Metallurgy of Stainless Steel and Cobalt – Chromium alloys

COMPARISON OF CLINICAL CHARACTERISTICS OF STAINLESS STEEL & Co-Cr WIRES

 

 

  Wire Springback Stiffness Formability Stored energy Biocompatibility Joinability
Friction
S.S Low High Good Low Good Sold./weld Low
Co-Cr Low High Good Low Good Sold./weld Low- mod

HEAT TREATMENT OF COBALT CHROMIUM NICKEL ALLOY

With the exception of red temper elgiloy, non heat treated Co-Cr wires have a smaller spring back than stainless steel f comparable sizes, but this property can be removed by adequate heat treatment. The ideal temperature for heat treatment is 900 F° (482 C°) for 7-12 minutes in a dental furnace. this causes precipitation hardening of the alloy, increasing the resistance of the wire to deformation, and results in a wire that demonstrates properties similar to stainless steel.

Heat treatment at temperature above 1200°F (749°C) results in a rapid decline in resistance to deformation because of partial annealing. Optimum levels of heat treatment are confirmed by a dark straw- coloured wire or by use of temperature indicating paste. This can be softened by heat soaking at 400oc to 1200oc followed by a rapid quench. The age hardening temperature range is 260-650oc for the alloy Elgiloy should be held at 482oc for 5 hours.

Ordinarily the wires are heat treated before being supplied to the user. In addition the orthodontist can heat treat the wires by placing them in an oven or by passing an electric current with the help of certain type of welders. A typical cycle could be 482oc for 7-12 min. this will increase the yield strength and decrease the ductility.

 

 

 

 

ADVANTAGES OF Co-Cr ALLOY (ELGILOY)

 

1)            Low cost, although greater than stainless steel.

2)            Proven biocompatibility from excellent  clinical use.

3)            Outstanding formability (heat treated to increase YS and resilience)

4)            Can be soldered and welded with joining characteristics similar to stainless steel.

5)            Excellent in vivo corrosion resistance.

 

DISADVANTAGES

 

1)            High elastic force delivery similar to stainless steel.

2)            Lower spring back than stainless steel

 

STERILIZATION

The stainless steel and Co-Cr alloy wires can be sterilized by using autoclave, cold solutions, dry heat or iodophor. Glenn Smith and
J.A. Von Frounhofer conducted a study to find out the effect of various sterilization and disinfection protocols on three types of stainless steel orthodontic arch wires. They found that there is no clinically significant differences were found between new and used arch wires.

ORTHODONTIC BRACKETS

Stainless steel and Co-Cr alloys can be used for the fabrication of orthodontic brackets. A typical stainless steel bracket can be made of up to four different metals. The bracket profile may be made of machinable s s AISI 303, and the mesh pad of    304&305 brazed with a braze to the mesh pad. 316, 317 Austenitic stainless steel is used for making orthodontic brackets. AISI 316 L and 317 L can also be used. But they are very hard. 17 – 4 PH stainless steel is widely used for “mini brackets”. The manufacturer ORMCO has used another steel from the same class PH 17 – 7 to make its edge lock brackets. The added metals lower their corrosion resistance.

 

The modern super ferritics which contains 19-30% Cr are used to make several nickel free brackets. Example, Forestadent (Fe 26% or 5% Mo 3% Co)

 

Co-Cr alloy can also be used for making orthodontic brackets. Nickel free Co-Cr – alloys are used primarily to manufacture attachments. Such as:

Prestige                       [pyramid orthodontics]

Nu edge LN                [TP orthodontics]

Elite opti MIM            [ortho organizers]

 

When compared to ceramic brackets stainless steel and Co-Cr orthodontic brackets produce less friction because of a smooth surface. The various types of manufacturing processes like sintering and Molten Metal Injection molding and powder injection molding techniques produces brackets with very smooth surfaces which produces less friction.

 

 

 

Kapila Angolkar and Duncanson conducted a study to find out the friction between Edgewise stainless steel brackets and four orthodontic arch wire alloys including stainless steel, Co-Cr, NiTi and β Ti. The aim of the study was to determine the effects of wire size and alloy on frictional force generated between bracket and wire during translatory displacement of bracket relative to wire. For most wire sizes NiTi and β Ti generated greater amounts of frictional forces than stainless steel or Co-Cr wires. When the size of the wire is increased the friction increased.

