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DIRECT RETAINERS

By Dr Shebin Abraham

Dept. of Prosthodontics

Contents

Introduction Definitions. Classification. Basic part of clasp assembly. Analysis of tooth contour. Basic Principles of clasp design. Cast circumferential clasp. Bar clasp. Other types of clasp. Intracoronal retainers. Conclusion.

Introduction

Types of retention

Primary retention.1. By clasps

Secondary retention.2. Acting through

polished surface of the denture.

3. Tissue coverage.

Definition:-

Direct retainer:-“That component of a partial removable

dental prosthesis used to retain and prevent dislodgment, consisting of a clasp assembly or precision attachment” GPT-8

Direct retention : “Retention obtained in a partial removable

dental prosthesis by the use of clasps or attachments that resist removal from the abutment teeth” –GPT-8

Classification Of Direct Retainers.

Intra coronal Extra coronal

Precision attachme

nt

Semi precision attachme

nt

Retentive clasp

assemblies

Attachments

Supra bulge clasp

Infra bulge clasp

Clinical removable prosthodontics:- STEWART’S 3rd edition

Intracoronal

Proposed by Dr Herman E S Chayes in 1906. Cast or attached within the contours of natural

teeth(abutment). Keyway and key…..Opposing vertical walls provides retention. Retention is achieved by frictional resistance.

1. Precision attachment manufactured by high precision technique and instruments

2. Semi precision attachment less intimate contact between matrix and matrix component. Unlike precision attachment they consist of tapering walls and are casted from wax or plastic pattern.

Extracoronal attachment.

First proposed by Henry H Boos 1900 later modified by Ewing F Roach in 1908.

Located outside the teeth. Retention from mechanical resistance. Permit vertical movement during vertical loading. Minimize potentially damaging forces to abutment

Stress breaking or stress directing effects.

Occlusally approaching /

Suprabulge / Ney Type I clasp /

Circumferential:

• Approaches the tooth undercut from an occlusal direction

• It is attached to metal framework above the height of contour.

Gingivally approaching / Infrabulge/ Bar/ Roach /

Ney Type II Clasp:

• The retentive arm originates from the metal base or denture framework traverses soft tissue and

• Approaches the tooth undercut area from a gingival direction.

EXTRACORONAL DIRECT RETAINERS

The basic parts of a clasp assembly include

the following:

It is the part of the clasp that lies on the occlusal, lingual or incisal surface of a tooth and resist (tissue ward) movement of the clasp by ensuring that the retentive terminals of the clasp remain fixed in the desired or planned depth of undercut.

REST

Body of the clasp

It is the part of the clasp that connects the rest and shoulder of the clasp to the minor connector.

It must be rigid. Above the height of contour.

Shoulder

It is the part of the clasp that connects the body to the clasp terminals.

The shoulder must lie above the height of contour and provide some stabilization against horizontal displacement of the prosthesis.

Reciprocal arm

A rigid clasp arm placed above the height of contour on the side of the tooth, opposing the retentive clasp arm.

Retentive arm

It is the part of the clasp comprising the shoulder which is not flexible and is located above the height of the contour

It is the terminal end of the retentive clasp arm. It is the only component of the removable partial denture that lies on the tooth surface cervical to the height of the contour. It possesses a certain degree of flexibility and offers the property of direct retention.

Approach arm It is a component of the bar clasp. It is a minor connector that projects from the

framework, runs along the mucosa and turns to cross the gingival margin of the abutment tooth; to approach the undercut from a gingival direction.

Analysis of tooth contour:-

Before moving on to principles of clasp design its vitally important to consider how tooth contour & RPD components must be related to allow a stable prosthetic function.

What is path of insertion??? path of removal ??and height of contour???

Point of maximum convexity or the term height of contour Dr Edward Kennedy in 1985.

This critical area of an abutment that provide for retention & stabilization can only be identified with the use of dental cast surveyor.

Prothero’s concept

Proposed “cone theory” of clinical crown in 1916.

Provided conceptual basis of mechanical retention.

Contours of clinical crown resembles two cones sharing a common base.

The line formed at the junction of this base represents the greatest diameter of the tooth.

This greatest diameter is called height of contour or point of maximum convexity.

