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The strength of Class II amalgam restorations with and without pins
The strength with
and
of Class II amalgam
without
r&orations
pins
Josd Mondelli, C.D., M&D.,* and Dioracy Fonterrada Vieira, C.D., M.S.D.,
Ph.D.**
Faculdades de Odontologia de Bauru e de SGoPaulo, Sa’o Paulo, Bras2
S
ince dental amalgam has excellent physical properties, cavities that are larger than usual have been restored with this material. Some authors recommended the restoration of the distobuccal cusp of the lower first molar with amalgam.lb3 Others advocate dental amalgam retained with pins for nontypical cavity preparations.‘, ’ Enthusiasm for pin-retained amalgam restorations has recently gained momentum.B Few investigations support the clinical practice of using pins. Some authors state that pins increase the strength of amalgam,7~ 8 others report that pins only increase retention,s* lo and still other investigators contend that pins lower the transverse and compressive strengths of ama1gam.2~ 8, lo The present data on amalgam strength were derived from atypical standards. This report presents measurements of amalgam strength from samples similar to those found in clinical practice. MATERIALS
AND
METHODS
A typical MOD cavity was prepared in a sound extracted lower first molar, and a silicone mold was made of it. Four models were made in polystyrene resin with this mold. Modifications of the MOD cavity were cut in the models in order to obtain four different cavity preparations. The resin teeth containing the MOD preparations were then reproduced in a chrome-cobalt alloy (Figs. 1 to 4) . The roots of the teeth were cut in the shape of a cone in order to adapt the models to a testing machine (Fig. 2, A). The crowns of the teeth had a planed gingival base which was prependicular to its long axis; this plane base rested on a metal base located on the lower platform of a compressive strength testing machine. Cavity I was a typical MOD cavity, prepared according to Strickland’ and Stibbs’ (Figs. 1, A, and 2, A). Cauity 2 had the same characteristics as Cavity 1, but the *Assistant
Professor
**Professor
and
and Chairman,
Chairman,
Dental
Operative Materials
Dentistry
Department.
Department.
179
180
Mondelli
Fig.
1. Occlusal
Fig.
2. Proximal
and Vieira
view view
of Cavity of Cavity
1 1
(A) and Cavity (A) and Cavity
2 (B). 2 (B) .
distobuccal cusp was reduced to provide for protection with an amalgam cusp 2 mm. thick (Figs. 1, B, and 2, B) . Cavity 3 was similar to Cavity 1, with an enlarged proximal portion. This change conformed to Black’s princip1es.l’ The isthmus was half as large as the distance between the mesiobuccal and mesiolingual cusps.Two pinholes were prepared on the gingival floor to receive metal pins (Fig. 3, B and C) . Cavity 4 was similar to Cavity 1 but represented a preparation in which the distobuccal and mesiobuccal cuspswere destroyed, thus increasing the size of the cavity preparation. This cavity had the largest area of all, and therefore its amalgam restoration would have greatest bulk. Pinholes for three pins were prepared in the gingival floor (Fig. 4,AandB). In order to standardize the procedures, one amalgam restoration was made for each cavity preparation using steel strips in a Tofflemire matrix holder. After finishing and polishing, self-curing resin matrices were made for each tooth (Fig. 5) . The matrices consisted of two halves which opened at the occlusal surface making it possible to condenseand carve a standardized amalgam restoration with controlled shape and thickness. Amalgam restorations made for Cavities 1 and 2 had no retentive pins. For Cavities 3 and 4, amalgam restorations were made with and without pins. When used, the pins were straight in half of the restorations (Fig. 6)) and in the remaining half, the pins were bent at the upper third and inclined toward the center of the
Volume Number
28 2
Strength
Fig. 3. Proximo-occlusal (R)
Occlusal
view
view of Cavity of Cavity 3 (C).
Fig. 4. Proximal
view
of
1 (A)
(A)
with
restorations
f or comparison
and occlusal
view
and
with
(B)
without
similar
of Cavity
view
jhs
of Cavity
181
1;
4.
occlusal surface (Fig. 7). These were self-threading pins, and they were cemented. Amalgam restorations were made with Caulk’s fine cut alloy using a 1: 1 mercuryalloy ratio and triturated in a Dentomat for 20 seconds. The amalgam was condensed with an electric mallet using a 1.5 mm. condensing point. Condensation pressure was 66 Kg./cm.Z The restorations were completed in 10 minutes. After condensation and carving, the dies containing the restorations were stored in a 37’ C., 100 per cent humid environment for 1 hour, 1 day, or 1 week. The restorations were not polished (Fig. 8). The dies containing the amalgam restorations were placed on the lower platform of a Riehle Universal testing machine in such a way that it was possible to apply stresses to the occlusal surface to the point of fracture. The stress was applied axially through a steel ball 2.4 mm. in diameter and at a speed of 0.5 mm. per minute (Fig. 9). The load was applied at the mesial or distal fossa for Restoration 1, at the distal fossa for Restoration 2, at the mesial fossa for Restoration 3, and at the central fossa or the distal triangular fossa for Restoration 4. The magnitude of the load was recorded at the time of fracture of the amalgam restoration. Five samples were tested for each type of cavity preparation; 165 amalgam restorations were made, and the
182
Mondelli
and
J, Prosthet. Dent. August, 1972
Vieira
Fig. 5. Cold-curing acrylic the amalgam restoration.
