Prosthodontics 2 (RPD) - Lecture 1: Rest and Rest Seats PDF

PROSTHODONTICS 2 (RPD) - LECTURE Lecture 1: REST AND REST SEATS 4th Year | 2024-2025 | MIDTERM REST AND REST SEATS The part of the RPD that is closer to the Appropriate use of teeth (as abutments) abutment, more...

PROSTHODONTICS 2 (RPD) - LECTURE Lecture 1: REST AND REST SEATS 4th Year | 2024-2025 | MIDTERM REST AND REST SEATS The part of the RPD that is closer to the Appropriate use of teeth (as abutments) abutment, more of the occlusal load is requires considerations as to how to best transmitted to the abutment tooth by means of engage teeth for the supportive qualities they the Rest. provide. When rests prevent movement of the denture The most effective resistance can be provided if apically, the clasps will maintain its position the tooth is stressed along its long axis. (Axial within the undercuts. force or axial loading) TYPES OF RESTS Various forms of rests have, as a main goal, a A. Occlusal Rest: form that allows for axial loading. This can be 1. Occlusal Rest (Conventional) achieved by some form of tooth modification. 2. Extended Occlusal Rest Vertical support must be provided for a 3. Interproximal Occlusal Rest (Embrasure Removable Partial Denture, and so, a REST is a Rest) component that should always be present, as it B. Lingual Rest (Cingulum Rest) provides such support. C. Incisal Rest A rest should be located on a properly prepared D. Implants as Rest tooth surfaces. This preparation is called the REST SEAT. As the name implies, it is where a FUNCTIONS OF A REST seat will rest. 1. To provide resistance against occlusal load A rest should restore the exact physical feature 2. To direct occlusal forces parallel to the long axis of the tooth before a rest seat was prepared. of the abutment tooth 3. To maintain the retentive terminal of the clasp PURPOSE OF A REST in its intended position MAIN PURPOSE – to provide vertical support 4. To prevent the denture base from seating past 1. Maintains components in their planned its intended position positions 5. To give lateral stability against undue horizontal 2. Maintains established occlusal relationships by stresses to the soft tissue of the ridges preventing setting of denture 6. To close small spaces between teeth by bridging 3. Prevents impingement of soft tissue that space 4. Directs and distributes occlusal loads to abutment teeth FORM OF THE OCCLUSAL REST AND REST SEAT Rests serve to support the position of a partial GUIDELINES: denture and to resist movement towards the tissue The outline form of an occlusal rest seat should be a rounded triangular shape with the apex OTHER FUNCTIONS OF REST toward the center of the occlusal surface. Transmit vertical forces to the abutment teeth It should be long as it is wide, and the base of the Direct vertical forces along the long axis of the triangular shape (at the marginal ridge) should teeth be at least 2.5 mm for both molars and Functions similar to FPD abutments premolars. ** rest seats of smaller dimension do not provide CHARACTERISTIC OF A REST adequate bulk Must be RIGID – for stability and positive The marginal ridge of the abutment tooth at the support. Under occlusal loading, the rest and the site of the seat must e lowered to permit a tooth remain stable contact and no independent sufficient bulk of metal for strength and rigidity movement or slippage should occur. of the rest and the minor connector. In Distal extension cases, the load is distributed The floor of the occlusal rest seat should be between the abutment and the supporting apical to the marginal ridge and the occlusal residual tissue. surface and should be concave, or spoon WHY? shaped. Because it is increasingly tissue supported as it gets farther away from the abutment. annie | 1  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 1: REST AND REST SEATS 4th Year | 2024-2025 | MIDTERM ** avoid creating sharp edges or line angles in the preparations of rest seat. All edges should be rounded off, to prevent possible breakage of the unsupported tooth structure. Use a finishing bur in rounding off edges. The angle formed by the occlusal rest and the vertical minor connector from which it originates should be less than 90 degrees. Only in this way can the occlusal forces be directed along the long axis of the abutment tooth. ** an angle greater than 90 degrees fails to transmit For mesially inclined abutment occlusal forces. This will also cause slippage of the Extended more then ½ of the mesiodistal width RPD from the abutment tooth. and 1/3 of buccalingual width Minimum 1mm metal How to check using an explorer? No sharp edges/angles Explorer will NOT slip off rest seat, when pulled proximally INTERPROXIMAL OCCLUSAL REST Explorer WILL slip off rest seat, when pulled Prepared as individual occlusal rest seats, but is proximally extended farther lingually than usual. Adjacent rests, rather than single rest are used to prevent interproximal wedging by the framework. This design will prevent food entrapment in between the abutment. In the preparation, removing of the contact points should be avoided. Removal of sufficient tooth structure must be pone for an adequate bulk of the Rest, for strength and to be shaped to prevent the Rounded triangular shape occlusion from getting altered. Base is 2.5mm Floor is apical to the marginal ridge Spoon shape floor Angle is < 90* EXTENDED OCCLUSAL REST Used when the most posterior abutment is mesially inclined or tipped. It prevents further tipping of the abutment. Used in Kennedy Class Il, mod1 and Class Ill. Extended lingually Ensures that the forces are directed down the long axis of the abutment. Adjacent rests rather than single This extends more than one half of the Like back to back rests or twin rests mesiodistal width, and one third of the Sufficient preparation for adequate bulk buccolingual width, of the tooth. INTERNAL OCCLUSAL REST Minimum of 1mm thickness of the metal. Used for RPD that is totally tooth supported. Should have no undercuts and sharp Used for both occlusal support and horizontal edges/angle. - Occlusal support-floor of rest seat - Horizontal stability-from near vertical walls of rest seat annie | 2  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 1: REST AND REST SEATS 4th Year | 2024-2025 | MIDTERM MAIN ADVANTAGES Facilitates the elimination of a visible clasp arm buccally, as its retention is provided by a lingual clasp arm. Permits the location of the rest seat in a more favorable position in relation to the tipping, horizontal axis of the abutment. Anterior teeth, preferably canines Aka: Cingulum rest Gradual not perpendicular Mandibular anterior teeth lack enamel bulk Measurements: - Base= 2.5 mm - Depth= 1.5 mm For tooth supported RPD - Lingual: labial distance = 2mm For Cast Restored abutment LINGUAL RESTS Retention is provided by Lingual clasp only, no