brit. D . J., 110-3, 95-100 (Feb.) 1961. 2Colcman. J. M . . and Kirk, E. E. J.-An assessment of a modified zinc oxide-eugenol cement. Brit. D. J., 118:11, 482-487 (June) 1965. 3 Chong. W. F.. Swartz. M. L., and Phillips. R. W.Displacement of cement bases by amalgam condensaction. J.A.D.A.. 74:l. 97-102 (Jan.) 1967. 4 Bryant. R. W. and Wing, G.-The rate of development of strength in base forming materials for dental amalgam. Austral. D. J . , in press.
quick-setting calcium hydroxide cements possess significantly lower strength values, a t the time of clinical importance, than those exhibited by zinc phosphate cements. The aim of this investigation was to determine whether materials possessing significantly lower compressive and tensile strengths than zinc phosphate cements were sufficiently strong to withstand the packing pressures during amalgam condensation. Experimental Method To investigate the possibility of either fracture or displacement of a base forming material occurring during amalgam condensation, cavities were prepared in extracted teeth and restored with amalgam after the placement of different base forming materials. Following sectioning, the restored teeth were examined microscopically. Class I, Class I1 and Class V cavities were prepared in extracted teeth to a depth considered
323
Australian Dental Journal, August, 1976 TABLE 1
Base forming materials investigated and proportions used for clinical consistency Category
Material
Clinical consistency gm powder/0.5 ml liquid
Manufacturer and/or Distributor
I
S.S. White Zinc Cement
2.30
IIA
Unmodified zinc oxideeugenol
2.50
IIB
Zinc oxide (1% zinc acetate)-eugenol Kalzinol Caulk ZOE B & T Opotow EBA Cement
2.25
Non commercial product
2.70 1.65
A.D. International Ltd., London L. D. Caulk Co., Milford, Del., Toronto, Ont.
2.50
-tow
S. S. White Dental Mfg. CO. (G.B.) Ltd,
Improved
London
Dental Mfg. Corp., Brooklyn, N.Y.
.
111 Dycal
Hydrex
Equal lengths of base and catalyst
L. D. Caulk Co., Milford, Del., U.S.A.
Equal lengths of base and catalyst
Kerr Mfg. Co., Detroit, Michigan
greater than ideal, both on the axial and pulpal walls. These cavities were then restored. Base forming material was placed on both the axial and pulpal walls to provide a cavity of approximately ideal depth into which amalgam was to be condensed. Eight base forming materials were tested and included materials from the three categories, zinc phosphate, zinc oxide-eugenol and calcium hydroxide cements (Table 1). With the exception of unmodified zinc oxide-eugenol, each material has been found'-5 to reach a state of clinical set at a time which would suggest its suitability for use as a base under an amalgam restoration. Excess cement was removed and a mechanical matrix applied. A load of approximately three kilograms was used to condense amalgam into the cavity using a 1 5mm condenser. Condensation of amalgam was commenced seven minutes after mixing of the base forming material and was carried out in a manner similar to that which would be used in a clinical situation. After completion of the packing of the restoration, the tooth was placed in distilled water for a period of at least one week to ensure setting of both the base forming material and the amalgam. Before sectioning for microscopic examination, the restored teeth were mounted in cavities prepared in lucite rod, using small amounts of autopolymerising resin. The teeth were positioned in such a way as t o allow investigation of the significant areas of the base and amalgam. The
teeth were positioned, so that after sectioning, the axial and/or pulpal walls of the cavities could be observed. Because of the considerable differences in the hardness of the materials under investigation, namely, tooth structure, base forming material, and amalgam, it was necessary to use a sectioning technique which would cut each of the materials at approximately the same rate without a tendency for flow or smearing to occur. The method employed was the metallographic polishing technique developed by Samuelse. 7. Silicon carbide abrasive papers in three grades, 220, 400 and 600, were used with a constant stream of running water. Following the silicon carbide papers, use was made of an alumina-wax lap using light hand pressure to produce a flat specimen. Finally the specimens were ground on a slowly revolving cloth-covered wheel, charged in turn with two grades of diamond paste. The diamond pastes used were in the grades of 4-8am and 0-lam. After the completion of specimen preparation, microscopic examination was carried out at magnifications ranging from IOX to SOX, using reflected light on a Zeiss Neophot I1 Metallographic microscope*. Photographic records at magnifications of 12.5X were made and these were examined for evidence
*
Carl Zeiss Jena, East Germany.