resin
matrix
in place
mean fracture strength of each variation subjected to statistical analysis.
to standardize
is presented
the thickness
in Table
I. The
and
carving
of
results were
RESULTSAND DISCUSSION Table I shows the results for fracture strength of amalgam restorations prepared for all cavities. Table II illustrates comparisons between the data presented according to student’s t test. Table I shows that the usual increase in amalgam strength with age occurred even with pins present and where amalgam size and shape simulated clinical practice. This increase in strength was most evident in the 1 day specimens; from 1 day to 1 week, the difference was slight. There was no difference in strength for Restoration 1 when the stresses were applied to the mesial or distal fossa. Table II shows that Restoration 2 presented stronger amalgams than Restoration 1 after 1 day and 1 week, confirming the clinical observations of Stibbsl and Strickland.2 They recommended that MOD cavities for lower first molars include the distobuccal cusp in order to increase the strength of the restoration. Table I shows that the strength of the restoration at 1 hour was generally low, which means that the patient should be alerted to shelter freshly inserted amalgam restorations. Restoration 3 demonstrated that straight pins tended to decrease the fracture strength of amalgam for 1 day and 1 week restorations when compared with specimens without pins. This difference did not exist for 1 hour restorations with or without straight pins. When the pins for Restoration 3 were bent occlusally, the fracture strength for 1 hour restorations was higher than the strength for amalgam restorations without pins. This finding suggests that bent pins are advantageous, since they increase the early strength of amalgam restorations. The fracture strength of 1 day and 1 week amalgam restorations with bent pins was higher than the strength of similar restorations with straight pins and equal in strength to those without pins. Table II compares Restorations 1 and 3 without pins. The stresses were applied to the mesial fossa. The amalgam restorations in Cavity 3 were stronger than those
Volume Numbrr
Fig. Fig.
28 2
Strength
6. Cavities
3
7. Cavities
4
Fig.
of restorations
with
and without
pins
183
(A) and 4 (B) with straight pins in place. (A) and 3 (B) with bent pins in place.
8. Amalgam
restorations
for cavities
1
(A), 2 (B), 3 (C), and 4 (D).
in Cavity 1. This means that the cavity which provides for greater bulk of amalgam leads to a stronger restoration. This is in agreement with mechanical principles in which the stressesare more homogeneouslydistributed throughout the masswhen the cavity preparation presents parallel and perpendicular walls, as in Cavity 3. This did not occur when the buccal and lingual walls of the proximal portion converged from gingival to occlusal (Cavity 1) as advised by Stibbsl and Bronner.12 In Restoration 1.
J. Prosthet. Dent. August, 1972
Mondelli and Vieira
104
Fig.
9. A steel
bal1
(B) .
Table
ball
applied
to the
restoration.
(A)
The
restoration
fractured
by
the
siteel
I. Stresscausing fracture of amalgam specimensof different ages Fracture MOD
Stresses applied
cavity
(Kg.)
Cavity 1 (typical)
Mesial
Pin inclusion fossa
strength
according in Kg.* (S.D.)
I hour
to amalgam
1 day
1
age
1 week
No
20.00
-I
2.52
75.40 2
5.73
80.00?
7.45
3.87
76.00
t
5.24
8.60
88.80
2
7.46
6.58
126.20
t
6.76
Distal
fossa
No
19.40 +
2.07
73.002
Cavity 2 (nontypical
Distal
fossa
No
22.60
+
4.16
88.00
Cavity 3 (nontypical)
Mesial
No
39.80
+
2.28
127.002
39.60 f: 56.20+
2.30 3.03
93.40 k 21.44 123.802 3.90
108.60 t 131.00?
8.76 3.67
117.602
4.51
119.602
7.44
3.84
146.405
11.89
160.602
10.90
6.69
105.OOL
4.95
94.40
fossa
Two Two Cavity 4 (nontypical)
Central
fossa
No Three
Distal triangular fossa
*Each
result
represents
straight bent
53.2Ok10.66 bent
95.50
2
No
37.802
1.92
101.40+
Three straight Three bent
36.10+
1.52
93.00
the mean
for
36.60f 5 samples
1.67 in the
102.602 same
2
t
5.98
experimental
lOl.OO+
7.21 2
2.88 3.32
condition.
there was a stressconcentration at the proximo-occlusal third of the restoration. It must be pointed out that the buccal and lingual extension of Cavity 3 will promote a decreasein the strength of the remaining tooth structure. However, the extension of the proximal portion as advised by Stibbs,’ Strickland,* and Bronnerl* but with parallel buccal and lingual walls, agreeswith present results. These results also agree with Correa’s opinion that the proximal portion of Class II amalgam preparations must have parallel buccal and lingual walls perpendicular to the axial wall.‘3
Volume Number
28 2
Strength
Table II. Some comparisons t = 2.306) Stresses
agplied
(Kg.)
between
Comparisons between cavities
of restorations with and without pins
data presented
Amalgam
in Table
I (two means test:
Interprctation
Test
age 3.20
Distal
fossa
1x2
1 week
Mesial
fossa
Central
1x3
fossa (no
4x4 pins, with
N.S.” L
1 hour
1 day
t L
15.00 3.40
=
4.41
s.t
t .=
12.80 z :~.9.5
3.14
S.
