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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014

Today’s lecture is about direct and indirect retainers

1- Direct retainers

It is that component of a removable partial denture that is used to retain and prevent dislodgment( vertical path away from the teeth and mucosa ), consisting of a clasp assembly or a precision attachment

The retainers function is retention which is:

That quality inherent in the prostheses which resists the forces that try to dislodge the denture away from the teeth and tissuesincluding

1 -force of gravity mainly affecting the maxillary denture.

2 -the adhesiveness of food (affect both the maxillary by pulling it downwards and the mandibular by pulling it upwards )

3 -and the forces associated with the opening of the jaws which are the muscles.

Classification of Direct Retainers

1-Extra coronal : which means clasps outside the contours of the crown .include

a- Extracoronal attachments (Dalbo)

b- Retentive clasp assemblies

2 -Intracoronal :which is on the crowns all are (precision attachments) , examples:

Stud attachment

Bar attachment

Intracoronal attachments

The crowns are prepared on their proximal surfaces so that certain slots placed on the prosthetic crowns can slide into the abutments

preparations precisely.

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Stud attachments

certain slots can be placed intra radiularly (inside the root canals) it’s composed of female component in the prosthetic tooth and male component in

the root.

Extra coronal Retainers

Clasp assembly components

Retentive arm

-Reciprocal element (arm, plate, or a combination of mesial and distal minor connectors)

-Rest

To connect clasp arms with the major connectorMinor connector(s)

-Shoulder (connects between minor connector and the arms in suprabulge clasps)

Body (connects the rest to minor connector)

*Clasp is composed of

A retentive arm which is the flexible part of clasp and the only part that goes under the survey line

While the retentive clasp arm is above it

*Shoulder connects the arm to the body.

*the guiding plate: is the part of the minor connector that is touching with the guiding plane of the abutment.

Principles of Clasp Design

*Occlusal rest should be designed to prevent tissue-ward movement and provide support for the clasp not to move during function. The retentive arm of occlusally approaching clasp should not approach closer than 1 mm to the gingival margin as it can be irritating to the gums and can injure the soft tissues.

-*Each retentive terminal should be opposed by reciprocal component a counter actingto the retentive tip moving up and down the survey line. The reciprocal elements should be placed at the

height of contour and the retentive elements below the height of contour(surveyline ) .

-Balanced retention: Where there are clasps on opposite sides of the arch, the retentive arms are best placed on opposing tooth surfaces, ie buccal/buccal or lingual/lingual

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014It means that retention should be on the same tooth surface on the two halves or sides of the arch as the RPD is a bilaterally balanced prosthesis

*Only the minimum amount of retention should be used not to put too many clasps on the RPD design as it makes it difficult to insert and remove , also a major rule in RPD design is simplicity.

Maximum 2 clasps at each side of the arch

Functional requirements of the clasp

-Retention

-Stability

-Support

-Reciprocation

-Encirclement

-Passivity

1 - Retention

The most important function of the clasp, provided by the retentive tip .

Note : the more retension is needed , the more rigid the clasp material and the less flexible of course

*Factors affecting retention:

1 -Depth of the undercut ( measured by the undercut gauge the deeper the clasp retentive tip is and the more retention

a- Buccolingual width: Determines the clasp alloy (more flexible alloys for deeper undercuts: 0.25 Co/Cr (most commonly used), 0.5 and 0.75 Gold and wrought wire stainless steel).

b- Occlusogingival height: Determines the length of the clasp and as it increases thus its flexibility is more but less retention

c- Mesiodistal depth: Determines the length of the retentive tip.

2 -Cross-sectional form of the clasp: Round clasps are flexible in all directions as opposed to half round which flex in one direction so, round is more flexible (round clasps are preferred for distal extensions)

3 -The approach of the clasp: Gingivally-approaching clasps provide better retention because they provide a push action .

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Dima AbuBakerProsthodontics – sheet no.5 18-3-20144 -Cross-sectional dimension of the clasp: thickness Inversely proportional with flexibility

5 -Uniformly tapered clasp are more flexible than non-tapered clasps ( more rigid).

6 -Curvature of the claps: A clasp curved in more than one plane has reduced flexibility .

2nd clasp requirement : Stability

to resist displacement by horizontal functional stresses.

Provided by the reciprocal element, the shoulder of an occlusally approaching clasp and the vertically oriented minor connector .

