Orthodontic Appliances: Types, Uses, and Effects

Posted on Jan 12, 2025 in Electromechanical Engineering

Center of Resistance in Orthodontics

  • Center of Resistance: The point on the tooth where, if a single force is applied, it would result in the tooth’s translation along the line of action of that force.

Factors Affecting the Center of Resistance

  1. Number of roots
  2. Degree of alveolar bone loss
  3. Degree of root resorption

Anchorage in Orthodontics

Anchorage: The resistance to unwanted tooth movement.

Absolute Anchorage: Appliances gaining anchorage from extraoral structures.

Example: Titanium screws implanted into the alveolar bone through the gingiva.

  • Intraoral Anchorage: The anchorage value of a tooth is proportional to the surface area of the root.
  • Different Types of Orthodontic Tooth Movement (OTM):
    • Controlled tipping
    • Uncontrolled tipping
    • Bodily movement
    • Rotation
    • Root uprighting
    • Intrusion
    • Extrusion

Forces Delivered by Orthodontic Appliances

  • Continuous Force (ideal spring)
  • Interrupted Force (removable active plates)
  • Intermittent Force (removable appliances)

Deleterious Effects of Orthodontic Force

  • Pain
  • Allergic reactions
  • Mobility
  • Gingival inflammation
  • Loss of vitality of the pulp
  • Root resorption

Types of Root Resorption

  • Slight Blunting
  • Moderate Resorption – up to ¼ of the root length
  • Severe Resorption – more than ¼ of the root length
  • Moderate Generalized Resorption
  • Severe Generalized Resorption
  • Severe Localized Resorption

Drugs that Enhance Orthodontic Tooth Movement

  • Vitamin D administration
  • Direct injection of prostaglandin into the periodontal ligament (PDL)

Drugs that Impede Orthodontic Tooth Movement

  • Bisphosphonates – for osteoporosis
    • Alendronate
  • Prostaglandin Inhibitors
    • Indomethacin
  • Tetracyclines
    • Doxycycline
  • Tricyclic Antidepressants
    • Doxepin
    • Imipramine

Orthodontic Appliances

An orthodontic appliance is a device used to change the relationships of teeth and skeletal structures.

Removable Appliances

Removable appliances are designed to be placed and removed by the patient as directed by the orthodontist.

Tooth Movement with Removable Appliances

  1. Increase arch perimeter (arch expansion).
  2. Repositioning of individual teeth within the arch.
  3. Intrusion or extrusion of teeth.

Types of Removable Appliances

Active (Mechanical appliances)

Passive

  • Retainers
  • Space maintainers

Or

  1. Intraoral
  2. Extraoral

Intraoral Appliances

Removable, such as:

  1. Functional appliances
  2. Active retainers
  3. Passive retainers and positioners

Or

Fixed: Space maintainers

Extraoral Appliances

An extraoral appliance is worn outside the mouth to gain force from the head and neck to change the differential growth rate (headgear).

Components of Removable Appliances

  1. Active component (spring, screw, elastics)
  2. Retentive components (clasps: Adam’s, C-clasp, ball clasp, lingual extension clasp)
  3. Acrylic base plate

Advantages of Removable Appliances

  • More acceptable to the patient
  • They are fabricated in the lab
  • Allow some types of growth guidance

Disadvantages of Removable Appliances

  • Heavily dependent on patient compliance
  • Difficult to obtain the two-point contacts on teeth necessary for certain movements

Indications of Removable Appliances

  • Growth modification during mixed dentition
  • Limited (tipping) tooth movements
  • Retention following orthodontic treatment
  • Adjunct to fixed orthodontic appliances

Fixed Appliances

Fixed appliances are attached to the teeth and are removed by the orthodontist.

Types of Retentive Appliances

  1. Positioners
  2. Hawley retainers
  3. Lingual retainers

Orthodontic Wires

  • NiTi wire (Nickel-Titanium wire)
  • TMA wires (Titanium-Molybdenum-Alloy)
  • Stainless steel wire

Ideal Orthodontic Wire Material

Deflection properties:

  • High strength
  • Low stiffness (usually)
  • High range
  • High formability

Three Basic Types of Stainless Steel

  1. Ferritic Stainless Steel
    • Good corrosion resistance
    • Not hardenable by heat treatment
    • Not readily work hardenable
  2. Martensitic Stainless Steel
    • Can be heat treated
    • High strength and hardness
    • Used for surgical and cutting instruments

Ideal Requisites of Orthodontic Wires

  1. Should be easy to fabricate
  2. Should be economical
  3. Should not damage any hard or soft tissue
  4. Should have sufficient stability
  5. Should have sufficient tensile and compressive strength

Other Uses of Steel in Orthodontics

  • Band material
  • Brackets
  • Molar tubes
  • Lingual attachments
  • Facebows