19.80
1 hour
S.
1 day
t =
51.60 3.90
=
13.~3 -
S.
1 week
t =
45.00 __ 4.49
=
lO.OL’
S.
1 hour
t =
42.30 3.06
x
8.35
S.
1 day
t=--=
5.07
S.
6.94
S.
pins)
28.80 5.68
1 week “N.S.
=
tS. =
significant,
185
t =
41.00 5.90
=
not significant.
In a practical sense, the increase in strength with bent pins, as in Restoration 3, occurred only in the 1 hour specimens. This may be explained by the material being less resistant and more plastic-like in the fresh state. In this case, the strength of the amalgam was related to the stiff pins which increased the strength of the entire restoration. However, after 1 day or 1 week, amalgam would be stronger and would not flow readily, and the stiffness of the pins would have no bearing on strength. This confirms an earlier report by Minelli. I4 It is interesting that bent pins did not decrease the strength of the amalgam restorations. In Restoration 4, the occlusal stresses were applied either at the central fossa or at the distal triangular fossa. For stresses applied to the central fossa, we did not use straight pins, since the stresses were localized far from the pins, and in such a situation, there was no wedge action into the amalgam. The presence of bent pins delinitely increased the strength of amalgam restorations when compared with samples without pins and when the stresses were applied far from the location of the pins (Table II). This result is in agreement with Markley,4 in that the pins increase the retention of large restorations, and also with Johns,‘j who stated that pins may be
J. Prosthet. Dent. August, 1972
Mondelli and Vieira
186
Table III. Summary of the analysisof variance for Cavities 3 and 4 (stressesapplied on the distal fossa) with or without pins
Stresses Cavity
applied
3
Mesial
4
Distal
*S. = significant tN.S.
=
age fossa
fossa
at the level
Analysis
Amalgam Observed
F
)
of
Critical
variance F
) Znter$retation
1 hour 1 day 1 week
69.11 10.16 14.90
3.89 3.89 3.89
s.* S. S.
1 hour 1 day 1 week
2.11 3.90 6.02
3.89 3.89 3.89
N.S.t S. 8.
of 5 per cent.
not significant.
consideredan extension of the retentive grooves. However, when comparing amalgam sampleswith bent pins in which the stresseswere applied to the distal triangular fossa, we note that the pins did not increase the amalgam strength when the stresses were applied to the area where the pins were located. With stressesapplied to the distal triangular fossa,there was a tendency of straight pins to decreasethe fracture strength of amalgam for 1 day and 1 week specimens.However, when these pins were bent, they did not decreasethe amalgam fracture strength. An analysis of variance was made to determine the influence of pins on the amalgam restorations for Cavities 3 and 4. For Restoration 3, there were two pins in the proximal portion, and the stresseswere applied to the mesialfossa.Restoration 4 contained 3 pins, and the stresseswere applied to the distal triangular fossa.Contrasts were studied between the means according to Tuckey’s test in casesof significance found in the analysis of variance. Tables III and IV present these results. It may be seen in Table III that, with the exception of 1 hour restorations in Cavity 4, there was a significant difference between sampleswith and without straight or bent pins. Contrasts for 1 hour restorations in Cavity 3 show that pins bent toward the occlusal surface increase the fracture strength of amalgam samples (Table IV) confirming Minelli’s observations on compressive and transverse strengths of pin amalgams.I4 In Cavity 3 restorations, straight pins tended to decreasethe fracture strength of the amalgam regardlessof age. Bent pins, in 1 day or 1 week restorations, did not seemto increase the strength of the amalgam when compared with restorations without pins. For Restoration 4, the analysis of variance showed no difference between 1 hour restorations with or without pins. For 1 day and 1 week restorations, the significant difference seemsto be due to the tendency of straight pins to decreasethe fracture strength (Table IV). Another interesting observation is that the more amalgam bulk, the greater the fracture strength. This was established by comparing the results for Restorations 1 and 2 with those of Restorations 3 and 4. The results partially agree with previous studies in that the pins increased retention.9l lo*l6 However, it must be pointed out that for Restoration 3 with 2 bent pins at 1 hour, for Restoration 4 with 3 bent pins, and for those restorations where
Volume Number
Strength
28 2
Table IV. Contrast
between
of restorations
means (Tuckey)
with
for Cavities
and
without
3 and 4
Comparisons
Amal-
_--.._
wm Cavity 3
Stresses applied
age (hr.)
Mesial fossa
1
24
168
4
Distal fossa
1 24
168
*N.S
=
+S. =
significant.
