Metal and Metal alloys for orthodontics

Alloy : is a metal containing two or more elements at least one of which is a metal and all of which are mutually soluble in the molten state.
                 Requirements of casting alloys:

1. They must not tarnish or corrode in the mouth;
2. They must be sufficiently strong for intended purpose.
3. They must be compatible ( nontoxic , non allergic)
4. They must easy to melt, cast, cut, grind (easy to fabricate)
5. They must flow well and duplicate fine details during casting
6. They must have minimum shrinkage on cooling after casting
7. They must easy to solder.

Applications of metal alloys

1.  Construction of metallic frame work of removable partial denture
2. Construction of metal core of crown and bridge
3. Making orthodontic wires ,bands ,brackets ,etc
4. Making endodontic instrument
5. Construction of metal implants

Classification of metals
Nobel metals: like gold, platinum, rhodium, ruthenium ,iridium ,osmium ,silver. However, in the oral cavity silver is not considered Nobel because of tarnish.
Non Nobel metals (base metals): like Chromium , cobalt ,nickel ,iron , copper , etc….

Classification of dental alloys

1. According to number of elements:
2. Binary ---2 elements
3. Ternary --- 3 elements
4. Quaternary ---4 elements

B-According to nobility

1. High Nobel alloys---contain 40% gold or more and 60% Nobel or more
2. Nobel alloys ---contain 25% or more Nobel metals
3. Base metal alloys ---contain less than 25% Nobel

C-According to major elements

1. Gold alloys
2. Silver alloys
3. Palladium alloys
4. Nickel alloys
5. Cobalt alloys
6. Titanium alloys

D- According to 3 major elements

1. Gold-palladium-silver alloys
2. Palladium-silver-tin alloys
3. Nickel-chromium-molybdenum
4. Cobalt-chromium-molybdenum
5. Iron-nickel-chromium alloys
6. Titanium-aluminum-vanadium alloys

GOLD
Gold foil filling (pure gold)
Pure gold is very malleable and ductile. Gold foil is in the form of very thin sheath or foil about 0.001 mm thickness. It is condensed into the cavity and each layer of foil become welded to the material already condensed.
Advantages of gold foil filling
Perfect corrosion resistance
Adequate mechanical properties
Very durable

Disadvantages
Highly expensive
Not esthetic
The technique is time consuming and depends on the skill of operator

GOLD ALLOYS

They are classified according to yield strength and percentage of elongation

Type 1 --- soft ---it is indicated for small inlay restoration not subjected to mastication stress like class 5 and class 3
Type 11 –medium – it is indicated for large inlay restoration
Type 111 – hard --- it is indicated for crown and bridge
Type 1v –extra hard – it is indicated for crown and bridge and removable partial denture frames
Composition of gold alloys
Gold, copper, silver, palladium, and some platinum ,zinc ,tin , and iron .
Copper –reduce melting point and density, increase hardness, gives red color to gold. If high % it reduce corrosion resistance and tarnish resistance.
Silver ----whiten the alloy .increase strength and hardness slightly. In large amount it reduce tarnish resistance.
Platinum -----increase strength corrosion resistance and melting point .has white color.
Palladium ----- it hardens and whitens the alloy, raise the fusing temperature increase tarnish resistance .it is less expensive than gold.
Zinc -----acts as scavenger for oxygen.
Properties
1. Color –it is yellow and there is white gold depending on the whitening elements present (silver, platinum, palladium)
2. Melting range _-920-960 C
3. Density–for pure gold is 19.3gm/cm3 .gold alloys have less density.
4. Yield strength ---for type 111 -207 MPa .for type 1v –275 MPa
5. Hardness ---for type 111 –121 MPa.  For type 1v -----149 MPa
6. Elongation ----type 111 –30%-40%.   For type 1v -----30%-40%
7. resistant to tarnish and corrosion because of high Nobel metal content
8. Casting shrinkage is less than 1.25—1.65%
9. they are relatively biocompatible.
10.gypsum bonded investment is used when the melting temp. is below 700 C.
Alternatives to gold alloys