 

RECYCLING OF DIRECT BOND ORTHODONTIC BRACKETS

Currently, there is increased interest in the recycling of metallic direct-bond orthodontic brackets. The steps in recycling or reconditioning involve removal of the bonding agent from the bracket, followed by electropolishing. A number of companies, including Esmadent (Company E), Ortho-Cycle (Company O-C), and Ortho Bonding (Company O-B), offer such a service, and the price per bracket is significantly lower than the price of new appliances.

The areas to be analyzed include the base torque angle (a), the slot width (b), and the mechanical properties of the appliances as reflected in the microstructure, hardness, theoretical tensile strength, and magnetic properties of the metal.

Removal of the acrylic bonding agent, which is usually a type of thermosetting filled resin, is the most critical part of the recycling process and requires either long exposure to heat or special procedures, such as the Use of a solvent. These methods are accomplished prior to any cleaning and polishing procedures.

Complete decomposition (pyrolysis) of acrylics occurs at around 770° C. Even at temperatures of up to 416° C., polymethyl methacrylate, a noncross-linked resin which is thermoplastic and less thermo resistant than most acrylics used in dental procedures, is only degraded and not pyrolyzed. In general, pyrolysis of acrylics forms acids, which are a possible source of metallic intergranular attack. Thus for complete decomposition of the bonding agent to occur when heat is used, the temperature of the process would likely be in the sensitization if not the heat-softening range of the metal.

Temperature thresholds are very important. Exposure to heat may lead to stress relieving or softening of cold-worked metal along with decreasing its corrosion resistance. At the same time, this may produce a layer of metal oxide, or scale, on the metallic surface6 which would have to be removed by electropolishing, thus leading to a possible slot widening in a bracket.

 

ORTHODONTIC BANDS

                        Stainless steel can be used for making orthodontic bands

 

DIMENSIONS OF THE BAND MATERIAL         (Annea1ed 18-8 Strips)                      

 

GRADE          THICKNESS IN mm.                        WIDTH IN mm.

No. 1                          0.07                                         4.0

No. 2                          0.13                                         4.5

 

After the material has been heated to 900° to 1000° C., it must be cooled immediately and only one side polished to a mirror like luster.

Band material of a soft nature is most suitable, and it is very important to leave one side of the strip unpolished; otherwise it is difficult to insure stability after it is cemented.

 

OTHER APPLICATIONS OF STAINLESS STEEL IN ORTHODONTICS

 

  • Stainless steel is used for making auxillaries Austenitic stainless steel is used for making these auxillariess. For example, Lingual button, Lingual Cleats, Lingual Sheath.
  • Ligature wires are made up of stainless steel wires

 

  • It can be used for making orthodontic instruments like different types of pliers. For this we are using Austenitic stainless steel. For making cutting pliers we are using more hardened Martensitic stainless steel.
  • Stainless steel wires find its use in making Uprighting springs and Minisprings.
  • For the fabrication of arch forming turrets. For making soft tissue x-ray shields etc. It is used to make parts of dental chair, instrument stand, instrument trays, impression trays etc. to name a few.
  • Different gauges of stainless steel round wires are used for the fabrication of different components of removable appliances like clasps, labial bows, springs etc.

It is now become an indispensable part of orthodontic profession.

REQUIREMENTS OF AN ORTHODONTIC SPRING

 

  1. Springs should deliver optimum force.
  2. It should possess high degree of elasticity.
  3. It should have a long range of action.

 

The force system delivered by an orthodontic spring depends upon
intrinsic and extrinsic factors. The intrinsic properties like modulus of elasticity yield strength which cannot be altered by the operator. The operation can exercise control over the extrinsic factors like the length of the wire, thickness of
the wire etc.

The force exerted by a spring can be summarized as

 

F  α    Edr4

l3

F = Force, d = deflection, r = radius, l = length.

 

The force exerted by a spring can also be expressed as

 

d  α      Pl3

r4

d = deflection, l = length, p = pressure, r = radius.

 

CONCLUSION

In the last few decades a variety of new wire alloys has been introduced into orthodontics. These wires demonstrate a wide spectrum of mechanical properties and have added to the versatility of orthodontic treatment. Appropriate use of all the available wires may

(a)          enhance patient comfort

 

(b)         Reduce chair side time

 

(c)          Duration of treatment

 

But the stainless steel alloys of which most orthodontic devices are made outstanding for the valuable properties they provide at a reasonable cost.