buccal clasp Semi precision attachment LINGUAL REST/CINGULUM REST If the Anterior teeth (Incisors and Canines) are the only abutment available for occlusal supp of the denture. Preferably Canines. Careful assessment of the incisal and lingual contact areas is needed. Sufficient space must be present to avoid interference in the placement of rests during occlusion. LINGUAL REST PREPARATION Root form and length, inclination of tooth and ratio of the crown and alveolar support must be I. REST SEAT TOOTH PREPARATION: considered. Slightly rounded V shape on the lingual surface A lingual rest is preferable to an incisal rest as it at the junction of the gingival and middle 1/3 of has a less tendency to tip the tooth. This is due the tooth. to its placement which is closed to the horizontal The apex of the V is directed incisally. axis of rotation. Start with an inverted cone shaped diamond Cingulum rest is if the lingual slope is gradual burs, then smaller tapered with round ends. rather than perpendicular, mostly in the case of Sharp edges and line angles should be rounded Maxillary canines with prominent cingulum off and polished. Perpendicular slope are likely on Mandibular Polish with rubber points and pumice. canines, this would need more rest support It is important to determine the path of Lingual rest seat preparations are rarely placement for the denture. satisfactory in mandibular anterior teeth due to Must be prepared as though it's going to be lack of enamel bulk. approached from a direction perpendicular to the lingual slope but angled towards the cingulum (the floor of the rest seat). annie | 3  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 1: REST AND REST SEATS 4th Year | 2024-2025 | MIDTERM Be careful of enamel undercut that will interfere At the incisal angles of anterior teeth the placement and removal of the denture. Rounded notch Deepest portion is apical to the incisal edge II. REST SEAT ON CAST RESTORATION: Placed on enamel The most satisfactory lingual rest from the standpoint of support is one that is placed on a More applicable in the mandibular canine prepared rest seat in a cast restoration. IMPLANTS AS REST Done most effectively by planning and executing Implants can also be considered to serve as a a rest seat in a wax pattern rather than by rest when one takes advantage of the vertical attempting to cut a rest inside the mouth. stiffness characteristic they possess. The rest seat is carved on the wax pattern on the When used as a rest, implants can serve to lingual surface of a cast veneer crown, three efficiently resist vertical movement and provide quarter crown, an inlay, a laminate veneer, a positive support. composite restoration or an etched metal It is important to consider the following if using restoration. implants as rest. INCISAL REST AND REST SEATS 1. Future use as a support to an implant fixed Placed at the incisal angles of anterior teeth and prosthesis. on prepared rest seats. 2. Current placement at the distal- most tooth Least desirable position to most effectively resist functional forces of occlusion. Used in selected patients when the abutment is sound and not indicated for a cast restoration. SUPPORT FOR RESTS Placed on the enamel Should be placed on sound enamel Used predominantly as auxiliary rest or as Can be placed on any restoration material that indirect retainers. can resist fracture and distortion when More applicable in the mandibular canine subjected to applied forces Provides definite support with relatively little Good oral hygiene is a prime factor for low caries loss of tooth structure and little display of metal. Proximal tooth surfaces are more prone to caries An incisal rest is more likely to lead to that occlusal surfaces supporting an occlusal orthodontic movements that the lingual rest, Restore any precarious fissures before preparing due to its unfavourable leverage factors. a rest REST SEAT is prepared in the form of a rounded Patients should be informed of the risks involved notch at the incisal angle of a canine or on the and of their responsibility for maintaining a good incisal edge of an incisor with the deepest oral hygiene and returning for periodic portion of the preparation apical to the incisal observations. edge In case a proximal preparation is needed to - The notch should be beveled both labially prevent undercuts or as needed for frameworks and lingually, and the lingual enamel should to easily pass through, occlusal rest seats must be shaped to accommodate the rigid minor follow proximal preparations, not precede it. To connector connecting the rest to the prevent irreparable damage to the tooth caused framework. by the marginal ridge being too low or to sharp, when occlusal rest seat is done first. Occlusal rest seats may be prepared with diamond round burs and poshed with polishing burs to round off sharp edges and smoothen the enamel. Large round bur - lower the marginal ridge and outline of the rest seat Smaller round bur - deepens the floor of the rest seat and forms the spoon shape inside the marginal ridge annie | 4  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 1: REST AND REST SEATS 4th Year | 2024-2025 | MIDTERM Fluoride gel must be applied after impression In the event that an existing restoration has been perforated, it should be repaired and modified to accommodate the rest. BASIC CONSIDERATIONS Forces that are applied to removable partial denture must be transferred to the supporting teeth and tissues in an atraumatic fashion Rest should be sturdily placed in properly prepared rest seat Rest seat should be within the confines of the greatest tooth mass Should not be placed on a downward inclined tooth surface No sharp angles, should be round in aspects No undercuts RESTS ACCORDING TO STEWART 1. Primary rest - A rest that is part of the retentive clasp assembly - Prevents vertical movement of a prosthesis toward the tissues and also help transmit applied forces to the supporting teeth - Should be shallow and should function like a ball and socket 2. Auxiliary or secondary rests - Responsible for additional support or as indirect retention - Used as indirect retainer in extension base RPD REST ACCORDING TO KRATOCHVIL 1. Anterior rests - Labial - Lingual 2. Posterior rests - Occlusal - Extended Occlusal - Interproximal annie | 5  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 2: DIRECT RETAINERS 4th Year | 2024-2025 | MIDTERMS DIRECT RETAINERS TYPES OF DIRECT RETAINERS Direct Retainers The direction of forces on an 1. Intracoronal retainer RPD can be toward, across or away from the Casted or attached totally within the contours of tissue. the abutment. The force of occlusion have the greatest Composed of prefabricated key and keyway, like intensity as it is the force acting to move a precision attachment, that is internal in prostheses toward and across the supporting nature. teeth and/or tissue. 