Retention/ Labial orthodontics

Clasp retention

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Component parts of RPD

Major connectorsMinor connectorsRestsDirect retainers Indirect retainersDenture bases


Forces acting on the partial denture

Removable partial denture is subjected to a composite of forces arising from three principal fulcrums.

Movement about an axis through the most posterior abutments (Principal fulcrum line).

Movement about the longitudinal axis (in the sagittal planes the distal extension moves in rotary direction about the residual ridge)

Movement about an vertical axis located near the center of the dental arch.



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Movement about an axis through the most posterior abutments .

(Principal fulcrum line)

(in sagittal plane)


Movement about an axis through the most posterior abutments .(Prnicipal fulcrum line)

Force activated by Resultant force

Counteracted by

Sticky foods Vertical lift Retention

Tongue and muscle forces

Vertical lift Adequate denture base coverage

Gravity (maxillary) Vertical lift Indirect retainers , Gravity (mandibular)

Occlusal load Movement towards the residual ridge

Rests , Adequate denture base coverage, choice of connector


Movement about the longitudinal axis (in the frontal plane as the distal extension moves in rotary direction about the residual ridge)

Force activated by

Resultant force

Counteracted by

Occlusal force on one side of the arch causes lifting forces on the contra lateral side of the arch

Twisting and tilting

Rigid connectorsDirect retainer designDenture base coverageDenture tooth placement and Contour of the denture base.


Movement about an vertical axis located near the center of the dental arch

(in horizontal plane)Force activated by

Resultant force

Counteracted by

Masticatory stress

Twisting and spreading of RPD

Adequate bracing: Rigid connectors Connector choiceDenture base coverageOcclusal balance contour of the denture base.


That quality inherent in the prostheses acting to resist the forces of dislodgement along the path of placement.

Retention –Definition (gpt-7)


Sources of retention in RPD

Physiologic retention Mechanical retentionFrictional retention


Physiologic retention

Proportional to the tissue covered by denture base.

Important in extension base partial dentures.

Maximum coverage is needed so that the primary retention line is so far anterior that indirect retention is ineffective.


Maximum coverage is needed so that the primary retention line is so far anterior that indirect retention is ineffective.


Mechanical retention

Provided by direct retainers

DIRECT RETAINERS

Intra coronal Extra coronal

Precisionattachments

Semi precision attachments

Retentive clasp

assemblies

Attachments

Suprabulge Infra bulge


Retentive clasp assemblies

Retention clasp assembly represent most common method of extra coronal direct retention.

First appeared in dental literature with W. G. A. BONWILL description in 1899.

Proper use of dental surveyor is only reliable method of effectively analyzing teeth for their contribution to the retention of partial denture.


Classification of the clasps

Based on design : Circumferential type (Akers, supra

bulge) clasp, Bar type clasp (Vertical

projection, Roach, infra bulge). Based on construction :

1. Cast clasp,2. Wrought wire clasp,3. Combination clasp


Parts of a clasp

Clasp assembly consists of : Rest, Reciprocal arm Retentive arm,, minor connectors



The component of the clasp that provides vertical support or prosthesis is called rest and the portion of the abutment tooth prepared to receive rest is called rest seat.

Properly prepared rest seats and rests serve to :

Resist displacement of prosthesis towards supporting tissues.

Transmit functional forces parallel to the long axis of abutments.


Retentive arm is only portion of the RPD that contacts the surface of an abutment apical to the height of the contour. Depending on their approach to the undercut region of the abutment they are termed

Supra bulge clasp arm and

Infra bulge clasp arm


Supra bulge clasp arm originates from a minor connector occlusal or incisal to the height of the contour.

The portion of the clasp arm arising from the minor connector is known as shoulder which compromises the proximal third in a clasp arm.

From the shoulder the clasp arm follows gently curved path on the surface of the abutment and the clasp arm passes over the height of contour and its tip contacts undercut and this portion of the clasp is terminus.


The infra bulge clasp arm consists of two distinct segments approach arm and terminus.

Approach arm is a minor connector that originate from frame work travels horizontally along the surface of the mucosa and then turns vertically to cross the free gingival margin at 90°.

The terminus arises from the vertical portion of the approach arm and engages undercut and surface of the abutment.


The component of the clasp assembly from that braces an abutment during prosthesis insertion and removal is called the reciprocal element. The reciprocal element may be a cast clasp, lingual plate or a combination of mesial and distal minor connectors.

