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Corrosion of endodontic silver cones in humans: a scanning electron microscope and X-ray microprobe study
JOURNAL OF ENDODONTICS ] VOL 1, NO 6, JUNE 1975
C o r r o s i o n of e n d o d o n t i c s i l v e r c o n e s in h u m a n s : a s c a n n i n g e l e c t r o n m i c r o s c o p e and X-ray microprobe study John M. Brady, DDS, MSPH, a n d Carlos E. del Rio, DDS, W a s h i n q t o n , DC
A n a l y s e s with the s c a n n i n g e l e c t r o n m i c r o s c o p e a n d the X-ray m i c r o p r o b e w e r e p e r f o r m e d o n 19 silver c o n e s r e m o v e d from 12 p a t i e n t s . The c o n e s s h o w e d c h a n g e s that r a n g e d from s u r f a c e d u l l i n g to b l a c k c o r r o s i o n a n d pitting. Sulfur a n d c h l o r i n e w e r e d e t e c t e d at the a p i c a l e n d of the c o n e s a n d in the b i o p s y s p e c i m e n s of p e r i a p i c a l tissues. The development of a fluidtight seal at the apical foramen is one of the primary objectives of endodontic therapy. Silver cones have long been accepted as endodontic filling materials for the apical seal when they are used in conjunction with cements. 1 According to one study, 2 the inability to secure a satisfactory apical seal was responsible for 59% of the endodontic failures. However, silver cones are not without inherent faults. The dissolution of the cementing mediums by tissue fluids and the lack of adaptability of the silver cone to the root canal walls are two impediments in the silver cone obturating technique. In addition, corrosion may accompany the unsuccessful obturation of root canals with silver cones. The microscopic ap-
pearance of the corroded surface ranges from small pitting to formation of deep craters with highly cytotoxic surface contaminants that contain sulfur. 3 Investigations in our laboratory have indicated that surface contamination of silver cones implanted in bone reaches a maximum after only two months; contamination is visible on the scanning electron microscope as a thin, loosely adherent layer. 4 Corrosion of the surface of silver cones is probably a result of the breakdown of the apical seal with its exposure to the upward percolation of tissue fluids. If this hypothesis is true, then corrosion should decrease with distance from the apex. To test this hypothesis and to gain more information about the reactions of silver cones in clinical situations, cones removed from a group of 12 patients were studied with the scanning electron microscope (SEM) and the X-ray microprobe.
M e t h o d s a n d Materials Thirteen silver points were removed from 12 patients; the points had been in place in the root canal from 9 months to 15 years (Table 1, Fig 1). Four cases (no. 2, 5, 8, 9) were asymptomatic with no radiographic
abnormalities. One case (no. 11) showed an indistinct lamina dura; it was normal otherwise. The termini of the silver cones were 0.5 to 1.0 mm from the radiographic apex (the acceptable location). These five cases were considered clinically and radiographically successful. The remaining eight cases were endodontic failures. Five cases were asymptomatic with radiographic evidence of pathosis (no. 1, 3, 4, 7, 12). Only two cases were symptomatic. In one case (no. 6), the terminus of the cone was 2.5 mm beyond the radiographic apex and there was a draining sinus tract. In the other case (no. 10), the terminus of the cone was 2.5 mm short of the radiographic apex. In this case, there was pain on percussion. After removal, the cones were fixed to aluminum stubs, coated with carbon, and examined in a scanning electron microscope.* Simultaneously, energy-dispersive X-ray analysis was performed with an electron microprobe.~ Elemental analysis of the surface of the point disclosed principally silver, sulfur, and chlorine. The sulfur and chlorine windows were counted to a fixed count of silver, at 20 kv on the SEM, at a tilt angle of
205
JOURNAL OF ENDODONTICS ] VOL 1, NO 6, JUNE 1975
Table 1 9 Clinical history of cases.
Symptoms
Appearance of silver cone
Reason for removal of silver cone
No.
