Lec 8 - Step-by-Step Technique for Placing Amalgam Restoration PDF

Preclinical Conservative Dentistry Dr. Maryam F. Ibrahim Lec:8 Step-by-Step Technique for Placing Amalgam Restoration Selection of the Alloy The selection of the alloy powder mainly depends on the operato...

Preclinical Conservative Dentistry Dr. Maryam F. Ibrahim Lec:8 Step-by-Step Technique for Placing Amalgam Restoration Selection of the Alloy The selection of the alloy powder mainly depends on the operator. The high-copper alloys are preferred for clinical use because they do not have the γ2 phase after the setting reaction with mercury, which is considered the weakest phase, responsible for the low strength of the low-copper alloys. Also, high-copper alloys possess good corrosion resistance and low creep. Finer particle sizes are used for the ease of handling and dispensing; in addition, they produce a smoother surface during carving and finishing. Spherical particle alloys are preferred over lathe-cut alloys, because they have more regular surfaces, require less condensation pressure, and require less mercury for trituration. Non-zinc containing alloys are used only in those cases where moisture control is very difficult. Proportioning (Mercury Alloy Ratio) Despite the various techniques prescribed, the most acceptable one is the minimal mercury technique (Eames’ technique). In 1960, Eames was the first to promote low mercury:alloy ratio. According to this technique, the recommended mercury-to-alloy ratio is 1:1 by volume. This amounts to the fact that the mercury content is 50%. However, for spherical alloys, the recommendations for mercury are closer to 42%, because spherical particles have lower surface-to-volume ratio and so they require less mercury to completely wet the particles. Dispensing A wide variety of dispensers are available through which mercury and alloy are dispensed for dental use. Some of them are as follows: 1. Bottles containing bulk of mercury (30 g) and alloy powder (225 g) are available. The proportioning is done by the dentist according to the amount of the mix required for the restoration to be done. Although this kind of dispensing is economical, inappropriate proportioning can lead to too grainy or too plastic mix, thus hampering the properties of the material. 2. Pre-weighed pellets: The pellets of alloy are available in different sizes, which can be mixed with appropriate amount of mercury to obtain a suitable mix. 3. Disposable capsules containing pre-proportioned aliquots of mercury and alloy are also available. These pre-proportioned capsules contain alloy particles and mercury in compartments separated by a disc or membrane. In the older type, before use, the membrane was ruptured by compression of the capsule and then the capsule was placed in an amalgamator. Newer types, called self-activating types, are now available which are more convenient but expensive. In these capsules, the membrane ruptures on its own once the capsule is activated in the amalgamator. 4. Reusable capsules are also available into which the amalgam alloy is dispensed as a pellet of pressed powder of standard weight and mercury is dispersed from an automatic dropper bottle. The capsule and its contents are automatically mixed using an amalgamator. The amalgamator is designed to oscillate in a pattern of figure ‘8’. Trituration Trituration is described as the process of mixing the amalgam alloy particles with mercury. The alloy and mercury can be mixed or triturated by hand with mortar and pestle or with a mechanical amalgamator, which saves time and standardizes the procedure. Objectives 1. To remove oxides from the powdered particle surface, facilitating direct contact between the alloy particles and the mercury 2. To achieve a workable mass of amalgam within a minimum time, leaving sufficient time for insertion into the cavity Hand Trituration This is done using mortar and pestle. These can be made of either glass or ceramic. The mortar should rest on a firm base and uniform pressure should be applied during mixing. Mixing time should be well controlled. The surface texture of the mortar and the pestle should be rough so as to produce enough friction to remove the oxide layer of the alloy particles and to ensure proper coating of the alloy particles with mercury. Since ceramic ones possess more rough texture in the area to be used for mixing, they are considered better. Mechanical Trituration This is done using a mechanical amalgamator. It can work on two major principles: (a) amalgamator without the use of capsules and (b) amalgamator with the use of capsules. Amalgamator without the Use of Capsules In this electrically driven system, alloy powder and mercury are put in bulk in their respective compartments provided on the top of the machine. The amount of material and time of