2. Extracoronal (clasp-type) retainer 3 Direction forces: uses mechanical resistance to displacement 1. Forces acting to displace the prosthesis from through components placed on the external the tissue can consist of the gravity acting surfaces of the abutment tooth. against a maxillary prosthesis 2. The action of adherent foods acting to CLASP ARM: displace the prosthesis on opening of the mouth Most common form of extracoronal retainer in chewing Flexible 3. Functional forces acting across a fulcrum to Engages an the external surface, in an area unseat the prosthesis. cervical to the convexity of the abutment tooth. **The first two forces are seldom at the It engages a depression that would receive the magnitude of functional forces, and the last one terminal tip of the arm. is minimized through the use of adequate REQUIREMENTS OF DIRECT RETAINERS support. 1. Support - resist vertical forces rests DIRECT RETAINER - is the RPD component part 2. Stability - the quality of a prosthesis to be applied to resist the movement away from the firm, stable, or constant and to resist teeth and/or tissue and provides retention for displacement by functional, horizontal, or the prosthesis. rotational stresses. A unit of a removable dental prosthesis that = rest, minor connector and bracing arm engages an abutment tooth to resist 3. Reciprocation - resistance to retentive forces, displacement of the prosthesis away from basal resistance to orthodontic movement of teeth seat tissue using reciprocal arms or elements placed against It's ability to resist movement is greatly guiding planes influenced by the stability and support of the = bracing arms, minor connectors prosthesis provided by major and minor 4. Retention - is that quality inherent in the connectors, rests, and tissue bases denture that resists the vertical forces of SUFFICIENT RETENTION IS PROVIDED BY TWO dislodgment. MEANS: Ex: force of gravity that may affect the maxillary 1. Primary retention - mechanical denture, adhesiveness of food or forces by placing retaining elements (direct retainers) associated with the opening of jaws. on the abutment teeth. = retentive arm 2. Secondary retention – connection close relationship of the minor connector contact with the guiding planes and denture bases of the major connector (maxillary) with underlying tissue. **The accuracy of the denture bases and involved contact area can be achieved by an accurate impression. mimi | 6  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 2: DIRECT RETAINERS 4th Year | 2024-2025 | MIDTERMS PLACEMENT OF RETENTIVE ARM: IV. Middle to cervical 1/3 of tooth Clasp retainers on abutment teeth adjacent to Considerations: distal extension bases should be designed to Tipping forces avoid direct transmission of tipping and Esthetics rotational forces to the abutment. They must be Occlusal interferences "stress breakers" BASIC PRINCIPLES OF CLASP DESIGN BASIC PRINCIPLES OF CLASP DESIGN The clasp assembly serves a similar function for an RPD that a retainer crown serves for a fixed partial denture In FPD, the principle of limiting the freedom of V. displacement refers to the effect of one Retentive clasp should be bilaterally opposed. cylindrical surface, which is the tooth. It is same Ex: buccal retention on one side of the arch as the basic principle of the clasp design called should be opposed by a buccal retention on the “THE PRINCIPLE OF ENCIRCLEMENT” other side, or lingual opposed by lingual on the other side. I. PRINCIPLE OF ENCIRCLEMENT More than 180 degrees in the greatest VI. circumference of the tooth, passing from The path of escapement for each retentive clasp diverging axial surfaces to converging axial terminal must not be parallel to the path of surfaces, must be engaged by the clasp removal for the prosthesis to require clasp assembly. engagement with the resistance to deformation The engagement can occur in the form of (retention) continuous contact, such as in a circumferential clasp, or discontinuous contact, such as in the use of a bar clasp VII. The amount of retention should always be the minimum necessary to resist reasonable dislodging forces. II. VIII. The occlusal rest must be designed to prevent Reciprocal elements of clasp assembly should be the movement of the clasp arms toward the located at the junction of gingival and middle cervical. thirds of the crowns of abutment teeth. III. Each retentive terminal should be opposed by a reciprocal component capable of resisting transient pressures exerted by the retentive arm during placement and removal. mimi | 7  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 2: DIRECT RETAINERS 4th Year | 2024-2025 | MIDTERMS IX. Passivity: at rest, a direct retainer should not exert force against a tooth BENEFITS OF THIS PRINCIPLE: 1. It ensures the stability of the tooth position because of the restraint from encirclement 2. Ensures stability of the clasp assembly because of the controlled position of the clasp in 3 dimensions 3 CONTACT AREAS OF A CLASP (Circumferential or AMOUNT OF RETENTION Bar Clasp) 1. The size of the angle of cervical convergence 1. The occlusal rest area (depth of undercut) and how far the clasp 2. The retentive clasp terminal area terminal is placed into the angle of cervical convergence 3. The reciprocal clasp terminal area TYPES OF CLASP ASSEMBLIES 1. Circumferencial (Suprabulge clasp) - used to designate a clasp arm that originates above the height of contour and approaches the tooth undercut from an occlusal direction When the abutment tooth is surveyed, any single tooth has a height of contour or an area of greatest convexity. There are certain areas of cervical convergence that may not be usable for placement of retentive clasps because of its proximity to the 2. Bar clasp (Roach clasp/Infrabulge) - a type of gingival tissue. extracoronal retainer that originates from the 2. The flexibility of the clasp arm: denture base or framework, traverses soft tissue o Clasp length and approaches the tooth undercut area from o Clasp relative diameter the gingival direction. o Class cross sectional form or shape (whether round or half round, or others) o The material used in making the clasp LENGTH OF CLASP ARM The longer the clasp arm, the more flexible it will be, all other factors being equal. The length of cast clasp arm is measured form COMPONENTS OF A CLASP ASSEMBLY the point of which a uniform taper begins Minor Connector: from which the clasp The retentive circumferential clasp arm should components originate be tapered uniformly from its point of origin Principal Rest: designed to direct stress along through the full length of the clasp arm the long axis of the tooth Dimensions at the tip are about half those at the Retentive Arm: engaging a tooth undercut. For point of attachment. most clasps, the retentive region is only at