Patient
Tooth
t
21-yearold man
Maxillary left lateral incisor
3
Asymptomatic
Black and rough with notching of cone at apical end
Construction of post crown
2
27-yearold man
Maxillary left central incisor
10
Asymptomatic
Black and rough
Construction of post crown
3
25-yearold man
Mandibular left central incisor
13
Asymptomatic
Black and rough; Fracture of apical 1.5 mm of cone on removal
Construction of post crown
4
35-yearold man
Maxillary right lateral incisor
9
Asymptomatic
Black and pitted
Construction of post crown
5
37-yearold man
Maxillary left central incisor
5
Asymptomatic
Black and rough
Construction of post crown
6
23-yearold man
Maxillary right lateral incisor
5
Draining sinus tract
Black and rough; Notching of cone at apex
Symptoms
7
25-yearold man
Maxillary right central incisor
13
Asymptomatic
Black and rough
Construction of post crown
8
60-yearold man
Mandibular right lateral incisor
15
Asymptomatic
Dull
Construction of post crown
9
25-yearold man
Maxillary left central incisor
10
Asymptomatic
Black and rough
Construction of post crown
10
45-yearold woman
Maxillary right lateral incisor
3
Pain on percussion and palpation
Black and rough
Symptoms
11
47-yearold man
Maxillary right first premolar
3
Asymptomatic
Dull
Construction of post crown
12
29-yearold man
Mandibular left central incisor
Asymptomatic
Dull Black at tip
Construction of post crown
40 ~, and at magnifications of 500 and 1000 times. A tissue section f r o m a biopsy specimen of case no. 1 was deparaffinized, coated, and examined for elemental composition in the Xray analyzer. Areas of the tissue were mapped for localization of silver and sulfur in the S E M . Results
A t the time of r e m o v a l of the cones, the silver surfaces were black, rough, and pitted. T h r e e cases (no. 8, 11, 12) had dull silvery surfaces. In two cases 206
Time of case (years)
9 months
(no. 1, 6), the points were notched on the apical extension. The appearance of a severely notched cone (case no. 1) in the scanning microscope can be seen in Figure 2. W h e n seen with the microprobe, there was a deep circumferential erosion 1.0 m m wide and 0.1 m m deep with a high sulfur concentration (Fig 3). W h e n seen with light microscopy, periapical area tissue f r o m this case contained black rounded masses s u r r o u n d e d by chronic inflammatory cells and fibroblasts (Fig 4). In the S E M , the section of
tissue was mapped for location of silver and sulfur, and these elements were localized to the same black areas ( F i g 5, 6). T h e majority of the silver cones showed only moderate pitting of the surface near the apex; X-ray microprobe examination disclosed surface deposits of sulfur and chlorine (Fig 7, 8). Sulfur was seen in high concentrations on surface elevations with the use of X-ray m i e r o p r o b e - S E M elemental mapping m o d e ( F i g 9, 10). All of the cones were measured
JOURNAL OF ENDODONTICS I VOL 1, NO 6, JI/NE 1975
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207
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Fig 2--Scanning electron micrograph of silver cone (case no. 1). There is severe notching at asterisk. Areas surveyed for sulfur and chlorine are a, b, and c (orig mag X27).
Fig 3 - - X - r a y microprobe display o] notch surface of case no. 1. Predominant X-ray peaks are from silver (Ag) and sulfur (S).
Fig 4 - - L i g h t micrograph of periapical tissue from case no. 1. Black deposits were probed for elemental composition on scanning microscope (orig mag ,'(300).
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@~:~:
Fig 5 - - P r o b e display of X-ray spectrum from area of black deposits in tissue section from case no. 1. Silver and sulfur are present in addition to calcium, potassium, and silicon from glass slide.
208
Fig 7--Scanning electron micrograph of apical area of silver cone (case no~ 5). Patchy areas o] corrosion cover surface (a and b refer to two of three areas measured for sulfur and chlorine with microprobe) (orig mag XIO0).
...
Fig 6--Elemental maps of silver and sulfur in tissue section (case no. 1). Elements are concentrated in black masses in tissue (orig mag X1500).
Fig 8--Electron microprobe display of X-ray spectrum from area a in Figure 7. Sulfur (S) and chlorine (Cl) are detected in addition to silver (Ag). A l u m i n u m on mounting stub accounts for first major peak o/ spectrum.
JOURNAL OF ENDODONTICS I VOL 1, NO 6, JUNE 1975
Table 2 9 Intensity (counts) of elemental X r a y at specific areas (mean + S.E.)
Area A Sulfur Chlorine
1851 + 624 279 ___ 152
Area B
Area C
1032 __+ 437 115 -4- 27
589 _ 373 99 39
*,Intensity of sulfur and chlorine X-ray emission, corrected to I0,000 counts due to silver, in 13 endodontic cones (in place from 1 to 15 years) at three areas on the surface (A--tip, B=0.2 mm from tip, C=3.0 mm from tig).