187
pins
Criti-
Conditions
Result
No pins-straight pins Bent pins-straight pins Bent pins-no pins No pins-straight pins Bent pins-straight pins No pins-bent pins No pins-straight pins Bent pins-straight pins Bent pins-no pins
39.80 56.20 56.20 127.00 123.80 127.00 125.20 131.00 131.00
-
39.60 39.60 39.80 93.40 93.40 123.80 108.60 108.60 125.20
= 0.20 = 16.60 = 16.40 =x 33.60 = 30.40 = 3.20 = 16.60 = 22.40 = 5.80
No pins-straight Bent pins-straight Bent pins-no No pins-straight Bent pins-straight No pins-bent
101.40 102.60 102.60 105.00 101.00 105.00
-
93.00 93.00 101.40 94.40 94.40 101.00
= = = = = =
pins pins
pins pins pins
pins
8.40 9.60 1.20 10.60 6.60 4.00
cal value
Interpre. tation
4.32 “ “ 22.“1 “ “ 11.97 ” “
N.S.” L5 .IJ. S. S. S. N.S. S. S. N.S.
9.99 “ “ 8.23 ‘< ‘(
N.S. N.S. N.S. N.S. S. N.S. N.S.
not significant.
the stresses were not directly applied fracture strength of the restorations.
to the pin site the pins definitely
increased
the
CONCLUSlONS There was no difference in the fracture strength of typical Class II (MOD) amalgam restorations when the compressive stress was applied to the mesial or distal fossa. MOD amalgam restorations for the lower first molar presented a higher fractun strength when the distobuccal cusp was restored. Straight pins located at the distoproximal portion of an MOD restoration decreased the 1 day and 1 week strength of amalgam restorations. Two bent pins located in the distoproximal portion of typical MOD restorations increased the 1 hour strength. The larger the bulk of amalgam in the proximal portion of an MOD cavity, the higher the fracture strength. In large MOD cavity preparations, 3 bent pins increased the fracture strength of the amalgam when the stresses were not applied at the pin sites. When the stresses were applied over the pins, the fracture strength was decreased. Straight pins decreased the fracture strength of amalgam restorations. In some instances, bent pins increased the fracture strength of amalgam restorations but did not decrease the fracture strength when compared with restorations without pins.
188
Mondelli
and
Vieira
J. Prosthet. August,
Dent. 1972
References 1.
Stibbs, G. D.: Cavity Preparation and Matrices for Amalgam Restorations, J. Am. Dent. Assoc. 56: 471-479, 1958. 2. Strickland, W. D.: Amalgam Restorations for Class I Cavity Preparations, in Sturdevant, C. M.: The Art and Science of Operative Dentistry, New York, 1968, McGraw-Hill Book Company, Inc., pp. 235-259. 3. Wolcott, R. B.: Failures in Dental Amalgam, J. Am. Dent. Assoc. 56: 479-491, 1958. 4. Markley, M. R.: Pin Reinforcement and Retention of Amalgam Foundations and Restorations, J. Am. Dent. Assoc. 56: 675-679, 1958. “Dentin,” Dent. Clin. North. Am., Nov., 1963, pp. 5. Courtade, G. L.: Creating Your Own 805-822. 6. Klein, R. E.: Steel Reinforced Restorations, J. PROSTHET. DENT. 14: 1140-1143, 1964. 7. Going, R. E., Maffa, J. P., Nostrant, G. W., and Johnson, B. E.: The Strength of Dental Amalgam, as Influenced by Pins, J. Am. Dent. Assoc. 77: 1331-1334, 1968. 8. Wright, R. W.: The Use of Stainless Steel Pins to Strengthen Amalgam Restorations, Aust. Dent. J. 3: 369-370, 1958. 9. Collard, E. W., Caputo, A. A., and Standlee, J. P.: Rationale for Pin Retained Amalgam Restorations, Dent. Clin. North Am. 14:43-51, 1970. 10. Watson, P. A., and Gilmore, H. W.: Use of Pins for Retaining Amalgam Restorations: A Synopsis, J. Can. Dent. Assoc. 36: 30-31, 1970. 11. Black, G. V.: Operative Dentistry, vol. 2, Chicago, 1908, Medico-Dental Publishing Company. 12. Bronner, F. J.: Mechanical, Physiological and Pathological Aspects of Operative Procedures, Dent. Cosmos 73: 577-584, 1931. 13. Co&a, A. A.: Los principios mecanicos ya las formas de resistencia y retention, in Pa&a, N. : TCcnica de Operatoria Dental, Buenos Aires, 1964, ODA, pp. 409-440. 14. Minelli, C. J.: Contribui$o ao estudo das resist&ncias B compress%o, B trat$o e ao dobramento do amalgama de prata, corn e sem inch&o de pinos metalicos, Faculdade de Farmdcia e Odontologia de RibeirZo P&o, Bras& Doctor’s thesis, 1967. 15. Johns, R. B.: Intra-coronal pins, Br. Dent. J. 127: 580-582, 1969. 16. Dyce, J. M., Dow, J. A., and Norcliffe, A.: The Restoration of Broken-Down Teeth. Pin Retention, Br. Dent. J. 125: 347-350, 1968. FACULDADE DE ODONTOLOCIA DE BAURU CAIXA POSTAL 73 17100 BAURU, Sjio PAULO, BRAZIL
and
of Class II amalgam
without
r&orations
pins
Josd Mondelli, C.D., M&D.,* and Dioracy Fonterrada Vieira, C.D., M.S.D.,
Ph.D.**
Faculdades de Odontologia de Bauru e de SGoPaulo, Sa’o Paulo, Bras2
S