Reciprocal elements must contact the abutment tooth at or occlusal to the height of the contact.


Incorrect Correct


Mechanism of action of reciprocal arm

As the retentive arm passes over the height of the contour the arm displace a slight amount of flexure. This flexure places lateral stress in the abutment. If the stress is not counter acted it may cause lateral displacement of the abutment and significant damage to the supporting periodontium. To prevent these effects, a rigid reciprocal element must be used.

During insertion of the prosthesis a reciprocal element should contact the abutment slightly before the retentive arm contacts the abutment and this contact should be maintained while the retentive terminus pass over the height of the contour and onto the undercut.


The abutment surface that will be contacted by the reciprocal element should be parallel to the path of the insertion of the removable partial denture.

Minor connectors join the elements of the clasp assembly to the other parts of RPD and they must be rigid depending on the philosophy of the design.

The minor connector serve as guiding plate to direct insertion and removal of RPD

Reciprocal element to counteract non axial forces.Approach arm for intra bulge clasp.


Cast circumferential claspWas introduced by Dr .N.B. NESBITT in (1916)Design of choice in tooth supported removable partial

denture.Advantages:Simple and easy to constructClose adaptation to the abutment – minimizes the

entrapment of the food and debris.Disadvantages:As the large amount of the tooth surface is covered by the

cast assembly ; if proper oral hygiene is not maintained, under lying enamel is at under increased risk of calcification.

Interfere with food flow and bolus during elimination during the masticatory process. As a result , the abutment and associated periodontium may be damaged.


Design rulesClasp should originate from

portion of frame work that lies above height of the contour

Retentive arm should extend apically and circumferentially in gently arcing manner. Terminal third of the retentive clasp should pass over the height of the contour and enter the infrabulge portion of the abutment. Retentive terminus should be directed occlusally , never towards the gingiva.

Clasp arm should terminate at the mesial or distal line angle never at the midfacial or midlingual surfaces.


Retentive arm should be placed as far as apically on the abutment as practical.

Reciprocal arm should be located at or slightly above the height of the contour on the opposite surface of the abutment teeth.

In an extension base partial denture clasp should not engage mesiofacial surface of the abutment adjacent to posterior edentulous space.

Distofacial surface of the abutment surface of the abutment adjacent to the anterior edentulous spaces


Simple circlet design

Clasp of choice to the tooth supported removable partial dentures.

Clasp originates from the proximal surface of the abutment adjacent to the edentulous space with the clasp arms away from the edentulous space.


AdvantagesEasy to construct Simple to repair.DisadvantagesIncreased tooth coverage may promote

decalcification and compromise esthetics .Interfere with the elimination of the food

from the occlusal table.It can be adjusted in only in one direction

(bl).


Reverse circlet clasp

Used when the available undercut is located at the facial or lingual line angle adjacent to the edentulous space.

Retainer of choice when the infrabulge clasp cannot be used


In distal extension applications ,it helps to control stresses transmitted to the abutment teeth during functional movement of the prosthesis

Not a clasp of choice in canine and premolar areas.

Main problem associated with the clasp is the strength, and food impaction between the proximal plate and surface of the abutment.


Embrasure clasp designIt is the two simple circlet

clasps joined at the bodies.Used in the side of the arch

where there is no edentulous space.

Clasp originates from the minor connector that traverse the marginal ridge between teeth

Retentive arms engage undercuts on opposing line angles.


Ring clasp designIndicated on a tipped

mandibular molarEngages the mesiolingual

undercut of the mesially tipped molar.

Clasp originates at the mesial rest and traverses the facial and distal surfaces of the tooth ,remaining occlusal to the height of the contour

And at the lingual surface the clasp arm engages the undercut at the mesiolingual line angle.

Additional support is provided by the bracing arm at the facial surface of the tooth.


Bracing armOriginates at the minor connector near used to

retain the acrylic denture base. Provide reciprocation Runs horizontally across the mucosa apical to the

abutment teeth and then turns occlusally to cross the free gingival margin and join the clasp assembly in the midfacial aspect.

An additional rest is placed on the disto occlusal surface if desired.

Not considered when There is soft tissue undercut.

Vestibular depth is minimal.


C clasp designReferred to as fish hook

or hair pin clasp.It is a simple circlet

clasp where the retentive arm loops back to engage an undercut apical to the point of the origin.

