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Modeling framework of clasp prosthesis

June 20, 2024

Modeling framework of clasp prosthesis.

Metal alloys for the manufacture of clasp dentures.

If you are missing only a few teeth scattered over either arch (upper or lower teeth), or even if you have a minimum of two teeth on both sides of the arch, then you can most inexpensively replace the missing teeth with a Removable Partial Denture (RPD). There are several types of RPD’s. All of them use standard plastic denture teeth as replacements for the missing natural teeth. The differences between them are the materials that are used to support the denture teeth and retain the RPD in the mouth.

Removable Partial Dentures with cast metal frameworks offer numerous advantages over the conventional partial dentures commonly used. The frameworks of these dentures are cast to fit the teeth. Since they sit on the teeth, as well as being attached to them, they are extremely stable and retentive. The teeth have to be altered slightly beforehand in order that the partial denture can rest upon them without interfering with the way the patient bites the teeth together.

The metal framework does not contact the gums. Thus, as the gums resorb, this type of partial denture does not sink with them and rarely requires relining. Because the teeth are altered beforehand, there are fewer limitations in the placement of clasps, and they are less likely to be seen than the wrought wire clasps of the conventional treatment partial denture. Modern frameworks are cast from an extremely strong alloy called chrome cobalt which can be cast very thin and are much less likely to break than the all plastic variety. They are also much less noticeable to the tongue.

The largest single advantage that cast metal framework partial dentures have over the other types of partials is that sore spots are almost never an issue since neither the framework, nor the plastic extensions contact the soft oral tissues with any force! Patients who exhibit the symptoms of TMJ, or who are known bruxers are much better off with cast metal partials than with flexible framework partials.

Cast Metal Partial Denture:

This type comprises cast metal frameworks and is more advantageous as compared to the flipper type. They are strong, rigid and durable, thus, are considered for the long run. Modern frameworks are cast from chrome cobalt (strong alloy), which is less likely to break. The metal used in these dentures is not seen to cause any allergies.

These partial dentures are attached to the teeth in a very stable fashion. The denture rests on the teeth and not on the gums, as in the case of flippers. This stability is achieved by slightly altering the natural teeth surface so that the dentures can fit properly. However, this modification will io way alter the way the teeth bite into each other or their grinding function.

The metal clasps are also not as visible as those in flippers, however, can be seen and often people find this aspect uncomfortable. Neither the metal framework nor the plastic extensions, come in contact with the soft oral tissues, thus sore spots are not an issue. Moreover, they are less noticeable to the tongue and most people find them more comfortable to wear than the flippers.

Denture Base

Base metal alloys- these are nooble metals which are reactive with their environment and are referred to as base metal

Cast metal is any metal that is melted and poured into a mould

Dental applications of cast and wrought base metals alloys

Cast Cobalt Chromium Alloys

( Vitalium &Nobilium).

Partial Denture Framework

Porcelain Metal Restorations

Cast Nickel- Chromium Alloys

(Ticonium).

Partial Denture Framework

Crowns & Bridges

Porcelain Metal Restorations.

Cast Titanium And Titanium Alloys

Crowns, Bridges, Partial Dentures, Implants.

Wrought Stainless Steel Alloys

Endodontic Instruments,Orthodontic Brackets

Preformed Crowns

Wrought Cobalt- Chromium Nickel Alloys

Orthodontic Wires

Endodontic Files

Wrought Nickel -Titanium Alloys

Orthodontic Wires

Endodontic Files

Wrought Beta- Titanium Alloys

Orthodontic Wires

General requirements of dental alloy

The metals and alloys used as substitutes for gold alloys in dental appliances must possess certain minimal fundamental characteristics.

• The alloys chemical nature should not produce toxicological or allergic effects

• The chemical properties of the appliance should provide resistance to corrosion and physical changes when in oral fluids

• The physical and mechanical properties such as thermal conductivity, melting temperatures, coefficient of thermal expansion should be satisfactory.

• Use should be feasible for both the dentist and the technician

• The metals, alloys and companion materials should be plentiful, inexpensive and readily available.

