# Cusp Fracture Resistance of Maxillary Premolars Restored with the Bonded Amalgam Technique Using Various Luting Agents

Authors: Shivaughn M. Marchan; Larry Coldero; Daniel White; William A. J. Smith; Reisha N. Rafeek
Journal: International Journal of Dentistry
 (2009)

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## Abstract

Objective. This in vitro study uses measurements of fracture resistance to compare maxillary premolars restored with the bonded amalgam technique using a new resin luting cement, glass ionomer, and resin-modified glass ionomer as the bonding agents. Materials. Eighty-five sound maxillary premolars were selected and randomly assigned to one of five test groups of 17 teeth each. One group of intact teeth served as the control. The remaining groups were prepared to a standard cavity form relative to the dimensions of the overall tooth and restored with amalgam alone or a bonded amalgam using one of three luting agents: RelyX Arc (a new resin luting cement), RelyX luting (a resin-modified glass ionomer), or Ketac-Cem μ (a glass ionomer) as the bonding agents. Each tooth was then subjected to compressive testing until catastrophic failure occurred. The mean loads at failure of each group were statistically compared using ANOVA with a post hoc Bonferroni test. Results. It was found that regardless of the luting cement used for the amalgam bonding technique, there was little effect on the fracture resistance of teeth. Conclusion. Cusp fracture resistance of premolars prepared with conservative MOD cavity preparations is not improved by using an amalgam-bonding technique compared to similar cavities restored with amalgam alone.


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## Body

## 1. Introduction

Amalgam has been  shown to  be an acceptable material with proven longevity for large
posterior restorations [1].
The major disadvantage of amalgam, however, is its inability to bond to dental
hard tissues preventing reinforcement or strengthening of prepared tooth
structure. This lack of bonding necessitates the use of macromechanical
retentive features which are inherently destructive and cause further weakening
of the remaining tooth structure [2].Resin bonding agents
have been used for a number of years as an adhesive liner between dental
amalgam and tooth structure.  In vitro tests have shown that these adhesive
systems form retentive bonds with amalgam that increase
the shear bond strengths of amalgam to tooth and reduce microleakage [2, 3]. Other studies
claim an increase in the fracture resistance of prepared teeth when restored
with bonded amalgam [4, 5].More recently,
glass ionomer formulations have been tested as an adhesive liner with dental
amalgam for the restoration of prepared teeth [6–8]. Certain glass
ionomer formulations have demonstrated increased shear bond strength
measurements of amalgam to dental hard tissue comparable to that achieved with
resin systems [6, 8]. The fracture resistance of teeth restored with the
bonded amalgam technique using restorative glass ionomer as an adhesive has
been shown to be greater than that of prepared teeth alone and teeth restored
with amalgam using a copalite liner [7].The objective of
this study was to assess the resistance of premolars with traditional MOD
preparations to cuspal fracture when restored with amalgam that has been
adhesively bonded to cavity preparations using various glass ionomer
formulations as the adhesive liner compared to cavities restored with amalgam
bonded with a resin luting liner and cavities restored with amalgam alone.


## 2. Methods and Materials

Three
groups of teeth restored with the amalgam bonding technique, 1 group of intact
teeth, and 1 group of teeth restored with amalgam only were included in this
study. The luting cements used as amalgam bonding agents utilized in this study
are shown in Table1. A power calculation determined
that a minimum of 10 teeth were needed in each group. A sample size of
17 per group was employed.Table 1
Mean fracture strengths and standard
deviations of tested teeth.
GroupNMean (S.D.)ANOVA/post hoc Bonferroni comparisonControl
(intact)152070.9(317.02)AAmalgam16917.4(198.04)BRelyX
Arc16834.5(323.73)BRelyX Luting17869.6(299.39)BKetac-Cemμ15917.8(344.73)BSimilar
letters are not significant atP=.05Eighty-five
human maxillary premolars extracted for orthodontic reasons were collected and
immediately stored in distilled water. The teeth were rinsed and any debris
removed with a sonic scaler (Titan SW Scaler, Dental EZ Group, StarDental
Division, Lancaster, Pa, USA).
The teeth were embedded in individual cylinders of autopolymerizing polymethyl
methacrylate (Fas-Tray, Harry J Bosworth Company, Stokie, Ill,
USA)
1.0 mm short of the cementoenamel junction so as to mimic alveolar support for
the tooth. After complete setting of the tray acrylic, the base was trimmed to
expose a cross-section of the root in its apical one third. This allowed
transmission of applied force entirely through tooth structure by preventing
settling of the tooth within the tray acrylic during testing. The mounted teeth
were randomly divided into five groups and stored in distilled water at37°C.