its The Bar clasp is longer than a circumferential terminal end. clasp, its flexibility will be less because its Non Retentive Arm: on the other side of the halfround form lies in several planes, this tooth for stabilization and reciprocation against prevents its flexibility from being proportionate horizontal movement of the prosthesis. Rigidity to its total length of this clasp is essential mimi | 8  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 2: DIRECT RETAINERS 4th Year | 2024-2025 | MIDTERMS MATERIAL USED FOR THE CLASP ARM Clasp Flexibility Material 1. Casted Clasp – less flexible 2. Wrought wire o greater tensile strength o flexibility without fatigue 3. Chromium cobalt alloys o greater rigidity with less bulk DIAMETER OF CLASP ARM The greater the average diameter of a clasp arm is, the less flexible it will be If its taper is absolutely uniform, the average diameter will be at a point midway between its origin and its terminal end. If the taper is not uniform, a point of flexure (a point of weakness) will exist, this will then be the determining factor in its flexibility, regardless of the average diameter of its entire length. (Flexure is the action of bending or curving. ) RECIPROCAL ARM FUNCTIONS ** Diameter is inversely proportional and with 1. Intended to resist tooth movements in uniform taper. response to the retainer / retentive arm, CROSS-SECTIONAL FORM OF THE CLASP ARM deforming as it engages a tooth height of The only universally flexible form is the round contour form, which is practically impossible to obtain by 2. Stabilize the denture against horizontal casting and polishing movement. Horizontal forces applied on one side of the dental arch are resized by the Half round form – flexibility is limited to one stabilizing components on the opposite side direction providing cross-arch stability Round form – 2 directions 3. It may act, to a minor degree, as an indirect Cast retentive clasp arms are more acceptable in retainer tooth supported partial dentures in which they STABILIZING / RECIPROCAL CASTED CLASP ARM are called on to flex only during placement and PROPERTIES removal of the prosthesis. Should be rigid A retentive clasp arm on an abutment adjacent Its average diameter must be greater than the to a distal extension base not only must flex average diameter of the opposing retentive arm during placement but also must be capable of A casted retentive arm is tapered in two flexing during functional movement of the distal dimensions, whereas a reciprocal arm should be extension base. tapered by one dimension only A round clasp is the only circumferential clasp form that may be safely used to engage a tooth undercut on the side of an abutment tooth away from the distal extension base, to avoid transmission of tipping stresses to the abutment tooth mimi | 9  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 3: DIRECT RETAINER SELECTION 4th Year | 2024-2025 | PRELIM DIRECT RETAINER SELECTION The retentive tip contacts the tooth from the undercut to the height of contour CLASS I AND II (TOOTH AND TISSUE BONE) This area of contact provides stabilization Stress releasing direct retainers through encirclement Abutments periodically involved The horizontal portion of the approach arm Displaceable mucosal support must be located at least 4mm from the gingival Extensive edentulous space margin and even farther if possible. CLASS III AND IV (TOOTH-BORNE) Non-stress releasing direct retainer STRESS RELEASING DIRECT RETAINERS FOR TOOTH AND TISSUE BORNE RPD CLASSIFICATION Denture base moves toward the tissue in function Rotation around rests 2 STRATEGIES ARE ADOPTED TO EITHER: 1. Change the fulcrum location and subsequently the “resistance arm” engaging R – rest (always mesial) effect (mesial rest concept) P – proximal plate (distal) 2. Use a flexible arm (wrought-wire retentive I-Bar – (buccal) arm) BAR CLASP REATAINER SELECTON Also known as Gingivally approaching clasp or RPI Roach clasp or Vertical protection clasp RPA A clasp that extends from major connectors or from withing the denture base, the arms pass COMBINATION CLASP adjacent to the soft tissues and approach the RPI – REST, PROXIMAL PLATE AND I-BAR point of contact on the tooth in gingivo-occlusal Mesio-occlusal rest with the minor connector direction placed into the mesiolingual embrasure, but not Approaches undercut area or retentive area contacting the adjacent tooth from gingival direction A distal guiding plane, extending from the Push type kind of retention marginal ridge to the junction of the middle and Should be strong flexible arm or approach arm. gingival thirds of the abutment tooth, is Tissue side of the arm should be polished well at prepared to receive a proximal plate is contacts the mucosa The buccolingual width of the guiding plane is Approach arm tapers uniformly from determined by the proximal contour of the attachment to the clasp terminal tooth It never bridges the soft tissue undercut as there The proximal plate, in conjunction with the can be chances of food impaction minor connector supporting the rest, provides Approach and should cross the gingival margin stabilizing and reciprocal aspects of the clasp at 90 degrees assembly It is placed as low as possible on the tooth The I-bar should be located in the gingival third surface and the clasp terminal should engage of the buccal or labial surface of the abutment in with the distal undercut a 0.01 inch undercut The whole arm of the I-bar should be tapered to it’s terminus (end) with no more than 2mm of its tip contacting abutment alliah | 10  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 3: DIRECT RETAINER SELECTION 4th Year | 2024-2025 | PRELIM TYPES OF BAR CLASP 1. T – Clasp Retention arm resembles T-shape 5. I-Bar Indicated when undercut is on the Distal Modification of I-clasp side and the Non-Retentive area is Has a mesial rest and I-bar above the height of contour For vertical support and horizontal Both retentive terminals face towards stability the occlusal surface Contact should be at passive area CONTRAINDICATIONS FOR A BAR-TYPE CLASP Deep cervical undercut Extensive buccal or lingual tilt of abutment tooth Shallow buccal or labial vestibule High frenum attachments 2. Modified T-Clasp Large soft tissue undercut (will cause food Non-Retentive terminal is absent impaction) Aesthetically preffered Disto-buccal undercut is less than 180° Compromises stability encirclement RPA – REST, PROXIMAL PLATE, AKER’S ARM Similar to RPI except that the suprabulge wrought wire is used instead of I-bar Used where infrabulge approach is not possible Indicated when a bar type clasp is contraindicated and a desirable undercut is 3. Y-Clasp located in the gingival third of the tooth, away Same considerations like T-clasp from the extension base area. Height of contour is located high in mesial and distal surface and the