PHYSICAL PROPERTIES

Melting Temperature of base metal alloys differs significantly from that of dental gold casting alloys. Most base metal alloys melt at a temperature 14000-15000 C as compared with cast gold alloys. Type I-IV have a melting range of 800-10500 C

Though One Commonly Used Nickel Chromium Alloy Ticonium Melts At 12750 C. The Addition Of 1-2% Beryllium Lowers Melting Temperature By About 100 0C.The melting temperature is important in the selection of casting equipment, control of casting technique .

Density-The Average Density Of Cast Base Metal Alloys Is Between 7-8gms /Cm3 ,which is approximately half the density of most dental gold alloys. Density is of some importance in bulky maxillary appliances in which the force of gravity causes the relative weight of the casting to place additional forces on the supporting teeth. With certain appliances, therefore the reduction of weight resulting from lower density of cast base metal alloys can be considered an advantage.

MECHANICAL PROPERTIES

• Yield Strength: gives an indication when a permanent deformation of a device or part of a device, such as a clasp will occur. As such, it is one of the most important properties of alloys intended for RPD restorations. It is believed that dental alloys should have yield strength of atleast 415Mpa to withstand permanent deformation when used as a partial denture clasps. It has been seen that base metal alloys have yield strength greater than 600Mpa

• Tensile Strength: the ultimate tensile strength of cast base metal alloys is Greater Than 800 Mpa.

• Elongation: The percent elongation of an alloy is important as an Indication Of Relative Brittleness And Ductility. The combined effect of elongation and tensile strength is an indication of toughness of material. Because of their toughness, partial denture clasps cast of alloys with a high elongation and tensile strength do not fracture in service as often as those with low elongation.

The percent elongation is one of the properties critical to accurate testing and to

proper control during test preparation .For e.g. a small amount of micro porosity in

test specimen will reduce elongation considerable whereas its effect on yield

strength, elastic modulus and tensile strength is rather limited. The nickel content

with a corresponding reduction in cobalt generally increases the ductility and

elongation. High values of elongation are also obtained by casting at the normal

melting temperature and by not heating the alloy 100 0C above its normal casting

temperature.

Elastic Modulus

The higher the elastic modulus ,more rigid the structure can be expected elastic modulus of base metal alloys is approx double the modulus of cast dental gold alloys

Hardness

Differences in composition of cast base metal alloys have some effect on hardness. In general cast base metal alloys have hardness about one third greater than gold alloys used for the same purpose.

Hardness Is An Indication of the ease of Finishing The Structure & Its Resistance To Scratching When In Service. The Higher The Hardness, Requires The Use Of Special Polishing Equipment, which may be considered as a disadvantage .It is common to use electrolytic polishing for a portion of finishing operation, which reduces the time and effort necessary for mechanical finishing operation.

Fatigue

The fatigue resistance of alloys used for partial denture is important, when it is considered that these appliances are removed and placed daily .At these times, the clasps are strained as they slide around the retaining tooth and the alloy undergoes fatigue. Comparisons of cobalt, chromium, titanium and gold alloys show that cobalt chromium alloys possess superior fatigue resistance as indicated by higher number of cycles requires to fracture a clasp .Any procedure that results in increasing the porosity or carbide content of the alloy will reduce fatigue resistance. Also soldered joints, which often contain inclusions or pores, represent weak links in fatigue resistance of the appliance

Passivation

Elements in some base metal alloys have high affinity for oxygen, but the oxide film formed can serve a protective layer against oxidation and corrosion. This formation of protective film by a reactive substance is called passsivation.

Three metals are known for their passivation potential Aluminum, chromium, titanium the most corrosion resistant of these titanium

FUNCTION OF ALLOYING ELEMENTS

Chromium-

Content is directly proportional to tarnish and corrosion resistance. It has a passivating effect which ensures corrosion resistance. 30% of chromium -upper limit for attaining maximum mechanical properties. When the chromium content of an alloy is higher than 30%, the alloy is more difficult to cast with this % of chromium, he alloy also forms a brittle phase, known as sigma phase, therefore cast base metal alloys should not contain more than 28-29% chromium.

