Introduction Direct retainers Clasps I bar removable partial denture RPI
RPI stands for:R RestP Plate (proximal)I I bar clasp
It is that component part of a RPD that is used toretain and prevent dislodgement, consisting of aclasp assembly or a precision attachment – GPT TYPES OF DIRECT RETAINERS INTRACORONAL RETAINERS EXTRACORONAL RETAINERS -External attachments - Occlusally approaching ( Aker’s -Internal attachments clasp) -Stud attachments -Gingivally approaching ( bar or -Bar attachments roach type of clasp) -Special attachments
Types on the basis of designCircumferential or Akers Vertical projection or Bar orclasps: roach clasps:-Simple circlet - T clasp-Reverse circlet - Modified T clasp-Fish-hook or hairpin - Y clasp-Embrassure clasp - I clasp and I bar-Multiple circlet-Ring clasp-Commbination clasp
It is a special type of removable partial denture Designed by Kratochvil in 1963 To reduce tooth contact of the retainer. I-Bar clasp system: mesial rest, I-bar retainer and long guide plane.
I- Bar RPD Regular RPD1. Mesial rest instead of distal rest. In 1. Distal rest other words, the rest is on proximal side away from edentulous space.2. I- bar retainer is used for 2. Occlusally approaching direct retention. retainer is used3. Long guide planes 3. Normal guide planes extending into tooth tissue junction
I-Bar Partial denture components:I. RestII. Proximal platesIII. Major connectorsIV. Minor connectorsV. Denture base connectorsVI. Retainers
Function of resta) Provide vertical support against occlusal forces andb) Control the relationship of the prosthesis to supporting structures. In order to perform this function rests must be of Mesial Rest sufficient bulk to withstand direct occlusal force and also the indirect force that they are subjected to as fulcrum points.
Anterior rest must meet the basic requirements of:a) Strength & positiveness.b) Esthetic considerations. Ideal anterior rest crescent shaped cingulum rest provides maximum stabilization. Cingulum rest can be prepared directly in enamel on bulky canines and maxillary central Cingulum rest for both maxillary and incisors or can be implemented mandibular anterior with a cast restoration. teeth. The incisal rest is used on mandibular anterior teeth when esthetics allow.
Posterior rests designed to direct vertical forces along the long axis of tooth;a) Premolar rests are prepared in marginal triangular ridges, andb) Molar rests extend into the central fossa. The rests have a ball- shaped terminus that allows ball-and-socket movement around the axis of rotation.
In distal extension cases the most distal rests are placed on the mesial aspect of the abutment teeth for the following 2 reasons:1. Anterior placement of the rest (fulcrum) helps verticalize the forces of occlusion on bearing mucosa under the denture base extension.2. The mesial rest directs tipping forces on the abutment mesially and tends to move the abutment tooth into firm contact with the support of the anterior teeth.
Additional proximal plate on distal surface. The proximal plate covers the guide plane from marginal ridge to the tooth tissue junction and extends onto the attached gingiva for 2mm. a b a. Extention of proximal plate in conventional RPD b. Extension of proximal plate in an I-bar RPD
Advantages:i. Provides horizontal stability.ii. Reunites & stabilizes the archiii. ↑ retention because of parallelism and because dislodgement is limited to the path of insertion.iv. Protects the tooth-tissue junction by preventing food impaction and because of mental coverage in the areav. Provides good reciprocationvi. Distributes occlusal force throughout the arch.
Retention in most partial denture clasps is achieved with a retentive arm that engages an undercut on an abutment tooth. In the conventional sense the I-bar provides retention against vertical displacement, but this retention is augmented considerably by the parallelism of guide planes that, in most situations, limit displacement to the path of insertion. The I-bar is an infra-bulge retainer with a configuration designed to minimize the deleterious effect that over-contoured retainers have on the health of both tooth & gingiva.
The arm is long and tapering with a half-round cross- section. The tip which flexes, engages an undercut at the height of mesio-distal contour. Proper positioning of I-bar allows the tip to move passively into the mesial embrassure space when the extension base recieves occlusal loading. The I-Bar retainer should extend 2mm above the gingival margin to engage an undercut mesial to the The retainer engages the mesio-distal height of contour. undercut area & resists vertical displacement.
Advantages of I-bar configuration:1. Because tooth contour is not altered , food accumulation against the tooth surface is minimized.2. I-bar is passive in relation to the abutment tooth except against vertical displacing forces. Disadvantages:1. Reduced retention2. Less horizontal stability than other retentive elements.
Other parts Major connector Minor connectors Denture base connectors
Rest, Proximal plate and I bar Rest, Proximal plate and Aker’s clasp
RPI is a modified I-bar retainer system. Krol devised it in 1973. All the components of the I-bar assembly were modified significantly to fulfill Krol’s design. Principle of Krol’s design was – “Stress control with minimal tooth & gingival coverage”.
The speciality of these dentures is that the direct retainer is modified such that the retentive and reciprocal units of the clasp act in the mesio-distal direction ( in conventional design the retentive & reciprocal units act buccolingually). The advantage of adopting this design is the minimal tooth contact. Coverage produced by it. Hence, these dentures are more easy to maintain.
Modification of each component of I-bar retainer required to form RPI system MESIAL REST MODIFICATION PROXIMAL PLATE MODIFICATION I-BAR MODIFICATION.
The mesial rest extends only into the triangular fossa even in molar preparations. Conservative rest seat preparation The canine rests are limited to triangular fossa circular, concave depressions prepared on the mesial marginal ridge. It does not cover the entire marginal ridge. Conservative circular lingual rest used instead of the usual cingular canine rest
One of the major disadvantages of the I-bar denture was the excessive tooth contact produced by the extra long proximal plate. Krol introduces 3 modifications to the proximal plate. Any one of these modifications may be followed while designing an RPI retainer.
The 3 different design modifications of the proximal plate are:i. Design modification Iii. Design modification IIiii. Design modification III
Here the proximal plate is designed to extend from the marginal ridge to the junction between the middle & cervical third of tooth (remember in an I-bar retainer the proximal plate extends about 2mm across the tooth tissue junction).
Here the proximal plate is designed to extend along the entire length of the proximal surface of the abutment with a minimum tissue relief. A relief is provided near the gingival margin at tooth tissue junction. This allows the proximal plate to disengage into proximal undercut during occlusal overloading.
Here, the proximal plate is designed to contact just about 1 mmof the gingival third of the guiding plane of the abutment tooth. The purpose of reducing the length of the proximal plate is to improve the gingival health.
The tip of the I-bar is modified to have a pod-shape in order to allow more tooth contact. It is placed more mesially so that it shifts towards the mesial embrasure space under occlusal load and increases reciprocation. Pod shaped I-bar retainer of RPI system The I- bar is designed to compensate for the decreased encirclement (tooth contact), provided by the proximal plate & mesial rest.
Clinical removable partial prosthodontics – Stewart Rudd Kuebker McCracken’s removable partial prosthodontics - Alan B. Carr - Glen P. McGivney - David T brown Internet