Fig 9--Scanning electron micrograph o/surface crater on apex of cone (case no. 5) (orig mag X750). Surface layer with dehydration cracks is absent in area X.
2000
9 9
tO00
i
chlorine 3~,--..-.x
*'~-~.~
*\ .., r,.. SALVER CONE
Fig lO--Elemental mapping o / s u l f u r in same area as Figure 9. Sulfur is more concentrated in fragile layer around periphery o/ area X.
I
SFTES OF ANALYSES
Fig l l - - X - r a y intensity due to sulfur and chlorine at three regions o/ the silver cones (A, at apex; B, 0.2 from apex; and C, 3.0 m m from apex). Intensity is expressed as number of counts in chlorine and sulfur windows for 10,000 counts due to silver9
for sulfur and chlorine content at three points: at the apex, at 0.2 m m from the apex, and at 3.0 mm from the apex (Fig 2). The results were expressed as the number of X-ray counts due to sulfur and chlorine in the areas for 10,000 counts of X rays due to silver. In this way, a relative concentration of these three elements can be expressed. The sulfur content at the apex was 79% higher than at 0.2 mm from the apex and 214% higher than at 3.0 mm from the apex. Chlorine at the apex was considerably less than sulfur, but it was 142% and 181% higher at the apex than at 0.2 rnm and 3.0 m m from the apex (Table 2, Fig 11). Discussion
The results of this study indicate that silver cones undergo moderate to severe deterioration when placed as endodontic filling materials. Defects range from pitting of the whole point surface in the apical region to severe circumscribed notching accompanied by aggregation of silver and sulfur in the apical inflammatory tissue. This type of breakdown, associated with sulfur deposition, leads to increased fragility of the apical region of the cone. It becomes a site for potential fracture and retention when endodontic failure necessitates removal of the cone. In case no. 3, a radiograph taken after removal of the silver cone showed retention of the apical portion of the cone within the canal (Fig 1, 12). There was periapical pathosis in all the cases of failure although most cases were asymptomatic. In most of the cases in this study, endodontic failures would probably have gone undetected if fixed prosthodontic procedures had not been contemplated. The most significant observations from this study are the differences in sulfur and chlorine content of the
209
JOURNAL OF ENDODONTICS 1 VOL I, NO 6, JUNE 1975
support the contention that corrosion is related to exposure to tissue fluids and to the percolation of this fluid along the canal-cone interface. Summary
:~iI'~ "i,' '
"
Fig 12--Radiograph o[ case no. 3 alter removal o/ cone. A p e x o/ cone has [ractured and remains in canal. surface of the cone at various areas. The highest concentration of these two elements was at the apex; concentration decreased with the distance from the apex. This would appear to
210
Thirteen silver cones from 12 patients, that were removed either for reasons of endodontic failure or for the construction of fixed prostheses, were studied with the scanning electron microscope and X-ray microprobe. Clinically, the cones showed surface changes ranging from dull to black and pitted. Electron microscopically, the cones showed corrosive changes from pitting to deep circumferential erosion. Sulfur and silver were localized in biopsy tissue from the periapex that was associated with severe surface erosion. Measurement of sulfur and chlorine with the X-ray microprobe showed that these elements are present with decreasing concentrations at greater distances from the apex. This is consistent with the hypothesis that corrosion results from the percolation of tissue fluid
through the apical seal and along the cone-canal interface. *Model AMR 1000, Advanced Metal Research Corp., Burlington, Mass. tEDAX-707A with EDIT II Data System, EDAX Int., Prairie View, Ill. Dr. Brady is chief, department of biophysics, and Dr. del Rio is chief, department of clinical operations, United States Army Institue of Dental Research, Washington, DC. Requests for reprints should be directed to Col C. E. del Rio, Army Institute of Dental Research, Walter Reed Army Medical Center, Washington, DC 20012. References
1. Grossman, L.I. Endodontic practice, ed 8. Philadelphia, Lea & Febiger, 1974, pp 302-306. 2. Ingle, J.I. Endodontics. Philadelphia, Lea & Febiger, 1965, p 64. 3. Seltzer, S., and others. A scanning electron microscope examination of silver cones removed from endodontically treated teeth. Oral Surg 33:589 April 1972. 4. Brady, J.M.; Zielke, D.R.; ana del Rio, C.E. Corrosion of silver cones in bone: a scanning electron microscope and microprobe analysis. J Endo. Submitted for publication.