center area is low or deep R – rest (always mesial) P – proximal plate (distal) A – Aker’s retentive arm (always wrought wire) 4. I-Clasp Placed on the distobuccal surface of maxillary canine The tip contacts just 2-3 mm to the area But compromises stability alliah | 11  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 3: DIRECT RETAINER SELECTION 4th Year | 2024-2025 | PRELIM COMBINATION CLASP DISADVANTAGES OF COMBINATION CLASP Involves extra steps in fabrication, particularly when high-fusing chromium alloys are used May be distorted by careless handling on the part of the patient Because it is bent by hand, it may be less accurately adopted to the tooth and therefore provide less stabilization in the suprabulge Wrought wire retentive clasp arm and cast portion reciprocal clasp arm May distort with function and not engage the Bracing and retentive arms originate from Distal tooth rest Guiding plane must not run entire NON-STRESS RELEASING RETAINERS occlusogingival height REST PLACEMENT: TOOTH-BORNE RPD ADVANTAGES OF COMBINATION CLASP Adjacent to Edentulous Space Flexibility Most effective placement of support Adjustability Ease of preparation Appearance Reduces minor connectors Minimum tooth covered Very rare exceptions Less fatigue failure RETAINER SELECTION: INDICATIONS OF COMBINATION CLASP Minimal rotation: The most common use of the combination clasp Cast Circumferential is on an abutment tooth adjacent to distal Ring clasp extension base where only mesial undercut Embrasure clasp (Double Aker’s) exists on the abutment or where a large tissue Reverse action (C – clasp) undercut contraindicates a bar-type retainer It is used when maximum flexibility is desirable, such as an abutment tooth adjacent to distal I. CAST CIRCUMFERENCIAL (AKER’S) extension base or on a weak abutment when a bar-type retainer is contraindicated May be used for its adjustability when precise retentive requirements are unpredictable and later adjustment to increase or decrease may be necessary Esthetic advantage over cast clasps. Wrought in structure, it may be used in smaller diameters than a casted clasp with less danger of fracture Recommended for anterior abutment of The most universally employed of all clasp posterior modification space in Kennedy Class II designs (clasp of choice) classification, where only a mesiobuccal A double arms clasp, relatively rigid undercut exists, to minimize the effects of first Retentive and bracing arms that originates from class lever system. rest alliah | 12  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 3: DIRECT RETAINER SELECTION 4th Year | 2024-2025 | PRELIM ADVATAGES OF AKER’S CLASP II. RING CLASP For tilted abutments Simple to construct Usually mesially and lingually tilted mandibular molars (with mesiolingual undercut) or mesially Hygienic and bucally tilted maxillary molars ( with Excellent stabilization and bracing mesiobuccal undercut) DISADVANTAGES OF AKER’S Undercut on same side as the rest ( adjacent to Less esthetic than bar clasps edentulous span) Increase the width of the occlusal surface of Supporting strut and auxillary rest (resist some teeth flexure) More difficult to adjust than wrought wire Excellent bracing IMPROPER DESIGNS OF CIRCUMFERENCIAL CLASP DISADVANTAGES OF RING CLASP Poor hygiene Very difficult to adjust Contraindicated with excessive tissue undercuts Use a cast circumferential clasp with lingual retention and buccal bracing, in preference to a ring clasp whenever possible, unless a severe tilt of the tooth will not permit, and cannot be approached with a bar clasp arm because of lingual inclination of the tooth alliah | 13  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 3: DIRECT RETAINER SELECTION 4th Year | 2024-2025 | PRELIM III. EMBRASURE CLASP COMPARISON STRESS RELEASING DIRECT RETAINERS Mesial Rest Concept Rotation: retentive tip, proximal plate Move mostly down (and forward) Into more undercut (release of tooth) NON-STRESS RELEASING DIRECT RETAINERS Distal Rest Concept Rotation : retentive tip, proximal plate Move mostly forward (tip rotates up) Toward height of contour (activate or bind) Long guiding planes (not advisable) Unmodified Class II or Class III RPD where no edentulous spaces are available on the opposite side of the arch to aid in clasping Two rests, two retentive arms, two bracing arms bilaterally or diagonally opposed Requires extensive preparation as the contact area should be eliminated entirely Rests must be positive to prevent wedging In deciding to use this clasp, age, caries index and oral hygiene should be considered Abutment protection with inlays or crowns is recommended IV. REVERSE ACTION CLASP (C-CLASP/HAIRPIN/FISH HOOK) Undercut adjacent to an edentulous space Almost impossible to adjust Poor esthetics especially on anterior abutment Poor maintenance on hygiene Clearance from opposing occlusion may be a problem Limited flexibility (esp. Short crowns) alliah | 14  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 4: INDIRECT RETAINERS 4th Year | 2024-2025 | MIDTERM These lateral stresses are the most damaging INDIRECT RETAINERS The component of a removable partial denture that assists the direct retainer in preventing base by functioning through lever action on the opposite side of the fulcrum line when the denture base moves away from the tissues in pure rotation around the fulcrum line TORSIONAL STRESS FULCRUM LINE It is a twisting rotational type of force. It’s a An imaginary line connecting occlusal rests, combination of vertical and horizontal force. around which a partial removable dental prosthesis tend to rotate under masticatory forces The determinants for the fulcrum line are usually the cross arch occlusal rests located adjacent to the tissue borne components ROTATION OF DENTURE BASES AROUND VARIOUS AXES VERTICAL STRESS 1. Displacing stresses: Those forces which are the result of downward stresses along the long axis of the teeth in a crown to apex direction and the relatively vertical stresses on the ridge mucosa. 