1. Silver-palladium alloys

They are cheaper than gold alloys .whiter in color and their properties are similar to type 111 and type 1v gold alloys but : lower density
Lower ductility
Lower corrosion resistance

1. Titanium and titanium alloys

       Metal ceramic alloys
They are alloys that are compatible   with porcelain and capable of bonding to it. A layer of porcelain is fused to the alloy to give it natural tooth like appearance. Porcelain is brittle so these alloys reinforce the porcelain (ceramics) .they should have coefficient of expansion and contraction (CTE) matching that of porcelain.

Requirements of metal ceramic alloys

1. The melting temperature of the alloy should be higher than the porcelain firing temp.
2. CTE should be compatible with that of porcelain
3. It should resist creep
4. It should be able to bond with porcelain
5. It should have high stiffness (high modulus of elasticity)
6. It should not stain or discolor porcelain.

 

       Types of metal ceramic alloys

1. High Nobel (gold alloys) like: gold-palladium ,  gold –palladium -silver
2. Nobel (palladium) like :palladium-silver, palladium gold,            palladium -copper
3. Base metal alloys like: nickel-chromium, pure titanium,           Titanium-aluminum-vanadium.

 

 

Removable denture alloys     
Requirements
Beside the requirements of metal casting alloys, partial denture alloys should:

1. Have low weight, because it is large volume structure.
2. Have high stiffness which help in making casting more thinner which is important in the palate of upper denture.
3. Have good fatigue resistance. It is important for clasp
4. Not react with denture cleansers
5. Have low cost

                       Types
Cobalt chromium, Nickel chromium alloys, and type 1v Gold alloy

Cobalt –chromium alloys
They are also called Satellite because of their shiny –star like appearance. They have high strength, excellent corrosion resistance, hard
Applications

1. Denture base
2. Removable partial denture framework
3. Bar connectors

Composition
Cobalt……..35-65%----decrease hardness, strength, and rigidity
Chromium ….23-30%----passivity effect, decrease the melting point      
Nickel………….0-20%----- decrease strength and hardness, increase ductility.  Molybdenum .0-7%-----increase hardness
Iron ……………..0-5%----increase hardness
Carbon…………. 0.4%
*** Nickel causes sensitivity in some patients
Properties

1. Density-------- 8 -9 gm/cm3 -------- half of gold
2. Fusion temperature -----1250 -1480 C, higher than gold alloys
3. Yield strength ----710 MPa, higher than gold alloys
4. Elongation --------1-12% , less than gold
5. Modulus of elasticity--- 225GPa, twice that of gold alloys
6. Hardness-----------432 VHN, thus cutting, grinding, finishing is difficult. Special high speed finishing tools are needed
7. Tarnish and corrosion resistance.

Passivation: it is the formation of chromium oxide layer on the surface of these alloys prevents tarnish and corrosion in the mouth.
Hypochlorite and other chlorine in some denture cleaning solutions should not be used because it will cause corrosion of the alloy.

1. Casting shrinkage is about  2.3%

      Advantages of metal alloys base

1. Lighter in weight
2. Better mechanical properties
3. Corrosion resistance as gold alloys because of passivating effect
4. Less expensive than gold

Disadvantages

1. More technique sensitive
2. Complexity in production of dental appliance
3. High fusing temp.
4. Extremely hard so require special equipment for finishing
5. High hardness causes wear of restoration and natural teeth contacting them

            Titanium and titanium alloys
They have excellent biocompatibility, light weight, good strength and passivation effect.
        Applications in dentistry