Devan [1955] referred to the surface occlusal to the height of contour as suprabulge, & the surface inclining cervically as infrabulge.

suprabulge

infrabulge

Angle of convergence

o When the surveyor blade contacts a tooth on the cast at its greatest convexity, a triangle is formed.

o The apex of this triangle is at the point of contact of surveyor blade with the tooth and base is towards the gingival tissues.

o This apical angle is called angle of cervical convergence. The importance of this angle lies in its relationship to the amount of retention.

Basic Principles of clasp design

According to Stewart basic principles are:-1. Retention.2. Stability.3. Support. 4. Reciprocation. 5. Encirclement.6. Passivity.

Retention “Retention is the inherent quality of the clasp

assembly that resists forces acting to dislodge components away from the tooth structure.”

No single component of a clasp assembly is solely responsible for prosthesis retention.

Rather, it is effective design and accurate construction that make the removable partial denture retentive.

The amount of retention designed into a removable partial denture should always be the minimum necessary to resist reasonable dislodging forces.

A rigid clasp flexing over the height of contour may transfer harmful stresses to an abutment during insertion, removal, and functional movement of the prosthesis.

An only a minimum area of contact should be seen.

Amount of retention

Factors that effect retention can be divided into -1. Tooth factors

Size of angle of cervical convergence How far clasp is placed in angle of cervical convergence.

2. Prosthesis factors Clasp length Clasp diameter Clasp cross-sectional form[ shape] Material used for making clasp[ alloy]

Prosthesis factors:-

Length of clasp arm- Longer the clasp arm the more flexible it will be. Circumferential clasps more retentive than bar clasp for a given

clasp length. The clasp arm should taper from the point of origin to provide its

flexibility.

Diameter of the clasp: The greater the diameter of a clasp arm the less

flexible it will be.(only in uniform taper) If its taper is absolutely uniform ,the avg diameter will

be at a point midway between its origin & its terminal end.

But if taper is not uniform a point of flexure –therefore a point of weakness will exist.

Cross-sectional form: Flexibility may exist in any form, but is limited to only one direction in the case

of the half-round form The only universally flexible form is the round form. Clasp arm should only flex away from tooth so half round is used. Round shaped clasp arm used only in distal extension denture bases so that it

can flex in all directions during functional movement and minimize stresses.

Material used for construction:Gold alloy greater flexibility than chrome alloys ,Disadvantage of cast gold alloys bulk of the

prosthesis costly.Chrome alloys have a higher modulus of elasticity &

therefore less flexibility.So in less undercut areas CoCr alloy can be used but

in cases of deep undercut wrought metal can be used.

Relative uniformity of retention:

1.Retention on all principal abutments should be as equal as possible.

2.Retentive clasp arms should be located so that they lie in the same approx.. Degree of undercut on each abutment tooth.

3.Retentive clasp positioning should also be same on the contralateral side of arch.(atleast for one teeth).

Support

“Support is the quality of a clasp assembly that resists displacement of a prosthesis in an apical direction.”

Provided by occlusal rest.1. A properly prepared rest seat and corresponding rest serve

to resist displacement of the prosthesis toward the supporting teeth and soft tissues, thereby ensuring that the clasp assembly maintains its intended relation to the abutment, and

2. Transmit functional forces parallel to the long axes of the abutments.

Stabilization.

“Stability is the quality of a clasp assembly that resists displacement of a prosthesis in a horizontal direction.”

It helps the denture be steady constant firm and resist displacement due to function stresses and also prevent change in position of the denture.

It is provided by :-1. Reciprocal element.2. The shoulder(s) of a cast circumferential retentive clasp.3. Vertically oriented minor connectors.

Reciprocation “Reciprocation is the quality of a clasp assembly that

counteracts lateral displacement of an abutment when the retentive clasp terminus passes over the height of contour.”

As the retentive arm passes over the height of contour it flexes creating lateral forces damage to the tooth.

The reciprocal element may be a 1. Retentive arm of clasp2. Lingual plating, 3. Combination of mesial and distal minor connectors.

Points to be remembered while providing reciprocation

1. To optimize reciprocation, the axial surface of an abutment should be prepared parallel to the path of insertion and removal.

