I – bar Concept in Removable

Partial Dentures

Evolution of the I-bar Clasp design Philosophy

Origin of the Concept of I – bar Clasp Assembly -

Kratochvil

Principles of designing I-bar clasp assembly

Physiologic relief of framework extension base

Removable Partial Denture

CONTENTS

➢ A distal extension removable partial denture derives support from

both the abutment teeth as well as tissues of the residual ridge.

Evolution of the I-bar Clasp design Philosophy

➢ This becomes of great significance when one considers the

masticatory forces placed on the extension base during

function. Differences in the tooth and soft tissue support

results in pivoting of the prosthesis on the abutment closest to

the extension base , generation of stresses in the prosthetic

framework and consequently non-axial loading of the

abutment tooth.

➢ The retentive clasp arm is the element of the removable partial

denture that is responsible for transmitting most of the destructive

forces to the abutments.

➢ In distal extension base RPDs, the distal extension act as a long

“effort arm” across the distal rest “fulcrum” to cause the clasp tip

“resistance arm” to engage the tissue undercut.

➢ This results in harmful tipping or torquing of the tooth, ultimately

causing periodontal breakdown of abutment tooth & failure of

prosthesis.

➢ This led to the introduction of numerous clasping philosophies for

distal extension applications in an attempt to establish the ideal

clasp assembly – that would proportionately load the teeth and the

soft tissues.

➢Two strategies which gained wide acceptance in this regard are:

1) Minimizing the lever action with the use of flexible wrought wire

retentive arm. [COMBINATION CLASP]

2) Changing the fulcrum and subsequently the “resistance arm”

position engaging effect through mesial rest concept clasp

assemblies. [I – BAR CLASP DESIGN PHILOSOPHY]

➢Rationale behind the Mesial rest concept based design

philosophies:-

1) When the clasp tip is placed clasp tip mesial to the axis of

rotation, the effect of the torque on the clasped tooth can be

compared to the action of a class I lever . Placing clasp distal to the

axis of rotation changes class I to Clas II lever minimizing the stress

on the abutment tooth.

During the application of occlusal load the rests serve as

rotational centers. As the distance from the rest to the denture

base is increased, the associated radius becomes larger, and the

arc of rotation becomes more linear. Hence, anterior placement of

the rest direct the forces more vertically onto the tissues of the

residual ridge.

Mesial rests direct tipping forces towards the mesial surfaces.

This makes the abutment in firm contact with adjacent teeth,

resulting in more favourable distribution of stresses.

➢In 1963, Kratochvil introduced an innovative clasp

assembly for controlling stress in distal extension base

removable partial denture.

➢ In developing his clasp assembly, he incorporated the idea

of remote rather than adjacent rest , adopted and

relocated I bar retainer and originated the proximal

plate.

Origin of the Concept of I – bar Clasp

Assembly - Kratochvil (1963)

➢ So Kratochvil’s original I bar Clasp assembly consisted of :-

1) An occlusal rest away from the minor connector on the side of the abutment

away from the edentulous space [MESIAL REST]

2) An I shaped bar clasp retaining arm placed mid bucally on the abutment.

[I -BAR]

3) A vertical plate contacting the distal and distolingual surface of the abutment

adjacent to edentulous space. [ PROXIMAL PLATE]

➢ Proponents of I bar Philosophy claimed that the resultant clasp

design minimized torquing forces and directed occlusal forces

parallel to the long axes of the abutment.

➢ The RPI clasp configuration was strategically designed for

allowing extension base removable partial denture some degree of

tissueward rotational freedom without applying torque to the

abutment tooth resulting from its potential to disengage during

functional movement .

➢Most accepted stress releasing clasp releasing design described in

literature

➢ Dr A.J. Krol(1973) along with certain modifications of original

Kratochvil’s design provided a valid theoritical rationale for

disenagement of the I-bar retentive clasp terminus during

rotational movement of the removable partial denture and supplied

the name ‘RPI Bar Clasp Design’.

RPI concept- Krol (1973)

Principles of designing I-bar clasp assemblyGuide surface preparation & Proximal plate

➢ Much of the controversies surround the guide surface preparation

for receiving Proximal Rests of I-bar Removable Partial Denture.

