Orthodontic Archwires Key considerations Orthodontic Archwires Key considerations 1-Stiffness ( Spring rate): magnitude of force at a given deflection? 2-Springback ( range of action): Will it deflect that far? 3-Strength: The highest amount of force delivered by the wire.
Factors affects the force wire exerts: Thickness LengthMaterial
1-Effect of thickness round wires Stiffness is proportional to (diameter) 4 Diameter Stiffness 14 1.00 16 1.71 18 2.73 20 4.16 Small increment in size= big increment in force 14 20
Effect of thickness Rectangular wires Stiffness is proportional to w x h 3 W h3h3
The Chronological Development of Archwires ( Evans,1996) Phase l : Gold and Stainless steel ( 1900-1960’s) Phase ll: Stabilized NiTi “ Stabilized Martensitic” ( 1970’s) Phase lll : Superelastic NiTi “ Active Austenitic” ( 1980’s) Phase lV : Thermodynamic NiTi “Active Martensitic” ( Early 1990’s) ( Early 1990’s) Phase V : Graded thermodynamic ( Late 1990’s)
Stainless steel archwires - SS was developed in World War l, only in the 1940’s was introduced to orthodontics. - Very rigid wire, good for space closure but not for alignment. - This was solved by: Wire bending and loops, the use of multistrand SS. - Multistrand SS has 20% of the stiffness and twice as range as SS.
Development of the High Technology Alloys -NiTi alloys were developed in early1960’s for space programs by W.Buehler in USA. -This metal was called “ The Memory Metal” -Very complex structure and mechanical behavior. -Mechanical properties and thermal behavior are highly affected by composition, machining characteristics and heat treatment during manufacturing.
NiTi Transformation Austenite Martensite High Temperature Low Temperature TTR In response to temp variation, the crystal structure undergoes deformations in which the molecular arrangement is modified without a change of atomic composition.
Properties of different phases Martensite NiTi Austenite NiTi HexagonalCubic Crystalline structure 31 GPa 98 GPa 98 GPa Elastic Modulus 138 MPa 379 MPa Yield Strength
NiTi Alloys -Martensitic NiTi is responsible for the lowering of the delivery force. -Austenitic NiTi is responsible for elasticity. -Modulus of elasticity of Austenitic NiTi is 3-4 times than Martensitic NiTi.
Transitional Transformation Range (TTR) Temperature Austenite 0 % 100 %
NiTi Alloys Development Stage l : Nitinol “Stabilized Martensetic” (1970’s) Stage ll : Superelastic NiTi “ Active Austenite” ( Mid 1980’s) Stage lll: Thermal Wires “ Active Martensite” (Early 1990’s) Stage lV: Development of Copper NiTi “CuNiTi” (Late 1990’s)
Stage l: Stabilized Martensetic “ Nitinol” -Composed of 55 Ni:45 Ti -Introduced to Orthodontic by Dr.Andreasen mid 1970’s. -No shape memory or superelasticity. -Deformation occurring during processing ( work hardening) suppress SME ( work hardening) suppress SME -It is passive “ Stabilized” alloy
Cont. Stabilized Martensitic wires ( Nitinol) Advantages: -Low stiffness ( 20% of SS) -Springy ( range 2.5 as SS) ( range 2.5 as SS) -Light, continuous and linear force delivery. S.S. NiTi Stress Strain
Stage ll: Superelastic NiTi (Japanese or Chinese Wires) -Developed by Dr.Burstone and Muira mid 1980’s -TTR below room temperature ( Cr, Nb additions) -Active Austenitic at room temperature -A f is lower than oral temperature so no thermoelastic properties.
Superelasticity -Occurs above TTR -Wire initially austenitic -Only stressed ares transform to martensite Stress Induced Martensitic Transformation ( SIMT). -Superelasticity only exists when both phases of metal are present. -Delivery of forces will be lowered in the needed areas only. Muira et al. AJODO 90: 1-10; 1986
Advantages of Superelastic NiTi archwires -Excellent springback (4-5 of SS) -Constant forces over large wire deflection Activation Deactivation
SE NiTi wires ?? -The slope of the graph starts with a slope three times that of Nitinol. -2 mm deflection is necessary for the formation of SIM in austenitic wires - Austenitic alloys only behave superelastically in very severe crowding cases. Muira et al. AJODO 90: 1-10; 1986
Effect of heat treatment on SE NiTi deformation Muira et al. AJODO 1986
Stage lll: Thermal Wires (Martensitic Active) -For the memory property to be clinically detectable, A f has to be slightly below oral temperature. -For every 150 ppm variation in composition, a 1°C change in TTR occurs. -Mainly Martensitic at room temperature-softish, ductile with shape memory -Austenitic with SIMT at 37˚ C -Deliver 25-30% of the force of SE NiTi and greater range of action. Room Temp Mouth Temp AUSTENITEAUSTENITE
Thermal Wires ( A f =37°) Iijima et al. Dental Material 18 ( 2002) 88-93 Stress Deflection 60°C 37°C 23°C
Thermal NiTi -The main benefit is that these wires generate lower forces at mouth temperature than the corresponding size of non-thermal wire. -Allow earlier progression to large dimension wires e.g. 18x25,20x20. -Allow control amount of force delivered to posterior and anterior teeth.