1. Dental implant
2. Root canal instruments

Properties of Titanium and Titanium alloys

1.  Color ------white gray color
2. Density---- 1-4 gm/cm3
3. Modulus of elasticity----110 GPa , half rigid as base metal alloys
4. Melting temp. is high---1668 C, special equipment should be used
5. Coefficient of thermal expansion CTE  8.3*10 -6 cm/cm C. it is low compared to porcelain 12.7—14.2 *10 -6 cm/cm C. special low fusing porcelain is used with it.
6. Biocompatibility--- it is nontoxic and excellent biocompatibility with soft and hard tissue
7. Tarnish  and corrosion resistance---passivation by the formation of  titanium oxide layer to protect the metal from further oxidation
8. phosphate  and ethyl silicate bonded investment is used for the casting

 

Nickel chromium alloys
They are used for metal ceramic crown and bridge
Composition
Nickel---------------61-81%
Chromium---------11-27%
Molybdenum-----2 – 9%
Minor elements like beryllium, aluminum, and silicate
properties

1. color------white
2. melting range------- 1155- 1340 C
3. Density----------------7.8 – 8.4 C
4. Hardness------------- 175 -360 VHN, the high hardness make them difficult to grind .cut and polish.
5. Yield strength-------310 – 828 MPa. It is stronger than gold
6. Modulus of elasticity- 150 – 210 GPa. This makes casting thinner and lighter
7. Elongation------------10 – 28 % .they are ductile but not easily burnishable
8. Porcelain bonding--------- this alloy forms adequate oxide layer which bonds to porcelain.
9. Esthetic--------------- dark oxide layer may be seen at porcelain metal junction.

 

             Wrought stainless steel alloys

Wrought alloys are defined as alloys which are shaped without applying heat (room temp.) by hammering cold working.
Stainless steel is an alloy of iron and carbon that contains chromium, nickel and manganese
Cold working: the alloy is hammered, drawn or bent into shape at room temp.
It is called wrought alloy and it is used for making instruments, burs, wires.
Among these alloys is 18-8 stainless steel alloy. The numbers refer to percentages of Cr and Ni respectively

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In modern history, metals have been used as implants since more than 100 years ago when "Lane" first introduced metal plate for bone fracture fixation in 1895. The first metal alloy developed specifically for human use was the “vanadium steel” which was used to manufacture bone fracture plates (Sherman plates) and screws. But it was no longer used in implants because its corrosion resistance is inadequate in vivo environment.

Advantages and disadvantages of metallic Biomaterials:
Advantages

• High strength.
• High hardness.
• Fatigue and impact resistance.
• Wear resistance.
• Easy fabrication.
• Easy to sterilize.
• Shape memory.
• inert

  Disadvantages

• High modulus.
• High corrosion.
• Metal ion sensitivity and toxicity.
• High density.

In general metallic biomaterials can be grouped in the following categories:

• Stainless steels.
• Co-based alloys.
• Titanium-based alloys.
• Specialty metallic alloys.
• Metals and their alloys are widely used as biomedical materials. On one hand, metallic biomaterials cannot be replaced by ceramics or polymers at present. Because mechanical strength and toughness are the most important safety requirements for a biomaterial under load-bearing conditions. On the other hand, metallic materials sometimes show toxicity and are fractured because of their corrosion and mechanical damages . Therefore, development of new alloys is continuously trialed.

STAINLESS STEELS
is stronger and more resistant to corrosion than the vanadium steel.

TABLE (1): Compositions of 316L Stainless Steel (American Society for
Testing and Materials, F139—86. p. 61. 1992).

          The Ni stabilizes the austenitic phase (g) face centered cubic crystal (fcc) structure at room temperature and enhances corrosion resistance.
The advantages of stainless steels, especially type 316 and 316L over other grades of steel:

• Biocompatible.
• This group of stainless steels is nonmagnetic and possesses better corrosion resistance than any others.

 

A wide range of properties exists depending on the heat treatment (annealing to obtain softer materials) or cold working (for greater strength and hardness).

(Co Cr) ALLOYS
There are basically two types of cobalt—chromium alloys:

• The castable CoCrMo alloy.