mixing are set manually and the amalgamator is turned on. The mix can be received from the small projection in the front of the amalgamator. Amalgamator with the Use of Capsules Alloy and mercury are dispensed into a reusable capsule or a disposable capsule system is used. In this kind of mechanical amalgamator, the capsule acts as mortar and a cylindrical metal or plastic piston of diameter and length smaller than the capsule (placed inside the capsule) acts as pestle. When the capsule is secured into the machine and the machine is turned on, the arm holding the capsule oscillates and thus trituration is accomplished. Automatic timer is present for controlling the mixing time, and most of the modern amalgamators have two or more mixing speeds. New amalgamators have hoods that cover the reciprocating arms holding the capsule. The purpose of this hood is to prevent the scattering of mercury due to its accidental spillage from the amalgamator. The three basic movements of mechanical trituration are as follows: 1. Mixing arm carrying a capsule moves back and forth in a straight line. 2. Mixing arm travels back and forth in a figure-of-eight pattern. 3. Mixing arm travels in a centrifugal fashion. The time for trituration ranges from 3 to 30 seconds. Variation of even 2–3 seconds can produce an undermixed or overmixed mass. After trituration, the mix should look homogenous and shiny (Fig. 16.6). The mix is then placed over a muslin cloth (Fig. 16.7) and squeezed to remove the extra mercury from the mix (Fig. 16.8). The mix is now placed in a dappen dish (Fig. 16.9) or in an ‘amalgam well’ and is ready for use. Mulling Mulling is actually a continuation of trituration. It is mainly done to improve the homogeneity of the mass and to ensure a consistent mix with improved texture. The mix is placed in a dry piece of rubber dam sheet and vigorously rubbed between the first finger and the thumb. This process should not exceed 2–5 seconds. Condensation Condensation refers to the placement of mixed amalgam in a specific manner into the prepared cavity in small increments. Each increment after placement is condensed thoroughly and then the second increment is placed on top of it. This procedure is continued till such time the entire preparation is filled. Objectives 1. To adapt the plastic amalgam mix to the cavity wall margins, thereby increasing retention and minimising microleakage 2. To bring the strongest phases of amalgam closer together, thereby increasing the final strength of the restoration 3. To displace the unreacted mercury out of the increments building up the restoration, thereby preventing entrapment of mercury 4. To reduce the number of voids and keep the matrix crystals to minimal dimensions and continuous Condensation should start immediately after trituration. Usually, 3–3.5 minutes can be given for condensation of the amalgam mix. Further condensation can create cracks in the already formed matrix. The field of operation must be kept absolutely dry during condensation. The presence of slightest moisture in a zinc-containing amalgam at this stage can result in delayed expansion and finally corrosion and loss of strength. The ultimate result of moisture contamination is premature failure of the restoration. Types Condensation can be done in two ways: 1. Hand condensation 2. Mechanical condensation Hand Condensation The amalgam mixture should never be touched with bare hands, because freshly mixed amalgam contains free mercury that can get absorbed via skin, and also the moisture on the skin is a source of contamination for the amalgam. Increments of alloy should be inserted in the prepared cavity with the help of an amalgam carrier. Amalgam Carriers Amalgam carrier is used to carry the mixed amalgam from the dappen dish to the prepared cavity (Fig. 16.10). It has a hollow end, into which the amalgam gets packed. This end is placed near the cavity preparation and the plunger at the back end of the carrier is pushed, which expresses the amalgam from the hollow end into the prepared cavity (Fig. 16.11). One can use two carriers at a time, while the operator places the amalgam into the prepared cavity; the assistant can load the second carrier and be ready to give it to the operator for the next increment. This procedure can save a lot of chairside time. 1. Once the increment of amalgam is inserted into the preparation, it should be immediately condensed with sufficient pressure for proper removal of voids. 2. Amalgam condensers are the instruments that are used to pack (condense) the amalgam into the prepared cavity. These are usually double-ended instruments, one end being larger and the other one smaller. They have working ends of various shapes and sizes. The working ends of the amalgam condensers are serrated. Various condenser shapes available are triangular, round, elliptical, rectangular and trapezoidal. The condenser shape is selected based on the ease of adaptation to the walls and margins. Serrated condensers bite into the material, resulting in better condensation, whereas smooth-faced condensers skid over the surface. 