Cobalt- increases the strength and hardness of alloys

Nickel- decreases strength and hardness. In general cobalt and nickel upto a percentage are interchangeable elements

Carbon- is one of the most effective ways of increasing The Hardness Of cobalt based Alloys is by increasing the Carbon Content. A change in carbon content of approx 0. 2% changes the properties to such an extent that the alloy would no longer be useable in dentistry. For e.g. if carbon content in increased by 0.2%over the desired amount, the alloy Becomes Too Hard And Brittle and should not be used for making any dental appliances. Conversely if decreased by 0.2% -reduces the yield and tensile strength.

Molybdenum 3-6% Increases The Strength of the alloy

Aluminium – in Ni containing compound forms a Compound of Nickel and Aluminium(Ni3Al)This compound increases the ultimate tensile and yield strength of the alloy.

Beryllium 1-2% Lowers The Fusion Range By 1000 C. However recent studies suggest that the concentration of beryllium may adversely affect ductility. Corrosion resistance is also compromised

Silicon And Manganese Increases The Fluidity And Castability.

HEAT TREATMENT OF BASE METAL ALLOYS

The early base metal alloys used in partial denture prosthesis were primarily cobalt chromium and relatively simple. Heat treatment of these alloys at 10000 c upto 1 hour did not change their mechanical properties appreciably. Base metal alloys available today for partial denture prosthesis however are more complex. Presently complex cobalt chromium alloys as well as nickel chromium alloys are used.

Studies have shown that heat treatment of cobalt based alloys reduce both the yield strength and elongation. If for some reason some soldering or welding must be performed on these partial dentures, the lowest possible temperature should be used with the shortest possible time of heat to elevated temperature.

COBALT CHROMIUM ALLOYS ALLOY

Edward Haynes Introduced Cr- Co Alloys With Some Ni to the Automobile Industry During Early 1900’s They Are Called Haynes Stellites or Stellite Alloys Because of Their Bright Shiny Star like Appearance COBALT-CHROMIUM AND NICKEL CHROMIUM CASTING ALLOYS

According To ADA Specifications No:14, The Weight Of Chromium Should Be No Less Than 20% And The Total Weight Of Chromium, Cobalt And Nickel Should Be No Less Than 85%.

COMPOSITION

Principal Elements- Cobalt, Chromium, And Nickel (82%-92%)

Minor Alloying Elements- Carbon, Molybdenum ,beryllium, tungsten and Aluminum.

Cobalt- 35-65% – Imparts strength & rigidity, has high melting temperature.

Chromium- 23-30%Cr content is directly proportional to tarnish and resistance30% is considered the upper limit for attaining maximum mechanical properties

Nickel-0-20% Co & Ni is interchangeable. It decreases strength, hardness, fusion temperature it increases ductility

Molybdenum or Tungsten-0-7%are effective hardeners .It decreases ductility to a lesser extent than tungsten. It refines grain structure

Iron, Copper,Beryllium- 0-5% are hardeners. in addition beryllium decreases fusion temperature and refines grain structure.

Carbon Upto 0.2 % .carbon is most critical .Small amounts may have pronounced effect on strength ,hardness and ductility.Carbon forms carbides with metallic constituents which is an important factor in strengthening the alloy. However excess carbon increases the brittleness. Thus control of carbon content is important.

Manganese And Silicon in traces primarily oxide scavengers to prevent oxidation of other elements during melting.

PROPERTIES

Cobalt chromium alloys have replaced type IV gold alloys especially for making RPD because of their lower cost and good mechincal properties

Density is half that of gold alloys,so they are lighter in weight (8-9 gm/cm3)

Elongation depends on composition, rate of cooling, fusion and mould temperature employed. Ductility is lower than that of gold alloys. They may break if bent too many times

Modulus of Elasticity-They are twice as stiff as gold alloys. Thus castings are made thinner,thereby decreasing the weight of the R.P.D to resist deflection high stiffness is an advantage because less undercut is used for the retentive arm of the clasp than when the gold alloys are used. But high stiffness is a disadvantage since excessive forces is necessary to remove and insert the denture. However this problem can be overcome by making clasps of thinner sections and also by engaging the clasp to a lesser distance

Hardness – These alloys are 50% harder than gold alloys. Thus cutting, grinding, finishing is difficult.