2. It should be placed above the height of contour.3. To provide true reciprocation, the reciprocal clasp

arm must be in contact during the entire period of retentive clasp deformation.

Encirclement.

“Encirclement is the characteristic of a clasp assembly that prevents movement of an abutment away from the associated clasp assembly”.

Clasp assembly 180 degrees contact.

The engagement can be in the form of continuous contact, such as circumferential clasp, or discontinuous contact, such as Bar clasp.

Both provide tooth contact in at least 3 areas encircling the tooth-

Passivity.

“Passivity is the quality of a clasp assembly that prevents the transmission of adverse forces to the associated abutment when the prosthesis is completely seated.”

When fully seated it should be passive. Should be activated only when dislodging forces are

applied.

Other principles1. Retentive clasp element should always be placed on facial

surface of tooth.(mainly in premolars)2. Only one retentive element should be used per tooth

opposed by a reciprocal element.3. Clasp retainers on the abutment adjacent to distal

extension bases should be designed so they will avoid direct transmission of tipping & rotational forces to the abutment.

4. The path of escapement for each retentive clasp terminal must be other than parallel to the path of removal for the prosthesis to require clasp engagement with resistance to deformation.

CRITERIA FOR CLASP SELECTION

Survey line Requirements of retention and stability

depending on the number, configuration of edentulous areas.

Nature of support Root size and form Esthetics Presence of excessive tissue undercut Oral hygiene and patient awareness

Cast circumferential

clasp.

First proposed by Dr N B Nebbit. Later modified by Dr Polk E Aker.

Simple and easy to construct Large amount of tooth surface that

is covered by the clasp assembly leads to enamel decalcification.

Design rules..

1. The clasp should arise from the main body of the clasp assembly above the height of contour. The retentive arm should extend cervically and circumferentially in a gently arcing manner.

2. All the components of the C clasp should be present above the height of contour except the retentive tip.

3. The retentive terminus should always be directed towards the occlusal surface never towards the gingiva.

4. It should always terminate at the mesial or distal line angle never at midfacial or midlingual surface.

5. The retentive arm should be positioned as far apically on the abutment as is practical.(not impinging the gingiva)

6. Special considerations in case of distal extension case:- A cast circumferential clasp should not be used to engage (a)

the mesiofacial surface of an abutment adjacent to a posterior edentulous space or

the distofacial surface of an abutment adjacent to an extensive anterior edentulous space.

Simple circlet clasp.

It is one of the most commonly used clasps retentive and stabilizing ability.

The basic design consist of 1. buccal retentive arm and2. a lingual reciprocal arm

originating from a common body.3. Occlusal rest.

With retentive arm projecting away from the edentulous space.

Advantages:-1. Fulfils the design requirements of support, stability,

reciprocation, encirclement, and passivity.2. Its uncomplicated design features make it easy to

construct and relatively simple to repair. Disadvantages:-

1. If used in distal extension base due to fulcrum rotation of the clasp it can cause damage to teeth.

2. Can increase the circumference of teeth and lead to food accumulation and decalcification.

Reverse circlet clasp

Used when undercut is located at the facial distoangle adjacent to an edentulous space.

Design It consists of a mesial occlusal rest, a horizontal reciprocal

arm, and a retentive arm engaging the distobuccal undercut adjacent to the edentulous area.

Advantages:- Decreases the harmful stresses to teeth.

Disadvantages:- Week clasp if sufficient preparation is not done. Poor aesthetics' if used in premolars and cuspids.

Multiple circlet clasp:

A multiple circlet clasp design involves two simple circlet clasps joined at the terminal aspects of their reciprocal elements.

Used in periodontally weekend teeth to splint them Disadvantages are same as circlet clasp.

Embrasure clasp

Also known as the Bonwill clasp.

fabrication of unmodified Class II or Class III partial denture situation;

when there are no edentulous spaces available on the opposite side of the arch to aid in clasping.

Design:- Double occlusal rests, two retentive clasp arms and

the two reciprocal clasp arms either bilaterally or diagonally opposed.

Prevent interproximal wedging by the prosthesis, which could cause separation of the abutment tooth and result in food impaction and clasp displacement.

In addition to providing support, occlusal rests also serve to shunt food away from the contact area.