### 2.1. Cavity Preparation

Mesio-occlusal-distal (MOD) cavities were prepared using a 245 bur (SS White, Lakewood,
NJ, USA) in a high-speed handpiece (BELLAtorque, KaVo Dental Corporation, Lake Zurich, Ill, USA) with water spray. A new bur was used for every 5 teeth. One operator (WS) completed all the preparations. The dimension of each preparation was made proportional to the dimensions of the tooth to minimize variations as a result
of the size of individual teeth.The occlusal isthmus had a facial-lingual width of 1/3 intercuspal distance. The pulpal floor was 2/3 of the height of the cusps measured from the cementoenamel junction. The facial-lingual width of the approximal boxes was 1/3 the maximum facial-lingual width of the tooth. The gingival floor of the approximal box was
established 1.0 mm above the cementoenamel junction. The depth of the approximal box (from gingival cavosurface to axial wall) was 1.0 mm. All internal line angles were rounded and no additional retentive features were cut.Prior to actual cavity preparation, the outline of each cavity was drawn on the
surface of the tooth as a preparation guide.


### 2.2. Adhesive Application and Restoration

#### 2.2.1. Control Group

The control
group, of unprepared teeth, was returned to water for storage until testing.


#### 2.2.2. Amalgam Group

Number 1 Tofflemire band
(Sullivan-Schein Dental, Melville, NY, USA) was placed in a
universal Tofflemire matrix retainer (Teledyne Getz, Elk Groove Village, Ill, 60007) and secured
around the tooth. The cavity walls were dried using clean compressed air. High
copper, platinum modified, precapsulated, spherical amalgam (Logic, SDI Ltd, Australia) was mechanically triturated (Wig-L-Bug, Crescent Dental Mfg Co, Lyons,
Ill, USA) following the manufacturer’s
instructions followed by hand condensation into the prepared cavity. The matrix
band was removed and the amalgam was carved and finished following the normal
anatomical contours of the tooth.


#### 2.2.3. Adhesive Application

The cavity walls of the remaining
three groups were coated with one of three luting cements: RelyX Arc (3M ESPE, St. Paul, Minn, USA), RelyX luting (3M ESPE, St. Paul, Minn, USA), or Ketac-Cemμ (3M ESPE, St. Paul, Minn, USA). All materials were dispensed
and handled according to the manufacturer’s instructions.Following
cement application, each tooth was immediately restored in the same manner as
the amalgam group, with the amalgam being condensed into the cavity while the
adhesive liners were still unset. The matrix band was removed to allow carving
of the amalgam to normal anatomical contours and removal of any adhesive flash
at cavosurface margins. The exact handling techniques utilized for each adhesive material
were as follows.RelyX Arc
The cavity was etched with 37% phosphoric acid for 20 seconds using the total etch
technique. The acid was then thoroughly rinsed for 20 seconds and the cavity
was dried, but not desiccated, with compressed air. Single bond (3M ESPE, St. Paul, Minn, USA) was applied to the cavity using a microapplicator tip and dried for 2 seconds with compressed air,
leaving glossy cavity surfaces. The dentine bonding agent was then light cured
for 10 seconds using visible blue light (Optilux, Demetron/Kerr 21 Commerce Drive,
Danbury, Conn, USA) at 460ηm wavelength. The RelyX Arc resin luting cement was dispensed in equal
amounts and mixed for 10 seconds and applied to the cavity using a microapplicator brush. Amalgam was condensed and finished as
previously described.RelyX Luting
The cavity was washed with distilled water and dried with
compressed air from the air-water tip, preventing desiccation of tooth tissue.
The powder was fluffed and the standard powder: liquid ratio of 1.6:1, by
weight, was dispensed on a glass slab. Mixing was completed with the aid of a
metal spatula for 30 seconds. The mixed cement was applied to the cavity walls
using a microapplicator brush. Amalgam was condensed and finished as previously
described.Ketac-Cemμ
the cavity was washed
with distilled water and dried with compressed air from the air-water tip,
preventing desiccation of tooth tissue. The powder was fluffed and the standard
powder: liquid ratio of 3.8:1, by weight, was dispensed on a glass slab. Mixing
was completed with the aid of a metal spatula for 30 seconds. The mixed cement
was applied to the cavity walls using a microapplicator brush. Amalgam was
condensed and finished as previously described.


