|Year : 2015 | Volume
| Issue : 2 | Page : 38-42
Evaluating the strengthening effect of combined amalgam-composite restoration on the weakened wall of a dental cavity: An in vitro study
Syed Nahid Basheer
Department of Restorative Dental Science, College of Dentistry, Jazan University, Gizan, Kingdom of Saudi Arabia; Department of Restorative Dental Science, Krishnadevaraya College of Dental Sciences, Bengaluru, Karnataka, India
|Date of Web Publication||22-Jun-2015|
Syed Nahid Basheer
Department of Restorative Dental Science, College of Dentistry, Jazan University, PO Box Number: 114, Gizan 45142, Kingdom of Saudi Arabia
Source of Support: None, Conflict of Interest: None
Background: Combined amalgam-composite restoration capitalizes on the advantageous properties of the two materials and nullifies their individual disadvantages. Aim: This in vitro study was conducted to evaluate the strengthening effect of combined amalgam-composite restoration on the weakened wall of a dental cavity by measuring the cuspal deflection. Materials and Methods: Ten human maxillary premolar teeth with similar crown size were selected. Each tooth was subjected to a respective procedure and tested in the following sequence: Group 1 (sound teeth), Group 2 (cavity-prepared teeth), Group 3 (amalgam-restored teeth), Group 4 (amalgam-removed teeth), and Group 5 (amalgam-composite restored teeth). The cuspal deflection was measured using strain-gauge methodology. Statistical Analysis: ANOVA test and Bonferroni test for multiple comparisons were used. The level of significance was fixed at P < 0.05. Results: The mean cuspal deflection and standard deviation values recorded for buccal cusps of each of the five groups respectively were: 28.70 (8.88), 112.03 (55.93), 86.56 (40.77), 120.50 (59.03) and 59.40 (32.10) and for lingual cusps were: 118.00 (37.46), 178.80 (50.86), 147.47 (47.76), 185.33 (48.29), 151.03 (46.11). A significant difference in the cuspal deflection between the groups with respect to the buccal cusp (P < 0.001) and lingual cusp (P < 0.01) respectively was noted. However, as compared to sound teeth, there was no significant recovery of lost strength in amalgam-restored as well as in amalgam-composite restored teeth. Conclusion: The combination restorative technique cannot be relied upon to reinforce unsupported enamel to a level similar to that provided by dentin.
Keywords: Amalgam, buccal cusp, combined amalgam-composite restoration, cuspal deflection, lingual cusp
|How to cite this article:|
Basheer SN. Evaluating the strengthening effect of combined amalgam-composite restoration on the weakened wall of a dental cavity: An in vitro study. Dent Med Res 2015;3:38-42
|How to cite this URL:|
Basheer SN. Evaluating the strengthening effect of combined amalgam-composite restoration on the weakened wall of a dental cavity: An in vitro study. Dent Med Res [serial online] 2015 [cited 2019 Jun 15];3:38-42. Available from: http://www.dmrjournal.org/text.asp?2015/3/2/38/159178
| Introduction|| |
A "combined amalgam-composite restoration technique" presented by Abu-Hanna and Mjör in 2004 was intended to conserve unsupported enamel cavity walls and weakened cusps in extensive class 1 and class 2 preparations.  Bonded-resin composites act as dentin replacement to reinforce undermined enamel cusp.  The purpose of combining the two restorative materials was to capitalize on the advantageous properties of these two materials. , From various studies it is clear that bonding of the restoration material to the remaining tooth structure increases the fracture resistance of the restored tooth. 
The structure of posterior teeth with cusps and fossae has a tendency to deflect the cusps under stress. While sound teeth rarely fracture, cusp fracture may occur in teeth that have been weakened by carious lesion and cavity preparation. One experimental approach to study the effect of cavity preparations and restorative procedures is to apply loads to fractured teeth.  Deformation of cusps under physiologically applied forces can be indirectly measured (nondestructively) using either strain-gauge methodology or direct current differential transformer.  Measuring cuspal deflection using strain-gauges has been previously reported by various researchers.  The strain-gauge method is based on the use of electrical resistance and provides both in vitro and in vivo measurements of strains under static and dynamic loads.  The strain-gauges provide a sensitive estimation of cuspal flexure or strain within cusps indicating the strength of the tooth. More the cuspal deflection more is the loss of tooth strength.
The aim of this in vitro study was to evaluate the strengthening effect of combined amalgam-composite restoration on the weakened wall of a dental cavity by measuring the cuspal deflection.
| Methods|| |
Ten extracted, intact, noncarious, human, maxillary first premolar teeth of similar crown size were selected after scaling with a curette. The strengthening effect of combined amalgam-composite restoration was evaluated by comparing its cuspal deflection with the cuspal deflection produced by sound teeth (Group 1), cavity-prepared teeth (Group 2), amalgam-restored teeth (Group 3) and amalgam-removed teeth (Group 4) Amalgam-composite restored teeth (Group 5).
