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Rpi and rpa concept

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This document discusses the RPI and RPA concepts for removable partial dentures. It begins by defining direct retainers and clasp functions. It then describes the components and mechanics of the RPI system, including the occlusal rest, proximal plate, and I-bar retainer. The document discusses the indications, advantages, and disadvantages of the RPI system. It notes the RPI is designed to minimize stresses on abutment teeth during function. The summary concludes by stating the RPI system utilizes an I-bar retainer to provide retention and aesthetics while placing minimal metal on tooth surfaces.

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RPI and RPA concept By – Dr RASHIKA SINGH MDS IInd YEAR
CONTENTS • INTRODUCTION • DIFFERENCE BETWEEN I-BAR AND I-CLASP • RPI SYSTEM  COMPONENTS  ADVANTAGE AND DISADVANTAGE  LIMITATIONS  INDICATION AND CONTRAINDICATIONS  MODIFICATIONS • RPA CLASP • REFRENCES
Introduction • A wide variety of clasp assemblies are available for clinicians to use. • This variety exists because of the imagination of clinicians and technicians who provided prosthesis when tooth modification was not or could not be provided. • Clasps were designed to accommodate distal extension functional movement and also without movement . • These categories are not mutually exclusive because any clasp assembly is used to retain and maintain a well supported prosthesis.
• Direct retainer is any unit of a removable dental prosthesis that engages an abutment tooth or implant to resist displacement of the prosthesis away from basal seat tissue. • Types of direct retainer – Intracoronal - Precision attachment Extracoronal - Clasp type • Function of a clasp – support stabilization reciprocation retention movement of abutment tooth. • Clasp design – Circumferential clasp arm – approaches the retentive undercut from occlusal direction. Bar clasp arm – approaches the retentive undercut from cervical direction
I bar removable partial denture • The I-bar removable partial denture design, introduced by Kratochvil in 1963 has achieved considerable status as a treatment modality. • The mechanical approach proposed by Kratochvil in the design concept of the I-bar has encouraged others to apply sound engineering principles in challenging the design. • As a result, the design has been subjected to a thorough evaluation, & in the process, several modifications have been suggested.
Difference between I-bar and I-clasp I-CLASP The I-clasp is an extracoronal retainer that is occasionally used on the distobuccal surface of the maxillary canines for aesthetic reasons . There is definite danger involved in using this clasp because the only contact of the retentive clasp with the tooth is the tip of the clasp, an area of 2-3 mm. Encirclement & horizontal stabilization are compromised. There must always be a posterior abutment tooth for this clasp to be successful.
I-BAR The I-bar is a component part of a partial denture design philosophy, the rest, proximal plate, I-bar retentive clasp (RPI) concept. It differs from the I-clasp in position on the abutment tooth that retentive undercut is selected. I-bar retention is normally near the centre of the facial surface of the tooth or on the mesiobuccal side .
RPI system
RPI stands for: •occlusal rest (R) •distal guide plate (P) •gingivally approaching I bar (I) Mesial Distal
•The RPI is a current concept of bar clasp design that refers to the rest, proximal plate and I-bar component parts of the clasp assembly . •Clasp assembly consists of a mesio occlussal rest and minor connector placed in the mesio-lingual embrasure and not contacting the adjacent tooth. •A distal guiding plane extending from the marginal ridge to the junction of the middle and gingival third of the abutment tooth is prepared to receive the proximal plate
Bucco-lingual width of guiding plane determined by proximal contour of tooth. The proximal plate and minor connector supporting the rest provide the stabilizing and reciprocal action of the clasp assembly. I-bar located on the gingival third of buccal/labial surface of abutment in 0.001 inch undercut.Arm of I-bar tapered to its terminus with 2mm of tip contacting the tooth.
The retentive tip contacts the tooth from undercut to height of contour . Area of contact along rest and proximal plate provide stabilization through encirclement. Three basic approaches to the application of RPI system – • Location of rest . • Design of minor connector (proximal plate) as it relates to guiding plane . • Location of retentive arm.
Rests Function •Vertical support against occlusal forces •Control relationship of prosthesis to supporting structures Rests must be •Positive •Sufficient bulk to withstand direct occlusal forces and indirect forces since they are subjected to as fulcrum points in distal extension .
Distal occlusal rests •The gingival part of the denture base adjacent to the posterior abutment moves in an arc parallel to the mucosa. This provides no support from the mucosa near the tooth. Also, the soft tissue adjacent to the tooth may be pinched with resultant tissue strangulation.