Tarnish and Corrosion Resistance-Formation of chromium oxide layer on the surface prevents tarnish and corrosion. This is called as passivating effect hypochlorite or other chlorine containing compounds that present in some denture cleaning compounds will cause corrosion in base metal alloys. Even the oxygenating denture cleansers will stain such alloys. Therefore these solutions should not be used to clean chromium-based alloys.

Yield Strength- Is indication of amount of stress necessary to cause permanent deformation in the appliance. This is important with regard to clasps deformation of clasps while in use or during removal or insertion of denture in and out of the mouth must always be elastic iature. That means the deformed clasp must go back to its original position. If not, displacement of clasp arm away from tooth permanent deformation would make the clasp ineffective in its bracing and retentive position. The yield strength is higher than that of gold alloys is 710mpa.

APPLICATIONS

Denture Base

Cast Removable Partial Denture Framework

Crowns And Bridges

Bar Connectors

Cobalt Chromium Alloys are ideal materials for major connectors, they are rigid and strong in thin section and allows a wide variation in major connectors.

They may also be used in thinner sections than acrylic resins and have good thermal conductivity and relatively low density so denture is fairly light

They Can Be Finished to High Polish.

They Have Excellent Life Span.They Are Rather Difficult to Adjust at Chair Side

NICKEL – CHROMIUM ALLOYS

Composition

Nickel 61-81%

Chromium 11-27%

Molybdenum 2-9%

Minor additions Beryllium,Aluminum, Iron, Silicon, Copper, Manganese, Cobalt and Tin.

PROPERTIES

Cost- Cheapest of all casting alloys

Density- ranges from 7.8-8.4gm/cm3 .They have half the density of gold alloys making them lighter.

Castability-extremely technique sensitive

Hardness and workability- ranges from 175-360 VKN. These alloys are extremely difficult to work with.

Their high hardness makes them very difficultto cut,grind and polish.in the mouth more chair time may be needed to adjust the occlusion.

Modulus of elasticity- Ranges From 150-210 Mpa. This property Denotes Stiffness of Metal, Twice As Stiff As Gold Alloys.

Percent elongation- Ranges from 10- 28% gives indication of ductility of alloys

Though they are ductile they are not easily burnishable.this may be related to additional factors like high hardness and yield strength.

Tarnish and Corrosion Resistance-They are highly resistant to tarnish and corrosion resistance. This is due to a property called passivation

PASSIVATION Is the property by which a resistant oxide layer forms on the surface of chrome containing alloys. This oxide layer protects the alloys from oxidation these alloys maintain their polish for years. Other self-passivating alloys are titanium and aluminum

SOLDERING- is necessary to join bridge parts long span bridges are often cast in two parts to improve fit and accuracy. Base metals are much more difficult to solder.

Nickel chromium alloys are strong in thin sections and so can be used for long span bridges and other situations where strength is important, particularly in production of minimum preparation bridges /resin bonded/Maryland splints.

They can be cast to fine margins and they can become easier to finish. However they produce a grayish discoloration of abutment teeth

BIOLOGICAL CONSIDERATIONS

The nickel chromium alloys can be divided into those containing and those not containing beryllium.Those alloys that contain beryllium, contain 1.6% -2% of the element.

Precaution should be taken to avoid exposure the metallic vapor ,dust or grindings containing beryllium and nickel.the presence of nickel is of greater importance. Because it is a known allergen and may produce allergic reactions in some individuals.

Incidence of nickel sensitivity reported to be 5-10 times more in females than males, with 5-8% of females showing sensitivity. However no correlation between the presences of intraoral nickel based restorations and sensitivity It is also a potential carcinogen. Inhalation of beryllium containing dust, or fumes is the main route. it causes a condition called beryllosis, characterized by flu like symptoms, and granulomas of lungs. Physiological response may range from contact dermatitis to severe chemical pneumonits. Beryllium dust is 2 ug/m3 of air in a time waited 8 hour day. Nickel is another Known Allergen.