Disadvantages:- Improper clearance can give rise thin section of

clasp leading to breakage.

Ring clasp

Circumferential clasp encircles nearly all of the tooth from its point of origin.

It is usually used when a proximal undercut cannot be approached by any other means.

In case of tilted molars A support strut is provided on

the non retentive arm.

Advantages:- Provides adequate

encirclement. excellent retention with

adequate flexibility due to increased length of clasp arm

Disadvantages:- Decalcification of teeth Increased occlusal table. Poor structure of clasp.

Hairpin clasp or reverse action or fish hook clasp

A simple circlet clasp in which the retentive arm loops back to engage an undercut apical to the point of origin.

Used when a distofacial undercut is present adjacent to the edentulous space.

Retentive arm has two horizontal components. 1. The occlusal portion minor connector and

must be rigid.2. The apical portion pass over the height of

contour to engage the desired undercut.

Consideration:- Sufficient clinical crown height. Space between occlusal and apical arm. Occlusal arm shouldn’t interfere within the occlusion

This clasp is indicated when the 1. soft tissue contour precludes use of a bar-type

clasp and 2. when the reverse circlet cannot be considered

because of a lack of occlusal clearance.

Half and half clasp It consists of a circumferential retentive arm arising from

one direction and a reciprocal arm arising from another. used only for unilateral partial denture design.

Combination clasp Given by O C Applegate. This type of clasp consists of a wrought wire retentive clasp

arm and a cast reciprocal clasp arm. Wrought retentive arm is circular in cross section. Used in deep undercut case.

Onlay clasp:-

Indicated when the occlusal surface of the abutment lies noticeably apical to the occlusal plane.

occlusal surfaces of the abutments are covered with crowns clasp arms arises

It establishes the occlusal plane. Indicated in caries free individual. Occlusal surface should be restored with gold acrylic

inserts.

Infrabulge clasp/bar clasp/roach clasp.

Popularized by Ewing Roach in 1930 called it the Bar Clasp.

An infrabulge clasp approaches the undercut region of an abutment from an apical direction.

Push type retention. Flexibility of clasp from length and taper. More aesthetic than c clasp. Ex :- y clasp, t clasp, I clasp.

Design rules…

1. The approach arm of an infrabulge clasp must not impinge on the soft tissues adjacent to the abutment.

2. The approach arm should cross perpendicular to the free gingival margin. It shouldn’t impinge the underlying gingiva.

3. Shouldn’t be used in area of tissue undercut.4. Uniform length and adequate taper should be given

for sufficient flexibility.5. The clasp terminus tip should be placed as apical as

possible on the abutment teeth.

T clasp

Name is from the shape of the retentive terminal.

Used in class 1 and class 2 situation. distofacial undercut is seen. The retentive terminal consist of

horizontal two projection the one on the distal side engages the undercut and the one on the mesial side is above the height of contour.

T clasp is contraindicated when the height of contour is at the occlusal one thirds.

In modified t clasp the non retentive arm is absent.

Y clasp is similar to t clasp with the approach arm ending cervical to the retentive arm.

I clasp lack the horizontal retentive arms but only a horizontal retentive tip.

Only the retentive tip contacts the abutment surface only at the undercut region.

The amount of contact is about 2 to 3mm in height and 1.5 mm in width.

Other types of clasp philosophies.

RPI, RPA DeVan’s Clasp VRHR clasp or Grasso clasp Euipose clasp

RPI concept.

RPI stands for Rest Proximal plate I bar. Introduced by Kratochvil in 1963 it consisted of

three different parts connected to the metal framework.

Mesial occlusal rest, a distal guide plan, and an I bar retainer.

The guide plan contacts the full length of the of the proximal surface of the tooth.

This design had certain basic disadvantages:

Physiologic relief was required to prevent impingement of gingival tissues during function.

Since the proximal plate covers a greater surface area of the tooth, the functional forces are directed in the horizontal direction, thus the tooth is located more than the edentulous ridge.

Krol in 1973 made certain modifications in the design under the “minimal coverage criteria”

Rest preparations are less extensive in the RPI system.

