SGD Orthodontic: Material Hamzi, Zulkhairi, Azizul, Haziq, Aishah, Anis, Asmat, Masyitah
Lecture outline: material Wire fracture Mechanics of spring Bauchinger effect How does the material affect stability and the stiffness of the component?
Wire fracture
Small loads → the stress is below the elastic limit of the material, reversible elastic strain occurs that disappears completely when specimen is unloaded. High stress A ductile material begins to undergo irreversible plastic or permanent deformation A brittle material will fracture without any significant permanent deformation
Stainless steel wire Orthodontic wire are generally shaped by bending and the wire should possess sufficient ductility to resist fracture during this bending procedure. The amount of residual ductility remaining in a wire depends in part on the ductility used up in its manufacture.
Possible to carry out soldered repair to an adam’s clasp if a fracture has occurred at tip of arrowhead. This is an unusual place of fracture unless the wire been overworked during construction. The arrowhead should be cleaned, fluxed and flushed with solder. Other attempts are not worth and replacement is more sensible.
On ocassion, a clasp that has broken where the wire crosses the embrasure may be cut away to leave one intact arrowhead, which can be pinched closed with a pair of pliers so no sharp end remains. The arrowhead may adjusted to provide retention. This is useful where retention provided by other wire is fairy good and especially when appliance is not going to be worn for much longer.
For broken springs, the remains of the spring are cut away and a recess drilled into the fitting surfaces of acrylic baseplate. A replacement spring may be bent up and embedded into this space using small amout cold cured acrylic. In case palatal finger spring, the presence of guard wire will help to hold new spring in place during this procedure.
Mechanics of spring
Force= (deflection)(radius 4 F=dr 4 /l 3 length 3 Force: – Single rooted: 25-40g – Excess force: delay movement, overload anchorage & discomfort. Deflection: – Common spring activation: 3mm – Greater activation -> pt insert it incorrectly -> unwanted movement – Smaller activation -> force applied decrease -> wanted tooth movement (1-2mm/month)
BAUSCHINGER EFFECT
NNamed after German Engineer, Johann Bauschinger. AApplies to very small deformations.
May be stated as follows “By applying a tensile or compressive load beyond the elastic limit, the elastic limit for compression or for tension, respectively, is reduced considerably, and the more the load exceeds the elastic limit, the greater the reduction”
T e n s i l e s t r e s s C o m p r e s s i v e s t r e s s Tensile strainCompressive strain In this graph, lets treat tensile stress and strain as POSITIVE and compressive stress and strain as NEGATIVE 0
If an annealed specimen is loaded from 0 to B beyond its elastic limit, Tensile stress Compressive stress Tensile strainCompressive strain 0 B designated by point A, A and unloaded, Its condition is represent by C. C Note that the elastic limit of the material in tensile is given by Se T Se T
Tensile stress Compressive stress Tensile strainCompressive strain 0 B A C Se T If the same specimen is next loaded in compression, it follows the path CDE, E where D is the elastic limit point on the compression curve, so that the elastic limit in compression is now S’e C D S’e C According to Bauschinger effect, S’e C < Se T
Tensile stress Compressive stress Tensile strainCompressive strain 0 B A C Se T E D S’e C If an annealed specimen instead of being loaded in tension and then in compression, as stated above, was directly loaded in compression, The elastic limit in compression of the annealed material should be Se C, Se C F And would be equal to magnitude to Se T. Hence, Se C = Se T, and S’e C < Se T and S’e C < Se C
SSimilar reasoning will happen if the annealed specimen was initially loaded in compression past the elastic limit, unloaded and loaded next in tension. TThe resulting elastic limit in tension would be smaller than the elastic limit of annealed material in compression
WWhereas Bauschinger effect was originally stated in terms of the elastic limit, the discussion of this effect in the literature has involve the use of terms elastic limit and yield strength interchangeably. TThe reason for this anomaly lies in the elastic limit and the yield point being located very close to each other on the stress-strain curve.
The important thing is that one should not lose sight of the fact that Bauschinger effect applies to VERY small strains only.
How does the material affect stability and the stiffness of the component?
The stability ratio of a spring in mechanical terms : Stiffness in the direction of unwanted displacement Stiffness in the intended direction of tooth movement The spring must be guided so that its action is exerted only in the appropriate direction by: Place the spring in an undercut of the tooth so that it does not slip occlusally during activation Use a guide to hold the spring in its position during activation Bond an attachment to the tooth surface to engage the spring
In Practice High stability spring eg. Finger spring Straightforward to adjust/movement Low stability spring eg. Buccal canine retractor Difficult to position precisely on the tooth to be moved The spring should be adjusted so that the point of application will give the desired direction of tooth movement.
Self supported spring These springs are made up of thicker wire to avoid distortion by the patient Supported spring These springs are made up of thinner wire, a guidewire maybe provided. Alternatively, they maybe supported by an additional sleeve or ‘boxed’ of acrylic – to ensure adequate stability
References 1. Kerr, W.J.S. (1984) Appliance breakages. British journal of Orthodontics, 11: 2. Munns, D. (1971) An analysis of broken removable appliances. Proceedings BSSO, 45-48
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