Dental Material Lecture 10 PDF

Second stage – lec. 10 Dental material Dr. Sahar Dental amalgam is an alloy produced by mixing liquid mercury with solid particles of silver, tin, copper and zinc. In dentistry, the amalgam has been successfully used for more than a century as a res...

Second stage – lec. 10 Dental material Dr. Sahar Dental amalgam is an alloy produced by mixing liquid mercury with solid particles of silver, tin, copper and zinc. In dentistry, the amalgam has been successfully used for more than a century as a restoration material for tooth decay mainly in posterior teeth. Supplied as: 1- Bulk powder and mercury. 2- Alloy and mercury in disposable capsule mixed amalgamator machine. Classification of amalgam alloys: 1. Based on copper content: * Low copper alloys: Contain less than 6% copper (conventional alloys). * High copper alloys: Contain more 13-30% copper. 2. Based on zinc content: * Zinc-containing alloys: Contain more than 0.01% zinc. * Zinc-free alloys: Contain less than 0.01% zinc. 3. Based on the shape of alloy particle: * Lathe cut alloys: irregular shape. * Spherical alloys. * Mixture of Lathe cut and spherical alloys. Amalgam alloy consist essentially of sliver and tin, in lesser amount cooper, zinc and a trace of gold, palladium, indium and selenium. Second stage – lec. 10 Dental material Dr. Sahar Properties of amalgam: 1- Dimensional change: In general, most amalgams expand or contract only slightly during setting. Expansion may result in post placement sensitivity or protrusion from the cavity. Whereas contraction would leave gaps between the restoration and the tooth prone to leakage and recurrent decay. The initial contraction after short time (the first 20 minutes after tirturation) is believed to be associated with the solution of mercury in the alloy particles. After this period an expansion occurs which is believed to be result of reaction of the mercury with silver and tin. Factors favoring contraction are: 1.Low mercury/alloy ratio. 2. Higher condensation pressure (squeezes out mercury). 3. Smaller particle size (accelerate mercury consumption). 4. Longer trituration times (accelerate setting). *The dimension becomes nearly constant after 6-8 hours, and thus the values after 24 hours are final values. *Modern amalgams show a net contraction, whereas older amalgams always showed expansion. Two reasons for this difference: A. Older amalgams contained larger alloy particles and were mixed with higher mercury/alloy ratios. B. Hand trituration was used before; modern amalgams are mixed with high-speed amalgamators. The effect of moisture contamination (delay expansion): If zinc containing amalgam contaminated with water during trituration or condensation, a large Second stage – lec. 10 Dental material Dr. Sahar expansion can take place. It is usually starting after 3-5 days and may continue for months. This is known as delayed or secondary expansion. H2O+Zn ------ ZnO + H2 (gas) The hydrogen gas does not combine with the amalgam, but collects within restoration, creating internal pressure and expansion of the mass. It may reach 4% and cause pressure on the dental pulp and post-operative sensitivity; this is known as delay or secondary expansion. Amalgam without zinc tends to be less plastic and less workable; used only for cases where it is difficult to control moisture, e.g.: patients having excessive salivation, subgingival lesions, etc. 2. Strength Hardened amalgam has good compressive strength but low tensile or bending strength. Therefore, the cavity design should be such that the restoration will receive compression forces and minimize tension or shear forces in service. Factors affecting strength: 1. Trituration affect: either under or over trituration will decrease the strength for amalgam. 2. Mercury content affect: sufficient mercury should be mixed with the alloy to wet each particle of the alloy. Otherwise, a dry, granular mix results which has rough and pitted surface that invites corrosion. Excess mercury in the mix can produce a marked reduction in strength because high mercury amalgam has more y2 content (which is weakest phase). 3. Condensation affect: higher condensation pressure results in higher compressive strength. Good condensation technique will minimize porosity, improves adaptation and remove excess mercury from lathe-cut amalgam. While Second stage – lec. 10 Dental material Dr. Sahar the spherical amalgam condense with lighter pressures produce adequate strength (If heavy pressure is used the condenser will punch through). 4. Porosity affect: voids and porosity reduce strength. Porosity is caused by: A. low and high Hg/alloy ratio & over and under trituration. B. Inadequate condensation pressure. C. Irregularly shaped particles of alloy powder. D. Insertion of too large increments. 