Rests extend only into the triangular fossa, even in molar preparations, and canine rest

2-3mm guide plane in which only 1mm contact was seen from the guide plate.

Arthur Krol JPD 1973;23;408-415

BASIC PRINCIPLES OF RPI CONCEPT

The mesiobuccal rest with the minor connector is placed

into the mesiolingual embrasure, but not contacting the

adjacent tooth.

A distal guiding plane, extending from the marginal ridge

to the junction of the middle and gingival thirds of the

abutment tooth, is prepared to receive a proximal plate

The proximal plate in conjunction with the mesial occlusal rest and minor connector provides the stabilizing and reciprocal aspects of the clasp assembly.

The I-bar contributes to the retentive aspect and should be located in the gingival third of the buccal or labial surface of the abutment in 0.01 inch undercut.

Three different approaches to RPI clasp

These approaches are based on the location of proximal plate ,location of the I bar

RPA clasps

The rest-proximal plate-Aker’s clasp was developed and described by Eliason in 1983.

It consists of a mesial occlusal rest, proximal plate and a circumferential clasp arm, which arises from the superior portion of the proximal plate and extends around the tooth to engage the mesial undercut.

De VAN CLASP Also called as the mirror view clasp Two occlusal rest on the lingual side of the

teeth.

M.M.DeVan JPD 1955;5,208-14

VRHR Clasp

The vertical reciprocal horizontal retentive arm concept was developed by Grasso in 1980 and is characterized by: A distal occlusal rest supported by a minor connector. A lingual vertical reciprocal component originating from

the major connector. A horizontal retentive arm attached to either the major

connector or the retention latticework for the denture base.

Joseph Grasso JPD 1980,43;618-21

Equipoise clasp

Proposed by J. J. Goodman in 1990, it is an esthetic retentive concept for distal extension situations.

Rests are placed away from edentulous span. Vertical inter-proximal reduction of 1 mm between abutment and adjacent tooth is done.

Optional bucco-lingual retentive groove at mid and gingival third junction on distal surface of abutment tooth is provided.

Quintessence Int. 1996 May;27(5):333-40.

Metal free-1. Natural-flex2. Optiflex3. Proflex clear wire4. Valplast5. Cu-sil

Occlusally and gingivally approaching clasps:

Relative merits and demerits

Bar claspThe bar clasp approaches

the undercut from below the height of contour pushes

towards the occlusal surface abutment tooth

• Easier to seat and more difficult to remove

Circumferential clasp

• Above the height of contour

• Pulls towards the occlusal surface from the undercut

to resist dislodgement.

retention

Bar clasp

It is more flexible because of which it provides less

bracing or stability against lateral stresses.

Circumferential clasp

• Because of its rigidity it provides very good stability or bracing

bracing

Bar clasp

It allows a certain degree of functional movement of the

distal extension base dissipate the stresses and

lessen the load on the abutment

Circumferential clasp

Potential to torque abutment teeth in distal extension

based partial denture situations.

Stress breaking effect

Bar clasp

Minimal tooth contact and less damage to

tooth

Circumferential clasp

More tooth contact leading to food accumulation

Contact with tooth structure

Bar clasp

Minimum relief can lead to tissue damage to mucosa under the

approach arm

Circumferential clasp

Damage to gingiva can take place during

improper removal of clasp

Damage to oral tissues

Bar clasp

Very less metal display so high aesthetics

Circumferential clasp

Increased metal display so poor Esthetics.

Esthetics

INTRACORONAL RETAINERS/

precision attachments

The intracoronal retainer is usually regarded as an internal attachment or precision attachment.

Definition:- “A retainer consisting of a metal

receptacle (matrix) and a closely fitting part (patrix); the matrix is usually contained within the normal or expanded contours of the crown on the abutment tooth and the patrix is attached to a pontic or the removable partial denture framework.”

INDICATIONS

To provide movable joint in Removable Bridgework, fixed removable bridges.

To stabilize unilateral saddles. Pier abutments. Titled molars. F.P.D's in severely misaligned

abutments. Use in Over dentures (different forms of retainer

are bar, telescopic, use of auxillary attachments). Fixed removable implant restorations.  