Occlusal portion of the clasp should be rigid and display consistent dimensions.

Retentive portion of the clasp should be tapered and flexible. www.indiandentalacademy.com

Abutment should Have sufficient vertical height.Adequate space between the occlusal and retentive

portion of the clasps.Occlusal aspect of the retentive arm should not

interfere with the opposing teeth in maximum intercuspation.

DisadvantagesGenerally yields inadequate flexibility,

abutment subjected to non axial forces.


Multiple circlet clasp

It involves two circlet clasps joined at terminal aspects of the reciprocal elements.

Indicated when the primary abutment tooth is periodontally compromised and stresses originating from prosthesis can be transmitted to the mutiple abutment teeth.


Onlay clasp

It consists of a rest the covers the entire occlusal surface and serves as origin to buccal and lingual arms.

Indicated when the occlusal surface of the abutment is apical to the occlusal plane .

Onlay rest serves as the vertical stop.

Done in caries resistant patients.


Half and half clasp

Consists of circumferential retentive arm arising from one direction and a reciprocal arm arising from other. second arm arises from the second minor connector and it may be used with or without the rest.

Reciprocation can be accomplished by minor connector avoiding the much tooth coverage


Combination clasp

O.C. Applegate introduced modified wrought wire clasp assembly known as the combination clasp.

It consists of occlusal rest, a cast metal reciprocal arm, and a wrought wire retentive arm.

It is used when maximum flexibility is desirable such as on abutment tooth adjacent to the distal extension basesWhen a bar type retainer is contraindicated on the abutment.Esthetic advantage over cast clasps.

.


The most common use of this clasp is on abutment tooth adjacent to the distal

extension base where only mesial undercut is present and

use of the bar clasp is contraindicated due to presence of large tissue undercut

When the undercut is on the side away from the extension base the tapered wrought wire retentive arm offers great flexibility than does the cast clasp arm and therefore better dissipates the forces


AdvantagesFlexibilityAdjustabilityEsthetic advantage over the cast clasp armsMinimum tooth surface covered because of its

line contact with the toothFatigue failures are less common than the cast

retentive armDisadvantagesExtra steps in fabrication of the retentive armDistorted by the careless handling of the

patientsAs it is bent with the hand the adaptation may

not be so accurate and less stabilization in the suprabulge portion.


Infrabulge clasps

Clasp that approaches the undercut region of the abutment from n apical direction.

Different types of infrabulge retainers:T claspModified T claspI clasp or I barY clasp


Design rulesApproach arm should not impinge on the soft tissues of

the abutment.Tissue surface of the approach arm should be smooth

and well polished.Approach arm should cross perpendicular to the free

gingival margin.Approach arm should never be designed to bridge a soft

tissue undercut since it will produce food entrapmentApproach arm should be tapered from its origin to the

terminusClasp terminus should be placed as far apically on the

abutment as is practical as it decreases the leverage induced stresses resulting from the movement of the prostheses.

Minor connector attaching the occlusal rest to the framework should be rigid and should contribute to the overall stabilization and bracing characteristics of the prostheses. www.indiandentalacademy.com

Advantages :Minimal tooth contact and minimal distortion of

the normal tooth contours.-improved tissue stimulation and oral hygiene, decreased caries and periodontal problems.

Improved estheticsIncreased retention because of the tripping

action.Decreased torquing forces applied onto the

abutment tooth in the extension bases.Improved adjustability because of the location of

the bend is less critical than for the circumferential clasps.


DisadvantagesCannot be used in presence of soft tissue under

cuts ,a shallow vestibule, or high frenal attachments.

Bracing action provided by bar clasps is less than circumferential clasps.

Esthetic s may be affected if the patient has a high smile line enough to expose the approach arm as it crosses the gingiva.

May not be retentive unless the rigid elements determine the specific path of insertion of the prostheses.


T clasp Used in Kennedy Cl I and II

situations when an undercut is present adjacent to the edentulous tissues.

From the point the approach arm turns vertically to cross the free gingival margin and contact the abutment two horizontal projections arise.

One projection passes the height of the contour and enters the undercut and the other remains occlusal to the height of the contour.

If the undercut is present on the mesio facial surface this clasp cannot be used in Kennedy Cl 1 and II partially edentulous application.


Mode of action of the T clasp


Modified T clasp design

It is identical to the T clasp except that it will act as non retentive horizontal projection.

