Wrought Alloys and Orthodontic Materials

• Components of orthodontic treatment (fixed):

  • Aim of treatment
  • Align and orientate teeth
  • To improve appearance and function
  • Achieved using:
  • Arch wires
  • Brackets/bands
  • Bonding agents
  • Auxiliary components - hooks, springs, ligatures, elastic bands
• Orthodontic wires are produced by cold work:

  • Work hardened, fibrous grain structure
  • High yield strength and hardness - springy
  • Reduced ductility
• Useful properties of orthodontic wires for orthodontics:

  • Stiffness
  • Controls the amount of force applied to teeth
  • The more stiff the wire is, the higher the force applied to the tooth and the more rapid the movement
  • Elastic range (also called Springback Ability)
  • How much the wire can be deformed (bent) without plastic deformation
  • Elastic deformation in bending (proportional limit/modulus)
  • Related to the total amount of tooth movement
  • Lowered by recrystallisation

  
• Treatment may require different levels of wire stiffness:

  • Wire stiffness controls the rate of tooth movement, but:
  • Rapid tooth movement may be painful
  • May be difficult to engage stiff wires with brackets in grossly misaligned dentitions
  • May lead to bracket debonding
  • So, treatment usually involves a series of wires:
  • Initially use low stiffness wires to move large distances, slowly
  • Wires replaced with increasingly stiff ones as treatment progresses
  • Higher stiffness leads to quicker movement, smaller distances involved
• What controls the stiffness of a wire:

  • Wire stiffness is related to:
  • Choice of alloy
  • Degree of cold working carried out
  • Dimensions of the wire
  • Effect of cross-sectional area on wire stiffness:
  • For round wires stiffness depends on the radius - bigger wires are stiffer
  • For rectangular wires stiffness depends on the width and height - bigger wires are stiffer
  • Multi-stranded wires - combination of smaller (more flexible) wires of the same alloy
  • Allows for very low stiffness wires to be used in standard brackets
• Wire length affects stiffness:

  • Effect of length on wire stiffness
  • The longer the wire the lower the stiffness - stiffness depends on 1/(length)³
  • This may mean that short lengths of wire need to be joined together
  • Joining can involve:
  • Loops - requires ductility, not always possible
  • Soldering - use eutectic alloy to join
  • Welding - use an electric current to increase temperature
  • Soldering and welding may lead to recrystallisation:
  • Reduction in mechanical properties
  • Potential treatment problems
• Further properties of wires:

  • Corrosion resistance
  • Wires will be in mouth for long periods
  • Corrosion results in ion release - may affect biocompatibility (e.g. Ni allergies)
  • Ion loss leads to loss of strength - will affect durability
  • Friction
  • Successful treatment requires the wire to slide through the brackets
  • High friction can lead to no sliding
  • Ceramic brackets have a higher coefficient of friction with metal wires - this friction may stop tooth moving and halt treatment
• Common alloys used for archwires:

  • Stainless steel
  • 18% Cr, 8% Ni
  • Ni/Ti
  • Ni with 45% Ti (50/50 atomic ratio)
  • Beta Titanium
  • Ti with ~ 11% Mo, 7% Zr, 4% Sn
• Stainless steel archwires - properties:

  • Relatively high modulus - leads to rapid movements
  • High proportional limit - has a moderate range (elastic range)
  • Ductility
  • Depends on manufacturing and heat treatments
  • Range offered by manufacturers
  • Various options eg hard (low ductility), half hard, soft (highest ductility)
• Stainless steel archwires:

  • After designing the device a stress relief anneal is needed
  • eg 450°C for 10 mins
  • Beware overheating - recrystallisation
  • Joining
  • Can be joined by soldering or welding
  • Beware overheating - recrystallisation
  • For welding: weld decay can occur
  • Above 500°C CrC (chromium carbides) forms at grain boundaries - brittle
  • Stabilised SS (stainless steel) - contain Ti or Nb used to prevent weld decay
  • Friction
  • Slides easily through brackets
  • Slides best through stainless steel brackets
• Ni/Ti (Nitinol):

  • Composition = Ni (55), Ti (45) - 50/50 atomic ratio
  • Mechanical properties
  • Low modulus, flexible - apply low forces
  • High proportional limit - good springback, large movements but slowly
  • Low ductility and cannot weld or solder - cannot be joined by looping, use as single strand
• Nitinol shape memory alloys - different grades available:

  • Conentional - mechanical behaviour is like other metals and alloys
  • Shape memory alloys - super-elastic behaviour
  • Addition of other materials such as Cu produces different behaviour
  • Capable of very large deformation still return to original dimensions
  • Related to complex microstructure
  • By controlling microstructure archwires can:
  • Have low stiffness below mouth temperature, high stiffness at mouth temperature
  • Be shaped below mouth temperature, want to return to ideal arch at mouth temperature
• Nitinol shape memory alloys:

  • Behaviour not ideal yet but developments being made
  • Two behaviours identified:
  • Stress related changes - termed pseudoplastic
  • Temperature related changes - termed thermoelastic
• Beta Ti:

  • Composition = Ti with ~ 11% Mo, 7% Zr, 4% Sn
  • Mechanical properties
  • Low modulus - flexible, applies low forces
  • High ductility - easily adjusted
  • No shape memory effect
  • Joining - can be welded
  • High friction - doesn't slide easily through bracket
  • No nickel - claimed biocompatibility advantage
• Co/Cr (Elgiloy) wires:

  • Composition = Co(40), Cr(20), Ni(15), Fe, Mo
  • Unique character:
  • Supplied in soft state - not work hardened
  • Hardened by heat treatment
  • Ductile - easy to join using loops
  • Stiffness - similar to stainless steel
• Gold alloy wires:

  • Composition
  • Au, Pd, Ag, Cu (as for type IV casting alloy)
  • High in palladium (Pd)
  • High melting point and recrystallisation temperature - prevents recrystallisation during soldering
  • Joining:
  • Can be soldered (Ag/Cu alloy)
  • Low electrical resistance - welding difficult
  • Ductility - can be adjusted
• Aesthetic wires:

  • Developed to meet patient desire for "invisible" treatment
  • Coated metal wires
  • Coating made from either white epoxy or PTFE (Teflon)
  • Limited success
  • Coatings removed due to friction - reduction in aesthetics and compromised function
  • Alloy wire will be thinner to allow for coating - relationship between wire thickness and stiffness
  • Non-metallic
  • Two main types developed: nylon-coated silica and glass fibre composites
  • Better performance than coated metal wires but limited compared to metallic wires
• Bracket materials:

  • Stainless steel
  • Most common type
  • Rough back surface to aid adhesion
  • Polymeric
  • Made to be white/transparent - improve aesthetics
  • However, they discoloured quickly
  • Modern materials have glass filler added - opaque, discolour less
  • Ceramic
  • Best aesthetics of all bracket types
  • Disadvantages:
  • Hardness may lead to wear of opposing teeth - rarely used on lower arch
  • Increased risk of enamel fracture on bracket removal
  • Increased incidence of bracket failure during use - brittle material
  • Higher friction with SS wires
  • Cost more than SS wires