C o r r o s i o n of e n d o d o n t i c s i l v e r c o n e s in h u m a n s : a s c a n n i n g e l e c t r o n m i c r o s c o p e and X-ray microprobe study John M. Brady, DDS, MSPH, a n d Carlos E. del Rio, DDS, W a s h i n q t o n , DC
A n a l y s e s with the s c a n n i n g e l e c t r o n m i c r o s c o p e a n d the X-ray m i c r o p r o b e w e r e p e r f o r m e d o n 19 silver c o n e s r e m o v e d from 12 p a t i e n t s . The c o n e s s h o w e d c h a n g e s that r a n g e d from s u r f a c e d u l l i n g to b l a c k c o r r o s i o n a n d pitting. Sulfur a n d c h l o r i n e w e r e d e t e c t e d at the a p i c a l e n d of the c o n e s a n d in the b i o p s y s p e c i m e n s of p e r i a p i c a l tissues. The development of a fluidtight seal at the apical foramen is one of the primary objectives of endodontic therapy. Silver cones have long been accepted as endodontic filling materials for the apical seal when they are used in conjunction with cements. 1 According to one study, 2 the inability to secure a satisfactory apical seal was responsible for 59% of the endodontic failures. However, silver cones are not without inherent faults. The dissolution of the cementing mediums by tissue fluids and the lack of adaptability of the silver cone to the root canal walls are two impediments in the silver cone obturating technique. In addition, corrosion may accompany the unsuccessful obturation of root canals with silver cones. The microscopic ap-
pearance of the corroded surface ranges from small pitting to formation of deep craters with highly cytotoxic surface contaminants that contain sulfur. 3 Investigations in our laboratory have indicated that surface contamination of silver cones implanted in bone reaches a maximum after only two months; contamination is visible on the scanning electron microscope as a thin, loosely adherent layer. 4 Corrosion of the surface of silver cones is probably a result of the breakdown of the apical seal with its exposure to the upward percolation of tissue fluids. If this hypothesis is true, then corrosion should decrease with distance from the apex. To test this hypothesis and to gain more information about the reactions of silver cones in clinical situations, cones removed from a group of 12 patients were studied with the scanning electron microscope (SEM) and the X-ray microprobe.
M e t h o d s a n d Materials Thirteen silver points were removed from 12 patients; the points had been in place in the root canal from 9 months to 15 years (Table 1, Fig 1). Four cases (no. 2, 5, 8, 9) were asymptomatic with no radiographic
abnormalities. One case (no. 11) showed an indistinct lamina dura; it was normal otherwise. The termini of the silver cones were 0.5 to 1.0 mm from the radiographic apex (the acceptable location). These five cases were considered clinically and radiographically successful. The remaining eight cases were endodontic failures. Five cases were asymptomatic with radiographic evidence of pathosis (no. 1, 3, 4, 7, 12). Only two cases were symptomatic. In one case (no. 6), the terminus of the cone was 2.5 mm beyond the radiographic apex and there was a draining sinus tract. In the other case (no. 10), the terminus of the cone was 2.5 mm short of the radiographic apex. In this case, there was pain on percussion. After removal, the cones were fixed to aluminum stubs, coated with carbon, and examined in a scanning electron microscope.* Simultaneously, energy-dispersive X-ray analysis was performed with an electron microprobe.~ Elemental analysis of the surface of the point disclosed principally silver, sulfur, and chlorine. The sulfur and chlorine windows were counted to a fixed count of silver, at 20 kv on the SEM, at a tilt angle of
205
JOURNAL OF ENDODONTICS ] VOL 1, NO 6, JUNE 1975
Table 1 9 Clinical history of cases.
Symptoms
Appearance of silver cone
Reason for removal of silver cone
No.
