CLASP USED FOR TOOTH BORNE PARTIAL DENTURES1. Circumferential Clasp
b. Minor connector.
c. Cast retentive clasp.
d. Cast bracing or reciprocal clasp.
Engages 0.01-0.02 inch undercut
Provides bilateral bracing
Commonly used in tooth borne segments.
The circumferential clasp arms does have the following
1. More tooth surface is covered than with a bar clasp arm because of its occlusal origin.
2. On some tooth surfaces, particularly the buccal surface of mandibular teeth and the lingual surfaces of maxillary teeth, its occlusal approach may increase the width of the occlusal surface of the tooth.
3. In the mandibular arch, more metal may be displayed than with the bar clasp arm.
4. As with all cast clasps, its half-round form prevents adjustment to increase or decrease retention.
2. Ring clasp.Ring clasp, which encircles nearly all of a tooth from its point of origin.
It is used when a proximal undercut cannot be approached by other means. For example, when a mesiolingual under cut on a lower molar abutment cannot be approach directly because of its proximity to the occlusal rest area and cannot be approached with a bar clasp arm because of lingual inclination of the tooth.
Because of the great length of the clasp, it must be designed with additional support, usually in the form of an auxiliary bracing arm ( supporting strut).
The entire clasp with the exception of the retentive tip must be placed above the height of contour.
Disadvantages1- a large amount of the tooth surface is covered with metal.
the contour of the crown is altered, and interference with normal stimulation of the surrounding mucosa may occur ( decalcification of tooth)
2-With or without the auxiliary bracing arm, the clasp is prone to get out of adjustment, and it is difficult to adjust or repair.
3. Embrasure clasp.In the fabrication of an unmodified Class II or Class III partial denture, there are no edentulous spaces on the opposite side of the arch to aid in clasping.
Sufficient space must be provided between the abutment teeth in their occlusal third to make room for the common body of the embrasure clasp, yet the contact area should not be eliminated entirely.
The embrasure clasp always should be used with double occlusal rests. This is done toavoid interproximal wedging by the prosthesis, which could cause separation of the abutment teeth and result in food impaction and clasp displacement. In addition to providing support, occlusal rests also serve to shunt food away from contact area
4. Back-action clasp.The back-action clasp is a modification of the ring clasp, which has all of the same disadvantages and no apparent advantages. It is difficult to justify its use. The undercut can usually be approached just as well using a conventional circumferential clasp,
5. Multiple clasp.The multiple clasp is simply two opposing circumferential clasps joined at the terminal end of the two reciprocal arms. It is used when additional retention and stabilization are needed, usually on tooth-supported partial dentures. It may be used for multiple clasping in instances in which the partial denture replaces an entire half of the dental arch. It may be used rather than an embrasure clasp when the only available retentive areas are adjacent to each other. Its disadvantage is that two embrasure approaches are necessary rather than a single common embrasure for both clasps
6-Half-and-half clasp7- Reverse-action clasp.
Bar claspthe bar clasp arm arises from denture framework or a metal base and approaches the retentive undercut from a gingival direction. The bar clasp arm has been classified by the shape of the retentive terminal. Thus been identified as a T, modified T, I, or Y. The form the terminal takes is of little significance as long as it is mechanically and functionally effective, covers little tooth surface as possible, and displays as little metal as possible.
Thus use of the bar clasp arm is contraindicated when a deep cervical undercut exists or when a severe tooth and/or tissue undercut exists, either of which must be bridged by excessive blockout. When severe tooth and tissue undercuts exist, a bar clasp arm usually is an annoyance to the tongue and cheek and also traps food debris.
Other limiting factors in selecting of bar clasp assembly include a shallow vestibule or an excessive bucaal or lingual tilt of the abutment tooth. .( A gingivally-approaching clasp is contraindicated if the buccal sulcus is less than 4 mm in depth)
Bar claspsRPI, and RPA
Clasp assemblies that accommodate functional prosthesis movement are designed to address the concern of a Class I lever. The concern is that the distal extension acts as a long "effort arm' across the distal rest "fulcrum" to cause the clasp tip "resistance arm" to engage the tooth undercut.
This results in a harmful tipping or torquing of the tooth & is greater with stiff clasps and more denture base movement. Two strategies are adopted change the fulcrum location and subsequently the "resistance arm" engaging effect (mesial rest concept clasp assemblies), or to minimize the effect of the lever by use of a flexible arm (wrought wire retentive arm).
Mesial rest concept clasps are proposed to accomplish movement accommodation by changing fulcrum location This concept includes the RPI clasps
this clasp assembly consists of a mesiocclusal rest with the minor connector placed into the mesio-lingual embrasure, but not contacting the adjacent tooth. A distal guiding plane, extending from the marginal ridge to the junction middle and gingival thirds of the abutment tooth prepared to receive a proximal plate. The buccolingual width of the guiding plane is determined by the proximal contour of the tooth. The proximal plate, in conjunction with the minor connector supporting the rest provides the stabilizing and reciprocal aspect of the clasp assembly
The arm of the I-bar tip should connecting the abutment no more than 2 mm. The horizontal portion of the approach arm must be at least 4 mm from the gingival margin and even farther if possible.
Combination ClaspThe most common use of the combination clasp is on an abutment tooth adjacent to a distal extension base where only a mesial undercut exists on the abutment or where a large tissue undercut contraindicates a bar-type retainer
The combination clasp has several disadvantages:
it involves extra steps in fabrication, particularly when high-fusing chromium alloys are used.
it may be distorted by careless handling on the part of fee patient.
because it is bent by hand, it may be less accurately adapted to the tooth and therefore provide less stabilization in the suprabulge portion.
it may distort with function and not engage the tooth.
The disadvantages of the wrought-wire clasp are offset by its several advantages, which are
its esthetic advantage over other retentive circumferential clasp arms.
a minimum of tooth surface covered because of its line contact with the tooth, rather than having the surface contact of a cast clasp arm.
a less likely occurrence of fatigue failures in service with the tapered wrought-wire retentive arm versus the cast, half-round retentive arm.