•As the denture base is followed posteriorly, the arc of movement becomes more nearly perpendicular to the mucosal surface. •The placement of the rest distal to the central axis of the posterior tooth will tend to tip the tooth posteriorly. When movement of the edentulous base occurs, the force exerted on the tooth can be compared to a precision- constructed & fitted ‘wrench’ that tends to pull the tooth backwards . This can result in tooth mobility, bone loss & bodily movement of the tooth.
The distal extension acts as a long effort arm and distal rest as fulcrum to cause clasp tip or resistance arm to engage the tooth undercut. This results in tipping or torquing of the tooth, which is greater with stiff clasps and increased denture base movement. Two strategies adopted to change fulcrum location and subsequently resistance arm engaging effect (mesial rest concept clasp assembly) , or to minimize the effect of the lever through the use of a flexible arm (wrought wire retentive arm).
With mesial occlusal rest •The direction of movement of the denture base adjacent to the tooth is more nearly perpendicular to the mucosal surface & is less likely to cause a pinching of the tissue. •More posteriorly also, the movement of the base is near perpendicular to the mucosal surface in each region under the base.
•As a result of transferring the rest to the mesial surface, the denture movement will force the tooth anteriorly & reverse the ‘wrench’ effect . All remaining teeth will combine to help withstand this forward force, so this will increase support & bracing assistance from the teeth anterior to the posterior abutment tooth.
Rest preparations are less extensive in the RPI system. Always located on the mesial aspect of the primary abutment tooth adjacent to edentulous area. The mesial rest extends only in the triangular fossa, even in the molars, & canine rests are often circular concave depressions in the mesial marginal ridge or cingulum rests.
For distal extension base partial dentures, the mesial rest in the posterior teeth can be prepared in the appropriate triangular fossa with a No. 6 carbide bur or diamond stone. The marginal ridge must be lowered about 1.5 mm & the preparation must have the deepest portion in the centre of the triangular fossa. The preparation should be rounded & fully polished to permit some rotation when depression of the extension base occurs.
For cuspids that serve as abutments, a mesiolingual rest is made. The rest seat must be deep enough to prevent the metal rest from slipping gingivally. As a general rule, mandibular cuspids have a thin enamel & penetration into dentin is inevitable during rest seat preparation. If dentin is exposed, the preparation should be deepened & modified to accept an amalgam or other restoration which can be properly contoured.
Parallel guide planes are prepared on all proximal surfaces adjacent to the edentulous spaces. Proximal plate covers the guide plane from marginal ridge to the tooth - tissue junction and extends onto the attached gingiva for 2 mm . One approach recommends guiding plane and proximal plate should extend the entire length of proximal tooth surface with physiologic tissue relief eliminating impingement on free gingival margin . PROXIMAL PLATES
Second approach suggests guiding plane and proximal plate extends from marginal ride to junction of middle and gingival third on proximal surface .
•Third approach , the superior edge of the proximal plate is located close to the bottom of the prepared guide plane which should be at the junction of occlusal one-third & middle one- third of the tooth. •The proximal plate contacts only 1mm of the gingival portion of the guide plane while the remainder of the proximal plate lies below the guide plane.
•The proximal plate extends lingually just far enough so that the distance between the minor connector & the proximal plate is less than the mesiodistal width of the tooth. The distance between the proximal plate & the mesial minor connector should be no less than 5 mm. (prevent lingual migration of the tooth). •It should be 1 mm thick & join the framework at a right angle. At the junction with the framework, the proximal plate is relieved so as not to contact the gingiva & is highly polished. A finish line is placed at the base of the proximal plate to permit a butt joint with the resin base
Mouth preparation for guide planes • A guide plane is prepared on the distal surface of the abutment tooth at the occlusal one-third as proposed by Potter et al. • The guide plane should be approximately 2 to 3 mm high occlusogingivally. • This guide plane will often permit the proximal plate & the mesial minor connector to contact the tooth simultaneously & reciprocate the force exerted by the retentive buccal clasp arm during the seating & the removal of the denture.
The minor connector carrying the mesial rest contacts the mesiolingual surface of the abutment tooth and, together with the distal plate, acts as a reciprocal for the tip of the retentive clasp which is positioned on or anterior to the midpoint of the buccal surface of the tooth.
• If the mesial minor connector & the proximal plate cannot contact simultaneously, as may occur with cuspid abutments, then the retentive I-bar should engage the mesiobuccal undercut & receive its reciprocation from the proximal plate alone.