Safety Standard for Nickel Is 15 Ug/m3 of Air for a 40 Hour Week.To Minimize Exposure To Metallic Dust Containing Nickel Or Beryllium Intraoral Finishing Should Be Done With A High Speed Evacuation System

TITANIUM

• Titanium was first discovered in 1791by Wilhelm

Gregor ,a clergyman, who found the metal in black magnetic sand, in Cornwall, England and was named as titanium by Klaproth in 1794 after the Greek mythological first sons of earth- the Titans

Titanium Can Exist in Two Crystal Form

Alpha Phase &Beta Phase

• The alpha phase has a hexagonal close pack structure. The alpha phase is stable at all temperatures upto 8820 c which is beta transus temperature .at this temperature ,the alpha phase gets transformed to beta phase. The beta phase is stable from 8820c to melting point of titanium at 16700c. This phenomenon can be used to produce alloys, which at room temperature can have alpha, beta or alpha-beta structure, and therefore different mechanical and chemical properties. Pure titanium has alpha crystalline structure; hence it is mostly used in chemical industry where the excellent corrosion resistance is more vital than good mechanical qualities.

• The need for mechanical qualities are more in orthopedic implants, dental prosthesis and implants. in these situations ,titanium is combined with vanadium and aluminum to obtain the most commercially known alloys,ti 6 al 4 v which is two phase (alpha +beta) alloy. The addition of alloying elements extend the stability field of different phases .The stability of alpha phase can be extended by aluminum, carbon, oxygen and nitrogen. The alloying elements stabilizing the beta phase includes: hydrogen,, manganese, chromium, molybdenum, iron and vanadium The presence of even small amounts of interstitial elements alter the mechanical properties, e.g.a very low hydrogen conc.(0.15%) can produce great brittleness Small quantities of carbon, nitrogen and oxygen can raise the strength of titanium and decrease its ductility.

COMMERCIALLY PURE TITANIUM

Available In Four Grades According to Purity.

According to American Society of Testing and Material 67/89 Standard Specification, as the

Grades Increase From 1to 4 ,the Amount of Contaminating Oxygen, Iron and Nitrogen Increases.

TITANIUM PROCESSING

• Titanium casting is highly technique sensitive dueto its tendency to interact with atmospheric components .The intrinsic properties of titanium make it a difficult metal to melt and cast.A heat source powerful enough to melt titanium is requiredThe molten metal should be isolated from air and the crucible should not react with molten Titanium.

CASTING MACHINES

• In all these machines ,the main characteristic feature ,is that the melting occurs inside an inert gas atmosphere such as argon gas, so that the molten titanium does not absorb oxygen and nitrogen form air. The electric arc or high frequency induction is used to reach the high melting point Titanium.a centrifugal method is used to drive the molten metal into the mold cylinder. Copper ceramic or carbon based crucibles have been used to contain the molten metal in the casting system, by gas pressure, the equipment consists of 2 chambers, an upper melting chamber, housing a copper crucible and a tungsten electrode lower casting chamber connected to the first one by a mold cylinder. Necessary melting power is generated by a electric arc. After placing the titanium load to be melted over the crucible, the air in both the chambers is drained off by means of a vacuum and argon gas is introduced in the melting chamber.the argon gas pressure is adjusted and then the arc generated between the titanium load and tungsten electrodes heats causing titanium to melt. a considerable amount of thermal energy is kept at the center of titanium load for 60 sec to compensate for low thermal conductivity of titanium.

ALTERNATIVES TO CASTINGSPlasticization of Metallic Surface With a Thin Film of Titanium,niobium and Ceramic.

Second Option Is Forming of Superplastic.

Third Option Is Titanium Processing by Milling and Spark Erosion

PROPERTIES

Density Is 4.5 G/cm3. Light weight metal

Modulus Is 100 Gpa.,half the value of other BMAMelting point Is Quite High 16680C. Therefore Special Equipment Is Required for Casting Titanium.

