23.4 Fixed Appliances, Anchorage, Adult Orthodontics and Retention
Key Takeaways
Preadjusted (straight-wire) brackets build tip, torque and in-out compensations into the bracket so that a straight wire expresses Andrews six keys of normal occlusion.
Superelastic nickel-titanium wires deliver light forces over a large range for initial alignment, stainless steel wires are stiff for working and space closure, and beta-titanium (TMA) has intermediate stiffness and is easily formed.
Temporary anchorage devices (miniscrews) provide skeletal anchorage that does not rely on teeth and are commonly placed between roots and loaded soon after placement.
Adults with treated, stable periodontitis can have orthodontic treatment with light forces, but active periodontal inflammation must be controlled first because tooth movement in inflamed tissue accelerates attachment loss.
Every orthodontic result needs retention; rotated teeth relapse because of supracrestal gingival fibers, which remodel slowly, and long-term or indefinite retention is usual.
This section links orthodontic mechanics to everyday general dentistry: recognizing appliance problems, planning adjunctive movement before restorations, and protecting results with retention.
Types of Appliances
| Appliance | Movement | Notes |
|---|---|---|
| Removable (Hawley-type with springs) | Tipping only | Simple crossbites, retention; depends on wear |
| Functional | Growth modification | Covered with Class II and III management |
| Fixed (brackets and wires) | Bodily, torque, rotation, intrusion and extrusion | Most precise control |
| Clear aligners | Mainly tipping and planned movements with attachments | Depend on wear of about 22 hours a day |
Preadjusted (straight-wire) appliances build tip (angulation), torque (inclination) and in-out offsets into each bracket so that a full-size straight wire expresses the planned positions. They were designed from Andrews' six keys of normal occlusion:
- Molar relationship: the distal surface of the distobuccal cusp of the upper first molar contacts the mesial surface of the mesiobuccal cusp of the lower second molar (and the mesiobuccal cusp sits in the buccal groove of the lower first molar).
- Correct crown angulation (mesial tip).
- Correct crown inclination (torque).
- No rotations.
- Tight contacts.
- A flat or slight curve of Spee.
Self-ligating brackets use a built-in clip instead of elastic or wire ligatures. Lingual appliances are bonded on the lingual surfaces for esthetics.
Archwires
| Wire | Properties | Typical use |
|---|---|---|
| Superelastic nickel-titanium (NiTi) | Low stiffness, very large working range, nearly constant light force | Initial leveling and aligning |
| Stainless steel | High stiffness, low friction, formable, can be soldered | Working wires, space closure, finishing |
| Beta-titanium (TMA) | Intermediate stiffness, good formability | Finishing bends, springs |
| Multistrand stainless steel | Flexible | Early alignment, bonded retainers |
Physics: for a round wire, stiffness rises with the fourth power of diameter (doubling the diameter makes it 16 times stiffer). For a cantilever, stiffness falls with the cube of the span length (doubling the span gives one-eighth the stiffness). This explains why loops and longer interbracket spans give lighter forces.
Sequence: small round NiTi, larger NiTi, rectangular NiTi, then rectangular stainless steel to control torque and close spaces, then finishing.
Anchorage
Every force has an equal and opposite reaction (Newton's third law); anchorage is the resistance to unwanted movement of the reactive teeth.
| Category | Description |
|---|---|
| Maximum anchorage | Anchor teeth should take up little of the extraction space (most space used to retract the anterior teeth) |
| Moderate | Space shared |
| Minimal | Most space closed by moving posterior teeth forward |
| Reinforcement | More teeth in the anchor unit, transpalatal arch, Nance button, lingual arch, headgear, intermaxillary elastics, TADs |
Temporary anchorage devices (miniscrews): titanium screws commonly about 1.2-2 mm in diameter and 6-10 mm long, placed between roots or in the palate; give near-absolute anchorage for retraction, intrusion and molar distal movement; can be loaded soon after placement; complications include failure (loosening), root contact and soft-tissue irritation.
Risks of Fixed Appliances
- Demineralization (white spot lesions): prevent with high-fluoride toothpaste, fluoride varnish and diet control; after debonding, remineralize first and consider resin infiltration for persistent lesions.
- External apical root resorption: usually small (on average about 1-2 mm), occasionally severe; risk increases with long treatment, heavy forces, intrusion and torque, previously traumatized teeth and blunt or pipette-shaped roots; take a radiograph at about 6-12 months in higher-risk patients.
- Gingival inflammation, enlargement and recession.
- Loss of vitality in previously traumatized teeth.
- Enamel damage at debond; soft-tissue ulcers; nickel allergy (rare).
- Relapse without retention.
Adult and Adjunctive Orthodontics
Adults cannot benefit from growth, often have restorations, missing teeth and periodontal disease, and tend to prefer esthetic appliances.
- Periodontal disease must be treated and stable before tooth movement; light forces; frequent periodontal maintenance during treatment.
- With bone loss, the center of resistance moves apically, so the same force causes more tipping.
- Moving teeth in inflamed tissue accelerates attachment loss.
Adjunctive (limited) orthodontics before restorative work:
| Procedure | Purpose |
|---|---|
| Molar uprighting | Upright a tipped molar before a bridge or implant, improve parallelism and remove a periodontal defect |
| Forced eruption (orthodontic extrusion) | Bring a subgingivally fractured tooth up to provide a ferrule and restore the supracrestal tissue attachment; a fibrotomy prevents bone following the tooth when bone level must stay |
| Space redistribution | Create ideal space for implants or veneers |
| Intrusion of overerupted teeth | Restore space for an opposing prosthesis |
| Closing diastemas, aligning before veneers | Minimize tooth reduction |
Retention and Relapse
- Causes of relapse: periodontal and gingival fibers (the supracrestal fibers remodel slowly, so rotations relapse most), soft-tissue pressures, continued growth (late lower incisor crowding), and occlusal factors.
- Retainers: removable Hawley, vacuum-formed (clear) retainers, and bonded lingual retainers (commonly canine to canine).
- Adjuncts: overcorrection of rotations and circumferential supracrestal fibrotomy for severely rotated teeth.
- Retention is usually long-term, and patients should know that teeth can move throughout life.
Exam Traps
- Removable appliances tip teeth; bodily and torque control need fixed appliances.
- Start orthodontics only after periodontal inflammation is controlled.
- Rotations relapse because of supracrestal gingival fibers.
Which archwire is most suitable for the initial leveling and aligning of crowded teeth?
A heavy round stainless steel wire with loops
A small round superelastic nickel-titanium wire
A full-size rectangular stainless steel archwire
A rectangular beta-titanium wire with torque
A 50-year-old has a mesially tipped tooth 37 after early loss of tooth 36, with a pseudo-pocket on its mesial surface. A fixed bridge is planned. What adjunctive orthodontic treatment would help?
Rapid maxillary expansion to correct the tipping of the mandibular molar
A functional appliance to stimulate mandibular growth in the molar region
Intruding tooth 37 to create space for a larger pontic of greater width
Uprighting tooth 37 to improve abutment parallelism and reduce the mesial defect
Six months after orthodontic correction of a severely rotated maxillary lateral incisor, the tooth is starting to rotate back. Which tissue is mainly responsible, and what helps prevent this?
Supracrestal gingival fibers; fibrotomy and long-term bonded retention
The apical periodontal ligament; no retention is needed after six months
Pulpal tissue; root canal treatment prevents rotational relapse
Alveolar bone at the apex; a heavier archwire prevents the relapse
Sections you finish are checked off in the contents.