LIMITATIONS

1. Large pulp size which is usually related to the age of patient.

2. Length of the clinical crown, not used in short or abraded teeth.(6mm crown)

3. Expensive

4. Distal extension denture bases.

ADVANTAGES

1. Esthetically acceptable, because not much of metal

display like extracoronal retainers.

2. It is preferred in many of the situation because of its

vertical support through a rest seat located more

favorable to the horizontal axis of the abutment tooth.

3. Stimulation to the underlying tissues greater

when internal attachment are used because of

the intermittent vertical massage.

4. It permits proper tooth form to be maintained and

allows for control of vertical, mesiodistal and

buccolingual displacement of the prosthesis.

5. It provides for excellent retention of the prosthesis on account of frictional resistance between opposing parallel vertical walls that serve to limit movement and resist removal of the partial denture.

Disadvantages of intra coronal retainers

1. They require preparation of abutment tooth

and casting.

2. Difficult clinical and laboratory procedure.

3. They eventually wear, result in loss of

frictional resistance to denture removal.

4. Difficult to repair and replace.

5. They are effective in longer teeth and least

effective in shorter teeth.

6. Difficult to place completely in the abutment

teeth.

MATERIAL OF CHOICE

Pt, Iridoplatinum, Gold and Pt, Gold and Pd.Type III and IV type of gold is to be used for

crown castings.Base metal alloys are also used now a days

as low cost 2 alternate ways to construct crown and rest

seat.Rest seat may be cast against the full coverage

restoration Rest seat may be soldered into place.

Classification

Classification by Good Kind and Baker in 1976 :1)Intra coronal a. resilient b. non resilient

Intracoronal retainersFrictional resistance.

Tapered and parallel-walled boxed and tubes. Adjustable metal platesSpringsStuds:Locks

Magnetic resistance.

How To Choose An Attachment?

It is the length of attachment, not its width that is main criteria in choosing attachments. For each length there are 3 different sizes (width) of precision

attachments (anterior, bicuspid, molar.) Width is measured from one side of rest to other. Full length of a precision attachment is 8 mm for full benefit of

bracing, support and retention a minimal of 5mm height is must. This means that the clinical length of crown must be at least 7

mm so as to accommodate an attachment of 5mm and in addition a minimum of 2mm between the gingival floor of attachment and gingival margin.

Otherwise a periodontal problem may be created.

Precision attachment selection

Kennedy’s class I and class II partially edentulous arches

The most difficult type of treatment plan. Some practitioners advocate non rigid and resilient attachments and

some advocate resilient attachment in distal extension to minimize rotation and torquing of the abutment tooth, when the components of an attachment are rigidly connected.

Int J Prosthodont. 1990 Mar-Apr;3(2):169-74

Another philosophy , known as the stable base precision attachment RPD concept or floating denture base concept recommends incorporation of rigid internal attachments and a cast metal base made from mucostatic impression of the residual ridge.

The male portion of the attachment is connected to the denture base , allowing the complete seating within the abutment.

Kennedy’s class III partially edentulous arch.• Rigid internal attachments are recommended .

• Provides good retention, support and brazing because of its rigid interlocking components.

• If the posterior abutment prognosis is questionable then a resilient type of attachments are recommended with anterior abutment.

Kennedy’s class IV partially edentulous arch

The ideal RPD design for such situation involves the use of a tissue bar placed close to the edentulous ridge and connected as a fixed unit to the abutment teeth on either side of the space using crowns.

Conclusion

Keep the prosthesis design as simple as possible…

Make RPD more comfortable, more efficient and affordable to the patient.

References

1. Clinical removable prosthodontics:- STEWART’S 3rd edition2. Mc cracken removable partial denture prosthodontics – 12th

edition.3. Davenport J.C., Baskar R.M., Heath J.R., Ralph J.P. “A color

atlas of RPD”, Wolfe Medical Publications Ltd., 1988.4. Krol A.J. “Clasp design for extension base RPD”. J. Prosthet.

Dent., 1973; 29 : 408-415.5. M.M.DeVan JPD 1955;5,208-146. Joseph Grasso JPD 1980,43;618-217. Quintessence Int. 1996 May;27(5):333-40.8. Burns DR, Ward JE. Int J Prosthodont. 1990 Mar-Apr;3(2):169-74

Thank you