Proximal plate serves the following functions : •Provides horizontal stability. •Reunites & stabilizes the arch. •Increases retention because of parallelism & because dislodgement is limited to the path of insertion. •Protects the tooth-tissue junction by preventing food impaction & because of metal coverage in this area. •Control against ridge crest gingival hyperplasia. •Provides reciprocation. •Distributes occlusal force throughout the arch.
The proximal plate can also provide a certain degree of retention by mean of friction (without engaging an undercut). These plates are also a major component determining the path of insertion, and giving the clasps their effectiveness in the undercuts they engage.
I-BAR The I-bar is an extracoronal, infrabulge retainer with a configuration designed to minimize the deleterious effects that over contoured retainers have on the health of the tooth & the gingiva. The arm is long & tapering with a half round cross-section. Also called roach clasp arm.
•The approach arm of the I-bar extends from the framework so as to remain at least 3 mm from the gingival margin & then crosses the gingival margin at right angles. •Approximately 2mm of the I-bar contacts the tooth surface, usually at the gingival one-third of the tooth. • The bottom portion of the I-bar contacting the tooth surface should engage 0.01 inch undercut. • The I-bar should taper slightly from the base to the tip . •The I-bar is usually placed at the greatest mesiodistal prominence on the buccal surface or towards the mesial, but not towards the distal. • This is necessary to permit movement of the I-bar away from the entire buccal surface in function. • When the I-bar is placed towards the mesial, it has the advantage that when properly adjusted, it brings the proximal plate into tight contact with the distal surface
Shape of the retentive terminal maybe – T , modified T, I or Y If a T-clasp is used, the metal framework rotates around the mesial rest & the mesial part of the clasp moves forward & slightly upward. It loses contact with the tooth & causes no adverse forces. The distal part of the clasp moves forward & downward . This seems acceptable until an occlusal view of the region is observed . The distal part of the T-clasp wraps around the tooth. Then, when the distal part moves forward, it engages the distal curvature of the tooth & exerts a torque that is detrimental to the periodontium. The solution to the problem lies in the I-bar clasp with its retentive tip at the point of greatest curvature of the buccal surface of the tooth.
•The RPI system is designed to allow vertical rotation of a distal extension saddle into the denture-bearing mucosa under occlusal loading. •As the saddle is pressed into the denture- bearing mucosa, the denture rotates about a point close to the mesial rest. •Both the distal guide plate and the I bar move (downward & forward away from the tooth) and disengage from the tooth surface. Potentially harmful torque is thus avoided. •When trying in the metal framework, it is advisable to check that it is able to rotate about the abutment tooth in the intended fashion. If this is found not to be the case, the framework should be carefully adjusted to allow this rotation.
•This minimizes the denture caused stresses on the abutment teeth, without damaging the supporting structures of the abutment tooth particularly in cases with free end saddle(s). •If the fulcrum is moved to the distal surface of the tooth with the same clasping situation above, the movement of the clasp tip will be upward & forward . It will engage the tooth undercut & produce a torque on the tooth. •The RPI system is normally used with the “skeletal design” .The skeletal design has very limited coverage on the surfaces of the remaining natural teeth.
•The I bar is utilized to provide direct retention and better aesthetics because of its minute surface area coverage. •If the clasp crosses the gingival margin at a right angle (90°), then its interruption of that margin is confined to a very small distance (1- 2mm). •Added to that, the RPI system places very little metal parts of the denture on the abutment teeth surfaces. This keeps the gingival margins of the natural teeth exposed to the self cleansing actions of the tongue and saliva.
The mechanics of the RPI also aim to exert minimal amount of torquing (tilting) forces on the abutment teeth.
INDICATIONS • Small degree of undercut (0.01 inch) exists in cervical third of abutment tooth which can be approached gingivally. • Used in tooth supported partial denture or tooth supported modification area • Distal extension case. • Esthetics concern
ADVANTAGES OF THE I-BAR •Vertical forces on the distal extension base during function cause both the I-bar & the proximal plate to disengage the abutment & thereby reduce the torquing of the tooth. •The mesial minor connector with the proximal plate provides for reciprocation & eliminates the need for a lingual arm. •An important but seldom mentioned advantage of the RPI clasp is its avoidance of contact with the lingual surface of the abutment tooth. Without a lingual arm, the high survey line on the lingual surface of many mandibular teeth is not a problem, making this design useful for tooth supported as well as distal extension removable partial dentures.
• The mesial rest eliminates the potential ‘pump handle’ effect that a force on the base often induces with a distal rest. • The RPI clasp contacts the tooth minimally & is advantageously used on caries prone patients. • Usually more aesthetic than most other clasp arms.
