L4 Types of Direct Retainers PDF

Types of Direct Retainers Lecture 4 Dr. Faten K. Al-Kadi BDS , MSc , PhD Prosthodontist Preclinical Prosthodontics IV Fall 2024 1 Comparison of retention: Infrabulge vs S...

Types of Direct Retainers Lecture 4 Dr. Faten K. Al-Kadi BDS , MSc , PhD Prosthodontist Preclinical Prosthodontics IV Fall 2024 1 Comparison of retention: Infrabulge vs Suprabulge Clasp Assemblies 2  Comparison of retention An infrabulge clasp approaches the associated undercut from an apical direction. Displacement of the clasp requires that the infrabulge retentive arm be “pushed” over the height of contour. In contrast, a suprabulge clasp approaches the associated undercut from an occlusal or incisal direction. Displacement occurs when the suprabulge clasp is “pulled” over the height of contour. 3 Types of clasp: Generally, there are three types: 1. Circumferential Clasp:- it is called so, because it is circumferentiate the tooth , other names of this type of clasp :- It is called occlusal approaching clasp since it approach the undercut from an occlusal direction. It is called supra bulge clasp since it is coming to the undercut area from the supra bulge area. It is called Pulling clasp since it is pulling the tooth during its action. Also called Akers clasp and clasp number I. 2. Bar clasp: since it approaches the undercut area from a gingival direction it is called gingivally approaching clasp, it is called infrabulge clasp since it reaches the undercut from infrabulge area. And since it pushes the tooth during its action it is called pushing clasp. Also called I-Bar clasp, Roach clasp and clasp number II. 3. Combination clasp. it is joining two types of clasp, it consists of tapered round wrought wire retentive clasp arm and cast reciprocal clasp arm, and it is usually in a form of a circumferential clasp type. The wrought wire retentive clasp arm is either soldered to the metal framework, or it is embedded in the acrylic resin base. Suprabulge Clasps: a. Cast Circumferential Clasp b c b. Simple Circlet Design c. Reverse Circlet Design d. Multiple Circlet Design e. Embrasure Clasp Design h f. Half and Half Clasp g. Ring Clasp Design h. Hairpin Design e i. Onlay Clasp Design i 5  A. Cast Circumferential Clasp The clasp is remarkable for its simplicity & ease of construction. It is the design of choice for tooth-supported removable partial dentures because of its excellent support, bracing, & retentive properties. ((Mostly used with tooth supported partial denture due to its retentive and stabilizing abilities)). A cast circumferential clasp displays close adaptation to the abutment &, therefore, minimizes the entrapment of food & debris. The course of the clasp arm begins from the minor connector near the occlusal rest and approaches the undercut area from an occlusal direction. 6 Disadvantages of the Cast Circumferential Clasp 1. One of the primary disadvantages of the cast circumferential design is the large amount of tooth surface that is covered by the clasp assembly. 2. If proper oral hygiene is not maintained, the underlying enamel is at increased risk for decalcification. 3. When in place, the cast circumferential clasp also alters the gross morphology of the clinical crown. This may interfere with food flow & bolus elimination during the masticatory process. As a result, the abutment & associated periodontium may be damaged 7  Design rules of the Cast Circumferential Clasp 1. A cast circumferential clasp should originate from a portion of the framework that lies above the height of contour (Occlusal ). The retentive arm should extend cervically & circumferentially manner. The terminal third of the retentive clasp should pass over the height of contour &Only the apical border of the retentive clasp terminus should engage the desired undercut. The reciprocal element should be located at or slightly above the height of contour on the opposite surface of the tooth & should prevent lateral displacement of the abutment when the retentive clasp flexes over the height of contour. 8 Design rules of the Cast Circumferential Clasp 2. The retentive terminus should be directed occlusally, never toward the gingiva. The long, curved clasp arm that results from this contour permits improved flexibility. 3. A cast circumferential clasp arm should terminate at the mesial line angle OR distal line angle of the abutment, never at the midfacial or midlingual surfaces. 9 4. A cast circumferential clasp should not be used to engage : (b) The (a) The distofacial mesiofacial surface of an surface of an abutment abutment adjacent to adjacent to a an extensive posterior anterior edentulous edentulous space. space. Lead to premature loss of abutments 10 b. Simple Circlet Design. The simple circlet clasp design is widely used. It is generally considered the clasp of choice for tooth-supported removable partial dentures, because its uncomplicated design features make it easy to construct & Simple circlet cast relatively simple to repair. circumferential retentive clasps are present on A simple circlet clasp usually originates on the facial surfaces of both the proximal surface of an abutment adjacent to first premolar & second an edentulous area, with the clasp arms molar abutments. projecting away from the edentulous space. 