During the study, the cuspal deflection of the teeth samples was tested in the following sequence:
- Group 1 (Sound teeth) - The load was applied to the unprepared, unrestored teeth, and cuspal flexure measurements were recorded. These were taken as baseline measurements
- Group 2 (Cavity-prepared teeth) - Cavity was prepared, and the cuspal flexure measurements were taken for each cusp of the prepared unrestored teeth
- Group 3 (Amalgam-restored teeth) - Tooth was restored with amalgam and cuspal flexure measurements were recorded
- Group 4 (Amalgam-removed teeth) - Amalgam-restoration was removed with care and cuspal flexure measurements were recorded for the unrestored teeth
- Group 5 (Composite-amalgam-restored teeth) - The tooth was finally re-restored with combined amalgam-composite and cuspal flexure measurements were recorded.
Each test tooth was rigidly mounted in a cylindrical base of auto-polymerising resin. Roots of the tooth were embedded in the acrylic resin, at 2 mm below the cemento-enamel junction. The load-strain behaviour of the sound teeth (Group 1) was evaluated when an axial force was applied. Thus, a baseline cuspal deflection or an ideal load-strain reading for each cusp (buccal and lingual) was established on the strain indicator.
Three microstrain (μ strain) measurements were taken for each tooth, and the average was recorded which represented the final reading for each tooth.
Once the baseline cuspal deflection of the sound tooth was established, a cavity was prepared on the same tooth using new carbide bur mounted in a high-speed hand-piece with air and water spray. A standardized mesial-occlusal-distal (MOD) cavity preparation was designed with a bucco-lingual width of the isthmus measuring half the width of the intercuspal distance. The buccal and the lingual axial walls of the proximal cavity were parallel to each other and perpendicular to the gingival wall. Cavity depth was standardized to 2.5 mm. The gingival seat with 1.5 mm width was maintained at 1 mm above the cementoenamel junction. Undermining was done by removal of dentin apical and lingual to the buccalcusp, measuring 2.5 mm gingival to the occlusal enamel of each tooth. A bur was used in a slow speed hand-piece, and a uniform thickness of cuspal enamel of 1.5 mm, as measured by thickness gauge, was maintained. Following this, cavity-prepared tooth (Group 2) was loaded, and cuspal deflection was recorded on the strain indicator.
After measuring the cuspal deflection of the cavity-prepared tooth (Group 2), it was restored with amalgam. Later loading was carried out following storage in water for 24 h at room temperature and the cuspal deflection was recorded (Group 3).
This was followed by removal of amalgam with great care without disturbing the cavity dimensions and then followed by loading. The measurements of cuspal deflection were noted (Group 4). After taking the measurements, the internal buccal enamel wall surface was etched with 37% orthophosphoric acid gel for 30 s, rinsed for 30 s, and blot dried with cotton. Then bonding was done with a single bond (3M ESPE) and photocured through the enamel walls for 40 s using a light-activating unit. A thick monolayer of hybrid composite (Filtek Z250) was applied to the undermined buccal cusp and photocured for 40 s through the cavity walls to direct the polymerization contraction toward the enamel. The rest of the preparation was restored with amalgam, followed by storage in water for 24 h at room temperature. The composite-amalgam-restored tooth (Group 5) was loaded, and cuspal deflection was recorded. Same procedure was followed for each of the ten teeth and mean cuspal deflection was determined for each group.
The cuspal deflection was measured using a nondestructive testing technique with strain gauges. The strain-gauges used in this study consisted of a single element precalibrated constantan grid fully encapsulated in a polyimide connected to a strain indicator (SI-30 Digital Strain Indicator, SYSCON Instruments Pvt. Ltd., Bengaluru, India) with accuracy of 1 μ. The mounted teeth were positioned on the lower plate of an Instron Testing Machine, which delivered an uniaxial compressive load of 100N comparable to the average chewing forces in the mouth (13 - 147N) at a crosshead speed of 5 mm/min using a metal rod (4 mm diameter) directly through the cusps, and outward flexure was measured in response to the applied force. The deflection was recorded in terms of μ strain unit for each cusp.
The mean cuspal deflection values of five groups were statistically analyzed using ANOVA test, and Bonferroni test was used for multiple comparisons. The level of significance was fixed at P < 0.05.
| Results|| |
The mean and standard deviation values of cuspal deflection in μ strain for each of the five groups are presented in [Table 1]. The buccal cuspal deflection readings of the sound teeth (Group 1 (28.70)) was found to increase when the teeth were subjected to cavity preparation (Group 2 (112.03)). Further, the cuspal deflection values decreased in the amalgam-restored teeth (Group 3 (86.56)) and increased in amalgam-removed teeth (Group 4 (120.50)). The combined amalgam-composite restored teeth (Group 5 (59.50)) showed a drastic reduction in the cuspal deflection values. A similar pattern was observed in mean lingual cuspal deflection values. It was observed that the baseline lingual cuspal deflection value was high compared to that of the buccal cusp value. ANOVA test was used to compare the cuspal deflection recorded in the five groups for buccal and lingual cusp respectively. The results suggested that there was a significant difference in the cuspal deflection between the groups with respect to the buccal cusp (P < 0.001) and lingual cusp (P < 0.01) respectively.