ince dental amalgam has excellent physical properties, cavities that are larger than usual have been restored with this material. Some authors recommended the restoration of the distobuccal cusp of the lower first molar with amalgam.lb3 Others advocate dental amalgam retained with pins for nontypical cavity preparations.‘, ’ Enthusiasm for pin-retained amalgam restorations has recently gained momentum.B Few investigations support the clinical practice of using pins. Some authors state that pins increase the strength of amalgam,7~ 8 others report that pins only increase retention,s* lo and still other investigators contend that pins lower the transverse and compressive strengths of ama1gam.2~ 8, lo The present data on amalgam strength were derived from atypical standards. This report presents measurements of amalgam strength from samples similar to those found in clinical practice. MATERIALS
AND
METHODS
A typical MOD cavity was prepared in a sound extracted lower first molar, and a silicone mold was made of it. Four models were made in polystyrene resin with this mold. Modifications of the MOD cavity were cut in the models in order to obtain four different cavity preparations. The resin teeth containing the MOD preparations were then reproduced in a chrome-cobalt alloy (Figs. 1 to 4) . The roots of the teeth were cut in the shape of a cone in order to adapt the models to a testing machine (Fig. 2, A). The crowns of the teeth had a planed gingival base which was prependicular to its long axis; this plane base rested on a metal base located on the lower platform of a compressive strength testing machine. Cavity I was a typical MOD cavity, prepared according to Strickland’ and Stibbs’ (Figs. 1, A, and 2, A). Cauity 2 had the same characteristics as Cavity 1, but the *Assistant
Professor
**Professor
and
and Chairman,
Chairman,
Dental
Operative Materials
Dentistry
Department.
Department.
179
180
Mondelli
Fig.
1. Occlusal
Fig.
2. Proximal
and Vieira
view view
of Cavity of Cavity
1 1
(A) and Cavity (A) and Cavity
2 (B). 2 (B) .
distobuccal cusp was reduced to provide for protection with an amalgam cusp 2 mm. thick (Figs. 1, B, and 2, B) . Cavity 3 was similar to Cavity 1, with an enlarged proximal portion. This change conformed to Black’s princip1es.l’ The isthmus was half as large as the distance between the mesiobuccal and mesiolingual cusps.Two pinholes were prepared on the gingival floor to receive metal pins (Fig. 3, B and C) . Cavity 4 was similar to Cavity 1 but represented a preparation in which the distobuccal and mesiobuccal cuspswere destroyed, thus increasing the size of the cavity preparation. This cavity had the largest area of all, and therefore its amalgam restoration would have greatest bulk. Pinholes for three pins were prepared in the gingival floor (Fig. 4,AandB). In order to standardize the procedures, one amalgam restoration was made for each cavity preparation using steel strips in a Tofflemire matrix holder. After finishing and polishing, self-curing resin matrices were made for each tooth (Fig. 5) . The matrices consisted of two halves which opened at the occlusal surface making it possible to condenseand carve a standardized amalgam restoration with controlled shape and thickness. Amalgam restorations made for Cavities 1 and 2 had no retentive pins. For Cavities 3 and 4, amalgam restorations were made with and without pins. When used, the pins were straight in half of the restorations (Fig. 6)) and in the remaining half, the pins were bent at the upper third and inclined toward the center of the
Volume Number
28 2
Strength
Fig. 3. Proximo-occlusal (R)
Occlusal
view
view of Cavity of Cavity 3 (C).
Fig. 4. Proximal
view
of
1 (A)
(A)
with
restorations
f or comparison
and occlusal
view
and
with
(B)
without
similar
of Cavity
view
jhs
of Cavity
181
1;
4.
occlusal surface (Fig. 7). These were self-threading pins, and they were cemented. Amalgam restorations were made with Caulk’s fine cut alloy using a 1: 1 mercuryalloy ratio and triturated in a Dentomat for 20 seconds. The amalgam was condensed with an electric mallet using a 1.5 mm. condensing point. Condensation pressure was 66 Kg./cm.Z The restorations were completed in 10 minutes. After condensation and carving, the dies containing the restorations were stored in a 37’ C., 100 per cent humid environment for 1 hour, 1 day, or 1 week. The restorations were not polished (Fig. 8). The dies containing the amalgam restorations were placed on the lower platform of a Riehle Universal testing machine in such a way that it was possible to apply stresses to the occlusal surface to the point of fracture. The stress was applied axially through a steel ball 2.4 mm. in diameter and at a speed of 0.5 mm. per minute (Fig. 9). The load was applied at the mesial or distal fossa for Restoration 1, at the distal fossa for Restoration 2, at the mesial fossa for Restoration 3, and at the central fossa or the distal triangular fossa for Restoration 4. The magnitude of the load was recorded at the time of fracture of the amalgam restoration. Five samples were tested for each type of cavity preparation; 165 amalgam restorations were made, and the
182
Mondelli
and
J, Prosthet. Dent. August, 1972
Vieira
Fig. 5. Cold-curing acrylic the amalgam restoration.