11 Simple Circlet Because of the half-round cross-sectional geometry of each clasp arm, adjustment is Design. difficult. L I M I TAT I O N S Also, the clasp assembly tends to increase the circumference of the clinical crown. This may interfere with the elimination of food from the occlusal table ,increased tooth coverage may promote decalcification & compromise dental aesthetics. 12 c. Reverse Circlet Design. A reverse circlet clasp is often used when the available undercut is located at the facial or lingual line angle adjacent to an edentulous space. The preferred method for engaging such an undercut involves the use of an infrabulge clasp that traverses the mucosa, turns vertically to cross the free gingival margin, & ultimately engages the undercut adjacent to the edentulous area. IF, however, the anatomic contours of the hard & soft tissues apical to the abutment prohibit use of an infrabulge clasp, a reverse circlet clasp may be the retainer of choice. 13  c. Reverse Circlet Design. In Kennedy Class I & Class II applications (i.e., distal extension applications), the mesial-to-distal projection of a reverse circlet clasp may help control stresses transmitted to an abutment during functional movement of the prosthesis. When occlusal load is applied the tip of the retentive clasp moves into an area of greater undercut (no load on abutment). But when a dislodging force is applied to the prosthesis, the retentive tip engages the undercut & the removable partial denture is retained. 14  Reverse Circlet Design. L I M I TAT I O N S 1. Biggest limitation is the strength of the clasp assembly. As the shoulder of a reverse circlet clasp originates from a minor connector & must pass through the marginal ridges of adjacent teeth where often occlusal contacts exists, it may be difficult to provide adequate room for clasp components without removing a significant amount of tooth structure from the abutments. (Failure to remove adequate tooth structure often results in thin portions of the clasp assembly that are susceptible to fracture). 15  Reverse Circlet Design. L I M I T A T I O N S 2. The prosthesis may damage the associated soft tissues, because there is no rest adjacent to an edentulous space. (The marginal gingiva also may be traumatized if food particles are forced between the proximal plate & the surface of the abutment). 3. Because a reverse circlet clasp is most commonly crosses the facial surface from mesial to distal, it is a poor choice from an esthetic viewpoint. (As a result, the reverse circlet clasp design is not the clasp of choice for canine & premolar abutments). 16  d. Multiple Circlet Design. A multiple circlet clasp design involves two simple circlet clasps joined at the terminal aspects of their reciprocal elements. ((It consists of two circumferential clasps with their reciprocal arms are united together at their terminal end)). This clasp design is primarily indicated when the principal abutment tooth is periodontally compromised & stresses originating from prosthesis retention can be favorably distributed between multiple abutment teeth. So, the Multiple Clasp used when only few teeth are left and multiple clasping is needed. The disadvantages of the multiple circlet clasp are the same as for simple circlet clasp: The clasp assembly tends to increase the circumference of the clinical crown. This may interfere with the elimination of food from the occlusal table.Increased tooth coverage may promote decalcification & compromise dental aesthetics. 17 e. Embrasure Clasp Design or Butterfly clasp An embrasure clasp is essentially two simple circlets joined at their bodies. This design is most frequently used on the side of the arch where there is no edentulous space. Clasp arms originate from a minor connector that traverses the marginal ridges between teeth. Especially used for bilateral bracing in a unilateral edentulous area (CL II and CL III without modification). It has two occlusal rests, two retentive arms and two reciprocal arms and positioned on two teeth. Note that the simple circlet retentive clasps are joined at their bodies, forming a minor connector that traverses the occlusal embrasure between the first & second molars. 18  Embrasure Clasp Design. L I M I TAT I O N S 1. Occlusal rests must be used to support the embrasure portions of the clasp. 2. Adequate preparation of the marginal ridges & adjacent facial inclines must be accomplished to ensure a sufficient metal bulk for clasp strength. 3. Fatigue failure of an embrasure clasp design is quite common, particularly when insufficient tooth preparation results in inadequate cross-sectional dimensions & compromised clasp strength.Therefore, the use of this clasp design should be avoided unless adequate tooth preparation can be achieved. 19 F. Half and Half Clasp A circumferential clasp consists of a retentive arm arising from a direction and a reciprocal arm arising from the other direction and each of which has its minor connector and rest. g. Ring Clasp Design. A ring clasp is most often indicated on a tipped mandibular molar. When the loss of mandibular posterior teeth results in the absence of mesial proximal contact for a remaining molar, that strut molar tends to tip in a mesiolingual direction. it is frequently used with isolated lingually tilted lower molar with an undercut located mesiolingually. The ring clasp permits engagement of this undercut through encirclement of the tooth. In most instances, the clasp originates from a mesial rest. The ring clasp encircles the entire tooth from its point of origin to the tip. The clasp arm then traverses the facial & distal surfaces of the tooth, remaining occlusal to the height of contour. At the middle of the lingual surface, the clasp arm passes apical to the height of contour & engages a measured undercut at the mesiolingual line angle. 