FAILURE OF AMALGAM RESTORATIONS Introduction Clinical failure is dentinal as a point at which the restoration is no longer serviceable or at which time the restoration causes other severe risks it is not replaced. For nearly 160 years dental amalgam has seen the most common and often used material in restorative dentistry. The title of father of amalgam goes to Sir Regnart as he worked extensively in 1818 by reducing the fusing temp of the alloys. The first form of amalgam was silver mercury paste advocated by Tayeaue in 1826 in Paris. Later G V Black carried out extensive studies and modification in amalgam until recently same composition was followed. But now there is a change in the copper percentage to improve the physical properties of amalgam major disadvantage of previous amalgam where strength, tarnish, corrosion creep etc. The reasons for amalgam failures can be generally identified and avoided. These failures commonly include retentive failure, marginal break down, fracture of the tooth or restoration, post operative, sensitivity, poor surface characteristics. The materials are least often the source of problem, where as most of the failure are attributed to the lack of attention in cavity preparation and handling of the materials. In today’s this discussion we will be discussing in detail the causes of failure of amalgam restorations.
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About amalgam in Brief: Between going in to details of failure of amalgam restoration let us discuss about amalgam in brief. By dentition dental amalgam is an alloy of mercury with silver, tin, copper, and some times zinc, mercury is liquid at room temperature. The mercury wets the particles of alloy to produce a plastic mass. That is condensed into prepared cavity and allowed to set. The setting amalgam is then smoothened and shaped to produce the final dental restoration. It is essential that clinical characteristics of dental amalgam be well understood when considering, use of material wtihin the plan of patients treatment. Some characteristics make amalgam most advantageous for the restoration of certain teeth, while other, characteristics may result in early restoration failure. All the indication and contraindication for use of amalgam must be considered when planning the treatment. Composition of amalgam alloys generally effects their clinical characteristics. Recently spherical high copper alloy considered to give good results. A copper tin compound is formed which either diminishes or eliminate the weak V2 phase, which often contribute to marginal failure. Even the mechanical and physical properties of amalgam play an important role in success of the restoration. Clinically it is evident that amalgam can with stand compressive loading for better than tensile. Compressive strength values for amalgam specimen and loading rates are about 5 times greater than that of tensile value. Because of this thinner sections of amalgam.
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Whether at margins or in the central restoration area have greater tendency to fracture than thicker sections. These thin sections of amalgam behave as a brittle material when subjected to high rates of loading induced during the masticatory forces. Types of Failure in Amalgam Restoration: 1. Fracture (i)
Marginal fracture
(ii)
Isthums fracture
(iii)
Bulk fracture
(iv)
Tooth fracture
2. Secondary caries. 3. Post operative sensitivity and pain. 4. Dislodgment of the restoration. 5. Contribution to periodontal diseases. 6. Discoloration of the teeth. 7. Tarnish and corrosion. 8. Pulpal damage. 9. Plaque formation 10. Occlusal interference. 11. Galuanison. 12. Amalgam fartoo Coming to the details of causes for failure of amalgam restorations.
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A. Causes for Fracture of amalgam restoration: Fracture of the amalgam restoration is one of the main failures seen. The amalgam restoration the fracture can be marginal, istherms bulk or tooth fracture. According to the study and reviews done by Ronald K. Harris which was published in journal of operative dentistry 1992, 17, 243 he stated that material themselves are least of the source of the problem, whereas most of the failure is due to lack of attention during operating and manipulative procedure. Here I am going to discuss few specific causes for the fracture of amalgam restoration. 1. Selection of the cases: (a) Extenisive tooth loss and undermined enamel. (b) Post endodontic restoration. (c) Cases with poor retention / resistance (d) Areas of high masticatory load where referred failure of amalgam restoration. 2. Selection of alloy. 3. Due to the improper cavity preparation. (a) Over cutting. (b) Under cutting. (c) Poor retention and resistance (d) Improper finishing of the cavity.