➢ Three concepts that have been reported in this regard are:

i) Kratochvil’s (1963) extensive guide surface preparation

ii) Krol’s (1973) RPI concept of guide surface preparation

iii) Demer’s principle of guide surface preparation

i) Kratochvil’s (1963) extensive guide surface preparation principle

✓ Maximum contact of the guide plate and the guide surface to eliminate space

between guide plate and abutment.

✓ A 2-3 mm metal foot extended from the base of the guide plate onto the mucosa of

the residual ridge.

✓ Binding of the guide plate against the abutment during function was prevented by

physiologic relief of the metal at the framework try-in stage.

ii) Krol’s (1973) RPI concept of guide surface preparation

✓ Recommended a much smaller guide surface preparation (2 to 3mm in height, located

in the occlusal third of the proximal surface) and a guide plate that contacted only the

bottom 1mm of the guide surface.

✓ Binding or the torquing of the abutment was prevented by retaining a small space

below the guide surface(into which the guide plate can move during function).

✓ The portion of the guide plate in contact with the gingival tissue was relieved.

iii) Demer’s principle of guide surface preparation

✓ He concurred with Krol that a slight undercut should be retained below the guide

surface. However he felt that that the more gingival location of the guide plate would

create the potential for food entrapment between the occlusal aspect of the abutment

and the artificial tooth.

✓ He recommended that the guide plate contact the proximal surface of the abutment

only at the top of the guide surface.

✓ Demer also extended the guide plate lingually far enough so that in connection with

the minor connector of the mesial rest,It would provide reciprocation and prevent

lingual migration of the abutment.

2) Rests

➢ In I bar applications, the clasp assembly consists of a mesio-

occlusal rest with the minor connector placed into the mesio-

lingual embrasure, but not contacting the adjacent tooth.

➢ Premolar rest seats are prepared in marginal and triangular ridges

& the molar rest seats extend into the central fossa.

➢ Anterior rests must satisfy esthetic

requirements.

➢ Cingulum rests are ideal for

anterior applications. A cingulum

rest forces closer to the rotational

center of associated tooth and

provide maximum stabilization.

They are prepared on canines and

maxillary incisors.

➢ Mandibular incisors lack

sufficient enamel for appropiate

placement of cingulum rests.

Therefore incisal rests should be

used whenever esthetics permit.

➢ Krol in his RPI concept however advocated less extensive rest

seat preparation. Rests extend only into triangular fossa, even

in molar preparations,and canine rests are often circular,

concave depressions prepared in mesial marginal ridges.

3) I- bar retainer

➢ Each I bar is designed to minimize the coverage of the teeth and soft tissues,

thereby promoting tissue health.

➢ The approach arm is long and tapering and displays a half round cross

sectional geometry.

➢ The horizontal portion of the approach arm must be located at least 4mm

from the gingival margin and even farther if possible.

➢ The I bar engages a 0.010 inch undercut

and terminates at or slightly above the

height of contour .

➢ The whole arm of the I bar should be

tapered to its terminus, with no more than

2mm of the tip contacting the abutment.

In the RPI system the tip is modified to

have pod-shaped in order to allow more

tooth contact.

➢ The retentive tip engages an undercut at

the height of the mesiodistal contour or

slightly mesial to it.

Physiologic relief of framework extension base

Removable Partial Denture

➢ To allow movement of the partial denture around the axis of rotation, it is

necessary to reduce the binding between the framework (minor connectors,

guide plates) and the abutments. This is accomplished by precise adjustment

of the removable partial denture framework under physiologic loading

conditions.

➢ The procedure is done at the framework try-in appointment.

➢ A mixture of chloroform and rogue or one of the commercially available

disclosing materials is applied to the guide plates and minor connectors.

➢ The framework is seated in the mouth and manual pressure is

used to place stress on the denture base retentive network. An

effort is made to equal or surpass the movement that might be

expected when biting forces are applied.

➢ The framework is removed from the mouth and the areas of

contact are relieved. The process is repeated until functional loads

produce no evidence of binding.

Criteria for the ideal functioning of I – bar

Clasp Assembly

➢ One of the major attributes of I-bar is their potential for

disengaging during functional movements of RPD by

allowing some degree of rotational freedom to RPD

framework.

1) Axis of rotation must pass through a mesially located rest .

2) The tip of the I-bar must be placed in an undercut located at or in front of the

greatest mesiodistal curvature of the facial surface of the abutment but behind

the axis of rotation.