5. Rate of hardening affect: strength increase with time. Amalgam do not gain strength as rapidly as might be desired. Patient should be cautioned not to bite too hard for least 8 hours after placement. The time at which at least 70% of its strength is gained. Second stage – lec. 10 Dental material Dr. Sahar 6. Cavity design affect: the cavity should be designed to reduce tensile stresses. Amalgam has strength in bulk; therefore increase thikness will increase strength, the cavity should has adequate depth. 3. Creep: Creep is defined as a time dependent plastic deformation. Creep of dental amalgam is a slow progressive permanent deformation of set amalgam which occurs under constant stress (static creep) or intermittent stress (dynamic creep). It is related to marginal breakdown of low copper amalgam. The higher the creep, the greater is the degree of marginal deterioration (ditching). Creep causes the amalgam to flow over time such that unsupported amalgam protrudes at the margin of the restoration. * Low copper amalgam creep= 0.8-8.0%. * High copper amalgam creep= 0.4-0.1%. * Creep should be below 3%. * The Y2 phase is associated with higher creep rates. * Increase in zinc content gives less creep. Effect of manipulative variables (for increase strength & low creep): - Hg/alloy ratio should be minimum. - Condensation pressure should be maximum for lathe-cut or admixed alloys. - Careful attention should be paid to timing of trituration and condensation. Either under or over trituration or delayed condensation tend to increase the creep rate. 4. Tarnish: Tarnish: means loss of luster from the surface of metal or alloy due to the formation of a surface coating. Second stage – lec. 10 Dental material Dr. Sahar - Cause no change in the mechanical properties of the alloy. - Amalgam is usually tarnish due to the formation of sulphide layer on the surface. - Tarnish increase in patients on a high sulfur diet. - Rough surface and moisture contamination during condensation increase tarnish. 5. Corrosion: Corrosion is the progressive destruction of a metal by chemical or electrochemical reaction with its environment that penetrate the body of the amalgam and cause the failure of the restoration - Excessive corrosion can lead to increased porosity, reduced marginal integrity, loss of strength, and the release of metallic product into oral environment. Factors related to excess tarnish and corrosion: 1. High residual mercury increase corrosion. 2. Contact of dissimilar metals, e.g gold, and amalgam increase galvanic corrosion. 3. High copper amalgam is cathodic in respect to a low copper amalgam so mixed high copper and low copper restoration increase galvanic corrosion (should be avoided). 4. Rough surface texture, small scratches and exposed voids increase corrosion. 5. Moisture contamination during condensation. 6. Patients on a high sulfur diet. 7. Type of alloy-low copper amalgam is more susceptible to corrosion (due to greater y2 content) than high copper. Corrosion of amalgam can be reduced by: 1. Smoothing and polishing the restoration. 2. Correct Hg/alloy ratio and proper manipulation. 3. Avoid dissimilar metal including mixing of high and low copper amalgams. Corrosion has one advantage that corrosion products gathered at the restoration- tooth interface (seal the gap) to prevent or decrease micro leakage. 6. Thermal properties: Amalgam has a relatively high value of thermal diffusivity. In large cavities it is necessary to line the base of the cavity with an insulating, cavity lining material prior to condensing the amalgam. The coefficient of thermal expansion value for amalgam is about three times greater than that for dentine. 7. Biological properties Certain mercury compounds are known to have a harmful effect on the central nervous system. The patient is briefly subjected to relatively high doses of mercury during placement, contouring and removal of amalgam fillings. A lower, but continuing, dose results from ingestion of corrosion products. Properly handled dental amalgam should be regarded as safe for general use as a direct restorative material. Mercury toxicity: Mercury is toxic, Free mercury should not be sprayed or exposed to the atmosphere. This hazard can arise during triturating, condensation, and finishing of restoration, and also during the removal of old restoration at high speed. Mercury vapors can be inhaled; skin contact with mercury should be avoided as it can be absorbed. It also reacts with gold. Mercury has a cumulative toxic effect. Dentists and dental assistant are at high risk. Though it can be absorbed by skin or by ingestion, the primary risk is from inhalation. The clinic should be well ventilated. All excess mercury and amalgam waste should be stored in well-sealed containers. Precautions: 1. The clinic should be well ventilated. 2. Mercury should be stored in well-sealed container. 3. Skin contact with mercury should be washed with soap and water. 