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4. Due to physical properties of amalgam. (a) Dimensional changes (Delayed expansion). (b) Strength. (c) Creep. (d) Tarnish and Corrosion. 5. Improper manipulation of the alloy. (a) Improper selection of the alloy. (b) Improper powder, liquid ratio. (c) Under trituration. (d) Over trituration. (e) Improper condensation. (f) Improper carrying (i) Over carrying. (ii) Under carrying. (g) Improper finishing and polishing. 6. Due to improper matrix adaptation: (a) Improper covering (b) Excess thickness (c) Improper wedging (d) Premature matrix bond removal
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7. Due to contamination: • Moisture • Instrument indication • Contamination during manipulation. 8. Oral environment • Excess stress. • Malposed teeth. • Poor oral hygiene. 9. Pin restoration (a) Fracture of tooth. (b) Improper placing. (c) Improper adaptation. I. Fracture due to the selection of the cases: Selection of the cases is one of the factor contributed in the success of final amalgam restoration. (a) Extensive loss of tooth and undermined enamel: In the cases of extensive loss of tooth structure prognosis of amalgam restoration is very poor, as it is not possible to achieve an ideal form of the cavity. Even the undermined enamel act as a week points. As it is not supported by underlying dentin. In these cases stress concentration will lead to fracture of tooth or restoration.
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(b) Post endodontic restoration: As the tooth becomes non vital there is good chance of fracture of tooth or restoration by concentration of masticatory stress. This is mainly due to the brittless of non vital tooth and in most of the cases of post endodontic treated teeth we can see extensive loss of tooth structure. (c) In the cases where retention and resistance that is not achievable the success of amalgam is questionable in such cases. Other means of restoration is preferred to overcome the failure of amalgam restoration. (d) Areas of high masticatory loads where there is repeated failure of amalgam restoration seen. In the areas like cusp tips marginal ridges and also in cases of deep bite where there is more stress concentration the success of the restoration becomes of vestinable. Especially if the restoration involving the functional cusps i.e. buccal cusps in lower and palatal of upper there is a good chance of fracture. As maximum load is taken by these cusps, so in these cases cast restoration is preferred. (e) In cases of large and extensive contact area and in cases with spacing between the teeth. In the cases of large contact area application of the matrix band is difficult and it is difficult to achieve same contact area or a point in the amalgam restoration. In cases where spacing either due to periodontal problem or natural spacing it will be difficult to achieve nice proximal contact of the restoration. Manipulation of these cases will lead to overhang of restoration and can lead or aggravate the gingival and periodontal problems.
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II. Role of selection of alloy and mercury It is the operation who causes the amalgam to be success or failure because the choice of alloy is personal preference. Basic lathe cut without any additions or modification are seldom used nowadays having given way to advanced modification in the alloys. Amalgam can have ultimate compressive strength of 40000 psi to 75000 psi it properly manipulated. These variations can be made by metallurgical modification in the constituents, heat treatment, particle size and shape and surface texture. The choice between zinc containing and zinc free alloy is controversial definitely zinc containing alloys create problems. In presence of moisture all the same time amalgam not containing zinc or any of its substitutes will tend to be less plastic and less workable and more susceptible to oxidation. Non zinc containing alloys should be chosen in cases where it is clinically imposible to eliminate moisture from the field of operation for e.g. Indium containing alloys one same exception to this role as Indium performs same role as zinc in addition to diminishing the V2 phase. III. Due to improper cavity preparation: (a) Over cutting of tooth structure by depth and surface area frequently compromises the potential success of the silver amalgam restoration. These preparations may result in failure through generalized weakening of remaining tooth structure. And also some times if cavity out line is placed in the maximum stress bearing areas like cusp tips or marginal ridges will lead to potential weakening of the tooth structure leading to concentration of functional stress resulting in fracture of the tooth condition called
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cracked tooth syndrome. This over cutting along with fracture can also precipitates unnecessary pulpal irritation which can lead to irreversible changes. (b) Under cutting of the cavity preparation also invites failure of the restoration through insufficient removal of tooth structure. As amalgam requires bulk for sufficient strength and due to decreased tensile property. There will be a very good chance for fracture if used in thinner sections. Traditionally also it is accepted that the bulk is required for amalgam restoration many silver amalgam restorations fail due to fracture at either isthmus or at the junction between the occlusal and proximal portion of the restoration because insufficient removal of dentin from pulpal axial or gingival walls. It is also ideal to keep the depth of the restoration 0.5 mm into the dentin as it gives sufficient bulk and also due to good resiliency of dentin as it gives a cushioning effect for amalgam restoration. (c) Due to poor resistance and retention form of the cavity resistance form may be defined as that shape and placement of the cavity that best enables both the restoration and the tooth to withstand the occlusal and masticatory forces. This form of the cavity plays an important role in preparation of fracture of amalgam restoration. Fundamentals of retention form are (i)
To utilize the box form with a flat floor which helps the tooth to resist occlusal loading by virtue of being at right angles to the forces of mastication.