E.-The use of diamond abrasives for a universal system of metallographic polishing. J. Inst. Metals, 81. 471478, 1952-1953. 7 Samucls. L. E.-Metallographic polishing by mechanical methods. Melbourne and London, Pitman and Sons. 1st ed.. 1967.
6 Samucls. L.
6
Bryant, R. W.. and Wing. G.-TI!@ effects of manipulative variables on base forming materials for amalgam restorations. Austral. D. J.. in press,
324
Australian Dental Journal, August, 1976
of fracture or displacement of the base forming material. Results Zinc phosphate cements A typical situation of amalgam packed over zinc phosphate cement is shown in Fig. I*. In general there was no evidence of displacement of the base
Fig. I.-Amalgam
with base of S.S. White Zinc Cement Improved.
Fig. 3.-Amalgam
with base of set, unaccelerated zinc oxide-eugenol.
this specimen because of the very obvious displacement that was occurring. In contrast with the unset material, unaccelerated zinc oxide-eugenol, when allowed to set fully (one week) before condensation of the amalgam, possessed adequate strength to resist displacement (Fig. 3).
Fig. 2.--Unsatisfactory restoration placed with base of unset, unaccelerated zinc oxide-eugenol.
Fig. 4.-Amalgam
with base of zinc oxide (1 per cent zinc acetate)-eugenol.
forming material, although at times very slight fractures of the base were detected as shown in this illustration on the axial wall. Almost without exception, this type of base forming material appeared to remain relatively intact during amalgam condensation. Unmodified zinc oxide-eugenol cement Gross displacement of the base material resulted when an attempt was made to pack amalgam on to unset, unaccelerated, zinc oxide-eugenol as seen in Fig. 2. The cement was grossly incorporated within the amalgam and also extruded from the cavity at the margins. In two places, seen in Fig. 2, it was observed that amalgam had penetrated the lining and was in direct contact with the dentine in spite of the fact that only a very light condensation load had been used for ______
*
~~
AU illustrations show restorations placed teeth. Original magnification X 12.5.
in extracted
Fig. 5.-Amalgam
with base of Knlzinol.
Modified zinc oxide-eugenol materials Figures 4 and 5 show amalgam condensed onto two typical zinc oxide-eugenol materials. There was no evidence of fracture or displacement of the base. The occasional presence of large voids in the base (Fig. 4) apparently did not weaken the material sufficiently to enable condensation
3 25
Australian Dental Journal, August. 1976 forces to crush the unsupported material. The commercial zinc oxide-eugenol material, Kalzinol, shown in Fig. 5 allowed good condensation of the amalgam with no evidence of fracture or displacement of the base. Opotow EBA Cement also appeared to allow good condensation of amalgam as seen in Fig. 6.
over base forming materials which are weak in compressive strength and contain air bubbles Some evidence of fracture and crushing has been observed in such situations. Because, in general, this is localized to the area of the void, it may not have an effect on the satisfactory performance of the material as a base.
--
~~
Fig. 6.-Amalgam
with base of Opotor EBA Cement.
Fig. 7.-Amalgam
with base of Hydrex.
c
Fig. E.-Amalgam with base of a thinly applied, relatively weak calcium hydroxide material with internal voids.
Fig. 9.-Amzlgzm with base of a calcium hydroxide material. Note evidence of crushing.