2. Dislodging stresses These are the forces which tend to lift the partial denture from the rest position HORIZONTAL STRESS They originate as a component of rhythmic chewing stroke. These forces are effective in mesio-distal and buccolingual direction. HIKUN - G | 15  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 4: INDIRECT RETAINERS 4th Year | 2024-2025 | MIDTERM INCLINED PLANE Forces against an inclined plane may result in deflection of that which is applying the forces or may result in movement to the inclined plane, neither of these is desirable. These rotational movements are resisted by the stabilizing components such as the reciprocal or bracing arm Also by the clasp arms and minor connectors that are in contact with the vertical tooth surface SNOWSHOE PRINCIPLE MECHANICAL PRINCIPLES APPLICABLE IN RPD This principle is based on distribution of forces Lever Principle to as large an area as possible. Inclined plane Like in a snowshoe which is designed to Snowshoe principle distribute forces on the entire base area of the L beam effect shoe, a partial denture should cover maximum area possible within the physiologic limits so as LEVER to distribute the forces over a larger area. A simple machine consisting of a rigid bar pivoted on a fixed point and used to transmit force, as in raising or moving a weight at one end by pushing on the other. These classes of levers (base on location fulcrum, resistance and direction of effort (force) o CLASS I L BEAM EFFECT This principle is applicable to the antero posterior palatal bar or strap major connector. In this component there are two bars /strap lying perpendicular to each other. The anterior and posterior bars are joined by flat longitudinal elements on each side of the lateral slopes of the palate. o CLASS II The two bars lying in two different planes produce a structurally strong L beam effect that gives excellent rigidity to the prosthesis. o CLASS III HIKUN - G | 16  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 4: INDIRECT RETAINERS 4th Year | 2024-2025 | MIDTERM PRINCIPLE OF INDIRECT RETAINER forces to the edentulous ridge in a more nearly vertical direction. Rotational movement away from the tissue or ridge, An indirect retainer consists of one or more rests The tip of the retentive clasp will act as fulcrum and the supporting minor connectors. thus lifting the distal extension base The proximal plates adjacent to the edentulous areas also provide indirect retention An indirect retainer should be placed as far from the distal extension base as possible Most effective location of an indirect retainer is commonly in the vicinity of an incisor tooth But incisor tooth may not be strong enough and may have steep inclines, not favourable to support a rest In such situation, nearest canine or mesio Sticky foods, tissues adjacent to the borders of occlusal surface of the first premolar – BEST the denture, such as tongue or buccinator LOCATION muscle may lift the denture base from the ridge Two indirect retainers closer to the fulcrum line when they are activated by speech chewing or are used swallowing Gravity exerts an unseating force on maxillary AUXILLARY FUNCTIONS OF INDIRECT RETAINERS prosthesis. 1. It tends to reduce anteroposterior tilting leverages If an additional rest is introduced anteriorly then on the principal abutments, this is particularly it will act as a fulcrum thus converting the Class important when an isolated tooth is being used as an I lever to Class II lever situation. abutment Now when the distal extension base is lifted the Proximal contact with the adjacent tooth auxillary rest will prevent this movement as it prevents such tilting of an abutment as the base will enhance the retention properly of the clasp. lifts away from the tissues. 2. Contact of its minor connector with axial tooth surfaces Contributes to support and stability of the RPD Counteracts horizontal forces applied to the denture 3. Anterior teeth supporting indirect retainers are stabilized against the lingual movements 4. It may act as an auxillary rest to support a portion of major connector facilitating stress distribution. As the rest moved anteriorly (MO), it moves the Ex: Lingual bar point of rotation anteriorly as compared to the Long span mandibular lingual bar major conventionally disto-occlusal rest. It also transits connector Indirect Retainers provides additional support and rigidity for lingual bar HIKUN - G | 17  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 4: INDIRECT RETAINERS 4th Year | 2024-2025 | MIDTERM Prevents impingement of the lingual bar on the DISTANCE FROM THE FULCRUM LINE mucosa during function. LOCATION OF THE FULCRUM 5. Provide the first visual indications for the need to reline an extension base partial denture. Deficiencies in basal seat support are manifested by the dislodgement of indirect retainers from their prepared rest seats when the denture base is depressed, and rotation occurs around the fulcrum. In Class I arch, fulcrum line passes through most The Indirect retainer also acts as a third point of posterior abutments contacting tooth structure to ensure accurate. In Class I Mod 1 arch, fulcrum line is diagonal, repositioning of the framework on the teeth passing through the abutment on distal during relining and rebasing. extension side and most posterior abutment on Occlusal rest on the terminal abutment teeth opposite side. acts as the other two of the three points of the tripod FACTORS INFLUENCING EFFECTIVENESS OF IR The principal occlusal rests on the primary abutment teeth must be reasonably held in their seats by the retentive arms of the direct If the abutment tooth anterior to the retainers, otherwise, the partial denture will be modification space lies far enough, it may be dislodged used for Indirect retainer In all Class I and Class II cases, the rest seat preparation must be saucer shaped, completely devoid of any sharp angles or ledges As forces are applied, the rests must be free to move within the rest seat prepared to dissipate the force, which would otherwise be transferred to the tooth. For better action of the Indirect Retainers, the Class IV arch fulcrum line passes through distance from it to the primary rest should be abutment adjacent to single edentulous space more than the distal extension base. Class III arch with posterior tooth on the right side (poor prognosis) The efficiency of an IR is affected by the relative lengths of the distance from the end of the denture base to the clasp (AB) and from end of the rest to the clasp (BC) HOW FAR BEYOND THE FULCRUM LINE THE INDIRECT RETAINER IS PLACED? The greater the distance between the fulcrum line and IR, the more effective, because the HIKUN - G | 18  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 4: INDIRECT RETAINERS 4th Year | 2024-2025 | MIDTERM mechanical advantage increases proportionately Most commonly used IR In a Class I mandibular arch, the location is usually mesial marginal ridge of the first ADVANTAGES OF MESIAL RESTS IN RPD premolar In distal rest, when distal extension move Bilateral rests on the first premolar is effective towards the tissue, the distal rest pulls over the tooth, whereas in the mesial case, it allows the distal extension base to move more towards the tissue and clasp to move more towards the retentive are of the tooth, thus applying less stress to the abutment tooth In Maxillary Class I, same principle is used Weaker single rooted teeth is not used Interference with tongue is less, minor connector is placed in the embrasure between the canine and premolar OTHER IMPORTANT THINGS TO CONSIDER IN IR Rigidity - All connectors must be rigid if the IR is to function as intended Effectiveness of supporting