### 2.3. Testing

All
specimens (control and restored teeth) were stored in 100% humidity at37°C
for 14 days prior to testing. The teeth were placed
in a stabilizing ring, to avoid movement, and tested in compression in a Hounsfield
H50KS tensometer (Tinius Olsen Ltd, Redhill, Surrey, UK) using a rounded, stainless steel, testing probe 5 mm in
cross-section. The profile of the testing probe ensured contact with buccal and
lingual inclines of cusp triangular ridges and not the amalgam restoration (see
Figure 1). The teeth were preloaded to a maximum force of 250 N at a speed of
5 mm/min for 10 times (with the speed of return at 50 mm/min), prior to
catastrophic testing. The preloading was performed to simulate clinical
situations where the occlusal surface of the tooth is subjected to repeated
chewing forces. Following this preloading procedure, the teeth were tested to
failure at a speed of 0.5 mm/min until cusp fracture occurred.Figure 1
Photograph of restored tooth within compressive testing probe in contact with
the cuspal inclines.Means, with standard deviations, were calculated for each
group. Group means were analyzed using a one-way analysis of variance (ANOVA)
followed by a post hoc Bonferroni multiple comparison test.


## 2.1. Cavity Preparation

Mesio-occlusal-distal (MOD) cavities were prepared using a 245 bur (SS White, Lakewood,
NJ, USA) in a high-speed handpiece (BELLAtorque, KaVo Dental Corporation, Lake Zurich, Ill, USA) with water spray. A new bur was used for every 5 teeth. One operator (WS) completed all the preparations. The dimension of each preparation was made proportional to the dimensions of the tooth to minimize variations as a result
of the size of individual teeth.The occlusal isthmus had a facial-lingual width of 1/3 intercuspal distance. The pulpal floor was 2/3 of the height of the cusps measured from the cementoenamel junction. The facial-lingual width of the approximal boxes was 1/3 the maximum facial-lingual width of the tooth. The gingival floor of the approximal box was
established 1.0 mm above the cementoenamel junction. The depth of the approximal box (from gingival cavosurface to axial wall) was 1.0 mm. All internal line angles were rounded and no additional retentive features were cut.Prior to actual cavity preparation, the outline of each cavity was drawn on the
surface of the tooth as a preparation guide.


## 2.2. Adhesive Application and Restoration

### 2.2.1. Control Group

The control
group, of unprepared teeth, was returned to water for storage until testing.


### 2.2.2. Amalgam Group

Number 1 Tofflemire band
(Sullivan-Schein Dental, Melville, NY, USA) was placed in a
universal Tofflemire matrix retainer (Teledyne Getz, Elk Groove Village, Ill, 60007) and secured
around the tooth. The cavity walls were dried using clean compressed air. High
copper, platinum modified, precapsulated, spherical amalgam (Logic, SDI Ltd, Australia) was mechanically triturated (Wig-L-Bug, Crescent Dental Mfg Co, Lyons,
Ill, USA) following the manufacturer’s
instructions followed by hand condensation into the prepared cavity. The matrix
band was removed and the amalgam was carved and finished following the normal
anatomical contours of the tooth.


### 2.2.3. Adhesive Application

The cavity walls of the remaining
three groups were coated with one of three luting cements: RelyX Arc (3M ESPE, St. Paul, Minn, USA), RelyX luting (3M ESPE, St. Paul, Minn, USA), or Ketac-Cemμ (3M ESPE, St. Paul, Minn, USA). All materials were dispensed
and handled according to the manufacturer’s instructions.Following
cement application, each tooth was immediately restored in the same manner as
the amalgam group, with the amalgam being condensed into the cavity while the
adhesive liners were still unset. The matrix band was removed to allow carving
of the amalgam to normal anatomical contours and removal of any adhesive flash
at cavosurface margins. The exact handling techniques utilized for each adhesive material
were as follows.RelyX Arc
The cavity was etched with 37% phosphoric acid for 20 seconds using the total etch
technique. The acid was then thoroughly rinsed for 20 seconds and the cavity
was dried, but not desiccated, with compressed air. Single bond (3M ESPE, St. Paul, Minn, USA) was applied to the cavity using a microapplicator tip and dried for 2 seconds with compressed air,
leaving glossy cavity surfaces. The dentine bonding agent was then light cured
for 10 seconds using visible blue light (Optilux, Demetron/Kerr 21 Commerce Drive,
Danbury, Conn, USA) at 460ηm wavelength. The RelyX Arc resin luting cement was dispensed in equal
amounts and mixed for 10 seconds and applied to the cavity using a microapplicator brush. Amalgam was condensed and finished as
previously described.RelyX Luting
The cavity was washed with distilled water and dried with
compressed air from the air-water tip, preventing desiccation of tooth tissue.
The powder was fluffed and the standard powder: liquid ratio of 1.6:1, by
weight, was dispensed on a glass slab. Mixing was completed with the aid of a
metal spatula for 30 seconds. The mixed cement was applied to the cavity walls
using a microapplicator brush. Amalgam was condensed and finished as previously
described.Ketac-Cemμ
the cavity was washed
with distilled water and dried with compressed air from the air-water tip,
preventing desiccation of tooth tissue. The powder was fluffed and the standard
powder: liquid ratio of 3.8:1, by weight, was dispensed on a glass slab. Mixing
was completed with the aid of a metal spatula for 30 seconds. The mixed cement
was applied to the cavity walls using a microapplicator brush. Amalgam was
condensed and finished as previously described.