Tarnish and Corrosion Resistance- Has Ability to Self Passivate

Biocompatibility- Is Non Toxic and Has Excellent Biocompatibility With Both Hard and Soft Tissues.

OTHER APPLICATIONS OF CAST BASE METAL ALLOYS

In Surgical Repair Of Bone Fracture .

Metallic Obturators And Implants.

Cobalt Chromium Alloys Have Been Implanted Directly Into Bone Structure For Long Periods

With Favorable Response Of The Tissue,Probably Due To Low Solubility And Electro- Galvanic Action Of The Alloy. The Product Known As Surgical Vitalliun Is Used Extensively For This Purpose .

WROUGHT BASE METAL ALLOYS

Stainless Steel

Cobalt Chromium Nickel

Nickel -Titanium

Beta- Titanium

STAINLESS STEEL

This was first discovered by Brearely of Sheffield, England in 1913.Stainless steel is an alloy

of steel containing minimum of 13% chromium.It is chromium which makes the steel resistant

to tarnish and corrosion Steel is an iron-carbon alloy, which contains less than 1.2%

carbon. The term stainless steel is applied to the alloys that contain chromium, nickel,

manganese and perhaps other metals to improve properties and give the stainless

quality to the steel.

TYPES

There are three types of stainless steel based upon the lattice arrangements of iron.

FERRITIC (Alpha Form)Solid iron in which carbon is insoluble,because of small space in between atoms.It has a Body centered cubic structureThis condition remains from room temperature to 912 C

Properties and Use

The ferritic alloys have good corrosion resistance because of the principle component that is chromium, but less strength and hardness. So, they -find little application in dentistry

AUSTENITE(Gamma form)Exists at Temperature Between 912-1394 0CAs Face Centered Cubic Structure in Which Carbon Is Soluble, Because the Space Between the Atoms Is Larger and So the Carbon Atoms Occupy These Spaces to Form Interstitial Solid Solution If Such a Austenitic Steel at a Temperature Above 9120 C Is Suddenly Cooled , It Undergoes Transformation in Its Structure to Form a Body Centered Tetragonal Structure Called Martensite. This Change Makes the Metal Hard, strong but brittle.

Advantages

Austenitic steel is preferable to the ferritic alloys because of the following properties.

 Greater ductility and ability to undergo more cold work without breaking

 Substantial strengthening during cold working

 Greater ease of welding

 The ability to readily overcome sensitization

 Less critical grain growth

 Comparative ease in forming.

Tempering- such hard steel can be heated to 200-450 C for short period and cooled rapidly to adjust the precise hardness and strength required to reduce the brittleness. This process is called as Tempering.

Thus there are three forms of steel

1)Ferritic- have BCC space lattice

2) Austentic- have FCC space lattice

3) Martensitic- have distorted BCC tetragonal space lattice

18-8 is a special and famous type of austenitic .It is called as 18-8 because it contains 18% chromium and 8% nickel

Sensitization of 18-8 Stainless steel

•It is the loss of corrosion resistance of 18-8 stainless steel due to removal of chromium from it

•This happens when it is heated to temp between 400- 9000 C,say during soldering or welding

•Therefore this condition is also called WELD DECAY

Loss of chromium from 18-8 stainless steel is likely to corrode in that area of stainless steel which has been heated .It happens because chromium combines with carbon to form carbides during heating .Thus its passivating chromium oxide layer is lost resulting in loss of corrosion resistance.

Prevention of weld decay

1)by using stabilized stainless steel – adding of titanium or columbium into its composition brings about stabilization of steel. Such stabilized stainless

steel when heated ,its titanium or columbium will react with carbon instead of chromium,thereby chromium remains as it is and provides a passivating effect

2) Cold work the steel instead of hot work

3) Polish the appliance thoroughly

4) Do not handle stainless steel wires with carbon steel pliers

5) Chlorine containing cleansers should not be used to clean removable appliances fabricated with stainless steel

MECHANICAL PROPERTIES

Tensile strength – 21 00 MPa

Yield strength – 1400 MPa

Hardness – 600 KHN

USES OF 18-8 STAINLESS STEEL

•In Prosthetics

For Partial Denture Clasps ,Bars

For Swaged Plates,implants.