DISADVANTAGES & CONTRAINDICATIONS OF THE I-BAR •Insufficient depth of the vestibule to permit the approach arm of the I-bar to be located at least 3 mm from gingival margin. •A tooth with severe lingual tilt & no buccal or labial tilt. •Tissue undercut so severe that the approach arm of the I-bar acts as a food trap or irritates the mucosa of the lip or cheek by being too far away from the tissue.
LIMITATIONS OF THE RPI CLASP •Because the approach arm of the I-bar arises from the framework well back from the tooth to protect the gingival tissue, in some instances a long & correspondingly flexible retentive arm results & expected retention is not obtained. •On recall examination, clasp arms are sometimes found to be distorted & permanently sprung away from the tooth. To avoid this, the first part of the clasp coming from the framework must be rigid & thick, thus making it difficult to set teeth aesthetically.
•For some patients, the RPI denture is difficult to manipulate as there is no convenient component to grasp with the fingers for its removal. This can be a real problem for patients with arthritis or other physical disabilities. • When the tooth undercut is located close to the gingival margin, it is difficult to approach gingivally & avoid tissue impingement even with proper relief. •Patients who show a great deal of gingival tissue when smiling may object to the appearance of the RPI clasp & prefer the conventional clasps that they are accustomed to.
MODIFICATIONS IN THE RPI CLASP
When two teeth are splinted, the mesial rest is placed on the anterior tooth & the proximal plate on the posterior tooth with the I-bar on the mesiobuccal surface of the posterior tooth . If a three unit fixed prosthesis is constructed, the mesial rest is placed in the anterior abutment & the proximal plate on the posterior abutment with the I-bar on the mesiobuccal surface of the posterior tooth.
In a distal extension partial denture where an isolated tooth is involved, the RPI clasp may be placed on the tooth anterior to the isolated tooth & proximal plates on both the mesial & distal surfaces of the isolated tooth with no rest . This design permits removal of stress on the isolated tooth. It is important that the superior border of the mesial proximal plate on the isolated tooth be located at or gingival to the level of the occlusal rest of the RPI clasp. The RPI clasp may be designed not to release in function by extending the proximal plate to contact the entire guide plane. This may be used where it is desirable to load the tooth more than the edentulous ridge.
RPA CLASP
• The RPA clasp (rest, proximal plate, Aker’s clasp) was developed by Dr. Charles A. Eliason & Dr. Arthur J. Krol in 1970 at the University of the Pacific School of Dentistry to overcome some of the problems of the RPI clasp. • The RPA clasp was developed to deal with the problems faced with the RPI clasp. • The mesial rest & the proximal plate are designed identically to those of the RPI clasp. • The only difference lies in the retentive arm. An Aker’s or circumferential clasp arm arises from the superior portion of the proximal plate & extends around the tooth to engage the mesial undercut.
• If a conventional Aker’s clasp is used with the retentive arm coming off the proximal plate above the survey line & crossing the survey line in the middle of the tooth to engage the undercut, then the vital releasing capacity will be lost. • The rigid portion of the arm is not able to move toward the gingiva, so the fulcrum point will in effect be moved toward the distal surface of the tooth. The components anterior to this fulcrum will now lift in function. • When occlusal pressure is applied on the denture bases, the mesial rest will move out of its seat, & the retentive arm will engage the undercut, thus torquing the tooth distally. • The design of the RPA clasp must avoid this problem to be used successfully in distal extension situations.
•For the Aker’s arm of the RPA clasp, the distal half of the facial surface of the abutment as well as the surface under the guide plane are blocked out on the cast. •When the Aker’s arm is waxed, the superior border of the retentive arm is placed on the survey line from the proximal plate to the middle of the tooth, where it then drops down to engage the necessary undercut (usually 0.01 inch) . •When the casting is made, the rigid portion of the clasp arm will contact the tooth only along its superior border at the level of the survey line. • When an occlusal load is applied to the denture base, the retentive arm can move into the undercut because of relief under its rigid section & release from the abutment tooth . • With this special retentive design, the RPA clasp provides essentially the same kind of tooth release that the RPI clasp provides.
ADVANTAGES OF THE RPA CLASP OVER THE RPI DESIGN •The circumferential type retainer is easier to grasp for the removal of the prosthesis •The clasp is simple in design with few variations among patients & can thus be easily & consistently fabricated by dental laboratories. •The RPA clasp may be used in place of the RPI clasp when there is insufficient depth in the buccal vestibule or when the buccal tissue undercut is too great.