-Allow more severely displaced brackets to be engaged by chilling the wire locally.
But Thermal wires: -More expensive. -Very sensitive to manufacturing process. -Offer little advantages in small diameters. -May give almost no force in the unloading curve if they are not formulated correctly, so may be inefficient. -Very sensitive to temperature changes in the oral cavity.
Effect of temperature changes on thermal archwires during activation T.Melling and J.Odegaard AJODO 2001; 119: 263-73
Effect of temperature changes on thermal archwires during deactivation T.Melling and J.Odegard AJODO 2001; 119: 263-73
Effect of repeated short-term exposure to ice cream on torsional stiffness of thermal archwires T.Melling and J.Odegaard Angle Orthod 1998; 68: 369-376
Stage lV: Development of Copper NiTi “’ CuNiTi” -5% Copper, 0.2-0.5% Chromium -The addition of Cu: Increase strength, reduce energy loss and allows greater control of TTR. -Long force plateau -Better manufacturing consistency -Tolerate repeated loading better -3 Types 27°, 35°, 40°. CuNiTi 40 ° CuNiTi 35 ° CuNiTi 27° Stress Deflection
CuNiTi 27˚ -A f at 27˚. -Superelastic wire - In patients : -with average or high pain threshold. -with average or high pain threshold. -Normal periodontal health. -Normal periodontal health. -where rapid tooth movement is required -where rapid tooth movement is required
CuNiTi 35˚ -A f at 35˚. -Thermoelastic wire - In patients : -with low to normal pain threshold. -with low to normal pain threshold. -Normal to compromised periodontal health. -Normal to compromised periodontal health. -where relative low forces are required -where relative low forces are required
CuNiTi 40˚ -A f at 40˚. -Thermoelastic wire - In patients : -who are sensitive to pain. -who are sensitive to pain. -with compromised periodontal conditions. -with compromised periodontal conditions. - Good as initial rectangular wire.
Stage V: Graded Thermodynamic NiTi archwires -Deliver different amount of force at different areas of the dentition according to the surface area of periodontium. - Controlled by specifying different TTR. -80 gm of force anteriorly and 300 gm posteriorly.
Beta-Titanium Alloy ( TMA) -Contains 80% Ti, 11% Mo, 7% Zr and 4% Sn. -Medium stiffness ( 1/3 of SS and twice of (Nitinol) -Produce gentler linear forces than SS -Has more range and greater springback -Has rough surface
Archwire application -Aligning arches -Working arches -Finishing arches More Stiffness Less Range
Springback and stiffness ratios of different materials* StiffnessSpringback 11 Stainless steel.131.5-2 Multistrand SS.361.75B-Titanium.172.5Nitinol.414-5 SE NiTi *Evans (1996), Profit (2000)
Aligning wires need: -Low stiffness: low forces on activation -High strength: prevent permanent deformation -Long working range : maximize activation
First aligning wire Which is the best? -15 Multistrand SS -12 SE NiTi -14 SE NiTi -16 SE NiTi -16 Thermal -18 Thermal -16x22 Thermal -14x25 Thermal -20x20 Thermal Physiological Force !?
Amount of force delivered by wires 307 gm 16x22 Nitinol 193 gm 16x22 NiTi SE 143 gm 16x22 Thermal 137 gm 16x22 CuNiTi 27 ˚ 100 gm 16x22 CuNiTi 35˚ 87 gm 18 thermal 73 gm 16 NiTi SE 60 gm 16 Thermal 43.1gm 17.5 Multistrand
Advantages of NiTi as aligning archwires compare to Multistrand SS: -Long working range -Damage resistance -Sustained forces! -Low Forces!