3. Condensation is usually started at the central portion of the prepared cavity floor and then the condenser point is stepped little by little towards the cavity walls. 4. Small-sized increments should be carried into the cavity. Large increments make it more difficult to reduce voids and adapt the alloy to the cavity walls. In Class II cavities, a parallelogram condenser can be used to start the condensation at the gingival floor of the proximal box. One can first finish the condensation process in the proximal box and then continue the same for the occlusal extension, although there is no hard-and-fast rule for the sequence. 5. After condensation of an increment, the surface should have a shiny appearance. This indicates that there is sufficient mercury present at the surface to diffuse into the next increment. The procedure of adding the increments is continued till the cavity is overfilled so as to keep some extra amalgam, which might get removed during the process of carving. Mechanical Condensation This is done using mechanical condensers, also called amalgam packers or vibrators. Mechanical condensers are more useful and more popular for condensing lathe-cut alloys when high condensation forces are required. Ultrasonic condensers are not recommended because during condensation they increase the mercury vapour level to values above the safety standards for mercury in dental office. Condensation Pressure The area of the condenser face and the force exerted on it by the operator govern the condensation pressure (force/unit area). Smaller condenser will produce greater pressure on the amalgam. For example: 1. A 2-mm diameter condenser results in a condensation pressure of 16.8 MPa when a thrust of 44 N is exerted. 2. A 3.5-mm diameter condenser results in a condensation pressure of 4.6 MPa when the same thrust is applied. 3. Forces of 66.7 N (15 lb) are recommended for condensation. The shape of the condenser points should conform to the area under condensation. For example, a round condenser point is ineffective in an area that is adjacent to the corner of an angle of the prepared cavity. In such areas, a triangular or rectangular point is indicated. The cavity is usually overfilled with amalgam. Burnishing Burnishing is a process in which a smooth, rigid instrument is used for smoothening the restoration surface that has become rough by carving. There is a conflict between what should be carried out first—burnishing or carving. If carving is done before burnishing, the effect of carving is lost after burnishing. If burnishing is done earlier, then carving leads to the production of rough surfaces. This has led to the concept of pre-carve burnishing and post-carve burnishing. Objectives 1. It is continuation of condensation, as it will further reduce the size and number of voids on the critical surface and the marginal area of the amalgam. 2. It brings any excess mercury to the surface, to be discarded during carving. 3. It will further adapt the amalgam to the prepared tooth margins. Pre-Carve Burnishing This is carried out before carving. It is actually a continuation of condensation process. Pre-carve burnishing is done with a large egg-shaped burnisher or ball burnisher. The burnisher is moved from the amalgam to the tooth surface faciolingually and mesiodistally. 1. It finalizes the process of condensation. 2. It removes the excess mercury. 3. It initiates the carving process. 4. It helps in smoothening of the margins of the restoration. Post-Carve Burnishing After carving, the rough surface that is produced is smoothened by final burnishing, which is known as post-carve burnishing. At this stage, the mass is hard enough to prevent any disturbance of anatomy formed by carving. Small-sized burnisher with light strokes is used. A scraping or ‘ringing’ sound should be heard, which indicates that the mass is ready for burnishing. 1. Post-carve burnishing reduces voids and helps in improvement of the marginal seal (Fig. 16.13). 