Patient
Tooth
t
21-yearold man
Maxillary left lateral incisor
3
Asymptomatic
Black and rough with notching of cone at apical end
Construction of post crown
2
27-yearold man
Maxillary left central incisor
10
Asymptomatic
Black and rough
Construction of post crown
3
25-yearold man
Mandibular left central incisor
13
Asymptomatic
Black and rough; Fracture of apical 1.5 mm of cone on removal
Construction of post crown
4
35-yearold man
Maxillary right lateral incisor
9
Asymptomatic
Black and pitted
Construction of post crown
5
37-yearold man
Maxillary left central incisor
5
Asymptomatic
Black and rough
Construction of post crown
6
23-yearold man
Maxillary right lateral incisor
5
Draining sinus tract
Black and rough; Notching of cone at apex
Symptoms
7
25-yearold man
Maxillary right central incisor
13
Asymptomatic
Black and rough
Construction of post crown
8
60-yearold man
Mandibular right lateral incisor
15
Asymptomatic
Dull
Construction of post crown
9
25-yearold man
Maxillary left central incisor
10
Asymptomatic
Black and rough
Construction of post crown
10
45-yearold woman
Maxillary right lateral incisor
3
Pain on percussion and palpation
Black and rough
Symptoms
11
47-yearold man
Maxillary right first premolar
3
Asymptomatic
Dull
Construction of post crown
12
29-yearold man
Mandibular left central incisor
Asymptomatic
Dull Black at tip
Construction of post crown
40 ~, and at magnifications of 500 and 1000 times. A tissue section f r o m a biopsy specimen of case no. 1 was deparaffinized, coated, and examined for elemental composition in the Xray analyzer. Areas of the tissue were mapped for localization of silver and sulfur in the S E M . Results
A t the time of r e m o v a l of the cones, the silver surfaces were black, rough, and pitted. T h r e e cases (no. 8, 11, 12) had dull silvery surfaces. In two cases 206
Time of case (years)
9 months
(no. 1, 6), the points were notched on the apical extension. The appearance of a severely notched cone (case no. 1) in the scanning microscope can be seen in Figure 2. W h e n seen with the microprobe, there was a deep circumferential erosion 1.0 m m wide and 0.1 m m deep with a high sulfur concentration (Fig 3). W h e n seen with light microscopy, periapical area tissue f r o m this case contained black rounded masses s u r r o u n d e d by chronic inflammatory cells and fibroblasts (Fig 4). In the S E M , the section of
tissue was mapped for location of silver and sulfur, and these elements were localized to the same black areas ( F i g 5, 6). T h e majority of the silver cones showed only moderate pitting of the surface near the apex; X-ray microprobe examination disclosed surface deposits of sulfur and chlorine (Fig 7, 8). Sulfur was seen in high concentrations on surface elevations with the use of X-ray m i e r o p r o b e - S E M elemental mapping m o d e ( F i g 9, 10). All of the cones were measured
JOURNAL OF ENDODONTICS I VOL 1, NO 6, JI/NE 1975
m 9"I 9 ,! 9 .~-,..,.~
"
',,~ ,,: ~'
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.
.
.
.
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,
"L.
3 ,! ,-
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I
II
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8
,
L
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12 Fig 1--Silver cones of 12 patients included in study o/corrosion9
207
JOURNAL O F E N D O D O N T I C S
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....
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6, JUNE 1975
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Fig 2--Scanning electron micrograph of silver cone (case no. 1). There is severe notching at asterisk. Areas surveyed for sulfur and chlorine are a, b, and c (orig mag X27).
Fig 3 - - X - r a y microprobe display o] notch surface of case no. 1. Predominant X-ray peaks are from silver (Ag) and sulfur (S).
Fig 4 - - L i g h t micrograph of periapical tissue from case no. 1. Black deposits were probed for elemental composition on scanning microscope (orig mag ,'(300).
9 '7: m
@~:~:
Fig 5 - - P r o b e display of X-ray spectrum from area of black deposits in tissue section from case no. 1. Silver and sulfur are present in addition to calcium, potassium, and silicon from glass slide.
208
Fig 7--Scanning electron micrograph of apical area of silver cone (case no~ 5). Patchy areas o] corrosion cover surface (a and b refer to two of three areas measured for sulfur and chlorine with microprobe) (orig mag XIO0).
...
Fig 6--Elemental maps of silver and sulfur in tissue section (case no. 1). Elements are concentrated in black masses in tissue (orig mag X1500).
Fig 8--Electron microprobe display of X-ray spectrum from area a in Figure 7. Sulfur (S) and chlorine (Cl) are detected in addition to silver (Ag). A l u m i n u m on mounting stub accounts for first major peak o/ spectrum.
JOURNAL OF ENDODONTICS I VOL 1, NO 6, JUNE 1975
Table 2 9 Intensity (counts) of elemental X r a y at specific areas (mean + S.E.)
Area A Sulfur Chlorine
1851 + 624 279 ___ 152
Area B
Area C
1032 __+ 437 115 -4- 27
589 _ 373 99 39
*,Intensity of sulfur and chlorine X-ray emission, corrected to I0,000 counts due to silver, in 13 endodontic cones (in place from 1 to 15 years) at three areas on the surface (A--tip, B=0.2 mm from tip, C=3.0 mm from tig).