The castable CoCrMo alloy has been used for many decades in dentistry and, relatively recently, in making artificial joints.
(2) The CoNiCrMo alloy which is usually wrought by (hot) forging.
The wrought CoNiCrMo alloy is relatively new, now used for making  the stems of prostheses for heavily loaded joints such as the knee and hip.

The corrosion products of CoCrMo are more toxic than those of stainless steel 316L.
The advantages and disadvantage of CoCrMo:
 wrought or forged forms has the highest strength/wear resistance,
 hardest to fabricate,
 may produce cobalt or chromium ion sensitivity/toxicity.
Ti ALLOYS
Ti-based alloys are finding ever-increasing applications in biomaterials due to their excellent mechanical, physical and biological performance.

• Pure Ti and Ti6Al4V

Commercially pure titanium (Ti CP) and Ti-6Al-4V are the two most common titanium base implant biomaterials. These materials are classified as biologically inert biomaterials.

Comparison betweenTitanium alloy (Ti-6Al-4V) versus Titanium (Ti) metal

• titanium alloy is stronger than titanium metal,

-both have the best corrosion resistance,
-both have excellent bone bonding.
The modulus of elasticity of these materials is about (110 GPa.) This is much lower than stainless steels and Co-base alloys modulus (210 and 240 GPa.) respectively.
The Ti6Al4V alloy has some disadvantages: its elastic modulus, although low, is 4 to 6 times greater than that of cortical bone and has low wear resistance that is a problem in articulations surfaces. Also, (V) can cause toxicity, neuropathy and adverse tissue reactions, and (Al) ions from the alloy might cause long-term Alzheimer diseases.

 

2- TiNi Alloys
The titanium—nickel alloys show unusual properties, that is, after it is deformed the material can snap back to its previous shape by following heating of the material. This phenomenon is called shape memory effect (SME).
(Nitinol) exhibits an exceptional SME near room temperature, if it is plastically deform below the transformation temperature, it revert back to its original shape as the temperature is raised.
In order to develop such devices, it is necessary to understand fully the mechanical and thermal behavior associated with the martensite phase transformation. A widely known NiTi alloy is (55-Nitinol) (55 wt% atomic Ni)
Some possible applications of shape memory effects alloys are orthodontic dental archwires, intracranial aneurysm, skull clip, contractible muscles for artificial heart, vascular stent, and catheter wire guide.
In general , The advantage and disadvantage of Ti alloys are :  
Advantages
•Easily formed.
•Highly biocompatible.
•Outstanding corrosion resistance.
•Better than stainless steel and cobalt-chromium alloys.
•Forms protective oxide (TiO2) layer.
•Low elastic modulus.
Disadvantages
•Poor wear resistance.
•Should not be used in articulated surfaces such as hip or knee joints unless surface-treated through ion implantation which improves wear resistance.
In General Titanium alloys used in implants present three main problems:
- High cost because the amount of processing energy and melting and casting difficulties.
- Higher elastic modulus compared to bone.
- Although the inert behavior of Ti is a good property, its bone attachment is difficult because it do not react with the human tissues.

The following table represents the comparison of mechanical properties of metallic biomaterials with bone.

Table (4): Comparison of mechanical properties of metallic biomaterial with bone.

 

                                        Ti alloys (Bio metals) application :

 

• DENTAL METALS

          Metals are used in dentistry for direct fillings in teeth (dental amalgams), fabricating crowns and bridges (noble metal and base metal alloys), partial denture frameworks (base metal alloys), orthodontic wires and brackets (stainless steel, Ti alloys and Ni-Ti alloys) and dental implants (CP Ti and Ti6Al4V).

• Amalgam: An alloy obtained by mixing silver tin alloy with mercury.