Provides improved esthetics in most of the applications.

So used when the canines or premolars will serve as abutment.


Y Clasp

Y clasp is formed when approach arm terminates in the cervical third of the abutment while the mesial and distal projections are positioned near the occlusal and incisal surfaces.

Mechanics are similar to T clasp.


I CLASP

The approach arm originates from the frame work and projects horizontally over the soft tissues adjacent to the abutment and a gradual turn allows this arm to assure vertical orientation and cross perpendicular to the free gingival margin.

The clasp arm contacts the abutment that extends from undercut to the height of the contour.


The contact area between the clasp and the abutment 2 – 3 mm in height and 1.5 -2 mm in width.Commonly used in class I and class II partially edentulous arches.The clasp terminus in these cases should be placed at or mesial to the mid facial prominence of the abutment.


RPI Concept

First introduced by Kratchovil and later modified by Krol

RPI clasp assembly consists of mesio occlusal rest with the minor connector in the mesiolingual embrasure, a distal guiding plane and the I bar clasp located at the 0.01 undercut in the gingival third of the buccal or labial surface.



Survey lines

Blatterfeins classification of buccal and lingual survey lines:

Medium survey line

High survey line

Low survey line

Diagonal survey line


Medium survey lineOn the buccal and lingual surface of the tooth

equidistant from the occlusal and gingival margin in the near zone and slightly nearer the gingival margin in the far zone.


Indicates use of an occlusally approaching arm and also gingivally approaching arm.

Various forms of bar clasps are appropriate, the length of the bar used depending upon the resilience required in the arm.

If a larger degree of the undercut is to be engaged more resilience is required and the length of the bar is increased.

It is often necessary to engage undercuts more deeply so that the end of the clasp will be replaced nearer to the gingival margin.

The more resilient the arm in these cases the less the brazing effect, this factor too makes it separate demand on bar length.


Diagonal Survey line

This line lies nearer the occlusal surface than a gingival margin in the near zone of the tooth but in the far zone the opposite condition exists and little undercut is present.

Most commonly found on buccal surfaces of the canines and premolars.


If an occlusally approaching arm is preferred reverse circlet and C clasp can be used.

When the gingival approaching arm clasp is preferred L or T bar clasp can be used.

If there is rather more undercut as seen in the figure , U shaped bar clasp can be used.


High survey line

This survey line appears much nearer to the occlusal than the gingival surface of the tooth in both near and far zones.

It may arise as a result of abnormal tooth form where the occlusal surface has a considerably larger perimeter than the cementoenamel junction and where only small degree of convexity of tooth is present.


It results from the inclination of the tooth frequently found in the lingual surface of the lower teeth and the buccal surfaces of the upper teeth.

If occlusally approaching arm is required three arm clasp is to be used but if the arm is to cover only one surface of the tooth wrought wire is generally preferred.

When high survey line results from inclined tooth it is generally found that the opposite surface of the tooth has little or no undercut.

Then reverse back action and back action clasps are used.


Gingivally approaching clasp arm find little application with high survey lines that result from tilted teeth.

In these circumstances the supporting ridge generally confirms in its slope to that of the tooth.

Assuming vertical path of insertion, a greater tissue undercut exists so only gingivally approaching clasp cannot be used.


Low survey line

The survey line is traced very low in the buccal or lingual surfaces of the teeth.

Frequently occurs in:

marked inclinations of the teeth,

High survey line associated on the opposite surface,

Conically shaped tooth.


Tooth having a low survey line cannot bear a retentive clasp arm as:

Insufficient Under cut exists to the effective in retaining the denture,

The clasp arm in such a undercut will be situated near the gingival margin.

If the tooth support is adequate it is possible to place the arm within the 1mm of gingiva, but if the arm is thick non self cleansing area is liable adjacent to the gingival line and if there has been gingival recession or gingivectomy has been done it is necessary to keep the clasp arm above the cemento enamel junction.


The tooth surface with low survey line can always face brazing arm which is of two kinds:

Reciprocal arm of the circumferential clasp

If the tooth is tilted wrought wire is usually placed in the opposite side.

Additional retention is required three methods exist for obtaining it:

Using the near proximal undercut of the tooth,

Using the extended arm clasp,Crowning the tooth and developing

suitable contours.


