Retentive Clasp

From WikiDent

Retentive Clasps are extracoronal direct retainers used to prevent the RPD from moving away from the hard and soft tissues.

Contents 1 Components 2 Clasp requirements 3 Clasp Design Principles 4 Types of Metal

Components

• Retentive arm
• Reciprocal arm
• Rest Seat
• Guiding plate

Clasp requirements

• As little retention as needed
• Reciprocation or bracing of retentive clasp engages an undercut; it is the only part of the assembly below the height of contour (HOC) on the tooth.
• At least 180 degrees of encirclement.
• Support with proper Rest Seat.
• Passive fit when fully seated; retentive portion of the clasp only engages tooth when dislodging forces are applied (prevent orthodontic forces that might move the tooth).
• Type of retentive clasps base on how the clasp approaches the undercut:
  • Suprabulge - Aker's or circumferential (C-Clasp)
  • Infrabulge - Roach or bar (I, J, T, L, etc.)

Clasp Design Principles

• Resiliency and flexibility of a clasp is determined by its length, diameter, taper, shape, and type of metal.
• Shape in cross section can be round or half circle.
• As taper increases, flexibility increases.
• As diameter increases, flexibility decreases in a cube ratio.
• As length increases, flexibility increases in a cube ratio.
• The more the clasp is bent, flexibility decreases.

Types of Metal

• Wrought wire - 0.020 in. undercut
  • More flexible than cast metal
  • Can be soldered, incorporated into the wax pattern before casting, or embedded in acrylic resin during processing.
• Cast metal - 0.010 in. undercut