Aligning Archwires -The smallest diameter archwire to be avoided at this stage : -Small amount of force -Small amount of force -Play between bracket and wires limits the accuracy of alignment produced -Play between bracket and wires limits the accuracy of alignment produced
“Clinical trials” -Superelastic NiTi vs Stabilized NiTi O’Brien et al, EJO 12 ( 1990) 380-384 -Superelastic NiTi vs multistrand steel West. Jones & Newcombe, AJODO 108 (1995) 464-471 -Thermal NiTi vs graded force NiTi vs multistrand steel Evans, jones & Newcombe, AJODO 114 ( 1998) 32-39 -Superelastic NiTi vs ion implanted NiTi vs multistrand steel Cobb et al, clin orth Res 1 ( 1998 ) 12-19 -Does the transition temperature of CuNiTi archwires affect the amount of tooth movement during alignment? Dalstra & Melsen Orthd. Craniof. Res. 7 (2004) 21-25
Results of clinical trials - Rates of tooth movement hardly affected by type of wire, any difference no clinically significant. - Pain experience not affected. - Results are related to the individual variations in metabolic response within the periodontal ligaments and bone.
2-Archwires Sequence -A recent RCT in Manchester by Mandall N. et al. EJO in press -Three randomly allocated archwire sequence in terms of : efficiency, patient discomfort, root resorption. -A=16 NiTi, 18x25 NiTi ( n=41) -B=16 NiTi, 16 SS, 20 SS ( n= 44) -C=16x22 CuNiTi, 19x25 CuNiTi ( n=44) The endpoint was the passive placement of 19x25 SS for at least 4 weeks
Results -No statistical difference for patient discomfort at hours 4 hrs, 24 hrs, 3 days and 1 week. -Root resorption was not statistically significant with average root resorpion between.96-1.39 mm
Time required to reach the working archwire No of visits Time ( Months) Archwire sequence 5.7 ( 2.1) 5.4 ( 2.1) 6.8 ( 2.5) 6.7 ( 3.5) A Lower Upper Upper 7.5 ( 1.9) 7.1 ( 2.6) 9.3 ( 4.4) 7.9 ( 3.5) B Lower Upper Upper 6.4 ( 2.2) 5.9 ( 2.8) 8.3 ( 4.2) 7.1 ( 3.4) C Lower Upper Upper
Can Thermal Rectangular wires be used as first aligning archwires?
First aligning archwires -Mild crowding: 15 Multistrand SS 14 Nitinol 14 Nitinol 18 Thermal 18 Thermal (20x20 CuNiTi) (20x20 CuNiTi) -Moderate crowding: 16 Thermal 14 SE NiTi 14 SE NiTi -Severe crowding: 14 Thermal 12 SE NiTi 12 SE NiTi
When to move to the next wire? -When the next wire can be engaged in all the slots -Look at the worst tooth to decide -Watch for rotation particularly -Give enough time for the wire to work especially the new high technology wires
Second aligning archwire -18x25 NiTi -20x20 CuNiTi
Possible uses of 20x20 CuNiTi -Final alignment wire after round NiTi wire -Sole aligning wire for mild irregularities ( few cases) ( few cases) -Realignment after bracket repairs or repositioning.
Finishing wires Options for close-fitting archwires (21x25): -Steel : Too stiff -NiTi: Not adjustable Poor torqueing Poor torqueing -B-Titanium: Ideal stiffness used to provide root paralleling used to provide root paralleling
Self-Ligating Brackets? Self- Ligation Low Force, Low Friction Active Ligation High Force, High Friction
19x25 Damon 4 Solid Walls Conventional Wire Out Of Slot Elastic Ligature or Metal Clip What Are The Limitations Of Conventional or Active Ligation? Poor Control – Less Effective Torque
-Friction is increased 500% over Damon, if using a conventional bracket with steel ligatures -Friction is increased 500% over Damon, if using a conventional bracket with steel ligatures Friction is increased 1500% over Damon, if using an elastic ligature - Friction is increased 1500% over Damon, if using an elastic ligature -There are 70 grams of frictional force, per tooth, when using an elastic ligature EJO 2004 Khandy Self-Ligating Brackets
Friction!! Sims, Birnie and Waters (1993) Frictional Resistance N/m
Archwires in Self-Ligating brackets -High Technology Wires should be used ( e.g. CuNiTi). ( e.g. CuNiTi). -Smaller dimensions ( Start with 14) -Give 10 weeks appointment interval. -Use 14x25 CuNiTi as second aligning archwires to correct rotations. - Then 18x25 CuNiTi to express additional torque.