Principle of clasp design
The basic principle of clasp design referred to as the principle of encirclement means that more than 180 degrees in the greatest circumference of the tooth, passing from diverging axial surfaces to converging axial surfaces, must be engaged by the clasp assembly (Figure 1) .
The engagement can be in the form of continuous contact, such as in a circumferential clasp, or discontinuous contact, such as in the use of a bar clasp. Both provide tooth contact in at least three areas encircling the tooth: the occlusal rest area, the retentive clasp terminal area, and the reciprocal clasp terminal area.
In addition to encirclement, other basic principles of clasp design are as follows:
1. The occlusal rest must be designed to prevent the movement of the clasp arms toward the cervical.
2. Each retentive terminal should be opposed by a reciprocal component capable of resisting anytransient pressures exerted by the retentive arm during placement and removal. Stabilizing and reciprocal components must be rigidly connected bilaterally (cross-arch) to realize reciprocation of the retentive elements (Figure 2).
3. Clasp retainers on abutment teeth adjacent to distal extension bases should be designed so that they will avoid direct transmission of tipping and rotational forces to the abutment. In effect, they must act as stress-breakers either by their design or by their construction. This is accomplished through proper location of the retentive terminal relative to the rest or by use of a more flexible clasp arm in relation to the anticipated rotation of the denture under functional forces.
4. Unless guiding planes will positively control the path of removal and stabilize abutments against rotational movements, retentive clasps should be bilaterally opposed, i. e., buccal retention on one side of the arch should be opposed by buccal retention on the other, or lingual on one side opposed by lingual on the other. In Class II situations, the third abutment may have either buccal or lingual retention. In Class III situations, retention may be either bilaterally or diametricall opposed (Figure -3).
5. The path of escapement for each retentive clasp terminal must be other than parallel to the path of removal for the prosthesis to require clasp engagement with the resistance to deformation that is retention.
6. The amount of retention should always be the minimum necessary to resist reasonable dislodging forces.
7. Reciprocal elements of the clasp assembly should be located at the junction of the gingival and middle thirds of the crowns of abutment teeth. The terminal end of the retentive arm is optimally placed in the gingival third of the crown (Figures 4) . These locations permit better resistance to horizontal and torqueing forces because of a reduction in the effort arm.