The thickness of the bonding agent is increased to 50 micron from 20 micron (Graig, 2002). Copal varnish was also utilized below amalgam restorations to reduce microleakage. However, undercuts were needed in the restoration as the material did not offer adhesive between the tooth structure and the amalgam restoration (Cenci, 2004). 2. If the bonding agent tends to flow in the prepared cavity, it could accumulate along the gingival margins especially in class II and class V prepared cavitations and in the future cause microleakage and secondary caries (Bryant, 1998).
However, some studies have shown that bonding agents could effectively improve the marginal seal and reduce the microleakage especially in class V restorations, compared to unbonded amalgam (Graig, 2002). Several factors such as the viscosity of the bonding agent and the thickness in which it has been utilized play a major role in this regard. 3. At present clinical trials with the use of bonding agents for amalgam have not be conducted. Only short-term effects of the bonding agents and its effect on the amalgam restoration have been observed. In the long-run, the properties of the bonding agent have to be predicted (Bryant, 1998).
4. In prepared cavities, it may be very difficult to control the flow of a bonding agent. The material tends to accumulate along the margin, which makes it very difficult to manipulate (condense and carve) the amalgam (Bryant, 1998). 5. Bonding systems utilized in amalgam does not ensure that the tooth achieves it original strength before the cavity was prepared or before the caries was present (Graig, 2002). 6. Many researchers feel that as bonding amalgam does not improve the resistance for the amalgam filling, several other retentive characteristics including pins, slots, holes, grooves, dove tails, etc.
have to be provided, especially in large amalgam restorations to ensure that the retentive form is achieved. However, by using these characteristics in the prepared cavity, neither is the strength of the tooth increased nor does it act as reinforcement for amalgam (Graig, 2002). 7. Many clinicians have looked into the possibility of utilizing bonding agents to repair amalgam restorations. Studies have shown that the strength provided by bonding agents between one amalgam mixture and another is very low and was a failure if utilized clinically (Graig, 2002). 8.
Preparing a bonded amalgam restorations would require a lot of time and expertise from the operator compared to an unbonded amalgam restoration (Zbynek, 2002). Besides, it is very technique-sensitive and the restorations are comparatively costlier. The main aim of utilizing a bonding agent for amalgam is to enable some amount of interaction between the bonding agent and the unset silver amalgam material (before it sets). The developments in the field of bonding agents have been limited due to several problems. Of these, the most important one is that amalgam is an opaque material and does not permit light to pass through it.
Hence, the bonding agent should be able to be cured through chemical or self-cure means and not by visible or ultraviolet light means. Hence, bonding amalgam has been considered a separate entity from the area of bonded composites, and not developed along with bonded composite resins (Graig, 2002). A study was conduced by Zbynek et al to determine the clinical performance of bonded amalgam restorations in relation to conventional amalgam restorations over a longer term period (that is 5 years). The main disadvantage of this study was that the sample size which consisted both of cases and controls was small.
The number of bonded amalgam restorations that was performed was about 75 and the number of unbonded restorations was about 62. Several criteria mentioned by the US Public Health Service Department such as occlusal anatomical form, proximal anatomical form, occlusal marginal adaptation, proximal marginal adaptation, sensitivity, surface quality and secondary caries, were thoroughly evaluate for the amalgam restorations (Zbynek, 2002). The restorations were prepared and filled using the bonded and unbonded technique by using the toss of a coin.
Those restorations that were filled using the bonded technique were not given retentive features such as undercuts, grooves, etc. Class II was the most common restoration prepared. The teeth that were bonded were first etched; blot dried and then bonded using five layers of the primer and the appropriate adhesive cement. During the five-year period, about 6. 5 % of patients were lost in the case side and the control side of the study due to various reasons. The study demonstrated that there were not many differences between the bonded technique and the unbonded technique.
However, the study demonstrated that to a certain extent bonded amalgams could be utilized when certain retentive characteristics otherwise placed in conventional amalgam restorations were removed (Zbynek, 2002). This was seen over a 5-year period. The study demonstrated placing bonded amalgam instead of a composite pit and fissure sealant was equally successful over a two-year period. Several other criteria were also recorded during this study (but were not proved specifically). These included:- 1. lower microleakage levels 2. lower secondary caries 3. better marginal seal 4. reinforcement of the tooth structure 5.
lower hypersensitivity occurrence (Zbynek, 2002) Dental sealants are usually utilized over a period of 5 to 7 years when composite resins are used. As Dental amalgam has a wear resistance almost identical to that of the natural tooth material, and can tolerate the natural occlusal forces, they could be considered for use as pit and fissure sealants. Zbynek et al demonstrated that bonding amalgams could help to compensate for retentive features made in the tooth. This study by Staninec Et al (1998) was to demonstrate that bonded amalgam could be utilized as a pit and fissure sealants in deep fissures located adjacent to a carried lesion.
The study was also to prove that bonded amalgam could survive in thin sections in the oral cavity and needed not special retentive features. The study (including cases and controls) were conducted in about 114 teeth that included about 26 patients. After the pit and fissure sealants were placed in the cases and the controls, the patients were monitored every the 6 month, one year and the two year period. The study demonstrated that both bonded amalgam and resin sealants were similar in clinical performance over a two year period.
However, bonded amalgams could not replace composite resin sealants as they were not superior to the material. The study showed that the when the amalgam debonded from the pits and the fissures, the resin adhesive may have been left behind. On the other hand, when the resin sealant was lost, it had taken the resin adhesive along, and had left the deep pits and fissures of the tooth. The study also helped to provide more details about giving a cavosurface angle during bonding amalgam restorations. There was the decreased need of preparing a cavosurface margin in case bonded amalgam was utilized to fill the tooth (Staninec, 1998).
Another study was conducted by Davis and Overton (2000) to study the difference between the efficiency of pin-retained restorations over bonded amalgam restorations. Several criteria such as pain on chewing and hypersensitivity were recorded after a fortnight, three months and a year after placing the restoration. 20 bonded amalgam restorations and 20 pin-retained restorations were performed in about 38 patients. The study demonstrated that the pain to cold foodstuff was reduced compared following 3 months and one year in the bonded amalgam restorations, significantly, than the pin-retained restorations.
Both restorations ensured that pain on chewing was relieved. The bonded amalgams did not ensure that the sensitivity to cold was reduced at the 2 week interval following the insertion (Davis, 2002). Some of the common bonding systems utilized to bond amalgam includes, All Bond 2, Amalgam Bond Plus with HPA, Optibond 2 and Panavia. All bonds involve application of 10 % phosphoric acid to both dentin and enamel for about 15 seconds. Nakabayashi et al from Japan developed Amalgam Bond Plus more than 10 years ago. It contains 10 % citric acid and 3 % ferric chloride to decalcify the dentin and get rid of the smear layer.
Panavia is delivered in a 2 component system (either paste-paste or paste-powder). This bonding agent could bond the amalgam and reduce the microleakage. Thus it can be said that although, bonding amalgam can provide several benefits in the restoration, such as reducing provision of retentive characteristics, improvement in the resistance form, reduction of post operative hypersensitivity, reduction in the marginal leakage, etc, there are also several limitations associated with the technique such as microleakage and secondary caries in the gingival margin, etc.
Further studies need to be conducted to determine the long-tem effects of bonded amalgams in preserving and restoring the tooth structure. Besides, the applications of bonded amalgam should also be expanded.
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