tooth surface - The IR must be placed on a definite rest seat on which slippage or tooth movement will occur Quality of support of IR – An indirect retainers effectiveness is reported to decrease when it is placed on an inclined plane because the tooth For Class II – Indirect retainers are placed on a moves and the retainer slips marginal ridge of the first premolar tooth on the opposite side of the arch from the distal Basis in choosing IR location extension base 1. Abutment tooth support 2. A crown from favoring adequate rest seat 2. CANINE REST preparation Used when premolar is too close to the fulcrum 3. Patients’ esthetic desires line Teeth are overlapped so that the fulcrum line is FORMS OF INDIRECT RETAINERS not accessible 1. AUXILIARY OCCLUSAL REST Canine rest may be made more effective by Consists of a minor connector and a rest placed placing the minor connector in the embrasure in rest seat anterior to the canine. HIKUN - G | 19  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 4: INDIRECT RETAINERS 4th Year | 2024-2025 | MIDTERM The closer to the centre of rotation, the more esthetic and less prone to breakage Rest seat is V shaped and half moon shaped in configuration 5. CINGULUM BAR (CONTINUOUS BAR) AND LINGUOPLATE They rest on unprepared lingual inclines. Aid the terminal rest in indirect retention 3. CANINE EXTENSIONS FROM OCCLUSAL REST Serves as splints for weak anterior teeth Effectively achieve IR retention by increasing the distance from the fulcrum line. Applicable when a first premolar must serve as a primary abutment Should always be used with terminal rests on the mesial marginal ridge of the premolar tooth In class I and II RPD especially, a continuous bar 4. LINGUAL REST retainer or the superior border of the Lingual rests are primarily used on Maxillary linguoplate should never be placed above the canines. If a canine is not present, multiple rests middle third of the teeth – therefore, are spread over several incisor teeth instead of orthodontic movement is prevented a single tooth Lingual rest seat preparations on enamel should 6. RUGAE AREA never be attempted on mandibular anterior Coverage of rugae area serves indirect retention teeth. This is because of the lack of thickness of – the rugae area is thought to be firm and will enamel. However, a lingual rest on cast also help in providing support restoration may be used on any anterior tooth, Not as effective as positive tooth support either maxillary or mandibular The use of rugae support for IR is usually part of Lingual or Cingulum rest is preferred over incisal palatal horseshoe design because posterior rest because of its mechanical advantage retention is inadequate Incisal rest represents a long lever arm that concentrates stresses transmitted to RPD on incisal edge of the abutment teeth This magnifies forces beyond that would take place if lingual reset were used HIKUN - G | 20  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 5: DENTURE BASE CONSIDERATION 4th Year | 2024-2025 | MIDTERM DENTURE BASE CONSIDERATION As the abutment gets farther, support from the A DENTURE BASE is that part of the RPD which underlying ridge tissues becomes increasingly rests on the oral mucosa or the foundation important tissues, and to which teeth are attached. Maximum support from residual ridge – using broad, accurate denture base, the occlusal load spreads equitably throughout the support area. MAXIMUM SUPPORT ACHIEVEABLE BY: Using knowledge of the limiting anatomic structures Histologic nature of the basal seat areas Accuracy of impression Accuracy of the denture base Support is the primary consideration, esthetics is FUNCTIONS OF DENTURE BASE IN THE CONTROL OF secondary PROSTHESIS MOVEMENT Main Function – Masticatory EXTENT OF A MAXILLARY DENTURE BASE Supports the artificial teeth Cover the tuberosity and the hamular notches Transfers the functional forces to the supporting The palatal border should terminate on the oral structures tissue that is resilient Cosmetic effect during replacement Anterior border of the buccal flange should be Stimulation of the underlying tissues of the tapered posteriorly residual ridge Should be concave to provide a grip for the buccinator muscle FOR ANTERIOR REPLACEMENT (CLASS IV) Peripheral borders – 2mm, thick, rounded and Provide desirable esthetics polished Supports and retains the artificial teeth – provides masticatory efficiency and assist in EXTENT OF THE MANDIBULAR BASE transferring occlusal forces to the abutment Cover the retromolar pads, extend laterally and teeth include the buccal shelf Prevents horizontal and vertical migration of the Distolingual border should extend vertically remaining natural teeth downward from the top of the retromolar pad Eliminates undesirable food traps into the alveololingual sulcus Stimulates the underlying tissues Vertical extension depends on anatomy of the mylohyoid ridge TOOTH SUPPORTED RPD BASE Peripheral borders of the buccal and labial Denture base-span between two abutments flanges should be extended into the vestibules supporting artificial teeth Occlusal forces – abutments through rests RETENTION Prevents horizontal migration of all the Primary: accomplished mechanically by placing abutment teeth in partially edentulous arch retaining elements on the abutment teeth Prevents vertical migration of teeth in opposing Secondary: provided by the intimate arch relationship of denture bases and major Relining and rebasing – not necessary due to the connectors with the underlying tissues rests placed at each abutment tooth end. Facilitated by: 1. Adhesion – the attraction of saliva to the DISTAL EXTENSION RPD BASE denture and tissues GOAL: minimizing functional movement and 2. Cohesion – the attraction of molecules of saliva improving stability of the prosthesis. Critical to to each other this goal is the non-support modification. 