Orthodontic Materials and Appliances
What the exam tests
Wire alloys, bracket and adhesive systems, elastomeric behaviour, and how material properties dictate wire sequencing and appliance choice.
Core rules
- Stainless steel (18-8) is stiff, strong, and work-hardenable; it is used for finishing and sliding arches.
- Nickel-titanium is superelastic with a stress plateau and shape memory; austenite transforms to martensite on cooling or stress. Thermal NiTi has an austenite finish temperature near mouth temperature.
- Beta-titanium (TMA) has stiffness between NiTi and SS and is weldable, useful for loops and auxiliary springs.
- Multistranded wire offers low stiffness with good resiliency.
- Adhesives: self-etch primers, RMGI for moisture tolerance, and conventional etch-and-bond; contamination is the leading cause of bond failure.
- Bracket slot size (0.018 vs 0.022) changes play and the wire sizes that express torque; ceramic brackets are harder and more brittle than metal.
- Elastomeric chains lose 40–60% of force within hours to days and stain; NiTi closed-coil springs deliver more constant force.
Worked example
First-premolar extraction case. Sequence: 0.014 NiTi for initial alignment → 0.016 NiTi → 0.016×0.022 NiTi → 0.019×0.025 SS for sliding mechanics, because the SS arch resists deformation under the applied force and provides a low-friction, dimensionally stable track.
Traps
- Expecting torque expression from a small round NiTi wire in a 0.022 slot.
- Assuming elastomeric force is constant over a month.
- Using TMA where rigidity is needed for anchorage.
- Overlooking moisture contamination during bonding of molars.