(ii)
Restrict the extension of the walls to allow strong cusp and ridge areas to remain with sufficient dentin support.
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(iii)
To provide enough thickness of restorative material to prevent its fracture under load along with these all the line angles and point angles of the cavity is to be rounded and walls are smoothened in order to prevent the stress concentration which may contribute for fracture of tooth / restoration.
In cases of proximal boxes along with above points rounding of axiopulpal line line angles, bevelling of the gingival cavosurface in order to remove the unsupported enamel rods which are directed apically increased and masticatory stress is transferred perpendicular to the gingival seat and even distribution of forces is possible. Retention form: Is that form of cavity that permits the restoration to resist through tipping and litting forces. This form of the cavity mainly helps in preventing dislodgement of the restoration than fracture. It will be discussed later with dislodgement of amalgam restoration. (d) Due to improper convenience form finish of the cavity: Convenience form is that form of the cavity that allows sufficient observation accessibility and ease of operation in the final restoration of the cavity. Improper convenience form might lead into insufficient condensation and void formation which might affect the strength of the restoration leading to fracture.
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IV. Fracture due to the physical properties of amalgam The qualities of amalgam restoration are usually expressed in terms of properties of the material. The quality of the amalgam restoration changes as its functions under oral condition. The properties of amalgam which is responsible to qualify of the restoration can be discussed under following headings. (a) Dimensional changes: The dimensional changes either expansion or contraction that result during the hardening or setting of amalgam is one of the most important characteristic properties which will affect the success of amalgam restoration. In an early survey of the causes of failure of amalgam restorations 16.6% of large group of defective restoration failed due to excessive expansion. There are several causes for the expansion few of them are insufficient trifurcation and condensation and another is delayed expansion brought about by contamination of zinc containing amalgam with moisture during trifurcation and condensation. These properties will be discussed later when we discussed the post operative pain and sensitivity in amalgam restoration. (b) Strength: A prime requisite for any restorative material is sufficient strength to resist fracture of even a small area especially at the margin promotes corrosion, secondary caries, and subsequent restoration failure. a lack of adequate strength to resist the masticatory forces has been recongnized as one of the iherent weakness of amalgam restoration.
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When a patient masticates amalgam restoration is subjected to compressive tensile and shear stress since the tensile strength is poor compared to other two there is a good chances of fracture. When used in tinner sections the tensile strength with both high and low copper is between 40 to 70 mpa, whereas compressive strength of satisfactory amalgam should be atleast 310 mpa when manipulated properly most of amalgam will give compressive strength in excess to the above given values. Since amalgam is stronger in compression and much weaker in tension and shear the prepared cavity should be designed so that the restoration will receive more compressive force rather than tension or shear forces. A common example where tension fracture occurs in the isthums of compared restorations. Another desirable feature of an amalgam restoration is that it should attain a high compressive strength as quickly as possible since to reduce the possibility of fracture by premature high contact stress by the patient between the final strength is reached. Following methods can be accelerate the rate of altering strength 1. Reducing particle size. 2. Requalecity and smoothness of the particle shape and surface. 3. Increased trifurcation energy. 4. Increased condensation energy. 5. Homogensity heat treatment of powder alloy during manufacturing.
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Creep: Creep of a dental amalgam is a slow progressive permanent deformation of set amalgam which occurs under constant stress or incremental stress when an amalgam specimen is placed under a constant load which is less than that needed to produce either instantaneous plastic deformation or fracture. It slowly develops i.e. it exhibits creep. Creep of amalgam is important in relation to maintain the shape of restoration of after setting it the amalgam filling shows dynamic creep under chewing stress then the enamel surface. This produces an opponent expansion of filling out of cavity. These thin margins will then fracture on further stress producing a ditched amalgam. Creep rate has been found to co-relate with marginal breakdown of traditional low copper amalgams i.e. higher the creep. The greater the degree of marginal deterioration. V. Due to improper manipulation of alloy Failure of amalgam restoration due to manipulation of the materials can be subdivided into 2 groups. i)
Fracture under control of manufacturer a. Composition of the alloy. b. Speed at which mercury will react with alloy. c. Particle size and shape. d. The form in which the alloy is supposed.
ii)
Fracture under control of clinician a. Selection of the alloy.