3. Atmospheric Pressure – dependent on a border Improves stability of the prosthesis seal and results in partial vacuum beneath the denture base when a dislodging force is applied annie | 21  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 5: DENTURE BASE CONSIDERATION 4th Year | 2024-2025 | MIDTERM 4. Physiologic molding of the tissues around the Metal base provides an intimacy of contact that polished surfaces of the denture contributes to a direct retention 5. Effects of gravity on the denture - Interfacial surface tension Adhesion and cohesion are effective when there - intimacy of contact cannot be achieved by is perfect apposition of the impressioned surface acrylic resin based and so may be of the denture to the mucous membrane jeopardized further by poor cleaning habits surfaces II. COMPARATIVE TISSUE RESPONSE METHODS OF ATTACHING DENTURE BASES Inherent cleanliness of the cast metal base Acrylic resins are attached to the partial denture contributes to the health of oral tissues when base framework by means of a minor connector compared with acrylic resin base Needs at least 20 gauge thickness relief, for Greater dentisty sufficient space to create a raised platform on Bacteriostatic activity contributed by ionization the investment cast and oxidation of the metal base Retentive framework – 1.5mm for sufficient Acrylic resin base tend to accumulate musinous thickness of resin, to avoid weakness or fracture deposits from food, as well as calcular deposits of acrylic base Calcular deposits do not accumulate as they do Open patterns are better than mesh patterns in acrylic bases, therefore it is cleaner. *Mesh patters are rigid but the bulk may III. THERMAL CONDUCTIVITY contribute to the weakening of the acrylic base Temperature changes are transmitted through IDEAL DB MATERIAL REQUIREMENTS the metal base to the underlying tissues – this 1. Accuracy of adaptation to the tissues, with maintains the health of these tissues minimal volume change Freedom of interchange of temperature 2. Dense, non-irritating surface capable of between the tissues covered and the receiving and maintaining good finish surrounding external influences: temperature of 3. Thermal conductivity liquids, solid foods, inspired air, contributes to 4. Low specific quality; lightweight in the mouth the patient’s acceptance of denture 5. Sufficient strength; resistance to fracture or May help avoid the feeling of the presence of a distortion foreign body 6. Easily kept clean IV. WEIGHT AND BULK 7. Esthetic acceptability Maybe casted thinner than acrylic resin 8. Potential for future relining 9. Low initial cost Gold must be more bulky; Chrome and Titanium have lesser weight and bulk ADVANTAGES OF METAL BASES Extreme loss of residual alveolar bone may need I. Accuracy and Permanence of Form bulkier denture base to restore normal facial II. Comparative Tissue Response contour and fill out the buccal vestibule to III. Thermal Conductivity prevent food entrapment IV. Weight and Bulk In the maxillary arch, an acrylic resin may be better to provide fullness in the buccal flanges I. ACCURACY AND PERMANENCE OF FORM For mandibular, to provide room for tongue and Cast metal bases – gold, chrome, or titanium cheek, the thinness of a metal casted base is alloys more favorable More accurate casting than resins Lingual surfaces – more concave (except in distal Can maintain accuracy of form palatal area) Internal strains that cause distortion are absent Buccal surfaces – more convex at gingival Modern cast alloys are more superior than the margins, over root prominences and other areas most superior resin processing that needs to be covered Distortion of acrylic resin base can manifest in - convexity will prevent food entrapment in maxillary RPD the cheek, but cannot be accomplished using metal bases. annie | 22  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 5: DENTURE BASE CONSIDERATION 4th Year | 2024-2025 | MIDTERM Denture bases may be designed to provide CAST METAL PONTICS almost total metallic coverage, yet the have Used where inter-arch space is extremely resin borders to avoid display of metal and to limited or where drifting of the remaining teeth add buccal fullness has resulted in a space that is too narrow for the placement of a denture tooth METHODS OF ATTACHING ARTIFICIAL TEETH Made from the same metal that forms the ANTERIOR TEETH remainder of the partial denture framework Acrylic resin or porcelain teeth on an acrylic Veneer of acrylic resin or composite resin may denture base also be used to improve esthetics Reinforced acrylic pontics STRESS BREAKERS (STRESS EQUALIZERS) POSTERIOR TEETH In distal extension cases – use of a rigid Acrylic resin, composite resin or porcelain teeth connection between the denture base and the Tube teeth supporting tissue must account for base Metal pontics movement without causing either tooth or tissue damage ACRYLIC OR PORCELAIN TEETH ON ACRYLIC RESIN Functional basing BASE Broad coverage Acrylic base and composite resin teeth display Harmonious occlusion wear characteristics that make them compatible Direct retainers to the natural teeth The concept of stress breaking insists on Easy to reshape separating the action of the retaining elements Porcelain teeth can cause wearing off of from the movement of the denture base by opposing natural tooth – mechanical abrasion allowing independent movement of the denture Difficult to adjust and susceptible to fracture base (or its supporting framework) and the Limitations: direct retainers Does not work well for single tooth replacement This form of stress breaker or equalizer – means Resin Denture Base requires additional bulk for to compensate for inappropriately design RPD adequate strength Regardless of their design, most of them dissipate vertical stresses which is the purpose REINFORCED ACRYLIC PONTICS for which they are used Exceptionally strong and aesthetics Consists of acrylic resin tooth attached to a NEED FOR RELINING centrally located metal reinforcing strut that Distal extension base is different from tooth projects from the RPD supported base, it should be made of a material Can be designed so that occlusion is confined to that can be relined or rebased when it becomes the acrylic resin portion of the tooth necessary to re-establish tissue support for the Can be used in areas restricted restorative space distal extension base Limitations: Acrylic denture base materials that can be Requires well healed and favorably contoured relined are generally used residual edentulous ridge Metal bases are difficult to reline, hence it is Difficult to reline limited to stable ridges that will change little over a long period TUBE TEETH Loss of support – distal extension cases may not It consists of a resin or porcelain denture tooth be visible, but manifestation can be assessed prepared by drilling a vertical channel from the over time ridge lap area towards the occlusal surface Loss of occlusion between distal extension base Frequently used for posterior tooth replacement and opposing dentition, resulting in heavy Intimate contact is established between the occlusal contact between the remaining natural gingival aspect and the soft tissues of the ridge. teeth. It indicates that relining is need to reestablish the original occlusion by annie | 23  PROSTHODONTICS 2 (RPD) - LECTURE Lecture 5: DENTURE BASE CONSIDERATION 4th Year | 2024-2025 | MIDTERM reestablishing supporting contact with the residual ridge. Sound manifestion of change is evidence of rotation around the fulcrum with the indirect retainer lifting from their seat and the base is pressed against the tissue After changes in the ridge form, which cause some loss of support, rotation occurs around the fulcrum line when alternating finger pressure is applied If occlusal contact has been lost and rotation around the fulcrum line is evident annie | 24