4. During removing old filling, water spray and high suction should be used.  5. The use of ultrasonic amalgam condenser is not recommended. Manipulation of Amalgam. 1. Proportion and Dispensers. 2. Trituration. 3. Condensation. 4. Carving. 5. Polishing. 1. Proportion and Dispensers: Because proportioning is important, manufacturers have developed some simple dispensers for alloy and mercury. Dispensers by volume are unreliable because it is affected by particle size and the degree of packing (trapped air and voids) in dispenser. Tablets: This is most accurate method of dispensers manufacturers compress alloy powder into tablets of controlled weight which is used with measured amount of mercury. Pre-proportioned capsules (disposable amalgam capsules) containing alloy particles and mercury in compartments separated by a membrane. Before use, the membrane is ruptured by compressing the capsule, and the capsule is then placed in a mechanical amalgamator. Proportions of alloy to mercury: Some alloy require Hg-alloy ratio in excess of 1:1, whereas other use ratio of less than 1:1. the percentage of Hg varies from 43% to 54%. 2. Triturating (Mixing of Amalgam): Triturating of amalgam alloy and mercury is done with: 1. Hand mixing by use mortar and pestle. 2. Mechanical mixing: device called an amalgamator. Spherical or irregular low-copper alloy may be triturated at low speed (low energy), but most high copper alloys require  high speed (high energy). Advantages of mechanical triturating: 1. Shorter mixing time. 2. More standardized procedure. 3. Requires less mercury when compared to hand mixing technique. Mixing time: There is no exact recommendation for mixing time, since amalgamators differ in speed, oscillating pattern, and capsule designs. Spherical alloys usually require less amalgamation time than do lath-cut alloys. A large mix required slightly longer mixing time than a smaller one. When amalgams with longer or shorter working times are desired, one should use amalgam alloys that are designed to react faster or slower and not attempt to achieve the change by altering the trituration time. Mixing Variables: Under mixing, normal mixing, or over mixing can result from variations in the condition of trituration of the alloy and mercury. The three types have different mechanical properties, dimensional change, strength and creep. Under mixed amalgam appears dull and is crumbly. A grainy under triturated mixture; restoration made of such a mixture has low strength and poor resistance to corrosion. The mixture may appear in solid mass, but the surface remains without luster. Normal mix appears shiny and separates in a single mass from the capsule. It appears rounded with a smooth shiny surface. Over mixed amalgam appears soupy and tends to stick to the inside of the capsule. It is shinier than that of the properly triturated one the mass appears flattened by the force of trituration.  Under Normal Over 3. Condensation The amalgam is placed in the cavity after triturating, and packed (condensed) using suitable instrument. It is adaptation of the amalgam mass to the cavity walls that controls the amount of mercury that will remain in the finished restoration. More mercury left in the mass after condensation, the weaker the alloy. Aims of condensation: a. To adapt it to the cavity wall. b. Remove excess mercury. c. Reduce voids. Irregularly shaped alloys need higher percentage of mercury while with spherical alloys the amount of mercury is lower. Increasing the condensation pressure results in a significant increase in compressive strength. Condensation could be: Manual condensation or Mechanical condensation. Manual Condensation: Circular condenser tips may prove adequate condensation. With irregularly shaped alloys small tip condenser (1 to 2) mm is use with high condensation forces in a vertical direction. While for spherical alloys one should use condensers with larger tips, almost as large as the cavity permits (particles tend to roll over one another). Mechanical condensation: These devices are useful for condensing irregularly shaped alloys when high condensation forces are required. Ultrasonic condensers are not recommended because during condensation they increase the mercury vapor level. 1. Carving: The filling is carved to reproduce the tooth anatomy. The carving should not  be started until the amalgam is hard enough to offer resistance to the cavity instrument. Burnishing: After the carving, the restoration is smoothened, by burnishing the surface and margins of the restoration. Burnishing is done with a ball burnished using light stroke proceeding from the amalgam surface to the tooth surface. 1. Polishing: Polishing minimize corrosion and prevents adherence of plaque. the polishing should be delayed for at least 24hours after condensation. For polishing wet abrasive powder in a paste form is used.