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b. Mercury alloy ratio. c. Trifurcation method and time. d. Condensation technique. e. Marginal integrity and anatomic characteristics. f. The final finish. (A) Improper selection of alloy: Proper selection of the alloy helps in the success of the restoration. High copper spherical alloys are chosen over low copper because of better strength and lesser creep of the material and it has also less corrosion properties. (B) Improper trifurcation: Under trifurcation – the property trifurcated amalgam mass will have shiny appearance under trifurcated amalgam leads to granular mix, their by affecting the strength of amalgam. This leads to less working time for condensation and carrying leading to improper condensation and carrying which intern affects the success of restoration. (C) Improper condensation: Delayed, prolonged or poor condensation of amalgam can adversely effect the adaptation of the material. This will increases the chance of microleakage, secondary caries, rough surface etc on proper condensation around 1- 1½ kgs of pressure is to be applied immediately when the amalgam
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is placed into the cavity otherwise it might lead to void formation and result in poor strength and fracture of the restoration. (D) Over and under carrying: Over carrying causes weakening of the material by reducing the bulk referred to resist the masticatory stresses which inturn might lead to fracture of the restoration. Under carrying places the restoration and functional areas in conditions of unnecessary stresses that may cause areas of stress concentration leading to the fracture of the restoration. It might also causes the trauma from occlusion. (E) Improper burnishing and furnishing: Burnishing is done to remove the excess mercury and proper adaptation of restoration to the cavosurface margin over burnishing might remove excess mercury and weaken the restoration. Improper burnishing might lead to improper marginal adaptability and inturn. Improper finishing will leave a rough surfaces which helps plastic adhesion and leads to periodontal problems and lead to failure of restoration. VI. Due to improper matrix adaptation: Restoration of the prepared cavities with amalgam involving two or more surfaces requires the use of matrix. Matrix mainly serves the following functions. a) Providing general contour of the restoration. b) Substituting for the lost wall till amalgam sets.
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i) Improper matrix adaptation is common causes of failure of amalgam restoration modification in shape of the tooth requires correct adaptation and modification in shape of contour of the band matrix bond adaptation should be tested with fine explorer so that there is no gap in between. The tooth and gingival cavosurface area otherwise it might lead to over hang of the restoration. ii) Excess thickness the ideal thickness of band should be 0.05mm thick band might give rigidity. They will lead to open contact which will cause food impaction. Thereby, causing periodontal problems thus leading failure of restoration. iii) Improper reinforcement and wedging Many matrix needs reinforcement with impression compound, self cure acrylic to prevent distoration of amalgam during condensation. Matrix reinstrument improves desired anatomic form so improper reinstrument of matrix might lead to improper condensation which inturm affect the strength and leads to fracture amalgam restoratives. Wedging is also an important step in proximal restoration which causes slight seperation of the tooth for easy placement of matrix band and also stabilizes it. This prevents overhanging of the restoration for better finishing and good result of proximal restoration. VII. Fracture due to contamination a) Moisture: It includes saliva blood and lubricant from the instrument, most restorative material are successful only if placed and finished in clean dry operating field contamination seriously effects the setting and strength of
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amalgam leading to failure. rubber dam provides the optimum dry field for preparation of teeth to receive amalgam and also helps in enhancing the physical properties. It is seen that zinc that containing low copper and high copper amalgams if contaminated with moisture during trituration or condensation a large expansion can take place the expansion usually starts after 3-5 days and may continue for months reaching a value greater than 400mm (4%) this type of expansion is known as delayed or secondary expansion. Delayed expansion is seen in zinc containing alloys. The effect is caused by reaction of zinc with water. It has been clearly demonstrated that the contamination substance mainly water and saliva. VIII. Fracture due to pins used for retention. a) Fracture of tooth / restoration: It is commonly seen as a simple cracking may develop from placing of the pin to close to DEJ causing both enamel dentin fracture. b) Pin position: Site of the pin placement is very critical in respect to possible perforation of pulp chamber so placing the pins properly is important for the success of final restoration. c) Length of the pin: Pins provide retention but doesn’t give (or) enhance the strength of the amalgam restoration. This should be zinc to achieve retention in amalgam restorations if the pin length exceeding 2mm may weaker the whole amalgam restoration leading to fracture of amalgam.