Bibliography • Mc cracken s Removable partial denture prosthodontics . • Clinical removable partial prosthodontics – stewart
Thank you

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Rpi and rpa concept

  • 1. RPI and RPA concept By – Dr RASHIKA SINGH MDS IInd YEAR
  • 2. CONTENTS • INTRODUCTION • DIFFERENCE BETWEEN I-BAR AND I-CLASP • RPI SYSTEM  COMPONENTS  ADVANTAGE AND DISADVANTAGE  LIMITATIONS  INDICATION AND CONTRAINDICATIONS  MODIFICATIONS • RPA CLASP • REFRENCES
  • 3. Introduction • A wide variety of clasp assemblies are available for clinicians to use. • This variety exists because of the imagination of clinicians and technicians who provided prosthesis when tooth modification was not or could not be provided. • Clasps were designed to accommodate distal extension functional movement and also without movement . • These categories are not mutually exclusive because any clasp assembly is used to retain and maintain a well supported prosthesis.
  • 4. • Direct retainer is any unit of a removable dental prosthesis that engages an abutment tooth or implant to resist displacement of the prosthesis away from basal seat tissue. • Types of direct retainer – Intracoronal - Precision attachment Extracoronal - Clasp type • Function of a clasp – support stabilization reciprocation retention movement of abutment tooth. • Clasp design – Circumferential clasp arm – approaches the retentive undercut from occlusal direction. Bar clasp arm – approaches the retentive undercut from cervical direction
  • 5. I bar removable partial denture • The I-bar removable partial denture design, introduced by Kratochvil in 1963 has achieved considerable status as a treatment modality. • The mechanical approach proposed by Kratochvil in the design concept of the I-bar has encouraged others to apply sound engineering principles in challenging the design. • As a result, the design has been subjected to a thorough evaluation, & in the process, several modifications have been suggested.
  • 6. Difference between I-bar and I-clasp I-CLASP The I-clasp is an extracoronal retainer that is occasionally used on the distobuccal surface of the maxillary canines for aesthetic reasons . There is definite danger involved in using this clasp because the only contact of the retentive clasp with the tooth is the tip of the clasp, an area of 2-3 mm. Encirclement & horizontal stabilization are compromised. There must always be a posterior abutment tooth for this clasp to be successful.
  • 7. I-BAR The I-bar is a component part of a partial denture design philosophy, the rest, proximal plate, I-bar retentive clasp (RPI) concept. It differs from the I-clasp in position on the abutment tooth that retentive undercut is selected. I-bar retention is normally near the centre of the facial surface of the tooth or on the mesiobuccal side .
  • 9. RPI stands for: •occlusal rest (R) •distal guide plate (P) •gingivally approaching I bar (I) Mesial Distal
  • 10. •The RPI is a current concept of bar clasp design that refers to the rest, proximal plate and I-bar component parts of the clasp assembly . •Clasp assembly consists of a mesio occlussal rest and minor connector placed in the mesio-lingual embrasure and not contacting the adjacent tooth. •A distal guiding plane extending from the marginal ridge to the junction of the middle and gingival third of the abutment tooth is prepared to receive the proximal plate
  • 11. Bucco-lingual width of guiding plane determined by proximal contour of tooth. The proximal plate and minor connector supporting the rest provide the stabilizing and reciprocal action of the clasp assembly. I-bar located on the gingival third of buccal/labial surface of abutment in 0.001 inch undercut.Arm of I-bar tapered to its terminus with 2mm of tip contacting the tooth.
  • 12. The retentive tip contacts the tooth from undercut to height of contour . Area of contact along rest and proximal plate provide stabilization through encirclement. Three basic approaches to the application of RPI system – • Location of rest . • Design of minor connector (proximal plate) as it relates to guiding plane . • Location of retentive arm.
  • 13. Rests Function •Vertical support against occlusal forces •Control relationship of prosthesis to supporting structures Rests must be •Positive •Sufficient bulk to withstand direct occlusal forces and indirect forces since they are subjected to as fulcrum points in distal extension .
  • 14. Distal occlusal rests •The gingival part of the denture base adjacent to the posterior abutment moves in an arc parallel to the mucosa. This provides no support from the mucosa near the tooth. Also, the soft tissue adjacent to the tooth may be pinched with resultant tissue strangulation.
  • 15. •As the denture base is followed posteriorly, the arc of movement becomes more nearly perpendicular to the mucosal surface. •The placement of the rest distal to the central axis of the posterior tooth will tend to tip the tooth posteriorly. When movement of the edentulous base occurs, the force exerted on the tooth can be compared to a precision- constructed & fitted ‘wrench’ that tends to pull the tooth backwards . This can result in tooth mobility, bone loss & bodily movement of the tooth.