## 2.2.1. Control Group

The control
group, of unprepared teeth, was returned to water for storage until testing.


## 2.2.2. Amalgam Group

Number 1 Tofflemire band
(Sullivan-Schein Dental, Melville, NY, USA) was placed in a
universal Tofflemire matrix retainer (Teledyne Getz, Elk Groove Village, Ill, 60007) and secured
around the tooth. The cavity walls were dried using clean compressed air. High
copper, platinum modified, precapsulated, spherical amalgam (Logic, SDI Ltd, Australia) was mechanically triturated (Wig-L-Bug, Crescent Dental Mfg Co, Lyons,
Ill, USA) following the manufacturer’s
instructions followed by hand condensation into the prepared cavity. The matrix
band was removed and the amalgam was carved and finished following the normal
anatomical contours of the tooth.


## 2.2.3. Adhesive Application

The cavity walls of the remaining
three groups were coated with one of three luting cements: RelyX Arc (3M ESPE, St. Paul, Minn, USA), RelyX luting (3M ESPE, St. Paul, Minn, USA), or Ketac-Cemμ (3M ESPE, St. Paul, Minn, USA). All materials were dispensed
and handled according to the manufacturer’s instructions.Following
cement application, each tooth was immediately restored in the same manner as
the amalgam group, with the amalgam being condensed into the cavity while the
adhesive liners were still unset. The matrix band was removed to allow carving
of the amalgam to normal anatomical contours and removal of any adhesive flash
at cavosurface margins. The exact handling techniques utilized for each adhesive material
were as follows.RelyX Arc
The cavity was etched with 37% phosphoric acid for 20 seconds using the total etch
technique. The acid was then thoroughly rinsed for 20 seconds and the cavity
was dried, but not desiccated, with compressed air. Single bond (3M ESPE, St. Paul, Minn, USA) was applied to the cavity using a microapplicator tip and dried for 2 seconds with compressed air,
leaving glossy cavity surfaces. The dentine bonding agent was then light cured
for 10 seconds using visible blue light (Optilux, Demetron/Kerr 21 Commerce Drive,
Danbury, Conn, USA) at 460ηm wavelength. The RelyX Arc resin luting cement was dispensed in equal
amounts and mixed for 10 seconds and applied to the cavity using a microapplicator brush. Amalgam was condensed and finished as
previously described.RelyX Luting
The cavity was washed with distilled water and dried with
compressed air from the air-water tip, preventing desiccation of tooth tissue.
The powder was fluffed and the standard powder: liquid ratio of 1.6:1, by
weight, was dispensed on a glass slab. Mixing was completed with the aid of a
metal spatula for 30 seconds. The mixed cement was applied to the cavity walls
using a microapplicator brush. Amalgam was condensed and finished as previously
described.Ketac-Cemμ
the cavity was washed
with distilled water and dried with compressed air from the air-water tip,
preventing desiccation of tooth tissue. The powder was fluffed and the standard
powder: liquid ratio of 3.8:1, by weight, was dispensed on a glass slab. Mixing
was completed with the aid of a metal spatula for 30 seconds. The mixed cement
was applied to the cavity walls using a microapplicator brush. Amalgam was
condensed and finished as previously described.


## 2.3. Testing

All
specimens (control and restored teeth) were stored in 100% humidity at37°C
for 14 days prior to testing. The teeth were placed
in a stabilizing ring, to avoid movement, and tested in compression in a Hounsfield
H50KS tensometer (Tinius Olsen Ltd, Redhill, Surrey, UK) using a rounded, stainless steel, testing probe 5 mm in
cross-section. The profile of the testing probe ensured contact with buccal and
lingual inclines of cusp triangular ridges and not the amalgam restoration (see
Figure 1). The teeth were preloaded to a maximum force of 250 N at a speed of
5 mm/min for 10 times (with the speed of return at 50 mm/min), prior to
catastrophic testing. The preloading was performed to simulate clinical
situations where the occlusal surface of the tooth is subjected to repeated
chewing forces. Following this preloading procedure, the teeth were tested to
failure at a speed of 0.5 mm/min until cusp fracture occurred.Figure 1
Photograph of restored tooth within compressive testing probe in contact with
the cuspal inclines.Means, with standard deviations, were calculated for each
group. Group means were analyzed using a one-way analysis of variance (ANOVA)
followed by a post hoc Bonferroni multiple comparison test.