So vertically situated components prevent horizontal forces effect

3 rd requirement : support

The resistance to the movement of the denture in a gingival direction

Provided by the rest part of the clasp assembly.

The shoulder and the minor connector might contribute to support to a much lesser extent .

So any part that’s on the horizontal or occlusal plane provides support

4 th requirement : Reciprocation

It should be provided on the opposite side of the tooth but in some cases this cant be done so other means of reciprocation are used and placed on mesio/disto lingually

Reciprocation should be provided on clasped tooth diametrically opposite the retentive clasp tip. If a reciprocating clasp is used it should be placed at the gingival end of a guide surface on the clasped toot or a plate connector is used, reciprocation can be obtained by a guide plate on the connector .

Picture 1

Reciprocating aem is exactly opposite to the retentive arm but also extend on the entire lingual surface

Picture2

Guiding planes are prepared to allow the reciprocating arm remain in contact with the tooth surface as it reciprocate the retentive arm up and

down movement

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014Picture 3

In maximum coverage RPD , a guiding plate is providing reciprocation .

5 th requirement Encirclement

The clasp must encircle more than 180 º of the abutment tooth the or otherwise with clasp movement it can dislodge RPD. It can be either continuous (circumferential clasps) or broken contact

(infrabulge clasps). This property prevents movement of the tooth away from the clasp assembly .

Infrabulge clasps must contact the tooth at 3 widely separated areas that encompass more than half of the tooth circumference .

6 th requirement : Passivity

The clasps placed on abutment should never exert an moving forces ( unlike orthodontic clasps)

The quality of or condition of inactivity or rest assumed by the teeth , tissues, and denture when the RPD is fully seated (not under masticatory pressure). RPD should not exert any tipping movement on abutments.

**Incomplete seating of the prostheses results in the retentive tip of the clasp applying harmful forces on teeth .

Types of Clasps

According to the approach to the undercut

*Occlusally approaching clasps (suprabulge clasps or from above the survey line)

*Gingivally approaching clasps (infrabulge claspsor from bellow the survey line)

1 - occulsally approaching

Mainly Circumferential clasp

All its components are above the survey line except for the retentive tip

Advantages

-Easiest clasp to make and repair.

-Less food retention.

Best to be applied in tooth supported RPD ( bounded saddles ).

-Drives excellent support, bracing (reciprocation) and retention.

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014Disadvantages

-Covers a large tooth surface area.

-Difficult to adjust with pliers.

Should never be used to engage the mesiobuccal undercut in distal extension RPD 9free end saddle) as the clasps are rigid and they injure the single abutment tooth.

Types of Occlusally approaching clasps

1-Simple circlet clasp.( most commonly used)

2-Reverse circlet clasp.(sometimes used)

3-Multiple circlet clasp.

4-Embrasure clasp( double akre)

5-Ring clasp

6-Fishhook or hairpin or reverse action clasp

7-Onlay clasp

8-Combination clasp

9-Half and half clasp

10-Back-Action clasp .

** -A retentive tip of occlusally-approaching clasps should be at least 15 mm in length( to provide the length needed for the gradual taper to the retentive tip ) , and engages an undercut of no more than 0.25 mm if it is constructed in cast Co/Cr so if using Co/Cr . Therefore, should be restricted to

molars ( so on a premolar there is only 7-8 mm which is not enough and so not used ) .

**A retentive clasp engaging a 0.5 mm undercut should be constructed in wrought wire. This should be at least 7 mm in length and hence suitable for premolars

-If an undercut on a tooth is less than 0.25 mm, then composite resin should be added to the tooth to create at least this amount of undercut.

Clasp types in details

1 - Simple circlet clasp

Best for tooth supported RPD

-It approaches the undercut from the edentulous space and from the occlusal

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014-Retentive arm should run from the side of the tooth with the least undercut to the side with the

greatest undercut

-the undercut should be away from the rest of the clasp meaning that the part under the survey line should be on the distal of the abutment.

2- Reverse circlet and reverse approach clasp

Used in distal extension RPD, if bar clasp is contraindicated, when the undercut is located adjacent to the edentulous area

in this picture reverse approch clasps are on molars and they reverse the direction to be located under the survey line which is next to the rest and the body of the clasp assembly

this pictures shows a free end saddle 9 distal extension ) RPD and a premolar in which the simple circlet cant be used so revese circlet is used instead

note : other option to use on premolars in general are gingival approaching clasps

3 - Multiple circlet clasp :

Used when the principal abutment has poor periodontal support ( because of periodontal diseases and compromised bone support )

Multiple circlet on two adjacent abutments

4 - Embrasure clasp (Double Aker’s )

Very common , two clasp assembly and joined at rests held on one minor connector

Used on the reciprocating side where there is no edentulous space.