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IX. Fracture due to oral environment a) Excessive stress: Amalgams should not be placed in the mouth where large amount of masticatory stress applied and where large amount of tooth structure is lost (or) in case of developmental disorders of the teeth, which will fracture easily due to the brittleness. b) Malposed teeth: In case of malposed teeth and mal occlusion if the opposite cusp is impinging on the restoration constantly will lead to the fracture of amalgam restoration. Post Operative Sensitivity or Pain Post operative pain is one of the common clinical finding seen in some of the cases. There may be many contributing factors but main cause discussed here is due to the dimensional changes. In an early survey shows dimensional changes are significantly contributed in failure of amalgam restorations there are several causes for dimensional changes but most common is insufficient treatment and condensation expansion due to zinc containing amalgam with moisture during manipulation. Delayed expansion is probably caused by interval pressure excerted by hydrogen gas that is one of corrosion product between the zinc in amalgam and incorporated moisture. Hydrogen is produced by electrolytic action involving zinc and water. This hydrogen does not combine with amalgam but rather collects within the restoration which increases the internal pressure. Pressure to high enough for amalgam to creep. This large expansion is seen in 4 or 5 days of condensation. This increased internal pressure due to expansion will cause post operative pain and sensitivity.
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One should also examine high point in the restoration which is also one of the main causes for post operative pain. It can be examined by using articulating papper and these symptoms will seen subside once high point is removed and tooth is relived from occlusion. Dislodgment of the restoration Dislodgment of the restoration is the second most failure seen next to fracture in all amalgam restorations. This type of failure is also known as retentive failure. this is mainly due to detective retention form of the cavity and the forces that try to displace or dislodge the restoration. Retention form is that form of the cavity that best permits the restoration to resist displacement through tipping or lifting forces. Retention of amalgam is mainly mechanical it is obtained generally by giving the undercuts grooves. Flaring of the wall will displace the restoration due to the masticatory forces acting on it. In the cases where it is not possible to achieve retention form bonded amalgam can be tried but in case repeated failure it is ideal to go for cast restorations. Tarnish and Corrosion Tarnish is a surface discoloration on a metal (or) even slight loss of alterations of the surface finish. Corrosion – is an actual deterioration of a metal by reaction with its environment.
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Amalgam restorations often tarnish and corrode in the oral environment. The degree of tarnish, corrosion and resulting discoloration appears to be dependent on the individuals oral environment and to certain amount to the particular alloy employed. The tendency towards tarnish, although perhaps anaesthetic because of black silver sulphide. Mostly active corrosion of newly placed restoration occurs within the interface between the tooth and the restoration. The space between the alloy and tooth permits micro-leakage of electrolyte and leads to corrosion product. The build up of this corrosion product gradually seals this space making dental amalgam a self sealing restoration. The most common corrosion products found with traditional amalgam alloys are oxides and chlorides of tin. They are found at the tooth amalgam interface and penetrating into the bulk of the old amalgam restoration. Thus the corrosion takes place. The fractures Related Excessive Tranish and Corrosion: 1. High residual mercury level can lead to increase in corrosion as a result of increase in V2 phase. 2. Surface texture small scratches and exposed voids will develop concentration cells with saliva as the electrolyte. 3. Galvanic action when two dissimilar metals come into contact. 4. Moisture contamination during condensation will cause air voids to develop and corrosion to progress at taster rate.
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X. Pulpal Damage of Amalgam Restoration Inadequate pulp protection might lead to pulp damage which might inturm lead to pulp necrosis and failure of the restoration. As amalgam is a good conductor of heat in deep cavities it is must to apply thermal insulating bases. In some of the cases even delayed expansion also causes pressure on the pulp chamber and cause damage to the pulp. XI. Galvanism It is a small amount of current produced when two dissimilar metals else where it might lead to failure of restoration. Amalgam Tattoo This is a macular and bluish gray or even black lesion usually seen in the buccal mucosa gingiva or palate. Importantly they are found in the vicinity of teeth with large amalgam restorations or crowned teeth that probably had amalgam restoration removed at the time of tooth preparation for fabrication of crown. This is most oftenly an iatrogenic in orgin mainly due to traumatically introducing flecks by an rotary instruments or some time metal particles may full on to extraction select and during healing phase amalgam becomes embeded within the connective tissue while re-epithelization. Conclusion It proper selection of cases, alloys and good cavity preparation is followed failure of amalgam restoration cause minimized or avoided.
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