Direct Retainers PDF

# Direct Retainers Dr. Kamel Mostafa Salah BDS 2011, MSc 2018, PhD 2024 Lecturer of prosthodontics ## Outline - Introduction. - Types of Direct Retainers. - Attachments. - Clasps. - Requirements of a Clasp. - Classifications of Clasps. ## Requirements of a Clasp - **Retention.** - A...

# Direct Retainers Dr. Kamel Mostafa Salah BDS 2011, MSc 2018, PhD 2024 Lecturer of prosthodontics ## Outline - Introduction. - Types of Direct Retainers. - Attachments. - Clasps. - Requirements of a Clasp. - Classifications of Clasps. ## Requirements of a Clasp - **Retention.** - A dislodging force may be activated by speech, muscle action, mastication, deglutition, sticky food, or gravity (in case of upper denture). - The clasp arm is retentive by the fact that its position on a surface of the tooth is cervical to its greatest circumference. - **Factors affecting the retentive action of the clasp:** - **Depth of the undercut engaged.** The greater the degree of undercut engaged, the greater is the retentive action of the clasp considering the same rigidity. - **Angle of approach.** A gingivally approaching clasp arm gives better retentive action than occlusally approaching clasp arm. - **Flexibility of the clasp arm.** The greater the flexibility of the clasp arm, the lesser is the retentive action of the clasp considering the same undercut. - The degree of flexibility depends on: - The length of the clasp arm. The greater the length, the greater the flexibility. - The diameter of the cross-section. The smaller the diameter, the greater is the flexibility. - The form of the cross-section. A round clasp arm is more flexible than half-round or oval. - Degree of tapering. The retentive arm should be uniformly tapered in such a way that the diameter at its origin is twice that at its tip. - Modulus of elasticity of the alloy used. Casted gold alloys may have greater resiliency than do casted cobalt-chromium alloys. Wrought wire clasp can be used to provide greater flexibility than casted clasps. - **Support.** Support is the property of the clasp which enables it to resist displacement in a gingival direction. The rest is the support unit. - **Stabilization.** Stabilization, or bracing is the resistance to displacement of the prosthesis in a horizontal plane. All the clasp components provide this property in varying degrees. The cast circumferential clasp is a better stabilizing clasp than is the bar clasp or the combination clasp. - **Encirclement.** The clasp should encircle more than half the circumference of the crown of the tooth, to prevent movement of the tooth out of the confines of the clasp arms as stresses are applied. - **Reciprocity.** During insertion and removal of the denture the retentive arm flexes over the height of contour and generates force. The effect of the retentive clasp arm on the tooth is countered by the action of a rigid clasp arm. - **Passivity.** The clasp should exert no pressure against the tooth at rest until it is activated either by denture movement in function, or during removal from the mouth. ## Classifications of Clasps - **Based on construction.** - **Casted Clasps.** The clasp is casted in gold or chrome-cobalt alloy, it is accurately fitting and easily varied in thickness, form and taper. - **Wrought wire clasps.** The wrought wire clasp is usually made of stainless steel or gold alloy wire. Available in different sizes: - 17 gauge = 1.15mm - 18 gauge = 1.024mm - 19 gauge = 0.9119mm - 20 gauge = 0.8128mm - 21 gauge = 0.7239mm - **Combination Clasps.** The combination clasp is a cast clasp in which wrought wire has been substituted for the usual cast retentive arm. It is made by either of the two following methods: - The wrought wire retentive arm is attached to the clasp with solder. - The wrought wire can be embedded in the wax pattern of the clasp before casting. - **Advantages of Combination Clasp:** - It combines the resiliency of the wrought retentive arm plus the better stabilizing feature of the cast clasp. - It has a stress breaking action. - It can be used in the anterior part of the mouth as it is less showing and can be placed near the gingival margin. - **Based on Design.** - **Occlusally approaching Clasps (Suprabulge).** - **Double arms clasps.** - Aker's clasp (Circumferential). The most universally employed of all clasp designs. It is a double-arms clasp; relatively rigid. The circumferential clasp is usually the most logical clasp to use with all tooth supported partial denture because of its retentive and stabilizing ability. The circumferential clasp is used on molars and premolars of normal shape and alignment. The tooth to be clasped is adjacent to the edentulous space and the undercut is far from the edentulous area. The undercut used is 0.01 inch (0.25mm). - **Limitations.** - More tooth surface is covered. - Aker's clasp can be adjusted to the tooth surface in an inward or outward (Bucco-lingual) and *not upward or downward* (occluso-gingival) direction, due to its half round cross-section. - Cannot be used in free-end saddle cases due to its rigidity. - **The One-Arm Aker's.