  • 16. The distal extension acts as a long effort arm and distal rest as fulcrum to cause clasp tip or resistance arm to engage the tooth undercut. This results in tipping or torquing of the tooth, which is greater with stiff clasps and increased denture base movement. Two strategies adopted to change fulcrum location and subsequently resistance arm engaging effect (mesial rest concept clasp assembly) , or to minimize the effect of the lever through the use of a flexible arm (wrought wire retentive arm).
  • 17. With mesial occlusal rest •The direction of movement of the denture base adjacent to the tooth is more nearly perpendicular to the mucosal surface & is less likely to cause a pinching of the tissue. •More posteriorly also, the movement of the base is near perpendicular to the mucosal surface in each region under the base.
  • 18. •As a result of transferring the rest to the mesial surface, the denture movement will force the tooth anteriorly & reverse the ‘wrench’ effect . All remaining teeth will combine to help withstand this forward force, so this will increase support & bracing assistance from the teeth anterior to the posterior abutment tooth.
  • 19. Rest preparations are less extensive in the RPI system. Always located on the mesial aspect of the primary abutment tooth adjacent to edentulous area. The mesial rest extends only in the triangular fossa, even in the molars, & canine rests are often circular concave depressions in the mesial marginal ridge or cingulum rests.
  • 20. For distal extension base partial dentures, the mesial rest in the posterior teeth can be prepared in the appropriate triangular fossa with a No. 6 carbide bur or diamond stone. The marginal ridge must be lowered about 1.5 mm & the preparation must have the deepest portion in the centre of the triangular fossa. The preparation should be rounded & fully polished to permit some rotation when depression of the extension base occurs.
  • 21. For cuspids that serve as abutments, a mesiolingual rest is made. The rest seat must be deep enough to prevent the metal rest from slipping gingivally. As a general rule, mandibular cuspids have a thin enamel & penetration into dentin is inevitable during rest seat preparation. If dentin is exposed, the preparation should be deepened & modified to accept an amalgam or other restoration which can be properly contoured.
  • 22. Parallel guide planes are prepared on all proximal surfaces adjacent to the edentulous spaces. Proximal plate covers the guide plane from marginal ridge to the tooth - tissue junction and extends onto the attached gingiva for 2 mm . One approach recommends guiding plane and proximal plate should extend the entire length of proximal tooth surface with physiologic tissue relief eliminating impingement on free gingival margin . PROXIMAL PLATES
  • 23. Second approach suggests guiding plane and proximal plate extends from marginal ride to junction of middle and gingival third on proximal surface .
  • 24. •Third approach , the superior edge of the proximal plate is located close to the bottom of the prepared guide plane which should be at the junction of occlusal one-third & middle one- third of the tooth. •The proximal plate contacts only 1mm of the gingival portion of the guide plane while the remainder of the proximal plate lies below the guide plane.
  • 25. •The proximal plate extends lingually just far enough so that the distance between the minor connector & the proximal plate is less than the mesiodistal width of the tooth. The distance between the proximal plate & the mesial minor connector should be no less than 5 mm. (prevent lingual migration of the tooth). •It should be 1 mm thick & join the framework at a right angle. At the junction with the framework, the proximal plate is relieved so as not to contact the gingiva & is highly polished. A finish line is placed at the base of the proximal plate to permit a butt joint with the resin base
  • 26. Mouth preparation for guide planes • A guide plane is prepared on the distal surface of the abutment tooth at the occlusal one-third as proposed by Potter et al. • The guide plane should be approximately 2 to 3 mm high occlusogingivally. • This guide plane will often permit the proximal plate & the mesial minor connector to contact the tooth simultaneously & reciprocate the force exerted by the retentive buccal clasp arm during the seating & the removal of the denture.
  • 27. The minor connector carrying the mesial rest contacts the mesiolingual surface of the abutment tooth and, together with the distal plate, acts as a reciprocal for the tip of the retentive clasp which is positioned on or anterior to the midpoint of the buccal surface of the tooth.
  • 28. • If the mesial minor connector & the proximal plate cannot contact simultaneously, as may occur with cuspid abutments, then the retentive I-bar should engage the mesiobuccal undercut & receive its reciprocation from the proximal plate alone.
  • 29. Proximal plate serves the following functions : •Provides horizontal stability. •Reunites & stabilizes the arch. •Increases retention because of parallelism & because dislodgement is limited to the path of insertion. •Protects the tooth-tissue junction by preventing food impaction & because of metal coverage in this area. •Control against ridge crest gingival hyperplasia. •Provides reciprocation. •Distributes occlusal force throughout the arch.