Dental amalgams are formed by adding Hg to dental amalgam alloys (alloys containing Ag, Cu and Sn plus some other minor elemental additions).
Dental amalgam is an alloy made of liquid mercury and other solid metal particulate alloys made of silver, tin, copper, etc. The solid alloy is mixed with (liquid) mercury in a mechanical vibrating mixer and the resulting material is packed into the prepared cavity. The final composition of dental amalgams typically contains 45% to 55% mercury,  35% to 45% silver, and about 15% tin after fully set in about one day.
Gold and gold alloys are useful metals in dentistry as a result of their durability, stability and corrosion resistance. Gold filling are introduced by two methods: casting and malleting.
Gold alloys are used for cast restorations, since they have mechanical properties superior to those of pure gold.

OTHER METALS
Several other metals have been used for a variety of specialized implant applications. Tantalum has been subjected to animal implant studies and has been shown very biocompatible. Due to its poor mechanical properties and its high density (16.6 g/crn3) it is restricted to few applications such as wire sutures used for plastic surgeons and neurosurgeons and a radioisotope for bladder tumors.
NiCu and CoPd alloys are of interest in cancer treatment since their magnetic properties enable them to be heated by a oscillating magnetic field.
Surface modification of metal alloys such as coatings by plasma spray, physical and chemical vapor deposition and ion implantation have been used in the industry.
Coating implants with tissue compatible materials such as hydroxyapatite, oxide ceramics and bioglass are typically application in implants.
Platinum and other noble metals in the platinum group are extremely corrosion resistance but poor mechanical properties. They are mainly used as alloys for electrodes such as pacemaker tips because of their high resistance to corrosion.

 

 

Bio metals applications

Table (3): Implants division and type of metals used

Corrosion of Metallic Implants
          Corrosion is unwanted chemical reaction of a metal with its environment, resulting in its continues degradation to oxides, hydroxides or other compounds. Tissue fluid in the human body contains water, dissolved oxygen, proteins, and various ions such as chloride and hydroxide. As a result, the human body presents a very aggressive environment for metals used for implantation. Corrosion resistance of metallic implant materials is consequently an important aspect of its biocompatibility.

Failure of metals for biomedical devices

•  Corrosion

Metal implant is prone to corrosion during its services due to corrosive medium of implantation site. Types of corrosion that frequently found in implant applications are fretting, pitting and fatigue. Fretting corrosion most frequently happens in hip joint prostheses due to small movement in corrosive aqueous medium. And in most cases subjected to cyclic loading.

 

• Fatigue and fracture

During its service most of metallic implants are subjected to cyclic loading inside the human body which leads to the possibility for fatigue fracture. Factors determine the fatigue behavior of implant materials include microstructure of the implant materials. It was reported that Ti6Al4V with equiaxed structure has a better fatigue strength properties.

• Wear

Together with corrosion process, wear is among the surface degradation that limits the use of metallic implant such as Ti alloy. Removal of dense oxide film which naturally formed on the surface of this metallic implant in turn caused wear process. In fact, the major factor that causing premature failure of hip prostheses is due to the wear process with multiple variables interact and thus increase the resultant wear rates.
Recent developments in metals for biomedical devices
Along with the advances in biomedical technology and tissue engineering, biomaterials are desired to exhibit low elastic modulus, shape memory effect or superelasticity, wear resistance, and workability. In addition, they are required to eliminate all possibility of toxic effects from leaching, wear and corrosion. One of the concerns is avoiding the use of Ni in fabricating metal alloys. This demand leads to the development of new generation of metallic biomaterials and their novel processing.
Type Ti alloy exhibits a lower elastic modulus than type and Ti type which makes it considered to be the first candidate for low elastic modulus metallic biomaterials. In Ti-Nb systems such as Ti29Nb13Ta4.6Zr and Ti35Nb4Sn, the elastic moduli can be reduced to 50-60 GPa which are closer to that of cortical bone (10-30 GPa).
Besides, the development in alloy’s composition and microstructure, the processing technology for metallic biomaterials is also progressed. Porous structure further reduces elastic modulus to get closer to that of cortical bone. This structure can be obtained through powder sintering.