resin
matrix
in place
mean fracture strength of each variation subjected to statistical analysis.
to standardize
is presented
the thickness
in Table
I. The
and
carving
of
results were
RESULTSAND DISCUSSION Table I shows the results for fracture strength of amalgam restorations prepared for all cavities. Table II illustrates comparisons between the data presented according to student’s t test. Table I shows that the usual increase in amalgam strength with age occurred even with pins present and where amalgam size and shape simulated clinical practice. This increase in strength was most evident in the 1 day specimens; from 1 day to 1 week, the difference was slight. There was no difference in strength for Restoration 1 when the stresses were applied to the mesial or distal fossa. Table II shows that Restoration 2 presented stronger amalgams than Restoration 1 after 1 day and 1 week, confirming the clinical observations of Stibbsl and Strickland.2 They recommended that MOD cavities for lower first molars include the distobuccal cusp in order to increase the strength of the restoration. Table I shows that the strength of the restoration at 1 hour was generally low, which means that the patient should be alerted to shelter freshly inserted amalgam restorations. Restoration 3 demonstrated that straight pins tended to decrease the fracture strength of amalgam for 1 day and 1 week restorations when compared with specimens without pins. This difference did not exist for 1 hour restorations with or without straight pins. When the pins for Restoration 3 were bent occlusally, the fracture strength for 1 hour restorations was higher than the strength for amalgam restorations without pins. This finding suggests that bent pins are advantageous, since they increase the early strength of amalgam restorations. The fracture strength of 1 day and 1 week amalgam restorations with bent pins was higher than the strength of similar restorations with straight pins and equal in strength to those without pins. Table II compares Restorations 1 and 3 without pins. The stresses were applied to the mesial fossa. The amalgam restorations in Cavity 3 were stronger than those
Volume Numbrr
Fig. Fig.
28 2
Strength
6. Cavities
3
7. Cavities
4
Fig.
of restorations
with
and without
pins
183
(A) and 4 (B) with straight pins in place. (A) and 3 (B) with bent pins in place.
8. Amalgam
restorations
for cavities
1
(A), 2 (B), 3 (C), and 4 (D).
in Cavity 1. This means that the cavity which provides for greater bulk of amalgam leads to a stronger restoration. This is in agreement with mechanical principles in which the stressesare more homogeneouslydistributed throughout the masswhen the cavity preparation presents parallel and perpendicular walls, as in Cavity 3. This did not occur when the buccal and lingual walls of the proximal portion converged from gingival to occlusal (Cavity 1) as advised by Stibbsl and Bronner.12 In Restoration 1.
J. Prosthet. Dent. August, 1972
Mondelli and Vieira
104
Fig.
9. A steel
bal1
(B) .
Table
ball
applied
to the
restoration.
(A)
The
restoration
fractured
by
the
siteel
I. Stresscausing fracture of amalgam specimensof different ages Fracture MOD
Stresses applied
cavity
(Kg.)
Cavity 1 (typical)
Mesial
Pin inclusion fossa
strength
according in Kg.* (S.D.)
I hour
to amalgam
1 day
1
age
1 week
No
20.00
-I
2.52
75.40 2
5.73
80.00?
7.45
3.87
76.00
t
5.24
8.60
88.80
2
7.46
6.58
126.20
t
6.76
Distal
fossa
No
19.40 +
2.07
73.002
Cavity 2 (nontypical
Distal
fossa
No
22.60
+
4.16
88.00
Cavity 3 (nontypical)
Mesial
No
39.80
+
2.28
127.002
39.60 f: 56.20+
2.30 3.03
93.40 k 21.44 123.802 3.90
108.60 t 131.00?
8.76 3.67
117.602
4.51
119.602
7.44
3.84
146.405
11.89
160.602
10.90
6.69
105.OOL
4.95
94.40
fossa
Two Two Cavity 4 (nontypical)
Central
fossa
No Three
Distal triangular fossa
*Each
result
represents
straight bent
53.2Ok10.66 bent
95.50
2
No
37.802
1.92
101.40+
Three straight Three bent
36.10+
1.52
93.00
the mean
for
36.60f 5 samples
1.67 in the
102.602 same
2
t
5.98
experimental
lOl.OO+
7.21 2
2.88 3.32
condition.
there was a stressconcentration at the proximo-occlusal third of the restoration. It must be pointed out that the buccal and lingual extension of Cavity 3 will promote a decreasein the strength of the remaining tooth structure. However, the extension of the proximal portion as advised by Stibbs,’ Strickland,* and Bronnerl* but with parallel buccal and lingual walls, agreeswith present results. These results also agree with Correa’s opinion that the proximal portion of Class II amalgam preparations must have parallel buccal and lingual walls perpendicular to the axial wall.‘3
Volume Number
28 2
Strength
Table II. Some comparisons t = 2.306) Stresses
agplied
(Kg.)
between
Comparisons between cavities
of restorations with and without pins
data presented
Amalgam
in Table
I (two means test:
Interprctation
Test
age 3.20
Distal
fossa
1x2
1 week
Mesial
fossa
Central
1x3
fossa (no
4x4 pins, with
N.S.” L
1 hour
1 day
t L
15.00 3.40
=
4.41
s.t
t .=
12.80 z :~.9.5
3.14
S.
