What factors affect the magnitude ofclasp retention?

Retention’s definition is “resistance to removal from the tissues or teeth”. The amount of retention provided by a clasp assembly is dependent upon many factors. These include the type of clasp used, flexibility of the retentive arm, the axial convergence of tooth surfaces apical to the height of contour. As a general rule the amount of retention designed into the RPD should always be the minimum necessary to resist reasonable dislodging forces.

• Type of clasp used:
  •         Circumferential clasp (Circle or Akers)
    - Ring Clasp
    - Embrasure Clasp (Double Akers)
    - C Clasp (Hair pin or reverse action)
  •         Bar Clasp
    - RPI Clasp
    - RPA Clasp
    - Combination Clasp

The type of clasp used determines how it’ll be placed in relation to undercut. Retentive portions of clasps enter undercuts by approaching from either gingivally (infra bulge) or occlusally (supra bulge). The gingivally approaching clasps exert a trip action which increases retention. Varying the angle at the contact with tooth increases or decreases the trip action.

• Axial convergence of tooth surfaces apical to the height of contour:
  •         Size/how far into the angle of convergence (undercut) the clasp terminal is placed
  •         The depth of undercut engaged or the location of clasps on the undercut of tooth is determined when surveying. The greater undercut, the greater retention. An undercut too big will strain the clasp and traumatise the teeth. An undercut too small will give insufficient retention
  •         Upon insertion and removal, the clasps arms will contact the axial surfaces of the abutments
    Clasp arms will flex as they pass over the greatest convexities of the abutments = Height of contour
    Once they pass the "height of contour" and the RPD is fully seated, the retentive arms return to its "passive" state
    The retentive terminus is placed in an undercut area relative to the path of insertion
  •         Undercut is measured perpendicular to the path or insertion. The degree of undercut is measured using gauges which considers the materials MoE

• Flexibility of the retentive arm: (related to length, diameter, cross sectional form and clasp material)
  •         Length, the length of clasp can affect clasp retention, increasing the length increases the flexibility, measured from the point where the taper begins, may be increased by curving.
  •         Diameter - Thicker the clasps, the more rigid they are. The thickness and length of clasps must be related to the MoE of the alloy used.
  •         Cross sectional form - round more flexible
  •         Clasp material - wrought wire has greater tensile strength than cast (can be used in smaller diameters) Materials: gold, steel,chrome all have different MoE. MoE: a greater MoE means a stiffer material. It would require more force to deflect it to the same degree as a material with a lower modulus. It would require more force to deflect it to the same degree as a material with a lower modulus.  

Clasp length & material      &   Depth of Undercut engaged     

  Cobalt /Chromium

0.25mm (10 thou. inch)

  Cast gold  (e.g. premolar clasps).

0.25 mm

   Long cast gold (e.g. ring clasps)

0.5mm (20 thou. inch)

    Wrought clasps (gold/ stainless steel)

0.75mm (30 thou. inch)

    Long wrought clasps (may engage)

0.75mm (30 thou. inch)