Direct Retainers Infra Bulge Direct Retainers Dr.Mohammad Al Sayed 26/4/2008

Infra Bulge Retainers: Bar clasp: (Roach) Bar clasp originate from denture framework or a metal base and approaches the retentive undercut from a gingival direction.

* Other bar clasps e.g. T bar or modified T bar do not have a tripping action since the retentive terminal engages the undercut from an occlusal direction. * The push type retention of bar clasps is more effective than the pull retention of a circumferential clasp.

Types of bar clasps: * The bar clasp arm is classified by the shape of the retentive terminal into: T, modified T, I, Y or any letter clasp arm. * Bar clasp arms generally are used on the buccal surfaces of teeth in combination with lingual circumferential clasp arms. Some types of bar clasps e.g. “I” bar are “Push” type having a tripping action Tripping Action: Is caused by a clasp arm that engages the undercut directly from a gingival direction.

Rules applied to bar type clasp: 1.The approach arm of a bar clasp must never impinge on soft tissues. Adequate relief must be applied under the arm. 2.The approach arm must always taper uniformly from its attachment at the frame to the clasp. 3.The approach arm must never cross a deep soft tissue undercut requiring a great deal of relief. This will result in a food trap or lead to injury of the mucosa of the lip or cheek.

4. A bar clasp usually engages 4.A bar clasp usually engages .02 inch undercut except for the “I” bar which engages .01 inch. 5.The occlusal rest of the clasp assembly must be thick enough to provide adequate support without breaking and to avoid impinging of the soft tissue due to settling of the approach arm. 6.The minor connector (proximal plate) should be rigid and strong to provide some bracing to the partial denture. 7.The Approach arm should cross the gingival margin at 90° angle to avoid irritation.

8.The distance between the vertical projection arm and other vertical component should be at least 5mm. 9.The retentive terminal of a bar clasp should point toward the occlusal surface never towards the gingiva. *It must be kept as low as possible on the tooth to reduce leverage. *Flexibility of bar clasps can be controlled by the taper and length of the approach arm. The greater the length and more taper the more flexible the clasp will be.

Advantages: 1.Greater retention than occlusally approaching due to the trip action. 2.It is more esthetic than the occlusally approaching clasps. 3.The flexibility of the bar clasp arm can be controlled by its taper and length. 4.Covers less tooth structure than the occlusally approaching clasp (less caries susceptibility).

Disadvantages: 1.Greater tendency to collect and hold food debris (more gingival irritation). 2.The retentive arm does not contribute to bracing and stability.

Contraindication: 1.When a deep cervical tooth undercut exists. 2.When a severe tissue undercut exists. 3.When there is a shallow vestibule. 4.When there is excessive buccal or lingual tilt of the tooth. 5.When the height of contour is close to the occlusal surface of the tooth.

Types of the gingivally approaching clasps: 1.Bar Clasp (Roach or vertical projection) Indications: 1.On abutment for tooth supported or tooth-mucosa supported RPD when there is distobuccal undercut. 2.In situations in which esthetics is important.

Design: 1.Component parts: It is usually used as a combination clasp form. a) Rest. b) Minor connector. c) A cast bar retentive clasp arm. d) A cast circumferential bracing clasp arm. 2.Provides unilateral bracing. 3.Engages 0.01 inch undercut. 4.The shape of the retentive terminal as a T,Y, or I classify the bar clasp

- The I and Y retentive clasp arm provide tripping action, which is called push type clasp. - The T and modified T clasps did not have tripping action since the retentive terminal actually engage the undercut from an occlusal direction. One terminal of the T-bar clasp lies above the survey line for bracing and the other terminal below the survey line for retention. The modified T-bar clasp has only one terminal below the survey line.

Lingual view Buccal view

2.RPI Clasp: Indication: Commonly used for tooth mucosa borne partial dentures. Design: 1.It consists of: a) Mesial rest. b) Minor connector; placed into the mesiolingual embrasure, but not contacting the adjacent tooth. c) Proximal plate contacts approximately 1 mm of the gingival portion of guiding plane. d) Cast I bar retentive clasp arm located at the mesiobuccal prominence of the tooth or mesial to it.

2. Engages 0.01 inch undercut. 3. Provide unilateral bracing. 4. The proximal plate and the minor connector provide stabilization and reciprocation. 5. During function, proximal plate and I-bar clasp arm move in a mesiogingival direction- disengaging tooth. This distribute more functional load to edentulous ridge.

Combination clasp: Indication: Commonly used for tooth mucosa borne RPD, when there is mesiobuccal undercut on the abutment tooth.

Design: 1.Component parts are: a) Rest. b) Minor connector. c) Wrought wire circumferential retentive clasp arm. d) Cast circumferential bracing clasp arm. 2.Engages 0.01-0.02 inch undercut. 3.Provides bilateral bracing, but less than a cast circumferential clasp.

Advantages: 1.Greater flexibility. 2.It is more esthetically acceptable because it is placed too much gingival. 3.It flex in all directions (round in cross section), Which allow it to dissipate forces exerted on the abutment tooth. 4.The retentive arm can be adjusted in all directions. 5.Makes little tooth contact (line contact, rather than the surface contact of the cast clasp arm)

Disadvantages: 1.It is easily distorted or fractured by careless handling by the patient. 2.It does not possess the bracing and stabilizing qualities of cast clasp arm. 3.It involves extra step in fabrication.

Criteria for clasp selection: The selection of the clasp depends mainly on the type of support, the presence of undercut areas, and esthetics. 1.For bounded saddle; the retentive undercut present is used with any acceptable clasp type (clasping for convenience).

2.For distal extension base; a stress releasing clasp that equitably distribute the force between the abutment and the ridge is used. a) If a mesiobuccal undercut is available on the terminal abutment, a combination clasp with the wrought wire, RPI, or RPA clasps are used. b) If the retentive undercut is located on the distobuccal surface, a bar clasp, reverse circlet (the minor connector and the occlusal rests are placed mesially) and the C-clasp are used. c) If mesiolingual undercut is present a wrought wire clasp is used.

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