5 - ring clasps

Used in isolated and tilted molar teeth

Usually in bounded saddle where maxillary molars tilt mesiobuccally and mandibular molars mesiolingually , so if using a simple circlet it will be above the survey line , so a ring that circles the

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014tooth from lingual to the buccal so the retentive tip reaches bellow the surveline . it needs a lot of support by having occlusal rests mesially and distally or a buccal strengthening arm.

6 - Fishhook or hairpin or reverse action clasp

it has a curvature to which rigidity is added and retentive tip is flexible and in the undercut

used in the undercut is adjacent to the edentulous area

and infrabulge clasps (simple circlet) cannot be used so we use fishhook or gingivally approaching clasps.

7 - onlay clasp

To restore the normal occlusion of submerged abutments

In which an onlay is added over a submerged tooth with clasps on the onlay to be placed on adjacent teeth which are not submerged.

8 - Combination clasp ( very commonly used)

Consist of a rest seat, a cast ( build in the lab) reciprocal arm and a wrought wire retentive arm (provides more flexibility because of the stainless steel )

*Severe undercut and Co/Cr can’t be used

*Maxillary Canines for aesthetics

9- Half and half clasp ( uncommon)

Consists of a minor connector attached to an occlusal rest and retentive arm, and a minor connector attached to a reciprocal arm which is similar to a bar clasp with or without an auxiliary rest

- Provides double retention

-it’s like half of it is an occlusally approaching and the other half is a gingivaly approaching clasp

10 - Back-Action clasp

- A modification of a ring clasp but the minor connector is attached to the end of the clasp arm and the occlusal rest is left unsupported

- Lacks sufficient support and not commonly used

- only difference from the ring clasp Is the second rest placed without a minor connector

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Gingivally approaching clasp:

It has an approach arm that crosses the gingival and bend on a 90 degree to reach the undercut from bellow the survey line

** have a push type of retention, so to remove it you need to push which is harder than the pull action used in the occlusally approaching clasp

**ginigvally approaching is more retentive because of its push action, although the long approach arm increase in flexibility therefore lowers the retention provided

All components of the clasp assembly are similar to those of suprabulge clasps except for the retentive arm which is comprised of:

1 -Approach arm ( a minor connector that connects the retentive terminal to the base of the denture )

2 -Retentive terminal

Advantages

-Easy to insert and difficult to remove.

-More aesthetic as the approaching arm is bellow the survey line

Disadvantages

-Tend to collect food debris

-Increased flexibility but reduced bracing and stability. ( reciprocation is through minor connectors rest , encirclement reciprocating arm but non- continuous 180 degrees so, inferior bracing than the

occlusally approaching clasps)

**the approach arm is in contact with gingival but is stable and doesn’t move against it so it’s not irritating

Types of Infrabulge Clasps

Accoding to the termenal’s shape

1 - T Clasp

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014Used for Class I and II when the retentive undercut is next to the ridge. One end provides retention and the other improves bracing ( which is above the survey line , so have of the horizontal T part is bellow the bulge ( retention ) while the other have is above it for reciprocation)

2- Modified T Clasp

Similar to T-clasp but lacks the non-retentive arm. Has better aesthetics and best for premolars and canines

3 - Y-clasp

Similar to T-clasp but slightly different morphology

**So shape of the tip follows the shape of the undercut

4 - I Clasp

Best for Class I and II, as part of RPI system, if undercut is located mesially, it provides stress-breaking effect as it contacts part of the ginigva and tooth so dissipates the forces.

Notes:

**A gingivally-approaching clasp should be used if a retentive cast cobalt chromium clasp is required on a premolar or canine tooth , as Co/Cr is too rigid and will distort

**A distal extension saddle should have a retentive I-bar clasp whose tip contacts the most prominent part of the buccal surface of the abutment tooth mesiodistally .