•In Orthodontics

For Wires, Ribbons, Bands, Bows, Hooks, Springs.

ADVANTAGES OF 18-8 STAINLESS STEEL

•Heat and Corrosion Resistance.

•More Ductile and Can Be Cold Worked.

•Can Be Easily Welded.

•Hardened by Cold Working.

•Can Be Stabilized Against Corrosion.

NICKEL-TITANIUM ALLOYS

These nickel-titanium alloy or (nitinol) represents the first product with significantly different propertiesthan other base metals. Nitinol wires have large elastic deflections or working range and limited formability because of their low stiffness and moderately high strength.

COMPOSITION:

Nickel 54%

Titanium 44%

Cobalt 2%

Properties

•Shape Memory effect- it is the ability of the alloy to retain and memorize a particular shape given to it

Mechanism of shape memory

•Structure of nitinol at room temperature is BCC(austentic phase) when this is heated it changes to HCP(martensitic phase)

•This change is responsible for shape memory effect.

Superelasticity•It is a large plastic deformation induced in a alloy which generates constant stress when the material undergoes transformational change

•Mechanism-BCC changes to HCP due to increase in stress. this gives the property of super elasticity

WROUGHT COBALT-CHROMIUM-NICKEL ALLOYS–Cobalt chromium nickel alloys drawn into wires can be used successfully in orthodontic appliances.

These wrought alloys were originally developed for use as watch springs (Elgiloy). Their properties are excellent also for ortho¬dontic purposes as orthodontic wires.

COMPOSITION

CO-40% Mn-2%

Cr-20% C-0.15%

NI-15% Be-0.04%

Mo-7% Fe-15.8%

Processing and manipulation:

Ordinarily the wires are heat treated before being supplied to the user and may be ordered in several degrees of hardness(soft ductile semi spring temper and spring temper) .

In addition the clinician can heat treat the wires by placing them in an oven or by passing an electric current through them with certain types of spot welders. A typical cycle would be 482 C for 7 to 12 min. this heat treatment would increase the yield strength and decrease the ductility.

PHYSICAL PROPERTIES

Tarnish and corrosion resistance is excellent. Hardness, yield, and tensile strength similar to those of 18-8 stainless steel.

WROUGHT BETA TITANIUM ALLOY

A titanium molybdenum alloy known as beta titanium was introduced in 1979 as a wrought orthodontic wire. As discussed previously c.p. titanium exists in a hexagonal closed packed crystal lattice at temp below 883C and in a body centered cubic crystal lattice at higher temperatures. These structures are reffered to as alfa titanium and beta titanium respectively.

Composition:

Titanium 78%

Molybdenum 11.5%

Zirconium 6%

Tin 4.5%

Supplied as wrought wire

PROPERTIES: :

•Lower Elastic Modulus

•Lower Yield Strength

•Good Ductility

Good Weldability

METALLIC DENTURE BASE• used in clinical situations where a single maxillary CD opposes a partial component of natural mandibular teeth heavy masticatory load directed in to a thin palatal resin plate which may result in denture fracture.

TECHNIQUE

Relatively thin metal base is cast to contact denture bearing surface. Acrylic resin is used to retain the denture teeth and provide buccal and labial flanges that enhances esthetic quality The processed resin is attached to cast metal base by a retentive meshwork.

MATERIALS used

• Cobalt chromium alloys

Nickel chromium alloys

Titanium alloys

•Cobalt and nickel containing alloys could be potential allergens ,biological risks of metal ions being released suggests use of titanium.

ADVANTAGES•

High Thermal Conductivity

•Decreased Bulk Across Palate

•Dimensional Stability, increased Fit

•Superior Biocompatibility, in Case of Ti

•Stronger Denture Bases

•Does Not Break Easily, Strength Is Many Times More, hence Can Be Made Thinner.