Fig 9--Scanning electron micrograph o/surface crater on apex of cone (case no. 5) (orig mag X750). Surface layer with dehydration cracks is absent in area X.
2000
9 9
tO00
i
chlorine 3~,--..-.x
*'~-~.~
*\ .., r,.. SALVER CONE
Fig lO--Elemental mapping o / s u l f u r in same area as Figure 9. Sulfur is more concentrated in fragile layer around periphery o/ area X.
I
SFTES OF ANALYSES
Fig l l - - X - r a y intensity due to sulfur and chlorine at three regions o/ the silver cones (A, at apex; B, 0.2 from apex; and C, 3.0 m m from apex). Intensity is expressed as number of counts in chlorine and sulfur windows for 10,000 counts due to silver9
for sulfur and chlorine content at three points: at the apex, at 0.2 m m from the apex, and at 3.0 mm from the apex (Fig 2). The results were expressed as the number of X-ray counts due to sulfur and chlorine in the areas for 10,000 counts of X rays due to silver. In this way, a relative concentration of these three elements can be expressed. The sulfur content at the apex was 79% higher than at 0.2 mm from the apex and 214% higher than at 3.0 mm from the apex. Chlorine at the apex was considerably less than sulfur, but it was 142% and 181% higher at the apex than at 0.2 rnm and 3.0 m m from the apex (Table 2, Fig 11). Discussion
The results of this study indicate that silver cones undergo moderate to severe deterioration when placed as endodontic filling materials. Defects range from pitting of the whole point surface in the apical region to severe circumscribed notching accompanied by aggregation of silver and sulfur in the apical inflammatory tissue. This type of breakdown, associated with sulfur deposition, leads to increased fragility of the apical region of the cone. It becomes a site for potential fracture and retention when endodontic failure necessitates removal of the cone. In case no. 3, a radiograph taken after removal of the silver cone showed retention of the apical portion of the cone within the canal (Fig 1, 12). There was periapical pathosis in all the cases of failure although most cases were asymptomatic. In most of the cases in this study, endodontic failures would probably have gone undetected if fixed prosthodontic procedures had not been contemplated. The most significant observations from this study are the differences in sulfur and chlorine content of the
209
JOURNAL OF ENDODONTICS 1 VOL I, NO 6, JUNE 1975
support the contention that corrosion is related to exposure to tissue fluids and to the percolation of this fluid along the canal-cone interface. Summary
:~iI'~ "i,' '
"
Fig 12--Radiograph o[ case no. 3 alter removal o/ cone. A p e x o/ cone has [ractured and remains in canal. surface of the cone at various areas. The highest concentration of these two elements was at the apex; concentration decreased with the distance from the apex. This would appear to
210
Thirteen silver cones from 12 patients, that were removed either for reasons of endodontic failure or for the construction of fixed prostheses, were studied with the scanning electron microscope and X-ray microprobe. Clinically, the cones showed surface changes ranging from dull to black and pitted. Electron microscopically, the cones showed corrosive changes from pitting to deep circumferential erosion. Sulfur and silver were localized in biopsy tissue from the periapex that was associated with severe surface erosion. Measurement of sulfur and chlorine with the X-ray microprobe showed that these elements are present with decreasing concentrations at greater distances from the apex. This is consistent with the hypothesis that corrosion results from the percolation of tissue fluid
through the apical seal and along the cone-canal interface. *Model AMR 1000, Advanced Metal Research Corp., Burlington, Mass. tEDAX-707A with EDIT II Data System, EDAX Int., Prairie View, Ill. Dr. Brady is chief, department of biophysics, and Dr. del Rio is chief, department of clinical operations, United States Army Institue of Dental Research, Washington, DC. Requests for reprints should be directed to Col C. E. del Rio, Army Institute of Dental Research, Walter Reed Army Medical Center, Washington, DC 20012. References
1. Grossman, L.I. Endodontic practice, ed 8. Philadelphia, Lea & Febiger, 1974, pp 302-306. 2. Ingle, J.I. Endodontics. Philadelphia, Lea & Febiger, 1965, p 64. 3. Seltzer, S., and others. A scanning electron microscope examination of silver cones removed from endodontically treated teeth. Oral Surg 33:589 April 1972. 4. Brady, J.M.; Zielke, D.R.; ana del Rio, C.E. Corrosion of silver cones in bone: a scanning electron microscope and microprobe analysis. J Endo. Submitted for publication.