** It is similar to the double arm-form except that the reciprocal is formed by an extension of the major connector. - Reversed Aker's clasp. Clasp occlusal rest located away from the edentulous area. Retentive arm that engages an undercut near the edentulous area. A rigid reciprocal arm. - **Indication:** In distal extension cases when the bar clasp is contraindicated due to the presence of soft tissue undercut. - **Advantages:** - The mesial occlusal rest, exerts a mesially directed force where it is reciprocated by an adjacent tooth. - Clasp disengagement. - Reduce torque transmitted to the abutment tooth. - Double Aker's clasp. Double Aker's clasp is also called " embrasure, compound, butterfly, back-to-back or interdental clasp ". Two Aker's clasps arising from a common body and from the same minor connector. The minor connector is located in the embrasure between the two clasped teeth. - **Indication:** Used on the dentulous side of unilateral edentulous (class II, III). Cases having no modifications, class IV. - **Advantages:** - Provide bilateral stabilization, in addition to retention. - It also splints the two teeth. - RPA clasp. RPA design is formed of three components; - Mesial occlusal rest. - Proximal plate. - A circumferential clasp arm arises from the superior portion of the proximal plate. - **Indication:** In distal extension cases when a bar type is contraindicated, and the undercut is located in the gingival third of the tooth away from the edentulous area. - **Single arm clasps.** - Ring clasp. It is a single-arm clasp used on buccally or lingually tilted isolated molars. It arises from the mesial surface of the molar and then encircles the tooth resembling a ring. Attached to the denture by minor connector at the mesial occlusal rest area. An auxiliary distal rest is preferably added to prevent further mesial tilting of the tooth. - Exhibiting a mesiobuccal undercut in case of upper molars and a mesiolingual undercut on lingually tilted lower molars. Engages undercut of 0.02 or 0.03 inch. A reinforcing supporting arm located on the non-retentive side is usually considered to limit the flexibility of the clasp. - **Disadvantages of ring clasp:** - Excessive tooth coverage that may result in enamel decalcification and caries. - Easily distorted because of length and difficult to adjust. - Reinforcing arm may cause marginal irritation and inflammation and may act as a food trap. - **Gingivally approaching Clasps (Infrabulge).** These are also called *infrabulge*, *roach*, or *bar clasps*. The bar clasp arm arises from the denture framework and approaches the undercut from a gingival direction. It covers less tooth surface with little display of metal. It is used with bounded and free-end partial dentures. - **Indications:** - The bar clasp is mainly indicated in distal extension partial dentures to help in reducing torque on the abutments. - It can be also used in bounded dentures especially when esthetics is considered. - **Contraindication:** - Severe buccal or lingual tilt of the abutment teeth. - Shallow buccal or labial vestibule. - Soft tissue undercut. - Gingival inflammation. - **Disadvantages:** - Less stabilization is provided due to its flexibility. - Tendency to gingival inflammation. - The approach arm of an infrabulge clasp must not impinge on the soft tissues adjacent to the abutment. - The approach arm should cross perpendicular to the free gingival margin. - The approach arm should be uniformly tapered from its origin to the clasp terminal to increase flexibility. - The bar clasp arm has been classified by the shape of the retentive terminal. - **RPI Clasp.** The components of this clasp assembly are: - "R" - mesial occlusal rest. - "P" - proximal plate. - "I" - I-bar (retentive arm). - Used for distal-extension base partial denture and provides stress-release. - **Occlusally approaching vs. Gingivally approaching.** - **Angle of approach.** - **Bracing.** - **Caries Susceptibility.** - **Retention.** - **Gingival Health.** - **Esthetics.** - **Based on stress breaking action.** - **Clasps having a stress breaking action.** - **Clasps having no stress breaking action (Rigid).** - **IV. Planning for Direct retention.** - **Clasps having a stress breaking action:** - Gingivally approaching clasps (RPI and Bar Clasps). - Occlusally approaching clasps (RPA, and Reversed Aker's clasp). - Combination clasp. - **Clasps having no stress breaking action:** - Circumferential Clasp (Aker's clasp) - Modifications of Aker's clasp: - One arm Aker's - Double Aker's - Extended arm clasp - Multiple Clasp - **Direct Retainer Selection.** - **Selection is based on:** - Classification of edentulous arch. - Abutment tooth type. - Amount of undercut. - Location of tooth undercuts. - Occlusion. - Presence of restorations. - Tilting of the abutment tooth. - Soft tissue or bony undercut. - Vestibular depth - Esthetics - **Wrought wire Clasps.** Wrought wire clasps are commonly used on a variety of interim and transitional prosthesis. The commonly used clasps for acrylic RPD are: - Embrasure wrought wire clasp (half Jackson). - Circumferential wrought wire clasp. - **Half Jackson.** This type finds its applications on molars and premolars when no edentulous space exists on one side of the tooth to be clasped. The embrasure clasp originates from the lingual or palatal side between two standing teeth. It passes against the lingual embrasure then cross the occlusal embrasure between the two teeth to engage the buccal undercut along the gingival margin of one of them. - **Circumferential WW clasp.** It is used for the teeth adjacent to the edentulous space. It starts lingually and passes over the relieved (1-2mm) ridge along the proximal surface of the tooth to engage the buccal undercuts. It should pass away from the proximal surface of the clasped tooth to allow for the adjustment of the denture during insertion.