19.80
1 hour
S.
1 day
t =
51.60 3.90
=
13.~3 -
S.
1 week
t =
45.00 __ 4.49
=
lO.OL’
S.
1 hour
t =
42.30 3.06
x
8.35
S.
1 day
t=--=
5.07
S.
6.94
S.
pins)
28.80 5.68
1 week “N.S.
=
tS. =
significant,
185
t =
41.00 5.90
=
not significant.
In a practical sense, the increase in strength with bent pins, as in Restoration 3, occurred only in the 1 hour specimens. This may be explained by the material being less resistant and more plastic-like in the fresh state. In this case, the strength of the amalgam was related to the stiff pins which increased the strength of the entire restoration. However, after 1 day or 1 week, amalgam would be stronger and would not flow readily, and the stiffness of the pins would have no bearing on strength. This confirms an earlier report by Minelli. I4 It is interesting that bent pins did not decrease the strength of the amalgam restorations. In Restoration 4, the occlusal stresses were applied either at the central fossa or at the distal triangular fossa. For stresses applied to the central fossa, we did not use straight pins, since the stresses were localized far from the pins, and in such a situation, there was no wedge action into the amalgam. The presence of bent pins delinitely increased the strength of amalgam restorations when compared with samples without pins and when the stresses were applied far from the location of the pins (Table II). This result is in agreement with Markley,4 in that the pins increase the retention of large restorations, and also with Johns,‘j who stated that pins may be
J. Prosthet. Dent. August, 1972
Mondelli and Vieira
186
Table III. Summary of the analysisof variance for Cavities 3 and 4 (stressesapplied on the distal fossa) with or without pins
Stresses Cavity
applied
3
Mesial
4
Distal
*S. = significant tN.S.
=
age fossa
fossa
at the level
Analysis
Amalgam Observed
F
)
of
Critical
variance F
) Znter$retation
1 hour 1 day 1 week
69.11 10.16 14.90
3.89 3.89 3.89
s.* S. S.
1 hour 1 day 1 week
2.11 3.90 6.02
3.89 3.89 3.89
N.S.t S. 8.
of 5 per cent.
not significant.
consideredan extension of the retentive grooves. However, when comparing amalgam sampleswith bent pins in which the stresseswere applied to the distal triangular fossa, we note that the pins did not increase the amalgam strength when the stresses were applied to the area where the pins were located. With stressesapplied to the distal triangular fossa,there was a tendency of straight pins to decreasethe fracture strength of amalgam for 1 day and 1 week specimens.However, when these pins were bent, they did not decreasethe amalgam fracture strength. An analysis of variance was made to determine the influence of pins on the amalgam restorations for Cavities 3 and 4. For Restoration 3, there were two pins in the proximal portion, and the stresseswere applied to the mesialfossa.Restoration 4 contained 3 pins, and the stresseswere applied to the distal triangular fossa.Contrasts were studied between the means according to Tuckey’s test in casesof significance found in the analysis of variance. Tables III and IV present these results. It may be seen in Table III that, with the exception of 1 hour restorations in Cavity 4, there was a significant difference between sampleswith and without straight or bent pins. Contrasts for 1 hour restorations in Cavity 3 show that pins bent toward the occlusal surface increase the fracture strength of amalgam samples (Table IV) confirming Minelli’s observations on compressive and transverse strengths of pin amalgams.I4 In Cavity 3 restorations, straight pins tended to decreasethe fracture strength of the amalgam regardlessof age. Bent pins, in 1 day or 1 week restorations, did not seemto increase the strength of the amalgam when compared with restorations without pins. For Restoration 4, the analysis of variance showed no difference between 1 hour restorations with or without pins. For 1 day and 1 week restorations, the significant difference seemsto be due to the tendency of straight pins to decreasethe fracture strength (Table IV). Another interesting observation is that the more amalgam bulk, the greater the fracture strength. This was established by comparing the results for Restorations 1 and 2 with those of Restorations 3 and 4. The results partially agree with previous studies in that the pins increased retention.9l lo*l6 However, it must be pointed out that for Restoration 3 with 2 bent pins at 1 hour, for Restoration 4 with 3 bent pins, and for those restorations where
Volume Number
Strength
28 2
Table IV. Contrast
between
of restorations
means (Tuckey)
with
for Cavities
and
without
3 and 4
Comparisons
Amal-
_--.._
wm Cavity 3
Stresses applied
age (hr.)
Mesial fossa
1
24
168
4
Distal fossa
1 24
168
*N.S
=
+S. =
significant.