  • 30. The proximal plate can also provide a certain degree of retention by mean of friction (without engaging an undercut). These plates are also a major component determining the path of insertion, and giving the clasps their effectiveness in the undercuts they engage.
  • 31. I-BAR The I-bar is an extracoronal, infrabulge retainer with a configuration designed to minimize the deleterious effects that over contoured retainers have on the health of the tooth & the gingiva. The arm is long & tapering with a half round cross-section. Also called roach clasp arm.
  • 32. •The approach arm of the I-bar extends from the framework so as to remain at least 3 mm from the gingival margin & then crosses the gingival margin at right angles. •Approximately 2mm of the I-bar contacts the tooth surface, usually at the gingival one-third of the tooth. • The bottom portion of the I-bar contacting the tooth surface should engage 0.01 inch undercut. • The I-bar should taper slightly from the base to the tip . •The I-bar is usually placed at the greatest mesiodistal prominence on the buccal surface or towards the mesial, but not towards the distal. • This is necessary to permit movement of the I-bar away from the entire buccal surface in function. • When the I-bar is placed towards the mesial, it has the advantage that when properly adjusted, it brings the proximal plate into tight contact with the distal surface
  • 33. Shape of the retentive terminal maybe – T , modified T, I or Y If a T-clasp is used, the metal framework rotates around the mesial rest & the mesial part of the clasp moves forward & slightly upward. It loses contact with the tooth & causes no adverse forces. The distal part of the clasp moves forward & downward . This seems acceptable until an occlusal view of the region is observed . The distal part of the T-clasp wraps around the tooth. Then, when the distal part moves forward, it engages the distal curvature of the tooth & exerts a torque that is detrimental to the periodontium. The solution to the problem lies in the I-bar clasp with its retentive tip at the point of greatest curvature of the buccal surface of the tooth.
  • 34. •The RPI system is designed to allow vertical rotation of a distal extension saddle into the denture-bearing mucosa under occlusal loading. •As the saddle is pressed into the denture- bearing mucosa, the denture rotates about a point close to the mesial rest. •Both the distal guide plate and the I bar move (downward & forward away from the tooth) and disengage from the tooth surface. Potentially harmful torque is thus avoided. •When trying in the metal framework, it is advisable to check that it is able to rotate about the abutment tooth in the intended fashion. If this is found not to be the case, the framework should be carefully adjusted to allow this rotation.
  • 35. •This minimizes the denture caused stresses on the abutment teeth, without damaging the supporting structures of the abutment tooth particularly in cases with free end saddle(s). •If the fulcrum is moved to the distal surface of the tooth with the same clasping situation above, the movement of the clasp tip will be upward & forward . It will engage the tooth undercut & produce a torque on the tooth. •The RPI system is normally used with the “skeletal design” .The skeletal design has very limited coverage on the surfaces of the remaining natural teeth.
  • 36. •The I bar is utilized to provide direct retention and better aesthetics because of its minute surface area coverage. •If the clasp crosses the gingival margin at a right angle (90°), then its interruption of that margin is confined to a very small distance (1- 2mm). •Added to that, the RPI system places very little metal parts of the denture on the abutment teeth surfaces. This keeps the gingival margins of the natural teeth exposed to the self cleansing actions of the tongue and saliva.
  • 37. The mechanics of the RPI also aim to exert minimal amount of torquing (tilting) forces on the abutment teeth.
  • 38. INDICATIONS • Small degree of undercut (0.01 inch) exists in cervical third of abutment tooth which can be approached gingivally. • Used in tooth supported partial denture or tooth supported modification area • Distal extension case. • Esthetics concern
  • 39. ADVANTAGES OF THE I-BAR •Vertical forces on the distal extension base during function cause both the I-bar & the proximal plate to disengage the abutment & thereby reduce the torquing of the tooth. •The mesial minor connector with the proximal plate provides for reciprocation & eliminates the need for a lingual arm. •An important but seldom mentioned advantage of the RPI clasp is its avoidance of contact with the lingual surface of the abutment tooth. Without a lingual arm, the high survey line on the lingual surface of many mandibular teeth is not a problem, making this design useful for tooth supported as well as distal extension removable partial dentures.
  • 40. • The mesial rest eliminates the potential ‘pump handle’ effect that a force on the base often induces with a distal rest. • The RPI clasp contacts the tooth minimally & is advantageously used on caries prone patients. • Usually more aesthetic than most other clasp arms.