➢ In order to accomplish this endeavour, following criteria needs to

be fulfilled:-

3) No rigid metal distal to the mesial rest may lie above the survey

line or it will preempt the planned mesial rest.

4) There should be no binding of the guide plate against the guide

surface during functional movement to allow for free rotation. If

the clasp tip had to disengage during functional movement of

distal extension base. This criteria must be fulfilled.

5)The mesial minor connector must have a freedom to move

between the abutment and the adjacent tooth during function.

6) It should be avoided on mesially inclined terminal abutments

because it is extremely difficult to achieve any “releasing” capacity

for the guide plate. During function, the guide plate will contact the

tooth, preempt the mesial rest, and act as a rest on an inclined

plane. Efforts to prevent contact with physiologic relief would

create a significant space between the abutment and the prosthesis.

Advantages of I-bar clasp assembly

1)Minimal tooth contact and minimum distortion of normal tooth

contours,leading to improve tissue stimulation and oral hygiene and

decreased caries and periodontal problems

2)Improved esthetics if the approach arm is not visible as it crosses

gingiva

3)Because the approach arm does not contact the abutment,lateral

forces are minimized.

4)The clasp terminal disengages from the tooth when occlusal load is

applied to adjacent distal extension base.

Disadvantages of I-bar Clasp Assembly

1)Cannot be used in cases of soft tissue undercut,shallow vestibule

and high frenal attachments.

2)Bracing action is considerably less than that provided by cast

circumferential clasps

3)Appearance may be adversely affected if the smile line is high

enough to expose the approach arm as it crosses the gingiva.

4)May not be retentive until rigid elements define specific path of

insertion /dislodgement.

RPA concept in Removable Partial Denture ➢ RPA stands for Rest, Proximal plate and Aker’s clasp.

➢ Represents a hybrid of RPI clasp and conventional circumferential

clasp assemblies.

➢ Consists of a mesial rest, a proximal plate and a circumferential

retentive arm originating from the proximal plate .

➢ It is indicated for distal extension RPDs when the tissue undercut

below the abutment contraindicates the use of a bar clasp.

➢ However, a very specific criteria exist if this retainer is to disengage during

function.

➢ The contour of the contour of the abutment must be such that the occlusal

border of the originating portion of the arm will lie exactly on the survey line

and the portion of the tooth below the survey line( except the tip) must be

blocked out.

➢ The rigid portion of the arm must not lie above the survey line or it will act as

a rest on an inclined plane and preempt the planned mesial rest.

➢ It should also be avoided when the terminal abutment inclines mesially.

I-Bar Clasp as a Direct Retainer

1)RETENTION

➢ push type retention

➢ If all factors remain same ( depth of undercut,

flexibility,length of the clasp arm), it is more retentive than

circumferential clasp.

2) SUPPORT

➢ well supported by mesially placed rest.

3) STABLITY/ BRACING

Proximal plate & the mesial minor connector for occlusal rest are the prime

components providing bracing action to the I-bar clasp assembly.

➢ The I bar engages a 0.010 inch undercut and terminates at or slightly above the

height of contour . This occlusal extension of I-bar aids in stabilization.

➢ However the bracing action of the I-bar clasp assembly is less than

circumferential clasps.

4) ENCIRCLEMENT

➢ The three point contact of the abutment tooth with the Proximal

plate, I-bar , Mesial minnor connector provide for broken

encirclement.

5) PASSIVITY

➢ The I-bar clasp remains passive when not in function.

➢ The greatest attribute of I-bar clasp is their potential for

disengagement during functional movement of prosthesis.

6) RECIPROCATION

➢ Provided by Proximal Plate of Clasp Assembly

➢ It is less effective compared to reciprocating action provided

by Circumferential Clasps

References:

1) Stewart’s Clininical Removable Partial Denture ,4th edition

2) Mc Cracken’s Removable Partial Prosthodontics. 12th edition

3) An atlas of Removable Partial denture design , Russell J Stratton & Frank J

Weibett.

4) Kratochvil et al influence of occlusal rest position and clasp design on

movement of abutment teeth J Prosthet Dent 1963:13;114

5) Extracoronal direct retianers for distal extension removable partial denture,

M A Aras , JIPS June2005 , Vol 5 Issue 2

6) Benson D Spolsky ,A clinical evaluation of the Removable Partial Dentures

with Ibar Retianers, Part I.JPD 1979 41;246-254

Thank

You