2. Obtaining the satin or velvet finish is the main aim of this step. Carving Carving is defined as the anatomical sculpturing of amalgam to re-obtain the original anatomy of the restored tooth. This is performed by using instruments called amalgam carvers. The cutting edges of these carvers are sharp, which are used to reproduce the anatomy of the restored tooth. Objectives The objectives of carving are as follows: 1. To produce a restoration with proper physiological contours and no overhangs 2. To produce a restoration with adequate marginal ridges, proper size and location of contact areas and embrasures 3. To produce a restoration with functional non-interfering occlusal anatomy Carving is begun soon after condensation, but the amalgam should be hard enough to offer resistance to the carving instrument. If carving is started too soon, the amalgam is pulled away from the margins. Carving is always done from the tooth surface to the restoration surface. This is done to avoid removal of the amalgam at the margins. Procedure Hollenback carver (Fig. 16.14), Ward’s carver, Frahm’s carver (diamond-shaped carver), cleoid and discoid carvers, etc., are some of the different types of carvers used. 1. First, the embrasures are carved with Hollenback carvers and then the triangular fossa is carved with the discoid/cleoid carver, which will enhance the marginal ridge (Fig. 16.15). 2. Then, the inclined planes as well as occlusal fossae and grooves are carved. 3. The occlusal contours are checked during centric occlusion and during lateral mandibular movements. 4. Carving is done by moving the instrument laterally and cutting the amalgam, while being guided by the intact tooth. 5. Post-carve burnishing is done to remove scratches and irregularities on the amalgam surface, facilitating easier and efficient finishing and polishing. Matrix Band Removal After carving the occlusal portion, the matrix retainer, matrix band and wedges are removed. Proper care needs to be taken while removing the band, since many a times, restoration can fracture (especially Class II) during this step. To prevent this failure, one should first ensure that the amalgam is set hard enough, which can be done with the help of a burnisher. Thereafter, the wedge and the retainer should be removed carefully, keeping the band in position. The ends of the band (facial and lingual) should be held with the fingers of the two hands and everted in the opposite direction (e.g. while restoring the distal surface of a mandibular second molar, the ends of the band should be everted towards the mesial surface of the third molar (Fig. 16.16) and the band should then be gently moved occlusally in a shoeshine motion (Fig. 16.17)). The proximal surface at this point should be well formed, with proper contact evident and minimal carving required (Fig. 16.18), except the possibility to remove a possible small amount of excess amalgam at the proximal facial and lingual margins (at the faciogingival and linguogingival areas) and along the gingival margin. The amalgam knives are ideal for removing gingival excess, preventing gingival overhangs. They are also ideal for refining embrasure form around the proximal contacts. Checking Occlusion Before discharging the patient, occlusion has to be checked for high points (if any) present on the restoration. For this, articulating papers are used, which are usually blue/green/red in colour. The paper is placed on the arch where the restored tooth is present and the patient is asked to bite on it. If any high points are present on the restoration, they will appear more dark in colour (the colour of articulating paper leaves imprints), as compared with the areas on the adjacent teeth. These high points can then be selectively scraped off using a carver. The process is repeated up to the time no high points are seen any more. Finishing and Polishing Finishing and polishing of amalgam is one of the important steps to be performed to ensure the success of restoration. Finishing and polishing reduces plaque accumulation and decreases the chances of secondary caries. This also decreases the risk of fatigue failure under masticatory load. The significance of polishing can be realized from the fact that the hardness and strength of amalgam improves significantly after polishing. Objective The objective of this procedure is to remove superficial scratches and irregularities and to reduce the overhangs, which in turn will decrease the accumulation or adherence of the plaque and thus decrease the incidence of concentration cell corrosion. Procedure 1. After placement, restoration is left undisturbed for 24 hours. This time period is recommended to allow for the complete setting of the silver amalgam. The patient is cautioned that heavy biting force should not be applied to the filling for 7–8 hours (70% of compressive strength is achieved in this time). 2. Finishing is done with the use of steel finishing burs or stones. This includes trimming overextending margins, creating contours and correcting occlusal disharmonies. 3. High points appear as shiny points, which are later reduced using carborundum stones. 