  • 41. DISADVANTAGES & CONTRAINDICATIONS OF THE I-BAR •Insufficient depth of the vestibule to permit the approach arm of the I-bar to be located at least 3 mm from gingival margin. •A tooth with severe lingual tilt & no buccal or labial tilt. •Tissue undercut so severe that the approach arm of the I-bar acts as a food trap or irritates the mucosa of the lip or cheek by being too far away from the tissue.
  • 42. LIMITATIONS OF THE RPI CLASP •Because the approach arm of the I-bar arises from the framework well back from the tooth to protect the gingival tissue, in some instances a long & correspondingly flexible retentive arm results & expected retention is not obtained. •On recall examination, clasp arms are sometimes found to be distorted & permanently sprung away from the tooth. To avoid this, the first part of the clasp coming from the framework must be rigid & thick, thus making it difficult to set teeth aesthetically.
  • 43. •For some patients, the RPI denture is difficult to manipulate as there is no convenient component to grasp with the fingers for its removal. This can be a real problem for patients with arthritis or other physical disabilities. • When the tooth undercut is located close to the gingival margin, it is difficult to approach gingivally & avoid tissue impingement even with proper relief. •Patients who show a great deal of gingival tissue when smiling may object to the appearance of the RPI clasp & prefer the conventional clasps that they are accustomed to.
  • 44. MODIFICATIONS IN THE RPI CLASP
  • 45. When two teeth are splinted, the mesial rest is placed on the anterior tooth & the proximal plate on the posterior tooth with the I-bar on the mesiobuccal surface of the posterior tooth . If a three unit fixed prosthesis is constructed, the mesial rest is placed in the anterior abutment & the proximal plate on the posterior abutment with the I-bar on the mesiobuccal surface of the posterior tooth.
  • 46. In a distal extension partial denture where an isolated tooth is involved, the RPI clasp may be placed on the tooth anterior to the isolated tooth & proximal plates on both the mesial & distal surfaces of the isolated tooth with no rest . This design permits removal of stress on the isolated tooth. It is important that the superior border of the mesial proximal plate on the isolated tooth be located at or gingival to the level of the occlusal rest of the RPI clasp. The RPI clasp may be designed not to release in function by extending the proximal plate to contact the entire guide plane. This may be used where it is desirable to load the tooth more than the edentulous ridge.
  • 48. • The RPA clasp (rest, proximal plate, Aker’s clasp) was developed by Dr. Charles A. Eliason & Dr. Arthur J. Krol in 1970 at the University of the Pacific School of Dentistry to overcome some of the problems of the RPI clasp. • The RPA clasp was developed to deal with the problems faced with the RPI clasp. • The mesial rest & the proximal plate are designed identically to those of the RPI clasp. • The only difference lies in the retentive arm. An Aker’s or circumferential clasp arm arises from the superior portion of the proximal plate & extends around the tooth to engage the mesial undercut.
  • 49. • If a conventional Aker’s clasp is used with the retentive arm coming off the proximal plate above the survey line & crossing the survey line in the middle of the tooth to engage the undercut, then the vital releasing capacity will be lost. • The rigid portion of the arm is not able to move toward the gingiva, so the fulcrum point will in effect be moved toward the distal surface of the tooth. The components anterior to this fulcrum will now lift in function. • When occlusal pressure is applied on the denture bases, the mesial rest will move out of its seat, & the retentive arm will engage the undercut, thus torquing the tooth distally. • The design of the RPA clasp must avoid this problem to be used successfully in distal extension situations.
  • 50. •For the Aker’s arm of the RPA clasp, the distal half of the facial surface of the abutment as well as the surface under the guide plane are blocked out on the cast. •When the Aker’s arm is waxed, the superior border of the retentive arm is placed on the survey line from the proximal plate to the middle of the tooth, where it then drops down to engage the necessary undercut (usually 0.01 inch) . •When the casting is made, the rigid portion of the clasp arm will contact the tooth only along its superior border at the level of the survey line. • When an occlusal load is applied to the denture base, the retentive arm can move into the undercut because of relief under its rigid section & release from the abutment tooth . • With this special retentive design, the RPA clasp provides essentially the same kind of tooth release that the RPI clasp provides.
  • 51. ADVANTAGES OF THE RPA CLASP OVER THE RPI DESIGN •The circumferential type retainer is easier to grasp for the removal of the prosthesis •The clasp is simple in design with few variations among patients & can thus be easily & consistently fabricated by dental laboratories. •The RPA clasp may be used in place of the RPI clasp when there is insufficient depth in the buccal vestibule or when the buccal tissue undercut is too great.
  • 52. Bibliography • Mc cracken s Removable partial denture prosthodontics . • Clinical removable partial prosthodontics – stewart