Orthodontics and Orthodontic Appliances
What this domain tests
Appliance fabrication - fixed and removable - plus the wire selection and force reasoning behind a treatment sequence.
Core rules
- Archwire sequence. Start with small round superelastic NiTi (0.014, then 0.016), progress to rectangular NiTi (0.016 × 0.022), then stainless steel (0.019 × 0.025) for sliding mechanics and finishing. TMA and beta-titanium sit between NiTi and stainless in stiffness and are used for auxiliaries and loops.
- Wire behaviour. Deflection rises with the fourth power of diameter, so a small increase in wire size sharply increases delivered force. NiTi gives light, continuous force over a wide range; stainless gives high stiffness with low springback.
- Bands and tubes. Band fit is judged by seating to the marginal ridge with the band fully adapted; cement with glass ionomer. Spot welding is for bands and attachments; soldering is for joints requiring flow.
- Removable appliances. Hawley retainer uses 0.036–0.040 inch wire; vacuum-formed retainers use 0.030–0.040 inch sheet. Expansion screws: one quarter turn (0.25 mm) per activation as prescribed.
- Models. Pour, trim with a symmetrical base, and articulate before any appliance is designed on the cast.
Worked example
A clinician asks for a retainer after a NiTi finishing archwire. The teeth are still slightly rotated. A vacuum-formed 0.030 inch retainer will not express fine rotational correction as well as a Hawley with an active labial bow, but it is more comfortable and easier to keep clean. Reasoning: choose based on the correction still needed and the patient's hygiene, then confirm the prescription with the clinician rather than deciding unilaterally.
Traps that cost marks
- Jumping straight to rectangular stainless steel.
- Over-activating an expansion screw on your own initiative.
- Trimming a study model without a symmetrical base, making the midlines unreadable.