REQUIREMENTS OF CLASP ASSEMBLY

RETENTIONSTABILITY,RECIPROCATION,ENCIRCLEMENT,PASSIVITY,SUPPORT


RetentionRetention is the quality of the clasp assembly that

resists forces acting to dislodge components away from the supporting tissues. Each part of the clasp contribute to some feature of retention;

The retentive arm must be designed so that only the clasp terminus engages the undercut.

Rest provide support maintaing the clasp terminus in the optimal location.

Minor connector must be rigid to ensure proper stability and function of the parts of the clasp assembly.


Reciprocal element must contact the abutment before the retentive elementcontacts the tooth .

Components must provide encirclement to prevent movement of the abutment away from the assosciated clasp.

Indirect retainers must resist forces acting to dislodge the denture.

Amount of retention provided by the clasp is provided by factors like:

Flexibility of the direct retainerDepth of the undercutLocation of the retentive terminal


Flexibility of the clasp arm

Maximum flexibility of the retentive arm is the greatest amount of displacement that can occur without causing permanent deformation of the clasp arm.

Flexibility of the retentive arm is dependent uponLengthDiameterCross-sectional formMaterial which the clasp is made.


Length of the clasp armLonger the clasp arm more flexible it will

be (all the other factors being equal.)Length of the clasp arm is measured from

the point which the uniform taper begins.


Permissible flexibilities of retentive cast circumferential and bar type clasp arms for chromium cobalt alloys

Circumferential clasp Bar clasp

Length (inches)

Flexibility (inches)

Length(inches)

Flexibility (inches)

0-0.30.3-0.60.6-0.8

0.010.020.03

0-0.70.7-0.90.9-1.0

0.010.020.03


Diameter of the clasp arm As diameter of the clasp arm decreases flexibility

increases. Flexibility of the half round cast clasp have shown width

to thickness ratio of 2:1 to be optimal. Uniformly tapered clasp is more flexible than non

tapered clasp of the same proximal dimensions. The cross sectional dimensions of the shoulder of the

clasp should be twice the cross sectional dimensions at the clasp terminus.


Cross section form of the clasp

A circular cross sectional clasp form imparts omni directional flexure while a half round forms allows only bi directional flexure.

Clasps exhibiting circular cross section may permit dissipation of the detrimental forces during functional movement of the prosthesis as it flexes in all spatial planes.

Half round clasp flexes in a plane perpendicular to the flat surface so stress dissipation is limited.

As a result retentive clasp in circular form provides advantages.


Material used for clasp arm

Alloys exhibiting greater elastic moduli exhibit greater stiffness and vis versa.

Cobalt chromium and nickel chromium alloys have higher elastic moduli than gold alloys.

Clasps made from chromium alloys are rigid, so gold alloys provide a good flexibility ;but are quite expensive, so a wrought wire alloy can be used when greater flexibility is required.


Wrought wire produced by drawing a cast metal through the die. This process produces elongation of alloy crystalline micro structure which imparts greater flexibility than a conventional crystalline structure.

Flexibility of wrought wire diminished if the material is allowed to re crystallize which can occur as a result of cold working and heating a wrought structure.


Location of clasp terminus

It is related to height of contour and may be described in two dimensions :

Horizontal,Vertical.The position of a clasp terminus with regard to

this dimensions will influence the retentive forces exerted by the clasp and this process is accomplished using surveyor.


Height of the contour can be traced on to the surface of the cast by surveyor and the survey line separates the supra bulge and infra bulge portions of the abutments.

The infra bulge portions represent the mechanical undercuts in the clasp terminus is positioned into the undercut and the proper position of the clasp terminus can be identified by measuring the undercuts using the undercut gauzes.

They are available in .01,.02,.03 configurations.


Depth of the undercut

The amount of the undercut used in given situation depends on many factors:

Keeping all other clasp related factors constant, position of the clasp terminus at the greater horizontal undercut will result in increased retentive force.


Angle of gingival convergence

Angle formed between the analyzing rod and tooth surface apical to the height of the contour.

Angle of gingival convergence becomes greater , force required from the removal of the abutment becomes greater.

This may provide advantages or disadvantages depending on the clinical situation.


If a severe angle of gingival convergence is present recon touring the tooth surface to reduce the angle or placing the clasp terminus in the smaller undercuts can be done both are directed at maintaining an optimal retentive force for chosen clasp assembly



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Retention/ Labial orthodontics