The RPI system (Rest, Plate, I-bar clasp) which is a system of the distal extension. in this case, should be used if the tooth and buccal sulcus anatomy are favourable

**If a cast gingivally approaching I-bar cannot be used for these cases, a wrought wire occlusally-approaching clasp might be or another clasp design (example,if the undercut is close to the saddle we use the reverse clasp design)

**special type of clasp design though it’s a Kinedy major conector is Continuous clasp composed of a metal bar on the lingual surface of teeth and in some cases a lingual bar is also used like in this

picture

Factors affecting the choice of clasps

1- Position of retentive undercut

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in this picture a bounded saddle is present and according to the survey line 1- on the molar a simple circlet can be used 2- on the premolar an I-bar is used.

second picture is a distal extension and a reverse circlet or a ring clasp are used.

2- The health of periodontal ligament

The peridontiom should not be inflamed it should be healthy or controlled health , if a compromised periodontal conditions a more flexible clasp if it allows a an I-bar gingivally approaching mainly , but if like this image the root is exposed and carious we can use

an occlusally approaching clasp which is a wrought wire type.

3 - The shape of the sulcus

In this image frenum is low attached so gingivally approaching is difficult , the only option is wrought wire occlusally approaching.

4 - The length of the clasp

On the molar it’s easy to have a 15mm gradual taper to the retentive tip under the surveline but on premolars only 8mm in measurement so , the clasp choice is either gingivally or wrought wire occlusally approaching

clasp.

**A Co/Cr clasp arm, approximately l5 mm long, should be placed in a horizontal undercut of 0.25 mm. If the undercut is less the retention will be inadequate. If it is greater, the clasp arm will be distorted because the proportional limit will be exceeded. A Co/Cr occlusally-approaching clasp

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014engaging the same amount of undercut on a premolar is likely to distort during function because it is too short. In such a situation, a gingivally-approaching clasp is recommended.

**proportional limit means the limit of forces after which the clasp starts to distort.

-Whether this choice is appropriate depends on certain clinical factors. Alternatively, an alloy with a lower modulus of elasticity but similar proportional limit, such as a platinum–gold–palladium wire, can be used. Another option is to use a material with a higher proportional limit but similar modulus such as wrought stainless steel and so used in deeper undercuts or shorter mesiodistal measurement of the abutment

5 - appearance

Occulusally approaching are of low aesthetics so are never used on maxillary premolars , gingivally approaching is the solution

New approach is a tooth colored clasps are being manuficatured but they don’t provide proper retention and rigidity and the remaning clasp properties mentioned.

6- Occlusion

An occlusally approaching clasp must begin, and have two thirds of its length, in the area bounded by the occlusal contacts of the opposing teeth and the survey line on the tooth to be clasped. Provision of an adequate space for the clasp may require tooth preparation. Occlusal contacts, however, have no influence on gingivally approaching clasps.

**in short it means occlusaly approaching should have enough length for encirclement !

The Indirect Retainer

A part of an RPD which assists the direct retainers in preventing displacement of distal extension bases

This picture shows the importance of the indirect retainers

In dislodgment of RPD in a vertical direction if there is no retainer so the fulcrum is created at the retentive tip and so, a half circle like rotation will sink the RPD towards the tissues

So, to prevent this an extension is added to place a rest occlusally it’s for retension and not the support , to prevent the sinking by becoming the fulcrum and is occlusally placed and will not allow sinking

**a cingulm rest may also be used as an indirect retainer

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014Effectiveness of indirect retainers

-Location of the fulcrum line

--Distance of the indirect retainers from the fulcrum line (the longer the distance the better the indirect retention) the more it is the more the effictivness ideally should be on a 90 degree from

the retentive tip to a distal tooth ( becoming an abutment).

-Rigidity of the connector.

-Effectiveness of the supporting tooth (premolars and canines are better than incisors).

This picture the premolar where the rest is placed is exactly on a 90 degree from the rotation axis or fulcrum ( the arrow)

Types of indirect retainers

Auxiliary occlusal rest or canine rest.

-Canine extension of the occlusal rest.

-Continuous bar and lingual plate.

-Indirect retention from the major connector.

-Rugae Support.

**note that an occlusal rest is place on a horizontal plane and so provides stronger support again sinking tissue-wards in comparison to cingulum rest ( as it’s on an inclined surface)

**this picture shows the indirect retainer canine rest placed from the same supportive rest on the adjacent premolar

**Rugae retainer from the lingual bar that prevents the anterior – ward sinking

It has an extension from the rests as does any major connector they are placed on inclined planes and though very difficult to provide retention so they extend rests

on occlusal horizontal to provide effective retention.

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Dima AbuBakerProsthodontics – sheet no.5 18-3-2014Good luck

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