Factors influencing the flexibility of clasp arm in RPD

The flexibility of the clasp arm changes the way RPDs are placed and held around the tooth. There are several factors that need to be kept in mind, which would influence the flexibility of the clasp arm. Here are the main factors.

Factors that influence clasp arm flexibility

Before we begin, this article is written with McCracken’s Removable Partial Prosthodontics textbook as reference but we have made it easier to understand.

Clasp ArmThe length of clasp arm

If all the other factors are kept equal and only the length is kept in the mind, you will see that the longer clasp will be more flexible than a shorter one. The length of a circumferential clasp arm is measured from the point at which uniform taper begins. The retentive circumferential clasp arm should be tapered uniformly from its point of origin. The length of this uniform taper is the full length of the clasp arm.

The length of a bar clasp arm also is measured from the point at which a uniform taper begins. Generally, the taper of a bar clasp arm should begin at its point of origin from a metal base or at the point at which it emerges from a resin base. While a bar clasp arm will usually be longer than a circumferential clasp arm, its flexibility will be less because its half-round form lies in several planes, which prevents its flexibility from being proportionate to its total length.

Based on a proportional limit of 60,000 psi and on the assumption that the clasp arm is properly tapered, the clasp arm should be able to flex repeatedly within the limits stated without hardening or rupturing because of fatigue. It has been estimated that alternate stress applications of the fatigue type are placed on a retainer arm during mastication and other force-inducing functions about 300,000 times a year.

Diameter of clasp arm

The greater the average diameter of a clasp arm, the less flexible it will be, all other factors being equal. If its taper is absolutely uniform, the average diameter will be at a point midway between its origin and its terminal end. If its taper is not uniform, a point of flexure and therefore a point of weakness will exist that will then be the determining factor in its flexibility, regardless of the average diameter of its entire length.

Cross-sectional form of clasp arm

Flexibility may exist in any form, but it is limited to only one direction in the case of the half-round form. The only universally flexible form is the round form, which is practically impossible to obtain by casting and polishing.

Since all cast clasps are essentially half round in form, they may flex away from the tooth, but edgewise flexing (and edgewise adjustment) is limited. For this reason, cast retentive clasp arms are more acceptable in tooth-borne partial dentures in which they are called on to flex only during placement and removal of the prosthesis.

A retentive clasp arm on an abutment adjacent to a distal extension base must not only flex during placement and removal but also must be capable of flexing during functional movement of the distal extension base. It must either have a universal flexibility to avoid transmission of tipping stresses to the abutment tooth or be capable of disengaging the undercut when vertical forces directed against the denture are toward the residual ridge.

A round clasp form is the only circumferential clasp form that may be safely used to engage a tooth undercut on the side of an abutment tooth away from the distal extension base. The location of the undercut is perhaps the most important single factor in selecting a clasp for use with distal extension partial dentures.

Material used for clasp arm

Although all cast alloys used in partial denture construction possess flexibility, their flexibility is proportionate to their bulk. If this were not true, other components of the partial denture could not have the necessary rigidity. The only disadvantages of cast gold partial dentures are that their bulk must be increased to obtain needed rigidity at the expense of added weight, and their cost. It cannot be denied that greater rigidity with less bulk is possible through the use of chromium alloys.

Although cast gold alloys may have greater resiliency than do cast chromium alloys, the fact remains that the structural nature of the cast clasp does not approach the flexibility and adjustability of the wrought-wire clasp. Having been formed by being drawn into a wire, the wrought-wire clasp arm has toughness exceeding that of a cast clasp arm. The tensile strength of a wrought structure is at least 25% greater than that of the cast alloy from which it was made. It may, therefore, be used in smaller diameters to provide greater flexibility without fatigue and ultimate fracture.

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