Lecture: Orthodontic Series - Orthodontic Materials and Appliances

Overview

• Focus on biomechanics
• Discussed biological response, force application, and tooth movement in previous videos
• Current discussion: How orthodontic treatment applies forces using wires and brackets

Key Components in Orthodontics

Wire and Bracket

• Wire: Moves the teeth
• Bracket: Tooth handle to which wire is affixed

Orthodontic Wires

Paperclip Example

1. Out of the Package: Untouched state
2. Permanent Bend: Deforming the wire for tooth movement
3. Engagement: Wire placed into bracket slot
4. Activation: Wire tries to return to form, moving teeth

Phases of Orthodontic Wire

1. Activation (Loading): Force applied to engage wire into bracket slot
2. Deactivation (Unloading): Wire returns to original shape, applying force to tooth

Mechanical Terms

• Strength: Loading

  • How easily the wire will break (measured in megapascals)

• Strength: Unloading

  • How much force a wire can deliver
  • Related to proportional limit, yield strength, and ultimate tensile strength

• Proportional Limit: End of elastic behavior

• Yield Strength: Start of permanent deformation

• Ultimate Tensile Strength: Max stress the material can handle

Stiffness

• Loading: Wire flexibility
• Unloading: Force wire delivers returning to original shape
• Represented by slope of elastic portion of stress-strain curve
• Vertical slope = stiffer wire; Horizontal slope = flexible wire

Range

• Loading: Elastic deflection distance before permanent deformation
• Unloading: Duration wire remains active (measured in millimeters)
• Springback: Wire tries returning to shape even post-yield point

Resilience and Formability

• Resilience: Area under stress-strain curve up to proportional limit; energy storage capacity
• Formability: Area from yield point to failure point; tolerance of permanent deformation

Material and Geometry Impact

• Materials Order (Least strong to strongest): Nickel titanium < TMA < Stainless steel
• Diameter: Increase = more strength/stiffness, less range
• Length: Increase = more range, less strength/stiffness
• Shape: Rectangular wires are stronger/stiffer than round wires
• Beam vs. Cantilever: Beam (supported between two brackets) is stronger/stiffer than cantilever (one bracket)

Brackets

Historical Perspective

• Metal bands were initially used
• Modern brackets have horizontal slots (edgewise brackets)

Pre-adjusted Edgewise Brackets

• Custom prescription for each tooth
• Invented to reduce needing individual wire bends
• Each bracket positioned on center of facial surface

Types of Brackets

1. Metal Brackets:

   • Less aesthetic, require elastic rings or chains
   • Made of stainless steel

2. Ceramic Brackets:

   • Aesthetic, match tooth shade, prefered by adults
   • Prone to fracture, increased friction

3. Self-ligating Brackets:

   • Built-in door for arch wire, no need for ligatures
   • Reduces friction, purported to shorten treatment time, more expensive

Closing Remarks

• Subscribe for more dentistry-related content
• Support via Patreon for slides and board exam practice questions
• Gratitude for patron support and viewers