|Table 1: Intra-group comparison of buccal and lingual cuspal deflection values using ANOVA|
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The results of the multiple comparisons for buccal cuspal deflection [Table 2] between groups suggested that there was a significant difference when Group 1 was compared with Group 2, 3 and 4 respectively and between Group 4 and Group 5. Similarly, multiple comparisons of lingual cuspal deflection [Table 3] between groups suggested that there was a significant difference between Group 1 and Group 4 only.
|Table 2: Multiple comparisons between groups for buccal cuspal deflection values|
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|Table 3: Multiple comparisons between groups for lingual cuspal deflection values|
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| Discussion|| |
The results of this study indicated that neither combined amalgam-composite restoration nor amalgam could substantially strengthen the weakened wall of the dental cavity. This is in consensus with earlier study reports, which have failed to demonstrate strengthening of premolar teeth with either bonded MOD or complex amalgam-restoration. , Some authors have found that bonded composites increase the fracture resistance of molars with class 1 and class 2 restorations; others, contend that the fracture resistance is sacrificed by the preparation and is independent of the restorative material. 
The results of the study suggested that although there was highly significant loss of strength buccally in the cavity-prepared teeth (Group 2) as compared to sound teeth (Group 1), no significant loss of strength was observed lingually in these two groups. Following cavity preparation, an increase in cuspal deflection buccally as compared to sound teeth was the evidence of the weakening effect observed on the teeth. The MOD cavity preparation resulted in the separation of the buccal and lingual cusps. This, in turn, increased outward cuspal deflection following cavity preparation compared to sound teeth indicating the importance of maintaining the integrity of connectivity and interaction between the buccal and lingual cusps of these teeth. The results obtained were in agreement with other studies. ,,
Buccally the loss of strength due to cavity preparation could not be recovered significantly by amalgam-restoration (P < 0.05). However, lingually, there was no difference in cuspal deflection of cavity prepared the tooth and amalgam-restored tooth (P > 0.05). Probably, the presence of sound dentin supporting occlusal enamel lingually must have attributed to the strength. Buccally and lingually there was a significant loss of strength in amalgam-removed teeth when compared to sound teeth. However, there was no significant loss of strength in amalgam-removed teeth when compared with amalgam-restored teeth buccally and lingually. This may be because either restoration or removal of amalgam did not significantly affect the strength of the teeth. Since there was no bonding between the cusps it was likely that amalgam-restoration served only as an obturator. The tooth restored with amalgam remained virtually unconnected.
There was no significant recovery of lost strength in composite-amalgam-restored teeth when compared with amalgam-restored teeth buccally as well as lingually. However, there was a significant recovery (P < 0.05) of lost strength in composite-amalgam restored teeth compared to amalgam-removed teeth buccally, but not lingually. Buccally, the minimal improvement in the strength between composite-amalgam-restored teeth and amalgam-restored teeth, could be because of the bond formation between the enamel and the composite resin due to acid etching and bonding procedure. The more cuspal deflection observed in the amalgam-removed teeth lingually could be related to further weakening of tooth structure as a result of the inadvertent removal of dentin while removing the amalgam.
Previously, the in vivo studies by Kaur and Samra  and Kaur et al.  has confirmed that apart from esthetics, the use of composite-amalgam-restoration technique requires less removal of the sound tooth structure and improves fracture resistance of teeth. The increase in fracture resistance is directly related to the recovery of lost strength of the tooth. Ideally, a restorative material plays an important role in returning the same load-strain behavior of the restored tooth as displayed by the sound tooth under all conditions. The bonded composite in vitro is known to have the elastic modulus comparable to that of the sound dentin.  In the present study, the recovery of the lost strength was not significant with composite-amalgam-restored teeth when compared with sound teeth. The combined restoration did not provide support for occlusal enamel in the buccal wall that was equal or close to that provided by sound dentin. These findings were consistent with study by Latino et al.  The results of the in vitro study cannot be compared with in vivo study design.
The nondestructive forces applied to measure cuspal deflection during the study allow sequential testing of the experimental variables on the same tooth, thus overcoming sample variation, eliminating the need for a control group and a large sample size.  However, one cannot sidestep the limitation, that the force applied is of static nature compared with the dynamic repeated loads that teeth are exposed to in the oral cavity. In the present study, large values of standard deviations were observed, which could be attributed to the individual variations in morphology among the teeth including angulations of cuspal inclines, thickness of enamel, inherent weaknesses, slight variations among the size of teeth, and slight variations in the level of contact of the metal rods with the cuspal inclines. 
However, within the limitation of the study it can be concluded that the combination restorative technique cannot be relied upon to reinforce unsupported enamel to a level similar to that provided by dentin.
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[Table 1], [Table 2], [Table 3]