The irregular contours of the base possibly resulted from flow during condensation of the amalgam. It was considered more likely however, that this was attributable to the virtual lack of trimming of the base prior to amalgam condensation. No trimming of the Opotow material was possible in the in v i m studies because of the material’s slow setting rate and “tacky” nature, when tested out of the mouths. Calcium, hydroxide cements Of all the materials examined for the development of strength, calcium hydroxide cements possessed the lowest compressive and tensile strengths‘. It was therefore somewhat surprising to find that Hydrex (Fig. 7) appeared to be one of the base forming materials showing least tendency to be fractured or displaced during the condensation of amalgam. Influence of internal voids Figures 8 and 9 illustrate amalgam condensed
The nature of materials such as Hydrex and Dycal and their method of placement is such that they should be used in thin layers only. If placed in an attempt to build up large deficiencies to provide thermal protection in extremely deep cavities, the material may exhibit layering and the presence of voids and bubbles between successive layers. A summary of the results of this investigation following the placement of amalgam restorations is shown in Table 2. Conclusions and Discussion The strength of a base forming material has been used frequently as an indication of its ability to resist fracture or displacement. Displacement of the base. resulting from amalgam condensation, may: (1) Result in contact between amalgam and dentine, thereby negating the insulating
3 26
Australian Dental Journal, August, 1976 TABLE 2
Assessment of base forming materials for clinical usage based on the microscopic examination ~ e s u ~ tof s microscopic examination
Category of base forming material
Seven ,,,inUtes kg/cm2
600
I. Zinc phosphate IIA. Unmodified zinc oxide-eugenol i. Unset ii. Set
Displacement ot base
Voids in and around base
Overall assessment of the strength Of the base
t
Nil
t
Adequate
Nil
ttt
***
t
Nil
t
Not adequate Adequate
t
tt t tt
~~~~t~~~of base
**
~
IIB. Modified zinc oxide-eugenol i. Non commercial ii. Commercial
315 150-350
Nil
Nil Nil
60
t
Nil
111. Calcium hvdroxide
* Approximate' ** Taka between five and *** Not possible to assess t Mild t t Moderate
ttt
Adequate Adequate Adeauate
48 hours lo set
scvera
effect of the base and giving rise to the possibility of post operative hypersensitivity to thermal stimuli. (ii) Lead to eventual marginal failure of the restoration. The extrusion of material at the margin of the cavity and its subsequent loss by washing out or dissolution may provide a site for recurrent caries. Fracture of the base is unlikely t o affect the restoration. In the absence of large voids and in the presence of a reasonable thickness of base, it is unlikely that forces exerted through the amalgam during condensation are sufficient to crush a cement lining. Unfortunately the majority of base forming materials are unable to be mixed without the incorporation of some air, producing internal voids. Fracture and subsequent crumbling of the material around a large void, may then render the base liable to failure for the reasons given above with reference to displacement of a base. Traditionally, zinc phosphate cements have been widely acknowledged as clinically satisfactory base forming materials, at least from the point of view of their resistance t o fracture and displacement, It is likely that slight fractures which may occur in these materials in the presence of small voids, do not constitute a threat to the satisfactory functioning of the lining. Displacement of the base on the other hand, may seriously reduce the effectiveness of a base forming material. On the basis of this simulated clinical investigation, it can be concluded that clinical consistencies of zinc phosphate cements, unmodified but set
zinc oxide-eugenol cement, modified zinc oxideeugenol cements and calcium hydroxide cements all possess sufficient early strength to be used successfully beneath amalgam restorations. The weakest of the materials tested has been found4 to possess seven minute compressive and tensile strengths of 60 kg/cml and 15 kg/cmz, respectively. That this material has been shown to possess strength adequate to withstand the placement of amalgam is consistent with the findings of Chong, Swartz, and Phillips3 who established that base forming materials require a compressive strength of only 7-12 kg/cm2. Unaccelerated zinc oxide-eugenol cement is of little use in the clinical situation, as a base for the immediate placement of amalgam, when unset. If allowed to set, as occurs when this material is used as a temporary restoration and then subsequently cut back to be left as a base, unaccelerated zinc oxide-eugenol cement is almost certainly an adequate base forming material to withstand the forces of condensation of amalgam. Irrespective of the strength values, the results of this investigation suggest that all base forming materials, which are clinically set at the time of amalgam condensation and have been placed so as to minimise the presence of large voids, possess adequate strength to withstand the condensation pressures during the placement of amalgam. Department of Operative Dentistry, University of Sydney, 2 Chalmers Street, Surry Hills, N.S.W. 2010. Australia.