187
pins
Criti-
Conditions
Result
No pins-straight pins Bent pins-straight pins Bent pins-no pins No pins-straight pins Bent pins-straight pins No pins-bent pins No pins-straight pins Bent pins-straight pins Bent pins-no pins
39.80 56.20 56.20 127.00 123.80 127.00 125.20 131.00 131.00
-
39.60 39.60 39.80 93.40 93.40 123.80 108.60 108.60 125.20
= 0.20 = 16.60 = 16.40 =x 33.60 = 30.40 = 3.20 = 16.60 = 22.40 = 5.80
No pins-straight Bent pins-straight Bent pins-no No pins-straight Bent pins-straight No pins-bent
101.40 102.60 102.60 105.00 101.00 105.00
-
93.00 93.00 101.40 94.40 94.40 101.00
= = = = = =
pins pins
pins pins pins
pins
8.40 9.60 1.20 10.60 6.60 4.00
cal value
Interpre. tation
4.32 “ “ 22.“1 “ “ 11.97 ” “
N.S.” L5 .IJ. S. S. S. N.S. S. S. N.S.
9.99 “ “ 8.23 ‘< ‘(
N.S. N.S. N.S. N.S. S. N.S. N.S.
not significant.
the stresses were not directly applied fracture strength of the restorations.
to the pin site the pins definitely
increased
the
CONCLUSlONS There was no difference in the fracture strength of typical Class II (MOD) amalgam restorations when the compressive stress was applied to the mesial or distal fossa. MOD amalgam restorations for the lower first molar presented a higher fractun strength when the distobuccal cusp was restored. Straight pins located at the distoproximal portion of an MOD restoration decreased the 1 day and 1 week strength of amalgam restorations. Two bent pins located in the distoproximal portion of typical MOD restorations increased the 1 hour strength. The larger the bulk of amalgam in the proximal portion of an MOD cavity, the higher the fracture strength. In large MOD cavity preparations, 3 bent pins increased the fracture strength of the amalgam when the stresses were not applied at the pin sites. When the stresses were applied over the pins, the fracture strength was decreased. Straight pins decreased the fracture strength of amalgam restorations. In some instances, bent pins increased the fracture strength of amalgam restorations but did not decrease the fracture strength when compared with restorations without pins.
188
Mondelli
and
Vieira
J. Prosthet. August,
Dent. 1972
References 1.
Stibbs, G. D.: Cavity Preparation and Matrices for Amalgam Restorations, J. Am. Dent. Assoc. 56: 471-479, 1958. 2. Strickland, W. D.: Amalgam Restorations for Class I Cavity Preparations, in Sturdevant, C. M.: The Art and Science of Operative Dentistry, New York, 1968, McGraw-Hill Book Company, Inc., pp. 235-259. 3. Wolcott, R. B.: Failures in Dental Amalgam, J. Am. Dent. Assoc. 56: 479-491, 1958. 4. Markley, M. R.: Pin Reinforcement and Retention of Amalgam Foundations and Restorations, J. Am. Dent. Assoc. 56: 675-679, 1958. “Dentin,” Dent. Clin. North. Am., Nov., 1963, pp. 5. Courtade, G. L.: Creating Your Own 805-822. 6. Klein, R. E.: Steel Reinforced Restorations, J. PROSTHET. DENT. 14: 1140-1143, 1964. 7. Going, R. E., Maffa, J. P., Nostrant, G. W., and Johnson, B. E.: The Strength of Dental Amalgam, as Influenced by Pins, J. Am. Dent. Assoc. 77: 1331-1334, 1968. 8. Wright, R. W.: The Use of Stainless Steel Pins to Strengthen Amalgam Restorations, Aust. Dent. J. 3: 369-370, 1958. 9. Collard, E. W., Caputo, A. A., and Standlee, J. P.: Rationale for Pin Retained Amalgam Restorations, Dent. Clin. North Am. 14:43-51, 1970. 10. Watson, P. A., and Gilmore, H. W.: Use of Pins for Retaining Amalgam Restorations: A Synopsis, J. Can. Dent. Assoc. 36: 30-31, 1970. 11. Black, G. V.: Operative Dentistry, vol. 2, Chicago, 1908, Medico-Dental Publishing Company. 12. Bronner, F. J.: Mechanical, Physiological and Pathological Aspects of Operative Procedures, Dent. Cosmos 73: 577-584, 1931. 13. Co&a, A. A.: Los principios mecanicos ya las formas de resistencia y retention, in Pa&a, N. : TCcnica de Operatoria Dental, Buenos Aires, 1964, ODA, pp. 409-440. 14. Minelli, C. J.: Contribui$o ao estudo das resist&ncias B compress%o, B trat$o e ao dobramento do amalgama de prata, corn e sem inch&o de pinos metalicos, Faculdade de Farmdcia e Odontologia de RibeirZo P&o, Bras& Doctor’s thesis, 1967. 15. Johns, R. B.: Intra-coronal pins, Br. Dent. J. 127: 580-582, 1969. 16. Dyce, J. M., Dow, J. A., and Norcliffe, A.: The Restoration of Broken-Down Teeth. Pin Retention, Br. Dent. J. 125: 347-350, 1968. FACULDADE DE ODONTOLOCIA DE BAURU CAIXA POSTAL 73 17100 BAURU, Sjio PAULO, BRAZIL