Primary retention : This is mechanical in action. Eg..Direct retainers Secondary Retention: Achieved by intimate relationship of the denture base, minor connector with the guiding planes and the rigid major connector to the underlying soft tissues.
The components of aremovable partial denturethat engage abutments andresist dislodging forces arecalled direct retainers.
1.Intra-coronal As its name implies, an intracoronal direct retainer resides within the normal contours of an abutment and functions to retain and stabilize a removable partial denture.2.Extra-coronalOutside the contour of the crown of the tooth e.g. clasp
All clasps must be designed so that they satisfy the following six basic requirements: 1.Retention 2.Support 3.Stability 4.Reciprocation 5.Encirclement 6.Passivity
It represents the greatest axial diameter of the clinical crown when viewed along the proposed path of insertion. Protheros cone theory. Conceptually, the axial contour of an abutment resembles two cones sharing a common base. The line formed at the junction of the two cones represents the height of contour.
Basic parts of a clasp assembly rest Retentive arm Body of the clasp Retentive terminalMinor connector Reciprocal arm
This clasp is usually the most logical to use with all tooth borne partial denture, because of its better retentive and stabilizing ability. Basic design of the clasp is a buccal and lingual arm originating from a common body.
Advantages :1) The clasp fulfills the requirement of support,stability reciprocation, encirclement & passivitybetter than any other type of clasp.2) It is easy to construct.3) It is simple to repair.Disadvantages :1. It tends to increase the circumference of the crown.2. It is not acceptable in the anterior region.3. It covers more tooth surface then the bar clasp and prone to caries.
This clasp is essentially two single circumferential clasps joined at the body. Used in Kennedy class II, III, IV cases where no edentulous space on opposite side of the arch. Contact area should not be eliminated completely. This clasp should be used with double occlusal rest, two retentive clasp arms and two reciprocal arms Abutment tooth should be protected with crowns or inlays if necessary. This depends upon the age of the patient, caries index and oral hygiene.
Disadvantages: Needs adequate cleanliness in occlusal surface. Breakage may occur if inadequate preparation and clearance Wedging action can be seen.
Indication:1. In tilted molar2. Single standing abutment3. When a proximal undercut cannot be approached by any other means. This ring clasp permits engagement of this undercut by encircling almost the entire tooth from its point of origin. The entire clasp except the retentive terminal shall be placed above the height of the contour. An additional occlusal rest can be placed may provide additional support.
•This type of clasp consistsof a wrought wireretentive clasp arm and acast reciprocal clasp arm.•The tapered wrought wireretentive arm offersgreater flexibility thandoes the cast clasp armand therefore betterdissipates functionalstresses(stress breakingeffect)
Advantages : Flexibility Adjustability Estheticappearance A minimum of tooth surface is coveredDisadvantages: 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 the patient.
Bar clasps /roach claspsThe bar clasp arm is classified by the shape of the retentive terminal into: T-clasp Modified T-clasp Y-clasp I-clasp RPI concept
Bar clasp originate from denture framework or a metal base and approaches the retentive undercut from a gingival direction.it provides push type retention which is more effective than the pull type retention of a circumferential 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.A bar clasp usually engages .02 inch undercut.4.The Approach arm should cross the gingival margin at 90° angle to avoid irritation.
1.Greater retention than occlusally approaching clasps.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).
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.
Kratochvil in 1963 developed the early clasp assembly consisting of mesial occlusal rest, a distal guide plate and an I- bar retainer.Krol in 1973 made certain modifications in the design of the proximal plate and named it the RPI bar clasp design.
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. 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.
“The component of a removable partial denture that assists the direct retainer(s) in preventing displacement of the distal extension denture 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.”-GPT
Fulcrumline: is a imaginary line around which a RPD tends to rotate under masticatory force.
An indirect retainer does not control the movement of the denture base in the tissueward directionAmount of movement =amount of compressible mucosaFulcrum line runs between the most posterior rests.
Movement away from ridge Sticky foods Functions of tissues at the borders Gravity (maxillary denture)Fulcrum line runs through retentive tip of direct retainer
IRs must be placed as far as possible from distal extension base ,for better leverage advantage Most effective location for IRs is incisor region ,but due to their poor strength canines or premolars are used ideally IR should be located at a point perpendicular to the midpoint of the fulcrum line IR should be placed on a definite rest seat .tooth inclines and weak teeth should not be used for support
Auxillaryocclusal rests Canine extensions from occlusal rests Cingulum rests Continoius bar retainers and Linguoplate major connector Rughae area
Auxillary Occlusal Rest Consists of a minor connecter and a rest placed in a rest seat Class I arch –bilateral rests on mesial marginal ridge of 1st premolar
Consists of a finger extension from premolar rest to the lingual slope of adjacent canine Applied when first premolar is the primary abutment
Terminal rests are actual IRs A continuous bar retainer or superior border of the linguo-plate should never be placed above the middle third of the teeth to avoid tooth movement(Kennedy class1 & 2) As they rest on unprepared lingual surfaces they indirectly provide indirect retention
-Rugae area is firm hence used for indirect retention esp for class I situations -less effective than a tooth supported IRNot as effective as positive tooth support.Usually as palatal arms, palatal bearing connectors