21  Ring Clasp Design. L I M I TAT I O N S strut Because of the length of the clasp arm, additional support must be available to ensure its rigidity. Due to its great length, it may become flexible so it is provided by a "Strut" in the non-retentive side to give it more rigidity. This support may be provided by an auxiliary bracing arm located at the facial aspect of the tooth. The bracing arm usually projects from the minor connector used to retain the acrylic resin denture base. The bracing arm runs horizontally across the mucosa apical to the abutment tooth & then turns occlusally to cross the free gingival margin & join the clasp arm at its midfacial aspect. Without this bracing element, the clasp assembly cannot provide effective reciprocation or cross-arch stability. An additional rest may be placed on the disto- occlusal surface. It should be supported by two rests to avoid the possibility of opening it. This rest may provide additional support for the prosthesis. 22  Ring Clasp Design. L I M I T A T I O N S 2. Since a ring clasp covers an extensive amount of tooth structure, oral hygiene becomes more complicated & the likelihood of demineralization is increased. In addition, the ring clasp design significantly alters the functional contours of the abutment & may interfere with the elimination of food from the occlusal table. This alteration may result in insufficient stimulation of the associated soft tissues & adversely impact gingival health. 3. The disadvantage of ring clasp is that it covers large area of the tooth making it unaesthetic, so it is used only in posterior teeth. 4. From a structural standpoint, the ring clasp is susceptible to distortion & fracture. 5. A ring clasp should not be considered when limited vestibular depth does not allow placement of an auxiliary bracing arm. 6. This clasp design is also contraindicated when the bracing arm must project across a soft tissue undercut area. As a general rule, the ring clasp should not be considered the clasp of choice when an 23 alternative design is feasible.  h. Hairpin Design, Reverse action clasp, Fish-hook Design , C -clasp Design It is essentially a simple circlet clasp in which the retentive arm loops back to engage an undercut apical to the point of origin. By structural design, the retentive arm has two horizontal components. The occlusal portion of the retentive arm (should be considered a minor connector & must be rigid), & the apical portion of the retentive arm. This retentive arm must pass over the height of contour to engage the desired undercut. Hence, this portion of the clasp must be flexible. Used when a proximal undercut must be used on posterior teeth and there is a severe tissue undercut prevents the use of I-Bar clasp. It is also called "hairpin clasp". 24  Hairpin Design. LIMITATIONS 1. To accommodate a Hairpin clasp, the clinical crown must exhibit sufficient vertical height. 2. The occlusal aspect of the retentive arm also should not interfere with the opposing teeth in maximum intercuspation. 3. The Hairpin design generally yields inadequate flexibility, leading to harmful non-axial forces on the abutment. 4. The Hairpin design generally causes greater accumulation of food & debris making it inappropriate for patients with poor oral hygiene. 5. The Hairpin design is often esthetically unacceptable. 25 When the only available undercut is located at the line angle adjacent to the edentulous space, there are three clasp designs from which to choose: 1. The infrabulge clasp, 2. The reverse circlet clasp, 3. The Hairpin OR C-clasp. The Hairpin clasp is indicated when the soft tissue contour does not allow the use of a bar-type clasp & when the reverse circlet cannot be considered because of a lack of occlusal clearance. 26  i. Onlay Clasp Design. An onlay clasp consists of a rest that covers the entire occlusal surface & serves as the origin for buccal & lingual clasp arms. This clasp design is indicated when the occlusal surface of the abutment lies noticeably apical to the occlusal plane. The onlay rest serves as a vertical stop & also aids in the establishment of an acceptable occlusal plane. 27  Onlay Clasp Design. LIMITATIONS 1. Because of extensive tooth coverage, the onlay clasp should be prescribed only for caries-resistant patients. 2. If the removable partial denture framework is to be constructed using a Chrome alloy & is opposed by natural tooth structure, the occlusal surface of the onlay clasp should be veneered with acrylic resin or gold. (Chrome alloys often induce rapid wear of enamel & dentin surfaces, while acrylic resin & gold display greater compatibility with natural tooth structure.) 3. Although an onlay clasp may be used successfully, it is frequently more appropriate to restore the occlusal plane with one or more fixed restorations & consider a more conventional design for the removable prosthesis. 28 Thanks