## 3. Results

During pretesting, a number of specimens were lost in each
group due to early fracture. Two teeth were lost in the sound group, 1 in the
amalgam group, 1 in the RelyX Arc group, none in the RelyX luting group, and 2
in the Ketac-Cemμ group. The analysis of variance revealed that there were no
significant differences in fracture resistance of teeth restored with amalgam
alone or amalgam bonded with various luting cements (P>.05). There
was however a highly significant difference between the intact teeth (control
group) and all the restored groups (P<.05). The means and standard
deviations, together with statistical significance, for the remaining teeth are
presented in Table 1. This study did not attempt to characterize the fracture
patterns of the failed specimens.


## 4. Discussion

Various studies
have shown that prepared teeth fracture more readily than sound intact teeth [9, 10]. Ideally any material that is used to restore
missing tooth structure should reinforce the tooth and minimize risk of cuspal
fracture.In this study,
the difference in resistance to catastrophic fracture between the sound
(unprepared) teeth and restored teeth was highly significant. This supports
previous findings that demonstrate the deleterious effect that cavity preparation
has on the fracture resistance of posterior teeth [11].The results of
this study showed no statistically significant differences in cusp fracture
resistance between teeth restored with the bonded amalgam technique that
utilized either a resin-based adhesive (RelyX Arc), glass-ionomer adhesive
(Ketac-Cemμ), or resin-modified-glass ionomer (RelyX luting) and a
conventional technique using amalgam alone. These findings are contrary to
those of other investigators, who have shown an increase in fracture resistance
of teeth restored with the bonded amalgam technique using both resin and glass-ionomer
luting cements when compared to teeth restored with amalgam alone [7, 12]. This difference may be explained by the size of
the cavity preparation utilized in this study. The isthmus preparation
dimensions of 1/3 the intercuspal width and the facial-lingual width of the
approximal boxes at 1/3 the maximum facial-lingual width of the tooth could be
considered conservative. This resulted in a significant portion of coronal
dentin remaining, the mechanical properties of which contributed to similar
fracture resistance values within the tested groups. Indeed, researchers that
use more conservative cavity preparations have found that fracture resistance
is not improved regardless of the restorative material or technique used [13]. In a similar study where amalgam was bonded into
cavities of similar dimensions using filled and unfilled resin systems, it was
noted that there was no increase in the fracture resistance of restored teeth [13]. This may explain the unexpected results shown by
the sole resin system used, RelyX Arc, to bond amalgam in this particular
study.Investigators who use larger cavity
preparations have consistently shown that the use of the amalgam-bonding
technique significantly contributes to the fracture resistance of the teeth [5, 14].This particular
research produced large variations in the standard deviation values among the
tested groups. This may be attributed to the normal mechanical and anatomical
variations of natural teeth, including, the ratio of enamel to dentin, cusp
position and angulation, cusp height, and undetectable flaws. Another
explanation of the high standard deviations may be due to the catastrophic
forces applied in vitro until failure occurred. Such in vitro forces are applied at a constant
direction and speed until failure occurs, which rarely mimics failure intraorally.Although the
glass ionomer and resin-modified glass-ionomer luting cements used as adhesives for amalgam alloy did not increase
the fracture resistance of the prepared teeth in this particular study, it must
be noted that both conventional glass ionomer and resin-modified materials have
shown increased measurements of shear bond strengths at the dentin/amalgam
interface which has the effect of increasing retention of amalgam to tooth
structure, thus diminishing the reliance on macromechanical features which has
the overall effect of conserving tooth
tissue [6–8].


## 5. Conclusion

Within the
limitations of this study, the conclusion may be drawn that maxillary premolars
restored with the bonded amalgam technique using glass ionomer, resin-modified
glass ionomer, and resin luting cements do not improve measurements of fracture
resistance. The clinical relevance of this is that amalgam bonded into
conservative MOD preparations of premolars does not offer increased strength to
the tooth and restoration but only unnecessarily increases the complexity of
the clinical procedure.



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*Source: 946830-2008-12-25.xml*