Archwire Selection, Insertion, and Adjustment
What this domain tests
Wire material properties, size progression through treatment phases, insertion and ligation, distal end management, and the force levels that keep movement efficient.
Materials
- Stainless steel: stiff, formable, low friction, can be soldered and welded - the finishing wire.
- Nickel-titanium: superelastic, shape memory, delivers light continuous force over a wide range of deflection; cannot be sharply bent or soldered - the leveling wire.
- Copper NiTi: thermal variant that changes stiffness with mouth temperature; the transition temperature printed on the package matters.
- Beta-titanium (TMA): stiffer than NiTi but formable like steel, useful for finishing bends.
- Cobalt-chromium and coated aesthetic wires: know them as alternatives with different friction and esthetics.
Size and sequence
Initial leveling uses round NiTi, typically 0.014 then 0.016 or 0.018. Working and finishing use rectangular wires, 0.016 × 0.022 up to 0.019 × 0.025. In a 0.022 slot the common finishing wire is 0.019 × 0.025; in a 0.018 slot, 0.017 × 0.025. Never jump from a light round wire straight to a heavy rectangular wire - the force spike causes pain, anchorage loss, and root resorption.
Insertion and adjustment
Seat the wire fully in every slot, ligate with elastomerics or steel ligatures (steel gives less friction and better hygiene when sliding mechanics are planned), then cinch the distal ends so the wire cannot migrate and poke. Cut with a distal end cutter, control the fragment with gauze and suction, and place wax if the end is near mucosa. Check arch form symmetry, molar rotation, midline, and occlusal interference with articulating paper before the patient leaves.
Force logic
Orthodontic movement needs low, continuous force - tens of grams for intrusion and light leveling, not hundreds. Heavy forces occlude the PDL, cause hyalinization, and slow movement rather than speeding it.
Worked example
Three days after a wire change the patient reports a poking cheek. Reasoning: as teeth aligned, the arch length changed and the wire slid distally. Check the opposite side, cut and cinch both ends, verify no occlusal interference, and document. Wax alone treats the symptom; the wire still needs shortening.
Traps that cost marks
Bending NiTi sharply, which permanently deforms or fractures it; cutting wires without eye protection or suction; assuming a thermal wire delivers its intended force at room temperature; and leaving a distal end long enough to lacerate the buccal mucosa.