4. For proximo-occlusal restoration, finishing begins at the cervical margins, followed by buccal and lingual proximal margins and then occlusal margins. The overhangs, if present, are reduced by using thin trimmers, amalgam knives, etc. 5. Finishing of the cervical areas is done by inserting fine finishing strips cervical to the contact area through the interdental space and then moving to and fro. 6. Facial and lingual proximal margins are smoothened by the cuttlefish sandpaper discs. 7. Superficial scratches and irregularities are also removed simultaneously. The abrasives are always used in descending order, i.e., coarse, medium, fine and ultrafine, and are available commercially as amalgam polishing kit (Fig. 16.19). 8. The final polish or metallic luster is obtained by the application of polishing agents, such as tin oxide, zinc oxide, chalk and pumice. 9. For polishing in cervical areas, polishing strips and dental tapes are used. 10. After polishing, a fine luster is obtained. Hazards During Finishing and Polishing of Amalgam Restorations 1. Aerosols are produced. 2. High temperature may damage the pulp. 3. High temperature may bring the mercury on the surface, causing early staining. 4. High temperature may produce mercury vapours, which may be hazardous when inhaled. Mercury in Dental Amalgam There are essentially two pre-requisites for mercury to be used in dental office. One, it should be arsenic free. Second, it should not have non-volatile residue of more than 0.02%. Mercury used in the dental office can be hazardous if proper care is not taken during its handling and disposal. It can enter the human body majorly via three routes—skin, gastrointestinal tract and respiratory tract. Of these, the respiratory tract is the most common one, as mercury gets converted into vapours that can be easily inhaled. If a person is continuously exposed to these vapours over a long period of time, then systemic changes start appearing in the body such as nervous system disturbances, irritation, tremors, verbal skill impairment, depression and memory loss. The threshold limit value for exposure to mercury vapour for a 40-hour work week is 50 µg/m3 of air. Therefore, it is absolutely essential to follow the strict guidelines during performance of amalgam restoration to avoid mercury-related health hazards. Sources of Mercury Hazards 1. Mercury vapour released from the bottles in which it is stored (if not sealed tightly) 2. Release of mercury from capsules during trituration 3. Spillage of mercury during manipulation 4. Mercury vapour released during placement, polishing and removal of the amalgam 5. Contamination of cotton rolls used for isolation of the tooth being restored 6. Collection of debris via vacuum suction into the plumbing system and the sewer system 7. Collection of amalgam remnants (amalgam scrap) in waste collection jar for recycling 8. Mercury trapped in small cracks between floor tiles and/or in carpet fibres Guidelines for Handling Mercury and its Disposal 1. All the personnel working in a dental office who come in contact with the restorative materials should be well educated about the mercury-related health hazards and should be trained regarding its proper disposal. 2. The operatory should be well ventilated so as to avoid accumulation of mercury vapours in the working area. 3. Preferably, amalgam capsules should be used instead of bulk bottles to avoid any leakage of the mercury from the bottles. 4. Use of an amalgamator is preferred instead of mortar and pestle, as the latter exposes more mercury to the environment than the former. 5. If using the manual technique of trituration, the squeezed mercury should be collected carefully in a container and not spilled. 6. Amalgam should always be handled with gloved hands only to avoid skin contamination. 7. High-volume evacuation should be used while performing the restoration as well as removal of old amalgam restoration, as both these procedures cause release of mercury vapour to one extent or the other. 8. Instruments used for inserting, finishing, polishing or removing amalgam restorations contain some amalgam material on their surfaces. During instrument sterilisation techniques, this material may be heated and can release mercury vapours. Therefore, it is advisable to properly isolate or specially vent the air from the sterilisation areas. 9. Mercury-contaminated cotton rolls should not be thrown out with regular trash. They should be stored in a tightly capped plastic container or closed plastic bag for separate disposal. 10. All excess mercury (squeezed out) and leftover amalgam (amalgam scrap) should be placed in used radiographic fixer solution, which can be kept in a dark bottle. Fixer solution contains sulphur, which can combine with mercury to form stable sulphides. Also, used fixer solution is a source of silver, which can react with the unused mercury to form stable compounds. 11. Spilled mercury should be cleaned up properly using trap bottles, tape or commercial cleanup kits. Household vacuum cleaner should never be used.